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-PV P block copolymers of relatively high areal chain density, SIGMA; thes e are designated 800/870, 510/540, and 580/220, where the numbers are the polymerization indices of the dPS and PVP blocks. The critical ene rgy release rate of the interfacial crack, G(c) (fracture toughness), was measured as a function of SIGMA using an asymmetric double cantile ver beam geometry, and fracture mechanisms were studied by transmissio n electron microscopy (TEM) and forward recoil spectrometry (FRES), wh ich permitted the location of the dPS block to be determined. For the asymmetric block copolymer (580/220), we observed spherical micelles o n the PS side of the interface at large SIGMA's and G(c) remained cons tant, indicating that these block copolymer micelles have no effect on G(c). FRES and TEM observations showed that fracture occurred by craz ing in PS followed by craze breakdown, and the locus of the fracture w as the PVP/PS craze interface. In contrast to these results for the as ymmetric block copolymer, we observed that the symmetrical block copol ymers (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 c overage of the interface with block copolymer chains, G(c) began to de crease and finally reached a constant value when the interface was ful ly covered with one additional block copolymer lamella. TEM observatio n showed that the fracture mechanism is crazing on the PS side followe d by craze breakdown at the PVP/PS craze interface. FRES analysis reve aled that for 800/870 fracture took place both within the PS lamella a nd at the interface between outer block copolymer chains of the lamell a 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 tou ghening that can be produced by adding diblock copolymers, especially for symmetric block copolymers. Adding more block copolymer than neede d to saturate the interface actually produces secondary (lamellar) int erfaces which are weaker than the original saturated homopolymer one.