Phase behavior of aqueous polyacrylic acid solutions using atomistic molecular dynamics simulations of model oligomers

- Atomistic MD simulations of polyacrylic acid (PAA) have been performed.
- We study the effects of tacticity, concentration, and pH on the phase behavior.
- Aggregates form at high concentrations and their water content increase with pH.
- Results of this study may be useful for the design of pH-responsive drug carriers.

We have performed fully atomistic molecular dynamics simulations of aqueous solutions of a weak, pH-responsive polyelectrolyte, polyacrylic acid (PAA). Model oligomers of PAA of different tacticities, molecular weights, degrees of deprotonation, and deprotonation patterns are simulated with water molecules. Deprotonation of PAA chains that occurs with an increase in pH results in an increase in Coulomb repulsion between chain segments on one hand, and a non-monotonic change in the hydrogen bonding between chain segments on the other hand. Consequently, at the single chain level, PAA chains are stretched at higher pH values, where the amount of stretching varies with chain tacticity. For the multiple chains case, PAA forms aggregates at higher concentrations, which are relatively denser and contain lesser water (solid-like) at lower pH than compared to higher pH (liquid-like). Such phase transitions of PAA aggregates with pH has possible implications in the design of pH-responsive polyelectrolytes for applications in drug delivery.

498