Molecular Dynamics Simulations Of Structure And Mechanical Properties Of Physically Crosslinked Poly(vinyl Alcohol) Gels

Physically Crosslinked Poly(vinyl alcohol)gels have good mechanical properties and have been applied widely in many fields.In this paper,Considering water content,temperature and various solvents,we employed molecular dynamics simulations to study the equilibrated structure,uniaxial tensile deformation mechanisms and the effect of fiber orientation on mechanical properties and diffusion anisotropy.Our researches have important academic value and application prospect.The Monte Carlo self-avoiding random walks method was used to build the initial complex three dimensional polymer network,then the solvents were added to the network and some molecular dynamics simulations were used to equilibrate the initial gel models.We firstly built PVA hydrogel models with different water content,then Dimethyl Sulphoxide(DMSO),water and a mixture of DMSO and water(4:1 by weight)were added to the network to build PVA/DSMO,PVA/W and PVA/DMSO/W precursor gels,respectively.The solvents in the precursor gels were then exchanged with water to obtain three kinds of PVA hydrogels.Solvent in the PVA/DMSO/W precursor gel was also exchanged with ethanol(EtOH)and DMSO to obtain PVA/EtOH and PVA/DMSO gel models,respectively.We studied the equilibrated structures of PVA gels by some common methods in molecular dynamics simulation fields.We found the DMSO and ethanol make the polymer congregate more easily by analyzing the radial distribution function for hydroxyl oxygen and oxygen in solvent molecule and dihedral angle distribution of PVA backbone carbons.The water content,temperature and various solvents also have great influence on the hydrogen bonds between hydroxyls and between hydroxyl and solvents.By calculating the mean square displacement(MSD)of PVA backbone carbons and solvents,we found various solvents have significant influence on the mobility of polymer chains.We then applied simulations of uniaxial tensile deformation on the equilibrated gel models,and the stress-strain curves we obtained are very close to the experiment results.The curve comprises the toe region,linear region and yield and failure region.We found the tensile strength of PVA hydrogel is decreasing with increasing water content or temperature.The hydrogel built by exchanging solvents in PVA/DMSO/W precursor gel with water has higher tensile strength.The tensile strength of hydrogel is also higher than PVA/DMSO or PVA/EtOH gels which build from same precursor gel.By analyzing the potential energy decomposition for polymer network,we found,in the toe region of stress-strain curve,the bond stretching and bond angel bending energy are almost constant and the non-bond energy dominates the region.But in the linear region,the bond stretching and bond angel bending energy increase rapidly,and dominate the whole region.In the yield region,potential energy changes a little.Water content,temperature and various solvents mainly influence the bond stretching and bond angle bending energy.We found the proportion of trans dihedral angle increase rapidly in the linear region.This indicates the torsional activity of polymer chains is much stronger,especially,the ethanol is more easily to promote the torsional activity than DMSO or water.The hydrogen bonds in PVA hydrogel are stable when the strain is small,and then the number of hydrogen bonds between hydroxyl and water decrease,the hydrogen bonds between hydroxyl increase with increasing strain.Finally,we obtained gel models with polymer chains mainly align along x axis by applying initial strain on the equilibrated PVA hydrogel model,then study the influence of degree of alignment on the mechanical properties anisotropy and diffusion anisotropy of water.The results show that higher degree of alignment of polymer chains leads bigger difference between stress parallel and perpendicular to x axis and bigger strength parallel to x axis.When the degree of alignment beyond a certain range,the difference between diffusion coefficient of water parallel and perpendicular to x axis become bigger with increasing degree.The whole diffusion coefficient of water increases with increasing degree of alignment very slowly.