Glass Transition Of Polymer Single-chain Nanoparticles And Their Composites With Linear Polymers

Human life and social development cannot be separated from materials,and the emergence of new materials often promotes the development of the society.Therefore,the directed synthesis and preparation of various new materials has been a major direction of the scientific research.The preparation of polymer nanocomposites is usually achieved by incorporating nanoparticles into polymers.It is also one of the most effective methods to improving material properties,such as mechanical,optical and electrical properties.In recent years,the emergence of a new type of nanoparticlepolymer single-chain cross-linked soft nanoparticle,has attracted widespread attention.After such nanoparticles are added as fillers to polymer materials,the glass transition and rheological properties of the materials can be largely alternated.For example,when polystyrene single-chain nanoparticles are added to linear polystyrene,the viscosity decreases,which is contrary to the prediction of Einstein's theory,and the glass transition temperature also decreases.The influencing factors in this type of composite system are complex,and the mechanism of interaction between the components and the mechanism of property changes are still not very clear.In this work,we experimentally used 4-vinylbenzocyclobutene monomer similar to styrene as the crosslinking agent,and prepared 6 polystyrene-based polymer precursors by free radical polymerization.Because the molecular weight of these 6 chains is similar,the microscopic size of each polymer is similar,but the content of crosslinking agent is different.The content of crosslinking agent is varied as 7%,10%,13%,16%,23%,and 36%,respectively.And under the condition of extremely dilute solution,the corresponding single-chain nanoparticles were prepared by means of intra-chain thermal cross-linking.From the results of size exclusion chromatography and dynamic light scattering,it can be seen that the apparent molecular weight and hydrodynamic radius of the nanoparticles decrease monotonically as the cross-linking ratio increases.This result indicates that the cross-linking rate is an effective means to control the degree of folding and collapse of the nanoparticles.From the results of differential scanning calorimetry measurement for both precursor chain and nanoparticle,it can be seen that as the crosslinking rate increases,the glass transition temperatures Tg of the precursor chains have no significant difference,while that of the SCNP gradually increases with the increase of the cross-linking ratio.the largest glass transition temperature increment of SCNP induced by intermolecular cross-linking is up to 52 K compared with the precursor chain.At the same time,in order to further explore its underlying mechanism,we also used large-scale molecular dynamics simulation methods to study the influence of single-chain cross-linking on the glass transition of polymer systems.The simulation results show that the cross-linking bonds in the SCNP system can effectively change the stacking structure in the SCNP system,and then affect its local dynamics,which is consistent with the trend predicted by the general entropy theory(GET)developed by Douglas et al.In addition,we fabricated composite systems by adding these SCNPs with different crosslinking ratios into polydisperse commercial polystyrene(molecular weight Mw = 190 KDa)with different loadings(2%,20%).By using differential scanning calorimetry,the effect of SCNP cross-linking ratio and load on the glass transition temperature of the material was systematically investigated.We found that the introduction of SCNP with a low crosslinking ratio(7%)will cause the glass transition temperature Tg of the material to increase to a certain extent.In contrary,the addition of other nanoparticles will lead to a decrease in the glass transition temperature Tg of the composite.At the same time,under a fixed loading,adding SCNPs with larger cross-linking ratio will cause a larger decrease in the glass transition temperature Tg of the composite.These results indicate that the degree of internal cross-linking of SCNP is an important factor influencing the composite materials properties.