Mobile-Ligand Nanoparticle/Polymer Composites:Morphology And Mechanical Properties

Polymer nanocomposites are widely used in mechanics,optics,electronics,biomedicine,etc.due to the diverse choices of nanofillers,which can be widely regarded as one of the most promising materials in the 21 st century.With the development of polymer nanocomposites,the problem about how the assembly morphology of nanofillers in polymer matrix affects the properties of materials is becoming more and more obvious,especially the interfacial wettability between nanofillers and matrix,multiscale hierarchical structure and the precise control of the topography,which are vital factors in solving the mechanical,thermal and electrical properties.Therefore,we focused on two aspects.The first one is the selection of the nanofiller.The polyoxometalate clusters with clear topology structure and diverse functions were chose due to their comprehensive applications in the fields of optics,electricity,magnetism,and catalysis,etc.The second is changing the interaction between nanoparticles by ligand modification.In this work,a movable ligand was used to balance the enthalpy and entropy of particle assembly,thus achieving sub-10 nm phase separation self-assembly.This method was extended into the polymer matrix,in which the self-assembly morphology of the particles was tuned and the relationship between the morphology and the mechanical response was studied.The main research content were summarized as the following three aspects:Firstly,the influence of movable ligands to the anisotropic assembly was investigated and a series of cluster-based supramolecular polymers with precise ligand structure were prepared.We synthesized a chain transfer reagent which was electrostatically applied to a 1 nm diameter Keggin type cluster,and then a series of degree of polymerization graft chains were synthesized by Reversible AdditionFragmentation Chain Transfer Polymerization.The ionic bond grafting method enables the grafting point to move on the surface of the polyoxometalate,which can reduce the entropy loss caused by the rearrangement of the grafted chain when particles are close to each other due to the core-core interaction,thus resulting in a greater degree of anisotropy.Then the volumetric ratio of the grafted chain to the polyoxometallate was precisely controled to achieve ordered assembly of the particles,which is similar to the phase separation behavior of the block polymer.As the volume ratio increased,the packed structure of nanoparticles translated from the layer to the hexagonal column and finally to the disordered sphere.Secondly,the changes of the structure and morphology of nanoparticles during the thermal movement in the matrix were explored.The block-like behavior of the above particles is equivalent to the transformation of the spatial arrangement of particles from two-dimensional to zero-dimensional,so we bring this ordered transformation into the polymer matrix.The basic three microscopic assembly forms were observed by transmission electron microscope and small angle X-ray scattering.Based on this,the morphology evolution of macroscopic assemblies was explored.Thirdly,the influence of macroscopic morphology on the mechanical properties of materials was studied.We obtained semi-flexible sheets with anisotropy,interconnected network structures and isotropic spheres.The mechanical differences of the above three structures were studied from Oscillatory shear rheology measurements and start-up tests,then the effect of anisotropy and the importance of the connected structure were pointed out.In brief,we reduce the entropy loss of graft chain rearrangement by movable ligands,so that the particles produce anisotropic ordered multilevel assembly in the matrix,realizing the controllable spatial arrangement of nanoparticles in the matrix.These results expand a new method and understandings for multiscale assembly of particles in polymer nanocomposites.