Responsive Properties
                              Shao Hua SHI1,2,*, Li Jian LIU1*
1Department of Polymer Science, College of Chemistry and Molecular Science, Wuhan University
       Key Laboratory of Biomedical Polymers (Wuhan University), Ministry of Education,
                                        Wuhan 430072
                  2R & D Center, Nhwa Pharmaceutical Group, Xuzhou 221007
Poly(N-isopropylacrylamide) (PNIPAM) hydrogel, which is insoluble but shrinkable or
swellable in aqueous media when temperature rises or drops across 33oC1,2, has been extensively studied due to its potential applications in the fields of controlled drug release3, enzyme immobilization4, tissue engineering5 and bioseparasion6. For these applications the rates of swelling and shrinking of the PNIPAM hydrogel were of significant importance, therefore a variety of approaches were explored to accelerate the responsive rates of PNIPAM hydrogels, including: (i) tailoring PNIPAM molecular architecture by grafting hydrophilic side groups such as poly(ethylene oxide)7, carboxy group8 and poly(N-isopropylacrylamide) oligomer groups9. (ii) using pore forming agents, for instance, polyethylene glycol10, CO2 bubbles11 and CaCO3 . (iii) changing   polymerization conditions, e.g., conducting polymerization above the lower critical
solution temperature (LCST)1,2, under freezing condition13 and in sucrose solution14 etc..
     Due to the homogeneity, high efficiency and low energy consumption of microwave heating, microwave-assisted chemical synthesis has attracted wide attention15, and microwave-assisted heating could accelerate the rates of polymerization reaction16 and improve the properties of the products.    In this communication, we report microwave-assisted preparation of PNIPAM hydrogel with improved responsive properties such as faster swelling and shrinking rates.
Preparation of PNIPAM hydrogels A microwave oven with working frequency at 2.45 GHz was used in our experiments.
800 mg N-isopropylacrylamide, 5 mg N, N′-azobisisobutyronitrile and 50 mg N, N′-
methylenebisacrylamide were placed into a glass ampoule, and 3 mL 1:1 (v/v) ethanol/water was added. The ampoule was induced N2 and degassed for three times and sealed.
The reaction mixture was irradiated under microwave until the temperature in the
reaction mixture reached 56oC. The hydrogel thus prepared was immersed in water,and was refreshed with fresh water every few hours to remove unreacted chemicals anddried at 40oC in vacuum. The thermal polymerization was carried out at 62oC for 10min. The hydrogel prepared under microwave irradiation and by thermal heating were designated as MH and TH.
     It was found that the polymerization/crosslingking reaction of MH under 75 W
microwave irradiation completed in 40 s with a yield of 99 %, whereas the reaction of TH at 62 C needed 5 min to form a bulk hydrogel with approximately the same yield.Kinetics of swelling and shrinking The kinetics of swelling and shrinking were gravimetrically measured. The dry gel disc or the gel sample swollen to equilibrium at 25oC was transferred into 25oC or 37oC water for swelling or shrinking.       At regular time intervals, the changes of the hydrogel weights were recorded after blotting the excess surface water with moistened filter paper
until constant weights were reached. Water ratio (WR) is defined according to the
following equation:
                             WR (%)＝100 x (Wt-Wd)/(W25-Wd)
Wt is sample weight at time t, Wd is weight of dry sample and W25 is weight of sample
equilibrated in 25℃.