Doctor of Philosophy
School of Mechanical, Material & Mechatronic Engineering
Nasrullah, Shideh, Synthesis and inter-network interaction in double network (DN) hydrogels, Doctor of Philosophy thesis, School of Mechanical, Material & Mechatronic Engineering, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3259
Hydrogels are macromolecular networks that swell, but do not dissolve, in water at physiological temperature, pH, and ionic strength. The ability of hydrogels to absorb water arises from hydrophilic functional groups attached to the polymeric backbone, while their resistance to dissolution arises from cross-links between network chains. Interpenetrating polymer networks (IPNs) are cross-linked polymer networks in which at least one network is synthesized and/or cross-linked in the presence of the other. They have been the subject of extensive study since their advance in the 1960. Despite being one of the most promising types of material for biomedical applications, their low mechanical strength is a significant disadvantage for their use in different applications which required high mechanical properties.
Double network (DN) hydrogels are a new class of IPNs that exhibits remarkable strength and toughness. The mechanism of the toughening of DN hydrogels is not clearly understood. Also one the challenges in the preparation of DN hydrogels is the highly sensitivity of their polymerization process in the presence of oxygen.
The first part of this thesis is the study of the contribution of hydrogen bonding to the mechanical properties of a range of DN hydrogels by varying its strength. The hydrogen bonding interaction was varied by using urea to disrupt its formation or by controlling the ionization degree of polyacid groups though variation of the pH of the swelling media. Uniaxial tensile and tearing tests, as well as swelling measurements, were used to study the mechanical strength of the different DN hydrogels before and after applying the variations. Also the characterisation tests, including techniques such as light transmission, particle size and rheology measurements, were conducted on mixed polymer aqueous solutions of the uncross-linked polymers based on the different DN gels. The characterisation tests were carried out to confirm the effect of urea and pH variation on the existence of hydrogen bonding in each system.
The mechanical test results showed a positive effect of hydrogen bonding on the enhancement of the mechanical properties of DN hydrogels. In particular, the effects of two types of hydrogen bonding, in the form of inter- or intrapolymer hydrogen bonds, were studied. The results indicated that interpolymer hydrogen bonding had a direct effect on the strength and toughness of DN gels. Also, intrapolymer hydrogen bonding within the second network had a similar positive effect on their mechanical properties. Intrapolymer hydrogen bonding contributed significantly to the toughness of the DN hydrogels. The results suggested that the extent of hydrogen bonds had a larger effect on the toughness of the DN gels than the strength of individual hydrogen bonds or their original type.
The second part of this thesis is a study of using the thiol-ene polymerization method to eliminate the sensitivity of the second network polymerization to the presence of oxygen. The second network of DN gels was successfully prepared in the presence of poly(acrylic acid) (PAAc) and poly(1-vinyl-2-pyrrolidinone) (PNVP) by polymerization of thiol-vinyl ether and thiol-acrylate systems. It was found that the polar aprotic solvents were the most appropriate solvents and the polymerization did not proceed with the monomer concentration of less than 20 wt%. The best pH range for thiol-ene polymerization was between pH 5.4 and 7.5. The complete conversion of thiol functional groups was important in order to achieve DN gels with high mechanical properties. The mass loss profile for most of the thiol-ene DN gels was less than 1 % for 30 days storage. The evaluation of this technique showed that it was not as effective to produce strong materials as traditional free radical polymerization, which has mainly been used to prepare DN gels.