Upendra Natarajan

Molecular dynamics simulations are used to study the structure and dynamics of poly(vinyl alcohol) and water in aqueous solution as a function of concentration at different temperatures in the range of 278-338 K. Simulations were performed using multiple oligomeric chains for facilitating interchain interactions as well as a direct comparison with experimental data. PVA chains fold and bundle up to form an aggregate in solution. The intermolecular spatial distributions show the structure of aggregate to be ordered. PVA chains show a high tendency to form intrachain hydrogen bonds between adjacent repeating units, instead of interchain H-bonds, indicating hydrophobic effect as the major driving force for aggregate formation. At all temperatures, the conformations of a single PVA chain by itself in solution are unstable, going back and forth between extended and folded states. However, interchain interactions among PVA chains in the aggregate stabilize the folded conformation. An increase in temperature results in faster motions and an increase in concentration results in slower dynamics. At higher concentration, the chains adopt a single folded state independent of temperature so that there is an insignificant effect on Rg. The competition between the formation of various hydrogen bonds such as intrachain, interchain, and PVA-water is the key to understand the solvation behavior of PVA. The activation energy for the conformational transition between the trans and gauche states of backbone dihedrals obtained from the simulations is 15.73 kJ/mol, which is close to the value of 13.4 kJ/mol obtained from experiments for 15 wt % PVA solution. The hydrophobic effect rather than interchain PVA hydrogen bonding is the major driving force for the aggregation of PVA in water.

Many polyurethanes are prepared and processed in solution to realize their applications related to coatings and nanofibers. An understanding of the molecular-level interaction between polyurethane and solvent is important for polymer preparation and processing. Atomistic molecular dynamics simulations of two polyurethanes, poly(MDI/EG) (PMDIE) and poly(TDI/EG) (PTDIE), in two polar aprotic solvents, namely, N,N-dimethylformamide (DMF) and tetrahydrofuran (THF), were performed to investigate various aspects of polymer-solvent interactions. The polyurethane chains simulated were composed of either methylene diphenyl diisocyanate (MDI) or toluene diisocyanate (TDI) as the hard segment and ethylene glycol (EG) as the chain extender. Chain conformational properties, hydrogen bonding between polyurethane and solvent, distribution of solvent around different segments of the polyurethane chain, and polymer-solvent interaction energy were obtained. The autocorrelation functions of different dihedral angles suggest that the dihedral dynamics is influenced by the chemical structure of the hard segment and solvent. Hydrogen bonding between polyurethane and solvent shows that the nitrogen atom of the urethane linkage is the major donor and the oxygen atom of the solvent is the major acceptor. The radial distribution curves show that the polyurethane chain in the absence of a soft segment solvated better in DMF in comparison to the THF solvent. The interaction energy of different polyurethane segments obtained from the simulations suggests that the ethylene segment is the major contributor to favorable interaction with the solvent. Flory-Huggins interaction parameters were calculated, and the estimated values are in reasonable agreement with available data in the literature. Our study suggests that the greater hydrogen bonding of urethane linkage with DMF could be responsible for enhancing the bonding between polyurethane fibers that causes the formation of smaller-diameter nanofibers in DMF than in THF. This modeling approach and the results of this study pave the way for an improved understanding that may be relevant for the design of polyurethane chemical structure and solvent selection for processing.

We have performed atomistic MD simulations to investigate structure and hydration properties of isotactic Poly(methacrylic acid) (i-PMA) at CCl4–H2O interface at different concentration up to monolayer coverage. Three concentration regimes dilute, semi-dilute and concentrated are shown to occur at molecular level detail for the first time for polymer systems at oil-water interface. i-PMA chains at low interface concentration exists in a planar conformation along with an ordered arrangement of side groups and changes to a random coil structure at high concentration, which is observed from the variation of components of Rg and decrease in preferential orientation of groups. An abrupt change in chain conformation marks the end of the dilute regime and a rapid increase in thickness of adsorbed layer marks the beginning of the concentrated regime. This study provides new molecular insights into conformational changes and the associated changes in interactions among different chemical groups with variation of the interface concentration of polymer.

Atomistic molecular dynamic simulations were performed to investigate the adsorption behavior of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA) at the CCl4-H2O interface for isotactic, atactic, and syndiotactic forms. The conformational, orientation, and solvation behaviors of PAA and PMA chains at the interface were studied as a function of the degree of ionization (f). The calculated density profiles show that adsorption occurs only when degree of ionization is less than a critical value (ionization of 20% groups). The density profiles of different groups show the existence of carboxylic acid and carboxylate groups toward the aqueous phase and methyl groups toward the oil phase, relative to the interface. The radius of gyration values and dihedral distributions of completely adsorbed chains (i.e., for f = 0) reveal their existence in an extended conformation at the interface, in contrast to their coiled structure in bulk aqueous solution. The size of adsorbed chains (f < 0.2) decreases with increase in degree-of-ionization due to looping of chain toward water; the extent of looping depends on the distribution of charge on the chain. The carbonyl and methyl groups of uncharged PAA and PMA show two set of orientations corresponding to direction toward water and oil phases and this preferential orientation decreases with increase in degree-of-ionization. Significant differences in orientation distribution, dihedral angle, and hydration were observed among different tacticities of PMA which primarily reflect the hydrophobic nature of isotactic PMA as compared to other tacticity. The number of hydrogen bonds between the polyelectrolyte and water is much lower at the interface relative to the bulk aqueous phase as determined by the population of water in the interface region as well as charge on the polyelectrolyte.

Conformation, intermolecular structure and hydration of different stereo-isomers of poly(methacrylic acid) and poly(acrylic acid), specifically isotactic-poly(methacrylic acid) (i-PMA), syndiotactic-poly(methacrylic acid) (s-PMA) and syndiotactic-poly(acrylic acid) (s-PAA) at vacuum-water interface were studied using molecular dynamics simulations. The adsorbed chain conformation was analyzed by radius-of-gyration components parallel and perpendicular to the interface, relative shape anisotropy parameter, distribution and autocorrelation function of dihedral angles, and orientation distribution of different groups relative to interface plane. i-PMA and s-PAA adsorb in planar conformation (i-PMA is curved compared to s-PAA) whereas s-PMA adsorbs in random coil conformation. Different chemical groups of s-PAA and i-PMA show preferential orientation whereas it is absent in the case of s-PMA. The random coil conformation of s-PMA exhibits greater intra-chain hydrogen bonding whereas extended chain of s-PAA exhibits greater polymer-water hydrogen bonding. The overall contribution to polymer-water hydrogen bonds by carbonyl oxygen atoms is greater than that by hydroxyl hydrogen atom. Water molecules hydrogen bonded to s-PAA show faster dynamics compared to those bonded to i-PMA and s-PMA. The slower dynamics of polymer-water hydrogen bonds is correlated to the slower dynamics of polymer conformations. Knowledge of orientation of chemical groups is useful for understanding binding of adsorbed polymer with other polymers and ligands.