Role of Chemical Linkage in Solvation of Polyurethanes in Organic Solvents Studied by Explicit Molecular Dynamics Simulations

Abstract

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.