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Applicability of Vapor-Deposited Thermoresponsive Hydrogel Thin Films in Ultrafast Humidity Sensors/Actuators
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posted on 2020-01-14, 13:09 authored by Fabian Muralter, Francesco Greco, Anna Maria CocliteThermoresponsive
polymers reversibly react to changes in temperature
and water content of their environment (i.e., relative humidity, RH).
In the present contribution, the thermoresponsiveness of poly(N-vinylcaprolactam) thin films cross-linked by di(ethylene
glycol) divinyl ether deposited by initiated chemical vapor deposition
are investigated to assess their applicability to sensor and actuator
setups. A lower critical solution temperature (LCST) is observed at
around 16 °C in aqueous environment, associated with a dramatic
change in film thickness (e.g., 200% increase at low temperatures)
and refractive index, while only thermal expansion of the polymeric
system is found, when ramping the temperature in dry atmosphere. In
humid environment, we observed a significant response occurring in
low RH (already below 5% RH), with the moisture swelling the thin
film (up to 4%), but mainly replacing air in the polymeric structure
up to ∼40% RH. Non-temperature-dependent swelling is observed
up to 80% RH. Above that, thermoresponsive behavior is also demonstrated
to be present in humid environment for the first time, whereas toward
100% RH, film thickness and index appear to approach the values obtained
in water at the respective temperatures. The response times are similar
in a large range of RH and are faster than the ones of the reference
humidity sensor used (i.e., seconds). A sensor/actuator hygromorphic
device was built by coating a thin flower-shaped poly(dimethylsiloxane)
(PDMS) substrate with the thermoresponsive polymer. The large swelling
due to water uptake upon exposure to humid environment at temperatures
below the LCST caused the petals to bend, mimicking the capability
of plants to respond to environmental stimuli via reversible mechanical
motion.