Dynamic Heterogeneity in the Trommsdorff Effect of Bulk Polymerization Probed by Diffracted X-ray Blinking

The Trommsdorff effect is a pronounced reaction acceleration during bulk radical polymerization, as reported for methyl methacrylate (MMA) [R. G. W. Norrish and R. R. Smith, Nature 150, 336 (1942)]. It has commonly been explained in terms of increasing viscosity and the resulting suppression of termination. However, the polymerizing medium does not simply become more viscous. It transforms from a monomer-rich liquid into a polymer-rich amorphous medium and eventually undergoes vitrification. Marked changes in amorphous structure during reaction acceleration have also been reported [Y. Suzuki et al., Polym. J. 56, 1005 (2024)]. Yet it remains unclear whether the reduction in local mobility proceeds uniformly or becomes increasingly heterogeneous, and how this behavior is related to amorphous structural evolution. Here, we investigate MMA bulk polymerization using diffracted X-ray blinking (DXB) [H. Sekiguchi et al., Sci. Rep. 8, 17090 (2018)]. Dispersed ZnO nanocrystals serve as local probes: their small rotational motions alter the Bragg condition and produce time-dependent fluctuations in diffraction intensity. The fluctuation timescales provide information on local mobility around the probes. The same diffraction images contain the broad amorphous halo from the MMA/poly(methyl methacrylate) (PMMA) matrix, allowing structural evolution and local mobility to be followed on the same time axis. Near the onset of the Trommsdorff effect, the amorphous halo exhibited an abrupt change, while ZnO diffraction blinking showed a broader range of fluctuation timescales, suggesting the coexistence of rapidly and slowly fluctuating components. These results suggest that reaction acceleration is accompanied by heterogeneous local mobility coupled with amorphous structural evolution. Small-angle X-ray blinking (SAXB) was also applied to examine its potential for probing mesoscale structural dynamics during polymerization.