Nanoparticle interface driven microstructural evolution and crystalline phases of polypropylene: The effect of nanoclay content on structure and physical properties

Significant differences in the microstructure occur when the polypropylene matrix dispersed with 4 wt.% and 8 wt.% nanoclay. In neat polypropylene, α-crystals nucleate in the low to moderate crystallization pressure range of (∼0.1-60 MPa), while γ-phase nucleates at crystallization pressure of ∼60-200 MPa. In contrast to neat polypropylene, the presence of nanoclay in the polymer matrix, promotes the formation of γ-phase at ambient crystallization pressure of 0.1 MPa. Nanoclay and pressure-induced crystallization governs phase evolution and microstructure of polypropylene and is associated with change in the thermodynamic and physical properties such as equilibrium melting point and glass transition temperature. This change in equilibrium melting point and glass transition temperature points toward thermodynamic interaction between polymer-nanoparticle and nanoparticle interface-driven microstructural evolution. The dependence of equilibrium melting point on crystallization pressure and spherulite size on undercooling delineates two distinct regimes which are ascribed to dominant α- and γ-phases of polypropylene. However, the effect of nanoparticles is less dramatic when nanoclay content is increased from 4 wt.% to 8 wt.% implying that the effect maximizes at ∼4 wt.% nanoclay.