Segmental mobility of a typical amorphous polymer, polystyrene, at the interface with a solid substrate was examined noninvasively by fluorescence lifetime measurement using evanescent wave excitation. Glass transition temperature (Tg) was discernibly higher at the interface than in the bulk. Measurements at different incident angles of excitation pulses revealed that Tg became higher the closer to the interface. This is the observation for a Tg gradient of polymers at the interface.
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