Temperature‐Induced Swelling and De‐swelling of Thin Poly(N‐Isopropylacrylamide) Gels in Water: Combined Acoustic and Optical Measurements

Abstract

The temperature‐induced swelling and deswelling of thin layers of poly(N‐isopropylacrylamide) gels in water was measured as a function of cross‐link density and thickness. The collapse behavior was probed via an in situ combination of a quartz‐crystal microbalance (QCM) and a surface plasmon resonance (SPR) spectrometer. The shifts in the SPR coupling angle are explained in terms of decrease of the refractive index inside the film. The evanescent optical wave mostly probes the film's interior properties. The acoustic shear wave emanating from the quartz resonator, on the other hand, propagates to the outer surface of the film, unless the film is very dilute. The acoustic data are dominated by the changes in thickness, rather than in its viscosity. The combination of acoustic and optical measurements, therefore, provides complementary information on the film that can be exploited for sensing applications.

Keywords:

• Quartz crystal resonator
• pNIPAm
• Swelling
• Surface plasmon resonance

Acknowledgments

We thank Bernhard Menges for help with the fabrication of the quartz plates with corrugation gratings. This project was supported by the International Max Planck Research School for Polymer Materials Science.

Notes

^aSee, for example Ref. [10].