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Int J Plast Technol (December 2011) 15(2):150–162 DOI 10.1007/s12588-011-9017-x R E S E A R C H A RT I C L E

Study on the effect of nanosilica particles on morphology, thermo-mechanical and electrical properties of liquid polysulfide modified epoxy hybrid nanocomposites P. K. Guchhait & S. Bhandari & S. Singh & M. Rahaman

Received: 3 August 2011 / Accepted: 15 November 2011 / Published online: 16 December 2011 # Central Institute of Plastics Engineering & Technology 2011

Abstract The modification of epoxy resin with polysulfide polymer has been carried out by keeping the ratio of polysulfide polymer and epoxy resin as constant in order to avoid phase separation. The nanocomposites have been prepared by an ex-situ technique. The effect of nanosilica loading on mechanical property, thermo— mechanical properties, morphology, and dielectric properties has been studied. It is found that the tensile properties decrease with the increase in nanosilica loading for all the nanocomposites system except for 3 wt% nanosilica loading. This has been attributed to the better dispersion of nanosilica particles in modified epoxy matrix at 3 wt % loading compared to other systems which has been observed from transmission electron microscopy (TEM) images. Dynamic mechanical analysis (DMA) study shows that the storage modulus value and tanδ peak height is decreasing with the increase in nanosilica loading in the modified epoxy matrix. There is the increase in thermal stability with the incorporation of nanosilica in base polymer system. Dielectric study reveals that there is only one relaxation peak, and the dielectric constant value decreases with the increase in frequency as well as nanosilica loading. Fourier transfer infra-red (FTIR) study has also been carried out for the composite systems. Keywords Nanocomposites . Mechanical property . Morphology . DMA . TGA . Dielectric property

Introduction Recently organic/inorganic nanocomposites have attracted considerable interest towards both the fundamental and applied researchers from application point of view P. K. Guchhait : S. Bhandari : S. Singh : M. Rahaman (*) Rubber Technology Centre, Indian Institute of Technology, Kharagpur, India e-mail: [email protected]

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including aerospace. One of the widely used organic/inorganic composites is an epoxy/silica system, since epoxy resins as organic matrix have thermal, moisture, chemical and electrical resistance, and good adhesion to many substrates. They are widely used in the area of coatings, adhesives, castings, potting compounds, laminates and encapsulation of semiconductor devices [1, 2]. The interest in modification of an epoxy system has increased in recent years due to improved adhesion, durability, flexibility, and impact resistance particularly to difficult substrates [3–8]. It is well known that thiol terminated polymer is highly compatible with epoxy resin. A liquid polysulfide, polythioether, being very flexible polymer, its addition and subsequent reaction with epoxy enhances the flexibility as well as toughness of epoxy polymer matrix. However, the effect of nanosilica with polysulfide modified epoxy on the properties like morphology, microstructure, and permittivity (dielectric constant) has not been reported in the literature to the best of our knowledge. In this study, our goal is to investigate the effect of commercial nanosilica (aerosil® 300 particles, average particle size=7 nm) on the composite properties, especially mechanical properties, morphology, dynamic mechanical behavior, thermal, and dielectric properties. In this present work, we have reported the preparation of epoxy hybrid nanocomposites by modifying epoxy resin with liquid polysulfide (Thiokol) rubber (molecular weight=1,000 g/mol) and oligomeric poly (ether) amine to simultaneously strengthen and toughen the epoxy matrix. The following time-temperature cure cycle was employed for the study [4, 7] (1) (2) (3) (4)

24 h at room temperature 2 h at 70 °C 3 h at 100 °C 1 h at 120 °C

Experimental Materials Liquid Polysulfide polymer (LP) [Grade : LP-3; -SH value, repeat unit ‘n’ value 6, trifunctional monomer (1, 2, 3 Trichloro propane) 2 mole%, average mercaptan content 2.06, average thiol content 0.00206 mole/g] was purchased from SPI Inc., U.S., liquid epoxy resin [Grade EPON 828, Epoxy Equivalent 180–190 g/equivalent, viscosity 12,000–14,000 cps at 25 °C] was donated by M/s. Hexion specialty chemicals U.S., Trimethylol propane Tris [poly(propylene glycol) amine terminated ether (average molecular weight 440) was procured from Aldrich, and nanosilica Aerosil® 300 (average particle size 7 nm) was supplied by Degussa, Germany. Preparation and characterization of polysulfide modified epoxy nanocomposites The polysulfide liquid rubber and liquid epoxy resin were dissolved separately in methyl ethyl ketone (MEK) using a mechanical stirrer. These solutions were stirred

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with uniform speed of 1,000 rpm for 1 h to make homogenous solution. The nanosilica was dispersed in MEK. The better dispersion was carried out in a sonicator (Model-ELMA D-78224) at room temperature for a time period of 15 min to make agglomeration free. The dispersed nanosilica was then mixed separately with both the polysulfide and epoxy homogeneous solution prepared earlier. Finally, nanosilica filled polysulfide and epoxy slurry were mixed and stirred together at 2,000 rpm to make uniform dispersion of nanoparticles. In the aforesaid mixture, LP-3 was added at level of 20 g per 100 g of epoxy resin. Stoichiometric quantity of curator polyether amine was added into the above mixture. The resultant mixture was degassed at 25 °C until most of entrapped bubbles were removed completely. The bubble free mixture was cast, dried, and cured in Petridis according to the cure cycle mentioned earlier in this paper. The cured samples were used for different types of characterization. Different formulations and their codifications used this study are depicted in Table 1. Tensile properties Tensile test of dumbbell shaped specimen were performed by Instron Universal tensile testing machine as per ASTM D 412–98 with cross—head speed of 5 mm/min at room temperature; the data presented here are averages of five specimen test data. Transmission electron microscope (TEM) The samples for TEM analysis were prepared by ultra cryomicrotomy with Leica Ultracut UCT (Leica Mikrosystems GmbH, Vienna, Austria). Freshly sharpened glass knives with cutting edges of 45° were used to obtain nanosized sample (thickness below 100 nm) at constant temperature −50 °C. The samples were collected in sugar solution and cast on copper grid of 300 meshes in size. Dispersability of nanoparticles in liquid polysulfide epoxy matrix was evaluated by means of transmission electron microscope (TEM, JEOL model JEM 2010 TEM instrument) operated at an acceleration voltage of 200 kV. Scanning electron microscopy (SEM) The cryo-fractured surface of hybrid nanocomposite samples (prepared after soaking the samples in Methyl ethyl ketone for more than a month to preferentially remove liquid thiol rubber followed by drying operation in an

Table 1 Different formulation and their codifications

Composition

Designation

LP-EPOXY control

LPENSO

LP-EPOXY +1 wt% aerosil 300

LPENS1

LP-EPOXY +3 wt% aerosil 300

LPENS3

LP- EPOXY +5wt aerosil 300

LPENS5

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oven at 40 °C for 4 days, followed by cooling by immersion in liquid nitrogen) were sputter coated with gold and examined by a scanning electron micrograph JEOL JSM 5800 where the temperature were kept constant at 50 and 60 °C respectively [these temperatures were well below the glass transition temperature of LPE & LPENS]. The images were analyzed at an acceleration voltage of 15 kV to examine fractured surfaces. Dynamic mechanical thermal analysis Dynamic mechanical properties of neat LPE and LPENSs were measured by dynamic mechanical analyzer (DMA from TA instruments, model 2980VI.7B) in tensile mode geometry at constant frequency 1 Hz and strain amplitude 1%. The heating rate was 3 °C/min. The sample dimensions were 6.25 mm wide×30 mm long×1 mm thick. Storage modulus (E) and loss tangent (tan δ) were measured as a function of the temperature for all the samples under identical conditions. The temperature corresponding to the peak in the tanδ versus temperature plot was taken as glass transition temperature (tg). Thermo gravimetric analysis (TGA) and FTIR study TGA were carried out in TA Instruments (model Q50) at a heating rate of 10 °C/min under oxygen environment at the temperature range from 50 to 800 °C. IR spectra were recorded on a Perkin-Elmer, Spectrum RX I; FT-IR spectrometer. Dielectric properties The dielectric constant (ε’) and dielectric loss of LPE polymer based nanosilica filled composites with frequency (f) range from 10 Hz to 106 Hz were measured at room temperature (~25 °C) by using QuaTech-7600 Precision LCR meter.

Results and discussion Mechanical property The stress–strain plots and different mechanical properties like tensile strength at yield, tensile strength at break, elongation at break, and young’s modulus of neat polysulfide—epoxy and their different LPENSCs have been shown in Fig. 1 and summarized in Table 2. Both the Young’s modulus and tensile strength at yield clearly decreased after modification with nanosilica except for 3 wt% loading. However, tensile strength and elongation at break of LPE progressively increased after nanocomposite formation with aerosol® 300 but the test values for tensile strength and elongation at break at 5 wt% nanosilica loading decreases significantly. The increase in tensile values including Young’s modulus for LPENS3 is the highest when compared to the neat LPE and with different nanosilica loading. Wilford et.al reported that the liquid polysulfide is compatible with epoxy at processing temperature, while phase separation occurs during cross linking [4]. Jun Ma et al.

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Fig. 1 Typical stress- strain curves for rubber (LP) modified epoxy and nanosilica filled composites

[5] reported that rubber particles with micro and nanoscale size are formed in in-situ synthesis process in epoxy matrix which is taking advantage of the reaction of an oligomer diamine (Trimethylol propane Tris [poly(propylene glycol)] amine terminated ether). Low Young Modulus of the matrix is due to poor interface of the LP-3 particles, poor matrix—filler interaction and aggregation of silica particles inside the LPE matrix. Scanning electron microscopy (SEM) SEM micrograph of fractured surface of LPE blend nanosilica composites for neat LPE, 1, 3, and 5 wt% nanosilica loading are shown in Fig. 2a-d, respectively. The resolution of the scanning electron microscope (SEM) is not high enough to see the size of single silica nano particle (having silanol groups on the surface) in polymer matrix. However, the SEM figure exhibits how the ductile fracture surface of neat LPE changes to brittle fracture with the progressive increase in nanosilica loading in the polymer matrix. The weak interfacial bonding between ductile LPE and nanosilica can not effectively cause interfacial stress transfer when subjected to increased load. This results the failure of interface leading to lowering of tensile properties with the progressive increase in nanosilica loading. It may be concluded

Table 2 Mechanical properties of controlled and modified epoxy nanocomposites: σT, tensile strength at yield σt, tensile strength at break; εT, elongation at break; E, Young’s modulus

Resin

σT

σt

εT

E

Codification

(%)

(GPa)

(MPa)

(MPa)

LPENS-O

3.70

11.74

75.74

1.71

LPENS-1

2.69

11.79

110.84

1.31

LPENS-3

8.60

15.85

81.22

3.94

LPENS-5

2.40

8.84

81.51

0.80

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Fig. 2 SEM of fractured surface of LPE blend nanosilica composites at a) no loading b) 1 wt% loading c) 3 wt% loading and d) 5 wt% loading

that the nanoparticles enhance the deformation of the elastomeric component and thus influence the crack propagation due to the formation of nano and micro scale shear bands. Transmission electron microscopy (TEM) The micro and nano scale morphology of LPE blend nanocomposites for 1, 3, and 5 wt% nanosilica loading have been shown by TEM at two different magnifications in Fig. 3a-f. In both the low and high magnification, the TEM micrographs display that the nanosilica particles are well dispersed within the sizes ranging from 20 to 100 nm in the hybrid polymer matrix. Similar observations were also made by various authors [7, 9]. However, the TEM images of 1 and 5 wt% nanosilica loading show the aggregation of silica particle (Fig. 3a, b, e, and f), indicating a dispersion capacity of silica particles embedded in LPE matrix. This is why the tensile property of 1 and 5 wt% nanosilica loaded composites is lower compared to neat LPE. However, the TEM image exhibits that the nanosilica particles are well dispersed and distributed in the polymer matrix for 3 wt% nanosilica loaded composite (Fig. 3c and d). This attributes to its higher tensile property compared to the other nanocomposites systems.

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Fig. 3 TEM micrographs of LPE blend nanosilica composites for a) 1 wt% loading at low magnification b) 1 wt% loading at high magnification c) 3 wt% loading at low magnification d) 3 wt% loading at high magnification e) 5 wt% loading at low magnification and f) 5 wt% loading at high magnification

Dynamic mechanical analysis (DMA) DMA is used to interpret the polymer—polymer or polymer—filler interaction at their interface. Generally, the strong/weak interaction reflects the change in the values of storage modulus and tanδ when measured against temperature in the polymer nanocomposites system. The strong interaction restricts the movement of polymer segments near the nanoparticles interface. The storage modulus of LPE nanocomposites in Fig. 4 shows the temperature dependence of dynamic mechanical properties. Figure exhibits that the storage modulus of synthetic hybrid materials is lower than that of pure LPE probably due to weak interfacial interaction between organic epoxy phase and inorganic silica phase. Figure 5 exhibits tanδ (damping) for

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Fig. 4 Storage modulus versus temperature plots for different LPE nanocomposites

the composite systems. The appearance of single peak reveals a single Tg for all the composite systems. The difference between the higher tanδ peak of neat LPENS-0 and LPENS-3 points to the damping capabilities; the damping lowers when fillers are better dispersed and bonded strongly with the matrix [9, 10]. This may be due to the decrease in the friction between filler- filler and filler- matrix and also large amount of –OH groups were formed as the ring opening polymerization of thiol group and amine group with epoxy group [11]. Thermo gravimetric analysis (TGA) Figures 6 and 7 shows the typical TGA curves of the hybrid materials with nanosilica particles as measured under air atmosphere and distinct stages of weight losses/degradation of LPENS were observed .In the TGA curve, all the polymer nanocomposites exhibited more or less similar degradation behavior except neat

Fig. 5 Tan ∂ versus temperature plots for different LPE nanocomposites

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Fig. 6 Percentage weight loss versus temperature of LPE nanocomposites

LPE. The weight losses of the samples are starting and ending at 355 and 570 °C [LPENS-O], 361 and 559 °C [LPENS-1], 370 and 557 °C [LPENS-3], 375 and 561 °C [LPENS-5] respectively which might probably correspond to evaporation of entrapped solvent, elastomeric and amine component post-curing reaction and decomposition of thermosetting component. Typically, the thermal decomposition and mass loss of the hybrid materials starts at about 303 °C temperature range accompanied with an increase in silica content, which confirms the enhancement of thermal stability of hybrid materials. Moreover, we observed that the thermal decomposition of nanosilica filled hybrid materials progressively increases with increase in silica content when compared with that of neat hybrid material which may be attributed with nano particle showing very high interfacial area of contact with LPE matrix.

Fig. 7 Derivative weight loss versus temperature of LPE nanocomposites

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Dielectric property The variation of dielectric constant (ε’) and dissipation factor with frequency (f) of the polymer nanocomposites measured at room temperature (~25 °C) have been shown in Figs. 8 and 9, respectively. It is evident from the Fig. 8 that the dielectric constant is gradually decreasing with the increase in frequency for all the systems under investigation. Actually the dielectric property of any polymer composite systems is mostly governed by the interfacial and orientation polarization when subjected to an alternating electric field. Interfacial polarization originates at the interface of different groups present in the system; whereas, in the case of orientation polarization, the polar groups present in the polymer composite systems will tend to orient themselves in the direction of applied field. The higher is the orientation of the polar groups the higher will be the dielectric constant. Like wise the higher is the interfacial polarization the higher will be the dielectric constant. At lower frequency the dipoles like S-epoxide, S-hydroxyl, and S-NH2 get sufficient time to orient themselves in the direction of applied electric field. But as the frequency increases the dipoles gets very less time to orient them in the direction of the applied electric field and hence the effect of both the interfacial as well as orientation polarization get reduce with the increase in frequency [12]. This is why the dielectric constant is decreasing with the increase in frequency. The dissipation factor against frequency exhibits maximum near log frequency of 5.5 Hz indicates that there is only one dielectric relaxation (Fig. 9). This relaxation may be associated with the dipoles present in the macromolecular chain of modified epoxy matrix. Both the dielectric constant and dissipation factor are decreasing with the increase in nanosilica loading in the polymer matrix. Nano silica is non polar in nature. Consequently the orientation polarization of polar dipoles present in the modified epoxy matrix is somewhat suppressed due to the hindrance created by nanosilica in their movement. However, nanosilica can increase interfacial polarization but it is dominated by orientation polarization. Thus, ultimately the suppression of orientation polarization Fig. 8 Dielectric constant versus frequency for LPENS nanocomposites

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Fig. 9 Dissipation factor versus frequency for LPENS nanocomposites

reflects the decrease in dielectric constant and dissipation factor with the increase in nanosilica loading. FTIR study The FTIR spectra of amine cure Epoxy resin/LP/nanosilica composite from 500 to 4,000 cm−1 have been shown in Fig. 10. Figure 10a shows a broad characteristic peak at 3,503 cm−1 for hydroxy group and a sharp peak near 1,029 cm−1 for ether linkage in epoxy resin. Other characteristic peaks are at 1,607 cm−1, 1,508 cm−1, 1,246 cm−1, 1,183 cm−1, 1,039 cm−1 and 828 cm−1, and the peak near to 2,900– 3,100 cm−1 shows the C–H stretching [13, 14]. Figure 10b shows a broad peak for the polyether amine cure epoxy resin. The main characteristic peak of N–H stretching is in the range 3,371–3,466 cm−1. The peaks at 1,605 cm−1 was for N–H bending. A sharp C–O stretching of ether was well visible at 1,305 cm−1. There was Fig. 10 FTIR spectra of (a) epoxy resin (b) amine cure epoxy resin (c) amine cure epoxy resin with liquid polysulphide (d) amine cure epoxy resin with liquid polysulphide and nanosilica (3 phr)

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a very small shift of the same characteristic peaks when liquid polysulphide was introduced in the amine cure resin matrix. The formation of more electronegative group thiol shifts the peaks downfield and a single broad peak appears in the range 3,532–2,717 cm−1. The peak near 3,081 cm−1 is the characteristic peak for liquid polysulphide (LP) shown in Fig. 10c. The absorption band at 456 cm−1 is due to the stretching of the polysulphide functional group [15, 16]. Other peaks at 1,200 cm−1, 1,026 cm−1 and 829 cm−1 also the characteristic peaks of liquid polysulphide [17, 18]. The addition of nanosilica in the resin matrix is responsible for the shifting of peaks toward downfield as showing in Fig. 10d. At 1,132 cm−1 Si–O–Si absorption peak is visible. The observed peaks at 828 cm−1 and 3,055 cm−1 are for Si–O bending and Si–OH stretching, respectively [19, 20]. In Figs. 10a, b, c and d there are some common characteristic peaks for C-H stretching at 2,850–3,000 cm−1, C–H bending for 1,350–1,480 cm−1 and C = C stretching at 1,400–1,600 cm−1, which are giving the evidence for the aromatic hydrocarbon linkage.

Conclusions 1. The epoxy-polysulfide-silica nano composites have been prepared successfully. Tensile property of the composite systems decreases with nanosilica loading except for 3 wt% nanosilica loading. 2. SEM study reveals that the ductile fracture surface of neat LPE changes to brittle fracture with the progressive increase in nanosilica loading. From TEM study it is seen that the dimensions of well dispersed silica nano particles is in the range of 20–100 nm. Moreover, the dispersion and distribution of nanosilica particles are better in 3 wt% nanosilica loaded composite compared to other composite systems. 3. The storage modulus value is decreasing with the addition of nanosilica in the polymer matrix. Tanδ value shows a single Tg for all the nanocomposites systems i.e. the thiokol rubber is fully reacted with epoxy resin. 4. TGA study shows that the thermal stability is increasing with the addition of nanosilica in the polymer matrix. However, the dielectric constant and dissipation factor value is decreasing with the increase in frequency as well as nanosilica loading in the polymer matrix. 5. FTIR study reveals that the addition of nanosilica in the LPE matrix shifts the peaks to downfield region.

Acknowledgements for financial grant.

The authors are thankful to Indian Space Research Organization (ISRO), Bangalore

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