The Origin of Heterogeneity of Polymer Dynamics near the Glass Temperature As Probed by Defocused Imaging

Single molecule defocused wide-field fluorescence microscopy (SMDWM) has been used to monitor the 3D reorientation of single molecules in a thin polymer film (similar to 300 nm) of monodisperse poly(n-butyl methacrylate) near the glass temperature (T-epsilon). Stroboscopic illumination allows for estimating reliable correlation times of single molecule rotational diffusion owing to the drastic lengthening of the observable trajectories. We demonstrate that homogeneity is restored similar to 19 K above the T-g determined with calorimetry. The rotational correlation times obtained from SMDWM show similar temperature dependence as the ones measured with established bulk measurements, such as dielectric spectroscopy and rheology, on the same polymer sample. Single molecular reorientation is coupled to the segmental rather than terminal relaxation of the surrounding polymer matrix. SMDWM revealed that spatial heterogeneity is more pronounced than temporal heterogeneity within the measurement time scale (hours to days), whereas this information is hidden in the bulk measurement.