Morphological Evaluation of PP/PS Blends Filled with Different Types of Clays by Nonlinear Rheological Analysis

The effects of various Cloisite nanoparticulate clays with different hydrophobicities on (80/20) polypropylene/polystyrene (PP/PS) blends were investigated using their linear and nonlinear rheological properties. In descending order of hydrophobicity the four Cloisite clays examined were C20A > C10A > C30B > CNa+. Clays with a wetting coefficient ωa of between 1 and −1, that is C20A and C10A, located at PP/PS polymer interfaces and suppressed coalescence, while clays with a ωa of >1, that is C30B and CNa+, accumulated inside the PS droplets and increased viscosity ratios. As a result, C20A and C10A caused PS droplet size reductions while C30B and CNa+ did not change morphology of the PP/PS blend. Linear rheological properties as determined by SAOS (small amplitude oscillatory shear) test and nonlinear rheological properties determined by LAOS (large amplitude oscillatory shear) test revealed C20A and C10A increased mechanical properties of PP/PS blends. Rheological properties from SAOS and LAOS tests decreased in the same order as clay hydrophobicities at same concentration. The nonlinear–linear viscoelastic ratio (NLR is defined as normalized nonlinear viscoelastic properties/normalized linear viscoelastic properties) was used to quantify degrees of PS droplet dispersion and to determine the compatibilizing effects of the four clays on PP/PS blends. PP/PS blends filled with hydrophilic clays (C30B and CNa+) exhibited constant NLR values (NLR ≅ 1) with increasing clay concentration. However, NLR values of PP/PS blends containing C20A and C10A were much larger than 1 (NLR > 1) and increased with clay concentration, which concurred with reductions in droplet sizes. Interestingly, NLR values and droplet size changes exhibited inverse relationships with clay hydrophobicity. In addition, PS droplet sizes in PP/PS/C20A blends in the medium amplitude oscillatory shear (MAOS) flow region (Q0 zone) were observed to check NLR validation. It was found that droplet sizes did not change in the MAOS region where NLR values are calculated and that thus NLR values corresponded with droplet sizes. Finally, inverse composition (20/80) PP/PS/C20A blends were studied, and a good correlation was obtained between PP droplet sizes and NLR values