Fracture toughness of polymer interface reinforced with diblock copolymer: Effect of homopolymer molecular weight
Journal
Macromolecules
Journal Volume
29
Journal Issue
23
Pages
7536-7543
Date Issued
1996
Author(s)
Abstract
We have measured the fracture toughness, Gc, 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, M̄n, of the polystyrene homopolymer, either monodisperse homopolymer PS (MPS) or polydisperse homopolymer PS (PPS). The dependence of Gc 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, Gc is independent of the molecular weight of MPS and PPS. In contrast in the crazing regime (for Σ ≤ Σsat, where Σsat is the saturation Σ for the copolymer at the interface), M̄n of MPS has a strong effect on the fracture toughness. For this case, Gc increases sharply around M̄n ≈ 100 000 and then levels off at higher M̄n values. The polydisperse PS/PVP interface has a fracture toughness consistent with its M̄n rather than its weight-average molecular weight, M̄w. When the interface is covered with copolymer lamellae (Σ ≫ Σsat), Gc is found to be independent of M̄n 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 Gc compared to that of the MPS/PVP interface. We have also measured Gc as a function of composition of a blend of high and low M̄n MPS, where M̄n 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 Gc. 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 models2,9 using the asymptotic stress field.
Type
journal article