Kinetics of UV Radiation-Induced Fast Collapse and Recovery in Thermally Cycled and Rehydrated Light- and Thermo- Double-Responsive Copolymer Films Probed by In Situ Neutron Reflectivity

The kinetics of UV radiation-induced fast collapse andrecoveryin thermally cycled and rehydrated light- and thermo- double-responsivecopolymer films of poly(oligo(ethylene glycol) methyl ether methacrylate-co-6-(4-phenylazophenoxy)hexyl acrylate), abbreviated asP(OEGMA(300)-co-PAHA), are probed by in situ neutron reflectivity (NR). The copolymer film isexposed to a thermal treatment starting at a temperature of 60 & DEG;C,which is well above its transition temperature (TT = 53 & DEG;C) beforethe temperature is rapidly decreased from 60 to 23 & DEG;C. Basedon the applied protocol, the initially collapsed P(OEGMA(300)-co-PAHA) film is rehydrated due to the switchingof polymer chains from a more hydrophobic to a more hydrophilic statewhen the temperature falls below its TT. The whole rehydration processcan be divided into 3 stages: D2O absorption, chain rearrangement,and film reswelling. After rehydration, the thermally cycled P(OEGMA(300)-co-PAHA) film is switched by UV irradiationvia setting the UV radiation on and off. Considering the UV-inducedcollapse and recovery, both processes are slower than those observedin freshly hydrated films without any thermal stimulus history. Therefore,the experienced thermal history of the film should be considered inthe design of sensors and detectors based on double-responsive copolymerfilms.

Automated summary

The kinetics of UV radiation-induced collapse and recovery in thermally cycled and rehydrated double-responsive copolymer films were studied using in situ neutron reflectivity. The thermal treatment of the film above its transition temperature followed by rapid cooling induced rehydration of the collapsed film. The rehydration process consisted of D2O absorption, chain rearrangement, and film reswelling. UV irradiation-induced collapse and recovery processes were slower in films with thermal stimulus history, suggesting the importance of considering the thermal history in the design of sensors and detectors based on double-responsive copolymer films.