Pressure-induced amorphization and disordering on cooling in semi-crystalline polymers: Calorimetric evidence for inverse melting in one component system

We report some unusual phase behaviour, of general implication for condense d matter, on the polymer poly-4-methyl pentene-1 (P4MP1) induced by changes in pressure (P) and temperature (T), as observed by in-situ X-ray diffract ion and high pressure DSC. Upon increasing pressure beyond a threshold valu e, the polymer, crystalline at ambient conditions, looses its crystalline o rder isothermally. The process is reversible. This behaviour is observed in two widely separated temperature regions, one below the glass transition t emperature (< 50 degreesC) and one close to the melting temperature (250 de greesC), thus showing solid state amorphization and inversion in the meltin g temperature with increasing pressure. This further suggests inverse melti ng, i.e. re-entrant of the two widely separated liquid and amorphous phases along the T-axis at fixed P. This is confirmed experimentally as disorderi ng in the crystalline structure on cooling. The inverse melting in P4MP1 ra ises the possibility of exothermic melting and endothermic crystallization as anticipated by Tammann (1903), see reference 1. The anticipated exotherm ic melting and endothermic crystallization is confirmed experimentally in t he one component system P4MP1. We are observing similar features in a range of polymers.