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, PSL and PSS, of degrees of polymerization P-L and P-S, respectively (P-S < P-L), and (2) blends of PSL 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/PSL hosts were smaller than those in PSS/PSL mixtures, for the same TMPC, or PS, weight fractions (P-S approximate to P-TMPC). This is due to more extensive mixing between the TMPC host chains, compared to the PSS 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 PSS or TMPC and the value of P-L. The fraction of TMPC, or PS, mixed with the corona decreased as P-L decreased from P-L = 15400 to smaller values.