We have prepared a polydomain smectic liquid crystalline (LC) elastomer by introducing 2 mol % 1,3,5-benzenetricarboxylic acid as a cross-linker into a main-chain BB-5 polyester. The elastomer designated as BB-5/E2 forms isotropic and smectic CA (SmCA) LC phases in order of decreasing temperature similarly to the un-cross-linked BB-5 polyester. BB-5/E2 in the isotropic liquid phase shows a typical rubberlike elasticity, whereas that in the SmCA phase shows a characteristic elongation behavior accompanying a polydomain−monodomain transition: the initial strain up to 100% orients the smectic layer with its normal parallel to the elongation direction, and the further elongation does not change the layer orientation and results in permanent deformation. The corresponding stress−strain curve shows a quasi-plateau, which is a so-called “soft stress plateau”, over a wide strain region from 100% to 450%. These indicate that the polymer chains folded in the oriented smectic LC elastomer at a strain of 100% are stretched on further elongation and locked by the smectic layer order. At a strain of more than 300%, the SmCA phase begins to transform into a crystal because of the entropy reduction due to direct stress to the fully extended polymer chains.

We examine the tensile deformation of the smectic melt of a main-chain liquid crystalline polymer. A film molded in isotropic melt is heated to a smectic temperature of 60 °C and then stretched at selected strain rates. In the initial stage of strain below 100% (defined as {(l − l 0)/l 0} × 100%), smectic layers align parallel to the stretching direction; i.e., polymer chains orient perpendicularly (so-called “perpendicular orientation”). This perpendicular orientation in the low-strain region is invariably observed at any strain rate. Upon succeeding elongation, however, two distinct types of orientation process are observed depending on the strain rate. On elongation at strain rates as low as 5%/min, the perpendicular orientation is improved until the sample breaks at 400% strain (process A). At a high strain rate of 100%/min, in contrast, the perpendicular orientation initially observed is eventually altered to the “parallel orientation” with polymer chains lying parallel to the tensile axis (process B). In this case, the well-known necking takes place, and the sample breaks at an extremely large strain more than 3000%. These two distinct elongation processes are also dependent on temperature; processes A and B are observed in higher and lower temperature regions, respectively. On the basis of these data, the molecular orientation mechanisms in smectic field are discussed by coupling together the nature of smectic liquid crystal and dynamics of polymer chains.

Summary: Anisotropy in molecular motions of a glassy smectic CA (SmCA) liquid crystal of a main‐chain BB‐5(3‐Me) polyester was examined by dynamic mechanical analysis (DMA) for fibrous monodomain samples with two distinct orientations of smectic layers perpendicular and parallel to the fiber axis. The α‐process attributed to micro‐Brownian motion of the polymer on the glass transition shows clear anisotropy explainable by the nature of the smectic layer structure. Time‐temperature superposition is applicable to the α‐process, so that the frequency of the micro‐Brownian motion in the SmCA phase can be estimated despite the narrow frequency range in DMA. The results suggest that the decoupling of the molecular motions in two characteristic directions are parallel and perpendicular to the layer. The amplitude of the micro‐Brownian motion in the layer normal direction is 3 times smaller than that in the layer direction, and the frequency in the layer normal direction is 2.5 times higher than that in the layer direction.Illustration of molecular packing structures: fiber A (left side) and fiber B (right side) with the orientations of polymers parallel and perpendicular to the fiber, respectively.imageIllustration of molecular packing structures: fiber A (left side) and fiber B (right side) with the orientations of polymers parallel and perpendicular to the fiber, respectively.

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.

No abstract