Papers by Author: Christopher J. G. Plummer

Abstract: Although addition of montmorillonite (MMT) clay to isotactic polypropylene (iPP) results in large increases in the melt viscosity at low shear rates, satisfactory impregnation of glass mats is obtained with iPP/MMT under conditions typical of industrial processing, and the MMT is well dispersed throughout the resulting composites. However, the MMT is also shown to retard relaxation of the glass mat, leading to non-uniform through-thickness glass fiber distributions. Thus, depending on the initial lay-up and hence on the initial surface fiber concentration, the bending modulus of the consolidated specimens may either increase or decrease with increasing matrix MMT content, whereas the tensile modulus remains consistent with the predictions of micromechanical models based on the assumption of a uniform glass fiber distribution. The presence of matrix-rich layers at surfaces of specimens containing MMT is also shown to promote crack initiation and induce premature failure in flexural tests.

Abstract: Nanocomposite films containing up to 50 wt. % laponite have been prepared from emulsion polymerized polystyrene (PS)/laponite latexes with the laponite attached to the PS particle surfaces. Below Tg, the observed stiffness increases are accounted for by classical models for mechanical reinforcement, but these severely underestimate the stiffness above Tg. An alternative model has therefore been proposed, in which the laponite and PS with reduced mobility form a cellular network, consistent with TEM observations. This cellular network is also argued to play an important role in the decrease in fracture resistance at high laponite contents, through decreases in matrix ductility and breakdown of the laponite stacks.