FAILURE MECHANISMS OF POLYMER INTERFACES REINFORCED WITH BLOCK COPOLYMERS

The fracture toughness (characterized by the critical energy release rate G(c)) of interfaces between polystyrene (PS) and poly(2-vinylpyridine) (PVP) reinforced with poly(styrene-b-2-vinylpyridine) was measured with a double cantilever beam test geometry. The effect of the PVP block degree of polymerization (N(PVP)) and the areal density of block copolymer chains at the interface (SIGMA) on the measured G(c) and on the fracture mechanisms was investigated quantitatively. The PS degree of polymerization (N(PS)) was kept >280, while N(PVP) was varied between 45 and 870. For N(PVP) below 200 the interfaces showed only a small increase in G(c) with increasing SIGMA and failed by pull-out of the short PVP chain. In this regime G(c) increases linearly with SIGMA and scales roughly with N(PVP)2, in reasonable agreement with a recently proposed model of failure by chain pull-out. 1 If N(PVP) was increased well above 200, corresponding roughly to the average molecular weight between entanglements for the PVP, two separate fracture mechanisms could be distinguished. At low values of SIGMA, G(c) increased only slowly with SIGMA and the interfaces failed by scission of the copolymer chains near the joint between the two blocks. At higher values of SIGMA, the interfaces fractured by first forming a stable craze ahead of the propagating crack tip, giving rise to much higher values of the measured fracture toughness. In this regime, G(c) scaled with SIGMA(eff)2, an areal density of chains with at least one "effective" entanglement, in very good agreement with a model recently proposed by Brown 2 for failure by craze fibril breakdown.