Nanometric Inhomogeneity of Polymer Network Investigated by Scanning Near-Field Optical Microscopy

The structural inhomogeneity of the poly(methyl methacrylate) (PMMA) network was studied by scanning near-field optical microscopy (SNOM), which provides us optical images and spectroscopic information in a local area with a spatial resolution of several tens of nanometers. The optically transparent PMMA network was labeled with fluorescent dyes either at the network chain or at the cross-linking points. The spatial distribution of the chain segments and cross-links could be directly visualized in real space, and the PMMA network was found to have an inhomogeneous structure in a scale of submicrons. Nanosecond dynamics of the energy transfer among the dyes introduced to the side chain was also examined in a nanometric area by the time-correlated photon counting system combined with SNOM. The fluorescence decay through the near-field excitation showed that the local segment density was ca. 10 times higher than the ensemble average density. The local segment density had little correlation with the structure in a submicron scale observed by SNOM. These findings indicated that the PMMA network has structural hierarchy.