Anomalous temperature dependence of speed of sound of bulk poly(N-isopropylacrylamide) hydrogels near the phase transition

Ezekiel Walker; Delfino Reyes; Arkadii Krokhin; Arup Neogi

doi:10.1016/j.ultras.2014.01.014

Anomalous temperature dependence of speed of sound of bulk poly(N-isopropylacrylamide) hydrogels near the phase transition

Ultrasonics. 2014 Jul;54(5):1337-40. doi: 10.1016/j.ultras.2014.01.014. Epub 2014 Feb 12.

Authors

Ezekiel Walker¹, Delfino Reyes², Arkadii Krokhin³, Arup Neogi³

Affiliations

¹ University of North Texas, Department of Physics, Denton, TX 76201, USA. Electronic address: EzekielWalker@my.unt.edu.
² Universidad Autónoma del Estado de México, Toluca 50120, Mexico.
³ University of North Texas, Department of Physics, Denton, TX 76201, USA.

PMID: 24589257
DOI: 10.1016/j.ultras.2014.01.014

Abstract

Bulk Poly(N-isopropylacrylamide) (PNIPAm) hydrogels are thermally responsive polymers that undergo a sharp volumetric phase transition around its lower critical solution temperature of 33 °C. The physical characteristics of bulk, micro-, and nano-form PNIPAm hydrogel have been well-studied, and have applications ranging from biomedical devices to mechanical actuators. An important physical characteristics which reveals lack of available information is speed of sound. Prior studies have utilized Brillouin scattering, multi-echo reflection ultrasound spectroscopy, the sing-around method, and others in measuring the speed of sound. We use a planar resonant cavity with bulk PNIPAm hydrogel in aqueous solution to determine the temperature dependent speed of sound around the lower critical solution temperature. The results show sharp nonmonotonic behavior of the sound velocity in vicinity of the phase transition.

Keywords: Acoustic cavity; Fabry–Pérot resonance.