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High-resolution l3C 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 inlH TIp 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.

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Previous work at Kimberly -Clark Corp. (R. S. Nohr and J. G. MacDonald, Kimberly -Clark Corp., unpublished results, 1990 ) demonstrated that the addition of an ultrafine-particle ( submicron-size), surface-modified silica predispersed in an alkyl silicone to isotactic polypropylene (iPP) results in dramatic improvements in the tensile properties of fibers and spunbonded fabrics. For both fibers and spunbonded fabrics prepared with a high concentration of the low melt flow index (MFI) resin the incorporation of the additive resulted in markedly improved tensile properties. In this article it is shown that under quiescent conditions the surface-modified silica/silicone copolymer additive acts as an effective nucleating agent for iPP. This results in a reduction in the size of the crystalline entities, hence a more homogeneously distributed crystalline phase and load-bearing ''tie'' molecules. Indeed, scanning transmittance electron microscopy (STEM ) studies show that the crystalline entities are substantially smaller at the surface and mid-radius of fibers prepared with additive containing iPP. Furthermore, the bond points of the resulting spunbonded fabrics have a dramatically increased nucleation density and smaller crystallite dimensions.

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