Length Scales Necessary for Proper Averaging to Characterize Polymerization in Nanosystems: Topochemical Polymerization of Diacetylene Nanocrystals Dispersed in a Polystyrene Matrix As Probed by Confocal Raman Microscopy

The scales necessary to make appropriate spatial averaging on solid-state polymerization were investigated by confocal Raman microscopy with a mapping resolution of 2 μm. Nanocrystals of an aliphatic diacetylene with an average size of 0.14 μm, each separated by 0.3 μm on average, were dispersed in a polystyrene matrix and were polymerized by UV irradiation. The distribution of nanocrystals was inhomogeneous over approximately 20 μm scale. A large crystal of the same monomer shows that photoinitiation is already averaged at the microscope resolution, while the color transition from the blue to the red form requires a scale greater than 5 μm. For the nanocrystals at low conversion, UV–vis absorption spectroscopy measured over a centimeter scale indicates linear polymerization kinetics and a higher polymer yield at a higher temperature. By contrast, the Raman microscopy reveals that, whereas the 20 μm region of high monomer concentrations yields more polymers at −24 °C, the region of low monomer concentrations gives more polymers at 20 °C. We propose thermal initiation, which is not efficient in the large crystals, as an additional initiation process for the apparent discrepancy, implying that the initiation process is not averaged below 20 μm scale for the dispersed nanocrystals.