Abstract
Thermoresponsive polymers undergo a reversible phase transition at their lower critical solution temperature (LCST) from a hydrated hydrophilic state at temperatures below the LCST to a collapsed hydrophobic state at higher temperatures. This results in a strong response to temperature when in aqueous environment. This study shows that hydrogel thin films synthesized by initiated chemical vapor deposition show fast and strong response to temperature also in water vapor environment. Thin films of cross-linked poly(N-isopropylacrylamide), p(NIPAAm), were found to have a sharp change in thickness by 200% in water vapor at temperatures above and below the LCST. Additionally, the stimuli-responsive poly(N,N-diethylacrylamide) was investigated and compared to results found for p(NIPAAm). Analysis of the swelling kinetics performed with in situ spectroscopic ellipsometry with variable stage temperature shows differences for swelling and deswelling processes, and a hysteresis in the thickness profile was found as a function of temperature and of temperature change rate.
Highlights
Smart stimuli-responsive artificial hydrogels are water insoluble polymer networks, which respond to stimuli in their environment, for example, temperature, pH, and light
(NIPAAm-co-DEGDVE) and p(DEAAm-co-DEGDVE) polymer thin films with different monomer ratios were synthetized by initiated chemical vapor deposition (iCVD)
The differences measured for NIPAAm could be attributed to the faster response of this polymer to temperature changes, affecting its chain rearrangement. This characteristic is crucial when designing sensors based on thin-film NIPAAm-based polymers, as the temperature rate variation should be limited to have a precise response of the thermoresponsive polymer
Summary
Smart stimuli-responsive artificial hydrogels are water insoluble polymer networks, which respond to stimuli in their environment, for example, temperature, pH, and light. A reason for this slower response may be the lack of NH moieties and that the hydrogel cannot form intermolecular hydrogen bonds in the dehydrated state.[25] p(DEAAm-co-DEGDVE) polymers cannot transport water in and out of the network as fast as p(NIPAAmco-DEGDVE) ones For the NIPAAm-based polymer, the LCST was found at 29 °C, whereas for the DEAAm-based polymer, the value was found at 35 °C (data not shown) These values are in line with what was reported in the literature for similar iCVD polymers.[21] Differences between the LCST in humidity and liquid water point out that the kinetic processes and chemical rearrangements the polymers undergo with temperature are influenced by the physical state of the water molecules. The slower kinetic profile can be attributed to the lack of free N−H moieties
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