Side-chain liquid crystalline ionomers: 3. Stress-induced orientation in blends with poly(vinyl chloride) as matrix

Abstract

The mechanically induced macroscopic orientation in two series of side-chain liquid crystalline ionomers was investigated in order to reveal the effects of the ionic aggregates. The ionomer samples, which contained a small amount of COO −Na + groups ( < 10 mol% ), were prepared through partial hydrolysis, using sodium hydroxide, of two liquid crystalline polyacrylates differing in the mesogenic units. The application of a mechanical field was rendered possible by blending these samples with an immiscible poly(vinyl chloride) matrix and stretching the blend films obtained by solution-casting. Comparisons were made between the ionomers having different ion contents and the initial polymers containing no ionic groups, and very distinct orientation behaviours were observed for the two series of ionomers. In the case of the ionomers prepared from a polyacrylate having a biphenyl moiety in the side group, the stress-induced orientation of the mesogenic units was found to be almost unchanged with respect to the polymer, judging from the orientation development upon extension, the level of the achievable orientation, and the common feature that a macroscopic orientation could only be obtained by stretching the sample in the nematic state. In contrast, for the ionomers based on a polyacrylate carrying a phenyl benzoate moiety, the ionic aggregates showed strong effects on the macroscopic orientation; the achievable orientation was found to be systematically lowered as the ion content in the ionomer was higher. Moreover, it was found, surprisingly, that the macroscopic orientation could be induced even when the samples were stretched in the isotropic state, with the same level as the samples stretched in the nematic state. Studying the underlying mechanisms, we found that when stretching the samples at T > T ni (the nematic to isotropic transition temperature), the observed macroscopic orientation was totaly achieved during the cooling of the samples under strain, while when stretching at T < T ni, a partial macroscopic orientation was first induced by the stretching and the orientation was further improved during the cooling. Different factors which could play a role in this peculiar orientation process are discussed.