OPTIMUM TOUGHENING OF HOMOPOLYMER INTERFACES WITH BLOCK-COPOLYMERS

We have investigated the fracture toughness and fracture mechanisms of planar interfaces between polystyrene (PS) and poly(2-vinylpyridine) (PVP), which were reinforced with a series of deuterium-labeled dPS-PVP block copolymers of relatively high areal chain density, SIGMA; these are designated 800/870, 510/540, and 580/220, where the numbers are the polymerization indices of the dPS and PVP blocks. The critical energy release rate of the interfacial crack, G(c) (fracture toughness), was measured as a function of SIGMA using an asymmetric double cantilever beam geometry, and fracture mechanisms were studied by transmission electron microscopy (TEM) and forward recoil spectrometry (FRES), which permitted the location of the dPS block to be determined. For the asymmetric block copolymer (580/220), we observed spherical micelles on the PS side of the interface at large SIGMA's and G(c) remained constant, indicating that these block copolymer micelles have no effect on G(c). FRES and TEM observations showed that fracture occurred by crazing in PS followed by craze breakdown, and the locus of the fracture was the PVP/PS craze interface. In contrast to these results for the asymmetric block copolymer, we observed that the symmetrical block copolymers (510/540 and 800/870) formed lamellae at the interface at large SIGMA's and that this lamellar structure affected G(c). In both cases, after exhibiting a maximum at SIGMA corresponding to the saturation coverage of the interface with block copolymer chains, G(c) began to decrease and finally reached a constant value when the interface was fully covered with one additional block copolymer lamella. TEM observation showed that the fracture mechanism is crazing on the PS side followed by craze breakdown at the PVP/PS craze interface. FRES analysis revealed that for 800/870 fracture took place both within the PS lamella and at the interface between outer block copolymer chains of the lamella and PS homopolymer, while fracture took place between the dPS block brush at the saturated interface and the dPS block of the lamella for 510/540. These results show that there are limits to the interface toughening that can be produced by adding diblock copolymers, especially for symmetric block copolymers. Adding more block copolymer than needed to saturate the interface actually produces secondary (lamellar) interfaces which are weaker than the original saturated homopolymer one.