Studies On Preparation And Properties Of Aqueous Solutions And Hydrogels Of Hydrophobically Modified Polyacrylamide

Hydrophobically associating polymers consist of a water-soluble polymer containing a small number of hydrophobic groups. In aqueous solution, intermolecular hydrophobic interactions lead to the formation of polymolecular association above critical associating concentration. As a result, the structure and properties of these copolymers are different from traditional water soluble polymers. Hydrophobically modified polyacrylamide (HMPAM) is an important species of hydrophobically associating polymers, and has been studied widely. These studies have focused primarily on the chemical structure and solution properties of HMPAM. However, much less attention has been paid to hydrogels containing HMPAM, which is limited primarily to hydrogels with poor mechanical properties.As is well known, weak mechanical strength of hydrogels is the main factor of restricting their applications. To overcome this shortcoming, up to now, various hydrogels with good mechanical strength have been developed. However, the lack of the capability of re-forming resulting from their permanent cross-linking structure, the complex synthesis process and a narrow range of reactor monomer selectivity restrict their industrial and biomedical applications. Therefore, the design of hydrogels with good mechanical strength, self-healing and remolding capability has been an important topic in the field of hydrogel.Hydrophobic interaction is a reversible physical role in HMPAM solutions. If structure of hydrophobic association is designed properly, water-soluble polymer backbone chain could construct three-dimensional cross-linked network and a novel physical hydrogels, hydrophobic association hydrogels (HA-gels), can be obtained. In HA-gels, a large number of hydrophobic association domains are formed by hydrophobic association of hydrophobic groups belonging to two or more polymer chains, and these hydrophobic association domains act as the physical cross-linking points in a network of hydrogels. As long as the hydrophobic interaction is sufficiently strong, HA-gels will exhibit good mechanical strength. Furthermore, the reversible property of hydrophobic interaction will endow the property of structural reorganization of HMPAM. Therefore, HA-gels will be possessed of the capability of self-healing and remolding. Based on this research idea, this paper explored the method achieved of structural design and performance for HA-gels with good mechanical strength, self-healing and remolding capability, which provides the theoretical and experimental basis for the basic and applied research of HA-gels.To begin with, surfactant macromonomer (surfomer), octylphenol polyoxyethylene acrylate (OP-10-AC), was synthesized using acryloyl chloride (AC) and octylphenol polyoxyethylene ether (OP-10), and then OP-10-AC was copolymerized with acrylamide (AM) to form novel HMPAM (P(AM/OP-10-AC)) through micellar copolymerization. Structural feature of P(AM/OP-10-AC) and rheology behavior of its aqueous solution were investigated in detail. The results of rheology study show that content of hydrophobic monomer in P(AM/OP-10-AC), polymer solution concentration and test methods powerfully influenced shear viscosity of aqueous solutions of P(AM/OP-10-AC). In addition, depending on the oscillatory experimental results, there is the critical hydrogel concentration for aqueous solutions of P(AM/OP-10-AC). Meanwhile, for P(AM/OP-10-AC), content of hydrophobic monomer and its solution concentration also strongly influenced complex viscosity of its aqueous solution.On the base of previous research results, octylphenol polyoxyethylene acrylate (OP-4-AC) was synthesized and a preparation method of HA-gels was explored on the condition of OP-4-AC as hydrophobic monomer. Finally a simple preparation method was established. HA-gels were prepared indirectly in mould by micellar copolymerization of AM and a small amount of OP-4-AC in an aqueous solution containing SDS at 50℃. HA-gel prepared was a novel physical hydrogel with excellent mechanical properties and transparency. A large number of associated micelles were formed by hydrophobic associations of SDS and hydrophobic groups belonging to two or more HMPAM chains. Such associated micelles acted as effective cross-linkers. Consequently, the three-dimensional network of HA-gels could be constructed. Especially, no external cross-linker was used for the formation of their network structure. This unique network endows various properties of HA-gels.The results show that mechanical properties of HA-gels strongly depended on the content of compositions in initial reaction solution. For HA-gels, with increasing OP-4-AC content in the range of OP-4-AC used, elastic modulus exhibited increases, the elongations at break showed monotonous decreased, and tensile strength and fracture energy first increased and then decreased; with increasing SDS content in the range of SDS used, tensile strength and elastic modulus decreased, elongations at break showed increases, and the general trend of fracture energy exhibited decreases; with increasing AM content in the range of AM used, tensile strength and elastic modulus increased, elongations at break decreased, and the general trend of fracture energy exhibited increases. On the base of the micellar copolymerization theory and uniaxial stretching data, the formation mechanism of HA-gels was also investigated. Similar to rubbery, HA-gels exhibited obvious thermoelastic behavior and also showed a good rubberlike elastic property.For HA-gels, the capability of self-healing and remolding results mainly from their unique network. Because the network of HA-gels is a reversible three-dimensional physical cross-linked network, the structure rearrangement of cross-linked network can be achieved through dissociation and reassociation of cross-linking points under the condition of external forces. The conclusion consisted with experimental result of stress relaxation for HA-gels. HA-gels possessed unusual swelling behavior. The swelling process of HA-gels can be divided into five stages: over-swelling (Ⅰ), self-deswelling (Ⅱ), dynamic equilibrium of disassociation and re-association (Ⅲ), disassociation enjoying the advantage (Ⅳ) and dissolution (Ⅴ). The stability of HA-gels in water strongly depended on their construction and external environment. In addition, the life of HA-gels in the immersing liquid, which starts on the beginning of swelling test and ends on the beginning of the fifth stage of swelling process, can be adjusted in accordance with application requirements by changing hydrogel chemical structure (the type and amount of hydrophobic monomer) and the immersion environment from a few days to 6 months or more.In this paper, the research results demonstrate the feasibility of research ideas, and HA-gels represented important study value. Therefore, as a sort of advanced and environmentally friendly soft materials, HA-gels would have broad application prospects.