Nanometer Photothermal Heating and Cooling Dynamics of Azo Polymer Film Elucidated by Analyzing Nanosecond Laser-Induced Expansion/Contraction Behavior

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Abstract

The nanosecond transient expansion and successive contraction dynamics of an azobenzene-substituted urethane-urea copolymer film were studied by nanosecond time-resolved interferometry. In addition, the photoisomerization dynamics of azo dyes during morphological change was investigated by transient absorption measurement. A transiently generated cis isomer absorbed a few photons during laser irradiation and its energy was effectively converted to heat. When the pulse width of the irradiated laser was 8 or 14 ns full-width at half maximum (fwhm), the expansion of the polymer film started with 3–6 ns delay from the temperature elevation, while the film contracted exponentially accompanying a different absorbance change of the transient species. The expansion dynamics was well explained not in terms of volume change due to isomerization dynamics but in terms of thermal expansion involving glass–rubber transition. The film-thickness-dependent contraction dynamics could be mostly interpreted in view of the heat flow toward the quartz substrate, heat dissipation via the film itself, and phase transition of the polymer film from the rubbery to glassy states. The laser-induced transient morphological change of the urethane-urea copolymer film observed below the threshold could be well explained in terms of nanometer heating and cooling dynamics.

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