The re-entry of disordered phases in a crystalline polymer (poly-4-methyl pentene-1)

In situ x-ray expts. are reported on a cryst. polymer, poly(4-methylpentene-1) (I) as a function of pressure (P) and temp. (T) carried out under isothermal and isobaric conditions. At room temp., when the cryst. tetragonal phase (Ct) of I is subjected to pressure, it passes through a range of mesomorphic phases leading to complete amorphization. The melting temp. (Tm) gradually increases with P up to 3Kb where the trend reverses on further increase in P leading to a decrease in Tm. This, combined with the pressure-induced amorphization at room temp. raises the possibility of reentrant of 2 widely sepd. regions of disordered phases (along the T axis) in the P-T phase diagram. This effect, namely a reentrant of disordered phases, i.e. disordering setting in on isobaric cooling, at appropriately elevated P, was in fact obsd. exptl. In the course of the above expts. a new hexagonal crystal (Ch) phase was identified lying between Ct and the low T disordered phase along the T axis of P-T phase diagram. This Ch phase is attainable along 2 routes with basically different characteristics. Also, there is a triple point where the liq., Ct, and Ch phases are in equil. Recently similar observations were reported for inorg. materials such as silicates, phosphates, sulfates, and ice-water (the latter at low T and high P), so that the newly recognized behavior pattern seems to be of wider generality beyond the subject of polymers. [on SciFinder (R)]