LIQUID-CRYSTALLINE POLYETHERS BASED ON CONFORMATIONAL ISOMERISM: CAN PRESSURE INDUCE SECOND-ORDER PHASE TRANSITIONS IN CRYSTALLINE PHASES PRECEDING THE NEMATIC PHASE?

摘要

PVT studies of liquid-crystalline polyethers based on conforma-tional isomerism have evidenced that both the shape of the PVT volume vs temperature curves as well as the phase transition temperatures depend on the mode of operation. Generally, the pressure influence on the first-order phase transitions is higher than on the glass transition. As a consequence, the first-order phases are much more enlarged when increasing the pressure. Additionally, a change of the slope is observed at higher pressures in the volume vs temperature curves of the crystalline phase preceding directly the nematic phase, independent of the order of the phases of the studied LC polyethers. This change of the slope of volume vs temperature is accompanied by both an increased range of the high temperature crystalline phase and a reduction—and sometimes even disappearance—of the subsequent nematic phase. Taking into account that changes in the slope of volume vs temperature curves are characteristic of second-order phase transitions, the question on the nature of these transitions arises: Can second-order phase transitions be induced really within crystalline phases by pressure? The answer may be of significant applicational relevance.