Polyethylene glycol (PEG) is an important polymer that has an enormous area of application especially in biomedical field such as drug carrier agent, hydrogel, and tissue engineering material. A thorough understanding of the molecular structure of PEG at different environments is important to devise better routes to develop materials with desired purposes. Although, PEG has been a widely studied material its molecular structure in bulk and in aqueous environment has been poorly investigated. In this work, we strive to perform multi-scale molecular simulations in order to study the physical behavior and molecular structure of PEG in dry environment and interacting with water. We initially perform Dissipative Particle Dynamics (DPD) simulations to create the structure of PEG. Later, we perform reverse-mapping of the atomistic coordinates to perform atomistic molecular dynamics simulations in order to observe the chain configurations and material properties of PEG.
DPD simulations result in successful prediction of the negative volume excess of PEG upon mixing with water. Here, we use a recent extension of the DPD method, where the non-bonded potential of DPD is modified by a Morse potential term to mimic the intermolecular attraction. The parameterization of the Morse potential is done by a mapping of the mixing energies of different hydrogen bonding bead pairs and from radial distribution functions (RDF) for the energy and equilibrium hydrogen bond length terms, respectively. DPD simulations yield preferential attraction of some chemical groups to water as quantified by radial distribution functions. Moreover, PEG chain structure is observed to deviate significantly from a random coil structure as a result of the end-to-end distances and radius of gyration values. Moreover, DPD simulations reveal that water plays a significant role in increasing the flexibility of PEG chains. The helical structures of PEG chains are quantified and a significant helicity of PEG chains is noted with a higher fraction in wet environment.
The reverse-mapped coordinates of DPD simulations of bulk PEG structure are used in atomistic simulations to compute the material properties such as coefficient of thermal expansion, elastic modulus and Poisson’s ratio. The computed coefficient of thermal expansion is in line with the experimental prediction. The computed values mainly reveal that PEG becomes more elastic and less compressible upon addition of water. In addition, the experimental aggregation of water in PEG is observed from MD simulations. In all, the multi-scale procedure predicts the structural and material properties of a widely used PEG in dry and wet environment. The procedure presented herein can be applied to similar materials to gain a molecular and macroscopic understanding of their properties.