New trends in polymer crystallization studies Part II. The role of transient mesophases in polymer crystallization

Studies of crystn. of polyethylene (sharp fractions) under hydrostatic pressure revealed that the previously identified mobile hexagonal phase (h) can arise as a metastable transient even in the more customary orthorhombic (o) stability regime. In fact, crystn. in the h phase controls the crystn. process even there, at least in the portion of the pressure-temp. phase diagram explored so far. Accordingly, crystn. (nucleation and growth) can only take place while in the h phase and ceases upon transformation into the o phase (occurring when the o phase is the stable one). Crystal growth includes both lateral and thickening growth. Thickening growth in particular involves chain refolding, envisaged by sliding diffusion, and can proceed to thicknesses far in excess to chain extension. All growth rates can be defined quant. and correlated with morphol. [on SciFinder (R)]