Isolating the effect of polymer-grafted nanoparticle interactions with matrix polymer from dispersion on composite property enhancement: The example of polypropylene/halloysite nanocomposites

Interfacial effects can significantly perturb the properties of nanocomposites. Modification of filler surfaces can improve filler dispersion and compatibilize the polymer-filler interface, leading to property enhancements relative to composites made with unmodified filler. Here, we have isolated the effect of interactions between matrix polymer and polymer-grafted nanoparticles (PGNs) from dispersion quality and studied how the interactions affect the properties of polypropylene (PP)/halloysite nanotube hybrids in the absence of entanglements between matrix PP and grafted PP. We prepared PP hybrids with pristine halloysite nanotubes (p-HNT) or PP-grafted halloysite nanotubes (PP-g-HNT) up to 10 wt% filler using melt mixing or solid-state shear pulverization (SSSP). For melt-mixed samples, PGN-type filler yields better property enhancement than pristine filler, consistent with the combination of improved dispersion and polymer-filler interactions. As shown by microscopy and rheology, hybrids prepared by SSSP with p-HNT and PP-g-HNT fillers have very similar, high levels of dispersion, eliminating dispersion quality as a factor in property improvements. In comparison with well-dispersed PP/p-HNT nanocomposites at the same filler level, well-dispersed PP/PP-g-HNT nanocomposites prepared by SSSP exhibit enhanced tensile strength but the same values within error of Young's modulus and elongation at break when there is no entanglement between matrix polymer and grafted chains. Notably, at the same filler level, well-dispersed PP/PP-g-HNT nanocomposites exhibit higher crystallization rate and nucleation efficiency (NE) that is 2.5–4.5 times higher than well-dispersed PP/p-HNT hybrids. The impact of grafted PP chains on NE is so strong that at the same filler level but with worse dispersion, PP/PP-g-HNT hybrids made by melt mixing exhibit NE values 2.1 to 3.4 times higher than well-dispersed PP/p-HNT nanocomposites made by SSSP.