Shear Mechanical Properties of Main Chain Liquid Crystalline Elastomers

Abstract

Summary: The behavior of the complex shear modulus G = G′ + iG″ of a main chain liquid crystalline elastomer film oriented with the director in the plane of the film is investigated for the first time by shearing the film in a direction perpendicular or parallel to the director. The film exhibits a small mechanical anisotropy around the N‐I transition, which disappears slightly above TNI. A hydrodynamic softening of both G′⟂ and G′∥ (symbols ∥ and ⟂ refer to the direction of the director parallel, respectively perpendicular, to the shear displacement) is observed in the isotropic phase near TNI which is ascribed to the reorientation of the SmC domains revealed by X‐rays. These SmC domains are frozen‐in the temperature range investigated. They relax with a characteristic time which is deduced from the frequency dependence of the mechanical response of the film. It is shown that the time‐temperature superposition method does not apply and that there is no indication in favor of soft or semi‐soft elasticity. The influence of a static compression induced either mechanically on the film or by the shape change of the film as a function of temperature is also investigated. Data on the polydomain analogue complement this study.Temperature dependence of G′∥ and G′⟂ at 0.2 Hz for the monodomain film.imageTemperature dependence of G′∥ and G′⟂ at 0.2 Hz for the monodomain film.

Main Chain Liquid-Crystalline Elastomers: Swelling Dynamics and Electromechanical Effects

“…In order to remove any soluble part, the sample was extracted with toluene. The glass transition T g is about À10 C and the clearing temperature T c is about 64 C. The monodomain MCLCE was obtained by mechanical stretching in order to obtain the uniform director orientation, n, parallel to the stretching direction as described in [11][12][13][14].…”

“…The polydomain MCLCE was synthesized in a one-step reaction [11][12][13][14], by dissolving the monomer 2-ethyl-1,4-phenylen bis [4-[4-(vinyloxi)buboxy] benzoate] (C 34 H 38 O 6 ), the chain extender 1,1,3,3,-tetramethyldisiloxane (C 4 H 14 OSi 2 ) and 2.5 mol-% of the cross-linking agent pentamethylcyclopentasiloxane (C 5 H 20 O 5 Si 5 ) in toluene, adding a Pt catalyst and reacting in a centrifuge cell at 5000 rpm and 70 C for about 4.5 h to form a film (Fig. 1).…”

“…Most of the experimental works, so far, have focused on SCLCEs because MCLCEs are usually more difficult to synthesize than SCLCEs [11][12][13][14]. We have also studied the electromechanical and electrooptical effects of polydomain MCLCE which shows much larger effects [15].…”

Influence of Swelling on the Shear Mechanical Properties of Monodomain Side‐Chain Liquid‐Crystal Elastomers: Gaussian Versus Non‐Gaussian Elasticity

“…The complex shear modulus G = G ′ + i G ″ was measured as a function of frequency with the piezorheometer used previously for studying the rheological properties of elastomers,13, 14, 17–22 polymers,23–26 uniaxial magnetic gels,27 suspension of magnetic particles,28 and polyelectrolyte films 29. This apparatus is a plate–plate rheometer that uses piezoelectric ceramics vibrating in the shear mode to apply a small strain ϵ to the sample and to measure the amplitude and the phase ϕ of the shear stress σ transmitted through the sample.…”

State of Order of the Crosslinker in Main‐Chain Liquid Crystalline Elastomers

“…[1,3,4] With regard to a future applicability of nematic elastomers in devices, much of the work during the last decade has focused on the development of systems whose clearing temperatures are close to room temperature. [5][6][7][8] However, a thorough understanding of the influence of the crosslinker topology on the liquid crystalline order and the mechanics of MCLCEs is vital for designing devices as well. In side-chain systems, rigid, mesogen-like crosslinkers were found to increase the moduli and the phase transformation temperatures.…”