Resumen:
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.