STRENGTHENING POLYMER INTERFACES WITH TRIBLOCK COPOLYMERS

We have measured the fracture toughness, G(o), of an interface between polystyrene (PS) and poly (2-vinylpyridine) (PVP) reinforced with tri block copolymers (PVP-b-dPS-b-PVP) as a function of the areal chain de nsity, Sigma, of the copolymers at the interface. The failure mechanis ms of the interface are studied by transmission electron microscopy an d forward recoil spectrometry. For triblock copolymers with long PVP b locks (N-PVP > N-ePVP, where N-ePVP is the entanglement polymerization index of PVP), a transition from chain scission at low Sigma to crazi ng at high Sigma is observed. By comparing the areal chain density Si gma for the transition from chain scission to crazing for the tribloc k copolymers (Sigma = 0.015 chains/nm(2)) to that for the diblock cop olymers, PVP-dPS (Sigma = 0.03 chains/nm(2)), we show that most of th e triblock copolymers form a ''staple'' structure at the interface wit h the dPS block making a loop on the PS side of the interface and the PVP ends anchoring the ''staple'' in the PVP side. As a result of the ''staple'' structure, the saturation areal chain density of the triblo ck copolymer (Sigma(sat)) at the interface is half of that for the dib lock copolymer of similar molecular weight. For Sigma < Sigma(sat) in the crazing regime, the fracture toughness of the interface is control led by the areal joint density, Sigma(cross), where Sigma(cross) is th e number density of the copolymer excursions across the interface. For Sigma > Sigma(sat), the triblock copolymers appear to reinforce the c raze fibrils at the crack tip better than the corresponding diblock co polymers, leading to an interface fracture toughness approaching that of the PS homopolymer itself. For a triblock copolymer with short PVP blocks (N-PVP < N-ePVP), there is a transition in the fracture mechani sm from pull out of the PVP block to crazing with increasing Sigma. Sh ort triblock copolymers can form two chain conformations: one in which two PVP blocks anchor the copolymer on the homopolymer PVP side (stap le structure) and one in which one PVP block anchors the copolymer on the PVP side (tail structure) of the interface. Comparison of G(c) bet ween the triblock copolymer and the corresponding diblock copolymer is made. The larger G(c) values of the triblock copolymer reinforced int erface in the crazing regime are observed as a result of enhancement i n entanglements between the dPS loops of the triblock copolymer and th e homopolymer PS.