| In this work, an attempt was made to understand the swelling and mechanical properties of the hydrogels as a function of certain key parameters and investigated the effect of these parameters on the properties. Two known biocompatible (and FDA approved) hydrogel systems were prepared using thermal polymerization or UV-initiated polymerization. One system is the neutral hydrogel of N-vinyl-2-pyrrolidinone (VP) and 2-hydroxyethyl methacrylate (HEMA) copolymer and the other system is the ionic hydrogel system of N-vinyl-2-pyrrolidinone (VP) and acrylic acid (AA) copolymer. The systematic control over the degree, rate, and direction (anisotropy) of swelling of the hydrogels was focused to study. The control of gel swelling was achieved by regulating the network structure and composition of gels as well as environmental conditions.;Using VP/HEMA hydrogels, the effect of comonomer composition, ultraviolet (UV) irradiation intensity controlled by using the photomasks developed in this lab, concentration of photoinitiator (2,2-Dimethoxy-2-phenylacetophenone (Irgacure 651)) and cross-linker (N, N-methylene bisacrylamide (BIS)), the type of crosslinker as well as polymerization methods on the swelling degree and kinetics, structural characteristics, and mechanical modulus of a neutral hydrogel were investigated. Additionally, the ionizable moieties of AA instead of neutral species of HEMA were incorporated into the gel network, and the obtained ionic VP/AA hydrogel displayed a much improved degree of swelling. Using the VP/AA hydrogels, the swelling behavior, structure and mechanical modulus of the hydrogels were investigated as a function of concentration of ionizable AA species (CAA), and the effects of pH and temperature on their equilibrium swelling ratios were analyzed. Through these experimental performances and characterizations of gel/hydrogel systems, the hydrogel system producing the maximum equilibrium swelling ratio and slow enough swelling rate was determined to be UV-initiated polymerized VP/AA gel with the CAA of 40 wt. %. Moreover, by investigating the effect of crosslinker concentration and type, initiator concentration and type, and polymerization method on the degree and kinetics of swelling and mechanical properties of the VP/AA hydrogel with the CAA of 40 wt. %, the optimized gel system was obtained to be the VP/AA gel crosslinked by thermal polymerization with the VP:AA feed weight ratio of 6:4, with the initiator AIBN of 0.4 wt. % with the crosslinker BIS of 0.2 wt. %, (TVAAB5b gel), which was used for anisotropic swelling studies.;Some reports of the observations on anisotropic swelling of gels have been presented. However, this has been not researched intensively and indeed systematic studies on control over the anisotropic swelling of hydrogels have not been reported. In this thesis, two novel methods to prepare the gel system showing controlled anisotropic swelling are presented. One method is to induce anisotropic swelling through a structural gradient VP/HEMA gels synthesized by UV polymerization using gradient photomasks. The other method is to obtain compressive stress induced anisotropic swelling. The cylindrical gels were compressed by controlling strain from 70% to 95%, strain rate from 0.0001 s-1 to 0.001 s-1 and temperature from 100°C to 150°C. Controlled anisotropic swelling of compressed gels was successfully obtained and the achieved maximum swelling anisotropy (DSA), which is ratio of the equilibrium height swelling ratio to equilibrium diameter swelling ratio, was 25 +/- 3.2. Among the tens of hydrogel systems developed for the use of applications such as cleft palate or syndactyly surgeries, TVAAB5b gel was decided to be the optimal gel system producing isotropic swelling base on the key properties such swelling rate, mechanical properties. The TVAAB5b gel compressed with strain rate of 0.0001 s-1 up to strain of 90% at temperature of 110°C was determined to be the most optimized gel system producing anisotropic welling and the DSA, swelling rate, and time to reach equilibrium state of the gel were determined to be 22.0, 0.0019 min-1, and 14.4, respectively. The anisotropic swelling behavior was explained by a thermodynamic cycle first proposed in this thesis. (Abstract shortened by UMI.). |
