The Rheological Behavior Of Ultra-small Particle/polymer Composite System

Polymer nanocomposites(PNCs)are composites formed by dispersing nanoparticles into the polymer matrix as the continuous phase,which combine the excellent processability of polymer materials and the exceptional functional properties of nanomaterials.At present,the effect of nanoparticles on the rheological behavior of polymer matrix has become an important research direction in the field of PNCs.By adjusting the kind,size,content of the nanoparticles,the rheological properties such as modulus,viscosity,relaxation time of the polymer matrix can be significantly changed.Moreover,these rheological properties are tigjtly related to the size of the particles.Ultra-small particles refer to a class of particles that are smaller than the tube diameter of entangled polymer mesh structure.Theoretical studies have predicted that ultra-small particles can produce special disentanglement effects on polymer networks.However,currently experimental studies on ultra-small particles and polymer complex system have been rarely reported.Furthermore,the relevant mechanism needs to be verified and clarified.Polyoxometalates(POMs),as a class of inorganic molecule clusters with the size of 0.5 to 6 nm,processing precise chemical composition and topology structure,are the ideal model for ultra-small particles.Inthis paper,we used POMs as ultra-small particles and improved the compatibility of POMs and the polymer matrix by polymerization grafting modification on the surface of POMs,thus achieving the uniform dispersion of POMs in polymer matrix.And then,we studied the influence of POMs on the rheological behavior of polymer matrix,and found the disentanglement effect of POMs on entangled polymer networks.The main research contents include the following three aspects:Firstly,polystyrene-grafted POMs were prepared.We designed and synthesized a cationic chain transfer reagent with end-group of trithiocarbonate,which was modified to the surface of Keggin-type POMs [CoW12O40]6-(CoW12)through electrostatic interaction,then polystyrene chains were linked to the surface of CoW12 by reversible addition-fragmentation chain transfer polymerization.Three kinds of grafted-clusters were prepared and their compositions and structures were determined by NMR,EA,MALDI-TOF,IR and so on.We confirmed that the polymerization degrees of polystyrene chains on three grafted-clusters are 9,18 and 28 respectively,and their grafting numer are all 6.We named them as CoW12-PS1,CoW12-PS2,CoW12-PS3.Secondly,the nanocomposites of grafted-clusters and polystyrene were prepared.We incorporated the grafted-clusters into the polymer matrix by solution blending,and prepared the transparent composites.The results show that the grafted clusters are uniformly dispersed in polystyrene matrix by TEM and SAXS.By the results of DSC,it was found that the incorporation of grafted-clusters could significantly decrease the glass transition temperature(Tg)of polystyrene matrix,and Tg decreasesd more obvioisly with the increase content and size of grafted-clusters.Thirdly,the influence of grafted-clusters on the rheological properties of polystyrene matrix were studied.We investigated the effects of the content and size of grafted-clusters on the rheological properties of polystyrene matrix with different molecular weights.It was found that the incorporation of grafted-clusters could significantly reduce the melt viscosity,storage modulus,loss modulus,and affect the relaxation dynamics of polystyrene chains: reduce the end relaxation time ?t,increase entanglement relaxation time ?e,thereby promoting the disentanglement of thepolystyrene network.At the same time,with the increase content and size of grafted-clusters,the phenomenon of disentanglement becomes more obvious.Based on the tube theory,we further analyzed the disentanglement effect of grafted-clusters on polystyrene matrix,and presented that the disentanglement was due to enlarged tube diameter of polystyrene network caused by POM cluster.In summary,we used POMs as the model of ultra-small particles and introduce them into polymer matrix,systematically studied the influence of POMs on the rheological properties of polymer matrix,and verified the disentanglement effect of ultra-small particles on polymer networks.These results provide a new understanding of the rheological behavior of nanoparticle/polymer composite systems.