Temperature-Induced Reversible Distortion along Director Axis Observed for Monodomain Nematic Elastomer of Cross-Linked Main-Chain Polyester

Abstract

A nematic elastomer was prepared by cross-linking a main-chain liquid crystalline polyester in the isotropic liquid phase. A polydomain elastomer in the nematic phase at 100 °C was stretched to a strain of 500% at a rapid rate of 100% min-1, relaxed to 330% after the release from the grips of the stretcher and transformed to a monodomain sample with an orientation parameter of 0.48. The monodomain nematic elastomer decreased in length in the orientation direction by 45% on heating in the nematic temperature range from 80 to 150 °C and recovered on cooling to 80 °C without a change in the degree of orientational ordering. This temperature-induced reversible distortion of the nematic elastomer is well explained by the conformational change of the polymer chain including several hairpin foldings, the probability of which is proportional to the Boltzmann factor of hairpin energy.

Internal constraints and arrested relaxation in main-chain nematic elastomers

“…The present findings, via analysis of a series of specially designed LCE, of the internal constraints due to the coupling between the nematic alignment and the polymer network topology through the hairpin dynamics unifies the equilibrium and non-equilibrium features known for decades in N-LCE, namely soft-elasticity 1 , 19 – 25 , slow relaxation 28 – 30 , and hysteresis 43 , 44 . The result highlights that the balance between elastic and hysteretic properties can be controlled by engineering polymer network design.…”

Lamellar Morphology of an ABA Triblock Copolymer with a Main‐Chain Nematic Polyester Central Block

“…Thus, the chain reversibly contracts in the director direction with increasing temperature. In fact, such a chain conformation change induces the reversible contraction of a main‐chain nematic LC elastomer along the director direction on heating 12. At the same time, the chain reversibly expands its dimensions in the direction perpendicular to the director direction with increasing temperature.…”

Large spontaneous deformation of uniaxially oriented main-chain liquid–crystalline elastomers composed of BB-nmesogens

“…It is seemingly curious. However, according to a previous calculation of the minimum energy cost based on the chain-bending elastic energy and orientational order in main-chain liquid-crystalline BB-n polyesters [22,39], the number of chain foldings increases (decreases) with increasing (decreasing) temperature. An increase in the number of chain foldings might enhance the formation of smectic layers in a main-chain LCP because the position of the mesogens is restricted by chain foldings as illustrated in Figure 5 (b).…”

“…Wermter et al pointed out that hairpin conformations of a polymer chain should contribute to the large shape change, in which a non-linear relationship between the order parameter S [S = (1/2)<3cos 2 α i −1>] and the relative length λ (λ = L/L iso ) was observed [19]. Recently, Tokita et al have explained the temperature-induced reversible deformation of a main-chain nematic elastomer by considering the Boltzmann factor of the hairpin energy of chain conformations [22].…”