Micelle formation of amphiphilic polystyrene-b-poly(N-vinylpyrrolidone) diblock copolymer in methanol and water-methanol binary mixtures

The micelle formation by the amphiphilic polystyrene-block-poly(N- vinylpyrrolidone) (PS₄₈-b-PNVP₉₉) copolymer is investigated in methanol and water-methanol binary mixtures of various compositions using ¹H NMR, fluorescence spectroscopy, static/dynamic light scattering (SLS/DLS), and transmission electron microscopy (TEM). Critical micelle concentrations (cmc) are determined by employing fluorescence spectroscopy and DLS measurements. The cmc of the PS₄₈-b-PNVP ₉₉ block copolymer increases with increasing methanol content in the water-methanol binary mixtures, suggesting that methanol is a better solvent for the PS₄₈-b-PNVP₉₉ block copolymer than water-methanol mixtures or pure water. The amphiphilic PS₄₈-b-PNVP₉₉ diblock copolymer forms spherical micelles of R_h ∼ 16 nm in pure methanol solution as revealed by DLS measurements. In contrast, significantly larger micelles having higher aggregation numbers are formed in water-methanol binary mixtures. Temperature dependent data reveal an increase in aggregation number and radius of gyration (R_g) concomitantly with temperature (10-40°C). In contrast, the overall size (R_h) of the micelles remains almost constant over the same temperature range. An explanation is tendered that PNVP coronas dehydrate/desolvate at higher temperatures counteracting the increase in micelle size (R_h) caused by increased aggregation numbers (N_agg).