For weak polyelectrolytes (PE) as well as for weak polyampholytes (PA), not only the pH-pK0 value but also the local electrostatic environment of the ionizable group determines the degree of ionization and structural properties of the polymer. The proximity of already ionized groups, which is enforced by the interconnectivity of the polymer chain, influences the probability of additional groups to become ionized. Therefore, the degree of ionization for polymers with different architectures differs from the ideal value, obtained for monomers at low concentrations. PA with a network topolygy are interesting, because of their possible applications for pH-triggered uptake and release of guest molecules. In this work, we investigate PA networks with different distributions of titratable groups within the network. We employ a bead-spring model of the polymers with ionizable beads, explicit counterions and implicit solvent. The model is solved using Monte Carlo simulations and we involve a special titration step, which includes both the intrinsic probability of ionization for the isolated monomer as well as the interaction of the charge with its environment in the acceptance criterion of the trial move. We assess the degree of ionization as well as the structure of the polymer in dependence of the pKA,base, pKA,Acid and pH values of the system for homopolymer polyelectrolyte networks as well as for ampholytic copolymers with different spatial distributions of acidic and basic monomers. Counterion distributions and the effective charge of the microgel were investigated as well. It is seen, that the degree of ionization can either be reduced or enhanced depending on the type and distribution of the monomers of different type in network. In accordance with experiments, two titration steps are found for polyamphoteric polymer networks with relatively close pKA,acid and pKA,base values. The degree of ionization differs, depending on the radial position of the titratable group within the microgel.

Monte Carlo simulations of weak polyampholyte microgels: pH-dependent ionization and volume change

255th National Meeting (2018)

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