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

• Published in: Ultrasonics Vol. 54; no. 5; pp. 1337 - 1340
• Main Authors: Walker, Ezekiel, Reyes, Delfino, Krokhin, Arkadii, Neogi, Arup
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.07.2014
• Subjects: Acoustic cavity; Actuators; Fabry–Pérot resonance; Holes; Hydrogels; Phase transformations; Reflection; Sound; Sound velocity; Ultrasound; Acoustic cavity; Fabry–Pérot resonance
• ISSN: 0041-624X; 1874-9968
• DOI: 10.1016/j.ultras.2014.01.014

•Direct measurement of temperature dependent sound velocity in hydrogel.•Anamolous dispersion of sound observed in PNIPAM hydrogel.•Compared to the propagation of sound in water, it is characteristically different in hydrogel which has over 90% water. 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.