The elastic and dielectric properties of poly-n-isopropyl-acrylamide (PNIPAM) polymer network can be modified due to the volumetric phase transition induced by the change in its temperature. PNIPAM based polymer network contains 95-99% of water and is therefore ideally suited for the design of underwater acoustic meta-material systems. The volumetric phase transition in PNIPAM is abrupt and occurs within 1OC change in temperature around 32-34 OC. The temperature change in the polymer can be induced by an external stimulus, which can be through physical contact such as thermal or electrical stimulation [1]. Non-evasive mode of external stimulation such as magnetic [2], optical [3] or electromagnetic (EM) mode is more attractive due to its adiabatic process of heat transfer in the hydrogel. The external stimulation can be made efficient by incorporating magnetic or dielectric nanoparticles into the polymer network. The alternate way is to control the frequency and power of the stimulating source to optimize the dissipation within the polymer network.

Our objective was to control acoustic wave-propagation through a medium using EM stimulus. Direct measurement shows that the velocity of the ultrasonic waves through the polymer is slower than its velocity through water [4]. It results in a difference in wave propagation through the polymer below and above the phase transition temperature [4]. The dielectric properties of the PNIPAM have also been measured to optimize the response of the polymer to the electromagnetic radiation. Based on the fundamental elastic response of PNIPAM to EM waves, we have fabricated periodic phononic crystal (PhC) structures with transmission bandwidth in the ultrasonic frequency range for potential biomedical application. The PhC structures are embedded with the hydrogel PNIPAM network and can be controlled using EM stimuli such as infrared (IR) or radio-frequency (RF) radiation. The absorption of far-IR light by the polymer results in volumetric phase transition in the PNIPAM matrix. Ultrasonic transducers were used under water as source and receiver for the appropriate impedance matching of the hydrogel filled PhC structures. By controlling the concentration of the polymer in the hydrogel network and the dimension of the PhC structure, the efficiency of the remotely modulated structure can be controlled. We demonstrate IR light stimulated ultrasonic filters with a modulation depth of 95%. [5] The novel polymer based PhC structure opens up a new class of acousto-optic device for ultrasonic applications.

[1] B. Garner, T. Cai, Z. Hu., A. Neogi, Optics Exp. 16, 19410 (2008)

[2] S. Ghosh, Y. Chao, T. Cai, Z. Hu, A. Neogi, J. Phys. D: Appl. Phys. 42, 135501 (2009).

[3] B. Garner, T. Cai, M. Kim, Z. Hu, A. Neogi, Appl. Phy. Exp., 2, 075001 (2009).

[4] E. Walker, D. Reyes, A. Krokhin, A. Neogi, Ultrasonics, 54, 1337 (2014)

[5] E. Walker, D. Reyes, A. Krokhin, A. Neogi, Appl. Phys. Lett. (2014) [In press]

Arup Neogi

University of North Texas

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