Anomaly in the coefficient of performance of the volume phase transition process of poly(N-isopropylacrylamide) gels induced by mechanical stress

Tochishita, Naoya; Horinaka, Jun-ichi; Takigawa, Toshikazu

doi:10.1038/pj.2016.17

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Published: 24 February 2016

Physical Properties of Polymers

Anomaly in the coefficient of performance of the volume phase transition process of poly(N-isopropylacrylamide) gels induced by mechanical stress

Naoya Tochishita¹,
Jun-ichi Horinaka¹ &
Toshikazu Takigawa¹

Polymer Journal volume 48, pages 741–744 (2016)Cite this article

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It is well known that poly(N-isopropylacrylamide) (PNIPA) hydrogels show the volume phase transition if the crosslink density and polymer content are appropriately controlled.^{1, 2, 3} In our previous paper,⁴ we defined the coefficient of performance (the efficiency) (c) for a gel actuator driven by the volume-phase transition. During the swollen-to-collapsed (that is, shrinking) transition of PNIPA gels due to weights, heat is absorbed, and the shrinkage generates mechanical work.^{4, 5, 6, 7} Hereafter, we refer to this latent heat as the transition enthalpy (ΔH) because a constant pressure condition is assumed for the transition. Using ΔH, c in this isothermal process can be defined as⁴

where, f is the force acting on the gel, and Δl is the length change of the gel due to transition. The product fΔl corresponds to the work performed by the weights (ΔW): ΔW=fΔl. During the shrinking transition of PNIPA gels due to weights, Δl is negative while ΔH is always positive. Thus, c>0. We also demonstrated that Δl and ΔH are related to each other, as shown below.⁴

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Department of Material Chemistry, Kyoto University, Kyoto, Japan
Naoya Tochishita, Jun-ichi Horinaka & Toshikazu Takigawa

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Naoya Tochishita
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Jun-ichi Horinaka
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Toshikazu Takigawa
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Correspondence to Toshikazu Takigawa.

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Tochishita, N., Horinaka, Ji. & Takigawa, T. Anomaly in the coefficient of performance of the volume phase transition process of poly(N-isopropylacrylamide) gels induced by mechanical stress. Polym J 48, 741–744 (2016). https://doi.org/10.1038/pj.2016.17

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Received: 20 November 2015
Revised: 05 January 2016
Accepted: 06 January 2016
Published: 24 February 2016
Issue date: June 2016
DOI: https://doi.org/10.1038/pj.2016.17

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