Theory And Application Of Enhanced Surface Crosslinking Of Hydrogels

Based on the Flory and Tanaka theories of hydrogels, a thermodynamic model of the equilibrium swelling of hydrogel was proposed in considering the crosslinking efficiency and the elastic contribution of physical crosslinking, and the model relating the diffusion coefficient - temperature relationship was modified, respectively. To improve the properties of thermosensitive hydrogels and superabsorbent polymers (SAPs), a new idea of enhanced surface crosslinking was presented on the basis of molecular design concept to optimize the hydrogel structure. The preparation, structure and properties of thermosensitive hydrogels and SAPs with enhanced surface crosslinking were investigated. The results of this dissertation will promote the theory development and application of hydrogels.Due to the lack of basic research on the structure and properties of hydrogels, the existing thermodynamic models can't describe and predict the swelling behavior of hydrogel directly. Considering the crosslinking efficiency and the elastic contribution of physical crosslinking, a novel thermodynamic model based on the Flory-Rehner theory was proposed and applied to interpret the swelling behavior of N-isopropylacrylamide-acrylamide (NIPA-AM) copolymer hydrogel. The model prediction fitted well with the experimental swelling data.The diffusion coefficient of hydrogels was obtained by fitting the experimental swelling data with Tanaka swelling kinetics theory, and the effect of the synthesis conditions on the diffusion coefficient of hydrogels was investigated. It was found that the diffusion coefficient decreased firstly and then increased with the increase of the concentration of N,N-methylene bisacrylamide (BIS) crosslinking agent. The diffusion coefficient increased with the increase of the initiator and monomer concentrations, and the environmental temperature. The model relating the diffusion coefficient-temperature relationship was modified in considering the variation of chain segments status in the solvent. The model prediction fitted with the experimental observation with more accuracy than the original theory presented byTanaka et al.In order to improve the properties of conventional hydrogels, such as responsive rate, mechanical strength and salt-resistance etc., BIS and divinylbenzene (DVB) were used as the multiple crosslinking agents to form enhanced surface crosslinking structure. The relationships between the polymerization conditions, structure and properties were investigated, and the thermosensitive hydrogels and SAPs with optimized properties were synthesized.Firstly, the existence of surface crosslinking structure of SAPs was confirmed by SEM images and ultraviolet spectrum. The results indicated that the adding time of DVB could affect the thickness of the surface layer. The gel fraction of SAPs increased remarkably with the amount of BIS added, but it hardly increased as the amount of added DVB increased.Secondly, the thermosensitive poly(7V-isopropylacrylamide) (PNIPA) hydrogels were synthesized by using BIS and DVB as the multiple crosslinking agents via an inverse suspension polymerization. The synthesis conditions were optimized, and the resulted hydrogels had a lower critical solution temperature (LCST) of 32-34°C. The response time of these hydrogel beads was about 100s. In comparison with the conventional hydrogel crosslinked only by BIS, the hydrogels had a higher response rate due to the enhanced surface crosslinking. The PNIPA hydrogels were applied in separation of poly(ethylene oxide) glycol (PEG) from aqueous solution. It was shown that the separation efficiency increased with the increase of the molecular weight of PEG, and the decrease of PEG concentration. Increasing the amount of crosslinkers can also improve the separation efficiency, and the introduction of hydrophobic DVB was a suitable way to improve the separation property of hydrogel without the loss of swelling ratio of hydrogel. Acrylonitrile (AN) was copolymerized with NIPA to synthesize thermosensitive hydrogels, and the on-off switch behavior of NIPA-AN copolymer hydrogels was investigated. The resultant copolymers exhibit a distinct thermosensitive property. It was found that the LCST could be adjusted by varying the composition of feeding monomers. Furthermore, the response rate of hydrogel wasimproved by treat of surface crosslinking.At last, a series of SAPs were synthesized by an inverse suspension polymerization, using BIS and DVB as the multiple crosslinking agents. The obtained SAPs exhibited better absorption rate, mechanical strength, anti-hydrolyzing property and salt-resistance than conventional SAPs. The effects of synthesis conditions on the stability of polymerization process and the properties of SAPs were investigated. The results indicated that both the water absorption and the absorption rate increased as DVB concentration increased when its concentration ranged from 0.02 to 0.03wt%. The average diameter of SAPs decreased with the increase of amount of Span-60 and oil/water phase ratio. The absorption rate increased with the decrease of diameter of SAPs. The resultant SAPs exhibited the better comprehensive performances when 0.02 wt% BIS, 0.03wt% DVB, 3:1 (v:v) phase ratio, 37.5 wt% total monomer concentration, 5wt% AM, 75% neutralization degree and 0.34wt% initiator were applied.