A Molecular Dynamics Study On Surface Welding And Recycling Of Covalent Adaptable Network Polymers

Crosslinked network polymers exhibit excellent mechanical strength,thermal stability,and solvent resistance.However,reprocessing and recycling crosslinked networks are extremely difficult.Once the polymerization reaction is completed,the crosslinked networks are unable to be melted,reshaped,or decomposed.The recently developed covalent adaptable networks provides opportunities to reprocess the crosslinked network polymers.Covalent adaptable networks can rearrange the network topology via bond exchange reactions.Such rearrangement allows the material exhibit unusual properties such as reshaping,surface welding and recycling,which cannot be seen in conventional crosslinked networks.In this work,molecular dynamics is used to study the construction of crosslinked network model,as well as the surface welding and solvent assisted recycling of covalent adaptable networks.The main research content and results are listed as follows:1.Molecular dynamics simulations are used to construct atomistic models of crosslinked epoxy networks.An improved algorithm,which considers both the cutoff distance criterion and the orientation criterion,has been developed to construct realistic,well-equilibrated crosslinked epoxy networks.Epoxy networks containing diglycidyl ether bisphenol-A as epoxy resin and tricarballylic acid as crosslinking agent are obtained using this algorithm.The results show that this algorithm can decrease the molecular configurations that have unrealistic newly formed bond angles.Some important properties such as glass transition temperature and coefficient of thermal expansion are calculated from the equilibrated structures.2.Surface welding of the Diels-Alder networks with thermally reversible bonds is investigated using molecular dynamics simulations.In the simulations,two separate crosslinked Diels-Alder networks are brought in contact to allow depolymerization and interdiffusion at elevated temperatures,as well as subsequent repolymerization and interfacial linkages formation at relative low temperatures.The extent of depolymerization is correlated to temperature utilizing the van't Hoff equation.The influences of welding temperature and welding time on the interfacial structure are investigated by analyzing the interfacial linkages distribution,interpenetration depth,and minimal path length.Results show that when the welding temperature is below the gel-point temperature,increasing welding time shortens the minimal path length and reduces the extensibility of the welded interface.The interfacial strength can reach the bulk value with increasing welding time when the welding temperature is above the gel-point temperature.3.Decomposition of an epoxy covalent adaptable network in ethylene glycol solvent is investigated using molecular dynamics simulations.In the simulations,a polymer film and a solvent film are constructed separately and placed in contact.The simulations describe the interdiffusion between the polymer film and the solvent film.Additionally,the degradation of polymer network is investigated by calculating the monomer number of each polymer chain and analyzing the evolution of ester conversion.Finally,the covalent adaptable network decomposition in octamethylene glycol solvent is also simulated to study the influence of solvent molecule size.Results show that,the network decomposition in small molecule solvent is controlled by the diffusion of decomposed chain segments into solvent,thus chain segments accumulate at the polymer-solvent interface and form a thick gel layer.The network decomposition in big molecule solvent is controlled by the diffusion of solvent into network.This leads to a slow decomposition with a thin gel layer at the polymer-solvent interface.4.Solvent evaporation from decomposed covalent adaptable network solutions along with repolymerization of decomposed chain segments are investigated using molecular dynamics simulations.The effects of solvent molecule size and evaporation rate on the evaporation and repolymerization processes are analyzed.Results show that a polymer-rich layer at the solution surface quickly forms when evaporating into a vacuum.The polymer-rich layer significantly inhibits the evaporation of solvent molecules with larger size.Molecules have sufficient time to diffuse when evaporating at a slow rate,thus a more homogenous network can be achieved.Overall,this thesis provides insights into the molecular-level details of the reprocessing of covalent adaptable network polymers,including surface welding,solventassisted decomposition and evaporation-induced repolymerization.This study advances the understanding of the micro-mechanism involved in reprocessing of covalent adaptable networks,and provides guidelines to optimize the processing conditions,which will promote engineering applications of covalent adaptable network polymers.