Upendra Natarajan

Structural and dynamic properties of aqueous solution of atactic poly(acrylic) acid (PAA) in dilute, semi-dilute and concentrated regimes were studied by fully atomistic molecular dynamics simulations with explicit solvent description, as a function of polymer concentration c (i.e. volume fraction φp) and charge density f. PAA size (Rg, R) decreases with φp in semi-dilute and concentrated regimes, due to increase in counter-ion condensation. For all values of f, in dilute regime (c < c*) chains are expanded and in semi-dilute regime (c*< c < c**) chains are in contact with each other, while for c ≅ c** aggregates comprising of few PAA chains occur (at f = 0.2, 0.4 and 0.7). Number of PAA intrachain h-bonds is greater than PAA–PAA interchain h-bonds at all values of f and φp. The number of h-bonds between carboxylic acid groups and carboxylate groups remain unaffected by φp. The Na+ ion self-diffusion coefficient shows linear decrease with concentration for f < 1 and exponential decrease for f = 1. The PAA self-diffusion coefficient shows power law decrease with concentration for f < 1 and exponential decrease for f = 1. Aggregation of chains is favoured due to PAA–PAA interactions with increase in concentration. Our simulation results are in agreement with experiments and coarse-grained simulations in the literature.

Inter-macromolecular interaction between uncharged polymers in aqueous solutions can be influenced by concentration of polymer, solvent quality, molecular weight, solution pH, temperature, salt concentration and type of salt. Atomistic molecular dynamics simulations of aqueous NaCl solution containing poly(acrylic acid) and poly(ethylene oxide) are presented. The size of the polymer chains decreases with an increase in salt concentration resulting from water becoming a poor solvent which enhances compositing of polymers. A close pair is formed between carboxylic hydrogen (−COOH) and ether oxygen (−O−) of the polymer at high salt concentration. The change in structure of the PAA/PEO complex is analysed using simulated SAXS and SANS profiles. The solubility of chains decreases and attains the minimum energy conformation at a particular salt concentration. The solvent accessible surface area (SASA) of the complex at higher salt concentration is less as compared to the lower. Increase in salt concentration enhances the cooperative interaction via hydrogen bonding between PAA and PEO and weakens the polymer-water-hydrogen bonding. The net result is a decrease in solubility of these polymers and thermodynamically stable polymer-polymer complex is formed at higher salt concentrations. Simulation results are in agreement with available experimental data in literature.

Atomistic modeling of amorphous trans-1,4-polybutadiene (TPBD), using molecular mechanics and molecular dynamics (MD) simulations, is performed to generate three-dimensionally periodic bulk and two-dimensionally periodic thin film condensed phases. The condensed structures are constructed using multiple polymer chains. Structural and energetic relaxations and sampling of properties are performed using MD in the canonical ensemble (NVT) by a procedure that relieves local high-energy spots and brings the system to realistic thermodynamic states. The calculated surface energy for TPBD, 30.72 erg/cm 2, is in excellent agreement with the reported experimental value of 31 erg/cm 2. The structure of the surface layers is probed in terms of the atomic mass density variations, bond-bond orientation function profiles, and the distribution of the dihedral angles about the rotatable backbone bonds. The thickness of the surface layer over which the density varies smoothly but rapidly is found to be approximately 15 Å. The level of agreement of the calculated surface energy with the experimental value is superior in comparison to previous investigations in the literature using the atomistic approach for flexible polymers. © 2012 Taylor & Francis Group, LLC.

Chain conformations, counter-ion structure, intermolecular hydrogen bonding structure and dynamics of atactic polyethacrylic acid (PEA) in salt-free aqueous dilute solution at 25°C are studied via molecular dynamics (MD) simulations with explicit-solvent and explicit-ion description for the first time. The intermolecular structure was analysed by the radial distribution functions (RDF) for specific atom types between PEA chain, water molecules and Na+ counter-ions, as well as by the hydration near the PEA chain in the solvated system. An increase in f provides an increase in 〈Rg〉 of the chain, consistent with the existence of the compact form of PEA. The simulations show expansion for radius-of-gyration with increase in f, as expected for flexible polyelectrolytes under salt-free condition. The extent of intermolecular hydrogen bonds (H-bonds) between PEA and water is enhanced by increase in f. Chains having a higher counter-ion density show higher values of 〈Rg〉, influenced by intermolecular interactions between PEA and water. The coordination of Na+ counter ions and water molecules to carboxyl oxygens of polyacrylic acid (PAA) increases with charge density of the chain. A comparison of the structure aspects is made with PAA and PMA polyelectrolytes in dilute solution, which brings out the hydrophobic effect of the ethyl side-groups in PEA on conformational properties and counter-ion condensation structure. © 2013 Taylor & Francis.