Abstract
Nanoparticles (mean size around 7 nm) of the standard pyrogenic Aerosil 1380 (Degussa) pregrafted by γ-irradiation with styrene were melt-compounded with the general purpose isotactic polypropylene homopolymer (PP) to prepare nanocomposites with filler volume contents up to 4.68%. Solid-state properties of the nanocomposites were characterized by wide-angle x-ray scattering (WAXS), small-angle x-ray scattering (SAXS), differential scanning calorimetry, and stretching calorimetry.
The nearly identical overall patterns, the angular positions of the crystalline reflections on the WAXS diagrams as well as the WAXS degrees of crystallinity of both the neat polymer PP-0 and all nanocomposites suggested that the structure of the crystalline PP lamellae remained unchanged, irrespective of the filler content. However, a well-resolved SAXS reflection seen for PP-0 was not detectable on the SAXS curves of nanocomposites with low filler contents due to the sharp increase of SAXS intensity in the same range of scattering vectors. These results implied a significant increase in structural heterogeneity due to the appearance of new and strongly scattering entities (presumably polymer-nanoparticle interfaces and microvoids) with a broad distribution of sizes. In contrast to the basically composition-invariant WAXS crystallinities for nanocomposites, higher the filler volume content, the calorimetric crystallinities for the polymer matrix tended to increase, while the apparent densities of the polymer matrix decreased. Moreover, the Young's moduli of nanocomposites were considerably in excess of, whereas thermal expansivities, limiting strains for elastic behavior, and breaking strains, were much below the reasonable theoretical predictions.
These experimental observations were explained by a model assuming that a nonnegligible portion of PP chains in the melt state would be anchored by each end to the available absorption-active sites of two different neighboring nanoparticles. The restricted chain mobility in these sites should facilitate the crystal nucleation in the undercooled PP melt; hence, the same PP chain might be involved in two nucleation events at the surfaces of two adjacent nanoparticles. Presumably, subsequent crystallization in the undercooled melts of both neat PP and nanocomposites would proceed via the usual growth of chain-folded lamellae (therefore, the WAXS patterns should be similar). However, the tie-chains in the interlamellar space of the neat PP are expected to remain in the relaxed, coiled state, whereas in the latter case, a simultaneous lamellar growth at fixed positions of the same PP chains on adjacent nanoparticles would end up with not only a considerable extension of tie-chains but also with a concomitant fall in the local packing density in the interlamellar space.
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Dedicated to Prof. F. J. Baltá Calleja on the occasion of his 65th birthday.