Synthesis and characterization of the swelling and mechanical properties of amphiphilic ionizable model co-networks containing hydrophobic blocks.

Amphiphilic ionizable model co-networks based on near-monodisperse, linear ABA and BAB triblock and statistical copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic ionizable) and n-butyl methacrylate (BuMA, hydrophobic non-ionic) were synthesized using group-transfer polymerization in tetrahydrofuran (THF) with the use of the hydrophobic ethylene glycol dimethacrylate (EGDMA) as cross-linker. Seven model co-networks were prepared in which the architecture and copolymer composition were varied systematically. One randomly cross-linked copolymer co-network was also prepared. The co-networks were characterized in terms of their degree of swelling in water as a function of pH and in THF. An increase in the aqueous degree of swelling was observed below pH 6 because of the ionization of the DMAEMA residues. The aqueous degrees of swelling at low pH decreased with the co-network composition in hydrophobic BuMA units. The maximum aqueous degrees of swelling of the copolymer co-networks were architecture-dependent, with the co-networks comprising the statistical copolymer chains swelling about 4 times more than their triblock copolymer counterparts. This was attributed to microphase separation in the triblock copolymer co-networks, which reduced the effective chain length between cross-links due to the collapse of the hydrophobic blocks. The mechanical properties of the water-swollen co-networks at pH 3 and 9 were investigated by determining the co-network uniaxial compression modulus. This modulus was higher at pH 9 than 3, and increased linearly with the BuMA content.