Study On The Cross-linked Structure And Properties Of Hydrophobic Association Hydrogels

Hydrogels have been widely applied in many fields because of their special properties, and the synthesis and modification of intelligent gels and high mechanical strength gels becomes a subject concerned by scientists all over the world. Recently, more novel hydrogels with high mechanical strength have been emerged constantly, which improved the mechanical strength and optical transparency of hydrogels well. However, the development, modifying, and mechanism researching about novel hydrogels is still a long-term task.At present, the study on the interaction between surfactants and hydrophobically modified water-soluble polymers most were focused on the semi-dilute solution system and the characterization method is limited, so that research progress is slow. How to describe and characterize the interaction between them directly is also a topic that needs of further research. Moreover, people paid little attention to the rubber elasticity and viscoelasticity of hydrogels, especially about chain entanglements effect, swelling effect, the limited tensibility of network chains, stress-relaxation behavior and so on. Hence, research on these contents needs more attentions and further developments.Last two years, a new kind of physical hydrogels has been synthesized in our lab, which is named hydrophobic association hydrogels (HA-gels). This kind of hydrogels not only exhibits ideal mechanical strength, swelling property and optical transparency, but also possesses self-recovery and remold property. In this paper, the HA-gels were prepared via a micelle copolymerization and acrylamide was adopted as main monomer, a small amount of octylphenol polyoxyethylene ether acrylate with10ethoxyl units (OP10-AC) was used as hydrophobic monomer, and sodium dodecylsulfate (SDS) was acted as surfactant. On the basis of our previous results, we made a further research on the cross-linked structure and various properties of HA-gels.Firstly, we studied the effects of surfactant content on the mechanical properties, network structure, and transparency of HA-gels. Three series of HA-gels with different OP10-AC concentrations were prepared in this test. We changed SDS contents and adopted the molar ratio of SDS and OP10-AC (R) as a variable. Tensile test was used to measure the mechanical properties of HA-gels, and obtained the strength parameters, i.e. tensile strengths and elongations at breakpoints. The results showed that the tensile strength of HA-gels exhibited a "down-up-down" trend as R increased, and at the same time, the minimum and maximum points on the variation curves for three series HA-gels occurred at the similar positions. The optical transparencies of HA-gels were measured through UV-vis spectroscopy and exhibited the same tendency as tensile strengths with R increasing. The results indicated that when R was different, the network structures of HA-gels were different, and the difference between operation and aggregation form of SDS and OP10-AC was great. Moreover, we also took an analysis on the effect of OP10-AC contents on the properties of HA-gels. Through the experiments of this chapter we found that the optimal property of HA-gels could be obtained when R was in the range of1-2and the OP10-AC content was around2%.The effect of SDS content on the swelling behaviors of HA-gels was also researched, and the used samples in the test were the same as mechanical test. The results of swelling test for hydrogels and re-swelling test for xero-gels showed that the swelling ratios for all three series gels exhibited an "up-down-up" tendency with R increasing (the same as tensile results), which illustrated that the network sizes of HA-gels under various R were different. At the same time, we also followed the weight losses of gels during the swelling process, which also showed an "up-down-up" tendency as R increased. The experimental results still indicated the weight losses of HA-gels during the initial stage of swelling process were mainly made up by redundant SDS molecules in the network. Furthermore, introducing more OP10～AC units could decrease the effective cross-linking density of HA-gels, so that the swelling rate would be accelerated and the swelling stability turned poor. In brief, when R was between1and2, and OP10-AC content was around2%, the obtained HA-gels possessed the optimal swelling property, which was well consistent with the experimental results in mechanical tests.Next, we made an analysis on the stress-relaxation behaviors of HA-gels, and adopted generalized Maxwell model to fit the stress-relaxation curves, and then calculated the relaxation times of HA-gels in all stages. Experimental results showed that under the constant external forces (drawing or compressing force), HA-gels would firstly undergo a rapid relaxation process of polymer chains, which was mainly decided by the conformational change of polymer chains, and we could evaluate it via various structure parameters. And then, because the disassociation process would occur for the associated micelles after a long enough time, HA-gels would undergo a lengthy relaxation process of cross-linking points and the stress would relax to zero. Moreover, the addition of SDS made a greatly effect on the relaxation behaviors of HA-gels, which must be closely related to the structure and strength of mixed micelles. All in all, through stress-relaxation test, we could get a more in-depth understanding on the viscoelasticity of HA-gels, and their properties could be adjusted well by changing the addition of SDS.A series of HA-gels with different monomer concentrations were prepared in this paper, whose concentration range was changed from10wt%to40wt%, and the molar ratio was fixed on1.23(R=1.23). First, the gel fraction could be obtained from swelling test, and then we could calculate the density and volume fraction of HA-gels. The parameters of elasticity, network chains density and molecular weight between adjacent cross-linking points were measured by tensile tests. These parameters could be used to further calculate the proportion of the contributions from hydrophobic association cross-linking points and physical chain entanglements in the network. An exponential equation about volume fraction of gels and network chains density of chain entanglements was built via iteration and self-consistent derivation method, and the exponential factor was2.684. Furthermore, we also calculated the entangled molecular weight and hydrophobic association molecular weight of HA-gels with different monomer concentrations. It can be seen that the tensile strength of HA-gels showed a growth by exponential form as monomer concentration increased, which must be related to the chain entanglements effect in the network of HA-gels. The experimental results could provide us more theoretical support for the further research on construction mechanism of HA-gels.Finally, thermosensitive hydrophobic associated hydrogels (NI-HA-gels) were prepared by free radical micelle copolymerization in aqueous solution of N-isopropylacrylamide (NIPAM), a small amount of hydrophobic monomer octylphenol polyoxyethylene4acrylate (OP4-AC), and nonionic surfactants octylphenol polyoxyethylene10ether (OP10). The effects of OP4-AC content on the various properties of NI-HA-gels together with phase transition behavior were studied via tensile test, UV-vis spectroscopy, and swelling/de-swelling kinetics. Experimental results indicated that with the increasing of OP4-AC content, the mechanical strength of gels enhanced markedly and the swelling ratio exhibited a down tendency. Furthermore, NI-HA-gels possessed a rapid response to changes in temperature, and the most prominent characteristic of NI-HA gels was that the change of OP4-AC content had no obvious effect on the response rate and transition temperature. We also followed the swelling/de-swelling behaviors of NI-HA-gels, and the results indicated that NI-HA-gels possessed excellent swelling/de-swelling switching behavior and swelling stability. In conclusion, we made a more in-depth research on the surfactant effect, chain entanglements effect, stress-relaxation behavior and intelligent modification of HA-gels, and then we got a more profound understanding on the cross-linked structure and construction mechanism of HA-gels.