Study On The Structure Evaluation And Environmental Sensitivity Of Hydrogels

Hydrogels have been widely used in agriculture, forest, hygiene, biochemistry, medical and the like because their extreme hydrophilicity, low toxicity, water-absorbed, biocompatibility. Intelligent hydrogels could undergo substantial and abrupt volume chang in response to the extenal stimulus, such as pH, temperature, ionic strength, electric field, etc. The volme transitions of hydrogels have prompted researchers to investigate the application of the smart materials in sensors, drug-delivivery systerms, tissue engineering, etc.Knowing the forming mechanism of hydrogels is beneficial to investigate interior structure, evaluate swelling-ratio, and develop the application. Intramolecular cyclization is the main reason that experimental result is not coincident with the gelation theory. The theories and experiments on interamolecular cyclization of polycondensation systems have been widely studied, while the experiment on interamolecular cyclization of addition polymerization has not been reported by now. The poor mechanical strength of hydrogels resiticts their applications in some areas. In order to improve this weakness, some novel hydrogels with excellent mechanical strength have been developed:topological hydrogels, double network hydrogels, nanocomposite hydrogels and macromolecular microsphere composite hydrogels. However, the permanent crosslinking network, complex preparation processess and few monomer kinds are still insufficiency. pH-and temperature-responsive hydrogels are the most important intelligent hydrogels which have been investigated most extensively. In these hydrogels, regulable phase transition temperature, fast response rate and reversible phase transition are the aim of researchers.In this paper, chemical hydrogel has been prepared by radical copolymeration of acrylamide(AAm) and polyethylene glycol(PEGDAC) using potassium persulfate-sodium acid sulfite as reductant-oxidant initiator. We mainly disscussed the gel fraction increase of hydrogels and the influence of monomer AAm concentration and crosslinking agent PEGDAC chain length on interamolecular cyclization during gelation. The result shows that the gel fraction increase of hydrogel divided into three steps with the increase of the crossling concentration. The critical gel point and interamolecular cylization decreased with the increase of monomer concentration and crosslinking agent chain length, which consistents with the analysis of terminal distribution function theory. The tendency of phase separation decreased with the increase of the cross-linker in the AAm/PEGDAC copolymerization system. And the swelling ratios of the gels got the maximum value where the behaviors of phase separation were vanished. The maximum swelling ratio decreased with the increase of the monomer AAm concentration and the cross-linker agent chain length.A novel type of hydrophobic association hydrogels (HA-gels) was prepared through micellar copolymerization of acrylic acid (AA), acrylamide (AAm) asbasic monomer and a small amount of octylphenol polyoxyethylene ether acrylate (OP7-AC) as hydrophobic association monomer. The hydrophobic monomer OP7-AC was solubilized by SDS and formed associated micelles with them, OP7-AC in a micelle were copolymerized to two or more P(AA-AAm) chains during the copolymerization, and the associated micelles acted as cross-linking points cross-linked the P(AA-AAm) chains and formed three-dimensional polymer network. P(AA-AAm) HA-gels exhibited desirable mechanical property that the tensile strength became reduced and the break elongation tended to augment along with increasing AA components. The swelling process of P(AA-AAm) HA-gels can be divided into five stages: over-swelling, self-deswelling, dynamic equilibrium of disassociation and re-association, disassociation is advantage and dissolution. With the increase of AAm component in the P(AA-AAm) HA-gels, the swelling rate first decreased and then increased at pH=2 while it showed the opposite behavior at pH=11.5. At the same time, the swelling rate increased with increasing temperature.P(AA-AAm) HA-gels exhibited reversible opacity-transparence phase transition behavior with changing temperature, and the transmittance change could not be weakened by multiple temperature repeating changes. The phase transition temperature of P(AA-AAm) HA-gels can be finely adjusted in a wide range by adding urea or changing AA:AAm ratio, AA+AAm concentration,OP7-AC dosages and SDS dosages:the phase transition temperature decreased with increasing urea, increased with increasing AA+AAm concentration, first fast decreased and then slowly increased with increasing OP7-AC content, linear decreased with increasing SDS dosages. Adding urea shows that the reason for phase transition of P(AA-AAm) HA-gels is the action of forming-dissociating hydrogen bonding between amide and carboxyl groups. The introduction of hydrophobic units OP7-AC to P(AA-AAm) copolymer would result in the adulterating and cross-linking effects on the transition temperature, the former sharply reduced the transition temperature while the later gradually raised it.Poly(maleic anhydride-co-acrylic acid) [P(MA-AA)] was synthesized by free-radical copolymerization of maleic anhydride with acrylic acid, and fast responsive pH-sensitive poly(maleic anhydride-co-acrylic acid)/polyethene glycol [P(MA-AA)/PEG] hydrogels were prepared by using PEG as macromolecular cross-linking agent. In order to make the hydrogels more sensitive to pH and ionic strength, maleic anhydride has been introduced into poly(acrylic acid) chains and long chain PEG was adopted as cross-linking agent. There was a hundredfold difference in the swelling ratios between the conditions of pH=2 and pH=10, and the swelling capability could not be weakened after multiple swelling-deswelling cycles. The response rate of P(MA-AA)/PEG hydrogels was very fast, the hydrogels could completely swell within 20 min from pH=2 to pH=10, while they also could contract completely within 10 min from pH=10 to pH=2. And the swelling ratio decreased dramatically as the ionic strength increased.In a word, we firstly studied the influence of monomer concentration and crosslinking agent chain length on the interamolecular cyclization during gelation. And then we researched the swelling capability of temperature-responsive hydrogels with high mechanical strength and pH-responsive hydrogels with fast response rate. The obtained results were significant for the synthesization and the application of hydrogels.