Dynamic Swelling Properties of a Poly(N-isopropylacrylamide) Gel Measured by a Magnetic Force-Driven Rheometer

Abstract

KEY WORDSPolymer Gel / Swelling / Poisson's Ratio / Dynamic Viscoelasticity / Polymer gels in solvent show an interesting response to external stimulus. For example, a gel changes the volume when mechanical stress is applied to the gel, and the rate of volume change strongly depends on the sample dimension. The volume change responding to the applied stress originates from the fact that the polymer network and solvent molecules form a thermodynamical semi-open system, and this phenomenon has been considered to be a typical example of the stress-diffusion coupling. 1 Studies on swelling kinetics of the polymer gels after the application of mechanical stimulus have been made by employing so-called static methods, 2-5 such as continuous monitoring of the gel dimension after imposition of a step stress, and have shown that in the long time region the volume change can be written by a single exponential function; i.e., a single characteristic (or, relaxation) time governs the volume change to a new equilibrium state under deformation in that time region. 2-5 On the other hand, little is known on the swelling kinetics for the gels at short times, where a multi-exponential character is considered to become dominant for the volume change. 3,6 Studies in terms of dynamic viscoelasticity will be very useful to give the full picture of the swelling kinetics of the polymer gels. Conventional rheometers, however, cannot be used for this purpose; the rheometers require relatively large gel specimens to detect the mechanical response but the large specimens move the relaxation time far away from the measurable frequency range of the rheometers. A new type of rheomter, applicable to very small gel specimens, is needed to investigate the dynamic swelling properties of polymer gels. In this study we have made a new apparatus to observe the dynamic swelling behavior of polymer gels, and examined the frequency dependence of the dynamic Poisson ratio for a poly(N-isopropylacrylamide) (PNIPA) gel. A phenomenological theory of linear viscoelas- † To whom correspondence should be addressed. ticity to analyze the dynamic swelling behavior is also described briefly in this paper.Consider a time (t) profile of strain in response to an applied elongational stress (σ) of the following form

Static and Dynamic Swelling Properties of Poly(N-isopropylacrylamide) Gels in the Swollen State

“…Recently, we built up a new type of rheometer to examine the dynamic swelling properties of the PNIPA gels. 8 In the previous paper 8 we have reported the dynamic swelling properties of a PNIPA hydrogel using the laboratory-made rheometer as a preliminary study. The rheometer used in the study was a prototype and had several shortcomings to be improved.…”

Creep Behavior of Poly(N-isopropylacrylamide) Gels in the Collapsed State

“…Figure 1 shows the schematic representation of the rheometer built up originally in the previous study. 5,6 One end of the cylindrical gel was pasted to a metal (tungsten carbide) sphere and the other end to an aluminum plate using a cyanoacrylate-type adhesive. The aluminum plate with a gel sample was mounted on a moving plate and the system was set in a double-jacketed solvent bath.…”

“…Details of the experimental conditions are the same as the previous paper. 5,6 In strain-controlled operations, one end of the gels was settled with a larger, hence heavier, metal sphere; in this condition, the gel is elongated by about 50%, and the metal sphere touches on the bottom of the bath. As a result, the strain in the stretching direction is fixed throughout the measurement.…”

“…Above T t the gels consist of polymer network and a very small amount of water, which is called ''collapsed gels'', or ''gels in the collapsed state''. In the previous papers 5,6 we reported the swelling behavior of the PNIPA hydrogels in the swollen state below T t under static or dynamic stress. Elongational stress applied to the fully swollen gels induces a further swelling as a result of the shift to another equilibrium state under deformation.…”

“…[9][10][11] In the present paper, we report the creep behavior of the collapsed PNIPA gels under static and dynamic stresses (or strains) revealed by the same method in the previous papers. 5,6 EXPERIMENTAL Preparation of Gel Samples Poly(N-isopropylacrylamide) (PNIPA) hydrogels were synthesized by radical copolymerization of Nisopropylacrylamide (NIPA) and N,N 0 -methylenebis-(acrylamide) (BIS). First, N-isopropylacrylamide and BIS (as a closslinker) were dissolved in distilled water under nitrogen atmosphere.…”