SAXS studies of polymer melting: Models for surface melting, sequential melting, and stack melting

Melting of semicrystalline polymers typically occurs over a temperature range of 10 degreesC or more, throughout which the long period L increases by 50%-100%. Less recognized is the evolution of the small-angle X-ray scattering (SAXS) pattern from peaked to monotonic before the final stages of melting. These two features are considered with SAXS patterns calculated for basal surface melting, for sequential melting by increasing thickness of randomly placed crystals, and for melting of separate stacks of lamellar crystals having different periods. Calculated intensities are further analyzed by the correlation function gamma(r) and the interface distribution function G(r). Neither surface melting nor sequential melting can account for the commonly observed increase in L, although the sequential mechanism provides monotonic scattering from single lamellae as melting is completed. Stack melting can display an increase in L, but not a conversion to monotonic scattering. A reasonable conclusion is that most melting is accompanied by extensive reorganization by melting and recrystallization or by lamellar thickening. Sequential melting is favored in systems with very stable crystals that are resistant to reorganization.