Synthesis And Adhesion Of Polymeric Hydrogels

Polymeric hydrogels are materials with three-dimensional network structure obtained by crosslinking hydrophilic polymers which are ideal candidates for biocompatible materials because of their ability to swell in water, their softness similar to living tissue. By controlling the crosslinking degree of the three-dimensional network of hydrogel, adjusting the functional groups of polymer chains, and changing the external environment, the swelling degree and adhesive property of hydrogel can be controlled. Therefore, biocompatible hydrogels have a wide range of applications in the field of contact lenses, burn dressings, controlled drug release system and clinical adhesives. Currently, most researches are focused on exploring the mechanical properties and swelling properties, as well as their stimulation response. However, the study on the adhesive performance of polymeric hydrogel is still limited.In this work, a variety of polymeric hydrogels by chemically crosslinking, physically crosslinking and mixed crosslinking methods were designed and synthesized. Their adhesive property was studied systematically. Hydrogels based on modified methyl vinyl ether-maleic anhydride copolymer, poly(acrylic acid), poly(acrylic amide) and poly(methacrylic acid) were synthesized by chemically crosslinking and physically crosslinking or both. Then rheological method was applied to study the gelling process of hydrogel, especially the changing trend of the elastic modulus and viscous modulus with frequency near the gel point and their correlation with the adhesion. Meanwhile, the surface adhesion energy of hydrogels was studied by using a self-made surface energy measurement instrument. The mechanism of adhesion of polymeric hydrogel was revealed by combining rheological property, macroscopic adhesive property and the surface adhesion energy, which lays a foundation for their application in the field of denture adhesives, tissue engineering, wound dressings and transdermal drug delivery. The main results of this thesis are summarized as follows:1. Poly(vinyl acetate-vinyl alcohol)(PVAc-VA) were prepared by partially hydrolyzing PVAc with acid catalyst in a mixture solvent of ethanol and water. The amphiphilic polymeric hydrogel with tunable adhesion was obtained by chemically crosslinking PVAc-VA with poly(methyl vinyl ether maleic anhydride)(trade name:Gantrez). The hydrogel which was both hydrophilic and hydrophobic possess an ability of binding both hydrophilic and hydrophobic surfaces, should be an ideal biocompatible adhesive. The hydrolysis degree of PVAc-VA was characterized by1H NMR and it was found that the hydrolysis degree of PVAc can be controlled by the amount of catalyst. The180°peel strength of the polymeric hydrogel reduced with the increase of hydrolysis degree of PVAc due to the increased cross-linking degree resulting from the excessive hydroxyl group produced by the hydrolysis caused the declining of flowability, which made the hydrogel could not penetrate the surface. In addition, it was found that the peel strength increased with increasing molecular weight of Gantrez while reduced with the increasing molecular weight of PVAc. The rheological results indicated that storage modulus (G’) and viscous modulus (G") parallel with each other in a wide frequency range and showed a power relationship with frequency in the vicinity of critical gel point. Here, the hydrogel was found with a moderate cross-linking degree and the system reached a balance between cohesion and flowability. Meanwhile, the hydrogel got the maximum peel strength and the best adhesive performance. Besides, another biocompatible hydrogel was synthesized by crosslinking poly(ethylene glycol)(PEG) with Gantrez, and the effect of PEG molecular weight and the molar ratio of these two components on rheological properties of the hydrogels were investigated. Rheological results indicated that hydrogel shows that elastic modulus (G’) and viscous modulus (G") were close to each other and the system was near the critical gel point when the molar ratio of Gantrez and PEG was10:1. Swelling result shows that the obtained hydrogels exhibited high swelling ratio and their swelling behavior were significantly influenced by pH.2. Uniform and transparent poly(acrylic amide)(AAm) nanocomposite hydrogels were prepared by in situ polymerization using Laponite as crosslinker. They possessed great tensile strength which increased with the increasing amount of Laponite and monomer (AAm) and elongation was generally above1200%. In addition, biocompatible and transparent poly(acrylic acid)/Laponite nanocomposite hydrogels were synthesized. They had both excellent ductility and adhesion. It was found that the elastic modulus (G’), viscous modulus (G") and viscosity were all increased with the increasing Laponite and mononer (AA) content; The180°peel strength was found to reach a maximum with increasing Laponite and AA content, which indicated that the adhesion obtained the best performance when the cohesion and flowability of hydrogels achieved a balance. Due to the presence of-COOH group in the system, the hydrogels showed pH response. JKR instrument was used to characterize the surface adhesion energy, and the surface adhesion of poly(acrylic acid)/Laponite nanocomposite hydrogel increased with AA and Laponite content within the experimental range.3. Poly(methyl acrylic acid)/Laponite nanocomposite hydrogels were prepared by using both chemically crosslinking agent N,N’-methylene-bis-acrylamide (BIS) and physically crosslinking agent Laponite. The transparency and extensibility of the system were decreased significantly by increasing the amount of BIS, while the addition of Laponite helped to increase the transparency and extension degree of hydrogel. The effect of crosslinking agent on the adhesive properties of hydrogel was also investigated. It was found that the increased amount of BIS apparently deteriorated the adhesion of nanocomposite hydrogels.4. Two generations of JKR surface energy instruments with different functions were made by ourselves in the laboratory based on Johnson-Kendall-Roberts (JKR) theory, and the surface adhesive property of the polymeric hydrogels was studied by using these instruments. In the test, the sample’s contact and separation speed could reach the level of μm/s by precisely controlling the instrument, and the deformation energy loss in the contact process was greatly reduced, which enabled a more accurate determination of consumed work in the separation process, namely the surface adhesion energy. The test result of the JKR surface energy instruments exhibited great repeatability. The instrument was utilized to study the surface adhesive properties of a series of polymeric hydrogels. The result showed that the JKR instrument should be an ideal tool for the characterization of surface adhesive property of soft materials such as polymeric hydrogels which could be able to supplement to the macroscopic adhesive performance such as180°peel strength. The development of such instruments provided a new method for studying materials’adhesive behavior. Furthermore, it has great significance on acquiring an insightful cognizance on the adhesion mechanism of soft materials and manipulating their adhesive performances more effectively.