Synthesis And Properties Of Functional "Nanoparticle Brush" And The Corresponding Composites Systems

Polymer nanocomposites (PNCs) as one of the composite materials remainsthe thermoplastic, thermoset, processable and operational flexibility properties ofmatrix under the premise of the addition of modified or functionalized nano-filler andexhibits additional excellent properties which the matrix haven’t. It has been thereforeused in the high-performance coatings, automotive parts, packaging and refractories,solar panels, dielectric materials， aerospace and other fields. However, themacroscopic properties of nanocomposites will be impacted tremendously by theaggregation of nano-fillers due to its type, size, shape, concentration, specific area andthe volume effect, and the complex reaction with matrix as well. For betterunderstanding the relationship between the nano-fillers and matrix and the effect ofnano-fillers on the macro-performance of polymer nanocomposites, the polystyrene(PS), poly(methyl methacrylate)(PMMA) and poly-(styrene-co-acrylonitrile)polymer (PSAN) are therefore grafted onto the surface of a series of silicananoparticles with different radius using surface initiated atom transfer radicalpolymerization（SI-ATRP）that was called by nanoparticle brush (NPB). The reactionkinetics, thermal dynamics of grafted chains within NPB and the correspondingpolymer nanocomposites were studied systematiclly.The effect of thermal self-initiated (TSI) on kinetics of SI-ATRP wereinvestigated based on two systems including N， N， N， N，N-pentamethyldiethylene-triamine (PMDETA) ligands, with Cu catalysts at70°Cand4,4-dinonyl-2,2-bipyridine (dNbpy), with Cu catalysts at90°C, respectively.Results showed that the effect of TSI was more effective on reaction kinetics ofSI-ATRP at high temperature and lower reactive PMDETA ligands. The free polymerchains derived from TSI were separated uniformly within the corner regions of theWigner-Seitz cell (WSC) and enhanced therefore the stability and thermal mechanicalproperties of NPB, for example, the glass transition temperature (Tg), elastic modulus(E) and fracture toughness (KIC).Compared with the linear polymer having the same degree of polymerizationwith grafting polymer, the glass transition temperature of the grafting polymer of NPBwill be increased due to the reduction of conformational entropy of the system fromspace effect leaded by increasing surface curvature (i.e., decreasing nanoparticle size)，and the increased amount (T) increases with the decrease of degree of polymerization of grafted polymer chains. This transitional trend of T is consistentof the transition from concentrated particle brush (CPB) to semi-dilute particle brush(SDPB) regime known by Daoud and Cotton (D-C) model when the degree ofpolymerization of grafted polymer NGis equal to the critical degree of polymerizationNc. It was further proven that there is a certain relationship between thethermodynamic properties of NPB and the structure of grafted polymer chains.The disperse principles of NPB in polymer matrix have been studied in twomixture systems. In athermal system (corresponding to equal chemical composition ofgraft and matrix polymer), polymer matrixes are compatible with NPB when thedegree of polymerization (NG) of grafted chains is greater than the degree ofpolymerization (NM) of matrix (NG≥aNM, a <1). Inversely, they are incompatiblewhen NG<aNM. Here，the prefactor shuold be decreased with the decrease of coreradius. In thermal system, enthalpic compatibilization that is facilitated by favorableinteractions between graft and matrix chains is found to facilitate particlecompatibilization even for entropically unfavorable conditions, such as NG NMandR0Re(Reis the length of polymer chain from starting point to ending point), thusprovides a viable strategy toward thermodynamically stable particle brush/polymerblend systems with high inorganic loading, well dispersion and polymer-likemechanical properties.The thermal properties of PNCs have been studies via adjusting the disperse stateand micro-structure of NPB in two mixture systems. For athermal mixtures which areentropic interaction between graft and matrix chains, the Tgof the resultingnanocomposites did not find obviously changes when tuning particle sizes, annealingtime and, composition range of the grafted and matrix chains under our experimentalcondition as expected. In contrast, the Tgof thermal mixtures (occurring enthalpicinteraction between grafted polymer chain and the polymeric host) increase asrealized in the case of PSAN grafted particles embedded within PMMA matrixincrease with increase of nanoparticles concentration. The interesting observation wasthat particle brush systems and thermal mixtures had similar Tgtrends with theincrease of inorganic fraction due to the enthalpic interaction between grafted chainsand matrix or neat nanoparticle and matrix. It provides a feasible and potential methodfor design and production of nanocomposites with high performance. In the liquid system, the scattering efficiency of NPB having core-shell structurethat is uniformed nanoparticle assumed by Maxwell-Gannet (M-G) effectivemedium theory has reduced due to the index-matched of dielectric constant betweennanoparticle and polymer matrix. However, the scattering efficiency of NPB isreduced with the increase of the degree of polymerization or core radius, and thisefficiency is decreased especially by10orders of magnitude when the core radius isas much as250nm.The scattering theory of NPB in liquid system was applied for transparentproperties of the solid system prepared by blending appropriate amounts of PMMAmatrix and NPB. Results show that the transparent properties of the resulting PNCshave greatly improved while reducing the polarization difference Δa between thenanoparticles and the matrix as well as improving the compatibility of the dispersionof NPB in the polymer matrix through the design and synthesis of NPB from M-Geffective medium theory and Rayleigh scattering theory. At the same time, thescattering properties of NPB in resulting solid system are futher confirmed by Miescattering theory. Based on analysis for the microstructure and properties of theresulting NPB and nanocomposites, we propose theoretically therefore a designed andsynthetic method for producing nanocomposites with uniform particle dispersion,high content, good thermal and mechanical properties, and lowing scatteringefficiency.