The Role of Polymer Architecture in Strengthening Polymer−Polymer Interfaces:  A Comparison of Graft, Block, and Random Copolymers Containing Hydrogen-Bonding Moieties

A series of styrene-d8/4-hydroxystyrene graft and block copolymers has been prepared by "living" radical and anionic techniques for use in interfacial strengthening studies at the polystyrene/poly(2-vinylpyridine), PS/PVP, interface. The following copolymers in which A and B segments represent poly(styrene-d8) and poly(4-hydroxystyrene), respectively, have been prepared: poly(A-graft-B), poly(B-graft-A), poly(B-block-A-block-B), poly(A-block-B-block-A-block-B-block-A). The poly(4-hydroxystyrene) segments were obtained by "living" radical polymerization of 4-acetoxystyrene or anionic polymerization of 4-methoxystyrene, followed by conversion to the phenolic derivative. In general, the amphiphilic copolymers when placed at the PS/PVP interface acted as interfacial reinforcers but were susceptible to the formation of microphases such as lamellae or micelles, and therefore the measured fracture toughness depended on both the copolymer/homopolymer interfacial strength and the toughness of the copolymer phase structure itself. The pentablock copolymer showed better strengthening behavior than the triblock copolymer especially at very low areal chain density. The strengthening ability of the graft copolymers was found to depend on the lengths of the polystyrene, PS, and poly(4-hydroxystyrene), PS(OH), segments. In both graft and block copolymers the PS(OH) segments were found to resist pull-out from the bulk PVP even at low degrees of polymerization (NPS(OH) = 29). The H-bonding interaction between the phenolic and pyridyl groups combined with the severe immiscibility of poly(4-hydroxystyrene) and polystyrene is the most likely cause for pull-out resistance.