Processing Polypropylene Nanocomposites with Silylated Organoclays: Coupling at Edges versus Gallery Faces

Polymer nanocomposites with layered silicates have two different types of interface sites: edges with hydroxyl groups and gallery faces with oxygen atoms. The polymer–particle interface at either site may be strengthened by silane coupling agents. The objective of this work was to investigate the morphology and rheology in the melt-compounded state and the reprocessed state of polypropylene-layered-silicate nanocomposites with reactive coupling by the silane and a long chain polymeric compatibilizer at different interface sites. Two different organically modified layered silicates with different aspect ratios and surfactants were treated with an aminoalkyldimethoxysilane; in one case, the silanols reacted only at the nanolayer edges while in the other case, the silanes entered the interlayer galleries. The effect of reactive coupling in both cases was noticeably improved dispersion with thinner stacks of nanolayers. The uniaxial extensional viscosity of these melts displayed greater strain hardening and more so in the case of reactive coupling at both faces and edges despite the aspect ratio of this organoclay being significantly lower. Upon reprocessing, while some degradation of the polymer matrix occurred in all cases, the effects on the nanostructure were quite different just as the effects on the rheology of the melts were quite different. These effects may be attributed to differing changes in the fraction of particle-attached polymer chains entangling with free polymer chains.