Unusual pressure‐induced phase behavior in crystalline poly‐4‐methyl‐pentene‐1

We report some highly unusual phase behavior, of general implication for condensed matter, on the polymer poly(4‐methyl‐pentene‐1) (P4MP1) induced by changes in pressure and temperature, as observed in situ by x‐ray diffraction. Upon increasing pressure beyond a threhold, the polymer, crystalline under ambient conditions, loses its crystalline order isothermally, passing through a continuously varying sequence of mesomorphic states, the process being reversible. This behavior is observed in two widely separated temperature regions, suggesting, for the first time in a single component system, the possibility of reentrant liquid‐crystal and amorphous phases. At the upper temperature region (ca. 250°C) there is a consecutive increase and decrease of melting point with pressure. In the lower temperature region (room temperature) the pressure converts the crystal into an amorphous‐like glass obviating the need for going through the melt first, and this in a reversible manner. The latter pressure‐induced disordered phase converts into crystal on raising the temperature, and reverts to the glassy, disordered phase on lowering the temperature. Some aspects of this behavior have been found quite recently in water‐ice and silica but the process of “melting on cooling” has no precedent in any known system. Other unexpected findings include a new pressure‐induced modification of P4MP1 with a one‐way only entry with temperature, but full reversibility with pressure leading to a triple point in the PT phase diagram. The above highly uncommon results are putting several prevailing preconceptions to test which are being scrutinized. In the course of it some early expectations on general phase behavior, allowing among others for reentrant phases in one component systems, are being invoked as potentially appropriate for certain polymeric systems, if not for condensed matter in general. © 1993 John Wiley & Sons, Inc. Copyright © 1993 John Wiley & Sons, Inc.