Abnormal fast dehydration and rehydration of light- And thermo-dual-responsive copolymer films triggered by UV radiation

Abnormal fast dehydration and rehydration of light- and thermo-dual-responsive copolymer films of poly(oligo(ethylene glycol) methyl ether methacrylate-co-6-(4-phenylazophenoxy)hexyl acrylate), abbreviated as P(OEGMA₃₀₀-co-PAHA), are triggered by UV radiation. Both rapid kinetic processes are probed byin situneutron reflectivity (NR). The transition temperatures (TTs) of P(OEGMA₃₀₀-co-PAHA) are 53.0 (ambient conditions) and 52.5 °C (UV radiation,λ= 365 nm). Thin P(OEGMA₃₀₀-co-PAHA) films show a random distribution of OEGMA₃₀₀and PAHA segments. They swell in a D₂O vapor atmosphere at 23 °C (below TT) to a swelling ratiod/d_as-prepof 1.61 ± 0.01 and exhibit a D₂O volume fractionφ(D₂O) of 39.3 ± 0.5%. After being exposed to UV radiation for only 60 s,d/d_as-prepandφ(D₂O) significantly decrease to 1.00 ± 0.01 and 13.4 ± 0.5%, respectively. Although the UV-inducedtrans-cisisomerization of the azobenzene in PAHA induces increased hydrophilicity, the configuration change causes a breaking of the intermolecular hydrogen bonds between OEGMA₃₀₀and D₂O molecules and unexpected film shrinkage. As compared to thermal stimulus-induced dehydration, the present dehydration rate is 100 times faster. Removal of the UV radiation causes immediate rehydration. After 200 s,d/d_as-prepandφ(D₂O) recover to their hydrated states, which is also 30 times faster than the initial hydration. At 60 °C (above TT), thin P(OEGMA₃₀₀-co-PAHA) films switch to their collapsed state and are insensitive to UV radiation. Thus, the UV-induced fast dehydration and rehydration depend on the existence of hydrogen bonds.