Synthesis and characterization of polyampholytic model networks: Effects of polymer composition and architecture

Group transfer polymerization (GTP) was used for the preparation of seven ampholytic polymer networks based on 2-(dimethylamino)ethyl methacrylate (DMAEMA) and methacrylic acid (MAA). The MAA units were introduced via the polymerization of tetrahydropyranyl methacrylate (THPMA) and the acid hydrolysis of the latter after network formation. Ethylene glycol dimethacrylate (EGDMA) served as the cross-linker. Six of the networks had linear segments of accurate molecular weight between cross-links; i.e., they were model networks. In the seventh network the lengths of the segments between cross-links had a wide distribution of molecular weights (randomly cross-linked network) since the, crosslinker was copolymerized at the same time with the monomers. Four of the six model networks were based on ABA triblock copolymers with DMAEMA a mid-blocks and MAA (THPMA) end-blocks. In these networks the degree of polymerization of the DMAEMA mid-block was kept constant at 20, while the overall degrees of polymerization were 25, 30, 40, and 60. The fifth model network was based on an equimolar BAB triblock copolymer with an MAA (THPMA) mid-block and a total degree of polymerization 40. The sixth model network was based on an equimolar statistical copolymer with a total degree of polymerization 40. All linear precursors to the networks were analyzed by gel permeation chromatography and proton nuclear magnetic resonance spectroscopy to characterize their molecular weights, composition, and monomer conversion. The degrees of swelling (DSs) of all the polyampholytic networks were measured as a function of pH and were found to present a minimum at a pH value which was taken as the isoelectric point. The isoelectric point increased with the network content in DMAEMA. basic units. The isoelectric DSs of most networks were constant at 5, independent of network composition and architecture. The DSs at basic pH (similar to 11) increased sharply with the network content in MAA units. It was also found that the DS depends on network architecture, with the statistical copolymer-based model network exhibiting higher basic and acidic DSs than its isomeric counterparts.