Isothermal Crystallization Behavior of Isotactic Polypropylene H/D Blends as Viewed from Time-Resolved FTIR and Synchrotron SAXS/WAXD Measurements

The melt isothermal crystallization behavior has been investigated for a series of blend samples of deuterated (D) and hydrogenated (H) species of isotactic polypropylene by means of time-resolved Fourier transform infrared (FTIR) spectral measurement and simultaneous small-angle X-ray scattering (SAXS)/wide-angle X-ray diffraction (WAXD) measurements using synchrotron radiation source in SPring-8. The SAXS data collected were analyzed in the three different time regions. The Guinier plot in the early stage of crystallization gave the average size of isolated domains formed in the melt. In the middle stage, the correlation between these domains was analyzed using the Debye−Bueche equation. In the later stage of crystallization, the correlation function was calculated to estimate the degree of crystallinity and the long period of the stacked lamellae. The IR data analysis clarified the formation process of regular helical chains in the isothermal crystallization phenomenon. The combination of FTIR, WAXD, and SAXS data has revealed a concrete structural evolution process as follows: (i) In the early stage, the short helices start to be formed, and they gather together to create an isolated domain of about 330 Å radius of gyration. (ii) The domain size decreases gradually to about 300 Å, and (iii) these domains start to have some weak correlation to each other with ca. 300 Å correlation length. (iv) In these domains, the regular chain segments grow to longer helices, and the correlation distance becomes shorter to ca. 200 Å, and finally, (v) the stacked lamellae with long period of ca. 200 Å are formed and stabilized. The crystallization behavior of the D/H 50/50 blend sample was also investigated, and the origin of cocrystallization phenomenon has been discussed.