Rubber‐modified polystyrene from multistage latexes: Rheological and physical properties

The combination of rubbery and rigid polymers in a multiphase structure using staged emulsion polymerization has yielded materials with properties ranging from reinforced elastomers to high impact plastics. The many different particle morphologies that result from multistage latexes include core/shell, domain, interpenetrating polymer networks, and various combinations thereof. The present work focuses on rubber modified polystyrene (polyacrylates (PA) and polystyrene (PS) in a 1:3 ratio). The nature of the rubber‐modified polystyrene from multistage latexes has been determined through a combination of microscopy and mechanical property analyses. The uncrosslinked PS shells that form around the crosslinked PA seed particles with grafted PS microdomains coalesce upon molding to form a continous thermoplastic PS matrix that may absorb impact energy through mechanisms of crazing and shear yielding. The crosslinking of the acrylate seed with butadiene enhances PS grafting through residual unsaturation and thus affects the effective particle volume fraction and particle‐matrix interaction in these rubber‐modified PS materials. The substitution of some of the uncrosslinked PS of the second stage with crosslinked PS in a separate intermediate stage, resulting in a three‐stage latex, also affects the effective volume fraction and the particle‐matrix interaction through enhanced PS grafting and the formation of interpenetrating networks. In flow the rubber modified PS exhibits a Newtonian plateau at low shear, unlike crosslinked particles which flow through a particle slippage mechanism, and an unexpected deviation from power law behavior at high shear with a viscosity plateau, or even a slight increase in viscosity. These materials may essentially flow through molecular deformation, depending upon the temperature, molecular weight, particle‐matrix interaction, and level of shear. The rubber‐modified PS from the multistage latexes are a potential high impact material whose unique structure and chemistry distinguish it from commercial high impact PS (HIPS), although its mechanical and impact properties are, at the very least, similar to those of HIPS.