Abstract
The applicability of the Clapeyron equation to the volume phase transition of cylindrical poly(N-isopropylacrylamide)-based gels under external force is reviewed. Firstly, the equilibrium conditions for the gels under tension are shown, and then we demonstrate that the Clapeyron equation can be applied to the volume phase transition of polymer gels to give the transition entropy or the transition enthalpy. The transition enthalpy at the volume phase transition obtained from the Clapeyron equation is compared with that from the calorimetry. A coefficient of performance, or work efficiency, for a gel actuator driven by the volume phase transition is also defined. How the work efficiency depends on applied force is shown based on a simple mechanical model. It is also shown that the force dependence of transition temperature is closely related to the efficiency curve. Experimental results are compared with the theoretical prediction.
Highlights
More than forty years have passed since the discovery of volume phase transition in actual polymer gels [1], and the volume phase transition appears to be familiar for stimuli-responsive gels [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
The phase transition occurs in a macroscopic scale, and is the matter of thermodynamics [1,2,3,4,5,6,7,8,9,10,11,12], so this is often analyzed and discussed with the analogy of the phase transition of van der Waals fluids [3,11,12], and “volume phase transition” was basically used to mean a discontinuous change in volume (V) upon an infinitesimal change in a control variable [1,2,3,4,5,6,7,8,9,10,11,12]
The volume phase transition belongs to the category of first-order phase transition
Summary
More than forty years have passed since the discovery of volume phase transition in actual polymer gels [1], and the volume phase transition appears to be familiar for stimuli-responsive gels [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. There are similarities between the volume phase transition of polymer gels and the liquid-gas phase transition of the van der Waals fluids, but there exist marked differences. Is introduced as an additional degree of freedom: In the V-T curves of the gels φion acts as T in the P-V curves of the van der Waals fluids This control variable becomes meaningful only when various gels differing in φion are prepared and examined. This difference affects the phase transition behavior of gels under tension In this mini-review, applicability of the Clapeyron Equation to the volume phase transition of PNIPA-based gels under external force (f as a vectorial quantity and f = |f |) is reviewed, this may be limited to the cylindrical geometry at present.
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