Hydration Kinetics of Thermo- and Light-Responsive Azobenzene-Based Copolymer Films under Water Atmosphere and UV Irradiation

Thin films from copolymers of N,N-dimethylacrylamide (DMAm) and 4-acrylamidoazobenzene (AzBnAm) are examined in situ for their photoresponsiveness. The kinetics and equilibrium states during swelling and irradiation in water vapor are investigated using time-of-flight neutron reflectometry (ToF-NR) and Fourier-transform infrared (FTIR) spectroscopy. When the polymer film swells in a humid atmosphere, FTIR results show that, in thicker films, the tertiary amides of the DMAm units are hydrated earlier and to a larger extent than the secondary amides of the AzBnAm units. Upon irradiation with ultraviolet (UV) light at 365 nm, the azobenzene groups undergo E-to-Z isomerization, and the polymer film experiences an increase in hydration, which is larger in the thicker film used for FTIR measurements. FTIR signals reveal that the increase in hydration mainly takes place in the vicinity of the azobenzene moieties, where the increased polarity of the Z- compared to the E-isomers is most likely the driving force of enhanced hydration.