Functionalizing polymer surfaces by surface migration of copolymer additives: role of additive molecular weight

Surface migration of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer additives in polystyrene (PS) hosts was investigated using a series of narrow molecular weight distribution polystyrenes (5×103<MPS<2×107 g mol−1). Dynamic contact angle (DCA) analysis and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy measurements were used to characterize the surface chemical make-up of polymer/additive blends created by solvent casting, precipitation, and melt annealing processes. Selective surface enrichment of methyl methacrylate (MMA) groups was observed at the air/polymer interface for PS hosts with molecular weights substantially larger than that of the PS-b-PMMA additives. Surfaces of PS/PS-b-PMMA blends containing high molecular weight copolymer were, on the contrary, found to be depleted of MMA groups. An approximate relationship between surface excess concentration of PMMA ΔΦPMMA and polystyrene molecular weight MPS, ΔΦPMMA∼MPS0.8±0.04 for MPS≤105 g mol−1, could be defined in blends containing PS-b-PMMA additives of fixed molecular weight Ma and concentration. A somewhat different scaling form ΔΦPMMA∼Ma−0.53±0.05 was found to describe the effect of PS-b-PMMA additive molecular weight on PMMA surface excess in PS/PS-b-PMMA blends with fixed MPS. These results are discussed in terms of surface migration mechanisms based on entropic driving forces and the molecular-weight dependence of the mutual diffusion coefficient of PS-b-PMMA in PS.