In situ measurement of humidity induced changes in the refractive index and thickness of polyethylene glycol thin films

Abstract

Humidity induced changes in the refractive index and thickness of polyethylene glycol (PEG) thin films are in situ determined by optical waveguide spectroscopy. PEG brushes are covalently attached to the surface of a thin gold film on a borosilicate crown glass (BK7) using a grafting-from chemical synthesis technique. The measurements are carried out in an attenuated total internal reflection setup. At low humidity levels, both the refractive index and the thickness change gradually due to swelling of the PEG thin films upon water intake. At around 80% relative humidity, a steep decrease in the refractive index and a steep increase in the thickness are observed as a result of a phase change from a semicrystalline state to a physical gel state. The hydrogenation of PEG films causes a less pronounced phase change from a semicrystalline state to a gel state. Due to fewer ether oxygen atoms available for the water molecules to make hydrogen bonding, the polymer has a more stable structure than before and the phase change is observed to shift to higher humidity levels. It is discussed that such a humidity induced change in the index of refraction can be utilized in constructing of a PEG based humidity sensor.

Study Viscoelasticity of Ultrathin Poly(oligo(ethylene glycol) methacrylate) Brushes by a Quartz Crystal Microbalance with Dissipation

“…For all the figures in this paper, we applied the dry film thickness instead of the wet film thickness for the following two reasons: (i) We found that the ratio ( R = t f,wet / t f,dry ) was a constant for poly(OEGMA) brushes of different thickness but the same density, where t f,dry and t f,wet are the thickness of poly(OEGMA) in the dry state and in contact with water, respectively. In Figure , the value of R was found to be ∼2 and the fitted A n values were close to the reported value . (ii) Practically, measurement of dry film thickness is more feasible than the measurement of wet film thickness.…”

High-Performance Humidity Sensing in π-Conjugated Molecular Assemblies through the Engineering of Electron/Proton Transport and Device Interfaces

“…44,49 This simplified scenario does not apply here for OEG-BTBT films, where a detailed determination of how water interacts with OEG-BTBT and the derivation of a simple expression for the calibration curve using such devices become very challenging. Regarding structural stability, PEG thin films are known to undergo a phase change into gel-like structure at RH > 80%, 50 thus suffering from a critical dewetting when exposed to elevated humidity (Figure S22a−d). 44 On the other hand, OEG-BTBT displays remarkable stability, as no macroscopic changes are detected upon exposure to high RH (Figure S22e−h).…”