Single molecule diffusion and its heterogeneity during the bulk radical polymerization of styrene and methyl methacrylate

The diffusion of perylene diimide dye molecules of clearly distinct size was observed during the thermally initiated polymerization of styrene and methyl methacrylate (MMA) at room temperature. A combination of fluorescence correlation spectroscopy and widefield fluorescence microscopy allowed us to cover the entire conversion range from free diffusion in the monomer solution to immobilization in the produced polymer glass. The evolution of diffusion coefficients differs significantly in both systems. The diffusion coefficient of both dyes decreases clearly more rapidly during the polymerization of MMA, i.e. at the same conversion, diffusion coefficients in the polymerizing MMA solution are clearly lower compared to the polymerizing styrene solution. Extrapolation of the diffusion of the dyes to the diffusion of polymer chains can explain the more pronounced Trommsdorff effect during the polymerization of MMA. In contrast to other methods, single molecule fluorescence microscopy allows for a direct visualization of heterogeneities in single molecule motion. Our measurements point to the fact that the reason for the distinct behaviour of both polymerization systems lies in structural heterogeneities evolving in the PMMA/MMA system already at rather low conversion. In contrast, during the polymerization of styrene, only minor heterogeneities were observed.