FRACTURE MECHANISMS OF POLYMER INTERFACES REINFORCED WITH BLOCK COPOLYMERS - TRANSITION FROM CHAIN PULLOUT TO CRAZING

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 deuterium-labeled dPS/PVP block copolymer, which has a long dPS block (polymerization index N(dPS) = 580) and an intermediate length PVP one (N(PVP) = 220). The critical energy release rate of an interfacial crack, G(c)(or fracture toughness), was measured as a function of the areal chain density of the block copolymer, SIGMA, using an asymmetric double cantilever beam geometry. The fracture mechanisms of the interface were studied by transmission electron microscopy (TEM) and forward recoil spectrometry (FRES) which permitted the location of the dPS block to be determined after fracture. At a critical areal chain density, SIGMA* = 0.04 chains/nm2, a transition in the fracture mechanisms from chain pullout of the PVP block (SIGMA < SIGMA*) to crazing (SIGMA > SIGMA*) followed by fracture of the craze was observed. At this SIGMA*, a discontinuous transition in G(c) was also observed. G(c) linearly increased with increasing SIGMA when SIGMA < SIGMA*, exhibited a jump of approximately 10 J/m2 at SIGMA = SIGMA*, and then remained approximately constant for SIGMA > SIGMA*. Using the transition value of SIGMA*, we calculated a static friction coefficient per monomer for the pullout of the PVP block to be f(mono) = 6.3 X 10(-12) N/monomer.