Functionalizing polymer surfaces by field-induced migration of copolymer additives. 1. Role of surface energy gradients

Field-induced migration of low surface tension polystyrene-b-poly(dimethyl siloxane) (PS-b-PDMS) block copolymer additives in a polystyrene (PS) host was investigated using a series of narrow molecular weight distribution pol ystyrenes (5 x 10(3) < M-W,(PS) < 2 X 10(7) g/mol). Dynamic contact angle a nalysis (DCA) and attenuated total reflection Fourier transform infrared (A TR-FTIR) spectroscopy measurements were used to characterize the surface ch emical make-up of polymer/additive blends created by solvent casting, preci pitation, and melt annealing processes. For all host PS molecular weights i nvestigated, selective dimethyl siloxane (DMS) enrichment of the air/polyme r interface was observed. Near saturation levels of DMS surface coverage we re found in PS/PS-b-PDMS blends with bulk copolymer additive compositions a s low as 2 wt %. The surface excess concentration of DMS groups Delta phi ( DMS) in these blends could be manipulated, however, by careful selection of the host PS and PS-b-PDMS additive molecular weight, annealing conditions, and physicochemical properties of materials comprising the bounding interf aces. The effect of the first two factors on Delta phi (DMS) can be summari zed by an approximate scaling relation, Delta phi (DMS) similar to M-wPS al pha, where the scaling exponent ct is a function of annealing conditions. F or unannealed PS/PS-b-PDMS blend samples, alpha approximate to -0.5, wherea s for samples annealed for prolonged periods at elevated temperatures a val ues in the range 0.1-0.15 were found. Surprisingly, PS/PS-b-PDMS blend samp les annealed at elevated temperatures near enhanced DMS concentration at th e polymer/metal of the molecular weight dependence of diffusivity, host pol ystyrene.