polymers Review
Nanostructured Composites Based on Liquid-Crystalline Elastomers Vanessa Cresta 1 , Giuseppe Romano 2 , Alexej Kolpak 2 , Boštjan Zalar 3 and Valentina Domenici 1, * ID 1 2 3
*
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Moruzzi 13, 56124 Pisa, Italy;
[email protected] Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA;
[email protected] (G.R.);
[email protected] (A.K.) Department of Condensed Matter Physics, Jozef Stefan Institute, Jamova Cesta 39, SI 1000 Ljubljana, Slovenia;
[email protected] Correspondence:
[email protected]; Tel.: +39-050-2219-215
Received: 18 June 2018; Accepted: 10 July 2018; Published: 14 July 2018
Abstract: Liquid-crystalline elastomers (LCEs) are the object of many research investigations due to their reversible and controllable shape deformations, and their high potential for use in the field of soft robots and artificial muscles. This review focuses on recent studies about polymer composites based on LCEs and nanomaterials having different chemistry and morphology, with the aim of instilling new physical properties into LCEs. The synthesis, physico-chemical characterization, actuation properties, and applications of LCE-based composites reported in the literature are reviewed. Several cases are discussed: (1) the addition of various carbon nanomaterials to LCEs, from carbon black to carbon nanotubes, to the recent attempts to include graphene layers to enhance the thermo-mechanic properties of LCEs; (2) the use of various types of nanoparticles, such as ferroelectric ceramics, gold nanoparticles, conductive molybdenum-oxide nanowires, and magnetic iron-oxide nanoparticles, to induce electro-actuation, magnetic-actuation, or photo-actuation into the LCE-based composites; (3) the deposition on LCE surfaces of thin layers of conductive materials (i.e., conductive polymers and gold nanolayers) to produce bending actuation by applying on/off voltage cycles or surface-wrinkling phenomena in view of tunable optical applications. Some future perspectives of this field of soft materials conclude the review. Keywords: liquid-crystal polymers; bilayers; composites; liquid single-crystal elastomers; actuators; artificial muscles; orientational order; NMR; nanoparticles; nanomaterials; photo-actuation; electro-actuation; thermal actuation
1. Introduction Liquid-crystal polymers (LCPs) are a class of materials that combine the mesophasic features of low-molar-mass liquid crystals with the versatile physical properties of polymers [1]. A liquid-crystal polymer not only possesses the individual properties of each of its constituents, but also exhibits intrinsic new features. LCPs may exhibit either nematic and/or smectic phases, depending on the chemical structure of the mesogenic molecules, the flexible spacers, and the polymer main chains [2]. Moreover, the mesomorphic behavior of LCPs can be influenced by external physical parameters, such as temperature and pressure [3]. Liquid-crystalline elastomers (LCEs) are a particular type of LCPs, where the polymer chains are cross-linked together via flexible or semi-flexible cross-linkers [4]. As in the case of LCPs, side-chain and main-chain systems can be distinguished: side-chain LCEs contain liquid-crystalline molecules as pendant units, while main-chain LCEs constituted mesogenic
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moieties a partthe of the main-chain polymer backbone 1a). network As reported several books reviews as [5–13], interplay between the elasticity of (Scheme the polymer andinthe spontaneous and reviews [5–13], the interplay between the elasticity of the polymer network and the spontaneous orientational ordering of the liquid-crystalline units results in a reversible thermo-mechanical orientational of the liquid-crystalline in a reversible thermo-mechanical response, response, asordering first predicted by de Gennes in units 1975 results [14] (Scheme 1b). Several synthetic strategies were asdeveloped first predicted by detoGennes 1975 [14] (Scheme 1b). Several synthetic strategies were developedof in order obtain in macroscopically aligned LCE samples [6,15–20]. The preparation inmacroscopically order to obtain macroscopically aligned LCE samples [6,15–20]. The preparation of macroscopically aligned LCEs, associated with a relatively homogeneous orientational order (S) of aligned LCEs, associated with a relatively orientationalfeature order (S) the local nematic (or the local nematic (or smectic) domains, homogeneous is indeed a fundamental forof their peculiar physical smectic) domains, is indeed a fundamental feature for their peculiar physical properties, such as the properties, such as the macroscopic, controllable, and reversible shape changes [21]. LCEs are macroscopic, and reversible shape changes [21]. LCEs aredue included in ability a type of included in controllable, a type of materials called “shape-memory” materials, to their to materials memorize called “shape-memory” due to their ability to memorize their macroscopic shape under their macroscopic shapematerials, under specific conditions. specific conditions.
Scheme Scheme1.1.(a) (a)Sketch Sketchofofthe themain mainconstituents constituentsofofaaside-chain side-chainliquid-crystalline liquid-crystallineelastomer elastomer(LCE; (LCE;on onthe the left) and of a main-chain LCE (on the right). (b) Top: scheme of the volume deformation of a typical left) and of a main-chain LCE (on the right). (b) Top: scheme of the volume deformation of a typical LCE LCEsystem systematatthe thephase phasetransition transitionfrom froma aspherical sphericalshape shape(disordered (disorderedphase) phase)totoananoblate oblateuniaxial uniaxial shape reported asas the ratio L/L 0 0versus shape(ordered (orderedphase). phase).Bottom: Bottom:the theuniaxial uniaxialelongation, elongation, reported the ratio L/L versustemperature temperature (T) (T)ofofa atypical typicalnematic nematicLCE. LCE.
commonmacroscopically macroscopically aligned (monodomain) LCEs, mechanical deformation is induced InIncommon aligned (monodomain) LCEs, mechanical deformation is induced by by increasing (or decreasing) the temperature the disordered–ordered transition and increasing (or decreasing) the temperature acrossacross the disordered–ordered phasephase transition and vice vice (Scheme versa (Scheme 1b)However, [15]. However, the perspectives of applications these materials be versa 1b) [15]. the perspectives of applications of theseof materials could becould widely widely expanded if the reversible shape variation was by induced stimuli than temperature, expanded if the reversible shape variation was induced stimulibyother thanother temperature, such as, such as, forexternal instance, external electric orfields, magnetic fields, as electro-active in common electro-active polymers [22– for instance, electric or magnetic as in common polymers [22–24]. Another 24]. Another possibility, explored by several groups, isofthe preparation LCEs, of photosensitive possibility, explored by several research groups,research is the preparation photosensitive in order to LCEs, in the order to modulate the shape deformation and morphing geometries of (IR) UV-vis modulate shape deformation and morphing geometries by means of UV-visby ormeans infrared lightor infrared (IR) light stimuli [25–29]. These are the of two main directions development in the field of stimuli [25–29]. These are the two main directions development in theoffield of LCE-based materials. LCE-based In this materials. review, the focus is on the actual state of the art concerning the preparation, In this review, and the the focus is on the actual state of the art concerning theonpreparation, the the characterization, actuation properties of composite materials based LCEs. In fact, characterization, and the actuation properties of composite materials based on LCEs. In fact, this field this field is the object of great expansion, and an increasing number of works was published in the is the object of great expansion, anstudies increasing number works was published in with the past five past five years. In particular, theand main about LCEs of prepared, or reprocessed, microyears. In particular,materials the main about LCEs prepared, or reprocessed, with and and nanostructured [30]studies are reviewed, as well as composite bilayers made of microLCE films nanostructured materials [30] arein reviewed, composite made of LCEof films covered covered by conductive materials the formasofwell thinasfilms. Somebilayers future perspectives functional by conductiveLCE materials in the form of in thin films. Some future perspectives ofasfunctional nanostructured composites are discussed view of their potential applications, such artificial nanostructured LCE composites are discussed in view of their(MEMS), potentialand applications, such as artificial muscles, smart surfaces, micro-electromechanical systems nano-electromechanical muscles, smart [31–34]. surfaces, micro-electromechanical systems (MEMS), and nano-electromechanical systems (NEMS) systems (NEMS) [31–34].
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2. LCE-Based Composites with Carbon Micro- and Nanostructured Materials The induction of electro-mechanical actuation in LCE matrices by applying external stimuli, other than thermal actuation, was the purpose of many researches in the past 15 years. The first attempts concerned the preparation of composites made from carbon materials, and macroscopically aligned liquid-crystalline elastomers were obtained according to the so-called “two-step” cross-linking procedure, first described by Kupfer et al. [35]. Various types of carbon materials are used to prepare these LCE-based composites, such as carbon micro- and nanoparticles [36–39], carbon nanotubes (CNT) [40–44], fullerene (C60) derivatives [45], and graphene and its derivatives [45–48]. In the following sections, details about the preparation, physico-chemical characterization, and actuation properties are reported and discussed. 2.1. Sample Preparation and Characterization of Carbon-Based LCE Composites Carbon micro- and nanomaterials are considered good candidates as polymer dopants due to their conductivity, adhesion, wide range of morphologies and sizes, and for their relatively low cost [49]. Most LCE-based composites are made by macroscopically aligned monodomain LCE films, prepared following the “two-step” cross-linking procedure [35]. These films are also named liquid single-crystal elastomers (LSCEs) to distinguish them from all other kinds of LCEs. Their preparation includes a first step involving a cross-linking reaction where a pre-polymerization mixture of the polymer main chains, monomers, and cross-linkers in a solvent (e.g., toluene) are put inside a rotor with a known and controlled temperature and rate until a gel-like film is obtained. The second step of the cross-linking reaction occurs by handing the gel-like film under progressive weights in order to align the monomers along the longitudinal direction of the films [35]. This procedure gives rise to uniaxially aligned LCE films with very reproducible thermo-mechanical and ordering properties [16]. Most known LSCE systems are formed starting from polysiloxane-based chains, and, depending on the chemistry, they can form ordered mesophases (typically the nematic phase), stable at room temperature. In order to induce a response to electric, magnetic, and/or light stimuli in standard LSCEs, which are highly insulating, various procedures were adopted. One method was proposed by Chambers et al. [36–39] with carbon-black spherical nanoparticles, with an average diameter of 30 nm, and carbon nanohorns of about 30–50 nm in length and 4 nm in diameter. Standard monodomain LCE films, namely LSCEs, were swollen in different solvents (methanol, cyclohexane, and toluene) with a concentration of nanoparticles ranging from 0.05 g/L to 5 g/L [36]. According to this swelling procedure [36,37], the carbon nanomaterials, which are dispersed in the solvents, are able to partially penetrate the LSCE matrix only in the gel-like swollen state. The optimal penetration depth and nanocarbon layer thickness were reached with a mixture of cyclohexane/toluene, and it was found to be a few micrometers. The process of absorption/desorption of the nanoparticles to the swollen LSCE films was studied by applying the Langmuir theory [50], and the conductivity of the obtained LSCE-based composites in the swollen state was evaluated through their effective resistivity (ρ, Ω/cm). As observed in Figure 1a, a minimum in resistivity, associated with a saturation of nanocarbon absorption, was observed around 7.5 g/L and 20 g/L [37] in LSCEs reprocessed with carbon black and carbon nanohorns, respectively. The swelling procedure described by Chambers et al. [36,37] produces LSCE films with integrated external layers of various thicknesses with a volume fraction of carbon nanoparticles (p) which is dependent on the initial nanoparticle concentration (c; Figure 1b). The mechanism of swelling/deswelling of both LCEs and LCE composites was the object of several experimental and theoretical investigations [51–55]. Most of them refer to pristine liquid-crystalline elastomers swelled in appropriate low-molecular-mass LCs (LMMLCs) to study different geometrical distortions of the nematic director, electrical deformations, and electro-optical and mechanical properties [51]. Torsional actuation was studied in hierarchically patterned LCE materials under the effect of chemical vapor in view of applications as chemo-responsive systems [52]. LCEs swollen with nematic solvents featuring different swelling ratios still retained their main elastic properties,
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such as the soft and semisoft elasticity typical of LCEs [53]. Recent theoretical investigations [54,55] confirmed confirmedthese thesefindings findings showing showingpotential potentialapplications applicationsof of swollen swollenLCEs LCEs as as sensors, sensors, actuators, actuators,and and micro-swimmers. micro-swimmers. Few Fewstudies, studies, however, however, shed shed light light on on swollen swollen LCE LCE composites composites [36–39]. [36–39].
Figure1. 1. Liquid Liquidsingle-crystal single-crystalelastomer elastomer(LSCE) (LSCE)reprocessed reprocessedwith withcarbon carbonnanoparticles. nanoparticles. (a) (a)Effective Effective Figure resistivity (ρ) LSCE as aasfunction of concentration (c) for(c) carbon black (blue squares) resistivity (ρ) of ofthe themacroscopic macroscopic LSCE a function of concentration for carbon black (blue and carbon (red circles) particles of a reprocessed side-chain siloxane-based LSCELSCE film. squares) and nanohorn carbon nanohorn (red circles) particles of a reprocessed side-chain siloxane-based Top: picture of carbon-black-reprocessed LSCE samples. Particle fraction (p) versus concentration (c) film. Top: picture of carbon-black-reprocessed LSCE(b)samples. (b) Particle fraction (p) versus for carbon black (black squares) carbon nanohorns (red circles). The(red best-fit linesThe of the Langmuir concentration (c) for carbon blackand (black squares) and carbon nanohorns circles). best-fit lines equation are alsoequation shown [50]. Adapted frompermission the authorsfrom of [37]. © IOP Publishing. of the Langmuir are also shownwith [50].permission Adapted with the authors of [37]. © Reproduced with permission. All rights reserved. IOP Publishing. Reproduced with permission. All rights reserved.
An approach to toproducing producingLCE-based LCE-basedcomposites composites was reported Marshall et[40] al. An alternative alternative approach was reported by by Marshall et al. [40] using carbon nanotubes (CNTs), which are able to absorb light in a wide range of wavelengths, using carbon nanotubes (CNTs), which are able to absorb light in a wide range of wavelengths, and byby the authors of of [40] consists of of a and which whichcan canconvert convertlight lightinto intoheat. heat.The Theprocedure proceduredescribed described the authors [40] consists functionalization of the CNTs with pirene-ending polymer chains acting as a dispersing agent to a functionalization of the CNTs with pirene-ending polymer chains acting as a dispersing agent avoid the the aggregation of CNTs. The functionalized CNTs CNTs are then in toluene at a known to avoid aggregation of CNTs. The functionalized aredispersed then dispersed in toluene at a concentration, and the and pre-polymerization constituents (polysiloxane chains, chains, mesogens, and crossknown concentration, the pre-polymerization constituents (polysiloxane mesogens, and linkers) are added. The obtained dispersion with all starting reactants is then introduced in the rotor cross-linkers) are added. The obtained dispersion with all starting reactants is then introduced following thefollowing “two-step”the cross-linking [35].procedure Unlike Chambers’s swelling method swelling [36–39], in the rotor “two-step”procedure cross-linking [35]. Unlike Chambers’s in this approach theapproach carbon nanomaterials arenanomaterials added beforeare theadded preparation LSCE films. method [36–39], [40], in this [40], the carbon beforeof thethe preparation of As a main result, the distribution of the functionalized CNTs within the LSCE matrix is more the LSCE films. As a main result, the distribution of the functionalized CNTs within the LSCE matrix homogeneous in the volume with respect to the to previous method. As in As many otherother cases,cases, the is more homogeneous in the volume with respect the previous method. in many concentration of the nanomaterials is rather crucial in determining the physical properties of the LCEthe concentration of the nanomaterials is rather crucial in determining the physical properties of the based composites, as discussed in thein later LCE-based composites, as discussed the sections. later sections. A synthetic procedure waswas reported by Lama et al. [44] to [44] produce epoxyAcompletely completelydifferent different synthetic procedure reported by Lama et al. to produce based LCE composites with multi-walled carbon nanotubes (MWCNTs). The synthesis of the epoxy epoxy-based LCE composites with multi-walled carbon nanotubes (MWCNTs). The synthesis of polymer (labeled “DOMS_SA”, Figure 2a) involves the mixing of the epoxy precursor with sebacic the epoxy polymer (labeled “DOMS_SA”, Figure 2a) involves the mixing of the epoxy precursor with ◦ C. The mixture acid in the presence of a catalyst, obtaining a viscousa mixture at 160 °C. is then poured sebacic acid in the presence of a catalyst, obtaining viscous mixture atThe 160 mixture is then between two Teflon-coated glasses, and is cured in the oven to finalize the polymerization. poured between two Teflon-coated glasses, and is cured in the oven to finalize the polymerization. The The synthesis epoxy-basedLCE-MWCNT LCE-MWCNTcomposites composites implies functionalization of MWCNTs the MWCNTs synthesis of of epoxy-based implies thethe functionalization of the with with the epoxy pre-polymer. The curing is then performed bysebacic addingacid sebacic to the the epoxy pre-polymer. The curing reactionreaction is then performed by adding to theacid dispersion dispersion of epoxy-functionalized MWCNTs in the oven, as described for the pure epoxy polymer of epoxy-functionalized MWCNTs in the oven, as described for the pure epoxy polymer [44]. Several [44]. Several samples the plates form (10 of thick plates (10×mm 10 mm 0.25 mm) were obtained with samples in the form ofin thick mm × 10 mm 0.25 ×mm) were×obtained with varying nanotube varying nanotube content, indicated as DS_xCNT, with x = 0.75 wt %, 1.5 wt % and 3.0 wt %. content, indicated as DS_xCNT, with x = 0.75 wt %, 1.5 wt % and 3.0 wt %. An important step in An the important in the preparation of homogeneous LCE composites is represented by the preparationstep of homogeneous LCE composites is represented by the functionalization of the MWCNTs functionalization and the formation of the as epoxy precursor–MWCNT adducts, as and the formationofofthe theMWCNTs epoxy precursor–MWCNT adducts, revealed by the optical, SEM and TEM revealed by the optical, SEM and TEM images (Figure 2b). The mesophase behavior of the LCE and images (Figure 2b). The mesophase behavior of the LCE and LCE-based composites indicates the LCE-based composites indicates occurrence of a calorimetry smectic phase (see differential scanninga occurrence of a smectic phase (see the differential scanning (DSC) in Figure 2c). However, calorimetry (DSC) Figure 2c). However, concentration, x, of MWCNTs higher than 1.5 wt% concentration, x, of in MWCNTs higher than 1.5 awt% destabilizes the mesomorphic behavior, as observed destabilizes the mesomorphic behavior, as observed in other LCE composites [37,40–44]. As shown in Figure 1d and Figure 1e, both dynamic mechanical analysis and stress–strain measurements confirm the physical properties typical of liquid-crystalline elastomers. The storage modulus (E’) and
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in other LCE composites [37,40–44]. As shown in Figures 1d and 1e, both dynamic mechanical analysis and stress–strain confirm the physical properties typical of liquid-crystalline elastomers. Polymers 2018, 10,measurements x FOR PEER REVIEW 5 of 27 The storage modulus (E’) and dissipation factor (tan δ) indicate an increasing stiffness of the LCE dissipation (tan δ) indicate an increasing stiffness of the LCE compositescurves when (σthe composites whenfactor the concentration of MWCNT is increased (Figure 1d). Stress–strain versus concentration of MWCNT is increased (Figure 1d).to Stress–strain (σ the versus ε%) refer to the ε%) refer to the uniaxial mechanical stress applied the rubbercurves state of samples, which is able uniaxial mechanical stress applied to the rubber state of the samples, which is able to align the to align the network. The typical soft-elasticity response of LCEs is also observed in the composites network. The typical soft-elasticity response of LCEs is also observed in the composites (Figure 1e). (Figure 1e). As reported and discussed in Reference [44], the presence of MWCNTs has interesting As reported and discussed in Reference [44], the presence of MWCNTs has interesting effects on the effects on the thermo-mechanical behavior. In particular, at x = 0.75 wt%, the LCE-based composites thermo-mechanical behavior. In particular, at x = 0.75 wt%, the LCE-based composites show a showdramatic a dramatic enhancement in thermal actuation veryorientational high orientational order, probably enhancement in thermal actuation and aand verya high order, probably due to due to alignment of the nanotubes along the smectic director. alignment of the nanotubes along the smectic director.
Figure 2. (a) Scheme of the synthetic procedure to prepare smectic epoxy-based elastomer film
Figure 2. (a) Scheme of the synthetic procedure to prepare smectic epoxy-based elastomer film samples samples (named DOMS_SA). (b) Optical image (left), SEM micrograph (center), and bright-field TEM (named DOMS_SA). (b) Optical image (left), SEM micrograph (center), and bright-field TEM image image (right) of a smectic film doped with multi-walled carbon nanotubes (sample DS_1.5CNT). (c) (right) of a smectic film doped with multi-walled nanotubes samples. (sample DS_1.5CNT). (c) Differential Differential scanning calorimetry (DSC) curves carbon of the investigated (d) Storage modulus (E’) scanning calorimetry (DSC) curves of the investigated samples. (d) Storage modulus (E’) and tan δ of and tan δof the investigated samples. (e) Stress–strain curves (σ versus ε%) of the investigated the investigated samples. Stress–strain (σ versus ε%) of the investigated samples. Adapted samples. Adapted with(e) permission fromcurves the authors of [44]. Copyright (2018) American Chemical with Society. permission from the authors of [44]. Copyright (2018) American Chemical Society. In addition to carbon nanoparticles and carbon nanotubes, recent studies reported the
In addition to carbon nanoparticles and carbon nanotubes, recent studies reported the preparation preparation of liquid-crystalline elastomers with graphene [45–47] and fullerene derivatives [45]. An of liquid-crystalline elastomers with graphene [45–47] and fullerene derivatives [45]. An original original synthetic approach was proposed by Meng et al. [45] to produce shape-memory elastomers synthetic approach was proposed by Meng al. [45] to produce shape-memory without without chemical cross-linkers, which areet substituted by physically cross-linkerelastomers points. These chemical cross-linkers, which are substituted by physically cross-linker points. These materials materials are main-chain liquid-crystalline polymers (labeled PBDPS, Figure 3) obtained using are main-chain liquid-crystalline polymers (labeled PBDPS, Figure 3) obtained using biphenyl-phenyl
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succinate derivatives, where the adjacent phenyl rings are stacked together forming strong π–π interactions [45]. PBDPS exhibits physical properties typical of chemically cross-linked polymers, biphenyl-phenyl succinate derivatives, where the adjacent phenyl rings are stacked togetherLC forming such strong as a π–π largeinteractions maximum strain (>220%) shape-recovery ratioof (>99%). addition of [45]. PBDPS exhibitsand physical properties typical chemicallyThe cross-linked low LC concentrations nanomaterials (~0.5 wt%) was investigated by means of27 several polymers, ascarbon aPEER large maximum strain (>220%) and shape-recovery ratio (>99%). The addition Polymers 2018, such 10, x of FOR REVIEW 6 of of low concentrations of carbon (~0.5 wt%) was microscopy investigated by means of several physico-chemical techniques, suchnanomaterials as TEM, polarized optical (POM; Figure 3), X-ray, biphenyl-phenyltechniques, succinate derivatives, wherepolarized the adjacent phenyl rings are stacked together forming physico-chemical such as TEM, optical microscopy (POM; Figure 3), X-ray, fluorescence spectroscopy (Figure 4a,b), and dynamic mechanical analysis (Figure 4c,d). strong π–πspectroscopy interactions [45]. PBDPS exhibits physical mechanical properties typical of chemically4c,d). cross-linked fluorescence (Figures 4a,b), and pristine dynamic analysis TheLCliquid-crystalline textures of the elastomer matrix,(Figures named PBDPS, and the polymers, such as a large maximum (>220%) and shape-recovery (>99%). The addition The liquid-crystalline textures of strain the pristine elastomer matrix, ratio named PBDPS, and the corresponding LCE composites containing 0.5 (~0.5 wt%wt%) of the of low concentrations of carbon nanomaterials wascarbon-based investigated by nanomaterials means of several can be corresponding LCE composites containing 0.5 wt% of the carbon-based nanomaterials can be characterized using POM, as reported Figure 3. The smectic A phase of the LCP gives rise to physico-chemical techniques, such as in TEM, polarized optical microscopy (POM; Figure 3), X-ray, characterized using POM, as reported in Figure 3. The smectic A phase of the LCP gives rise to typical fluorescence spectroscopy (Figures singularities. 4a,b), and dynamic mechanical analysis (Figures 4c,d). typical Schliren textures with fourfold The presence of the carbon nanomaterials sensibly Schliren textures with fourfold singularities. The presence of the carbon nanomaterials sensibly does The liquid-crystalline textures of the pristine elastomer matrix, named PBDPS, and the does not notchange changethe the textures, thus indicating thatbirefringence the birefringence properties are retained [45]. textures, thus indicating that the properties are retained [45]. corresponding LCE composites containing 0.5 wt% of the carbon-based nanomaterials can be characterized using POM, as reported in Figure 3. The smectic A phase of the LCP gives rise to typical Schliren textures with fourfold singularities. The presence of the carbon nanomaterials sensibly does not change the textures, thus indicating that the birefringence properties are retained [45].
Figure 3. Liquid-crystalline polyesters reprocessed with various carbon nanomaterials. Polarized
Figure 3. Liquid-crystalline polyesters reprocessed with various carbon nanomaterials. Polarized optical microscopy (POM) photographs captured at 30 °C: (a) PBDPS, (b) PC60 (0.5%), (c) PCNT optical microscopy (POM) photographs captured at 30 ◦ C: (a) PBDPS, (b) PC60 (0.5%), (c) PCNT (0.5%), (0.5%), and (d) PG (0.5%). Labels: PBDPS, phenyl succinate-based main chain LC polymer; PC60, and (d) PG (0.5%). Labels: phenyl succinate-based main composite chain polymer; PC60, composite Figure 3. Liquid-crystalline polyesters reprocessed carbonLC nanomaterials. Polarized composite sample made PBDPS, of PBDPS including fullerenewith C60; various PCNT, sample made of PBDPS optical microscopy (POM) photographs captured at 30 °C: (a) PBDPS, (b) PC60 (0.5%), (c) PCNT sample made carbon of PBDPS including PCNT, sample made of PBDPS including including nanotubes; PG, fullerene compositeC60; sample madecomposite of PBDPS including graphene. Reproduced (0.5%), and (d)PG, PG composite (0.5%). Labels: PBDPS, phenyl main chain LC polymer; PC60, with carbon nanotubes; sample made ofsuccinate-based including graphene. Reproduced with permission from the authors of [45]. Copyright ©PBDPS 2018 Elsevier. composite sample made of PBDPS including fullerene C60; PCNT, composite sample made of PBDPS permission from the authors of [45]. Copyright © 2018 Elsevier. including carbon nanotubes; PG, composite sample made of PBDPS including graphene. Reproduced with permission from the authors of [45]. Copyright © 2018 Elsevier.
Figure 4. Liquid-crystalline polyesters reprocessed with various carbon nanomaterials.
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Fluorescence spectra of PBDPS and PBDPS/carbon nanofiller composites at concentrations of 1 mg/mL (a) and 5 mg/mL (b). Dynamic mechanical results of the testing samples: (c) recovery stress–strain curves at 40 ◦ C; (d) storage modulus as a function of temperature. Labels: PBDPS, phenyl succinate-based main chain LC polymer; PC60, composite sample made of PBDPS including fullerene C60; PCNT, composite sample made of PBDPS including carbon nanotubes; PG, composite sample made of PBDPS including graphene. Adapted with permission from the authors of [45]. Copyright © 2018 Elsevier.
The fluorescence emission of PBDPS doped with small concentrations of carbon nanofillers, in particular C60 and graphene (see Figure 4a), is enhanced, thus confirming that the presence of nanomaterials acts as an additional physical cross-link of the polymer network. At higher concentrations, the quenching phenomena are more pronounced due to the energy transfer activated by the strong interactions between the nanocarbon materials and the π–π groups. This effect is also evident from dynamic mechanical analysis, showing that the addition of CNTs, C60, or graphene (at low wt%) increases the recovery stress (Figure 4c) and the storage modulus (Figure 4d), especially in the case of graphene. When the concentration of graphene is higher than 0.5 wt%, however, the effect is opposite and the LC ordering is partially destroyed, probably due to the relatively bigger size of graphene with respect to other carbon nanofillers [45]. 2.2. Applications of the Carbon-Based LCE Composites The incorporation of carbon nanomaterials into the LCE matrix was often motivated by the possibility of extending the actuation mechanisms of LCE-based composite systems in view of possible applications as artificial muscles and electro-active actuators. For instance, the swollen LSCE/carbon-black-integrated layer systems described by Chambers et al. [36–39] showed actuation properties via resistive “Joule” heating under several cycles of direct current (DC) electric power. When a square-wave current–voltage function is applied to the reprocessed LSCE film, the length of the film decreases, reaching the isotropic (paranematic) phase (actuation state: L0 ) due to the “Joule heating effect”. When the current is off, the film recovers its pristine length assumed at room temperature (relaxed state: L). A maximum elongation of 50% is obtained and the process is reproducible after hundreds of cycles [39]. This value of elongation is similar to that obtained by thermo-mechanical measurements in standard LSCEs, showing that the presence of a carbon-black layer does not prevent the actuation mechanism. A completely new actuation process was observed in the case of LCE-based composites prepared with functionalized CNTs, as reported by Marshall et al. [40]. In this case, the reversible and controllable shape variation of the LCE-composite films is obtained when light is used as actuation stimulus. The photo-actuation of these composites is maximum in the infrared-visible region due to the selective absorption of these wavelengths by the CNTs. In Figure 5a, a typical thermo-mechanical behavior of an LCE-based composite with 0.1 wt% of functionalized CNTs is reported, while Figure 5b illustrates the stress measured on the same system when a uniform light excitation of 620-nm wavelength at various power densities is applied [43].
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Figure 5. Actuation embedded carbon carbon Figure 5. Actuation curves curves of of aa stretched stretched LCE LCE sample sample containing containing 0.1 0.1 wt% wt% of of embedded nanotubes (CNTs). (a) Contraction ratio (L/L 0 ) under uniform heating measured as a function of nanotubes (CNTs). (a) Contraction ratio (L/L0 ) under uniform heating measured as a function temperature. Pictures in the inset demonstrate the contraction/elongation phenomenon at the of temperature. Pictures in the inset demonstrate the contraction/elongation phenomenon at the mesophase mesophase transition. transition. (b) (b) Actuation-stress Actuation-stress curves curves measured measured under under the the application application of of various various power power densities. densities. The The inset inset presents presents the the scheme scheme of of the the set-up set-up used used for for testing. testing. Adapted Adapted with with permission permission from from the authors of [43]. Copyright © 2018 Elsevier. the authors of [43]. Copyright © 2018 Elsevier.
The use of various concentrations of functionalized CNTs in the LCE matrix is indeed the basis The use of various concentrations of functionalized CNTs in the LCE matrix is indeed the of several applications in the field of micro-optical devices [40–43], Braille-based displays [56], solarbasis of several applications in the field of micro-optical devices [40–43], Braille-based displays [56], energy harvesting [57], and tactile and piezoelectric devices [42,58]. The possibility of using LCE solar-energy harvesting [57], and tactile and piezoelectric devices [42,58]. The possibility of composites as light concentrators and heat collectors was recently demonstrated by showing the using LCE composites as light concentrators and heat collectors was recently demonstrated by artificial heliotropism for solar cells, utilizing fiber-network/SWCNT/LCE actuators [57]. These showing the artificial heliotropism for solar cells, utilizing fiber-network/SWCNT/LCE actuators [57]. systems can be directly driven by sunlight instead of relying on other power-consuming components These systems can be directly driven by sunlight instead of relying on other power-consuming for tracking the sun and actuation. Further studies are in progress to optimize several parameters, components for tracking the sun and actuation. Further studies are in progress to optimize several such as the contraction ratio, the threshold of response to the sunlight, and the loading capability. parameters, such as the contraction ratio, the threshold of response to the sunlight, and the loading Another concrete application is related to tactile and haptic displays [56,58], where micro-patterning capability. Another concrete application is related to tactile and haptic displays [56,58], where techniques were used to create arrays of dots able to change their shape either due to LED micro-patterning techniques were used to create arrays of dots able to change their shape either illumination or variations in temperature. due to LED illumination or variations in temperature. 3. 3. LCE-Based LCE-Based Composites Composites with with Other Other Nanomaterials Nanomaterials In as as fillers and dopants of In addition addition to to the thelarge largeuse useofofcarbon-based carbon-basednanostructured nanostructuredmaterials materials fillers and dopants LCE systems, several researches report LCE-based composites prepared using a wide variety of LCE systems, several researches report LCE-based composites prepared using a wide variety of of nanomaterials, ferroelectric ceramic ceramic nanoparticles nanoparticles [59,60], [59,60], conductive conductive molybdenum-oxide molybdenum-oxide nanomaterials, such such as as ferroelectric nanowires magnetic iron-oxide nanoparticles [62–65], and gold gold nanomaterials nanomaterials [66–68]. [66–68]. In nanowires [61], [61], magnetic iron-oxide nanoparticles [62–65], and In the the following sections, the synthesis, properties, and applications of these composites are reviewed. following sections, the synthesis, properties, and applications of these composites are reviewed. 3.1. 3.1. Preparation Preparation of of Nanomaterial-Based Nanomaterial-Based LCE LCE Composites Composites One produce composite materials combining the thermo-mechanic and One of of the thefirst firstattempts attemptstoto produce composite materials combining the thermo-mechanic thermo-elastic properties of LCEs theand ferroelectricity of nanoparticles was reported Reference and thermo-elastic properties of and LCEs the ferroelectricity of nanoparticles was in reported in [59], where the possibility of inducing a macroscopic shape deformation Reference [59], where the possibility of inducing a macroscopic shape deformationusing using non-spherical non-spherical ferroelectric ferroelectric nanoparticles nanoparticles aligned aligned along along the the main main direction direction of of monodomain monodomain LCE LCE films filmswas wasdescribed. described. Several Several nanostructured nanostructured materials materials were were chosen chosen as as potential potential candidates candidates to to produce produce electro-mechanical electro-mechanical active as as conductive nanowires made of molybdenum oxidesoxides (MoO3−x(MoO ) [61]3(Figures active composites, composites,such such conductive nanowires made of molybdenum −x ) [61] 6a,b), and ferroelectric ceramic nanoparticles, namely lead titanates (PbTiO 3 ) [59,60] (Figure 6c). (Figure 6a,b), and ferroelectric ceramic nanoparticles, namely lead titanates (PbTiO3 ) [59,60] (Figure 6c).
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Figure 6. Structured nanomaterials used to prepare LCE-based composites. (a) TEM image of the Figure 6. 6. Structured (a) (a) TEM image of the Figure Structured nanomaterials nanomaterialsused usedtotoprepare prepareLCE-based LCE-basedcomposites. composites. TEM image of MoO3−x nanowires, and (b) SEM image of the MoO3−x nanowires, revealing their electrical conductivity MoO 3−x nanowires, and (b) SEM image of the MoO3−x nanowires, revealing their electrical conductivity the MoO3−x nanowires, and (b) SEM image of the MoO3−x nanowires, revealing their electrical under 10-keV conditions. Adapted with permission from the authors of [61]. Copyright © 2018 under 10-keV conditions. Adapted with permission from thefrom authors of [61].ofCopyright © 2018 conductivity under 10-keV conditions. Adapted with permission the authors [61]. Copyright © Springer Nature. (c) A field-emission SEM picture of PbTiO3 quasi-spherical nanoparticles with an Springer Nature. (c) A field-emission SEM picture of PbTiO 3 quasi-spherical nanoparticles with an 2018 Springer Nature. (c) A field-emission SEM picture of PbTiO3 quasi-spherical nanoparticles with average diameter of 100 nm. Adapted with permission from the authors of [60]. Copyright (2018) average diameter of of 100 Copyright (2018) (2018) an average diameter 100nm. nm.Adapted Adaptedwith withpermission permissionfrom fromthe the authors authors of of [60]. [60]. Copyright American Chemical Society. American Chemical Society. American Chemical Society.
The preparation method proposed in References [59–61] is similar to that adopted by Marshall The preparation method proposed in References [59–61] is similar to that adopted by Marshall The method proposed in References [59–61] or is similar to that adoptedwere by et al. [40].preparation The nanomaterials, either ferroelectric nanoparticles conductive nanowires, et al. [40]. The nanomaterials, either ferroelectric nanoparticles or conductive nanowires, were Marshall [40]. The either ferroelectric nanoparticles or conductive dispersedetinal. toluene at ananomaterials, known concentration, and the pre-polymerization componentsnanowires, (polymer dispersed in toluene at a known concentration, and the pre-polymerization components (polymer were dispersed in toluene at a known concentration, pre-polymerization components (polymer chains, monomers, and cross-linkers) were addedand in the stoichiometric amounts before adding the chains, monomers, and cross-linkers) were added in stoichiometric amounts before adding the chains, monomers, and cross-linkers) were added in stoichiometric amounts before adding the catalyst. catalyst. The mixture was introduced to the rotor at 75 °C for about 1 h until a gel-like film was catalyst. The mixture was introduced to the ◦rotor at 75 °C for about 1 h until a gel-like film was The mixture was introduced to the rotor at 75 C for about 1 hinside until aangel-like film obtained, and obtained, and then, it was hanged with progressive weights oven at 80 was °C for about three obtained, and then, it was hanged with progressive weights inside oven at 80 °C for about three ◦ Can then, it was hanged with progressive weights inside an oven at 80 for about three days in order to days in order to get a monodomain LCE-composite stripe. days in order to get a monodomain LCE-composite stripe. get a Similar monodomain LCE-composite stripe. approaches, which refer to the “two-step” cross-linking reaction proposed by Kupfer et Similar approaches, which refer to the “two-step” cross-linking reaction proposed by Kupfer et Similar to the “two-step” cross-linking reaction proposed by al. [35], wereapproaches, followed to which obtain refer magneto-active LCE composites made of magnetic iron-oxide al. [35], were followed to obtain magneto-active LCE composites made of magnetic iron-oxide Kupfer et al. [35], were followed to obtain magneto-active LCE composites made of magnetic nanoparticles and side-chain polysiloxane-based LCEs [62–65]. In most cases, the iron-oxide nanoparticles and side-chain polysiloxane-based LCEs [62–65]. In most cases, the iron-oxide iron-oxide nanoparticles and of side-chain [62–65]. In mostwith cases,oleic the iron-oxide nanoparticles were made a core polysiloxane-based of magnetite (Fe3LCEs O4) surface-coated acid, Nnanoparticles were made of a core of magnetite (Fe3O4) surface-coated with oleic acid, Nnanoparticles were made of a core of magnetite (Fe with acid, N-oleylsarcosine, oleylsarcosine, or analogous functionalization chemicals. A scheme of theoleic preparation and the main 3 O4 ) surface-coated oleylsarcosine, or analogous functionalization chemicals. A scheme of the preparation and the main or analogous functionalization chemicals. A scheme the preparation and the chemical chemical components of these magneto-active LCEs, asofdescribed in Reference [65],main are shown in chemical components of these magneto-active LCEs, as described in Reference [65], are shown in components of these magneto-active LCEs, as described in Reference [65], are shown in Figure 7. Figure 7. Figure 7.
Figure 7. 7. Scheme of of the preparation preparation of the the composites made made from magnetic magnetic iron-oxide nanoparticles nanoparticles Figure Figure 7. Scheme Scheme of the the preparation of of the composites composites made from from magneticiron-oxide iron-oxide nanoparticles and a side-chain LCE matrix. Chemical structure of the liquid-crystalline elastomer precursors: the and LCE matrix. matrix.Chemical Chemicalstructure structure liquid-crystalline elastomer precursors: and aa side-chain LCE of of thethe liquid-crystalline elastomer precursors: the polysiloxane chain (PMHS), thethe flexible cross-linker (11UB), the polysiloxane chain (PMHS), flexible cross-linker (11UB),and andthe themesogenic mesogenicmonomer monomer (MBB). (MBB). polysiloxane chain (PMHS), the flexible cross-linker (11UB), and the mesogenic monomer (MBB). Reproduced with with permission from from the authors authors of [65]. [65]. Copyright © © 2018 Elsevier. Elsevier. Reproduced Reproduced withpermission permission fromthe the authorsof of [65].Copyright Copyright ©2018 2018 Elsevier.
A new synthetic strategy was recently proposed by Wjjcik et al. [68] to prepare LCE-based A new synthetic strategy was recently proposed by Wjjcik et al. [68] to prepare LCE-based composites including gold nanoparticles (GNPs) in the LCE matrix. The approach, shown in Figure composites including gold nanoparticles (GNPs) in the LCE matrix. The approach, shown in Figure 8, consists of the synthesis of gold nanoparticles functionalized with an olefin mesogenic ligand (L2) 8, consists of the synthesis of gold nanoparticles functionalized with an olefin mesogenic ligand (L2)
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A new synthetic strategy was recently proposed by Wjjcik et al. [68] to prepare LCE-based Polymers 2018, 10, x FOR PEER REVIEW 10 of 27 in composites including gold nanoparticles (GNPs) in the LCE matrix. The approach, shown Figure 8, consists of the synthesis of gold nanoparticles functionalized with an olefin mesogenic to obtain metal nanoparticles acting as cross-linking agents (GNP@L2) the polymer backbones ligand (L2) to obtain metal nanoparticles acting as cross-linking agentsfor (GNP@L2) for the polymer (PMHS). backbones (PMHS).
Figure 8. 8.Scheme composites(label (labelLCE-GNP) LCE-GNP)made madeof of Figure Scheme ofof the the preparation preparation of of the the LCE-based LCE-based composites functionalized gold nanoparticles(label (labelGNP@L2) GNP@L2) and and side-chain chains functionalized gold nanoparticles side-chainLCEs LCEsbased basedon onpolysiloxane polysiloxane chains (PMHS). Mesogenic monomers (label L1) used are used as pendant Standard LCEsalso were also (PMHS). Mesogenic monomers (label L1) are as pendant units. units. Standard LCEs were prepared prepared with cross-linkers L3 or L4. Reproduced with permission the authors [68]. Copyright with cross-linkers L3 or L4. Reproduced with permission from thefrom authors of [68].ofCopyright © 2018 © 2018 Wiley Online Library. Wiley Online Library.
Following a synthetic procedure similar to the “two-step” cross-linking reaction, the final LCEFollowing a synthetic procedure similar to the “two-step” cross-linking reaction, the final based composites contain metal nanoparticles covalently attached to the polymer chain (LCE-GNP), LCE-based composites contain metal nanoparticles covalently attached to the polymer chain without any problems of uncontrolled aggregation, ensuring a homogeneous distribution of (LCE-GNP), without any problems of uncontrolled aggregation, ensuring a homogeneous distribution nanoparticles within the polymer matrix [68]. of nanoparticles within the polymer matrix [68]. 3.2. Alignment of Nanomaterials in the LCE-Based Composites and Orientational-Ordering Properties
3.2. Alignment of Nanomaterials in the LCE-Based Composites and Orientational-Ordering Properties As seen for LCE-based composites made of carbon nanomaterials (Section 2), the distribution of As seen for LCE-based composites made of carbon nanomaterials (Section 2), the distribution of nanostructured fillers in the polymer matrix and the degree of orientational order in the LC phase nanostructured fillers in the polymer matrix and the degree of orientational order in the LC phase formed by LCE composites are fundamental features in view of the physical properties of these formed by LCE composites are fundamental features in view the of the physical properties of these materials. The main experimental techniques used to investigate alignment of the nanomaterials materials. The main experimental techniques used to investigate the alignment of the nanomaterials and the surface morphology of the composites are SEM, TEM, and AFM, while the degree of ordering andinthe morphology of the order composites are SEM, TEM, and AFM, while thestudied degree by of ordering thesurface bulk and the orientational parameters in the mesophase are typically means in the bulk and the orientational order parameters in the mesophase are typically studied by means of of X-ray scattering (wide-angle (WAXS) and small-angle (SAXS)) and NMR spectroscopy. X-ray scattering (wide-angle (WAXS) and small-angle and NMR spectroscopy. composites In the case of PbTiO3/LCE-based composites [60](SAXS)) and MoO 3−x nanowire/LCE-based In the thenanomaterials case of PbTiO composites [60] in and [61], were3 /LCE-based not homogeneously distributed the MoO volume the LCE matrix, but 3−xof nanowire/LCE-based composites [61],concentrated the nanomaterials notthe homogeneously in the surface volumelayer of thewas LCE instead, they on one were side of LCE film. The distributed nanomaterial-rich clearly by means of SEMon and AFM 9 and 10).The In both cases, the nanomaterials matrix, butobservable instead, they concentrated one side(Figures of the LCE film. nanomaterial-rich surface layer distributed in aby layer of about 10-μm showing9aand particular at the top waswere clearly observable means of SEM anddepth, AFM (Figures 10). In alignment, both cases,visible the nanomaterials surface, with respect to theofnematic director (parallel to the elongation direction). were distributed in a layer about 10-µm depth, showing a particular alignment, visible at the top
surface, with respect to the nematic director (parallel to the elongation direction).
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Figure 9.(a) (a) Field-emissionSEM SEM image obtained on aaon small piece of LCE stripe doped Figure 9. (a) Field-emission SEM image obtained a small of a monodomain LCE stripe Figure 9. Field-emission image obtained on small piecepiece ofaamonodomain monodomain LCE stripe doped with 5% of PbTiO 3 nanoparticles. The cross-section of the sample reveals an inhomogeneous doped with of PbTiO nanoparticles. The cross-section of the sample reveals an inhomogeneous with 5% of 5% PbTiO 3 nanoparticles. The cross-section of the sample reveals an inhomogeneous 3 distribution of the nanoparticles along the thickness of the stripe. (b) Field-emission SEM image of distribution of the distribution of the nanoparticles nanoparticles along along the the thickness thickness of of the the stripe. stripe. (b) (b) Field-emission Field-emission SEM SEM image image of of the PbTiO3 nanoparticles distributed on the top surface of the composite at 50-μm magnification. The the PbTiO33 nanoparticles nanoparticlesdistributed distributedonon the surface of the composite at 50-µm magnification. the PbTiO the toptop surface of the composite at 50-μm magnification. The direction of the nematic director (n) is indicated by a white arrow. Adapted with permission from the The direction of nematic the nematic director is indicated a white arrow. Adapted permission from direction of the director (n) is(n) indicated by aby white arrow. Adapted withwith permission from the authors of [60]. Copyright (2018) American Chemical Society. the authors of [60]. Copyright (2018) American Chemical Society. authors of [60]. Copyright (2018) American Chemical Society.
As reported in Reference [60], the presence of PbTiO3 nanoparticles in various percentages (up
As reported in Reference Reference [60], the presence PbTiO 3 nanoparticles in various percentages (up reported in presence ofofPbTiO various percentages to to As 5 wt%) is responsible for a[60], shiftthe in the transition temperature of a fewin degrees with respect to(up the 3 nanoparticles to 5 wt%) is responsible for a shift in the transition temperature of a few degrees with respect to the 5 wt%) is responsible for awith shiftthe in same the transition ofcomposition. a few degrees with respect standard LCE prepared proceduretemperature and chemical This means that to thethe standard LCE prepared prepared with the the same same procedure and chemical chemical composition. This means that the nanomaterials do not significantly influence the mesomorphic properties. The LCE-based films arethe standard LCE with procedure and composition. This means that nanomaterials do notthus significantly influence the mesomorphic mesomorphic properties. The LCE-based films are indeed transparent, indicatinginfluence a good macroscopic alignment of the nematic However, nanomaterials do not significantly the properties. Thedirectors. LCE-based films are indeed transparent, thus indicating a good macroscopic alignment of the nematic directors. However, as shown in Figure 9, the distribution of PbTiO 3 nanoparticles on top surface is regular and indeed transparent, thus indicating a good macroscopic alignment of the nematic directors. However, as anisotropic. shown in in Figure Figure 9,the thenanoparticles distribution of of PbTiO 3 nanoparticles on the top surface is regular and In fact,9, arePbTiO distributed along parallel stripes perpendicular to the as shown the distribution 3 nanoparticles on the top surface is regular and nematic director (n)the with a typical period few micrometers. anisotropic. In nanoparticles are along perpendicular to the anisotropic. In fact, fact, the nanoparticles areofdistributed distributed along parallel parallel stripes stripes perpendicular to the nematic director (n) with a typical period of few micrometers. nematic director (n) with a typical period of few micrometers.
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(c) Figure 10. Atomic force microscopy (AFM) topography image (a) and phase image (b) of the surface (c) dipping into the polymer matrix. (c) Surface of of a composite LCE film showing MoO3−x nanowires the composite film with MoO(AFM) 3−x nanowires showing characteristic undulations. Adapted with Figure 10. Atomic AtomicLCE force microscopy topography image image (a) (a) and and phase phase image image (b) (b) of of the the surface surface Figure 10. force microscopy (AFM) topography permission from the authors of [61]. Copyright © 2018 Springer Nature. of aa composite composite LCE LCE film film showing showingMoO MoO3−x nanowires dipping into the polymer matrix. (c) Surface of of 3−x nanowires dipping into the polymer matrix. (c) Surface of the composite LCE film with MoO 3−x nanowires showing characteristic undulations. Adapted with the composite LCE film with MoO3−x nanowires showing characteristic undulations. Adapted with permission from from the the authors authors of of [61]. [61]. Copyright Copyright © © 2018 2018 Springer Springer Nature. Nature. permission
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A similar anisotropic distribution of nanomaterials was observed in MoO3−x nanowire/LCE-based composites [61]. Most of the MoO3−x nanowires are indeed distributed on the top surface of the composite films. This is due to the fact that, during the first cross-linking step of the “two-step” cross-linking reaction, the nanowires tend to separate from the other pre-polymerization components, as noted in the case of ferroelectric nanoparticles [60], because of the different relative weight. The analysis of SEM and TEM images [61] reveals a uniform alignment of the nanowiresPolymers along2018, the thus indicating that the adopted procedure allowed 12usof 27 to induce a 10,director x FOR PEER n, REVIEW preferred alignment of the nanowires into the top surface of the LCE matrix. Topography and phase similar anisotropic distribution of nanomaterials was observed in MoO3−x nanowire/LCEAFM imagingAmodes were used to investigate the surface of composite films. This study indicates based composites [61]. Most of the MoO3−x nanowires are indeed distributed on the top surface of the that the nanowires are close tofact thethat, surface (Figurestep 10a,b), well-embedded composite films.located This is due to the duringof thethe firstLCEs cross-linking of the “two-step” cross- into the surface layer of the polymer matrix. Moreover, characteristic surface deformations are observed on the linking reaction, the nanowires tend to separate from the other pre-polymerization components, as noted in the casecomposites of ferroelectric nanoparticles becausedue of the relative weight. The typical of top surface of the LCE (Figure 10c), [60], probably to different Euler strut instabilities analysis of SEM and TEM images [61] reveals a uniform alignment of the nanowires along the director adjacent systems having a different Young’s modulus [61]. n, thus indicating that the adopted procedure allowed us to induce a preferred alignment of the Despite the inhomogeneous distribution of nanomaterials in phase the LCE the formation of nanowires into the top surface of the LCE matrix. Topography and AFMmatrix, imagingand modes were a surface layer with a highthedensity ofcomposite nanofillers (either MoO nanowires or PbTiO nanoparticles), used to investigate surface of films. This study indicates that the nanowires are located 3−x 3 close to the surface of the LCEs 10a,b), well-embedded theare surface layer to of the polymer the thermo-mechanical properties of(Figures the LCE-based compositeinto films similar those of the pristine matrix. Moreover, characteristic surface deformations are observed on the top surface of the LCE LCE materials (Figure 11a). The addition of 1 wt% and 5 wt% of lead-titanate nanoparticles reduces composites (Figure 10c), probably due to Euler strut instabilities typical of adjacent systems having a the maximum elongation from[61]. 68% to about 55% in both cases [60]. Similarly, the presence of different Young’s modulus molybdenum-oxide nanowires reduces the maximum elongation ofmatrix, the LCE films by about 10% Despite the inhomogeneous distribution of nanomaterials in the LCE and the formation of to a surface layer with[61]. a high of nanofillers (either MoO3−x nanowires or PbTiO3 has little with respect standard LCEs Thedensity increased stiffness of the LCE composites, however, nanoparticles), the thermo-mechanical properties of the LCE-based composite films are similar to effect on the mesomorphic properties, except for a slight shift in the paranematic-to-nematic phase those of the pristine LCE materials (Figure 11a). The addition of 1 wt% and 5 wt% of lead-titanate transition nanoparticles (see Figure reduces 11a). the maximum elongation from 68% to about 55% in both cases [60]. Similarly, An important aspect of these LCE-based systems related to the degreeofof the presence of molybdenum-oxide nanowires reducesisthe maximum elongation theorientational LCE films by order as about 10% with respect to standard LCEs [61]. The increased stiffness of the LCE composites,(2 H) NMR the temperature is varied from T > TN-PN to room temperature and vice versa. Deuteron however, has little effect on the mesomorphic properties, except for a slight shift in the paranematicstudies of selectively labeled LCE and LCE-based composites were of help in deeply investigating this to-nematic phase transition (see Figure 11a). 2 H NMR spectra, and the trends aspect [21,60,69–74]. The temperature dependence of the measured An important aspect of these LCE-based systems is related to the degree of orientational order of the quadrupolar splitting can from be analyzed obtain the local parameter, S(T), as the temperature is varied T > TN-PN toto room temperature andorientational-order vice versa. Deuteron (2H) NMR studies the of selectively LCE LCE-based composites mesophase were of help indirector deeply investigating which indicates averagelabeled degree of and order of the principal (n). A typical trend this aspect [21,60,69–74]. The temperature dependence of the measured 2H NMR spectra, and the of S(T) in an LCE system (standard or composite) is reported in Figure 11b, showing the continuous trends of the quadrupolar splitting can be analyzed to obtain the local orientational-order parameter, increase inS(T), local order at the paranematic–nematic upon decreasing the temperature. which indicates the average degree of order of thetransition principal mesophase director (n). A typical The continuous characteristics the temperature dependence of S is ainclear of the trend of S(T) in an LCEof system (standard or composite) is reported Figureindication 11b, showing the criticality continuous increase in local order at the paranematic–nematic transition upon decreasing the (or super-criticality) of the paranematic–nematic transition [69,70], which can be modeled through a temperature. The continuous characteristics of the temperature dependence of S is a clear indication modified Landau–de Gennes theory of the free-energy density of LCE systems. This approach [69,70] of the criticality (or super-criticality) of the paranematic–nematic transition [69,70], which can be gives rise to a complete of the microscopic properties of LCEs, and of it LCE associates modeled throughdescription a modified Landau–de Gennes theory of the free-energy density systems.the critical characteristics of the mesophase transition to the presence ofmicroscopic internal mechanical fields in the LCE This approach [69,70] gives rise to a complete description of the properties of LCEs, and it associatesby thea critical characteristics of the mesophase transition the presence of internal network, quantified parameter, g. According to this analysis, thetopresence of nanomaterials in the mechanical fields in the LCE network, quantified by a parameter, g. According to this analysis, the LCE matrix increases the parameter g to values five times larger than the critical value (gc ) shifting the presence of nanomaterials in the LCE matrix increases the parameter g to values five times larger mesophasethan transition to a supercritical one. the criticalfrom valuea(gcritical c) shifting the mesophase transition from a critical to a supercritical one.
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Figure 11. (a) Thermo-mechanical behavior of the standard monodomain LCE film (blue symbols),
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and LCE films doped with 1% (red symbols) and 5% (black symbols) PbTiO3 nanoparticles. Temperature transitions between the paranematic and nematic phases are indicated by vertical lines. (b) Temperature dependence of the orientational-order parameter (S) determined by analyzing the 2 H NMR quadrupolar splittings in terms of the Landau–de Gennes modified theory, as described Polymers 2018, 10, x FOR PEER REVIEW 13 of 27 in References [21,69,70]. A dashed line corresponds to the transition temperature obtained from the Figure 11. (a) Thermo-mechanical behavior the standard from monodomain LCE film fitting of the orientational order. Adapted withofpermission the authors of (blue [60].symbols), Copyright (2018) and LCE films doped with 1% (red symbols) and 5% (black symbols) PbTiO3 nanoparticles. American Chemical Society.
Temperature transitions between the paranematic and nematic phases are indicated by vertical lines. (b) Temperature dependence of the orientational-order parameter (S) determined by analyzing the NMR quadrupolar splittings in terms of the Landau–de Gennes modified order theory, as in temperature Moreover,2Has shown in Figure 11b, the value of the orientational (S)described at room References [21,69,70]. A dashed line corresponds to the transition temperature obtained from the is comparable with that measured in standard LCEs [60,69–72]. Relatively high values of the fitting of the orientational order. Adapted with permission from the authors of [60]. Copyright (2018) orientational-order parameter (S), obtained from X-ray diffraction analysis, were also found in American Chemical Society.
magneto-active LCE-based composites prepared with iron-oxide nanoparticles [65]. Another common Moreover, as shownthe in Figure 11b, the value of theparameter orientationalin order (S) atisroom temperature technique used to determine orientational-order LCEs indeed based on X-ray is comparable with that measured in standard LCEs [60,69–72]. Relatively high values of the diffraction (XRD) as reported in Figure 12. LCEs with and without the inclusion of magnetic orientational-order parameter (S), obtained from X-ray diffraction analysis, were also found in nanoparticles, for instance, have composites S values prepared betweenwith 0.5 and 0.6 atnanoparticles low temperatures (Figure 12d). magneto-active LCE-based iron-oxide [65]. Another common to determine orientational-order in order LCEs isin indeed based on The high value oftechnique S in theused ordered phasethe indicates that the parameter degree of the low-temperature X-ray diffraction (XRD) as reported in Figure 12. LCEs with and without the[65]. inclusion magnetic phase is not significantly reduced by the addition of the nanoparticles The ofX-ray patterns can nanoparticles, for instance, have S values between 0.5 and 0.6 at low temperatures (Figure 12d). The give information not only about the average order, but also about the homogeneity of distributions, high value of S in the ordered phase indicates that the degree of order in the low-temperature phase and aboutisthe of the mesophases. Both Figure 12a,b evidence thepatterns occurrence of lamellar not nature significantly reduced by the addition of the nanoparticles [65]. The X-ray can give informationfor nottwo onlygold-nanoparticle about the average order, also about the homogeneity of distributions, and (smectic) structures LCEbut composites, namely LCE-GNP1 and LCE-GNP2 [68], about the nature of the mesophases. Both Figures 12a,b evidence the occurrence of lamellar (smectic) with layer spacings in the range of 39–44 Å (Figure 12c), thus indicating the presence of a partially structures for two gold-nanoparticle LCE composites, namely LCE-GNP1 and LCE-GNP2 [68], with interdigitated structure. The diffuse XRD signal observed at high angles, corresponding to 4.5 Å, layer spacings in the range of 39–44 Å (Figure 12c), thus indicating the presence of a partially reflects theinterdigitated liquid-like structure. order of The mesogens within theobserved smecticatlayers. Moreover, temperature-dependent diffuse XRD signal high angles, corresponding to 4.5 Å, reflects the liquid-like order and of mesogens the smectic layers. temperatureX-ray measurements (upon cooling heating within the samples) show that Moreover, the gold nanoparticles remain dependent measurements heating the samples) show without that the gold well-dispersed in theX-ray polymer network,(upon with acooling highlyand homogeneous distribution the formation nanoparticles remain well-dispersed in the polymer network, with a highly homogeneous of nanoparticle agglomerations. distribution without the formation of nanoparticle agglomerations.
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Figure 12. (a,b) Two-dimensional (2D) X-ray diffraction (XRD) patterns taken in the broad-angle
Figure 12. range (a,b)for Two-dimensional (2D) X-ray diffraction (XRD) patterns taken in the broad-angle range smectic phases of polymers. (a) LCE-GNP with a hybrid gold-nanoparticle cross-linker; (b) for smecticstandard phases LCE of polymers. (a) LCE-GNP with a hybrid gold-nanoparticle cross-linker; (b) standard prepared with L3 as the cross-linker. (c) Temperature dependence of smectic-layer LCE prepared with L3 as the cross-linker. (c) Temperature dependence of smectic-layer thickness for LCE-GNP (black circles) and LCE (red circles). Reproduced with permission from the authors of [68]. Copyright © 2018 Wiley Online Library. (d) Order parameter (S) as a function of the reduced temperature
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(T/Tni ) of LCE composites containing various percentages of iron-oxide nanoparticles, as obtained usingthickness small-angle X-ray scattering (SAXS) pristinewith matrix ( ), 0.2 from wt% the (), 0.5 for LCE-GNP (black circles) andmeasurements: LCE (red circles).LCE Reproduced permission authors [68]. Copyright © 2018 Wiley OnlineReproduced Library. (d) Order parameter (S)from as a function of the wt% (∆), andof0.7 wt% (u) nanoparticle content. with permission the authors of [65]. reduced temperature Copyright © 2018 Elsevier.(T/Tni) of LCE composites containing various percentages of iron-oxide nanoparticles, as obtained using small-angle X-ray scattering (SAXS) measurements: LCE pristine matrix (●),conformation 0.2 wt% (□), 0.5and wt%chain (Δ), and 0.7 wt% (◆) nanoparticle content.elastomers Reproduced were with polymer anisotropy of liquid-crystalline permission from the authors of [65]. Copyright © 2018 Elsevier.
The also the object of several studies, based on a combination of X-ray techniques (namely SAXS and WAXS) [75,76] and small-angle neutron scattering and (SANS) However, to our knowledge, no such The polymer conformation chain[77–79]. anisotropy of liquid-crystalline elastomers were studies also the were performed to several investigate the based ordering structuralofproperties of nanomaterial/LCE composites. object of studies, on aand combination X-ray techniques (namely SAXS and WAXS) [75,76] and small-angle neutron scattering (SANS) [77–79]. However, to our knowledge, no such
3.3. Actuation Properties and to Applications Nanomaterial-Based LCE Composites studies were performed investigateofthe ordering and structural properties of nanomaterial/LCE
composites. As seen in the previous section, the addition of nanomaterials does not alter the thermo-mechanical behavior, preserving the ability of the LCE-based composites to act as 3.3. Actuation Properties and Applications of Nanomaterial-Based LCE Composites shape-memory actuators subject to temperature variations. However, in some cases, the nanostructured seen new in theproperties, previous section, addition of nanomaterials does not alter thermodopants As induce such the as the magneto-mechanical response of the LCE-composite mechanical behavior, preserving the ability of the LCE-based composites to act as shape-memory films prepared with iron-oxide nanoparticles [62–65]. Kaiser et al. [62] reported a reversible actuators subject to temperature variations. However, in some cases, the nanostructured dopants magnetic-field-induced shape change in LCE-doped systems under on/off cycles of various induce new properties, such as the magneto-mechanical response of LCE-composite films prepared electromagnetic-field amplitudes (fromKaiser 8.5 toet 42.6 kA/m), reaching a contraction of the starting with iron-oxide nanoparticles [62–65]. al. [62] reported a reversible magnetic-field-induced composite films comparable to the thermally induced contraction. Herrera-Posada et al. [65] confirmed shape change in LCE-doped systems under on/off cycles of various electromagnetic-field amplitudes the potential LCEs doped with 0.5 and 0.7 wt% nanoparticles (from 8.5 of to 42.6 kA/m), reaching a contraction of theoleic-acid-coated starting compositeiron-oxide films comparable to the as thermally induced contraction. Herrera-Posada et temperature al. [65] confirmed the potential of magnetic LCEs doped with magneto-active elastomers. In Figure 13a–d, the evolution of the elastomers 0.5 and 0.7 wt%thermal oleic-acid-coated magneto-active elastomers. of In Figures determined from infrarediron-oxide images nanoparticles is reported, aswhile the superposition the instant 13a–d, the temperature evolution of the magnetic elastomers and determined from thermal infrared temperature of the magneto-active elastomers during actuation, the contraction percentage profiles images is reported, while the superposition of the instant temperature of the magneto-active are shown in Figure 13e. The application of an alternating field of 48.24 kA/m and an oscillating elastomers during actuation, and the contraction percentage profiles are shown in Figure 13e. The frequency of 298 kHz causes relaxation and hysteresis phenomena in the magnetic nanoparticles, application of an alternating field of 48.24 kA/m and an oscillating frequency of 298 kHz causes leading to energy dissipation or heat loss [65]. This mechanism is at the origin of the thermally driven relaxation and hysteresis phenomena in the magnetic nanoparticles, leading to energy dissipation or isotropic–nematic transition from is the generated the magnetic heat loss [65]. This mechanism atheat the origin of theby thermally driven nanoparticles. isotropic–nematic transition In this [65], the possibility of using magnetic iron-oxide nanoparticle/LCE composites in from thework heat generated by the magnetic nanoparticles. biologicalInenvironments studying theiron-oxide magneto-mechanical response in deionized this work [65],was the investigated possibility of by using magnetic nanoparticle/LCE composites in environments was investigated by studying theThe magneto-mechanical response in of waterbiological and in a model cell-growth medium (labeled DMEM). time profiles of the contraction deionized water andand in athe model medium DMEM). profiles the LCE-based composites, fluidcell-growth temperature close to(labeled the films and atThe the time bottom of theofcontainer contraction of LCE-based composites, and the fluid temperature close to the films and at the bottom are reported in Figure 13f,g, respectively. As a main result, the LCE-composite systems have a lower of the container are reported in Figures respectively. As a main result, theasLCE-composite contraction in deionized water than in air, 13f,g, because the surrounding water acts a heat sink which systems have a lower contraction in deionized water than in air, because the surrounding water acts dissipates the heat generated by the magnetic nanoparticles in the LCE matrix. When DMEM fluid is as a heat sink which dissipates the heat generated by the magnetic nanoparticles in the LCE matrix. used When instead of water, the magneto-mechanical response increases during each successive cycle, and DMEM fluid is used instead of water, the magneto-mechanical response increases during each this issuccessive probablycycle, due to presence of salts and Even though the magnetically-induced andthe this is probably due to theelectrolytes. presence of salts and electrolytes. Even though the contractions in DMEM are lower thaninin air, they reproducible, and are thereproducible, temperatureand of the magnetically-induced contractions DMEM are are lower than in air, they the fluid ◦ C. remains lower than 41 temperature of the fluid remains lower than 41 °C.
Figure 13. Cont.
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(f)
(g)
Figure 13. Thermal images of LCE-based composites with 0.5 wt% of iron-oxide nanoparticles when
Figure 13. Thermal images of LCE-based composites with 0.5 wt% of iron-oxide nanoparticles when subjected to an alternating magnetic field of 48.24 kA/m and 298-kHz oscillating frequency at (a) 0, subjected to an alternating magnetic field of 48.24 kA/m and 298-kHz oscillating frequency at (a) 0, (b) 27, (c) 159, and (d) 926 s. (e) Superposition of the temperature (unfilled symbols) and contraction (b) 27,(filled (c) 159, and (d) 926 s. (e) Superposition of the temperature (unfilled symbols) and contraction symbols) profiles of the LCE composites with 0.5 wt% (●) and 0.7 wt% (■) of iron-oxide (fillednanoparticles symbols) profiles LCE compositesexperiment. with 0.5 Crosses wt% ( in ) and 0.7 images wt% ( ) of iron-oxide obtainedof in the a magnetic-actuation thermal represent the nanoparticles in athe magnetic-actuation experiment. ininfrared thermalcamera’s images software. represent the reference obtained points where temperature data were collectedCrosses using the reference pointsdeformation where the (f) temperature data increments were collected the infrared camera’s Mechanical and temperature (g) of using LCE-based composites with 0.5 software. wt% of iron-oxide nanoparticles in temperature response to anincrements applied alternating magnetic field of 34.73 kA/m at 298 Mechanical deformation (f) and (g) of LCE-based composites with 0.5 wt% of kHz over five eight-minute on/off cycles submerged in water (■) and in afield model medium iron-oxide nanoparticles in response to an applied alternating magnetic ofcell-growth 34.73 kA/m at 298 kHz DMEM; ●). on/off Filled and unfilled symbols in in water the bottom figure to temperatures over (labeled five eight-minute cycles submerged () and in correspond a model cell-growth medium measured close to the surface of the elastomer and at the bottom of the reservoir, respectively. T0 (labeled DMEM; ). Filled and unfilled symbols in the bottom figure correspond to temperatures corresponds to the initial temperature at t = 0. Reproduced with permission from the authors of [65]. measured close to the surface of the elastomer and at the bottom of the reservoir, respectively. Copyright © 2018 Elsevier. T0 corresponds to the initial temperature at t = 0. Reproduced with permission from the authors of [65]. Copyright © 2018 Elsevier.in Reference [65] demonstrate that collagen treatment applied to the Additional results reported surface of LCE films, with and without magnetic nanoparticles, allowed for the effective attachment and proliferation of reported a special kind of fibroblast. study opensthat up collagen several new applications in theto the Additional results in Reference [65]This demonstrate treatment applied field of bio-medical devices, overcoming the limitations due to the inherent hydrophobicity of surface of LCE films, with and without magnetic nanoparticles, allowed for the effective attachment polysiloxane matrices. and proliferation of a special kind of fibroblast. This study opens up several new applications in The addition of gold nanoparticles to LCEs was investigated by several groups [66–68,80], with the field of bio-medical devices, overcoming the limitations due to the inherent hydrophobicity of the aim of producing photo-thermal effects related to the localized surface plasmon resonance of gold polysiloxane matrices. nanoparticles. Several works [66,67,80] proposed the doping of LCE micro-particles or LCE microThe addition of nanoparticles gold nanoparticles LCEsdeformations, was investigated byand/or several groups [66–68,80], pillars with gold to inducetoshape rotations, translations using with the aim of producing photo-thermal effects related to the localized surface plasmon resonance optical manipulation. Liu et al. [80] fabricated micro-sized polyacrylate-based LCEs doped with 1 of gold nanoparticles. Several(and works the doping LCEamicro-particles wt% of gold nanoparticles gold[66,67,80] nanorods),proposed and they were able to of induce maximum strainor of LCE micro-pillars with gold nanoparticles induce shape deformations, translations 30% of the initial LCE length using to a laser of 635-nm wavelength. Sunrotations, et al. [66] and/or used a different approach based onetaal. soft lithography technique to produce micrometer-sized usingsynthetic optical manipulation. Liu [80] fabricatedmolding micro-sized polyacrylate-based LCEs doped with 1 cylindrical LCE actuators. theand mechanism manipulation the shape strain of these wt% of gold nanoparticles (andThey golddiscussed nanorods), they wereofable to induce aofmaximum of 30% colloidal birefringent elastomeric micro-particles [66] using infrared laser beams and polarized-light of the initial LCE length using a laser of 635-nm wavelength. Sun et al. [66] used a different synthetic optical tweezers. In this work [66], a 1064-nm laser source was selected, since this wavelength is about approach based on a soft lithography molding technique to produce micrometer-sized cylindrical LCE double that of the maximum absorption wavelength of the pristine gold nanoparticles, thus allowing actuators. They discussed the mechanism of manipulation of the shape of these colloidal birefringent an efficient two-photon absorption process. Moreover, infrared light has several advantages with elastomeric micro-particles [66]it using infrared laser beams and polarized-light opticalAtweezers. respect to visible light, since can penetrate into the bulk of LCEs, reducing the scattering. special In this work [66], a 1064-nm laser source was selected, since this wavelength is about double that of profile of the local nematic director (n) was achieved by means of nonlinear optical polarizing the maximum absorption wavelength of theRaman pristine gold nanoparticles, thus allowing an The efficient microscopy, including coherent anti-Stokes scattering polarizing microscopy (Figure 14). two-photon Moreover, light has several advantages with respect to shrinkingabsorption of the LCE process. micro-particle’s lengthinfrared and an increase in its diameter are normally observed when heated, to the increase into in local across the nematic–isotropic phase transition. visible light, sincedue it can penetrate thetemperature bulk of LCEs, reducing the scattering. A special profile When the LCE micro-particles are doped with gold nanoparticles, the same effect can be observed of the local nematic director (n) was achieved by means of nonlinear optical polarizing microscopy, due tocoherent photo-thermal transfer of energy into heat. Interestingly, the shape variation theshrinking micro- of including anti-Stokes Raman scattering polarizing microscopy (Figure 14). of The cylinder quickly relaxes back to the original shape within about one second. As demonstrated by Sun the LCE micro-particle’s length and an increase in its diameter are normally observed when heated, et al. [66], optical manipulation of LCE micro-particles doped with gold nanoparticles can be achieved
due to the increase in local temperature across the nematic–isotropic phase transition. When the LCE micro-particles are doped with gold nanoparticles, the same effect can be observed due to photo-thermal transfer of energy into heat. Interestingly, the shape variation of the micro-cylinder quickly relaxes back to the original shape within about one second. As demonstrated by Sun et al. [66],
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optical manipulation LCE doped with gold nanoparticles can be 16achieved with Polymers 2018, 10, xof FOR PEER micro-particles REVIEW of 27 robust and reproducible spatial (translational and rotational) control, opening up an opportunity for with robust and reproducible spatial (translational and rotational) control, opening up an new technological applications. opportunity for new technological applications.
Figure 14. (a–f) Examples of reversible morphing of LCE micro-particles including gold nanoparticles,
Figure 14.shaped (a–f) Examples of reversible morphing of LCE micro-particles including gold nanoparticles, by means of unidirectional laser-beam scanning along the blue arrows. The inset in (e) shows shaped bya typical meanscoherent of unidirectional laser-beam along theimage blueofarrows. The inset in anti-Stokes Raman scattering scanning polarizing microscopy an LCE goldmicro-particle thatRaman was bent scattering using a scanning laser beam. (g) Scheme ofimage directorof an LCE (e) shows nanoparticle-doped a typical coherent anti-Stokes polarizing microscopy distributions in the LCE micro-particlethat during the bent bending induced by the laserlaser beam. beam. (h) Scheme gold-nanoparticle-doped micro-particle was using a scanning (g) Scheme showing the effect of local manipulation of LCE orientational ordering by a scanned laser beam along of director distributions in the LCE micro-particle during the bending induced by the laser beam. the direction marked by the blue arrow. The scanning causes nonreciprocal unidirectional motion of (h) Schemea molten showing the(red), effect manipulation LCE orientational scanned laser region andofa local modification of the LCEof polymeric network in theordering vicinity ofby thea“hot” scanning region (orange), leading to the modification the particle shape, beam along the direction markedeventually by the blue arrow. Thestable scanning causes of nonreciprocal unidirectional the (red), laser isand turned off. Reproduced permission from the authors in of the [66].vicinity of motion of persisting a moltenafter region a modification ofwith the LCE polymeric network Copyright © 2018 AIP Publishing. the “hot” scanning region (orange), eventually leading to the stable modification of the particle shape, persisting A after laserwork is turned off. with Reproduced with permission from authors [66]. Copyright verythe recent [81] deals anisotropy shape deformation of the swollen LCE of micro-pillars © 2018doped AIP Publishing. with gold nanoparticles, with the possibility of using either UV or IR light stimuli. Additional potential applications of gold-nanoparticle-doped LCEs range from contact-free actuation of colloidal micro-particles to micro-fluidics and opto-mechanics, as also supported by recent theoretical A very recent work deals with anisotropy shape deformation of swollen LCE micro-pillars investigations [82].[81] Very recent works on nanostructured LCE-based materials were published doped with goldtonanoparticles, with thenanoparticles possibility [83] of using either UV or IR [84] lightinto stimuli. aiming introduce copper-sulfide and silver nanoparticles the LCEAdditional matrix to induce mechanisms. LCEs range from contact-free actuation of colloidal potential applications ofphoto-actuation gold-nanoparticle-doped
micro-particles to micro-fluidics and opto-mechanics, as also supported by recent theoretical 4. LCE-Based Bilayer Composites investigations [82]. Very recent works on nanostructured LCE-based materials were published aiming Another field of research related to LCE composites concerns the preparation of bilayered or to introduce copper-sulfide nanoparticles [83] and silver nanoparticles [84] into the LCE matrix to multilayered systems obtained by coupling monodomain LCE films, mainly prepared according to induce photo-actuation mechanisms. the “two-step” cross-linking procedure [35], and one or two layers of conductive materials deposited onto the top surface of LCEs. These bilayered (or multilayered) systems were the object of several works in the past six years [85–95]. Several new technological applications were explored in detail based on unique phenomena characterizing the LCE-based Another field of research LCE composites preparation of bilayered or bilayer composites, such asrelated bending to actuation [85,87,89,91–95]concerns and surfacethe wrinkling [85,86,88,90,92– 94], as described in the sections below. multilayered systems obtained by coupling monodomain LCE films, mainly prepared according to experimental theoretical 4. LCE-Based Bilayerand Composites
the “two-step” cross-linking procedure [35], and one or two layers of conductive materials deposited 4.1. Preparation of Bilayered (or Multilayered) Systems onto the top surface of LCEs. These bilayered (or multilayered) systems were the object of several The first bilayer LCE-systems were produced by depositing conductive nanostructured thin experimental and theoretical works in the past six years [85–95]. Several new technological applications polymer layers on the top surface of standard polysiloxane-based LCE films (see the general schemes were explored in detail based on unique phenomena characterizing the LCE-based bilayer composites, such as bending actuation [85,87,89,91–95] and surface wrinkling [85,86,88,90,92–94], as described in the sections below. 4.1. Preparation of Bilayered (or Multilayered) Systems The first bilayer LCE-systems were produced by depositing conductive nanostructured thin polymer layers on the top surface of standard polysiloxane-based LCE films (see the general schemes
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reported in Figure 15). Agrawal et al. [90] used nanolayers of poly(styrene) (PS), while Domenici, 2018, 10, x FOR PEER REVIEW 17 of 27 Greco Polymers et al. [86–88] chose the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conductive polymer. reported in Figure 15). Agrawal et al. [90] used nanolayers of poly(styrene) (PS), while Domenici, Nanoscale thinchose filmsthe were prepared [90] by spin-casting a PS solution on a(PEDOT:PSS) silicone-based Greco et al. PS [86–88] poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) plate, conductive cleaned bypolymer. UV/ozone treatment, placed on the top surface of LCE films, before being put in water at a Nanoscale known temperature, off, and dried vacuum. PS layers of varying thickness were PS thin filmspealed were prepared [90] byinspin-casting a PS solution on a silicone-based plate,ranging cleaned from by UV/ozone on the top surface of LCEwas films, before being putscale in of prepared, 30 nm totreatment, 400 nm, placed while the LCE-film thickness typically on the water at a known temperature, pealed off, and dried in vacuum. PS layers of varying thickness were 0.3/0.4 mm [90]. ranging 30 nm to 400 nm,bilayers, while the LCE-film waswere typically on the scale[85–89]. of Inprepared, the case of from PEDOT:PSS/LCE several thickness strategies optimized 0.3/0.4 mm [90]. The PEDOT:PSS material was chosen for its excellent properties of conductivity, and mechanical In the case of PEDOT:PSS/LCE bilayers, several strategies were optimized [85–89]. The and chemical stability, and for the possibility of fabricating nanolayers with very high control PEDOT:PSS material was chosen for its excellent properties of conductivity, and mechanical and and reproducibility [96,97]. The exposed to air chemical stability, and for themonodomain possibility of polysiloxane-based fabricating nanolayersLCE withfilms very were high control and plasma to improve the wettability of their surface by aqueous solutions. A filtered dispersion reproducibility [96,97]. The monodomain polysiloxane-based LCE films were exposed to air plasma of PEDOT:PSS in water was deposited with a by micropipette over the LCE films. Homogeneous layers to improve the wettability of their surface aqueous solutions. A filtered dispersion of PEDOT:PSS in water was with a micropipette over theusing LCE films. Homogeneous layers of PEDOT:PSS of PEDOT:PSS of deposited varying thickness were prepared a spin-coating deposition, and were then thickness were prepared using a spin-coating deposition, and were then at room of dried of at varying room temperature [88]. An improved approach [97], used to enhance thedried conductivity temperature [88]. An improved approach [97], used to enhance the conductivity of the PEDOT:PSS the PEDOT:PSS nanolayers, was adopted via the introduction of dimethylsulfoxide (DMSO) in the nanolayers, was adopted via the introduction LCE-based of dimethylsulfoxide (DMSO) in thewere preparation preparation procedure. To fabricate electro-active bilayers, the LCE films exposed to procedure. To fabricate electro-active LCE-based bilayers, the LCE films were exposed to plasma, as plasma, as described above, and thin copper wires were placed on the top surface to provide a good described above, and thin copper wires were placed on the top surface to provide a good electrical electrical contact for applying voltage to the actuator. Deposition of the DMSO-doped PEDOT:PSS contact for applying voltage to the actuator. Deposition of the DMSO-doped PEDOT:PSS nanolayer nanolayer was done by drop-casting of the solution LCEand stripes and the wires, resulting was done by drop-casting of the solution onto theonto LCEthe stripes the wires, resulting in a good in a good adhesion and electrical contact between the wire and the conductive layer. In all cases adhesion and electrical contact between the wire and the conductive layer. In all cases [85–89],[85–89], the the PEDOT:PSS depositedonto ontothe the surface of the at room temperature, i.e., in PEDOT:PSSmaterial material was was deposited surface of the LCELCE filmsfilms at room temperature, i.e., in the nematicphase phase(Figure (Figure 15b). 15b). the nematic Zupancic et al. [92] reported the preparation of bilayers (and multilayers) of LCE films Zupancic et al. [92] reported the preparation of bilayers (and multilayers) of LCE films reprocessed reprocessed gold with conductive gold nanolayers. Theofdeposition of gold nanolayers using sputtering with conductive nanolayers. The deposition gold nanolayers using sputtering technology technologyafter was performed after creation oflayer an adhesion layer ofon chromium the LCEThe surface. was performed the creation of the an adhesion of chromium the LCEon surface. average The average thickness of the gold layer, as determined from SEM investigations, was on the scale of thickness of the gold layer, as determined from SEM investigations, was on the scale of ~100 nm, while ~100 nm, while the Cr adhesion layer was about a few nanometers thick. Single-sided samples, with the Cr adhesion layer was about a few nanometers thick. Single-sided samples, with a layer deposited a layer deposited on one side of the LCE film, and double-sided samples, with layers deposited on on oneboth sidesides of the LCE film, and double-sided samples, with layers of an LCE film, were prepared using this procedure [92].deposited on both sides of an LCE film, were prepared using this procedure [92].
15.Scheme (a) Scheme theLCE-based LCE-based bilayer made usingusing monodomain side-chain LCE FigureFigure 15. (a) of ofthe bilayercomposites composites made monodomain side-chain films and a nanostructured conductive layer [92,93]. Different temperature-induced bending LCE films and a nanostructured conductive layer [92,93]. Different temperature-induced bending geometries of the LCE-based bilayers depending on the temperature of deposition of the conductive geometries of the LCE-based bilayers depending on the temperature of deposition of the conductive layer: (b) T TN-PN (T = 80 °C), LCE layer: (b) T TN-PN (T = 80 ◦ C), in the paranematic or isotropic (disordered) phase. LCE in the paranematic or isotropic (disordered) phase.
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As reported in several works, whether experimental and theoretical [88–94], the final physical properties of the LCE-composite bilayers depend on two parameters: (1) the ratio between the nanolayer thickness (dnl ) and the LCE thickness (dLCE ), and (2) the temperature of the conductive nanolayer deposition. The ratio (dnl )/(dLCE ) is a crucial parameter, since, as discussed in the next section, two different physical behaviors can be observed: bending actuation or surface wrinkling. Polymers 2018, 10, x FOR PEER REVIEW 18 of 27 The temperature of deposition is also an important parameter. If the layer is deposited on the As reported in below several works, whether experimental and theoretical [88–94], physical (ordered) LCE film at temperatures the isotropic–nematic transition, namely in the thefinal elongated properties of the LCE-composite bilayers depend on two parameters: (1) the ratio between the state, or above the isotropic–nematic transition, namely in the contracted (disordered) state, different nanolayer thickness (dnl) and the LCE thickness (dLCE), and (2) the temperature of the conductive geometries are observed. InThe Figure for a different bending geometry nanolayer deposition. ratio 15b,c, (dnl)/(dLCE ) isinstance, a crucial parameter, since, as discussed in the of nextthe bilayer systems issection, obtained depending on the temperature of the nanolayer deposition. two different physical behaviors can be observed: bending actuation or surface wrinkling. The temperature of deposition is also an important parameter. If the layer is deposited on the LCE film at temperatures below the isotropic–nematic Phenomena transition, namely in the elongated (ordered) state, 4.2. Bending Actuation and Surface Micro-Wrinkling
or above the isotropic–nematic transition, namely in the contracted (disordered) state, different
The bending systems was the objectbending of several investigations geometries actuation are observed.ofInbilayer Figures 15b,c, for instance, a different geometry of the bilayer [98–103]; is obtained of depending the temperature the nanolayer deposition. however, systems the fabrication robuston actuators, withofgood electrical contacts, and reproducible and durable systems after thousands of on/off cycles still represents an unresolved issue. One of the main 4.2. Bending Actuation and Surface Micro-Wrinkling Phenomena problems is related to the compatibility (chemical and physical) between the two layers of different The bending actuation of bilayer systems was the object of several investigations [98–103]; materials and their intrinsically different stiffness. As demonstrated by Greco et al. [88,89], excellent however, the fabrication of robust actuators, with good electrical contacts, and reproducible and results were obtained the case ofofLCE/PEDOT:PSS bilayers. PEDOT:PSS indeed durable systemsin after thousands on/off cycles still represents an unresolved issue. One of the has maina Young’s problems is related to the compatibility (chemical andthe physical) betweenThis the two layersisofadifferent modulus and Poisson’s ratio very similar to those of LCE films. aspect great advantage materials and their intrinsically different stiffness. As demonstrated by Greco et al. excellent with respect to other bilayered systems [91,92], since PEDOT:PSS acts as an[88,89], incompressible “skin” results were obtained in the case of LCE/PEDOT:PSS bilayers. PEDOT:PSS indeed has a Young’s during the temperature-induced actuation, inducing the elongation/contraction of the LCE films. modulus and Poisson’s ratio very similar to those of the LCE films. This aspect is a great advantage The perfect adhesion the bilayered PEDOT:PSS nanolayer to the surfaceacts of the results in“skin” the bending of with respect toofother systems [91,92], since PEDOT:PSS as anLCE incompressible the temperature-induced inducing the of the LCE films. The the wholeduring bilayered structure when actuation, the temperature iselongation/contraction varied across the nematic-to-isotropic phase adhesion of the the PEDOT:PSS nanolayer actuation to the surface theLCE/PEDOT:PSS LCE results in the bending of the transition.perfect In Figure 16a, thermal-bending ofofan bilayer is reported. whole bilayered structure when the temperature is varied across the nematic-to-isotropic phase This particular sample was prepared by depositing the PEDOT:PSS layer at T = 25 ◦ C (T
