Micellar formation and organization in thin film polymer blends

We study the formation and organization of micelles, of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer chains, in thin film hosts composed of (1) blends of long and short chain polystyrenes, PS _L and PS _S, of degrees of polymerization P _L and P _S, respectively (P _S < P _L), and (2) blends of PS _L and a homopolymer with which it is compatible, tetramethyl bisphenol-A polycarbonate (TMPC), of degree of polymerization P _TMPC (P _TMPC < P _L). The role of competing entropic and enthalpic interactions on the block copolymer micelle formation and organization is examined. We show that the average size of the micelle cores, D _core, decreased with increasing weight fraction of the shorter chain PS component in PS(P _S = 125)/PS(P _L = 15400) mixtures, for P _L>N _PS, N _PS is the degree of polymerization of the PS block (the micelle corona). D _core also decreased with increasing weight fraction of TMPC in TMPC(P _TMPC = 122)/PS(P _L = 15400) mixtures. The values of D _core in the TMPC/PS _L hosts were smaller than those in PS _S/PS _L mixtures, for the same TMPC, or PS _S, weight fractions (P _S ≈ P _TMPC). This is due to more extensive mixing between the TMPC host chains, compared to the PS _S chains, and the micelle corona. Furthermore, we show that the size and the organization of the micelles within the films may be controlled independently, through changing the relative fractions of PS _S or TMPC and the value of P _L. The fraction of TMPC, or PS _S, mixed with the corona decreased as P _L decreased from P _L = 15400 to smaller values.