LIQUID-CRYSTALLINE POLYETHERS BASED ON CONFORMATIONAL ISOMERISM - CANPRESSURE INDUCE 2ND-ORDER PHASE-TRANSITIONS IN CRYSTALLINE PHASES PRECEDING THE NEMATIC PHASE

PVT studies of liquid-crystalline polyethers based on conformational i somerism have evidenced that both the shape of the PVT volume vs tempe rature curves as well as the phase transition temperatures depend on t he mode of operation. Generally, the pressure influence on the first-o rder phase transitions is higher than on the glass transition. As a co nsequence, the first-order phases are much more enlarged when increasi ng the pressure. Additionally, a change of the slope is observed at hi gher pressures in the volume vs temperature curves of the crystalline phase preceding directly the nematic phase, independent of the order o f the phases of the studied LC polyethers. This change of the slope of volume vs temperature is accompanied by both an increased range of th e high temperature crystalline phase and a reduction - and sometimes e ven disappearance-of the subsequent nematic phase. Taking into account that changes in the slope of volume vs temperature curves are charact eristic of second-order phase transitions, the question on the nature of these transitions arises: Can second-order phase transitions be ind uced really within crystalline phases by pressure? The answer may be o f significant applicational relevance.