engleski

PHASE STRUCTURE AND MORPHOLOGY OF POLYPROPYLENE COMPOSITES WITH VARIOUS SILICA

Binary iPP/silica composites with silica contents 0-8 vol% were prepared by melt compounding. The effects of particle size (micro- vs. nanosilica) and surface modification (untreated vs. treated surface) of various silica grades on structural characteristics of composites were studied by wide-angle X-ray diffraction (WAXD), infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), optical and scanning electron microscopy. Microsilica particles and aggregates of nanosilica particles that were homogeneously incorporated into the iPP matrix, influenced phase structure and final supermolecular structure of the iPP/silica composites. The most significant effect was observed in spherulitic morphology of the iPP matrix due to nucleation ability of silica fillers and restrained crystallization (spherulitization). Nanosilica particles and their agglomerates are more cohesively connected to the iPP matrix than microparticles. WAXD revealed that silica particles act as -nucleants and partly as -nucleants, whereas their influence on the orientation of -crystallites was negligible. Moreover, due to the nucleation and steric hindrance effects the particles of all silica grades progressively disturbed and transformed well-developed spherulitic morphology of iPP matrix to gross morphology with dark branched iPP grains. No indication of interactions between silica particles of any grade was observed in FTIR spectra. DSC thermograms showed that the composites containing silica particles with polar surfaces exhibited less supercooling and stronger nucleation effect. The supercooling effect of hydrophylic nanosilica was somewhat higher than of hydrophylic microsilica. FTIR spectra of those samples indicate that this effect may be due to lower concentration of hydroxyl group in nanosilica than in microsilica. Two important effects on the spherulitization of the iPP matrix should be emphasized. Firstly, spherical silica fillers disturb spherulitization of the iPP matrix to lesser extent than previously studied anisotropic silicate fillers like talc and wollastonite. Secondly, the differences in the spherulite size between composites containing hydrophobic and hydrophilic silica fillers (with treated and untreated surfaces) are greater than between composites with microsilica and nanosilica fillers. The spherulites were more affected by the character of silica particle surfaces (hydrophobic vs. hydrophilic surface) than by the size of filler particles. This fact indicates that, at least for isotropic particles, the surface character of filler particles and strength of cohesion to polymer matrix may be more influential than the difference in particles size.

Morphology; Phase structure; Polymer-matrix composites; Polypropylene; Silica.

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