Rapid Shrinking of Microporous, Thermosensitive Hydroxypropyl Methyl Cellulose Hydrogel Prepared by Freeze-Drying

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Shrinking rates of thermosensitive hydroxypropyl methyl cellulose (HPMC) gel were easily accelerated by using a freeze-drying technique. Freeze-drying and subsequent water hydration of HPMC gels made the gels microporous and fast-responsive. A time for the first half shrinking (T_1/2) in response to abrupt temperature-change can be reduced to 10 s while that for non-porous gel is approximately 3,000 s. Microphotographs of scanning electron microscope revealed that different sizes of micropores and struts were formed when the swollen gels with different swelling degrees were frozen at -20°C. The freeze-drying treatment is a versatile technique to prepare the fast response gel; the validity of the technique was demonstrated with HPMC gel besides hydroxypropyl cellulose gel and poly(N-isopropylacrylamide) gel reported in our previous papers. To maximize the effects of freeze-drying on cellulose ether hydrogels, the equilibrium swelling degree should be set 20 or more. The minimum response time of microporous HPMC gel on shrinking is probably limited by the convective flow of pore water through the interconnected-pores because heat transfer as well as network motion of micropore walls on the order of micron or sub-micron is estimated to be faster than the convection.