jp9b01696_si_001.pdf (3.25 MB)
Effect of pH and Molecular Length on the Structure and Dynamics of Short Poly(acrylic acid) in Dilute Solution: Detailed Molecular Dynamics Study
journal contribution
posted on 2019-04-09, 00:00 authored by Dimitris
G. Mintis, Vlasis G. MavrantzasLong
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 ⟨Rg2⟩0.5changes with N as ⟨Rg2⟩0.5 ∼ Nν, 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 Lp 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 ∼ Nν, 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.