Mechanical Properties of New Main-Chain Liquid-Crystalline Elastomers

Mechanical Properties of New Main-Chain Liquid-Crystalline Elastomers
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  Author's personal copy Thermal and mechanical properties of new Main-ChainLiquid-Crystalline Elastomers Antoni Sa´nchez-Ferrer a , b , * , Heino Finkelmann b a Institute of Supramolecular Science and Engineering, University Louis Pasteur, 8, alle´e Gaspard Monge, 67083 Strasbourg, France b Institute for Macromolecular Chemistry, Albert Ludwigs University, Stefan-Meier-Str. 31, 79104 Freiburg, Germany a r t i c l e i n f o  Article history: Received 22 November 2008Received in revised form19 April 2009Accepted 10 January 2010Available online 18 January 2010 Keywords: Liquid-crystalline elastomersUniaxial stress–strain deformationsThermoelastic experiments a b s t r a c t New Main-Chain Liquid-Crystalline Elastomers (MCLCEs) were synthesized based on reacting vinyloxy-terminated mesogens under hydrosilylation conditions with a flexible crosslinker. These main-chainsystems showed smectic and nematic mesophases and their anisotropic properties were mechanically andthermally analysed as function of the crosslinking density. Due to the suitable chemistry used in this worklow crosslinking densities have been achieved (2.5 mol-%) with low soluble content (5%). For the first time,the degree of crosslinking could be adjusted and nematic or smectic MCLCEs with tuneable thermal andmechanical properties were obtained.   2010 Elsevier Masson SAS. All rights reserved. 1. Introduction In the early 1980s, Aguilera and Ringsdorf  [1] succeeded in syn-thesising Main-Chain Liquid-Crystalline Polymers (MCLCPs) basedon three aromatic rings mesogenic units and oligosiloxanes as chainextenders. This kind of chemistry reduced the glass transition ( T  g )and clearing temperatures ( T  c ) of those MCLCPs due to the flexibilityof the siloxane spacer. Years later, Donnio et al. [2] obtained the firstMain-Chain Liquid-Crystalline Elastomers (MCLCEs) by crosslinkingthe liquid-crystalline polymer chains with a flexible siloxane-basedcrosslinker. In both studies, low degrees of polymerisation wereobtained, from 6 to 9 repeating units, asit could be noticed from themolecularweightsofthepolymersandfromthehighpercentagesof soluble content and crosslinker used in the networks.MCLCEs are of interest due to the coupling of the polymerbackbone to the director [3,4]; the monodomains of these systemshave huge thermal expansion, up to 300% [5,6], with respect to theSide-Chain Liquid-Crystalline Elastomers (SCLCEs). This impressiveproperty is of high interest for the scientific community because of the potential uses as actuators [7,8] or artificial muscles [9–11]. In the process, external stimuli are needed, like thermal gradients,magnetic or electrostatic fields, or light. Several approaches havebeen taken for the integration of conductive particles in order toincrease their conductivity [12] or photo-isomerisable dies thatdestroy the liquid-crystalline phase upon change their shape from trans - to  cis -isomer [13,14].Main-chain elastomers are much more difficult to fabricate andwork with than side-chain elastomers. A key point is the solublecontent of the material during the polycondensation reaction,possibly determined by side reactions in the polymerisation andimpurities that did not allow obtaining well-definednetworks[2,5,6,15] and directlyaffected the mechanical and thermalproperties of these materials.We present the synthesis of well-defined monodomain samplesofMCLCEsobtainedbyhydrosilylationofnewmesogens.Inordertosynthesize networks, the solution was found in the use of vinyloxycarbon chains. The use of vinyloxy alkyl chains allowed the forma-tion of networks where the soluble content was minimised withrespect to the common vinyl ones. The mechanical properties andthethermalactuationoftheseelastomersarepresentedandstudiedin detail in order to understand better these fantastic materials. 2. Results and discussion  2.1. Synthesis of MCLCEs In the synthesis of all MCLCEs, a flexible crosslinker with 5reacting groups was used for preparing samples with 10, 5 and2.5 mol-%ofcrosslinker.Then,forsampleswith10,5and2.5 mol-%,the number of mesogens on average in the polymer chains ( DP  ) *  Corresponding author. Institute of Supramolecular Science and Engineering,University Louis Pasteur, 8, alle´e Gaspard Monge, 67083 Strasbourg, France. E-mail addresses: (A. Sa´nchez-Ferrer), heino. (H. Finkelmann). Contents lists available at ScienceDirect Solid State Sciences journal homepage: 1293-2558/$ – see front matter    2010 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.solidstatesciences.2010.01.017 Solid State Sciences 12 (2010) 1849–1852  Author's personal copy would be 4, 8 and 16 respectively. From the experience in thesynthesis of MCLCPs, the degree of polymerisation for the linearpolymers is higher than for the networks. Thus, well-definedelastomerswereobtainableusingthesecrosslinkerquantities,eventaking possible defects on building the network into consideration.The procedure in the synthesis of MCLCEs was the same as forSCLCEs, but with longerreaction times. Mixtures of mesogen, chainextender (TMDSO), crosslinker (PMPOPS) and azobenzene co-monomer (Azo) – for photoactive elastomers – were prepared andpartially crosslinked, by adding some tiophene free toluene andsome 1%-Pt cyclooctadiene platinum (II) chloride (Pt(COD)Cl 2 ) indichloromethane (30  m L per 0.4 mmol of mesogens). The mixturewas placed in the spinning Teflon-cell form and heated at 60   C for4 h at 5000 rpm. The swollen elastomer strip was removed fromthe spin-casting cell and aligned by applying a strong uniaxialstress and then elongated to uniformly align with the director.Then, the unswollen sample was completely cured to fix themonodomain orientation in the case of nematic elastomers and theconical distribution of domains in the case of smectic-C elastomers.In Scheme 1, the molecules involved in the synthesis of the fourMCLCEs are shown.Thefinalcompositionsofthefourelastomerswerethefollowing:SmC-MCE-10 ( a ¼ 11.8 mol-%;  b ¼ 88.2 mol-%;  d ¼ 100 mol-%), SmC-MCE-2.5 ( a ¼ 2.6 mol-%;  b ¼ 97.4 mol-%;  c  ¼ 5 mol-%;  d ¼ 95mol-%),N-MCE-10 ( a ¼ 11.8mol-%;  b ¼ 88.2mol-%;  c  ¼ 5 mol-%;  d ¼ 95mol-%), and N-MCE-2.5 ( a ¼ 2.6 mol-%;  b ¼ 97.4mol-%;  c  ¼ 5 mol-%; e ¼ 95mol-%); where the values for  a  and  b  are the mol-% related tothetotalamountofsilaneunits,and c  , d and e arethemol-%relatedtothe total amount of mesogenic units.  2.2. Thermal properties of MCLCEs MCLCEs have larger uniaxial thermal responses (up to 300%from literature) [5] than side-chain systems (Fig.1). Thermoelastic experiments were done for correlating the crosslinking density of the samples with the uniaxial thermal expansion ( L / L ISO ) and thesoluble content. Prolate monodomain samples of MCLCEs withvinyloxy chains showed both mesophases: smectic and nematic(Fig.2), dependingonthe chemistryof themesogen. On decreasingthe quantity of crosslinker, the clearing temperature of the smecticelastomers decreased, while the opposite took place for thenematic elastomers (Table 1).All mesophases were identified by XRD (Fig. 2 insets), wheresmectic samples had strong reflexes in the SAXS region with corre-lationlengthsof  x ¼ 30nmasanevidenceofthesmecticlayering.Onthe contrary, nematic samples had very weak reflexes with lowcorrelation lengths  x ¼ 5 nm (cybotactic). The order parameter ( S  ) of all samples was calculated from the azimuthal intensity distributionof the WAXS region [16]. DSC measurements showed latent heats of  D H  SI ¼ 8 Jg  1 for smectic elastomers and  D H  NI ¼ 2 Jg  1 for thenematic ones, an indication of high order for smectic systems.The uniaxial thermal expansion was indirectly proportional tothe crosslinking density: more crosslinker, less expansion. Nematicsamples with the same percentage of crosslinker expanded lessthan the corresponding smectics due totheirlowerorder( S  ¼ 0.73)as compared to the smectic layering order ( S  ¼ 0.83) where thepolymer chains are more stretched.  2.3. Mechanical properties of MCLCEs To answer the very important questions whether MCLCEs differfrom SCLCEs in their mechanical properties, and how the cross-linking density influences the liquid-crystalline phase, stress–strainexperiments on nematic and smectic MCLCEs were performed onsamples with different crosslinking densities as basis for the corre-lation of opto-mechanical and opto-thermoelastic experiments.In a recent communication [17], the stress–strain non-linearbehaviour of nematic MCLCEs using benzophenone moieties ascrosslinkers was reported. A single exponential function witha linear correction term was assumed. For our samples, an expo-nential growth is introduced in order to analyse the pre-stressregion in smectic elastomers and give a general function for bothkind of samples, nematic or smectic. Thus, the equation that SiOOSiSiOSiOSiOHHHHH a ) PMPOPSSiOSiHH b )TMDSO c ) AzoONNO d ) main-chain mesogen 1R = (CH 2 ) 4 OOOOORR e ) main-chain mesogen 2OOOOOORRR = (CH 2 ) 4 O Scheme 1.  Chemical structures of the flexible crosslinker, chain extender, azobenzenedye, and main-chain monomers for the obtaining of MCLCEs. Fig. 1.  Change in length of a main-chain liquid-crystalline elastomer: from the liquid-crystalline phase to the isotropic phase.  A. Sa´nchez-Ferrer, H. Finkelmann / Solid State Sciences 12 (2010) 1849–1852 1850  Author's personal copy describestheincreaseoftruestressasfunctionoftheappliedstrainis given by s ð l Þ ¼  s lin  þ  s exp  þ  s pre ¼  a ð l   1 Þ þ  b  e c  ð l  1 Þ  1  þ  d  1  e   f  ð l  1 Þ   (1) The fitting parameter ‘‘ a ’’ corresponds to the linear behaviour of Young’s modulus, ‘‘ b ’’ and ‘‘ c  ’’ are the fitting parameters of theexponential growth, and ‘‘ d ’’ and ‘‘  f  ’’ are the fitting parameters of the pre-stress region that can be applied for smectic elastomers.The Young’s modulus is calculated from the derivative of Eq. (1)giving the following equation: E  ð l Þ ¼  v s t v l  ¼  E  lin  þ  E  exp  þ  E  pre ¼  a  þ  b $ c  $ e c  ð l  1 Þ þ  d $  f  $ e  f  ð l  1 Þ (2) Nematic MCLCEs (N-MCE-10 and N-MCE-2.5) showed highermechanical responses than side-chain systems. For example,SCLCEs had polymer chain lengths of 30 Å (8 repeating units) for10 mol-% of crosslinker. Their corresponding Young’s moduli werein the range of 0.6 MPa at  T  red ¼ 0.92 in these systems. MCLCEs hadpolymer chain lengths of 137 Å (4 repeating units) for 10 mol-% of crosslinker (N-MCE-10) and 632 Å (16 repeating units) for 2.5 mol-% of crosslinking density (N-MCE-2.5). Their corresponding initialYoung’s moduli were  E  0 ¼ 2.6 MPa and 0.3 MPa, respectively, at T  red ¼ 0.92 (Fig. 3). Thus, MCLCEs had a clear different mechanicalbehaviour due to their chemical polymer chain constitution withrespect to SCLCEs.Smectic elastomers presented a difference in their mechanicalproperties with respect to the nematics, where all them had a pre-stresstransformationregion.Thesesamples–10 mol-%(SmC-MCE-10) and 2.5 mol-% (SmC-MCE-2.5) of crosslinker – showed initialYoung’s moduli of   E  0 ¼ 11.2 MPa and 17.4 MPa, respectively, at T  red ¼ 0.92 (Fig. 4), and only the samples with 10 mol-% of cross-linker presented a linear behaviour.TheexplanationgivenbyKrauseandFinkelmann[17]canbealsoappliedinthiscase.Themaindifferencewithrespecttotheirsystemis that the crosslinking process takes place at the same time as thepolymerisation. Thus, the polymer chain distribution is narrowerthan the distribution of chains between the crosslinking pointsunder photo-irradiation conditions. The final result is also a hard-ening of the sample: less than the UV-photo-crosslinked systems,but more than the hairpins model by Adams and Warner [18].Smectic samples presented almost the same behaviour but withand extra factor: the pre-stress transformation region. The fact thatsmectic samples showed this behaviour can be explained by anextra energetic factor due to the smectic layering.In nematic elastomers, the Young’s modulus is proportional tothe degree of crosslinking due to the entropy elasticity alreadydescribed by Warner, Terentjev and Lubensky [19–21]. For smecticsamples, the elasticity of the systems is inversely proportional totheircrosslinking density, wherethe enthalpyelasticity factorfrom 0.951.       L    /       L    O   S   I T  red  SmC-MCE-10 SmC-MCE-2.5 N-MCE-10 N-MCE-2.5 NSmC Fig. 2.  Uniaxial thermal expansion ( L / L ISO ) as function of reduced temperature( T  red ¼ T  / T  c ) for the four MCLCEs. The insets are the two XRD patters for bothmesophases.  Table 1 Clearing temperature ( T  c ), thermal expansion differences ( L / L ISO ) at the reducedtemperature  T  red  ¼ 0.92, order parameter ( S  ), and soluble content ( sc  ) of the fourelastomers.Samples  T  C  (  C)  L / L ISO  (0.92)  S sc   (%)SmC-MCE-10 53 (SI) 2.02 0.83 5.0SmC-MCE-2.5 47 (SI) 3.07 0.78 5.7N-MCE-10 48 (NI) 1.78 0.73 3.8N-MCE-2.5 55 (NI) 2.02 0.73 4.9  E  0  = 0.3 MPa N-MCE-10N-MCE-2.5     (   M   P  a   )  E  0  = 2.6 MPa Fig. 3.  Uniaxial stress–strain curves for the samples N-MCE-10 and N-MCE-2.5 at T  red ¼ 0.92 (nematic phase).  E  0  = 17.4 MPa SmC-MCE-10 SmC-MCE-2.5     (   M   P  a   )  E  0  = 11.2 MPa Fig. 4.  Uniaxial stress–strain curves for the samples SmC-MCE-10 and SmC-MCE-2.5 at T  red ¼ 0.92 (smectic-C phase).  A. Sa´nchez-Ferrer, H. Finkelmann / Solid State Sciences 12 (2010) 1849–1852  1851  Author's personal copy the smectic layers plays an important role [22–24]. This can beexplained by the fact that at high concentration of crosslinker thesmectic layers get disturbed because the polymer chains betweencrosslinking points are smaller and have less mobility for theobtaining of smectic phases: as a consequence the enthalpydiminishes. Thus, the mechanical response of highly crosslinkedsmectic elastomers was less than that of the low crosslinked ones.In principle, nematic mesogens would give nematic elastomericsystems and smectic mesogens would result into their corre-sponding smectic ones. The presence of 5 mol-% of azo-compoundas a doping molecule was only affecting highly crosslinked smecticelastomers, where one could see this effect on passing from thesmectic-C sample SmC-MCE-10 to the cybotactic nematic N-MCE-10. It is known that mixtures of mesogens with different numberof carbons in the chains induce disorder in the systems, and nematicphasescan be obtained bymixing mesogens thatgive crystalline orsmectic elastomers when their homopolymer networks aresynthesized [25]. Thus, 5 mol-% of the azobenzene derivative didnot influence the phase behaviour at low crosslinking density butnematic phase was induced at higher crosslinker concentration.Inanewcommunication [26],thepropertiesof smectic-Cmain-chain elastomers under uniaxial and shear deformations are dis-cussed in detail because of the new physics involved in theseprocesses. 3. Conclusions New Main-Chain Liquid-Crystalline Elastomers (MCLCEs) weresynthesized from vinyloxy-terminated monomers using the spin-casting technique. This new chemistry avoids side reactions andallows obtaining well-defined networks with low soluble contentcompared to the previous systems with olefinic mesogens.Thermoelastic experiments confirmed that all samples studied(MCLCEs) presented a prolate conformation of the backbone poly-mer chains, which induced a strong anisotropy along the stretcheddirection parallel to the director. All elastomers expanded oncooling and contracted on heating, because of the direct pro-portionality between the uniaxial thermal expansion and the orderparameter of the sample.In the case of MCLCEs the thermal expansion differences wentfrom 80% to 210%. Two types of elastomers were studied: smecticandnematic.Smecticelastomershadadiscontinuityintheclearingtemperature, while a continuous behaviour of the change in lengthwasobservedfornematicelastomers.Anotherfactnoticedwasthatthe presence of flexible crosslinker increases the clearing temper-ature in main-chain systems.Uniaxial stress–strain deformations weredone in order tostudymechanicalpropertiesontheseLCEs.Anexponentialbehaviourwasobserved for nematic elastomers and a pre-stress transformationregionwasanalysedforthesmecticones.Stress–strainexperimentson nematic elastomers were correlated with the chemical consti-tution of the samples, showing a direct proportionally relationbetweenthedegreeofcrosslinkingandtheelasticmodulus,andtheexact opposite for the smectic elastomers.Most importantly, the mechanical properties (elastic modulus)and the spontaneous contraction (uniaxial thermal expansion) cannow be tuned/trigged as function of the chemistry of the mesogenand crosslinking density. 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