Water and solute transport in responsive hydrogels of poly(ethylene glycol) and poly(methacrylic acid)

Interpolymer complexation occurs by hydrogen bonding between carboxylic groups on poly(methacrylic acid) (PMAA) and ether groups on poly(ethylene glycol) (PEG). This complexation is sensitive to the surrounding environment as complexes only form at pH levels low enough to insure substantial protonation of PMAA acid groups. At high pH, the acid groups become neutralized and do not form complexes. Copolymers containing PMAA and PEG exhibit pH-sensitive, reversible complexation within their own structure. Grafted poly(methacrylic acid-g-ethylene glycol) (P(MAA-g-EG)) copolymers were synthesized and their pH sensitivity investigated as a function of copolymer composition and PEG graft molecular weight. P(MAA-g-EG) membranes showed pH-sensitivity due to complex formation and dissociation. Uncomplexed equilibrium swelling ratios were 40 to 90 times higher than those of the complexed states and varied according to copolymer composition and PEG graft length. Mesh sizes in the two states were determined. Swelling under oscillatory pH conditions revealed the dynamic sensitivity of P(MAA-g-EG) membranes as well as the diffusional mechanisms causing network expansion and collapse. Network collapse (complexation) occurred more rapidly than network expansion (decomplexation) because of the rates of diffusion of specific ions causing the responses. A Boltzmann superposition model was used to analyze this behavior. Solute diffusivity was higher in uncomplexed than in complexed P(MAA-g-EG) membranes and decreased as solute size increased. Lower diffusivities resulted in membranes containing longer PEG grafts since in the uncomplexed state the PEG grafts dangled into the polymer mesh space. Membrane permeability was responsive to changing pH conditions and separation of solutes was achieved. Mechanical testing helped evaluate the strength of P(MAA-g-EG) membranes and further elucidated the mesh size under various conditions. ATR-FTIR spectroscopy and SEM were utilized to investigate hydrogen bonded complexes and the differences in membrane surfaces due to complexation