Design of Hygro-Swelling Polymer Films Exhibiting Thickness-Based Color Shifts

Thin-film hydrophilic polymer coatings represent an effective platform for the development of optical humidity sensors based on color changes. Hydrophilic polymers absorb water molecules from the environment, which leads to spontaneous swelling and a proportional increase in the thickness of the film. When film is deposited on reflective substrate even small changes (a few nanometers) in thickness are sufficient to shift the reflectance spectrum and cause a distinct change in the perceived color. This allows visual and contactless detection of humidity with high sensitivity, the process being fully reversible upon drying. Combined with the ease of fabrication, low cost, and the possibility of chemical tuning, thin-film hydrophilic polymers represent a promising material for integration into next-generation humidity optical sensor platforms.

A new hygrosensitive poly(vinyl alcohol) derivatives comprising grafted poly(N,N-dimethylacrylamide) chains of varied length and graft density were developed recently and has been shown to be a promising materials for humidity detection. The influence of the grafting density and chain length on the sensing properties for two copolymers annealed at three different temperatures (60◦C, 120◦C and 180◦C) was systematically studied. Polymer films deposited by spin-coating method with a thickness of 140-200 nm were investigated spectrophotometrically and the optical constants (refractive index and extinction coefficient) were determined. When exposed to humidity in the range of 5-95% RH the films demonstrated a relative change in thickness between 50% and 100%, as well as a change in the refractive index. It has been shown that hysteresis is influenced by both - the annealing temperature and the structure of the copolymer. The moisture absorbed in the films at 95% RH humidity was evaluated by using the quartz microbalance method. Both structural variations and annealing temperature were found to have an impact on copolymers’ optical and sensing properties.

Acknowledgements: The authors acknowledge the financial support from the Bulgarian National Science Fund under the Grant No. KP-06-COST/4 under COST Action CA21121 “European Network for the Mechanics of Matter at the Nano-Scale” (MecaNano). Research equipment of Distributed Research Infrastructure INFRAMAT, part of Bulgarian National Roadmap for Research Infrastructures, supported by Bulgarian Ministry of Education and Science was used in this investigation.