Fracture Toughness of Polymer Interface Reinforced With Diblock Copolymer: Effect of Homopolymer Molecular Weight

We have measured the fracture toughness, G c, of an immiscible polymer/polymer [polystyrene (PS) and poly(2-vinylpyridine) (PVP)] interface reinforced with deuterium-labeled dPS- b -PVP diblock copolymers as a function of the number-average molecular weight, M̄ n, of the polystyrene homopolymer, either monodisperse homopolymer PS (MPS) or polydisperse homopolymer PS (PPS). The dependence of G c on the PS molecular weight was investigated at different areal chain densities, Σ of the copolymers. These values of Σ were chosen to be in either one of two fracture mechanism regimes: chain scission or crazing. In the chain scission regime, G c is independent of the molecular weight of MPS and PPS. In contrast in the crazing regime (for Σ ≤ Σ sat, where Σ sat is the saturation Σ for the copolymer at the interface), M̄ n of MPS has a strong effect on the fracture toughness. For this case, G c increases sharply around M̄ n ≈ 100 000 and then levels off at higher M̄ n values. The polydisperse PS/PVP interface has a fracture toughness consistent with its M̄ n rather than its weight-average molecular weight, M̄ w . When the interface is covered with copolymer lamellae (Σ ≫ Σ sat ), G c is found to be independent of M̄ n of the MPS and is substantially larger than that for the PPS/PVP interface at the same Σ. For the PPS/PVP interface, the low molecular weight portion of PPS swells the lamellar structure, resulting in a decrease in G c compared to that of the MPS/PVP interface. We have also measured G c as a function of composition of a blend of high and low M̄ n MPS, where M̄ n of the low molecular weight PS is below the entanglement molecular weight of PS. Dilution of the entanglement density of the homopolymer polystyrene results in a strong decrease in G c . Our results are compared with recent models for craze failure. A continuum craze model using the full stress field proposed by Sha et al. 20 predicts the fracture toughness better than models 2,9 using the asymptotic stress field.