ABSTRACT
I. INTRODUCTION The swelling of gels, such as that observed when rubber is placed in ben zene, has long been a well-known phenomenon. As early as 1948, Vermaas and Hermans [1] studied the behavior of lightly cross-linked polymer chain networks with ionic charges, i.e., polyelectrolyte gels. Flory [2] and Katchalsky [3], in collaboration with their colleagues, also made great contribu tions to theories and experiments in the fields of ionic and neutral gels. In early studies by these pioneers, however, little attention was paid to the existence of a critical endpoint in the phase equilibria, although in 1968 Dusek and Patterson [4] published a theoretical paper on this subject. The earliest report of the phase transition in a gel was in 1978 by Tanaka [5], who discovered a discontinuous volume collapse of poly(acrylamide) (PA Am) gels in acetone-water mixtures when varying the temperature or the composition of the mixture. This phenomenon is now called “volumephase transition” (or simply “phase transition” ) of gels and is observed in many gel systems consisting of synthetic and natural polymers. The phase transition is accompanied by reversible, discontinuous (or in some cases, continuous) volume changes, often as large as several hundred times, in response to small variations in the solution conditions surrounding a gel. Variables that trigger the transition include solvent composition [5-11], pH [6], ion concentration [12], temperature [5,10,13], and small electric fields [14,15]. (Related literature published in the 1990s will be cited later as the need arises, since a number of studies focused on stimulus-sensitive polymer gels after the publication of these pioneering works.)
When setting a limit to the phase transitions in polyelectrolyte gels, two studies by Tanaka et al. are of historical importance; these were done with ionic gels of partially hydrolyzed PAAm [6] and copolymer of acrylic acid
(AAc) with A-isopropylacrylamide (NIPA) [13]. The former [6] is the ear liest paper on ionic gel transition, demonstrating that the introduction of charges into the network chain results in an increase in the gel volume at transition as well as a change in the transition threshold. Further demon strated in the latter [13] is the ionization effect on the transition temperature; that is, a rise in the temperature leading to a transition when increasing the charge density of NIPA-AAc copolymer gels. In particular in the NIPA-based ionic gel system, a discontinuous volume change was observed with a small variation in temperature, as well as in other solution conditions such as pH and salt concentration. Thus, the polyelectrolyte gel of this sort is called a “thermo-shrinking” or “temperature-sensitive” gel, the properties of which have extensively been studied from both fundamental and technological standpoints (e.g., see Ref. 16).
