Shear mechanical anisotropy of side chain liquid-crystal elastomers: Influence of sample preparation

Abstract

We study the mechanical anisotropy of a series of uniaxial side chain nematic elastomers prepared with the same chemical composition but with different preparation protocols. For all the compounds, the experiments performed as a function of temperature show no discontinuity in both G' (//) and G' ( perpendicular) (the labels // and perpendicular stand for the director parallel, respectively perpendicular to the shear displacement) around the nematic-isotropic (N-I) phase transition temperature determined by DSC. They also all show a small decrease in G' (//) starting at temperatures well above this temperature (from approximately 4( degrees ) C to approximately 20( degrees ) C depending on the compound studied) and leading to a small hydrodynamic value of the G' ( perpendicular)/G' (//) ratio. The measurements taken as a function of frequency show that the second plateau in G' (//) and the associated dip in G (//)" expected from dynamic semi-soft elasticity are not observed. These results can be described by the de Gennes model, which predicts small elastic anisotropy in the hydrodynamic and linear regimes. They correspond to the behavior expected for compounds beyond the mechanical critical point, which is consistent with the NMR and specific heat measurements taken on similar compounds. We also show that a reduction in the cross-linking density does not change the non-soft character of the mechanical response. From the measurements taken as a function of frequency at several temperatures we deduce that the time-temperature superposition method does not apply. From these measurements, we also determine the temperature dependence of the longest relaxation time tau(E) of the network for the situations where the director is either parallel or perpendicular to the shear velocity. Finally, we discuss the influence on the measurements of the mechanical constraint associated with the fact that the samples cannot change their shape around the pseudo phase transition, because of their strong adherence on the sample-bearing glass slides.

Shear Mechanical Properties of Main Chain Liquid Crystalline Elastomers

“…To answer these questions, we have recently studied the thermal behavior and the frequency dependence of the complex shear modulus G of various side chain liquid crystalline elastomers (SCLCE). [3,4] Our results show that SCLCEs are uniaxial rubbers with a small mechanical anisotropy in the hydrodynamic regime and a Rouse-like frequency dependence in the viscoelastic regime. We have also shown that no separation between the time scales of the director and the network occurs and that the time-temperature superposition method does not apply.…”

“…The complex shear modulus G ¼ G 0 þ iG 00 was measured as a function of frequency using the piezorheometer used previously for studying the rheological properties of elastomers [3,4,[8][9][10][11][12] and polymers. [13,14] The principle of this apparatus consists in applying a small strain e to the film by means of a piezoelectric ceramic vibrating in the shear mode, and measuring the amplitude and the phase f of the shear stress s transmitted through the film using a second piezoelectric ceramic.…”

“…We have also shown that no separation between the time scales of the director and the network occurs and that the time-temperature superposition method does not apply. [4] In this paper we report on a similar study of the complex shear modulus G carried out for the first time on main chain liquid crystalline elastomers (MCLCE). In MCLCE, the mesogens form the core of the polymeric chains.…”

Semisoft Nematic Elastomers and Nematics in Crossed Electric and Magnetic Fields

“…There is now strong evidence [11,12] that samples prepared with the KF technique are supercritical. In addition, C 5 measured in linearized rheological experiments is not particularly small [12].…”

“…Thus the phase structure of the Z plane is replicated in the X plane (h 0) and the Y plane ( xx h) with Before considering the xx -u xx stress-strain curve, it is useful to look more closely at elastic response in the vicinity of the Z plane and the nature of order in the Y plane. Throughout the h > 0 Z plane, the equilibrium state is prolate uniaxial with order parameter S S 0 , and thus strains u [12,19]. C 3 and C 5 are easily calculated as a function of r and h. In reduced units, the ordered pair (C 3 ,C 5 ) takes on the value ( ) in the supercritical regime at r;h r c ; 2h c .…”