(193v) Effect of Freezing Polymerization in Poly(N-isopropylacrylaide)-Alginate Hydrogels Preparation on Its Mechanical Strength and Thermoresponsive Properties

Poly N-isopropylacrylamide (PNIPAM) hydrogel is known as one of the representative thermoresponsive hydrogels having volume phase transition temperature (VPTT) of around 32°C in aqueous solution. Therefore, the poly NIPAM hydrogel shrinks and swells when the solution temperature is above and below the VPTT, and the transition is reversible. Because of the property, various applications such as drug delivery, actuator and microreactor etc., are expected. However, improvement of very low mechanical strength and slow shrink/swell rate of pure PNIPAM hydrogel is a key for its practical use. To improve thermoresponsive shrinking/swelling rate, creation of porous structure inside the gel has been tried. For example, phase separation synthesis in water/oil phase medium, application of freezing polymerization to the synthesis of PNIPAM hydrogel have been studied. Especially, in the case of study using freezing polymerization, Strachotova at el reported that swelling or shrinking equilibrium of thermoresponsive hydrogel was achieved within 1 minute. Freezing polymerization consists of two steps; at first, radical polymerization is carried out at constant temperature (e.g., 15 °C) for a few minutes (STEP1). Then, the gel in a reaction vial is transferred to a container in which the temperature is below 0°C and the reaction is continued for 24 hours (STEP2). During the process, both of ice crystal formation and polymerization is carried out at the same time. On the other hand, various composite in which PNIPAM combined with organic polymers or inorganic materials have been reported to improve mechanical strength. For example, Strachotova et al., tried to improve mechanical strength of PNIPAM hydrogel by incorporating silica, and Depa et al. attempted to incorporate both silica and polyaniline. However, in spite of various attempt, achievement of both high mechanical strength and fast shrinking/swelling rate is still a difficult task, and especially, mechanical strength is only up to a few kPa. Meanwhile, by incorporating alginate (Alg) in PNIPAM hydrogel, it was reported that Young’s modulus became 10 time higher than the original pure PNIPAM hydrogel. Since alginate is a natural polymer and has high biocompatibility and mechanical strength, it is expected to be used for various applications. Therefore, in this study, PNIPAM-Alg hydogels were prepared using freezing polymerization to obtain thermoresponsive hydrogels having high mechanical strength and fast shrink/swell rate. In the experiment, firstly, 2wt% Alg hydrogels were prepared by dropping sodium alginate into calcium chloride solution. Then, Alg hydrogel beads were soaked into solution containing NIPAM, N,N’-methylenebisacrylamide and ammonium persulfate with stirring for 1 h at 10°C. After that, by transferring the Alg hydrogel beads to oil phase (n-octane) with emulsifier and accelerator, polymerization of NIPAM inside the gel is initiated. The reaction was carried out at 10°C for 10-20 minutes under N₂ atmosphere. Finally, the reaction vial containing the gels transferred to a circulator which was kept at -20°C and continued to react for 24 h. The resulting samples were then washed with distilled water. For comparison, the sample without freezing polymerization was also prepared. The results showed that the mechanical strength of the PNIPAM-Alg hydrogels prepared by freezing polymerization increased c.a. 50% more than that of PNIPAM-Alg hydrogels prepared by normal polymerization. Especially when the freezing polymerization was carried out after reacting at 10°C for 20 minutes, the sample showed maximum strength of c.a. 240 kPa. In addition to the improvement of mechanical strength, shrinking/swelling rate of the sample was also improved 10.5% and 23.4%, respectively, by using freezing polymerization technique.