Shrinking Kinetics of Poly(N-isopropylacrylamide) Gels T-Jumped across Their Volume Phase Transition Temperatures Mitsuhiro Shibayama* and Kohji Nagai Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan Received May 7, 1999; Revised Manuscript Received August 24, 1999 ABSTRACT: The shrinking kinetics of poly(N-isopropylacrylamide) aqueous gels due to temperature jumps (T-jumps) across its volume phase transition temperature has been investigated as a function of the monomer concentration (C NIPA), the destination temperature (Td), and the cross-linker concentration (CBIS). The following facts were disclosed. (1) The shrinking rate is Td dependent and is slowest for 40 e Td e 45 °C. (2) By reducing CNIPA to the lowest concentration for gel formation (i.e., CNIPA ≈ 300 mM), an anomalous rapid shrinking is observed. (3) The shrinking rate becomes smaller with increasing CBIS. (4) Whenever a rapid shrinking takes place, characteristic changes in morphology, i.e., bubble formation, are observed on the surface of the gel. A mechanism of rapid shrinking of gels is proposed by taking account of bubble formation and phase separation. 1. Introduction Responsive gels, such as temperature-sensitive and pH-sensitive polymer gels, have been developed for applications to drug delivery systems and actuators. 1 Poly(N-isopropylacrylamide) (NIPA) gels have been most extensively studied as a temperature-sensitive gel. 2,3 One of the drawbacks is that the rate of response is rather slow (a few hours for a millimeter-sized gel). This is, in principle, due to the fact that the kinetics of gel swelling and shrinking is governed by cooperative (collective) diffusion of the gel network. 3,4 Recent ex- perimental studies, however, have shown that the rate of gel shrinking can be accelerated by designing special gels, such as (i) a gel which has a strong shrinking tendency introduced by dangling chains, 5-7 (ii) mi- croporous gels prepared by γ-ray irradiation, 8 or (iii) a gel having a pathway for water molecules by incorporat- ing hydrophilic chains or a high degree of porosity. 9 In the case of (i), a dangling chain in a gel can easily collapse by an external stimulus because one side of the dangling chain is free. With regard to (iii), Hirotsu developed a NIPA gel, for which the shrinking rate is 10 6 times faster than that for the conventional NIPA gels. 9 This gel was prepared from an NIPA monomer solution in the presence of poly(acrylamide) (PAAM). Although the hydrophilic PAAM chains are physically trapped in the NIPA network, the PAAM chains are thought to serve as a water reservoir as well as a pathway for rapid shrinking. However, the mechanism for rapid shrinking in the methods described above has not been fully elucidated. In addition, studies on the destination temperature, T d , dependence of shrinking kinetics have seldom been reported in the literature although the shrinking rate is known to be affected by T d of temperature jump (T-jump). In our previous paper, 10 we discussed the shrinking/ swelling behavior of cylindrical gels made of poly(N- isopropylacrylamide-co-acrylic acid) (NIPA/AAc) and poly(N-isopropylacrylamide) (NIPA) upon T-jump across the volume phase transition temperature, T c . It was found that the shrinking process of NIPA homopolymer gels was strongly decelerated due to phase separation, while the swelling process was well reproduced by the Tanaka-Fillmore theory. 11 In this paper, we discuss the shrinking kinetics of NIPA gels as a function of the destination temperature (T d ), the cross-linker concen- tration (C BIS ), and the monomer concentration (C NIPA ). Then a possible mechanism for gel shrinking will be proposed. Furthermore, the cooperative diffusion coef- ficient on swelling is compared with that obtained by dynamic light scattering. 2. Experimental Section 2.1. Samples. Poly(N-isopropylacrylamide) (NIPA) homo- polymer gels were prepared by redox polymerization. Aqueous solutions of NIPA monomers were polymerized in the presence of N,N′-methylenebisacrylamide (BIS; cross-linker). The mono- mer concentration was varied from C NIPA ) 300 to 400, 690, and 1000 mM. The cross-linker concentration was also varied from CBIS ) 4.00 to 8.62, 12.8, and 31.4 mM, to study effects of CBIS on shrinking kinetics. The polymerization was carried out in a micropipet of 471 µm diameter for 20 h at 20 °C. The prepared gels were cut to 5 mm long cylindrical pieces and washed with an excess amount of distilled water for several days. 2.2. Swelling Degree Measurement. The sample was immersed in a thermostated chamber filled with distilled water. The T-jump experiments were carried out by exchang- ing the circulating water of which temperatures were set to two desired temperatures (i.e., the initial, T i, and the destina- tion temperature, Td). The time required for T-jump was about 1 min. Images of gels during T-jumps were captured by an inverted microscope (TMD300, Nikon) coupled with an image processor (Algas 2000, Hamamatsu Photonics), and were recorded on videotapes. The degree of swelling, d/d 0, was obtained by measuring the diameter of the cylindrical gel, d, where d0 is the gel diameter at preparation (i.e., the mold diameter) ()471 µm). When the shape of a gel became irregular on shrinking, the maximum width of the gel was measured. The measurement was repeated at least three times and its average was used as the value of d. 3. Results and Discussion 3.1. Shrinking Process. 3.1.1. Destination Tem- perature Dependence. Parts a-c of Figure 1 show * To whom correspondence should be addressed. 7461 Macromolecules 1999, 32, 7461-7468 10.1021/ma990719v CCC: $18.00 © 1999 American Chemical Society Published on Web 10/07/1999