Temperature and time dependence of copolymer mixtures on interfacial adhesion at PS/PMMA interfaces

The effect of copolymer mixtures on the interfacial adhesion between slabs of PS and PMMA was investigated as a function of composition, time and temperature using the asymmetric double cantilever beam (ADCB) method. The nature of the interface was further probed using atomic force microscopy (AFM) and dynamic secondary ion mass spectroscopy (D-SIMS). The results show that mixtures of graft and block copolymers are much more effective than pure block copolymers in enhancing the interfacial adhesion. The most effective mixture consisted of a block copolymer of molecular weight 70 K and a copolymer with two PS grafts of molecular weight 30 K. This mixture yielded an interfacial fracture toughness of G_c = 127.5 J/m² as compared with G_c = 38.2 J/m² and G_c = 3.5 J/m² for the pure block and graft copolymer, respectively. G_c at the PS/PMMA interface reinforced only with block copolymer was maximal after an annealing temperature of 150°C for 1 hr. It decreased by an order of magnitude when the temperature was increased to 180°C or the joining time was increased from 1 to 10 hours. Go at the interface reinforced with a graft/diblock copolymer mixture was also maximum at an annealing temperature of 150°C but it decreased only by a factor of 2 with increasing joining time or temperature. Dynamic Secondary Ion Mass Spectroscopy (DSIMS) data show that this effect may be due to decrease in the diffusion of the copolymer from the interface when the mixture is present, i.e, the diblock copolymer is trapped within the graft copolymer.