Effect of pH and Molecular Length on the Structure and Dynamics of Short Poly(acrylic acid) in Dilute Solution:...

Long MD simulations are carried out using a detailed all-atom force field to investigate the effect of pH or, equivalently, degree of ionization α^- (= 0, 50, 100%) and degree of polymerization N (= 20, 23, 46, 70, and 110) on the structure and dynamics of poly(acrylic acid) (PAA) at infinite dilution. To ensure the validity and add to the reliability of our research conclusions, a systematic validation of several molecular mechanics force fields is performed. It is observed that the generalized AMBER force field in combination with the RESP charge fitting method best describes both the structural and dynamical behavior of PAA in comparison to experimentally obtained data. It is found that ⟨ R_g²⟩^0.5changes with N as ⟨ R_g²⟩^0.5 ∼ N^ν, with ν = 0.27 at α^- = 0% degree of ionization (acidic conditions), ν = 0.94 at α^- = 50% degree of ionization (neutral conditions), and ν = 0.87 at α^- = 100% degree of ionization (basic conditions), which is in perfect agreement with theory. The global shape of the PAA chain in the solution is quantified in terms of the three eigenvalues of the average radius-of-gyration tensor, the relative shape anisotropy κ², and the asphericity parameter b. It is revealed that at α^- = 0%, the chain adopts a spherelike conformation, while at α^- = 50 and 100%, its conformation is flattened and flexible. In addition, it is revealed that as the degree of ionization increases, the persistence length L_p increases, which suggests that PAA chains become stiffer with increasing pH. The global and local conformational changes of the PAA chain with the degree of ionization are found to be highly related to the solvation of the polymer. Finally, it is revealed that the diffusion coefficient D of the center of mass of PAA also exhibits a power law scaling with N, D ∼ N^ν, with ν = 0.25 at α^- = 0% degree of ionization, ν = 0.46 at α^- = 50% degree of ionization (neutral conditions), and ν = 0.44 at α^- = 100% degree of ionization (basic conditions), in excellent agreement with recent experimental data and theoretical predictions.