Interfacial fracture toughness between glassy polymer networks

The interfacial fracture energy, G_c, between glassy crosslinked polystyrene samples was studied with the symmetric double-cantilever beam test. Compression-molded polystyrene slabs were crosslinked by γ irradiation in vacuo. The fracture samples were prepared by the welding of two crosslinked polystyrene slabs under contact pressure for 1 h at 150°C, which was above the glass-transition temperature for polystyrene (≅ 104 °C). The fracture toughness tests were performed at room temperature. The interfacial fracture energy was studied as a function of the crosslink density of the slabs. It was found that the interfacial fracture energy decreased with increasing crosslink density. A least-squares fit of the data showed that the interfacial fracture energy scaled as N̄_c^1.35±0.27, where N̄_c is the weight-average chain length between crosslinks (in number of monomer units). A comparison with the predictions made by different models on weak interfaces was made. It was found that if the loops of the network were identified as the connectors responsible for adhesion at the glassy network-network interfaces, the model by Xu et al. predicted that G_c would scale as N_c^3/2, consistent with the trends in the data. Using the expression of G_c from this model to fit the data the value for the static friction constant per monomer for polystyrene was found to be f_mono = 3.5 ± 0.7 × 10^-12 N/monomer, in good agreement with the values found in the literature.