Investigation of secondary crystallization of polymers by means of microbeam X-ray scattering

The kinetics of secondary crystallization during spherulite growth of isotactic poly(propylene) (iPP) and poly(vinylidene flouride) (PVF2) is studied using a novel technique that employs a micron size X-ray beam. The data are combined with separate conventional simultaneous on-line SAXS/WAXS measurements and optical microscopy studies. In our experiments, crystallization takes place at low undercooling so that slowly growing large single spherulites are obtained. The data reveal that the main mechanism of secondary crystallization is the growth of new lamellae stacks within remaining amorphous regions. It is shown that a substantial amount of crystallites form as a result of secondary crystallization while the spherulite is growing. Furthermore, secondary crystallization is strongest directly behind the boundary of the spherulite and is independent of its size or growth state. A separate, off-line microfocus study on a quenched spherulite sample confirms this observation; the crystallinity is higher in the main body of the spherulite and lower near the boundary, where crystallization progressed to a lesser degree.