A Molecular Simulation Study On The Structure-property Relationship Of Polymeric Hydrogels

Most parts of human body except bones are composed of hydrogels.Hydrogels are soft materials that can find wide applications in biomaterials such as artificial cartilage and drug delivery system.To develop gels with desired mechanical properties,it is important to know the relationship between micro network structure and macro properties of gels.While at present theories that describes gel properties especially mechanical behaviors are still insufficient.The reason is:(1)Most experiments do not allow access of all the microscopic parameters;(2)Theoretical descriptions are very complicated and usually contain several adjustable parameters.Computer simulations can control Details of the microscopic topology and is possible to isolate there effect on macroscopic properties.Thus it is a powerful tool in investigating the complex hydrogel system.With the combination use of computer simulation,experiment and theory,this thesis investigated the following issues:(1)In this work,we report three-dimensional(3-D)coarse-grained Monte Carlo algorithms to simulate the conformations of swollen hydrogels formed by copper(I)-catalyzed azide-alkyne cycloaddition(CuAAC).The simulation consists of three successive steps including diffusion,cross-linking and relaxation.The cross-linking of multifunctional reaction sites is simulated instantly followed by fast crosslinking.In order to explore the validity of this approach pristine poly(ethylene glycol)(PEG)hydrogels with tri and tetra-functional reaction sites(G3 and G4 respectively)were prepared and characterized.The data from the simulations were found to be in good agreement with experimental results such as PEG2k lengths between crosslinks,pore volume and pore radius distribution,indicating the validity of the modeling algorithm.The calculated PEG2k lengths in G3 and G4 networks are close(?4.6 nm).The 3-D visual topological structure of the hydrogel network suggests that the "ideal"hydrogel is far from cubic,diamond or any well defined structures of regular repeating cells.(2)In this study,controlled amount of dangling ends is introduced to the two series of poly(ethylene glycol)-based hydrogel networks with 3 and 4 cross-linking functionality by using click chemistry.The structure of the gels with regulated defect percentage is confirmed by comparing the results of low-field NMR characterization and Monte Carlo simulation.The mechanical properties of these gels were characterized by tensile stress-strain behaviors of the gels,and the results are analyzed by Gent model and Mooney-Rivlin model.The shear modulus of the swollen gels is found to be dependent on the functionality of the network,and decreases with the defect percentage.Furthermore,the value of shear modulus well obeys the phantom model for all the gels with varied percentage of the defects.The maximum extension ratio,obtained from the fitting of Gent model,is also found to be dependent on the functionality of the network,and does not change with the defect percentage,except at very high defect percentage.The value of the maximum extension ratio is between that predicted from phantom model and the affine model.This indicates that at the large deformation,the fluctuation of the cross-linking points is suppressed for some extend but still exists.Polymer volume fractions at various defect percentages obtained from prediction of Flory-Rehner model are found to be in well agreement with the swelling experiment.All these results indicate that click chemistry is a powerful method to regulate the network structure and mechanical properties of the gels.(3)In this study,the effects of dangling end defect on inhomogeneity and mechanical properties of end-linked polymer networks are studied by molecular simulation.The effect of the pre-solution concentration is also discussed.Dangling end is found to have a larger effect on inhomogeneity of 4-functional network than 3-funcional network.Spatial inhomogeneity is demonstrated to have minor effect on mechanical properties.The large statistical error of maximum elongation ratio originates from uneven distribution of chain length,even an end-linked polymer network with a unique pre-polymer is used.(4)In this work,we conducted a combined experiment and simulation study on effect of pores on gel's mechanical properties.Poly(ethylene glycol)-based hydrogels with or without pores are prepared by cross-linking linear polymers using click chemistry.Pore structures are introduced using SiO2 particles with different diameter as pore forming agent.The pore structures are confirmed by morphology and thermoporosimetry.The mechanical properties of these gels were characterized by tensile stress-strain behaviors of the gels,and the tensile curves are fitted by Gent model.The maximum extension ratio is found to depend on pore size.Simulation result reveals that pores have an inverse effect on network mechanical properties by enhancing stress concentration around pores in equilibrated network.This effect vanishes when pore radius is below about two times that of network mesh size.