Nematic elastomers beyond the critical point

Disch, Stefan; Schmidt, Claudia; Finkelmann, Heino

doi:10.1002/marc.1994.030150402

Cited by 83 publications

(67 citation statements)

References 14 publications

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“…The difference in the heat capacity at the glass transition is ∆C p = 0.36 J/(g K) (±0.05 J/(g K)). The phase transformation of LSCEs is frequently super-critical due to internal stresses associated with the creation of monodomains by two-stage linking, as demonstrated by Disch et al for a similar elastomer [18]. Here, an apparent latent enthalpy at a nematic-to-paranematic (high temperature) phase transformation is found and is ∆H = 2.1 J/g (±0.5).…”

Section: Resultsmentioning

confidence: 71%

The elastic anisotropy of nematic elastomers

Finkelmann

Greve

Warner

2001

The European Physical Journal E

137

134

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We examine the robustness of order in nematic elastomers under mechanical strains imposed along and perpendicularly to the director when director rotation is prohibited. In contrast to electric and magnetic fields applied to conventional nematics, mechanical fields are shown theoretically and experimentally to greatly affect the degree of nematic order and related quantities. Unlike in liquid nematics, one can impose fields perpendicular to the director, thereby inducing biaxial order which should be susceptible to experimental detection. Nematic elastomers with unchanging director and degree of order should theoretically have the same elastic moduli for longitudinal and transverse extensions. This is violated when nematic order is permitted to relax in response to strains. Near the transition we predict the longitudinal modulus to be smaller than the transverse modulus; at lower temperatures the converse is true, with a crossover a few degrees below the transition. The differences are ascribed to the different temperature dependence of the stiffness of uniaxial and biaxial order. We synthesised side chain single-crystal nematic polymer networks, performed DSC, X-ray, birefringence, and thermo-mechanical characterisations, and then obtained linear moduli from stress-strain measurements. PACS. 61.30.-v Liquid crystals -82.70.-y Disperse systems; complex fluids -81.40.Jj Elasticity and anelasticity, stress-strain relations.

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Section: Resultsmentioning

confidence: 71%

The elastic anisotropy of nematic elastomers

Finkelmann

Greve

Warner

2001

The European Physical Journal E

137

134

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“…Such an effect is expected on theoretical grounds in a weak first-order transition system in an external field -however, in ordinary liquid crystals, electric and magnetic fields are usually too small to create a noticeable shift. On the other hand, there is no trace of supercritical behaviour or paranematics phase above T ni [27]: the transition remains abrupt and well defined; the small increase of λ above T ni is purely due to the rubberelastic response to the applied load.…”

Section: Thermo-mechanical Actuationmentioning

confidence: 91%

Spontaneous thermal expansion of nematic elastomers

Tajbakhsh

Terentjev

2001

The European Physical Journal E

245

228

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We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium thermal expansion on cooling from isotropic phase. At higher concentration of main chains their behaviour dominates the elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures, one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber, r(T) proportional to the effective nematic order parameter Q(T), is given by the average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators are discussed in the end

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“…The phase transformation seems to be supercritical. A supercritical phase transformation for a similar elastomer, proven by nuclear magnetic resonance (NMR) experiments, is reported by Disch et al [15] But also differences are found concerning the broadness of the phase transformation and the absolute value of the birefringence in the nematic phase depending on the different types and concentrations of the crosslinkers of the elastomers. To characterise the phase transformation, its broadness is derived in the following way.…”

Section: Nematic-to-isotropic Phase Transformation Behaviourmentioning

confidence: 92%