13C CP/MAS NMR studies of morphological changes in polypropylene: 1—Annealing of quenched films

High‐resolution 13C cross‐polarization/magic angle spinning (CP/MAS) spectra of the crystalline component of quench‐cooled isotactic polypropylene (iPP) were obtained by using a pulse sequence that incorporates a delay period with a reduced spin locking field prior to cross‐polarization, thus exploiting the differences in 1H T1p relaxation times between the various phases. The spectra of samples annealed at high temperature have resolved splitting, ranging between 0.2 and 1.1 ppm, for all carbon resonances. The methyl resonance is composed of three resolved peaks whose relative areas, as obtained through peak deconvolution, depend on the anneal temperature. Large changes in these peak areas coincide with an exothermic transition observed by differential scanning calorimetry. The spectra of the interfacial and amorphous phases, obtained by subtraction, indicate a reduction in the amorphous content at anneal temperatures >100°C while the interfacial component appears to increase owing to recrystallization and the formation of more amorphous/crystal interfaces.