Spatial Heterogeneity in a Polymer Thin Film Probed by Single Molecules

Spatial heterogeneity in a polymer melt has been divided into two distributions that are distinguished by different relaxation behaviors of single-molecule probes near the nominal calorimetric glass-transition temperature (T_g). The fluorescence intensity and lifetime of individual molecules in a polymer film each exhibited a bimodal histogram that involved fast- and slow-relaxation sites at T_g − 8 K and T_g + 7 K, whereas a monomodal histogram composed of fast-relaxation sites was observed at T_g + 30 K. Temperature has no important effect on the center of each distribution in these histograms. Instead, occurrences of slow-relaxation sites decreased as the temperature increased from T_g − 8 K to T_g + 7 K and disappeared at T_g + 30 K. Furthermore, a change in the single-molecule fluorescence intensity was traced at fast- and slow-relaxation sites in a polymer film. At T_g − 8 K and T_g + 30 K, no substantial change in fluorescence intensity was observed at each site for ≥3 h, whereas, at T_g + 7 K, the fluorescence intensity varied with an average time constant of 200−300 s at fast- and slow-relaxation sites. This variation comes most likely from a cooperative rearrangement of fast- and slow-relaxation sites, not from a migration of the single molecules between static fast- and slow-relaxation sites.