The role of local conformations in the stretching of a poly(ethylene oxide) chain in solution

We have conducted molecular dynamics simulations of the stretching of a poly(ethylene oxide) (PEO) chain in water and n-tridecane in order to elucidate the mechanisms of elastic response of the amphiphilic PEO chain in hydrophilic and hydrophobic environments. Quantitative agreement was found for the restoring force as a function of the degree of chain extension between simulation and recent atomic force microscopy measurements in the highly and severely stretched regimes. In these regimes the free energy of the PEO chain and the restoring force are determined by changes in local conformational populations and geometries. In the highly stretched regime stretching of the PEO chain results in an increased trans population of C-C and C-O dihedrals as well as the deformation of dihedral geometries. Here the restoring force is greater in water than in n-tridecane due to the greater free energy (mostly enthalpic) penalties for C-C trans dihedrals and the deformation of C-C gauche dihedrals in the polar solvent. In both solvents the conversion of gauche C-C and C-O dihedrals to trans in the highly stretched regime makes a significantly entropic contribution to the chain free energy and restoring force. No tendency toward extended gauche helical conformations of the PEO chain in water, as has been previously speculated, was observed in this regime. An estimation of the PEO free energy as a function of the degree of chain extension based upon conformational populations relative to equilibrium (unstretched) chains was found to lead to an overestimation of the free energy cost of chain extension in the highly stretched regime in n-tridecane due to neglect of the influence of deformation of dihedral geometries on chain dimensions. In water, the influence of the deformation of dihedral geometries was largely offset by an increase in their free energy due to decreased polar interactions of the distorted dihedrals with water. In the severely stretched regime, the PEO chain was found to be in an all trans conformation, and the restoring force, which is independent of solvent in this regime, was found to be due to deformation of backbone valence angles and the trans dihedral geometry. (C) 2003 American Institute of Physics.