Examining an Alternative Molecular Mechanism To Toughen Glassy Polymers

Contrasting the conventional rubber-toughening mechanism, we show that the new PMMA-based rubber-toughened nanocomposite (PMMA-rt/nc) achieves the desired rubber toughening through molecular-level interactions between the glassy PMMA chains and the nanosized rubbery domains. PMMA-rt/nc, as an “inverted” polymer nanocomposite, is found to be sufficiently rigid to be treated as a polymer glass, yet ready to undergo plastic deformation like a ductile polymer glass. In parallel, we study high-impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) to elucidate the conventional rubber-toughening mechanism. Unlike PMMA-rt/nc, HIPS and ABS cannot undergo yielding and plastic deformation at room temperature. The delayed brittle fracture in HIPS and ABS is accomplished through crazing-initiated rubbery cavitation, manifested in the form of whitening of the specimens at the point of apparent yielding. Further experiments show that HIPS and ABS can be made to avoid crazing and consequently whitening as well as brittle fracture when they have undergone adequate premelt stretching that can enhance the chain networks in the glassy polymer matrix. The evidence of plastic deformation in the premelt-stretched HIPS and ABS comes from the fact that such specimens, unlike the untreated counterparts, contract in their transverse dimensions during the extension.