Glass transition behavior of polystyrene blocks in the cores of collapsed dry micelles tethered by poly(dimethylsiloxane) coronae in a PS-b-PDMS diblock copolymer

The micellization of a polystyrene-b-poly(dimethylsiloxane) (PS-b-PDMS) diblock copolymer with the number-average molecular weights (M-n) of 193 000 g/mol for PS block and 39 000 g/mol for PDMS block and the glass transition behavior of the PS blocks in the cores of collapsed dry micelles with PDMS tethered coronae were studied. A range of micelle morphologies was observed by varying the micellization conditions and varying the ratio of the methylene chloride (MC) solvent and the n-dodecane (D) selective nonsolvent. MC is a solvent for both of the blocks, but D is a selective solvent for only the PDMS block. After completely removing the solvents below the glass transition temperature (T-g) of the initially solvated PS blocks in MC via extraction and vacuum-drying, the collapsed dry micelles, which were originally spheres and cylinders, became "tablets" or "ribbons". The diameters of the "tablets" and widths of the "ribbons" were approximately 50 and 100 nm, respectively. The thicknesses of these textures ranged from 20 to 40 nm. The PS block cores of the dried micelles exhibited a Tg very close to that of the homo-PS bulk samples with the same M-n. However, for a broad temperature range below the T-g(PS), the collapsed dry micelles possessed heat capacity (C-p) values higher than the addition scheme calculated Cp based on PS and PDMS in their solid and liquid bulk states; whereas the C-p values of the liquid state above the T-g(PS) fit well with the calculated data. These observations support the idea of PS interface layers with mobility induced by the liquid PDMS coronae such that only the inner cores of the PS blocks were in the glassy state. In a temperature region between 0 and 50 degreesC, the lower limit of the interface thickness (i.e., assuming that the mobile PS layers possessed the same mobility as the PS liquid) was estimated to be approximately 2 nm for the collapsed dry micelles. The interface thickness slightly increased with increasing temperature.