Osmotic Poisson’s Ratio and Equilibrium Stress of Poly(acrylamide) Gels

Abstract

KEY WORDSPolymer Gel/ Swelling under Elongation/ Osmotic Poisson's Ratio/ Equilibrium Stress/ Polymer gels have been recognized as a unique and interesting soft material, and the physical and mechanical properties of polymer gels have been investigated by many researchers. 1 -3 Experimental and theoretical studies, especially, on the volume phase transition 1 and swelling kinetics, 1. 2 have been carried out intensively. The physical properties of polymer gels have become clear. We have already shown 4 that the swelling kinetics of polymer gels can be written by two types of diffusion equations, i.e., the equations of longitudinal and transverse modes. The former determines the time evolution of the volume of gels from the initial to final state, and the latter the time dependence of the change of shape. We also found that the uniaxially stretched gel in solvent swells gradually until equilibrium under the deformation is attained. 4 When a gel sample is deformed quickly in solvent, the state just after the application of the strain, which we call hereafter the initial state, must be determined by the material nature of the gel used. For example, in the case that a gel specimen is stretched uniaxially, the initial dimension of the specimen perpendicular to the elongation is determined by Poisson's ratio as a material constant (µ 0 ). This holds so long as the time for swelling is long enough compared with the time required for the mechanical measurement to estimate Poisson's ratio, and this situation is easily attained by employing a larger size of polymer gel.On the other hand, the final state, i.e., the state realized at long time limit after the application of the deformation, will be determined not only by the mechanical properties of the gel (for example, µ 00 of the gels), but also by thermodynamic properties of the system under deformation. The polymer gel and surrounding solvent molecules compose a semi-open system and the solvent molecules can move through the interface between gel and solvent during long time when the solvent motion is favored. Osmotic Poisson ratio (µ 00 ) defined for uniaxially deformed gels in solvent at long time limit reflects the thermodynamic properties of the semi-open system.The equilibrium elongational stress (<1 00 ) has also been affected by the thermodynamic properties of the system. Using the initial elongational stress (<1 0 ) and <1 00 , we can define the relative stress reduction (<1 0 -<1 00 )/<1 0 . This quantity is also affected by the thermodynamic properties of the gel system. Although µ 00 , and <J 00 or (<10-<1 00 )/<10 are fundamental quantities for describing t To whom correspondence should be addressed. 1012the physical and mechanical properties of polymer gels at equilibrium, there are few studies on those values. 4 -7 The aim of the present study is to investigate the values of µ 00 and (<1 0 -<1 00 )/<1 0 for poly(acrylamide) (PAAm) gels, and to compare those with the theoretical predictions shown in a previous paper. 4 EXPERIMENTALPAAm gels used in this...

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