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Morphological and Tribological Properties of PMMA/Halloysite Nanocomposites Zina Vuluga 1, *, Mihai Cosmin Corobea 1, *, Cristina Elizetxea 2 , Mario Ordonez 3 , Marius Ghiurea 1 ID , Valentin Raditoiu 1 , Cristian Andi Nicolae 1 , Dorel Florea 1 , Michaela Iorga 1 , Raluca Somoghi 1 and Bogdan Trica 1 1

2 3

*

National Research and Development Institute for Chemistry and Petrochemistry-ICECHIM, Bucharest 060021, Romania; [email protected] (M.G.); [email protected] (V.R.); [email protected] (C.A.N.); [email protected] (D.F.); [email protected] (M.I.); [email protected] (R.S.); [email protected] (B.T.) Fundacion Tecnalia Research and Innovation, Donostia-San Sebastian 20009, Spain; [email protected] Maier Technology Centre, R&D Department, Polígono industrial Arabieta 48320, Spain; [email protected] Correspondence: [email protected] (Z.V.); [email protected] (M.C.C.); Tel.: +40-21-316-3068 (Z.V. & M.C.C.)  

Received: 14 June 2018; Accepted: 21 July 2018; Published: 25 July 2018

Abstract: From an environmental and cost-effective perspective, a number of research challenges can be found for electronics, household, but especially in the automotive polymer parts industry. Reducing synthesis steps, parts coating and painting, or other solvent-assisted processes, have been identified as major constrains for the existing technologies. Therefore, simple polymer processing routes (mixing, extrusion, injection moulding) were used for obtaining PMMA/HNT nanocomposites. By these techniques, an automotive-grade polymethylmethacrylate (PMMA) was modified with halloysite nanotubes (HNT) and an eco-friendly additive N,N 0 -ethylenebis(stearamide) (EBS) to improve nanomechanical properties involved in scratch resistance, mechanical properties (balance between tensile strength and impact resistance) without diminishing other properties. The relationship between morphological/structural (XRD, TEM, FTIR) and tribological (friction) properties of PMMA nanocomposites were investigated. A synergistic effect was found between HNT and EBS in the PMMA matrix. The synergy was attained by the phase distribution resulted from the selective interaction between partners and favourable processing conditions. Modification of HNT with EBS improved the dispersion of nanoparticles in the polymer matrix by increasing their interfacial compatibility through hydrogen bonding established by amide groups with aluminol groups. The increased interfacial adhesion further improved the nanocomposite scratch resistance. The PMMA/HNT-EBS nanocomposite had a lower coefficient of friction and lower scratch penetration depth than PMMA/HNT nanocomposite. Keywords: PMMA; halloysite nanotube; nanoscratch; friction; morphology

1. Introduction Over the past 10 years, consumer attraction to black and high-gloss products increased considerably. A key property that influences consumer perception of the overall quality of the product is scratch resistance, besides gloss, colour and the sensation felt when touching the surface. Scratch resistance of plastics is becoming increasingly important due to the growing use of plastics in many areas of the industry (automotive, electrics, electronics, furniture, etc.) and the trend to eliminate paint in favour of moulded-in-colour [1,2]. A product with a few visible scratches does not

Polymers 2018, 10, 816; doi:10.3390/polym10080816

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meet consumer expectations and is perceived as a low-value product [3]. Instead, products that do not show visible damage after use are quality products, and the consumer remains faithful to the product manufacturer in the event of a new purchase. Most of plastic materials like polyamide (PA) or styrene-based polymers (ABS, SAN) have good mechanical properties but low-gloss appearance and a limited scratch and abrasion resistance, resulting in negative aesthetic properties. For automotive parts, poly(methyl methacrylate) (PMMA) is preferred because it is transparent, has good injection moulding characteristics, chemical resistance, very good weather resistance, the pleasant acoustics, good mechanical properties and low cost [4,5]. However, non-modified PMMA behaves in a brittle manner to impact and is not very scratch-resistant [6]. To meet the requirements of the automotive industry, the surfaces of plastic parts need to be modified, either with paints, or with thin polymeric films. Although unpainted products have lower scratch properties than their counterparts [7], however, for cost reduction and compliance with environmental regulations, it is necessary to find new additives to remove the painting step and significantly and permanently improve the scratch resistance of products [8]. Organically modified polysiloxanes, slip additives (oleamide, erucamide, stearyl erucamide, etc., that migrate to the surface) and special mineral fillers are the most used categories of additives in the automotive industry to improve the scratch resistance of plastics [1]. The effectiveness of these additives depends on particle size, quantity, basic polymer matrix and processing technology and has been proved mostly for polyolefins [9–13]. For maximum efficiency, more than one additive of the same category or mixtures of different categories are used [8]. Often, obtaining the desired properties means a series of compromises. For example, to obtain improved scratch resistance, it is necessary to increase the tensile strength and rigidity of the polymeric composite which unfortunately affects hardness and impact resistance or induces the loss of optic and rheological properties and vice versa. Migratory additives such as erucamide, behenamide and stearamide have traditionally been used to improve scratch resistance of PMMA [14]. Fatty acid amides migrate to the surface of plastics and form a thin film, which has as effect reducing the coefficient of friction and increasing scratch resistance. However, amides provide only short-term scratch resistance and their migration influences the final appearance of the surface regarding the gloss. Nanoparticles like nanosilica, zinc oxide (ZnO), zirconium oxide (ZrO2), nanoclay, calcium carbonate, talc, kaolin or wollastonite have a significant effect on scratch performance [1,15–18]. Surface treatments with silane may improve dispersion and therefore improve scratch resistance. To achieve large improvements in stiffness and/or strength properties, important for scratch resistance (in terms of residual scratch depth), whilst keeping as many as possible of the other properties, nanoreinforcing additives are increasingly used. Halloysite nanotube (HNT) is harmless, can be simply processed, has a high degree of structural perfection and superior mechanical properties (such as high rigidity, good tensile and flexural strength), as well as high susceptibility for dispersion in polymer matrices [19,20]. HNTs are composed of several siloxane groups, but only few hydroxyl ones, which provide the ability of material to form hydrogen bonds and thus better dispersion potential [21]. In each halloysite nanotube (HNT), the external surface is composed of siloxane (Si–O–Si) groups, and the internal surface consists of an array of aluminol (Al–OH) groups [22]. One way to improve the dispersion of HNT in a polymer matrix and obtaining nanocomposites with enhanced thermal stability and mechanical properties is the surface modification of HNT with quaternary ammonium salts [23], organosilane such as γ-aminopropyltriethoxysilane [22,24] and N,N 0 -Ethylenebis(stearamide) (EBS) [19,25]. EBS is a synthetic wax used as a dispersing agent or internal/external lubricant for benefits in plastic applications to facilitate and stabilize the dispersion of solid compounding materials to enhance processability, to decrease friction and abrasion of the polymer surface. The amide groups (–CONH–) from EBS can interact with the hydroxyl groups from the HNT surface, contributing also to better dispersion of nanotubes [19]. EBS was reported also to induce different structures and chain packaging in polar polymers like PLA [26]. This work refers to the characterization of PMMA/HNT and PMMA/HNT/EBS nanocomposites. Several papers already investigated some composites with PMMA matrix and HNT fillers, dealing with

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very particular cases (like biomaterials). Such applications included PMMA bone cement obtaining with over 5 wt % HNT (modified with gentamicine) [27]. In this case, PMMA (obtained by an initial polymerization process) mixed with modified HNT, was used to a second direct polymerization process with monomer to embed with fresh polymer and to “freeze” the material mixture. Few data were found on materials with commercial grades PMMA based on pristine HNT. Fewer data were found on automotive grades (or other thermoplastic) PMMA materials obtained by injection moulding. It is noteworthy to mention that the effects of (co)addition of HNT and EBS into PMMA matrix have not been reported. Furthermore, relatively very few studies were focused on nanoscratch damage in PMMA nanocomposites. Over time, to define the appropriate parameters that can be used to evaluate the scratching behaviour of amorphous polymers, the characteristic strain on PMMA during scratch tests at various temperatures [28], as well as the scratch characteristics on PMMA as a function of the concentration of a slip agent [7] have been studied. In this paper, addition of pristine HNT (or in combination with a specific additive) can bring for PMMA-injected materials, several improved features in what concerns the materials design, properties enhancement and especially the balance between properties (needed in real applications). To the best of our knowledge, this research can be considered a first example for proving how simple polymer processing technique can be driven, to exclude supplementary synthesis processes (for filler functionalizing with organic moieties, use of solvents, purification and so on). By this approach the entire process (i.e., for an injected sample) can be managed towards favourable interface interaction between phases, in a synergistic manner for final properties design. In this paper, the relationship between the morphology/structure (XRD, TEM, FTIR) and the conventional and nano mechanical properties of PMMA nanocomposites is discussed. Based on possible interactions between the components, an explanation of the mechanical and nanomechanical properties as well as the nanoscratch damage of PMMA/HNT nanocomposites was attempted. In dynamical conditions by the melt processing method, nanocomposites with 2 wt % HNT or HNT modified with EBS were obtained. 2. Materials and Methods 2.1. Materials PLEXIGLAS® 8N black 9V022, an amorphous thermoplastic moulding compound PMMA (automotive parts grade), was supplied by Evonik Röhm GmbH, Darmstadt, Germany. Halloysite nanotubes (HNT), natural aluminosilicate clay with a hollow tubular morphology (nanoscroll) [29], was supplied by NaturalNano Corp, Rochester, NY, USA, in white colour powder form. N,N 0 -ethylenebis(stearamide) (EBS), with molecular weight of 593.02 g/mol, density of 0.97 g/cm3 and melting range of 144–146 ◦ C, was supplied by Sigma Aldrich, Saint Louis, MO, USA. 2.2. Preparation of Nanocomposites and Samples 2.2.1. Modification of HNT with EBS The surface of HNT was modified by treatment with EBS, in dynamical conditions using a Brabender Plastograph ( Brabender GmbH & Co KG, Duisburg, Germany), at 80, 120 and 160 ◦ C, 1 h and 100 rpm. The weight ratio between HNT and EBS was 2.33: 1 (70:30). The ratio between HNT and EBS was chosen in agreement with other already existing reports like Pluta et al. [26] (described for more polar matrices like polylactic acid). Before use, the HNT was dried under vacuum for 2 h at 70 ◦ C. The samples were denoted: (HNT-EBS)80, (HNT-EBS)120 and (HNT-EBS)160, respectively. 2.2.2. Obtaining of PMMA/HNT Nanocomposites Before use, PMMA and HNT were dried at 80 ◦ C for 4 h and at 70 ◦ C under vacuum for 2 h, respectively, to minimize the water content. To achieve a uniform dispersion of HNT/surface modified

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HNT into the polymer matrix, masterbatches based on PMMA with 20 wt % HNT/surface modified HNT have been prepared in dynamical conditions using a co-rotating twin screw extruder type DSE 20 Brabender (Brabender GmbH & Co KG, Duisburg, Germany) at a screw rate of 180 rpm. Extruder temperature profile from hopper to die was 220, 230, 240, 240, 250 and 240 ◦ C, respectively. The extruded filaments were cooled down with air and then were granulated. The masterbatches were denoted as CPA-80, CPA-120 and CPA-160, respectively. Prior to feeding into the extruder, the granules of PMMA and masterbatches were mixed in a rotating mixer at RT for 15 min. The dilution of masterbatches with PMMA was done using the same conditions as for masterbatches obtaining. The extruded filaments were cooled down in a water bath and then were granulated with a Brabender Pelletizer, the take-off speed being 9 ± 1 m/min. A neat PMMA (denoted as PMMA) and PMMA with 0.86 wt % EBS (denoted as PMMA-EBS) were obtained in the same conditions and served as control samples. The nanocomposites with 2 wt % HNT/surface modified HNT were denoted as PMMA-HNT, PMMA-(HNT-EBS)80, PMMA-(HNT-EBS)120 and PMMA-(HNT-EBS)160, respectively. For clarity, the formulations and the processing conditions for all prepared and characterized samples are presented in Table S1. For mechanical testing, nanocomposite granules were injection-moulded using Engel ES 40/22 injection-moulding machine, into standard test specimen for tensile, impact and wear tests. The temperature profile ranged from 220 to 240 ◦ C and the mould temperature was kept at 80 ◦ C. 2.3. Characterization 2.3.1. Thermal Analysis TGA was performed on TA-Q5000IR (TA Instruments, New Castle, DE, USA) using nitrogen as the purge gas at a flow rate of 50 mL/min. The thermograms were acquired between 25 and 700 ◦ C at a heating rate of 10 ◦ C/min. For each measurement, duplicate samples weighed in platinum pan, about 7–8 mg, were used. Differential Scanning Calorimetry (DSC) measurements were performed on samples of 8–12 mg, by using a DSC Q2000 (TA Instruments, New Castle, DE, USA) under helium flow of 25 mL/min. The glass transition temperature (Tg ), melting temperature (Tm ), maximal temperature (Trelax ) for the enthalpic relaxation and the corresponding enthalpies were determined from the first and second heating scans as follows: equilibration at 30 ◦ C, modulate ±0.796 ◦ C every 30 s, isotherm at 30 ◦ C for 3 min, heating from 30 to 160 ◦ C at a rate of 10 ◦ C/min−1 , isotherm at 160 ◦ C for 3 min, then cooling back to 25 ◦ C, isotherm at 25 ◦ C for 3 min and, reheating to 160 ◦ C at a rate of 10 ◦ C/min−1 . PMMA/HNT samples were sealed into Tzero aluminum pans and an empty Tzero aluminum pan was used as reference. 2.3.2. Structure/Morphology XRD X-ray diffraction measurements were carried out with a Rigaku Smartlab diffractometer (Rigaku Corporation, Tokyo, Japan) using a Cu Kα radiation. In this experiment, the accelerating voltage of the generator radiation was set at 45 kV and the emission current at 200 mA. The diffractograms were recorded in parallel beam geometry over 2θ = 5◦ to 50◦ continuously at a scan rate of 4◦ /min. FTIR FTIR spectra were recorded on a Jasco FTIR 6300 spectrometer (JASCO Int. Co., Ltd., Tokyo, Japan) equipped with a Golden Gate ATR (diamond crystal) from Specac Ltd., London, UK, in the range 400–4000 cm−1 (30 scans at a resolution of 4 cm−1 ).

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TEM Transmission electron microscopy was performed using a Tecnai™ G2 F20 TWIN Cryo-TEM, 2015-FEI Company™ equipment, Hillsboro, OR, USA. Sample evaluation was performed in classical TEM, electron diffraction and scanning transmission electron microscopy mode as well. EDX analysis was carried out in scanning transmission electron microscope (STEM) mode using an SDD Oxford Instruments X-Max detector (Oxford Instruments, Oxford, UK). HNT samples were prepared by mixing the dried samples with ethanol and then ultrasonicated for 30 min for a better dispersion. A small droplet of dispersion was poured on a holey carbon grid, without staining. Polymer-based samples were embedded in Epon Resin and sectioning small slices of samples, at room temperature, with the system Leica Ultracut EM FC7 microtome (Leica Microsystems GmbH, Wetzlar, Germany). EDX analysis allowed a proper identification of the organic phase (rich in N element for EBS, rich in Al and Si for the HNT, respectively only C and O for the polymer phase). 2.3.3. Mechanical Properties Conventional Tensile and Impact Testing Tensile properties of the composites were determined according to ISO 527, with 5 mm/min for the tensile strength and 1 mm/min for the modulus of elasticity, using an Instron 3382 Universal Testing Machine (Instron Corporation, Norwood, MA, USA) and seven specimens for each test. The Charpy unnotched impact strength of the composites was determined according to ISO 179-1, using a Zwick HIT5.5 Pendulum Impact Testers (Zwick Roell AG, Ulm, Germany) and seven specimens for each test. Nanomechanical Characterization Nanoindentation Nanoindentation tests have been performed at room temperature on a TI Premier system (Hysitron Inc., Minneapolis, MN, USA) using a three-side pyramidal Berkovich tip with total angles of 142.35 deg (radius of curvature of 150 nm). Four automated indentations have been performed on the sample surface, 2 horizontal and 2 vertical, with 1 µm the distance between the indents, using the trapezoidal load function. The normal load was linear-loaded during 5 s until the maximum load of 10 mN was reached. The peak load was held for 2 s in order to minimize the viscoelastic effects of the polymer during the unloading (creep or relaxation). The force was then unloaded for another 5 s. Based on this experimental set-up, the load-displacement curves were obtained. The nanomechanical properties were obtained by the values of hardness (H) and reduced modulus (Er ), calculated using Oliver-Pharr method [30]. All values were taken as the average of 4 indentations. Nanoscratch The nanoscratch experiments have been performed using the same Hysitron TI Premier and employing the same Berkovich tip used for nanoindentation. A normal load of 5 mN was applied in a controlled manner to the indenter tip and the lateral force and lateral displacement were recorded as a function of time. In the first stage, the tip was brought in contact with the sample for indenting, during 5 s, until the specified load was attained. The peak load was held for 3 s and in the second stage the indenter started sliding on the sample surface, during 50 s. After another 3 s of force holding the tip was unloaded during 5 s. Each scratch consisted in a 20 µm scratch length and a 66 s duration. The surface topography of the samples was characterized using the scanning probe microscopy (SPM) mode of the TriboIndenter (Hysitron, Inc., Minneapolis, MN, USA). After scratching, a representative 25 µm in situ SPM image was obtained on each sample for post-test qualitative surface characterization, and for quantitative nanoscratch depth analysis (coefficient of friction, cross profile topography, residual deformation of material during the scratch, surface roughness and depth of scratch).

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3. Results and Discussion 3. Results and Discussion 3.1. Thermal Properties 3.1. Thermal Properties 3.1.1. TGA 3.1.1. TGA Thermogravimetric analysis (TGA) emphasized the structural changes for different processing Thermogravimetric analysis (TGA) emphasized the structural changes for different processing stages and the reflection of these changes in the thermal properties of the composite materials stages and the reflection of these changes in the thermal properties of the composite materials (Figures 1–3). Thermal properties were found in close correlation with the behaviour reflected in the (Figures 1–3). Thermal properties were found in close correlation with the behaviour reflected nano and macro scale properties of PMMA composites. Despite the small concentration, HNT (2 wt in the nano and macro scale properties of PMMA composites. Despite the small concentration, %) modified with EBS, improved PMMA thermal stability. Like in the mechanical properties section, HNT (2 wt %) modified with EBS, improved PMMA thermal stability. Like in the mechanical EBS or HNT alone are inducing lower thermal stability for the PMMA matrix but together a properties section, EBS or HNT alone are inducing lower thermal stability for the PMMA matrix synergistic effect was attained. Hydrogen bonds between organic compounds and aluminol groups but together a synergistic effect was attained. Hydrogen bonds between organic compounds and from alumino-silicates can lead to strong interfaces like in covalent bonding in certain cases [31]. The aluminol groups from alumino-silicates can lead to strong interfaces like in covalent bonding in main vector was proven to be the alumina sites from the alumino-silicates. In our case, the charge certain cases [31]. The main vector was proven to be the alumina sites from the alumino-silicates. compensation (for HNT 1:1 structure) is lower if compared with 2:1 layered structures like In our case, the charge compensation (for HNT 1:1 structure) is lower if compared with 2:1 layered montmorillonite, therefore a certain increase in activity should be expected. In the case of HNT structures like montmorillonite, therefore a certain increase in activity should be expected. In the case decorated with EBS, the acid sites are acting as catalyst for the EBS thermal decomposition. The of HNT decorated with EBS, the acid sites are acting as catalyst for the EBS thermal decomposition. stronger the interaction by hydrogen bonding was, the better the catalytic effect became. A faster The stronger the interaction by hydrogen bonding was, the better the catalytic effect became. A faster decomposition and a lower thermal stability of the HNT modified with EBS at 120 °C was noticed decomposition and a lower thermal stability of the HNT modified with EBS at 120 ◦ C was noticed (Figure 1a). The different degradation behaviour of the three hybrids is probably due to different (Figure 1a). The different degradation behaviour of the three hybrids is probably due to different forms forms of EBS crystallization (mainly alpha and beta phase transitions) induced by the mixing of EBS crystallization (mainly alpha and beta phase transitions) induced by the mixing temperature [32]. temperature [32]. Crystallization forms of EBS may favour coupling to the HNT body which will be Crystallization forms of EBS may favour coupling to the HNT body which will be reflected in the reflected in the improvement of interface with the polymer matrix and, consequently, in the improvement of interface with the polymer matrix and, consequently, in the enhancement of properties. enhancement of properties. The char obtained from the organic compound then fastens the The char obtained from the organic compound then fastens the inorganic dehydroxylation stage of the inorganic dehydroxylation stage of the aluminosilicate between 400 and 500 °C. aluminosilicate between 400 and 500 ◦ C.

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Figure 1. TGA 1. TGA results effect of (HNT) halloysite nanotubes (HNT) for 0 -ethylenebis(stearamide) Figure results (a) catalytic(a) effectcatalytic of halloysite nanotubes for N,N N,N′-ethylenebis(stearamide) thermal decomposition in synergistic HNT-EBS effects decorated hybrids (b) (EBS) thermal decomposition in(EBS) HNT-EBS decorated hybrids (b) between EBS and synergistic effects between EBS and PMMA decomposition fractions able to improve masterbatches PMMA decomposition fractions able to improve masterbatches maximal decomposition temperature. maximal decomposition temperature.

The TGA of the masterbatches revealed the same synergistic effect (EBS-HNT) for the PMMA, The TGA of the masterbatches revealed the same synergistic effect (EBS-HNT) for the PMMA, the thermal resistance of the polymer being improved following the trend of that of HNT-EBS hybrids the thermal resistance of the polymer being improved following the trend of that of HNT-EBS in initial processing conditions (HNT modified with EBS at 120 ◦ C, then HNT modified with EBS at hybrids in initial processing conditions (HNT modified with EBS at 120 °C, then HNT modified with 160 ◦ C and then HNT modified with EBS at 80 ◦ C) (Figure 1b). The thermolytic radical decomposition of EBS at 160 °C and then HNT modified with EBS at 80 °C) (Figure 1b). The thermolytic radical PMMA [33] starts around 300 ◦ C, by forming tertiary and secondary carbon radicals. These radicals are decomposition of PMMA [33] starts around 300 °C, by forming tertiary and secondary carbon able to recombine in mirror (in the same thermal event and temperature around 300 ◦ C) with the in situ radicals. These radicals are able to recombine in mirror (in the same thermal event and temperature EBS thermal scission products [34,35]. EBS decomposition occurs in the most probable position (in the around 300 °C) with the in situ EBS thermal scission products [34,35]. EBS decomposition occurs in beta carbon position) of the carbonyl group (lowest energy) giving one radical, then the rest of the chain the most probable position (in the beta carbon position) of the carbonyl group (lowest energy) giving remains with a reactive double bond able also to interact with PMMA radicals. Therefore, the reactivity one radical, then the rest of the chain remains with a reactive double bond able also to interact with

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PMMA radicals. Therefore, the reactivity and the reaction place provided almost immediate recombination for almost both immediate partners (EBS and PMMA molecules subjected to and the reactionopportunity place provided recombination opportunity for both partners decomposition) and the overall stabilization for degradation occurs [36]. The improvement of (EBS and PMMA molecules subjected to decomposition) and the overall stabilization for degradation thermal stability in the case of (compared neat PMMA) (compared was increased occurs [36]. The improvement of masterbatches thermal stability in the casewith of masterbatches with with neat almost 30 °C for the maximal decomposition temperature. The mirroring effect of the radical ◦ PMMA) was increased with almost 30 C for the maximal decomposition temperature. The mirroring recombination (from EBS and PMMA) reflected that the onset wasonset less effect of the radical recombination (from [37] EBS can and be PMMA) [37]by canthe be fact reflected by the factstage that the affected theaffected synergistic effect; meanwhile, main event stage) was stage wasbyless by the synergistic effect;the meanwhile, the (maximal main eventdecomposition (maximal decomposition considerable affected by the radical recombination. Thermal decomposition of PMMA is stage) was considerable affected by the radical recombination. Thermal decomposition of described PMMA is in Scheme S1: (a) homolytic bond dissociation and (b) char production. Reactions suggested for described in Scheme S1: (a) homolytic bond dissociation and (b) char production. Reactions suggested thermal degradation of EBS are presented in Scheme S2. for thermal degradation of EBS are presented in Scheme S2. In %% HNT, lessless than 1 wt waswas not In the the final finalPMMA-HNT-EBS PMMA-HNT-EBSnanocomposites nanocompositeswith with2 wt 2 wt HNT, than 1% wtEBS % EBS enough to highlight an improvement in the thermal stability (Figure 2). As2).it can seen, not enough to highlight an improvement inoverall the overall thermal stability (Figure As itbecan be both HNT and EBS are inducing a faster degradation of the PMMA but like in the mechanical seen, both HNT and EBS are inducing a faster degradation of the PMMA but like in the mechanical properties brings a synergistic effect. EBSEBS alone starts the properties the the combination combinationof ofthe thetwo two(EBS (EBSand andHNT) HNT) brings a synergistic effect. alone starts degradation earlier than PMMA (Figure 1b)1b) when PMMA the degradation earlier than PMMA (Figure when PMMAradicals radicalsare arenot not available. available. HNT HNT alone alone (without EBS) because of a high hydroxyl content and acid alumina sites induces a faster (without EBS) because of a high hydroxyl content and acid alumina sites induces a faster degradation degradation of PMMA [38,39]. The combination EBS-HNT the effect synergistic effect and the of PMMA [38,39]. The combination of EBS-HNTofprovides theprovides synergistic and the stability of stability of PMMA remains unchanged (in the final composites compared with neat PMMA). PMMA remains unchanged (in the final composites compared with neat PMMA).

Figure 2. Synergistic effects between EBS and HNT, acting on PMMA decomposition fractions. Figure 2. Synergistic effects between EBS and HNT, acting on PMMA decomposition fractions.

3.1.2. DSC 3.1.2. DSC Thermal events in DSC revealed significant features of the phase distribution in the anisotropic Thermal events in DSC revealed significant features of the phase distribution in the anisotropic bulk materials and the fine-tuning of the structure, in order to achieve the highlighted properties (in bulk materials and the fine-tuning of the structure, in order to achieve the highlighted properties nano and macro scale). First, the alpha and beta phase transitions of EBS were confirmed by DSC, in (in nano and macro scale). First, the alpha and beta phase transitions of EBS were confirmed by DSC, good agreement with the same intervals highlighted by FTIR and XRD (Figure 3). These transitions in good agreement with the same intervals highlighted by FTIR and XRD (Figure 3). These transitions confirmed the speculated processing approaches, drawn by the experimental set-up in Brabender confirmed the speculated processing approaches, drawn by the experimental set-up in Brabender mixing stage (80, 120, 160 °C for EBS-HNT). The initial EBS as received at room temperature mixing stage (80, 120, 160 ◦ C for EBS-HNT). The initial EBS as received at room temperature (without further thermal modification) consists in a mixture of alpha and beta crystals, which after (without further thermal modification) consists in a mixture of alpha and beta crystals, which after 60 ◦°C starts an endothermic event specific for the alpha to beta full transition. These data were in 60 C starts an endothermic event specific for the alpha to beta full transition. These data were in good good agreement with the ones found in literature [26]. Around 80 °C, the EBS can be found mainly in agreement with the ones found in literature [26]. Around 80 ◦ C, the EBS can be found mainly in the the beta form, which around 110 °C converts again in the alpha form. At 120 °C, the EBS was mainly beta form, which around 110 ◦ C converts again in the alpha form. At 120 ◦ C, the EBS was mainly in the alpha form again, and then at around 140 ◦°C the complete melting of EBS begins in a classic in the alpha form again, and then at around 140 C the complete melting of EBS begins in a classic endothermic event. In the HNT presence (HNT has no transition on the mentioned interval), the endothermic event. In the HNT presence (HNT has no transition on the mentioned interval), the phase phase changes of EBS were influenced due to the specific interaction and the specific form at the changes of EBS were influenced due to the specific interaction and the specific form at the processing processing temperature. For the sample processed in the Brabender mixer at 120 °C, EBS was temperature. For the sample processed in the Brabender mixer at 120 ◦ C, EBS was coupled on HNT coupled on HNT mainly in the alpha form. This form was also favourable for the later interaction mainly in the alpha form. This form was also favourable for the later interaction with PMMA. At the

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samePMMA. time, this had the this largest amount EBS amount (again important for(again further interactions with Atsample the same time, sample had of thefree largest of free EBS important for with PMMA. At the same time, this sample had the largest amount of free EBS (again important for with PMMA). further interactions with PMMA). further interactions with PMMA).

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Figure 3. 3. Phase transition transition of EBS in in the HNT-EBS HNT-EBS fillers obtained obtained in in different different processing processing conditions conditions Figure Figure Phase 3. Phase transitionofofEBS EBS inthe the HNT-EBS fillers fillers obtained in different processing conditions compared with initial neat EBS and HNT. (a) DSC thermograms; (b) enthalpies of phase transformations compared thermograms; (b) (b) enthalpies enthalpiesofofphase phase comparedwith withinitial initialneat neat EBS EBS and and HNT. HNT. (a) (a) DSC DSC thermograms; (alpha-beta, beta-alpha and melting) of EBS in HNT-EBS fillers. transformations (alpha-beta, beta-alpha and melting) of EBS in HNT-EBS fillers. transformations (alpha-beta, beta-alpha and melting) in HNT-EBS fillers.

PMMA TTggTand Trelax (Figures 44and 5)5)were affected the processing condition the filler and PMMA gand and relax (Figures were affected by the condition ofofthe filler PMMA TTrelax (Figures 4and and 5) were affected by processing the processing condition of theand filler the plasticizing effect brought byfree free EBS. This phenomenon was and also onon the the plasticizing effect brought by wasexpected expected andreflected reflected also the and the plasticizing effect brought byEBS. freeThis EBS.phenomenon This phenomenon was expected and reflected also relaxation chain temperatures (T relax ) level. T relax were decreased in the presence of HNT-EBS, all relaxation chain temperatures (T relax ) level. T relax were decreased in the presence of HNT-EBS, all on the relaxation chain temperatures (Trelax ) level. Trelax were decreased in the presence of HNT-EBS, thermal events indicating the improvement of processing ability of PMMA materials next to the thermal events indicating thethe improvement of of processing ability all thermal events indicating improvement processing abilityofofPMMA PMMAmaterials materialsnext nextto to the the properties enhancement. properties enhancement. properties enhancement.

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Figure 4. 4.PMMA in the the HNT–EBS HNT–EBS masterbatches. masterbatches. DSC thermograms: Figure PMMAchain chain transition(T (Tgg,, TTrelax relax)) in thermograms: (d)DSC (b)transition (a)(a) thethe 1st1st heating and and Reversible ReversibleHeat HeatFlow) Flow)and andTrelax Trelax (from the Total and heating and(b) (b)the the2nd 2ndheating). heating).TTgg (from Figure 4.the PMMA chain transition (Tg(c) ,(c) Trelax in heating the HNT–EBS masterbatches. (from the Non-Reversible Heat the and (d) (d)the the 2ndheating). heating).DSC thermograms: (from Non-Reversible HeatFlow): Flow): the)1st 1st heating and 2nd (a) the 1st heating and (b) the 2nd heating). Tg (from the Total and Reversible Heat Flow) and Trelax (from the Non-Reversible Heat Flow): (c) the 1st heating and (d) the 2nd heating).

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Figure PMMA chain transition (Tg, ) in Trelax in the HNT–EBS final nanocomposites. DSC Figure 5.5.PMMA chain transition (Tg , Trelax the) HNT–EBS final nanocomposites. DSC thermograms: g (from the Total and Reversible Heat thermograms: (a) the 1st heating and (b) the 2nd heating). T (a) the 1st heating and (b) the 2nd heating). Tg (from the Total and Reversible Heat Flow) and Trelax Flow) Trelax (from the Non-Reversible Heat (c) and the 1st (d) the 2nd heating). (from and the Non-Reversible Heat Flow): (c) the 1stFlow): heating (d) heating the 2nd and heating).

The effect seen in masterbatches (high filler content) in DSC analyses was preserved also in the The effect seen in masterbatches (high filler content) in DSC analyses was preserved also in the final nanocomposites (less than 2 wt % filler). The results were in good agreement with elastic and final nanocomposites (less than 2 wt % filler). The results were in good agreement with elastic and plastic behaviour seen in mechanical properties (tensile, axial strain, impact and modulus) reflected plastic behaviour seen in mechanical properties (tensile, axial strain, impact and modulus) reflected in in DSC by the small modifications of the Tg and Trelax of the PMMA in the presence of HNT-EBS. DSC by the small modifications of the Tg and Trelax of the PMMA in the presence of HNT-EBS. 3.2. Structure/Morphology 3.2. Structure/Morphology 3.1.1. 3.2.1. XRD XRD Diffraction Diffraction patterns patterns XRD XRD for for the the HNT HNT (Figure (Figure 6) 6) revealed revealed that that the the interplanar interplanar distance distance for for the the nanoscroll wall structure at around 7.4 Å. EBS crystalline structures were also revealed at nanoscroll wall structure at around 7.4 Å. EBS crystalline structures were also revealed at room room temperature crystallization. In temperature with with the the coexistence coexistence of of the the alpha alpha and and beta beta form form of of crystallization. In the the presence presence of of EBS, HNT wall seemed very little affected with differences of less than 0.3 Å, indicating EBS, HNT wall seemed very little affected with differences of less than 0.3 Å, indicating that that EBS EBS penetration inside the the HNT HNTtube tube(for (forthe theinner inneraluminol aluminol groups) was a very improbable event. In penetration inside groups) was a very improbable event. In this this context, a partial debonding the exterior HNT(scrolled sheet (scrolled layer)not should not be laterlater context, a partial debonding of the of exterior HNT sheet layer) should be excluded. excluded. For the PMMA masterbatches and final composites, the presence of all partners was For the PMMA masterbatches and final composites, the presence of all partners was confirmed even confirmed even at low concentration (2 wt % HNT), confirming by this way the relation between at low concentration (2 wt % HNT), confirming by this way the relation between structure and the structure and the induced (Figure 7). induced properties (Figureproperties 7).

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◦ C). Figure Structuresofofthe theEBS-HNT EBS-HNT hybrids obtained 160160 °C). Free Figure 6. 6.Structures obtained at atdifferent differenttemperature temperature(80, (80,120, 120, Figure 6. Structures of the EBS-HNT hybrids obtained at different temperature (80, 120, 160 °C). Free EBS occurrence in in different crystallizing forms inin HNT modified with EBS. Free EBS occurrence different crystallizing forms HNT modified with EBS. EBS occurrence in different crystallizing forms in HNT modified with EBS.

(a) (a)

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Figure 7. XRD patterns for the masterbatches (a) and final PMMA nanocomposites (b). Figure Figure7.7.XRD XRDpatterns patternsfor forthe themasterbatches masterbatches(a) (a)and andfinal finalPMMA PMMAnanocomposites nanocomposites(b). (b).

For the different modification of HNT with EBS correlated with the nano and macro scale For the the different differentmodification modification of of HNT HNT with with EBS EBScorrelated correlated with with the the nano nano and and macro macroscale scale For mechanical properties, it seems that alpha form of EBS was able to promote the best interaction in mechanical properties, it seems that alpha form of EBS was able to promote the best interaction mechanical properties, it seems that alpha form of EBS was able to promote the best interactionin both masterbatches and final nanocomposites in relation with the PMMA matrix (from Scheme 1). (from Scheme Scheme 1). 1). inboth bothmasterbatches masterbatchesand andfinal finalnanocomposites nanocompositesininrelation relation with with the the PMMA PMMA matrix matrix (from The correlation between tensile strength and the occurrence of the alpha/beta form of EBS for the Thecorrelation correlation between tensile strength the occurrence the alpha/beta form the The between tensile strength andand the occurrence of theofalpha/beta form of EBSofforEBS the for initial initial HNT treatment reflects the tuning possibilities given by the processing route for the final initial HNT treatment reflects the tuning possibilities given by the processing route for the final HNT treatment reflects the tuning possibilities given by the processing route for the final material material properties. Like shape memory, even if the subsequent processing temperatures highly material properties. shape memory, even if the subsequent processinghighly temperatures highly properties. Like shape Like memory, even if the subsequent processing temperatures exceed the EBS exceed the EBS melting point, the induced properties can be controlled. exceed point, the EBS melting point, the induced melting the induced properties can beproperties controlled.can be controlled. XRD results also showed an increase in HNT crystallites dimensions, confirming indirectly the XRDresults resultsalso alsoshowed showedan anincrease increaseininHNT HNTcrystallites crystallitesdimensions, dimensions,confirming confirmingindirectly indirectlythe the XRD interactions between aluminol sites and amide groups (from Scheme 2). The decoration of HNT with interactionsbetween betweenaluminol aluminolsites sitesand andamide amidegroups groups(from (fromScheme Scheme2). 2).The Thedecoration decorationofofHNT HNTwith with interactions EBS was also confirmed by the TEM details reflecting EBS crystals on HNT surface. In the case of EBS was also confirmed by the TEM details reflecting EBS crystals on HNT surface. In the case EBS was also confirmed by the TEM details reflecting EBS crystals on HNT surface. In the case ofof PMMA final nanocomposites and their initial masterbatches, the crystallite dimensions were PMMAfinal final nanocomposites masterbatches, the crystallite dimensions were PMMA nanocomposites andand theirtheir initialinitial masterbatches, the crystallite dimensions were expected expected to increase even more, but the peak overlapping and the complex packaging in the increase even but the peak the complex packaging region in the toexpected increase to even more, but themore, peak overlapping andoverlapping the complexand packaging in the amorphous amorphous region made the assessment very difficult and the interpretation too subjective. amorphous region made the assessment very difficult and the interpretation too subjective. made the assessment very difficult and the interpretation too subjective.

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Scheme 1. Idealized structures of the most probable interactions available between partners (competitiveness of the structures PMMA-EBS vs. PMMA-HNT). Scheme the most Scheme 1.1. Idealized Idealized structures ofof the most probable probable interactions interactions available available between between partners partners (competitiveness (competitivenessofofthe thePMMA-EBS PMMA-EBSvs. vs.PMMA-HNT). PMMA-HNT).

Scheme 2. Idealized structures with the strongest interaction available between EBS and HNT (and the Scheme Scheme2.2.Idealized Idealizedstructures structureswith withthe thestrongest strongestinteraction interactionavailable availablebetween betweenEBS EBSand andHNT HNT(and (and most probable based on sites reactivity aluminol vs. silanol). the themost mostprobable probablebased basedon onsites sitesreactivity reactivityaluminol aluminolvs. vs.silanol). silanol).

3.2.2. FTIR 3.1.2. 3.1.2.FTIR FTIR EBS through hydrogen bonds established by amide groupsgroups with HNT aluminol residues, EBS interacts through hydrogen bonds established by with HNT EBSinteracts interacts through hydrogen bonds established by amide amide groups with HNT aluminol aluminol asresidues, can be observed in Figure 8. The amide band I (A I) corresponds to the stretching vibration of the residues, asascan be observed in Figure 8. The amide band I (A I) corresponds to the stretching can be observed in Figure 8. The amide band I (A I) corresponds to the stretching carbonyl group coupled with the in-plane bending of the amino group and the stretch of the C–N vibration the vibrationofofthe thecarbonyl carbonylgroup groupcoupled coupledwith withthe thein-plane in-planebending bendingofofthe theamino aminogroup groupand and the bond. Amide II (A II) originates in the bending vibration of the N–H bond and stretching of the stretch stretchofofthe theC–N C–Nbond. bond.Amide AmideIIII(A (AII) II)originates originatesininthe thebending bendingvibration vibrationofofthe theN–H N–Hbond bondand and C–N groupofof inthe equal proportion. The proportion. intensities of theintensities amide absorption bands I and II are sensitive stretching C–N group The ofofthe absorption bands I Iand stretching the C–N groupininequal equal proportion. The intensities theamide amide absorption bands and to environmental conditions and vary because of the hydrogen bonding of the carbonyl and IIIIare ofofthe aresensitive sensitivetotoenvironmental environmentalconditions conditionsand andvary varybecause becauseofofthe thehydrogen hydrogenbonding bondingamino the moieties. Therefore, the ratio was attributed to differences in secondary and possible tertiary structure carbonyl and amino moieties. Therefore, the ratio was attributed to differences in secondary and carbonyl and amino moieties. Therefore, the ratio was attributed to differences in secondary and ofpossible EBS (similar H–bonds like in proteins) possible tertiary structure ofofEBS (similar H–bonds tertiary structure EBS (similar[40]. H–bondslike likeininproteins) proteins)[40]. [40]. Analysing the ratios between the amide I and II band intensities of EBS andand HNT-EBS, therethere was Analysing amide I Iand IIIIband intensities ofofEBS Analysingthe theratios ratiosbetween betweenthe the amide and band intensities EBS andHNT-EBS, HNT-EBS, there − 1 − 1 awas decrease in the band II intensity at 1554 cm cm with respect toto amide I at cm ,, due −1−1with was aadecrease ininamide the band IIIIintensity atat1554 respect I Iat1636 cm toto decrease theamide amide band intensity 1554 cm with respect toamide amide at1636 1636 cm−1−1 ,due dueto associations through hydrogen bonds involving the N–H groups, as it can be seen in Figure 8a. associations associationsthrough throughhydrogen hydrogenbonds bondsinvolving involvingthe theN–H N–Hgroups, groups,asasititcan canbe beseen seenininFigure Figure8a. 8a.

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Figure 8. FTIR-ATR spectra of (a) HNT-EBS composites; (b) band at “1248 cm−1”. −1”. 1 ”. Figure 8. FTIR-ATR spectra of (a) HNT-EBS composites; band at “1248 cm Figure 8. FTIR-ATR spectra of (a) HNT-EBS composites; (b) (b) band at “1248 cm−

In the case of HNT, there are two sharp bands at 3693 and 3625 cm−1, respectively. The first is In the case of HNT, there are two sharp bands at 3693 and 3625 cm−1, respectively. The first is due to the case stretching the inner surface OHand bonds, second to The the first inner OH In the of HNT,vibration there are of two sharp bands at 3693 3625 and cm−1the , respectively. is due due to the stretching vibration of the inner surface OH bonds, and the second to the inner OH groups near the aluminium atoms. Decreasing the intensity of the second band in the case of to the stretching vibration of the inner surface OH bonds, and the second to the inner OH groups near groups near the aluminium atoms. Decreasing the intensity of the second band in the case of composites indicates these groups are involved hydrogen with amide groups. the aluminium atoms.that Decreasing the intensity of thein second bandbonding in the case of EBS composites indicates composites indicates that these groups are involved in hydrogen bonding with EBS amide groups. Regarding of crystalline formswith of EBS the thermal treatment, as was that these groupsinter-conversion are involved in hydrogen bonding EBSduring amide groups. Regarding inter-conversion of crystalline forms of EBS during the thermal treatment, as was already reportedinter-conversion [32] three bandsofare characteristic inof the FTIR spectrum, situated at 1248, 955 and Regarding crystalline forms EBS during the thermal treatment, as was already reported [32] three bands are characteristic in the FTIR spectrum, situated at 1248, 955 and −1 −1 940 cm reported . The presence of thebands band are at 955 cm in the in FTIR and thesituated absence at of 1248, the other two already [32] three characteristic thespectrum FTIR spectrum, 955 and 940 cm−1. The presence of the band at 955 cm−1 FTIR spectrum and the absence of the other two −1ininthe bands to of thethe alpha crystalline while the absence ofand the band at 955 cm and the 940 cmis−1characteristic . The presence band at 955 cmform, the FTIR spectrum the absence of −1the other bands is characteristic to the alpha crystalline form, while the absence of the band at 955 cm−1 and−the appearance of the other two bands is characteristic to the beta form. Analysis of the HNT-EBS two bands is characteristic to the alpha crystalline form, while the absence of the band at 955 cm 1 appearance of the other two bands is characteristic to the−1 beta form. Analysis of the HNT-EBS composites shows the disappearance of the peak at 1248tocm when theyAnalysis are prepared 120 °C, and the appearance of the other two bands is characteristic the −1 beta form. of the at HNT-EBS composites shows the disappearance of the peak at 1248 cm when they are prepared at 120 °C, − 1 which demonstrates the presence of the alpha crystalline form in these composites, as it can ◦be composites shows the disappearance of the peak at 1248 cm when they are prepared at 120 C, which demonstrates the presence of the alpha crystalline form−1in these composites, as it can be observed from Figure Unfortunately, in crystalline the range form 940–960 cm composites, the characteristic of the which demonstrates the8b. presence of the alpha in these as it canbands be observed observed from Figure 8b. Unfortunately, in the range−1940–960 cm−1 the characteristic bands of the crystalline forms cannot be identified to 940–960 the superimposing with the existent HNT bands. from Figure 8b. Unfortunately, in the due range cm the characteristic bands of the crystalline crystalline forms cannot be identified due to the superimposing with the existent HNT bands. the molecular of the partner involved (PMMA matrix, HNT-filler and formsConsidering cannot be identified due tostructures the superimposing with the existent HNT bands. Considering the molecular structures of the partner involved (PMMA matrix, HNT-filler and EBS) Considering and XRD and FTIR results, several combinations of interactions can HNT-filler occur during the the molecular structures of the partner involved (PMMA matrix, and EBS) EBS) and XRD and FTIR results, several combinations of interactions can occur during the processing stage. have different energies can andoccur formation and XRD and FTIRThese results,interactions several combinations of interactions during probabilities the processing(when stage. processing stage. These interactions have different energies and formation probabilities (when competing in the same [41,42]. Theand steric hindrances are playing alsocompeting an important role for These interactions have space) different energies formation probabilities (when in the same competing in the same space) [41,42]. The steric hindrances are playing also an important role for molecules confinement the composite structure. theseforstructures be reflected space) [41,42]. The stericin hindrances are playing also Furthermore, an important role moleculescan confinement in molecules confinement in the composite structure. Furthermore, these structures can be reflected indirectly by the associated interface and the induced properties of the final materials (Schemes 1 the composite structure. Furthermore, these structures can be reflected indirectly by the associated indirectly by the associated interface and the induced properties of the final materials (Schemes 1 and 2). and the induced properties of the final materials (Schemes 1 and 2). interface and 2). 3.2.3. TEM 3.1.3. 3.1.3. TEM TEM results reflected the HNT morphology induced by the tubular structure and the changes TEM results reflected the HNT morphology induced by the tubular structure and the changes trials. Initial HNT,HNT, a natural grade, had a relatively dimension during the theprocessing processing trials. Initial a natural grade, had a broad relatively broaddistribution, dimension during the processing trials. Initial HNT, a natural grade, had a relatively broad dimension with length starting from 60–100 nm up to 1.5–2 microns. The exterior diameter starts from 50 starts up to distribution, with length starting from 60–100 nm up to 1.5–2 microns. The exterior diameter distribution, with length starting from 60–100 nm up to 1.5–2 microns. The exterior diameter starts 200 nm 9 and The obtained results are in good agreement with the one already from 50(Figures up to 200 nm 10). (Figures 9 and 10). The obtained results are in good agreement withreported the one from 50 up to 200 nm (Figures 9 and 10). The obtained results are in good agreement with the one in literature [41]. in literature [41]. already reported already reported in literature [41].

Figure magnifications. Figure 9. 9. TEM TEM images images for for HNT HNT particles particles before before modification modification at at different different magnifications. Figure 9. TEM images for HNT particles before modification at different magnifications.

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Figure 10. TEM-STEM (bright field) imaging and EDX data for HNT phase identification. Figure and EDX EDXdata datafor forHNT HNTphase phaseidentification. identification. Figure10. 10.TEM-STEM TEM-STEM(bright (bright field) field) imaging and Figure 10. TEM-STEM (bright field) imaging and EDX data for HNT phase identification.

After HNT modification at 120 °C with EBS, the crystals of EBS were identified on HNT ◦ C with After HNT modification at 120 °C with EBS, the crystals of EBS identified were identified on HNT After HNT modification at 120 EBS,EBS, the increase crystals of EBS on modification HNT After HNT modification at 120 with the crystals ofwere EBS onstructure. HNT structure. These data confirmed the°C crystallites observed in were XRD identified after structure. These data confirmed the crystallites increase observed in XRD after modification These data11). confirmed theconfirmed crystallitesthe increase observed in XRD after modification (Figure 11). structure. These data crystallites increase observed in XRD after modification (Figure (Figure 11). (Figure 11).

Figure onHNT HNTstructure structure(sample (sample (HNT-EBS)120). Figure11. 11.TEM–STEM TEM–STEMdetails details of of EBS EBS crystallized crystallized on (HNT-EBS)120). Figure 11. TEM–STEM details of EBS crystallized on HNT structure (sample (HNT-EBS)120). Figure 11. TEM–STEM details of EBS crystallized on HNT structure (sample (HNT-EBS)120).

TEMdetails details forthe the finalPMMA PMMA composites (Figure dispersed in in the InIn TEM (Figure12), 12),HNT HNTappear appearwell well dispersed In TEM detailsfor for thefinal final PMMA composites composites (Figure 12), HNT appear well dispersed in thethe In TEM details for the final PMMA composites (Figure 12), HNT appear well dispersed in the polymer matrixhighlighting highlighting the interface interface assistance promoted involving polymer matrix promotedby byEBS. EBS.The Theassumption assumption involving polymer matrix highlightingthe the interface assistance assistance promoted by EBS. The assumption involving polymer matrixdelaminating highlightingofthe interface promoted assumption involving the superficial a small partassistance of the sheet forming by the EBS. HNTThe nanoscroll was confirmed thethe superficial delaminating of a small part of the sheet forming the HNT nanoscroll was confirmed superficial delaminating of a small part sheet forming the HNT nanoscroll was confirmed the superficial delaminating of a small part of theand sheet forming HNT nanoscroll was confirmed by TEM details. The interaction between EBS HNT (thenthe EBS-PMMA) was strong enough byby TEM details. EBS and and HNT HNT (then (thenEBS-PMMA) EBS-PMMA)was wasstrong strong enough TEM details.The The interaction interaction between between EBS enough by TEMmelt details. The interaction between EBS (like and HNT (thentoEBS-PMMA) was strong enough during processing and high shear process extrusion) promote such a debonding and, during meltprocessing processingand andhigh high shear shear process process (like a debonding and, during melt (like extrusion) extrusion)totopromote promotesuch such a debonding and, during meltsome processing and high shear to process (likeinextrusion) to promote such aby debonding and, even more, of the HNT nanotubes be found smaller dimension (induced local cracking even more, someofofthe theHNT HNTnanotubes nanotubes to to be be found in smaller dimension (induced byby local cracking even more, some in smaller dimension (induced local cracking even more, somecondition). of the HNTAtnanotubes smaller dimension (induced by local cracking after processing this point,toifbe wefound refer in again to the mechanical properties (at nano and after processingcondition). condition).At Atthis thispoint, point, if we refer again to the mechanical properties (at(at nano and after processing refer again to the mechanical properties nano and after this point, if we refer again to the mechanical (atfound nano and macroprocessing scale) we condition). can observeAt the force of the interface interaction, since manyproperties HNT were in a macro scale) wecan canobserve observethe theforce force of of the the interface interface interaction, since many HNT were found in ain a macro scale) we interaction, since many HNT were found macro scale) weand cantherefore observe the forcedecrease of the interface interaction, since many were found in a damaged state a strong in mechanical properties was toHNT be expected. damaged state andtherefore therefore a strongdecrease decrease in mechanical mechanical properties was totobe expected. damaged state propertieswas wasto expected. damaged stateand and thereforeaastrong strong decrease in in mechanical properties bebeexpected.

Figure 12. TEM images for HNT in PMMA materials obtained by favouring the interaction of EBS in Figure 12. TEM images for HNT in PMMA materials obtained by favouring the interaction of EBS in Figure 12.12. TEM obtainedby byfavouring favouringthe the interaction EBS Figure TEMimages imagesfor forHNT HNTin in PMMA PMMA materials materials obtained interaction of of EBS in in the alpha form. the alpha form. thethe alpha form. alpha form.

TEM and XRD confirmed the structures interaction and explained how the synergistic effect TEM and XRD confirmed the structures interaction and explained how the synergistic effect TEM and and XRDEBS confirmed the structures structures interaction and the effect between HNT can contribute to PMMA modification. All resultshow were insynergistic good agreement TEM and the interaction andexplained explained synergistic effect between HNTXRD and confirmed EBS can contribute to PMMA modification. All results how were the in good agreement between HNT and EBS can contribute to PMMA modification. All results were in good agreement with theHNT properties improvement at nano and macro scale. between and EBS can contribute to PMMA modification. All results were in good agreement with the properties improvement at nano and macro scale. with the properties improvement at nano and macro scale. with the properties improvement at nano and macro scale.

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3.3. Mechanical 3.3. MechanicalProperties Properties Mechanical properties initially showed some unexpected results involving PMMA materials with Mechanical properties initially showed some unexpected results involving PMMA materials HNT HNT high (theoretical value-140 GPa GPa (230–340 GPa)) [43],[43], PMMA HNT withfiller. HNTDespite filler. Despite HNTmodulus high modulus (theoretical value-140 (230–340 GPa)) PMMA HNT nanocomposite a decrease in strength tensile strength (with 10%) almost 10%)compared when nanocomposite materialmaterial showedshowed a decrease in tensile (with almost when compared with (Figure neat PMMA (Figure result 13a). Another result (more expected) for was registered blend, for with neat PMMA 13a). Another (more expected) was registered PMMA-EBS PMMA-EBS highlighting the polymer matrix weakening, the tensile strength highlighting theblend, polymer matrix weakening, by the tensile strength by decrease. Yet, at a closerdecrease. inspection, Yet,only at a the closer inspection, only the small molecular weight additivein(EBS) was used, the when small molecularwhen weight additive (EBS) was used, the decrease mechanical properties decrease in mechanical properties seems to be limited, when compared with PMMA samples seems to be limited, when compared with PMMA samples obtained with HNT only. Up to this point, obtained with HNTmatrix only. Up thisto point, it seems PMMA with matrix was ablewith to interact more it seems that PMMA wastoable interact morethat favourably EBS than HNT (the later favourably with EBS than with HNT (the later offering a weak interaction with the polymer offering a weak interaction with the polymer matrix—only one level of interaction between negatively matrix—only one level of interaction between negatively charged carbonyl, and edge aluminium charged carbonyl, and edge aluminium sites positively charged). Moreover, HNT was expected to be sites positively charged). Moreover, HNT was expected to be found in lower dispersion degree (in found in lower dispersion degree (in comparison with other samples) and the eventual aggregates comparison with other samples) and the eventual aggregates to act as stress concentrators in to act as stress concentrators in polymer matrix [44]. The results were also confirmed by the TEM polymer matrix [44]. The results were also confirmed by the TEM data (presented in this study). data (presented in this study). After this first model of the filler-matrix interaction, other paths were After this first model of the filler-matrix interaction, other paths were speculated by using the HNT speculated by using the HNT treated with EBS. This treatment had to cover two directions: one to treated with EBS. This treatment had to cover two directions: one to promote the energy needed for promote energyconformation needed for the EBS molecule for coupling on the HNT surface the EBSthe molecule for coupling on theconformation HNT surface (polar amide segment orientation (polar amide segment orientation in the opposite direction of the hydrophobic-stearyl tails, to reduce in the opposite direction of the hydrophobic-stearyl tails, to reduce the steric hindrance) (Scheme 2) theand steric hindrance) (Scheme 2) and second, a favourable crystallization for the EBS molecules able second, a favourable crystallization for the EBS molecules able to assemble on the interface. to assemble the interface. Overall mechanical highlighted the distinct induced Overall on mechanical properties highlighted theproperties distinct behaviour induced by the behaviour two major forms by(αthe two (α and [32]. β) forTherefore, EBS crystallizing [32].forms Therefore, different forms EBS and β) major for EBSforms crystallizing the different of EBS the crystallization shouldofbe crystallization should be considered the main actors for the PMMA-HNT interface, EBS can considered the main actors for the PMMA-HNT interface, since EBS can provide: (i) the since polar-polar provide: the HNT; polar-polar interface with HNT; between (ii) hydrophobic interaction between and interface(i)with (ii) hydrophobic interaction stearyl and PMMA main chainstearyl and (iii) amide-carbonyl interaction from free EBS molecules (uncoupled on the HNT surface). The later PMMA main chain and (iii) amide-carbonyl interaction from free EBS molecules (uncoupled on the described form oflater EBS described (iii) can provide also the(iii) coupling with another molecule confined HNT surface). The form of EBS can provide also theEBS coupling with anotheron EBS HNT surface (by hydrophobic chain interaction, stearyl-stearyl) for inducing different packaging of molecule confined on HNT surface (by hydrophobic chain interaction, stearyl-stearyl) for inducing hydrophobic chainsof(segment) of PMMA. different packaging hydrophobic chains (segment) of PMMA.

(a)

(b)

Figure 13.13.Tensile nanocomposites compared Figure Tensilestrength strength(a) (a)and and axial axial strain strain (b) (b) of of PMMA/HNT PMMA/HNT nanocomposites compared to to PMMA and PMMA with EBS or HNT. PMMA and PMMA with EBS or HNT.

Thetensile tensile strength strength ofofthe final nanocomposites increases for all for threealltypes of initial The the final nanocomposites increases three types thermal of initial treatment of the HNT filler with EBS when compared with PMMA-EBS and PMMA-HNT. This thermal treatment of the HNT filler with EBS when compared with PMMA-EBS and PMMA-HNT. improvement confirmed the structural interaction between partners on the interface based on the This improvement confirmed the structural interaction between partners on the interface based on theoretical molecular dynamics. The improvements should be regarded more in contrast with the theoretical molecular dynamics. The improvements should be regarded more in contrast with PMMA-EBS and PMMA-HNT (and less with PMMA alone which possesses a lower properties PMMA-EBS and PMMA-HNT (and less with PMMA alone which possesses a lower properties profile profile i.e., scratch, hardness in the nanomechanical section). Therefore, the balance between i.e., scratch, hardness in the nanomechanical section). Therefore, the balance between properties was properties was easier to establish by use of both EBS and HNT. The synergistic interactions were easier to establish ofPMMA both EBS and HNT. Theand synergistic interactions up the to the preserved up to by theuse final nanocomposites depend on EBS alphawere formpreserved available for final PMMA nanocomposites and depend on EBS alpha form available for the initial HNT modification (see (PMMA-HNT-EBS)120, 160, 80 from Figure 13 and EBS alpha/beta content Figure 3).

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initial HNT modification (see (PMMA-HNT-EBS)120, 160, 80 from Figure 13 and EBS alpha/beta content Figure 3). Theplastic plasticdeformation deformation of of PMMA PMMA (Figure (Figure 13b) interface The 13b) was wasalso alsoinfluenced influencedbybythe thecomplex complex interface provided by EBS confined on HNT; free EBS in different crystallizing states and PMMA interaction provided by EBS confined on HNT; free EBS in different crystallizing states and PMMA interaction with the two states of EBS crystals. In the case of PMMA with neat HNT, a decrease in axial strain with the two states of EBS crystals. In the case of PMMA with neat HNT, a decrease in axial strain was was expected, since inorganic fillers increase in general the stiffness of the final material [45,46]. expected, since inorganic fillers increase in general the stiffness of the final material [45,46]. However, However, the axial strain was kept almost unchanged when compared with PMMA matrix. This the axial strain was kept almost unchanged when compared with PMMA matrix. This interesting interesting phenomenon can be explained by two directions: one by the fact that weak PMMA-HNT phenomenon can be explained by two directions: one by the fact that weak PMMA-HNT interactions interactions (weaker than in PMMA-PMMA) worked in tandem with the contribution of stiffness (weaker than PMMA-PMMA) worked in tandem with the with contribution stiffness by HNT brought by in HNT nanotubes (with high modulus compared PMMA) of and second,brought by the recent nanotubes high research moduluswhich compared withthe PMMA) andflexibility second, byofthe advances HNT advances(with in HNT proved extreme therecent nanotubes (by inreal research which proved the extreme flexibility of the nanotubes (by real experimental technique, not experimental technique, not by simulations) [47]. This later flexibility of the HNT should be by simulations) [47]. This also laterfor flexibility of the be seriously considered also for axial seriously considered axial strain andHNT othershould elasticity-driven properties or phenomena. Lustrain et and elasticity-driven properties or phenomena. Lunanotubes et al. [47] proved by experimental bending al.other [47] proved by experimental bending of single HNT that bends up to almost 90° were of single HNT nanotubes that bends up to almost 90◦ were possible without damage. possible without damage. An was observed observedfor forthe thematerials materialswhich which used HNT Animportant importantincrease increase in in axial axial strain strain was used thethe HNT ◦ modification withEBS EBSatat120 120 °C. This increase could of of both interface modification with C. This couldbe beassigned assignedtotothe thecontribution contribution both interface elasticity (PMMAchains chainsconfined confinedon on the the EBS crystals) highlighting thethe elasticity (PMMA crystals)(see (seealso alsoPMMA-EBS PMMA-EBSsample sample highlighting same behaviour) and some contribution of the HNT nanotubes elasticity. This behaviour should be be same behaviour) and some contribution the HNT nanotubes elasticity. This behaviour should defended also by other mechanical properties, but only to a certain extent, since the interface itself defended also by other mechanical properties, but only to a certain extent, since the interface itself can can overtake on rigid both rigid and elastic segments when break occurs, weakest bonds overtake loads loads on both and elastic segments (and(and when break occurs, thethe weakest bonds will will fail). The synergistic interactions observed in the tensile strength section were in good fail). The synergistic interactions observed in the tensile strength section were in good agreement with agreement with ones from axial strain.also, In this also, the PMMA preferenceon to EBS assemble onthan EBS on ones from axial strain. In this section thesection PMMA preference to assemble rather rather than on HNT surface was confirmed again. HNT surface was confirmed again. The impact properties (Figure 14a) account more about the behaviour and nature of the The impact properties (Figure 14a) account more about the behaviour and nature of the complex complex interface generated by HNT, EBS decorated on HNT, free EBS, different EBS conformation interface generated by HNT, EBS decorated on HNT, free EBS, different EBS conformation and PMMA and PMMA chains (generating both elastic and rigid domains acting together on different chains (generating both elastic and rigid domains acting together on different mechanical loads). mechanical loads). Usually in polymer materials, improving one mechanical property (like tensile Usually in polymer materials, improving one mechanical property (like tensile strength for example) strength for example) means the decreasing of another one (like impact strength). However, in this means the decreasingthe of impact anotherproperties one (like were impact strength). However, this case, unexpectedly case, unexpectedly either kept constant withinrespect to neat PMMA thematrix impact properties were either kept constant with respect to neat PMMA matrix either either increased, based probably on the previously observed contribution of theincreased, elastic based probably theofpreviously contribution of for the the elastic domains. This of behaviour domains. Thison kind behaviourobserved was generally observed blends or alloys ofkind thermoplastics was generally observed for the blends or and alloys of thermoplastics with thermoplastic [48,49] with thermoplastic elastomers [48,49] their composites with silica or layeredelastomers silicates. Even and their composites with silica or layered silicates. Even without thermoplastic elastomers in the case without thermoplastic elastomers in the case of PMMA-HNT-EBS nanocomposites, the occurrence of of PMMA-HNT-EBS nanocomposites, the occurrence ofpseudo-rigid both elastic domains (EBS-HNT phase, EBS-PMMA both elastic (EBS-HNT phase, EBS-PMMA phase) and (HNT agglomerates, different crystals, small EBS crystals aggregates) can different be found.EBS In good agreement with crystals axial phase) andEBS pseudo-rigid domains (HNT agglomerates, crystals, small EBS strain, PMMA-EBS interaction playsagreement an important role to shock andinteraction improves impact aggregates) can be found. In good with axial strain,overtaking PMMA-EBS plays an strength role by a to discrete behaviour. The energy at break (not presented here)elastic follows as well theThe important shockelastic overtaking and improves impact strength by a discrete behaviour. same observed tendencies and correlations. energy at break (not presented here) follows as well the same observed tendencies and correlations.

(a)

(b)

Figure 14. Impact strength (a) and Young modulus (b) of PMMA/HNT nanocomposites compared to PMMA and PMMA with EBS or HNT.

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Figure 14. Impact strength (a) and Young modulus (b) of PMMA/HNT nanocomposites compared to PMMA and816 PMMA with EBS or HNT. Polymers 2018, 10, 16 of 23

The macro scale modulus (Young Modulus obtained by mechanical analysis) follows the The macro scale modulus (Young but Modulus mechanical analysis) the impact and tensile strength behaviour not inobtained a simple by expected correlation (as follows expected forimpact a fair and tensile strengthmaterial). behaviour in the a simple correlation (asPMMA expected for a fair isotropic isotropic polymer Inbut ournot case, accessexpected to the modulus of the matrix or HNT was polymer material). case,anisotropic the access to the modulus of modification the PMMA matrix HNT was very very difficult, givenIn theour strong structure and the of the or structures during difficult, given anisotropic structure and modulus, the modification the structures during processing. processing. In the fact,strong the data reflected an overall as sumofvalue of all interface areas. The In fact,decrease the datain reflected an should overall be modulus, as sum value of context all interface areas. The induced small decrease in small modulus considered also in the of: high share by HNT modulus should be(two considered also in the with context of:shear, high share bythe HNT duringstructures); processing during processing extrusion process high able toinduced influence involved (two extrusion process high shear, involved structures); EBS acts notPMMA; only as EBS acts not only as with slip agent, but able also to asinfluence a bridgethe agent for new interactions with slip agent, but also as a bridge for new interactions EBS-PMMA brings EBS-PMMA interaction bringsagent also more isotropy in thewith bulkPMMA; composite material interaction and provides the also more phases isotropyon in the composite phases on the interface, elasticity the bulk interface, seenmaterial also inand theprovides rest of the the elasticity mechanical properties. Again, seen also in the rest to of be thethe mechanical properties. PMMA-EBS seems to be thecan driving force for PMMA-EBS seems driving force for theAgain, discrete elastic behaviour (which be amplified the different discrete elastic behaviour (which be amplified by different crystallization to HNT by EBS crystallization next can to HNT and PMMA in the EBS masterbatch used next for the final and PMMA in the masterbatch used for the final nanocomposites). nanocomposites). EBS-HNT and EBS-PMMA EBS-PMMA interactions interactions were were the the driving driving forces forces not not only only in the final bulk but also alsoininmolten moltenstate. state.TheThe synergistic effect present the processability of materials, but synergistic effect waswas present and and the processability of both both masterbatches and nanocomposites final nanocomposites (indiluted the diluted improved compared masterbatches and final (in the form)form) were were improved compared with with neat neat PMMA. These results should be considered very relevant thecontext contextofofprocessing processing parameters parameters PMMA. These results should be considered very relevant ininthe improvement, energy energy consumption, consumption, and and ability ability of of melt melt composition composition to to incorporate incorporate inorganic inorganic filler. filler. improvement, Since this study assumed a PMMA grade for automotive applications, the flow index improvement will have several benefits also for the in-mould flow, allowing a good fill, especially for complex geometries or quality aspect parts. Back Back to each partner structure interaction, interaction, again the best results ◦ (like in tensile section) were attained when 120 °C C temperature was used for HNT decoration with EBS in in the theBrabender Brabendermixer mixer(Figure (Figure 15). Surprisingly, again same improvements in properties 15). Surprisingly, again the the same improvements in properties were were EBS confinement and probably induced crystallization the HNT surface linkedlinked on the on EBSthe confinement and probably induced crystallization on the HNTonsurface initially and initially and inmasterbatch. the PMMA masterbatch. in the PMMA

Figure 15. Improvements of PMMA melt flowability by the presence of HNT decorated with EBS at Figure 15. Improvements of PMMA melt flowability by the presence of HNT decorated with EBS at different temperature (80, 120,◦ 160 °C for both masterbatches-CPA and final PMMA different temperature (80, 120, 160 C for both masterbatches-CPA and final PMMA nanocomposites). nanocomposites).

3.4. Nanomechanical Properties 3.4. Nanomechanical Properties 3.4.1. Nanoindentation Test Results 3.4.1. Nanoindentation Test Results The variation of Er and H for PMMA nanocomposites with 2 wt % unmodified HNT or EBS The variation of Er and H for PMMA nanocomposites with 2 wt % unmodified HNT or EBS surface modified HNT is shown in Figure 16a. surface modified HNT is shown in Figure 16a.

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6,8 Contact Depth, μm

Reduced Modulus, GPa

0,50

1,04

5,6

0,96

0,40

H, GPa

0,45

1,00

6,0 hc, μm

Er, GPa

6,4

0,55

Hardness, GPa

1,08

0,92 0,35 0,88

5,2

0,30

0,84 0,80

0 T S A 20 60 PMM MMA-EB MMA-HN NTEBS)8 TEBS)1 BS)1 N NTE P P A-(H A-(H A-(H PMM PMM PMM

0,25 0 0 0 T S A PMM MMA-EB MMA-HN NTEBS)8 TEBS)12 TEBS)16 N N P P A-(H A-(H A-(H PMM PMM PMM

(a)

(b) 10000

PMMA PMMA-EBS PMMA-HNT PMMA-(HNT-EBS)80 PMMA-(HNT-EBS)120 PMMA-(HNT-EBS)160

Force, μΝ

8000 6000 4000 2000 0

0

200

400

600

800

1000 1200 1400

Displacement, nm

(c) Figure of (a) modulus; (b) contact and hardness; (c) load–displacement Figure 16.16.Comparison Comparison ofreduced (a) reduced modulus; (b)depth contact depth and hardness; (c) plots of PMMA andplots PMMA/HNT nanocomposites. load–displacement of PMMA and PMMA/HNT nanocomposites.

The addition addition of toto a decrease of of both Er and H, compared to to The of only only 0.86 0.86wt wt%%EBS EBSininPMMA PMMAled led a decrease both Er and H, compared neat PMMA. Thus, the Er and H values (lower for PMMA-EBS), 5.55 and 0.408 GPa, compared to neat PMMA. Thus, the Er and H values (lower for PMMA-EBS), 5.55 and 0.408 GPa, compared to neat neat PMMA, 6.35 and 0.431 GPa, prove the lubricating effect of EBS. EBS possesses two large PMMA, 6.35 and 0.431 GPa, prove the lubricating effect of EBS. EBS possesses two large hydrophobic hydrophobic stearyl segments able to promote similar effects like the related fatty acid (well known stearyl segments able to promote similar effects like the related fatty acid (well known as dispersing as dispersing agents or lubricants to enhance processability of polymers) [50]. These results for EBS agents or lubricants to enhance processability of polymers) [50]. These results for EBS in PMMA were in PMMA were promising and could recommend EBS, also in other application for PMMA promising and could recommend EBS, also in other application for PMMA formulation. The addition formulation. The addition of 2 wt % unmodified HNT in PMMA resulted in an increase of hardness of(approximately 2 wt % unmodified HNT in PMMA an the increase of unchanged hardness (approximately 5% compared 5% compared to neat resulted PMMA) in and almost modulus (a decrease of toapproximately neat PMMA)3.5% andmay thebe almost unchanged modulus (a decrease of approximately 3.5% may considered within the limits of experimental error). Similar results were be considered within the limits of experimental Similar results were also obtained other also obtained by other researchers. For example, error). K. Pal [6] reported an improved dispersion of by 3 wt researchers. For example, K. Pal [6] reported an improved dispersion of 3 wt % HNT in PMMA % HNT in PMMA matrix and, in consequence, an increasing in mechanical properties (an increase of matrix and, in approximately consequence, an increasing properties (an increase of with modulus modulus with 14%, comparedintomechanical PMMA). Surface modification of HNT EBS iswith approximately 14%, compared PMMA). Surface modification with EBS is reflected reflected in increasing, more or to less, of Er and H, respectively. It of canHNT be noticed a variation of in increasing, or less,onofthe Er and H, respectively. It can benanocomposites, noticed a variation of properties depending properties more depending technology of obtaining the more precisely on how strong the interaction is betweenthe HNT and EBS and the interface adhesion with PMMA. From the is on the technology of obtaining nanocomposites, more precisely on how strong the interaction three samples which contain HNT modified with EBS, sampleFrom PMMA-(HNT-EBS)120 presented between HNT and EBS and the interface adhesion withthe PMMA. the three samples which contain the smaller value for E r but the highest value for H. These values correlate well with the values HNT modified with EBS, the sample PMMA-(HNT-EBS)120 presented the smaller value for Er but obtained forvalue Young strength and Tensile strength (Figures andmodulus, 14a). the highest formodulus, H. TheseCharpy valuesimpact correlate well with the values obtained for 13a Young The sample PMMA-(HNT-EBS)160 presented the highest value . The values obtained for Charpy impact strength and Tensile strength (Figures 13a and 14a).for TheErsample PMMA-(HNT-EBS)160 hardness correlate quite well with contact depth. The lower the hardness of the sample, thewith greater presented the highest value for Er . The values obtained for hardness correlate quite well contact the contact depth and vice versa (Figure 16b). Figure 16c presents load versus indentation depth. The lower the hardness of the sample, the greater the contact depth and vice versa (Figure 16b). displacement at peak loads of 10 mN for each of the samples. It can be noticed that the ”softest” Figure 16c presents load versus indentation displacement at peak loads of 10 mN for each of the sample is PMMA-EBS, with a peak depth of almost 1295 nm, while the ”hardest” is samples. It can be noticed that the ”softest” sample is PMMA-EBS, with a peak depth of almost PMMA-(HNT-EBS)120, which was penetrated to a depth of about 1107 nm. However, 1295 nm, while the ”hardest” is PMMA-(HNT-EBS)120, which was penetrated to a depth of about PMMA-(HNT-EBS)120 showed the highest value for tensile strain (Figure 13b) and the highest 1107 nm. However, PMMA-(HNT-EBS)120 showed the highest value for tensile strain (Figure 13b) and degree of elastic recovery during unloading while PMMA-EBS showed the lowest degree of elastic the highest degree of elastic(Table recovery recovery during unloading 1). during unloading while PMMA-EBS showed the lowest degree of elastic recovery during unloading (Table 1).

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Table 1.The The residual depth after final unloading (final depth) for PMMA and Table depth unloading (final for Table1.1. Theresidual residual depthafter afterfinal final unloading (finaldepth) depth) forPMMA PMMAand andPMMA/HNT PMMA/HNT PMMA/HNT nanocomposites. nanocomposites. nanocomposites. Sample Sample Sample hf,hf, nm nm

hf, nm

PMMA PMMA PMMA 555 ± 6± 6 555

555 ± 6

PMMA-EBS PMMA-EBS

PMMA-EBS

PMMA-HNT PMMA-HNT PMMA-HNT

552 ± 1± 1 552

550 ± 2± 2 550

552 ± 1

550 ± 2

PMMA-(HNT PMMA-(HNT PMMA-(HNT PMMA-(HNT PMMA-(HNT PMMA-(HNT PMMA-(HNT PMMA-(HNT PMMA-(HNT -EBS)80 -EBS)120 -EBS)160 -EBS)80 -EBS)120 -EBS)160 -EBS)80 -EBS)120 -EBS)160 526 ± 8± 8 600 ± 3± 3 553 ± 5± 5 526 600 553

526 ± 8

600 ± 3

553 ± 5

3.4.2. 3.4.2.Nanoscratch NanoscratchTest TestResults Results 3.4.2. Nanoscratch Test Results Nanoscratch Nanoscratchprovides providesthe thecapability capabilitytotoinvestigate investigatemodes modesofofdeformation deformationand andfracture fractureofof Nanoscratch provides the capability to investigate modes of deformation and fracture of samples. Load-displacement plots, generated automatically using TriboScan software (Figure samples. Load-displacement plots, generated automatically using TriboScan software (Figure17), 17), samples. Load-displacement plots, generated automatically using TriboScan software (Figure 17), can provide more information about structure/morphology and interface characteristics than a can provide more information about structure/morphology and interface characteristics than a can provide moreplot information about structure/morphology and interface characteristics than a load–displacement this data, information such load–displacement plotobtained obtainedfrom froman anindentation indentationtest. test.From From this data,useful useful information such plot obtained from an indentation test. From this data, useful information such as asload–displacement critical load, L c (the load at which change in a displacement curve occurs) and critical as critical load, Lc (the load at which change in a displacement curve occurs) and critical critical load, Lhcch, (the at which Usually, change insamples asamples displacement curve occurs) and critical displacement, displacement, bebemeasured. with highest LcLand hchare bebeconsidered displacement, ccan , canload measured. Usually, withthe the highest c and c are consideredtoto hhave be measured. Usually, samples with the highest L and h are be considered to have optimal have optimal scratch-resistance properties. Figure 18 shows plots of L c and h c data from 5000 μN c , can c c optimal scratch-resistance properties. Figure 18 shows plots of Lc and hc data from 5000 μN scratch-resistance properties. Figure 18 shows plots of L and h data from 5000 µN ramping force ramping force nanoscratch tests performed on all samples. From this figure, it can be noticed c c this figure, it can be noticedthe ramping force nanoscratch tests performed on all samples. From the nanoscratch tests performed on all samples. From this figure, can be noticed the highest values for highest for and the ones for PMMA. AArepresentative 2525μm, highestvalues values forPMMA-HNT-EBS)120 PMMA-HNT-EBS)120 and thelowest lowest onesit for PMMA. representative μm, PMMA-HNT-EBS)120 and the lowest ones for PMMA. A representative 25 µm, 2-D topographical 2-D topographical in situ SPM image obtained on all samples after a 5000 μN ramping force 2-D topographical in situ SPM image obtained on all samples after a 5000 μN ramping force in situ SPMtest image obtained on all samples after a 5000 µN ramping force nanoscratch test is shown nanoscratch 1919and images, including 3-D images, also used nanoscratch testisisshown shownininFigures Figures and20. 20.These These images, including 3-D images,were were also used Figures S1 and S2. These images, including 3-D images, were also used to calculate surface toin calculate surface roughness, residual deformation of material during the scratch and depth to calculate surface roughness, residual deformation of material during the scratch and depthofof roughness, residual deformation of material during the and depth of scratch forineach sample. scratch The data, LcLcand chccorroborated with post-test SPM scratchfor foreach eachsample. sample. Thequantitative quantitative data, andhscratch corroborated with post-test insitu situ SPM The quantitative data, Lc and hc corroborated with post-test including inincluding situ SPM images provides additional images provides information for the ability totoconfirm critical images providesadditional additional information forcharacterization, characterization, the ability confirm critical information for characterization, including the ability to confirm critical event identification. event eventidentification. identification.

(a) (a)

(b) (b)

Figure plots ofofnormal displacement (a)(a)and lateral force (b) versus time from a Figure17. 17.Representative Representative Figure 17. Representative plots plots ofnormal normaldisplacement displacement (a)and andlateral lateralforce force(b) (b)versus versustime timefrom froma 5000 μN ramping force nanoscratch test ononPMMA. Critical events in the data (showing point pointofof of a5000 5000μN µNramping rampingforce forcenanoscratch nanoscratchtest test onPMMA. PMMA.Critical Criticalevents events in in the the data data (showing (showing point distinct material piled up) are circled and correspond to h c and Lc. distinct material material piled piled up) up) are are circled circled and and correspond correspond to to hhc and and LLc.. distinct c c Critical Load Critical Load Critical Displacement Critical Displacement

Lc, mN Lc, mN

1700 1700

1100 1100

1000 1000 hc, nm hc, nm

1800 1800

1600 1600

900 900

1500 1500

800 800

1400 1400 0 0 0 S T A PMMMMM A A-EB S A-HN-HNNTTEBS)8S)8T0EBS)1S2)12T0EBS)1S6)160 P P MMMA-EPBMMMMA A H NTA E-(BHN NTEAB-(HN NTEB P PMM -(A-(PH MM MA-(H MM MA-(H P P M M P PM PM

700 700

Figure 18. Plots of Lc and hc data from 5000 µN ramping force nanoscratch tests on PMMA and hc data from 5000 μN ramping force nanoscratch tests on PMMA and Figure ofofLcLand c and hc data from 5000 μN ramping force nanoscratch tests on PMMA and Figure18. 18.Plots Plots PMMA/HNT nanocomposites. PMMA/HNT PMMA/HNTnanocomposites. nanocomposites.

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Looking at these images, it seems that all samples have similar topographical features of the nanoscratch. In fact, there is a clear distinction in the scratch behaviour with different microstructures. It can be noticed formation of three new surfaces by the materials piled up during nanoscratching: at the rear end of the tip, on both sides of the scratch and in front of the tip. Depending on the resistance properties of sample as well as the composition and technology employed for obtaining of sample, there are different residual widths (side pile-up) of scratch and different scratch depths. The roughness of the surface influences the lateral force/frictional force measurements and, therefore, the value of coefficient of friction. The RMS roughness (root mean square roughness, Rq ), the coefficient of friction (µ), the scratch depth (SD) and the scratch pile-up (rear, side and front) measured for all samples are presented in Table 2. Table 2. The root mean square (RMS) roughness, the coefficient of friction, the scratch penetration depth and the scratch pile-up measured for all samples. Sample PMMA PMMA-EBS PMMA-HNT PMMA-(HNT-EBS)80 PMMA-(HNT-EBS)120 PMMA-(HNT-EBS)160

Rq (nm)

µ

SD (nm)

Rear Pile-Up (nm)

Side Pile-Up (nm)

Front Pile-Up (nm)

432 ± 0.2 174 ± 0.02 193 ± 0.04 234 ± 0.02 194 ± 0.09 148 ± 0.09

0.34 ± 0.01 0.31 ± 0.005 0.32 ± 0.005 0.33 ± 0.001 0.32 ± 0.001 0.30 ± 0.01

486 ± 5 345 ± 5 395 ± 5 465 ± 5 365 ± 5 320 ± 5

196 ± 5 320 ± 5 126 ± 5 228 ± 5 300 ± 5 400 ± 5

275 ± 5 430 ± 5 390 ± 5 450 ± 5 450 ± 5 650 ± 5

188 ± 5 75 ± 5 260 ± 5 200 ± 5 100 ± 5 50 ± 5

As observed from Table 2 there is a very good correlation among the roughness of surface, the coefficient of friction and the scratch penetration depth. The highest values for surface roughness, coefficient of friction and scratch penetration were obtained for PMMA and the lowest ones for PMMA-(HNT-EBS)160 nanocomposite. However, PMMA sample presented the lowest total pile-up of material (659 nm) compared to the highest presented by PMMA-(HNT-EBS)160 (1100 nm), given that both samples have the highest values for Er and H, 6.35 and 0.431 GPa for PMMA and 6.39 and 0.446 GPa, respectively for PMMA-(HNT-EBS)160 (Figure 16a,b). In the case of PMMA, it is rather a sink-in of plastically torn material on both sides of the scratch. The side surfaces showed some ”undulations” as scratching depth increased and even a distinct material piled up (Figure S1a). This behaviour of PMMA, noticed also by G. Mallikarjunachari and P. Ghosh [51], is mainly due to its high stiffness, 3838 MPa (Figure 14b). In PMMA-EBS, which presented the lowest stiffness, 2896 MPa (Figure S1b), the presence of EBS is reflected in decreasing in surface roughness, coefficient of friction and scratch depth (174, 0.31 and 345 respectively, compared to PMMA, 432, 0.34 and 486, respectively). The EBS contribution to reducing friction and abrasion is well known. It was reported elsewhere that by using fatty acid amides up to 0.1 wt % a decreasing of friction coefficient of PMMA can be obtained, the bulk properties of PMMA being almost unaffected [14]. The scratch depth decreased in time and also the frontal material piled up decreased (75 nm for PMMA-EBS, compared to 188 for PMMA). Compared to PMMA, the presence of HNT in PMMA-HNT sample, reduced the coefficient of friction to 0.32, from 0.34 in PMMA. The reduction of the friction coefficient generated a more elastic contact and a higher amount of elastic recovered at the rear end of the tip (126 compared to 196 nm for PMMA). The lateral and frontal amount of material piled up was higher compared to PMMA. Furthermore, it can be noticed a strain in the material, like a “splint” at the end of lateral displacement of tip moving across the sample (Figure S1c). A possible explanation of this behaviour may be a weak interaction between PMMA matrix and the stiff phase. The samples which contain HNT modified with EBS behaved differently depending, on one hand, on the interaction between HNT and EBS and, on the other hand, on the interaction of PMMA with both HNT and EBS. The behaviour of sample which contains HNT modified with EBS at 80 ◦ C is closer to that of PMMA (high values for surface roughness, coefficient of friction and scratch depth). Just like PMMA, this sample had high Er , H and Young Modulus but low tensile strain and therefore a low amount of elastic recovery

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(Figure S2a). Compared to PMMA, the scratching depth for PMMA-(HNT-EBS)80 sample was constant in time. The PMMA-(HNT-EBS)120 sample had the highest value for axial strain. This value was reflected in a reduction of the friction coefficient and a slight increase of the amount of elastic recovery frontal the tip (Figure S2b). The behaviour of sample which contains HNT modified with EBS at 160 ◦ C (PMMA-(HNT-EBS)160) (Figure S2c) is closer to that which contains only EBS (PMMA-EBS). Both samples presented low values for surface roughness, coefficient of friction and scratch depth and the highest amount of elastic recovery frontal the tip (75 nm for PMMA-EBS and 50 nm for PMMA-(HNT-EBS)160). From these results, it may be concluded that the behaviour to scratching of PMMA/HNT nanocomposites depends on many factors that should be considered in relation with each other (structure/morphology, bulk mechanical resistance and surface properties). For this reason, it is necessary to analyze in depth all these factors to better understand each one’s contribution to increasing the scratch resistance. 4. Conclusions HNTs were proven as viable fillers base for automotive grade PMMA and the related nanocomposites. The use of HNT in PMMA matrix can be seen as an alternative choice for greener nanotubes, interaction with eco-friendly additives like EBS, for automotive (polymer) materials. Future research in this direction could be interesting for reducing the use of carbon black, glass fibres and so on. All results were found in close correlations, highlighting the strong influence of the conditions used for the material processing. Using a simple processing route (mixing, followed by extrusion and injection moulding) the materials properties and features can be tuned on both nano and macro scale. EBS was proven as a promising additive also for PMMA. EBS plays a complex role for the material anisotropic nature. EBS molecules were more competitive in establishing bidentate hydrogen bridge interaction on HNT than PMMA macromolecule; were able to decorate HNT surface and even to crystallize on the nanotubes; were able to interact further with PMMA macromolecules (by both functional groups of PMMA and the hydrophobic chains) establishing rigid and flexible domains reflected in the mechanical properties; were able to influence the PMMA flow and processing ability. HNT and EBS can produce a synergistic effect reflected in mechanical and thermal properties (for PMMA nanocomposites). The initial processing route temperature of HNT with EBS can tune the masterbatches and final PMMA nanocomposites properties. This was possible by the different temperatures used to influence the EBS crystallizing (in alpha or beta form). The most influential on properties was found to be the EBS alpha form. The synergistic effect was valued also in the nanomechanical properties, found in good agreement with the macro scale ones. Nanomechanical analysis also highlighted the different contributions of the complex interface able to promote both rigid and flexible domains in the anisotropic material. The improvements obtained in nanoscratch, friction and indentation behaviour for PMMA-HNT-EBS nanocomposites (in comparison with neat PMMA—automotive grade), recommend this new class of nanocomposites and the associated technology (simple regular route for polymer processing) as an alternative material for further developments in the automotive industry. Therefore, future research should also consider other concentrations and combinations with other fillers to achieve even more progress over the properties. We used this work to demonstrate some first possibilities: without chemical functionalizing of HNT through solvent or synthesis routes; using simple polymer processing technology; for the low filler content (2 wt % HNT) able to induce properties in PMMA nanocomposites; to improve scratch behaviour (using two agents: one hardening HNT and a slip agent like EBS) especially without negative effect on the rest of the properties—important for polymer materials used in the automotive parts industry. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/10/8/816/s1. Table S1: Formulations and processing conditions of all prepared and characterized samples, Scheme S1: Thermal decomposition of PMMA: (a) homolytic bond dissociation and (b) char production, Scheme S2: Reactions suggested for thermal degradation of EBS—scission in the β position of the carbonyl group (able to generate

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species to react with PMMA degradation species), Figure S1: 2-D and 3-D topographical in situ SPM images obtained on: (a) PMMA; (b) PMMA-EBS; (c) PMMA-HNT, Figure S2: 2-D and 3-D topographical in situ SPM images obtained on: (a) PMMA-(HNT-EBS)80; (b) PMMA-(HNT-EBS)120; (c) PMMA-(HNT-EBS)160. Author Contributions: Z.V. and M.C.C. conceived, designed the experiments, performed the mechanical and nanomechanical tests, collected the data, wrote the paper and reviewed the manuscript; C.E. and M.O. designed the experiments, provided the raw material and validated the results; M.G. performed the XRD analysis and contributed to the interpretation of the results; V.R. performed the FTIR analysis and contributed to the interpretation of the results; C.A.N. performed the thermal analysis (TGA and DSC) and contributed to the interpretation of the results; D.F. and M.I. made an analysis of the literature database and dealt with the melt processing of the samples; R.S. and B.T. performed the TEM–STEM analysis and contributed to the interpretation of the results. Funding: This research received funding from the EU Commission through the H2020-686165 Project. Conflicts of Interest: The authors declare no conflict of interest.

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