| As a kind of temperature-sensitive material, the application of Poly(N-isopropylacrylamide) (PNIPA) intelligent hydrogels has been achieved primary development and got much intention in recent years. At the present time, studies are focused both on theoretic explanation and experimental verification for the stimuli-response mechanism, and on the copolymerization of NIPA with other monomers to modify their properties and obtain multi-sensitive hydrogels which are applied in wide fields.Usually, the intelligent properties of hydrogels are shown into two aspects: (1) The hydrogels have discontinuous swelling ratios with little change of temperature near the LCST. (2) The hydrogels have rapid stimuli-response velocities. PNIPA hydrogels prepared by traditional methods have very slow stimuli-response velocities, which limits their application range.Furthermore, the stimuli-response of PNIPA hydrogels is a very important parameter in application. The rapid stimuli-response velocity is needed in many cases. Therefore, the preparation and application are important task in research of intelligent hydrogels with rapid response velocity.The reason of the limited research on theory and application for rapid response velocity is ascribed to the complicated influencing factors and little of characterization methods. There are several methods reported to increase the response speed in literature.(1) Preparation of microgels by emulsion polymerization or other methods. But there are less techniques and methods for their characterization and development in theory, accordingly, there is little breakthrough in the practical application. Therefore, the use of this method is very limited.(2) Synthesis of porous PNIPA hydrogels. Though the response speeds are enhanced, the reduced hydrogel intensity limits their application in much fields. For example, some bio-macromolecules can be absorbed into the hydrogel pores in separation, which reduces the separation efficiency.(3) Manufacture by Phase Separation Technique. The PNIPA hydrogels have rapid response velocities, but the control of reaction time and temperature in preparation is much critical. Sometimes, the same hydrogels could hardly be achieved with different reaction time, and the hydrogels are often opaque and fragile. Thus, the hydrogels prepared by this method cannot be used practically too.(4) Graft copolymerization. NIPA monomer is grafted onto the defined matrix to synthesize the functional material. Obviously, such hydrogels have rapid response velocities compared with the large bulk hydrogels, but there are also two shortcomings in this method. One is the complicated preparation of defined structure; the other is the seriously reduced swelling ratio near LCST due to the limited content of PNIPA.In this paper, more attention was paid to two respects-swelling ratio and stimuli-response velocity to improve intelligent properties of PNIPA hydrogels.(1) By traditional chemical initialing method, a series of macrogels with different diameter (3. 12~12. 8mm, wet state) and thickness (3~12mm, wet state) were obtained in solution polymerization. Some of these macrogels were smashed into particles with different diameter(0. 154~0. 9mm, dry state).Firstly, the effects of polymerization conditions on the macrogels properties were conducted. The experiment results showed that the transparent, high swelling ratio and uniform macrogels could be manufactured at the optimized conditions (T^IO'C, Secondly, the relationship between the macrogels dimension and the swelling-deswelling, dehydration-soakage process was studied by different method. The results showed that the macrogels could form a shell-core structure easily under the deswelling or dry state. In this structure, some water was packed into the macrogels which could not diffuse out of the shell due to the outer compact shell. With the increase of diameter and thickness, the water content packed in the macrogels increased sharply. However, the swelling ratio was hardly affected by dimension for the smaller macrog... |
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