Simultaneous Determination of Critical Micelle Temperature and Micelle Core Glass Transition Temperature of Block Copolymer-Solvent Systems via Pyrene-Label Fluorescence

The critical micelle temperature (CMT) and micelle core glass transition temperature (T-g) for a poly(methyl methacrylate) (PMMA)-poly(tert-butyl methacrylate) (PtBMA) diblock copolymer system were measured by fluorescence via single temperature (T) ramps. Synthesis yielded identical block lengths in unlabeled and pyrene-labeled diblocks, the latter with dye at the PMMA block terminus. Studies were conducted at 5-18 wt % diblock in 2-ethylhexanol (2EH) with a trace of labeled diblock (0.2 wt 96 of total copolymer). The T dependence of pyrene-label fluorescence intensity yielded the CMT and micelle core T-g in systems where the PMMA-block and the 2EH within the cores constituted 1.9-7.8% of sample mass. While the CMT can be measured by many methods, this is the first direct measurement of micelle core T-g at low core content (e.g., 1.9 wt %) in a block copolymer/solvent system. Differential scanning calorimetry was done on diblock samples, showing severe limitations for sensing and characterizing core T-g. Fluorescence from trace levels of labeled diblock was used with 5-20 wt % PMMA-poly(n-butyl acrylate) PMMA triblocks in 2EH. The micelle core T-g is important in triblock systems that form thermoreversible gels because it fundamentally underlies the viscoelastic to elastic gel transition. Fluorescence results demonstrated the dependence of the CMT and the near invariance of the micelle core T-g on core-block molecular weight in these diblock and triblock systems for PMMA blocks with M-n = 15-25 kg/mol and solvent in the micelle core.