Physical Studies of a Series of Main-Chain Liquid Crystalline Polymers With Long Soft Segment
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BING WU, C. PETER LILLYA AND RICHARD S. STEIN University of Massachusetts, Chemistry Department, Amherst, MA 01003
ABSTRACT A series of main-chain liquid crystalline polymers with long and polydisperse soft segments has been synthesized. The thermal properties of these polymers have been characterized by differential scanning calorimetry, optical microscopy and wide angle X-ray diffraction. Unexpectedly, these polymers have a strong tendency to form purely smectic mesophases. The degree of ordering of these polymeric mesophases is much higher than that of the mesophase formed by a low molecular weight model compound with the same mesogen. The soft segment length effects on thermal behavior of such segmented copolymers have been elucidated. A semi-quantitative theory has been formulated to interpret these observations. A phase diagram relating thermal properties to soft segment length has been proposed.
INTRODUCTION Semiflexible main chain liquid crystalline polymers, which comprise alternating hard and soft segments, have received a lot of attention during the past decade. One major objective of these studies is to investigate the soft segment length effects on thermal behavior of liquid crystalline polymers. In most cases, the soft segments used are of the monodisperse polymethylene type and the number of methylene units in the soft segment main chain is from 4 to 18. As for many series of LCP's with such soft segments, the following effects are observed as the soft segment length increases [1]: (a) transition temperatures decrease; (b) the temperature range over which the mesophase is stable is decreased; and (c) the nature of the mesophase is changed from predominantly nematic to purely smectic. To interpret these observations, structural studies have been conducted [2,3]. By using various techniques, most researchers determined that in the mesophase the soft segments have highly, if not fully, extended conformations. However, a generally accepted structure has not been obtained. Elucidation of the behavior of LCP's with long, polydisperse soft segments should enhance our understanding of how structure determines thermal phase behavior of semiflexible main chain LCP's. In this case, an open question is what kind of mesophase will be formed by such polymers. Since the soft segments with different lengths are randomly distributed along the polymer chains, formation of a regular smectic layer structure with fully extended soft segments is not possible. Therefore, these polymers might be expected to form nematic mesophases. However, the observations on semiflexible LCP's with monodisperse soft segments show that formation of smectic mesophases is favored by long soft segments. Publications on liquid crystalline polymers with polydisperse long soft segments are very few and are principally on synthesis. Quite recently, Bilibin [4] and Kolb [5] have shown it is possible to make LCP's with increased hard and soft segment length. However, to our knowledge the nature of the mesophases formed by such LCP's has not been
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