Simulation Studies On Structural And Mechanical Properties Of Associating Polymers Network Under Start-up Shear Flow

Supramolecular polymer gel networks are formed by the physical association of linear or branched polymers through reversible non-covalent bonds(such as hydrogen bonds,?-?stacking,metal-ligand,and ionic interaction).The reversibility of crosslinking provides them with unique capabilities that can be used as self-healing,stimuli-responsive and shape memory materials.Compared with traditional polymers and chemical networks composed of covalently cross-linked polymers,they also have superior processing and recycling properties due to the sharp decrease in viscosity when the temperature increases or the concentration decreases.The potential applications of supramolecular polymer networks have stimulated a strong interest in understanding the physical mechanisms of their structure,dynamics and rheological properties.The structure,dynamics and rheological properties of the associative polymer transient network are mainly determined by the nature of the non-covalent interaction between the associative groups and the composition of the polymer chain,especially the chemical distribution of the stickers along the backbone,which lead to different relaxation mechanisms in the systems.Computational simulation can directly observe the evolution of the network structure through the cluster analysis method and explore the relationship between structure and properties.In this dissertation,we have separately studied the properties of linear telechelic polymers,star telechelic polymers and multi-stickers linear associative polymers during the startup shear flow.The main contents are as follows:1.The structural and mechanical properties of linear telechelic polymers under startup shear:In this work,we adopt Brownian dynamics to explore the stress-structure relationship of telechelic reversible associating polymer gel during start-up shear flow,with shear strengths varying from W4)=12.6 to W4)=12640.At weak shear flow W_i=12.6,the shear stress proportionally increases with shear strain in short times,followed by a strain hardening behavior and then passes through a maximum(?_max,?_max)and finally decreases until it reaches the steady state.During the evolution of stress,the gel network is only slightly broken and essentially maintains its framework with time,the strain hardening behavior originates from the excessive stretching of chains.On the other hand,the stress-strain curve at intermediate shear strength W_i=505.6 shows two differences from those at W_i=12.6,namely,disappeared strain hardening and dramatic increment of stress at large strains,which is caused by the rupture of gel network at small strains and the network recovery at large strains respectively.Finally,at the strong shear flow W_i=6319.7,the gel network is immediately broken with the performance of shear and the stress-strain curve has the similar behaviors with those of classical polymeric liquids.2.The structural and mechanical properties of telechelic star polymers under startup shear:We adopt the Brownian dynamics simulation to investigate the structural properties and stress behavior of two concentrations of telechelic star polymers solution during the start-up shear flow.The shear rates vary from 0.0001 to 0.05.The stress-strain curves are obtained by choosing four shear rates(?=0.0001,0.001,0.01,0.05).Studies have shown that,at low shear rate?=0.0001,the stress increases with the strain without obvious strain hardening behavior at small strains compared with linear telechelic polymers,followed by the maximum stress(stress overshoot point),then starts to decrease and finally reaches a stable value.During the entire network evolution process,the flow field induces clusters of small size to aggregate into larger clusters,which causes the size distribution of the clusters to shift significantly.When the shear rate increases to?=0.01,the stress of the high concentration system increases before reaching the stable value at large strain,which is due to the reassociation of clusters.Finally,at the strong shear rate?=0.05,the clusters rupture immediately at onset of shear flow,and the larger clusters rupture into more smaller clusters and free stickers.In the high concentration system,the stress decrease caused by cluster rupture is less than the stress increase caused by the flow field.3.The structural and mechanical properties of multi-sticker associative polymers under startup shear:In this work,we adopt the Brownian dynamics simulation to investigate the structural properties and stress behavior of two concentrations(?=0.2,0.85)of multi-sticker associative polymers during the start-up shear flow.By selecting four shear rates(0.0001,0.001,0.01,0.05),simulation results demonstrate that the stress-strain curves under different shear rates show the stress overshoot phenomenon.The difference is that the stress recovery occurs at large strain under strong shear flow for high-concentration system.In the process of stress evolution,changes in the network structure dominate.When?=0.2,the number of clusters increases at small strain,and the number of clusters decreases at large strain under the strong shear flow.When?=0.85,the number of clusters decreases at onset of shear flow,and at the intermediate shear strength,the number of clusters will increase at large strain.Due to the different changes of the network structure in the two concentrations,the dependence of the orientation angle of the polymer chain on the shear rate is more obvious for the low concentration system.The simulation results can provide a better understanding the macro-mechanical properties and microstructural changes of the associative polymer systems,and play a guiding role in the application of physical gel networks in various fields.