Stress–strain behavior in main chain liquid crystalline elastomers: effect of crosslinking density and transverse rod incorporation on “Poisson's ratio”

Abstract

Stress–strain results are reported for two series of smectic C main chain liquid crystalline elastomers (MCLCEs). The structural variable in one series is crosslink density; in the other, transverse rod content. With increasing crosslinker content, the LCEs exhibit increased modulus, a shorter polydomain‐to‐monodomain (P–M) plateau, and a lower elongation at break. Increasing transverse rod content leads to lower clearing temperatures and lower modulus. For each series, stress–strain measurement was made simultaneously with Poisson's ratio determination. For the variable crosslinker series, typical behavior showed the Poisson's ratio increased at small strain, rose to a maximum about 50% strain, then decreased with greater strain. For the transverse rod series, typical behavior also showed the Poisson's ratio increased at small strain, rose to a maximum about 50% strain, then decreased with greater strain. In both series, the maximum in the Poisson's ratio corresponded to the onset of the P–M transition. Possible deformation mechanisms are presented.

Soft Elasticity in Main Chain Liquid Crystal Elastomers

“…To study this effect as a function of cross-linking density, several samples were prepared with cross-linking concentrations from 5% to 25%. Increasing crosslinkage density gave rise to higher rigidity and an increase in glass transition temperature [60]. Also, the P-M plateau became shorter with increasing crosslinker content, and at higher concentrations, a yield point was observed, Figure 8.…”

“…This concept of molecular auxetic design was also attempted by incorporating TR3 into the main chain of LCE2. Unfortunately, it did not lead to auxetic behavior [60] due to the hindrance to transverse rotation in the presence of smectic layers. A longer transverse rod group and a shorter spacer length might be necessary to realize auxetic effects in MCLCEs.…”

“…Polydomain smectic MCLCEs have been extensively studied [20,[60][61][62][63][64][65][66][67]. Sánchez-Ferrer et al [64] reported the polydomain to monodomain (P-M) transition caused by uniaxial strain in these materials.…”

“…The chain extension and cross-linking reactions for preparing the elastomers were performed at room temperature. LCE1 was chosen as the parent elastomer because its cross linking concentration is optimum for efficient liquid crystalline and rubbery network properties, and presence of the plateau [60] in the stress-strain curve might correspond to the soft elastic behavior.…”

Negative Poisson's ratio and semisoft elasticity of smectic-Cliquid-crystal elastomers

“…To our knowledge this mechanism for negative Poisson's ratio has not been reported before. Alternative mechanisms of producing auxetic behavior based on modifying the attachment of mesogens to the polymer backbone in smectic LCEs have been proposed and investigated experimentally [38,39]. The microstructure formed by LCEs during deformation may prevent the observation of negative Poisson's ratio for some deformations.…”