Study On The Dynamic And Rheological Behavior Of Carboxy-terminated Polybutadiene-based Dynamically Crosslinked Polymers

Dynamically crosslinked polymers have attracted considerable attention due to its potentials in wide application of polymeric materials.It mainly introduces dynamic covalent bonds into the polymer network to enable the materials to undergo structural rearrangement and then obtain processability.Current understanding on the linear rheology of dynamically crosslinked polymer mainly focuses on the coupling between the dynamics of active network chain and dissociation kinetics of crosslinking points.In this work,a series of dynamically crosslinked polymers based on carboxy-terminated liquid polybutadiene rubber were prepared by changing the equivalent ratio of functional groups and catalyst contents,and then the influences of curing kinetics and rheological properties were studied.Furthermore,we also studied the influence of chain conformations on both the linear and nonlinear viscoelasticity in the dynamically crosslinked network.A simplified network model was developed with two types of chain conformation,i.e.,active chains with both ends confined to the network through reversible bond and pendent chains with one end free.The main results are as follows:(1)The model accurately predicts two relaxation modes in linear rheology and weak strain overshoot in nonlinear oscillatory rheology in dynamically crosslinked liquid polybutadiene rubber.At the same time,for the prediction of strain overshoot,the influences of the nonlinear association coefficient and dissociation coefficient on peak and peak strain of fundamental out-of-phase modulus overshoot are clarified.(2)Molecular dynamic polymers were prepared by crosslinking reaction of epoxy and carboxyl groups.The rate coefficients of association and dissociation under quiescent condition and under shear flow,determined by fitting the rheological experiments with the model,illustrate the effect of catalyst and the flow on the reversible reactions.It was found that catalyst can accelerate disassociation reaction by increasing its rate coefficient and decreasing its activation energy,while deaccelerate association reaction by decreasing its rate coefficient and increasing its activation energy.Furthermore,by monitoring the infrared curves of the reaction between epoxy and carboxyl groups,it was found that the temperature and catalyst were conductive to the improvement of the reaction rate.(3)Particle-containing dynamic polymers were prepared through the crosslinking reaction of epoxy-modified nano-silica and carboxyl groups.The increase of particle concentration will reduce the dissociation rate and inhibit the dissociation reaction due to the steric hindrance of particles on the network.And,through comparison with the nonlinear weak strain overshoot of the molecular crosslinked system,it was found that particles have a negative effect on flow-accelerated association.