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 isotactic polypropylene (iPP) as a model system were investigated via simultaneous synchrotron small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) and laser light scattering (LS) techniques. Since the crystallinity in the early stages is very low, the issue of the crystallinity detection limit of WAXD was addressed. This was done by using solutions of n-paraffin (C₃₃H₆₈) in dodecane (C₁₂H₂₆) at different concentrations (as low as 1%). The precipitated fraction simulated the degree of crystallinity in polyethylene since n-paraffin essentially completely crystallized and dodecane remained liquid at the measurement temperature. A modeling method was also used 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 stages of iPP isothermal crystallization, noticeable short-range density fluctuations with average periods from 20 to 24 nm (by SAXS) were seen prior to the observation of three-dimensional ordering of the crystalline α-form (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, which is consistent with the formation of a finite lamellar structure. Furthermore, larger objects with dimensions growing from 300 nm were observed with the more sensitive technique of polarized light scattering, prior to the detection of the lamellar period by SAXS. The development of the crystallinity as measured by WAXD as well as SAXS and light scattering are all consistent, which follow the same Avrami equation, suggesting that the early stages of crystallization as measured here follow classical nucleation and growth.