Preparation And Structural Control Of PMMA-based Nanocomposites And Their Microcellular Foams

Polymer-based nanocomposite microcellular foams have been widely investigated because it obtains the advantages of both polymer-based nanocomposites and microcellular foams. Among them, the polymer-based nanocomposite microcellular foams with conductive nano-phase as enhancer has the multifunction of lightweight with high strength, static electricity and electromagnetic shielding properties. So it has broad application prospects in electronics industry and aerospace field. The preparation of polymer-based nanocomposite microcellular foams includes two technologies:polymer-based nanocomposites preparation technology and supercritical fluid foaming technology. In this paper, PMMA as polymer matrix and Ag nanoparticles, CNTs-Ag nanoparticles as enhancer were used to prepare PMMA-based nanocomposite. The PMMA-based nanocomposite microcellular foams were prepared by using the supercritical fluid foaming technology. The preparation process, foam structure, mechanical and electrical properties before or after foaming were investigated.Firstly, Ag/PMMA nanocomposites were prepared by using in-situ reduction and anti-solvent precipitation technology. The effect of preparation process on microstructure and electrical properties were investigated. The mechanism of the growth and shape conversion of Ag nanoparticles were also analyzed. The results showed that it was successful to prepare Ag/PMMA nanocomposite with mono-dispersion when there was a little PVP as stabilizer. The size and morphology of Ag nanoparticles can be precisely controlled by changing reaction time. Analysis showed that the growing of Ag nanoparticles was mainly controlled by Ostwald ripening. The shape changed of Ag nanoparticles was due to the preferential adsorption of PVP on Ag (100) crystal surface. The electrical properties of Ag/PMMA nanocomposite were not significant improved. This was mainly because the lower content of Ag nanoparticles and it was hard for single crystal Ag nanoparticles to form the electron transport pathway. In addition, the Ag/PMMA nanocomposite microcellular foams were prepared by using supercritical CO2as blowing agent. The influence of foaming conditions and the size and shape of Ag nanoparticles on the foam structure were investigated. The heterogeneous nucleation mechanism and the mechanical properties were also analyzed. The results showed that the cell density of Ag/PMMA nanocomposite microcellular foams can be improved1～2orders when compared with PMMA microcellular foams. The cell size can be controlled in4～30μm while the cell density can be controlled in5×107～1010cells/cm3by changing the foaming conditions. It seemed that the cell size can be reduced at low temperature, high pressure and a short foaming time. The foaming temperature played a significant role. Calculation results showed that the Ag nanoparticles can play the role of heterogeneous nucleation agent when the particle size was about1～2nm which was coincided with the experimental results. The smaller the Ag nanoparticles were, the higher the theory nucleation density was. Finally, the cell density became higher. The nucleation efficiency can be improved when the Ag nanoparticles were shape changed. But the growth of particles size can reduce the cell density of microcellular foams. The compressive strength of Ag/PMMA nanocomposite microcellular foams can be improved about84%while the Young's modulus can be improved65%when compared with PMMA microcellular foams in the same relative density. According to the foam mechanical constitutive modeling analysis, the mechanical property is a little higher than the predictive value of the constitutive model when the relative density is higher than0.2. This is mainly due to the Ag nanoparticles enhance the PMMA matrix and heterogeneous nucleation reduces the cell size in supercritical fluid foaming process. The latter aspect plays a major role.Moreover, CNTs-Ag/PMMA nanocomposites were prepared by using ultrasound-solution blending method to disperse CNTs in Ag/PMMA suspension which was prepared by in-situ method. The effect of preparation process and CNTs content on microstructure and electrical properties of the materials were investigated. The results showed that by using anti-solvent precipitation technology can successful disperse the amino-modified CNTs in Ag/PMMA nanocomposites. It was difficult to disperse CNTs when the content was high. It was almost impossible to achieve the CNTs well-dispersed in the Ag/PMMA suspension by ultrasounding for6h when the CNTs content was higher than2wt.%. The TEM results indicated that the Ag nanoparticles in Ag/PMMA suspension tended to deposit on the surface of CNTs and bond with a lot of CNTs. The increasing of the content of CNTs will lead to a poor dispersion. The investigation of electrical properties indicated that the conductivity of CNTs-Ag/PMMA nanocomposites can be improved1-3orders when compared to Ag/PMMA nanocomposites. This was mainly due to the conductivity behavior can be explained by the tunnel effect theory. Ag nanoparticles play the role of conductive center and participate in conduction.At last, CNTs-Ag/PMMA nanocomposite microcellular foams were prepared by using supercritical fluid foaming technology. The influence of foaming temperature and CNTs contents on foam structure were investigated. The relationship between foam structure and mechanical and electrical properties was further explored. The results showed that the cell density of CNTs-Ag/PMMA nanocomposite microcellular foams can be further improved2～4times when compared to Ag/PMMA nanocomposite microcellular foams. When the content of CNTs was higher than0.5wt.%, there was little influence on the foam structure of CNTs-Ag/PMMA nanocomposite microcellular foams when increased CNTs content further. The cell size can be controlled at3～20μm and cell density at2×108～1011cells/cm3by changing foaming temperature. The CNTs-Ag two-phase nanoparticles have a better nucleating effect. This was mainly due to the CNTs have a weaker interface with PMMA which will easy to get heterogeneous nucleation. The CNTs-Ag/PMMA nanocomposite microcellular foams have a better compressive strength and Young's modulus when compared with Ag/PMMA microcellular foams in the same relative density. The conductivity of CNTs-Ag/PMMA nanocomposite microcellular foams reduced along with the increasing of cell size. And it gradually transformed into the insulator. This was due to the electrical path was disconnected when the cell size became bigger and the cell wall became thinner.