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

Abstract

The shear mechanical properties of nematic side-chain liquid-crystal elastomers swollen with a nematic solvent are studied. The results show that the elasticity of the network is Gaussian for elastomers oriented before crosslinking and non-Gaussian for the usual twicecrosslinked elastomers oriented by stretching the network formed after the fi rst crosslinking step. For systems oriented before crosslinking, a crossover between Gaussian and nonGaussian behavior is observed at a particular weight concentration of solvent. In contrast to the current belief, the neoclassical model based on Gaussian rubber elasticity cannot provide a correct description of the twice-crosslinked systems. 0,0 0,2 0,4 0,6 0,8 1,0 0 1 2 3 non-Gaussian behavior Gaussian behavior T=81 °C T=54 °C G' (x10 4 Pa)Weight concentration X of solvent direction, and fi nally crosslinked again to fi x the orientation of the director in this direction. [ 4 ] Recently, we have studied the mechanical properties of monodomain SCLCEs prepared in a completely different way, which is to orient the director by an electric or a magnetic fi eld before crosslinking, whereas in the usual two-step crosslinking procedure, the nematic director is oriented by stretching the network formed after the fi rst crosslinking step. The experiments, we have performed on these materials show that the shear mechanical anisotropy and the temperature range where the mechanical anisotropy extends beyond the transition region are strongly reduced compared to the samples oriented with the usual mechanical stretching procedure. [ 5 ] These changes strongly suggest that the polymer chains forming the network are less elongated for the elastomers oriented by E or H fi eld than for the elastomers prepared through a two-step crosslinking procedure. These results prompted us to make a detailed analysis of the stressstrain curves associated with the two types of elastomers, using the anisotropic Gaussian rubber elasticity

Nematic fluctuations and semisoft elasticity in swollen liquid-crystal elastomers

“…We show that the shear constant, contrary to what was observed in Ref. [18], decreases with swelling concentration as expected from the microscopic picture of Gaussian elasticity. The measured elastic response also remains semisoft with a typical stress-strain relation as in PHYSICAL REVIEW E 91, 042502 (2015) the dry samples.…”

“…However, to draw any conclusions regarding the validity of the semisoft description for samples analyzed in Ref. [18], an analysis similar to that presented in this paper should be made. The semisoft elasticity is characterized by the zeroing of the relaxation rate of the nematic fluctuations, accompanied by the zeroing of the shear modulus C5 at the critical strain Xc.…”

“…It could be that the samples in Ref. [18] were stretched beyond what was really necessary to obtain a good director alignment, hence making them non-Gaussian. However, to draw any conclusions regarding the validity of the semisoft description for samples analyzed in Ref.…”

“…Yet, some believe that the sem isoft model cannot describe system s that were synthesized using the tw o-step crosslinking method in which large elastic deformations are present during the synthesis [18,19], Their main argument is that the trace formula is derived from a microscopic picture of Gaussian elasticity in which polymer chains are treated as entropic springs. Furthermore, polymer chain conformation is assumed to be isotropic with a Gaussian distribution in the absence of the nematic field (in the isotropic phase).…”

“…In Ref. [18] it was shown that the shear constant of samples prepared according to the two-step crosslinking process increases if the sample is swollen in a nematic solvent. On the other hand, the shear modulus of samples prepared by orienting in electric field before crosslinking does decrease with the increase of the swelling concentration.…”