Probing the early stages of melt crystallization in polypropylene by simultaneous small- and wide-angle X-ray scattering and laser light scattering

The early stages of polymer melt crystallization using fractionated isotact ic polypropylene (iPP) as a model system were investigated via simultaneous synchrotron small-angle X-ray scattering (SAXS)/wide-angle X-ray diffracti on (WAXD) and laser light scattering (LS) techniques. Since the crystallini ty in the early stages is very low, the issue of the crystallinity detectio n limit of WAXD was addressed. This was done by using solutions of n-paraff in (C33H68) in dodecane (C12H26) at different concentrations las low as 1%) . The precipitated fraction simulated the degree of crystallinity in polyet hylene since n-paraffin essentially completely crystallized and dodecane re mained liquid at the measurement temperature. A modeling method was also us ed to simulate the WAXD profiles to check the effect of crystal size at low crystallinity. With these two methods, we conclude that our WAXD procedure is capable of detecting crystallinity from 0.5% to 1%. During the early st ages of iPP isothermal crystallization, noticeable short-range density fluc tuations with average periods from 20 to 24 nm (by SAXS) were seen prior to the observation of three-dimensional ordering of the crystalline alpha-for m (by WAXD). The spacing associated with the peak of the SAXS was found not to increase with time, being constant or a possible initial decrease, whic h is consistent with the formation of a finite lamellar structure. Furtherm ore, larger objects with dimensions growing from 300 nm were observed with the more sensitive technique of polarized light scattering, prior to the de tection of the lamellar period by SAXS. The development of the crystallinit y as measured by WAXD as well as SAXS and light scattering are all consiste nt, which follow the same Avrami equation, suggesting that the early stages of crystallization as measured here follow classical nucleation and growth .