Self-associating networks of poly(methacrylic acid-g-ethylene glycol)
Abstract
Graft copolymers of poly(ethylene glycol) with poly(methacrylic acid) were prepared by reaction of poly(ethylene glycol) methacrylate macromonomer with methacrylic acid in the presence of tetraethylene glycol dimethacrylate crosslinking agent. Solution polymerization reactions were initiated by ammonium persulfate and sodium bisulfite and polymerization took place at 37$\sp\circ$C. The poly(ethylene glycol) monomers required were synthesized by esterification reaction of methacryloyl chloride with poly(ethylene glycol). The ensuing materials were swollen in aqueous solutions with the pH ranging from 1.5 to 12.0 as well as in methanol and ethanol. It was found that the degree of swelling changes with pH, solvent composition, temperature and copolymer composition. In aqueous solvents, large increases in swelling were observed when pH was changed from acidic to basic and as temperature was reduced. Minima in the degrees were observed at acidic pH for copolymers containing a 1:1 ratio of ethylene oxide to methacrylic acid repeating units. Alcohol swollen networks exhibited a high degree of swelling; they were insensitive to changes in temperature or polymer composition. Based on these results, it was determined that swelling of poly(methacrylic acid-g-ethylene glycol) networks is regulated by hydrogen-bonded, hydrophobically stabilized complexes between the poly(ethylene glycol) and poly(methacrylic acid). Further verification of complex formation and structure was accomplished by $\sp1$H NMR spectroscopy using nuclear Overhauser enhancement (NOE) between the complexing constituents in solution. Complexes formed in water in graft copolymers containing all poly(ethylene glycol) molecular weights. Intermolecular hydrophopic stabilization of the complexes arose primarily from $\alpha$-methyl/ethylene interactions. Dynamic swelling rates were lowest in copolymer networks with 1:1 ratios of ethylene oxide to methacrylic acid. The dynamic swelling of complex forming networks was linear with time when swollen in basic solutions. These results were attributed to the combined effects of diffusion and complex dissociation during swelling. Lowest solute release rates were observed in complex-forming networks, verifying the relationship between complexation, swelling and solute permeability. The effect of polymer grafting on complexation was illustrated using a statistical thermodynamic model. Grafting promotes polymer complexation by reducing the configurational entropy change accompanying adsorption of a homopolymer from solution. Unlike homopolymer mixtures, graft copolymer complexation is largely insensitive to dilution or molecular weight.
Degree
Ph.D.
Subject Area
Chemical engineering
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