2562
2008, 112, 2562-2565 Published on Web 02/12/2008
Phase Separation Dynamics of Aqueous Solutions of Thermoresponsive Polymers Studied by a Laser T-Jump Technique Yasuyuki Tsuboi,* Yasuhiro Yoshida, Kensaku Okada, and Noboru Kitamura* DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan ReceiVed: NoVember 23, 2007; In Final Form: January 10, 2008
Poly(N-isopropylacrylamide) and poly(vinyl methyl ether) are well-known thermoresponsive polymers. The aqueous solutions of these polymers exhibit a phase transition followed by phase separation with LCST ∼ 305-310 K. In the present study, the dynamic behavior of the phase separation was analyzed by a laser T-jump method. Two different T-jump methodologies were employed: the first was a dye-photosensitized T-jump technique (indirect heating) using 532 nm laser pulses, while the other was a direct heating T-jump technique using 1.2 µm laser pulses. Both methods gave similar results. The time constants (τ) of the phase separation were systematically determined for 1-10 wt % aqueous solutions of the polymers, and a hydrodynamic radius (R) dependence for τ was clearly observed. The values of τ increased linearly with increasing square of R. The present behavior is interpretable in the framework of Tanaka’s model for the volume phase transition of a gel, since each of the polymer chains are entangled in the present sample solutions, which can be regarded as approximating to a gel in solution.
Introduction Since the discovery of the thermal-reversible phase separation of an aqueous PNIPAM solution,1,2 interest in the chemistry of thermoresponsive artificial polymers such as poly(N-isopropylacrylamide) (PNIPAM), poly(vinyl methyl ether) (PVME), and their related derivatives has experienced phenomenal growth over the past three decades.3-5 When homogeneously dissolved in water, these polymers adopt coiled structures at room temperature. With increasing temperature (∆T > 10 K), the coils turn into globules accompanied by dehydration of the polymer chains (i.e., phase transition) and, subsequently, the solution undergoes phase separation due to aggregation of the globules by hydrophobic interactions. Since such behavior is quite intriguing from the viewpoint of basic physicochemical polymer science and the various potential applications, much effort has been expended to analyze these phenomena. Various experimental approaches, such as differential scanning calorimetry (DSC),6,7 dynamic light scattering,8 absorption/fluorescence spectroscopy,9,10 vibrational spectroscopy,11,12 and so on have revealed the thermodynamic behavior and structural changes that occur upon phase separation in detail. By contrast, little is known about the dynamic behavior (temporal evolution) of the phase separation that follows the phase transition process. Although a DSC study has suggested that phase separation was not a rapid process,13 the precise time constant of the phase separation process and factors regulating its dynamics still remain unclear. Therefore, it is important to reveal the details of these dynamic processes in order to understand the fundamental mechanisms of phase separation and to apply the polymers to functional materials: drug delivery, selective extraction, and so on. * Corresponding authors. E-mail:
[email protected] (Y.T.);
[email protected] (N.K.).
10.1021/jp711128s CCC: $40.75
In the present study, we performed laser-induced temperaturejump (T-jump) experiments to analyze the phase separation dynamics of aqueous PNIPAM and PVME solutions. As we have shown in our previous work, laser-induced methods are a powerful tool for shedding further light on the science of thermoresponsive polymers.14-16 In the present study, we successfully revealed the phase separation dynamics of the polymer samples that we were studying, and the time constant of the phase separation process was discussed in terms of the size of the polymers. PNIPAM samples with different molecular weights (Mw) were synthesized by radical polymerization, and the averaged Mw of each sample was estimated on the basis of the relationship between the hydrodynamic diameter of the polymer and Mw17 (the averaged Mw e 1 × 106 at most). The lower critical solution temperatures (LCSTs) of the polymers were 305-306 K. PVME (Scientific Polymer Products, LCST ) 308 K) was purified by the same technique as reported previously.16 The polymers were dissolved in distilled water (Advantec, GSR-200) with concentrations ranging from 1.0-10 wt %, and in this paper, we mainly concentrate on the results of 2.0 wt % solutions. The commercially supplied dyes, rhodamin B (RhB), acid red 88 (AcR88), and acid red 52 (AcR52) were used without further purification. The hydrodynamic radii (R) of the polymers in their coiled states were evaluated by dynamic light scattering measurements (Otsuka Electronics, FDLS-3000) for very dilute solutions (
