Synthetic pH sensitive polyampholyte hydrogels: A preliminary study

Abstract

A combined capillary drop viscometer-pH cell has been constructed and used to study the solution properties of some polymeric weak acids, bases, and ampholytes containing methacrylic acid (MAA), dimethylaminoethyl methacrylate (DMAEMA), and 2-hydroxyethyl methacrylate (HEMA). The behavior of polyelectrolytes based on MAA or DMAEMA is consistent with established thinking, and is dominated by the chain expansions which accompany ionization of acidic (MAA) or protonation of basic (DMAEMA) functionality. The solution behavior of the polyampholytes is complex and was studied as a preliminary to investigations of crosslinked hydrogel analogues. Briefly, it was found that changes in polymer coil dimensions were dictated by the formation or “breakage” of intramolecular salt bridges by, for example, lowering the solution pH to protonate weakly acidic, or raising the pH to deprotonate weakly basic functionality. Similar effects were observed on adding counterions which preferentially complex with one of the bound ions. The pH “sensitivity” of polymer coil dimensions increased with the concentration of charged functionality in the polymer. Chemically crosslinked polyampholytes were prepared and equilibrium water contents were measured as a function of pH. In contrast, due to network constraints, the pH “sensitivity” of the hydrogels, as measured by changes in equilibrium water uptake, increased with a decrease in the level of charged functionality in the polymer.