Mechnical Behavior Of Solvent-Assisted Thermally-Activated Covalent Adaptable Networks Polymer

Thermosetting polymer materials have been widely used in the fields of wind energy,aviation and automotive lightweighting due to their superior mechanical properties,dimensional stability and thermal stability.The networks are “set” when the polymerization reaction is complete,and the polymer networks thus formed become insoluble and non-meltable.Therefore,their conventional recycling techniques are confined within mechanical grinding and pyrolysis,which are ineffective disposal approaches due to their low recycling efficiency or environmentally unfriendly features.Covalent Adaptable Networks(CANs)are the forefront of the recent recyclable thermoset polymer research.Researchers have discovered that CANs can undergo reversible network rearrangement through bond exchange reactions(BERs),providing a microscopic mechanism to achieve macroscopic flow and stress relaxation,which was previously inaccessible in thermoset materials.Furthermore,a green solvent-assisted recycling method of thermosets have been developed recently.From the perspective of sustainable development,CANs will attract numerous interests from the society in the long run.Due to the relative short development history of CANs,researchers lack comprehensive understanding on this novel polymer.The mechanism behind their features have not been well explored and discovered,which results in the development and application of CANs being seriously limited.In addition,with the increasing attention to sustainable development in recent years,the commercialization of thermoset material recycling is also imminent.In order to better carry out the design,preparation and application of CANs materials,guide the recovery,reuse thermosetting polymers,and promote the sustainable development of thermoset polymer materials,a systematic theoretical model is urgently needed to reveal the depolymerization and repolymerization process of thermoset polymers with covalently adapted networks.Firstly,the thermomechanical properties of CANs were studied in this work.When the temperature was high enough,BERs were activated to rearrange the network topology,accompanied by notable volume expansion and stress relaxation.However,the microstructure was unable to rearrange instantaneously in response to an external stimulus,and thus the CANs were in the nonequilibrium state.The thermomechanical behavior of the CANs was studied through the thermal expansion test and the stress relaxation test.To assist the discussion,a theoretical model was defined based on the concept of fictive temperature.And the non-equilibrium behavior of the CANs during the topological transformation was analyzed based on the experimental results.Then the influence of temperature program on the volume expansion of CANs was explored.Furthermore,a stress relaxation model coupled with structural relaxation was further established and the coupling influence mechanism of structural relaxation and BERs on the stress relaxation behavior of CANs was studied.Subsequently,we demonstrated the feasibility of solvent-assisted depolymerization and repolymerization of engineering epoxy with different crosslink density at reasonably low temperature(160°C)and ambient pressure.The evolution of the thermomechanical behaviors and change in chemical structure of CANs during the repolymerization process were performed on dynamic thermomechanical analysis,fourier transform infrared spectroscopy and viscosity rheometer.A multiscale chemomechanics modeling framework was developed to link the microscopic BERs kinetics to the evolution of macroscopic thermo-viscoelastic properties of re-polymerizing thermosets.Assisted by the theoretical model,it was further applied to perform parametric studies to examine the influences of BERs catalyst content,choice of solvent,and network crosslinking density on the re-polymerization rate.Based on the multiscale chemomechanics model,a diffusion-reaction-kinetics model was established,including the diffusion-reaction-depolymerization model and diffusion-reaction-repolymerization model.The diffusion-reaction-depolymerization model was concluded that the overall network depolymerization rate was therefore determined by the rates of three time-dependent processes:(1)the diffusion of solvent and catalyst molecules,(2)the cleavage and reconnection of chain segments,and(3)disentanglement rate of chain segments.The influences of various processing and material properties were investigated through a series of dissolution experiments.The diffusion-reaction-repolymerization model was established to study the evolution of the CANs residual stress and mechanical properties of the CANs along the material thickness direction during the repolymerization process combined with dynamic thermomechanical analysis,static mechanical test frame.The evolution of residual stress during the composite manufacturing with particle reinforcements was further investigated.The presented study advances the understanding of the structural-processing-property relationships during the recycling of thermosets and their composites.Finally,a solvent assisted interfacial welding method of engineering thermosetting polymer was developed.A diffusion-depolymerization-repolymerization reaction kinetics model was established,and the mechanism of welding was that the solvent and catalyst molecules diffused into the polymer network.During the interfacial welding,the solvent molecules break the polymer chains into short segments.Then the chain segments subsequently disentangle from the network and reconnect on the interface.Assisted by the theoretical model,combined with lap-shear tests,the influences of various processing and material properties were investigated,including the processing time,processing temperature,and catalyst content.Furthermore,the welding method was extended to reprocess the epoxy scarps from the powder state.The influence of processing time on the mechanical properties of the reprocessed welding samples were studied by static mechanical test,to reveal the influence of processing time on the mechanical strength of the material obtained by powder welding.