PMMA基纳米复合材料及其微发泡材料的制备与结构控制
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摘要
聚合物基微发泡纳米复合材料兼具了聚合物基纳米复合材料和微发泡材料的优点而被广泛研究。以导电纳米相作为增强剂的聚合物基微发泡纳米复合材料,具有轻质高强和静电、电磁屏蔽的多功能性,在电子工业和航空航天领域具有广泛的应用前景。聚合物基微发泡纳米复合材料的制备技术,包括聚合物基纳米复合材料的制备技术和超临界流体发泡技术。本文以PMMA作为聚合物基体,以Ag纳米粒子和CNTs-Ag纳米粒子作为增强剂,制备PMMA基纳米复合材料,再结合超临界流体发泡技术制备PMMA基微发泡纳米复合材料,重点研究其制备工艺、显微结构以及发泡前后的力学和电学性能。
首先,采用原位还原技术和反溶剂沉淀技术,制备Ag/PMMA纳米复合材料,研究制备工艺对其显微结构和电学性能影响,分析Ag纳米粒子长大和异形化机理。结果表明当有少量PVP作为稳定剂时,可以成功制备出具有单分散性的Ag/PMMA纳米复合材料,通过控制反应时间,可以对Ag纳米粒子的粒径和形貌精确调控。分析表明在高温缺少Ag源条件下,Ag纳米粒子的生长主要受Ostwald熟化控制,异形化主要是由于PVP对Ag(100)晶面优先吸附的结果。Ag/PMMA纳米复合材料的电学性能没有得到明显的改善,归因于Ag纳米粒子的含量低,弥散分布难以形成电子传输的通路。
其次,采用超临界流体发泡技术制备Ag/PMMA微发泡纳米复合材料,研究发泡工艺参数以及Ag纳米粒子粒径、形貌对泡孔结构的影响,分析异相成核机理,并研究Ag/PMMA微发泡纳米复合材料的力学性能。结果表明,与PMMA微发泡材料相比,Ag/PMMA微发泡纳米复合材料的泡孔密度提高了1-2个数量级。通过调节发泡工艺参数,实现了对泡孔结构的控制,制备出平均泡孔直径为4-30μm、泡孔密度为5×107-1010cells/cm3可调的微发泡纳米复合材料。低温、高压和较短的发泡时间有利于减小泡孔直径,发泡温度对泡孔结构的影响最为显著。计算表明当Ag纳米粒子粒径在1～2nm时,都可以起到显著的异相成核作用,这与本研究的实验结果吻合。Ag纳米粒子的粒径越小,理论成核密度越高,泡孔密度越高；Ag纳米粒子的异形化,可以提高其成核效率,但粒径的长大导致其泡孔密度降低。在相同体积密度条件下, Ag/PMMA微发泡材料比PMMA微发泡材料的压缩强度提高约84%,弹性模量提高约65%。泡沫力学本构模拟的分析表明,微发泡纳米复合材料的相对密度高于0.2时,力学性能略高于闭孔本构模型的预测值。这是由于Ag纳米粒子的存在,一方面增强了PMMA基体,另一方面在发泡过程中的异相成核作用减小了泡孔直径,提高了力学性能,其中后者起主要贡献。
再次,采用超声-溶液共混法将CNTs分散到原位合成的Ag/PMMA溶胶中制备出CNTs-Ag/PMMA纳米复合材料,研究制备工艺、CNTs含量对显微结构和电学性能的影响。结果表明,经过反溶剂沉淀技术可以将氨基改性的CNTs均匀分散在Ag/PMMA纳米复合材料基体中,CNTs含量越高,其均匀分散越难；CNTs含量超过2wt.%时,6h的超声条件无法实现CNTs在Ag/PMMA纳米复合材料基体中的均匀分散。TEM分析结果表明,Ag纳米粒子倾向于沉积到CNTs表面,并与多根CNTs发生结合,随着CNTs含量增加导致其分散性变差。电学性能研究表明,与CNTs/PMMA纳米复合材料相比,CNTs-Ag/PMMA纳米复合材料的电导率提高了1-3个数量级,主要归因于其导电行为可以由隧道效应理论解释,Ag纳米粒子承担起电子迁移的载体而参与导电。
最后,采用超临界流体发泡技术制备CNTs-Ag/PMMA微发泡纳米复合材料,研究发泡温度以及CNTs含量对泡孔结构的影响,探索力学和电学性能与泡孔结构的关系。结果表明,与Ag/PMMA微发泡纳米复合材料相比,CNTs-Ag/PMMA微发泡纳米复合材料的泡孔密度进一步提高2-4倍。当CNTs含量超过0.5wt.%后,继续增加CNTs的含量对CNTs-Ag/PMMA微发泡纳米复合材料的泡孔结构的影响较小；通过控制发泡温度可以将CNTs-Ag/PMMA微发泡纳米复合材料的泡孔直径控制在3-20μm,泡孔密度控制在2×108～1011cells/cm3。CNTs-Ag两相纳米粒子具有更好的成核效果,主要源于添加的CNTs与PMMA基体的结合界面更弱,更有利于发生异相成核。在相同体积密度条件下,CNTs-Ag/PMMA微发泡材料比Ag/PMMA微发泡纳米复合材料的压缩强度和弹性模量都有所增加。CNTs-Ag/PMMA微发泡纳米复合材料的电导率随着泡孔直径的增大而逐渐降低,并逐渐转变成绝缘体性质,本研究认为是孔径增加,孔壁厚度减薄,导电通路逐渐被破坏。
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.
首先,采用原位还原技术和反溶剂沉淀技术,制备Ag/PMMA纳米复合材料,研究制备工艺对其显微结构和电学性能影响,分析Ag纳米粒子长大和异形化机理。结果表明当有少量PVP作为稳定剂时,可以成功制备出具有单分散性的Ag/PMMA纳米复合材料,通过控制反应时间,可以对Ag纳米粒子的粒径和形貌精确调控。分析表明在高温缺少Ag源条件下,Ag纳米粒子的生长主要受Ostwald熟化控制,异形化主要是由于PVP对Ag(100)晶面优先吸附的结果。Ag/PMMA纳米复合材料的电学性能没有得到明显的改善,归因于Ag纳米粒子的含量低,弥散分布难以形成电子传输的通路。
其次,采用超临界流体发泡技术制备Ag/PMMA微发泡纳米复合材料,研究发泡工艺参数以及Ag纳米粒子粒径、形貌对泡孔结构的影响,分析异相成核机理,并研究Ag/PMMA微发泡纳米复合材料的力学性能。结果表明,与PMMA微发泡材料相比,Ag/PMMA微发泡纳米复合材料的泡孔密度提高了1-2个数量级。通过调节发泡工艺参数,实现了对泡孔结构的控制,制备出平均泡孔直径为4-30μm、泡孔密度为5×107-1010cells/cm3可调的微发泡纳米复合材料。低温、高压和较短的发泡时间有利于减小泡孔直径,发泡温度对泡孔结构的影响最为显著。计算表明当Ag纳米粒子粒径在1～2nm时,都可以起到显著的异相成核作用,这与本研究的实验结果吻合。Ag纳米粒子的粒径越小,理论成核密度越高,泡孔密度越高；Ag纳米粒子的异形化,可以提高其成核效率,但粒径的长大导致其泡孔密度降低。在相同体积密度条件下, Ag/PMMA微发泡材料比PMMA微发泡材料的压缩强度提高约84%,弹性模量提高约65%。泡沫力学本构模拟的分析表明,微发泡纳米复合材料的相对密度高于0.2时,力学性能略高于闭孔本构模型的预测值。这是由于Ag纳米粒子的存在,一方面增强了PMMA基体,另一方面在发泡过程中的异相成核作用减小了泡孔直径,提高了力学性能,其中后者起主要贡献。
再次,采用超声-溶液共混法将CNTs分散到原位合成的Ag/PMMA溶胶中制备出CNTs-Ag/PMMA纳米复合材料,研究制备工艺、CNTs含量对显微结构和电学性能的影响。结果表明,经过反溶剂沉淀技术可以将氨基改性的CNTs均匀分散在Ag/PMMA纳米复合材料基体中,CNTs含量越高,其均匀分散越难；CNTs含量超过2wt.%时,6h的超声条件无法实现CNTs在Ag/PMMA纳米复合材料基体中的均匀分散。TEM分析结果表明,Ag纳米粒子倾向于沉积到CNTs表面,并与多根CNTs发生结合,随着CNTs含量增加导致其分散性变差。电学性能研究表明,与CNTs/PMMA纳米复合材料相比,CNTs-Ag/PMMA纳米复合材料的电导率提高了1-3个数量级,主要归因于其导电行为可以由隧道效应理论解释,Ag纳米粒子承担起电子迁移的载体而参与导电。
最后,采用超临界流体发泡技术制备CNTs-Ag/PMMA微发泡纳米复合材料,研究发泡温度以及CNTs含量对泡孔结构的影响,探索力学和电学性能与泡孔结构的关系。结果表明,与Ag/PMMA微发泡纳米复合材料相比,CNTs-Ag/PMMA微发泡纳米复合材料的泡孔密度进一步提高2-4倍。当CNTs含量超过0.5wt.%后,继续增加CNTs的含量对CNTs-Ag/PMMA微发泡纳米复合材料的泡孔结构的影响较小；通过控制发泡温度可以将CNTs-Ag/PMMA微发泡纳米复合材料的泡孔直径控制在3-20μm,泡孔密度控制在2×108～1011cells/cm3。CNTs-Ag两相纳米粒子具有更好的成核效果,主要源于添加的CNTs与PMMA基体的结合界面更弱,更有利于发生异相成核。在相同体积密度条件下,CNTs-Ag/PMMA微发泡材料比Ag/PMMA微发泡纳米复合材料的压缩强度和弹性模量都有所增加。CNTs-Ag/PMMA微发泡纳米复合材料的电导率随着泡孔直径的增大而逐渐降低,并逐渐转变成绝缘体性质,本研究认为是孔径增加,孔壁厚度减薄,导电通路逐渐被破坏。
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.
引文
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