Structural characteristics and swelling behavior of pH sensitive hydrogels
Abstract
The equilibrium swelling behavior of hydrogels sensitive to pH or ionic strength changes of the swelling medium was studied by deriving structural models based on the Flory-Huggins thermodynamic theory, the rubber elasticity theory, and ionic interaction deviations therefrom. Models were derived to describe porous or non-porous, anionic, cationic or neutral hydrogels, prepared by crosslinking in the presence or absence of solvent, and for chains with gaussian or non-Gaussian distribution. In all cases, the number average molecular weight between crosslinks, M$\sb{\rm c}$, was related to equilibrium swelling characteristics, the thermodynamic compatibility of the polymer/swelling agent system, the porosity, the ionic charges, the pK$\sb{\rm a}$ of the ionizable polymer moieties, and the pH and ionic strength of the surrounding medium. The models have predictive, correlative, and design capabilities. Copolymers were prepared by bulk polymerization of 2-hydroxyethyl methacrylate (HEMA) with any one of the following comonomers: methacrylic acid, methyl methacrylate, maleic anhydride, N-vinyl-2-pyrrolidone, methacrylamide, N,N-dimethacrylamide, N-isopropylacrylamide, and diacetone acrylamide. The HEMA content varied from 0 mol% to 100 mol%, the reaction was initiated with 0.5 wt% benzoyl peroxide, the crosslinking agent was 0.5 mol% ethylene glycol dimethacrylate, and the temperature program followed started at 40$\sp\circ$C and ended at 80$\sp\circ$C in step intervals. Equilibrium swelling studies were performed in methanol, acetone, and various buffered, aqueous solutions from pH 1.2 to 10.0. It was shown that several copolymer structures were very sensitive to pH changes. The models could predict these changes. Dynamic swelling changes were also performed in buffers of various pH values, in order to establish the mechanism of water transport in these pH sensitive structures. It was shown that relaxation-controlled mechanisms prevailed in alkaline solutions. Surface instabilities were also detected by polarized videomicroscopy. The associated effects of the structural collapse due to pH change on the diffusive behavior of the pH sensitive hydrogels were followed by performing controlled release studies of theophylline and proxyphylline in different solutions of varying acidity. These results were used to calculate the Deborah and Swelling Interface numbers, which were correlated to the pH of the solution.
Degree
Ph.D.
Advisors
Peppas, Purdue University.
Subject Area
Chemical engineering
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