From Polymer Nano Composite Particles To Bulk Nanocomposites: The Preparation And Structure Control

The study of nanocomposite materials offers the possibility of substantialimprovements in material properties, ranging from mechanical to functionalproperties, with nanoparticles dispersed in polymer matrices. Unfortunately, ithas often proved difficult to form stable uniform dispersion of nanoparticles inpolymer matrices, slowing the rate of progress. In order to obtain polymernanocomposites with desired properties, the key problem is how to realize andkeep the stable uniform dispersion of nanoparticles in matrices.In-situ polymerization and melt intercalation solve the dispersion ofnaturally layered inorganics in polymer matrices. However, there is a limitednumber suitable for nanocomposite preparation. Comparing with inorganicmaterials, it is easier to design and control the structure of macromolecules.Polymer nanoparticles and nanocomposites with various novel properties can beobtained by different methods and procedures of polymerization.The aim of this research is to resolve the problem of the stable uniformdispersion of nanoparticles in polymer matrices. It is focused on the preparationand structure control of polymer nanoparticles, nano composite particles andbulk nanocomposites. The approach is to synthesize crosslinking poly(methylmethacrylate)(PMMA(CL)) nanoparticles at first, then to synthesize nanocomposite particles by emulsion polymerization with the nanoparticles as seeds.By directly melting the nano composite particles, the shell polymer forms thecontinuous phase and the PMMA(CL) nanoparticles uniformly and stablydisperse in the matrix. In this paper, the influences of various polymerizationparameters on the size, its distribution and morphology of polymer nanoparticles, nano composite particles are discussed. Then the influences of the interfacialstructure of nano composite particles on bulk nanocomposites structure arediscussed. Furthermore, the mechanism of stable uniform dispersion of polymernanoparticles in matrix is explored. The research results are as follows:(1) By controlling the proper polymerization parameters, PMMAnanoparticles with well monodispersity can be successfully obtained by emulsionpolymerization at very low surfactant level. For example, when thepolymerization conditions, are controlled as: reaction temperature 80℃, themonomer contration＜10g/100mlH₂O, and the weight ratio of the monomer tosurfactant 0.005～0.025, PMMA nanoparticles can be prepared, whose averagenumber size is between 40～75nm, PDI＜0.1, Dv/Dn≤1.18, and the conversion ofthe monomer is more than 94%.(2) The morphology of composite particles can be controlled bythermodynamic and kinetic factors during the seeded emulsion polymerization.The equilibrium morphology of PMMA(CL)/P(AN-MMA)(shell withAN/MMA＝9/1 weight ratio) composite particles synthesized by batch-seededemulsion polymerization should be hemisphere structure when SDS is used assurfactant. However, the morphology of the composite particles can change fromhemisphere, sandwich to core-shell structure with the slowing of the adding rateof the second stage monomer. And though the equilibrium morphology ofPMMA(CL)/PS composite particles should be inverted core-shell structure, asthe matter of fact, the final morphology can be core-shell structure by changingthe interfacial structure. When there is low crosslinking poly(butyl acrylate)PBA(LC) interfacial layer which leads to more PS grafted on PMMA(HC) coreand decreases the interfacial tension between core and shell, or when there isgradient copolymer P(MMA-St) interfacial layer which also decreases theinterfacial tension between core and shell, PMMA(CL)/PS composite particleshave core-shell structure.(3) In order to characterizing the morphology of polymer composite particles by transmission election microscopy (TEM), it is necessary to enhanceimage contrast for polymers by using a selective staining agent. Unfortunately,neither(OsO₄) nor (RuO₄) is suitable for PMMA/PAN composite particles. In ourresearch, it is founded that pH 6.4 PTA solution can be a selective staining agentin the TEM for PMMA/PAN and PMMA/P(AN-MMA)(shell with AN/MMA=9/1weight ratio) composite particles.(4) Neither Brownian motion nor molecular forces between nanoparticles tothe ^coalescence can be ignored because the size of dispered phase is very small,and the distance of a nanoparticle from its nearest neighbour is also very small inpolymer/polymer nanocomposites. In order to keep the stable uniform dispersionof polymer nanoparticles in matrix, the coalescence resulting from Brownianmotion should be prevented besides decreasing the interfacial tension. Theexperiments show that PMMA(HC) nanoparticles can stably dispersed in thematrices without obvious coalescence for PMMA(HC)/PS andPMMA(HC)/PBA(LC)/PS bulk nanocomposites, but they happen seriouscoalescence in the matrix for PMMA(HC)/P(MMA-St)/PS. The reason is that thegrafted macromolecules on PMMA(HC) nanoparticles are polystyrene(PS) forPMMA(HC)/PS and PMMA(HC)/PBA(LC)/PS, which have stretchedconformation in melt PS matrix, and can provide enough steric stabilization toprevent the coalescence. And the grafted macromolecules on PMMA(HC) nano-particles are gradient copolymer P(MMA-St) for PMMA(HC)/P(MMA-St)/PS,the macromolecular chains collapse on the nanoparticles. The stable uniformdispersion of polymer nanoparticles in matrix should be contributed to the stericstabilization of nanoparticles resulting from the grafted macromolecules.(5) Polymer/polymer nanocomposites show that the viscosityη* decreaseswith the increasing of the shear stress. And they also show a solidlikeviscoelastic behavior when the volume fraction of polymer nanoparticles isbetween 10% and 15%, which is much higher than inorganicnanoparticles/polymer nanocomposites. It means that polymer/polymer nanocomposites have better processability than inorganic nanoparticles/polymernanocomposites.