Thermomechanical analysis and pH-triggered elastic response of charge-balanced sulfonated poly(tertiary amine-methacrylate)-based terpolymer cryogels

pH-responsive terpolymer poly(hydroxyethyl methacrylate-2-acrylamido-2-methyl-propanosulfonic acid-diethylaminoethyl methacrylate) cryogels and hydrogels (named as CGs-PHA/Dm and HGs-PHA/Dm) were synthesized in ethanol-water mixture using different monomer feed compositions. By the combination of three monomers, an interesting system was designed with tunable pH/temperature responsiveness and elasticity. ATR-FTIR spectrums obtained after swelling swelling in pH buffer solution of 2.1 indicated –R₂NH⁺ stretching of DEAEMA moities in terpolymer matrices. By incorporation of DEAEMA units with ionizable amino groups, terpolymer CGs-PHA/Dm and HGs-PHA/Dm gels exhibited pH-responsive swelling behavior. Inclusion of DEAEMA dramatically altered pH-responsiveness of terpolymers in simulated intestinal fluid at pH 6.6. Dependence of interaction parameter χon terpolymer volume fraction in gel ν₂by a linear function and its enthalpic/entropic components with swelling pH were determined. At pH < 6.6, dependence of χ parameter on the swelling was evaluated as χ=1.097-2.965ν₂, while at basic pH region, pH > 6.6, χ=0.576+0.459ν₂. Both pH of swelling medium and the amount of DEAEMA in terpolymer composition affected the compression moduli of terpolymer gels. Flory-Rehner theory extended to correlate the compressive modulus with swelling pH and the dependence of the effective crosslink density ν_eon mol% DEAEMA was expressed in both acidic and basic conditions. pH-triggered swelling/shrinking of terpolymer gels were studied as reversible cycles in buffer solutions of pH 2.1 and 9.8. DEAEMA-rich terpolymer gels showed overshooting phenomena due to dynamic conformational changes of diethylaminoethyl units. Terpolymer cryogels exhibited much faster swelling kinetics than that of terpolymer hydrogels in simulated gastric fluid at pH 2.1. The values of diffusion coefficient seemed to be strongly dependent of terpolymer composition.