An proton NMR method, based on simple Bloch-decay spectra in the solid state, is presented that enables one to follow, with excellent sensitivity, structural changes associated with aging in semicrystalline polymers whose Tg is well below the aging temperature. The method is demonstrated for 2 representative isotactic polypropylene (iPP) samples, a Ziegler-Natta and a metallocene product. Starting with samples that had been melt crystallized at a cooling rate of 1 C/min and then aged at ambient temperature for long periods of time, subsequent mild heating cycles between ambient temperature and temperatures below 90 C were applied. Such heating cycles remained more than 70 C below the major crystalline melting temperature for iPP. Aging at 20 C was monitored by NMR over aging times, 10 min < tage < 4 d, following those heating cycles. It was shown that changes in the Bloch-decay spectra, corresponding to a transformation of 2 % to 3 % of the mass of iPP from mobile to rigid componentsaccompanied the aging process over the 4 d period. Moreover, the time dependence was linear in log(tage). It was further shown that the population of those non-crystalline (NC) stems with the highest mobility was most strongly reduced by aging; this observation does not, however, establish that these same chains were the actual stems participating in the newly formed structures. The aging process was also shown to be reversible in the sense that the structures formed could be completely destroyed by repeating the mild heating cycle. Attention was paid to the definition of crystallinity and an operational definition of crystallinity for the NMR measurements was based on the component with a long (> 150 ms) value of T1xz, the relaxation along the quantization axis of the toggling frame in a multiple pulse (MP) experiment.
Issue: No. 13
Pub Type: Journals
aging, isotactic polypropylene, morphology, NMR, proton, reversible, secondary crystallization