Structures of the cylindrical and vesicular micelles of an P4VP-longer asymmetric PS-b-P4VP

Abstract

The control of the self-assembled micellar morphology of amphiphilic block copolymers in a selective solvent has engrossed substantial interests in the theoretical and applied research fields. The micellar morphology of the block copolymers can be controlled by solvent selectivity and block copolymer composition. There are three main factors for controlling these morphologies, i.e., the stretching of core blocks, the repulsive interaction among corona chains, and the surface tension of the core/corona interface at the onset of micellization. The block copolymer can selfassemble into spherical, cylindrical, and vesicular micelles in a given solution. Among those micelles, the vesicle has fascinated scientists because of its diverse applications such as drug delivery diagnostics, electronics (chip nanopatterning), and catalysis (enzyme entrapment). Vesicles (generally kinetically trapped and non-equilibrium) are usually formed by low molecular materials such as surfactants or lipids. Wittmann et al. for the first time reported the polymeric vesicles of poly(styrene-b-isoprene) (PS-b-PI) and poly(styrene-b-ethylene oxide) (PS-b-PEO) in a mixture of ethyl benzene (a selective solvent for both blocks) and n-decane (a PI and PEO selective solvent). Schrage et al. investigated vesicular morphologies of quaternized poly(1-methyl-4vinyl pyridine-b-styrene) and poly(1,2-butadiene-b-(cesium methacrylate)) which were self-assembled in THF (nonselective for PS and poly(1,2-butadiene) but selective for charged blocks). Kesselman et al. also studied the change in the vesicular morphologies of PS-b-PI by controlling solvent selectivity in diethyl phthalate (less PS-selective) and dimethyl phthalate (highly PS-selective) mixtures. Putaux et al. investigated the vesicular morphologies of PS-b-PI in n-heptane and n-decane, both of which are selective solvents for PI. The PS-b-P4VP vesicles have been observed when the P4VP blocks were complexed with other small molecules. The vesicles of PS-b-P4VP in chloroform were observed when perfluorooctanoic acid (PFOA) was added to be complexed with P4VP. Peng et al. also reported another interesting research on the vesicular morphology of PS-b-P4VP in a complex form with low molecular-weight aliphatic acids (stearic acid, decanoate, acetic acid, and formic acid) in chloroform (non-selective for PS-b-P4VP and aliphatic acids but selective in a complex form). But, PS-bP4VP vesicles without complexation were rarely observed. The cylindrical morphology of an asymmetric PS(19600)b-P4VP(5100) (PS longer) in P4VP selective solvents was reported. However, the vesicles with asymmetric PS-b-P4VPs (P4VP longer) in PS selective solvents have not been reported. The block length of P4VP of the PS-b-P4VP is of prime importance because a number of valuable metals or small molecules could be complexed with the P4VP core of micelles (in the PS selective solvents) which is utilized as a nanoreactor. The screening of the solvent selectivity is necessary to check the possible micellar morphologies of the given block copolymer. Recently the micellar structures and their ordered structures of PS(12K)-b-P4VP(11.8K) and PS(19.6K)-b-P4VP (5.1 K) in the toluene/ethanol solvent system were reported. However, we believe that solvent selection for PS-b-P4VP is important in this study. We used the toluene/ethanol solvent system where toluene and ethanol are selective solvents for PS and P4VP, respectively. The toluene/ethanol system covers from PS-selective, to neutral, and to P4VPselective solvents although our previous THF/toluene system covers only from neutral to PS-selective solvents. Thus, we expected that a wider range of the micellar morphologies can be formed in the toluene/ethanol solvent system. The solvent selectivity was narrowly screened by changing the mixing ratio of the two solvents. But the vesicular structure was not found probably due to the less-asymmetric composition of the block copolymer and/or the less selectivity of the used solvent mixtures. The research in this article used a more-asymmetric block copolymer of PS (3.3K)-b-P4VP(18.7 K) (P4VP longer) in the toluene/ethanol mixtures to screen the solvent selectivity.