Applications Of Mesoporous Silica In Polymer Supercritical Carbon Dioxide Microcellular Foaming

For preparing polymeric microcellular foam materials, inorganic nanoparticles were usually used as heterogeneous nucleate agents to reduce cell size, increase cell density, and improve cell morphology and their mechanical properties. To date, numerous inorganic fillers ranging from micro inorganic filler, such as mica, calcium carbonate to nano particles, such as clay, carbon nanotubes and graphene have been developed and used as heterogeneous nucleate agents. It has been demonstrated that the size, morphology, and surface characteristics of these inorganic particles, as well as their interactions with polymer matrix can notably affect the nucleation efficiency. Therefore, designing and optimizing the structure and surface characteristics of the inorganic particles for the purpose of increasing their heterogeneous nucleation efficiency has become one of directions of critical importance.In this dissertation, an unique type of inorganic particles, i. e. silica particles with mesoporous structure, were proposed to applied as nucleating agents for polymer supercritical carbon dioxide(scCO2) microcellular foaming. The porous structure and high specific area of these particles are expected to supply reservoirs for scCO2local enrichment. In addition, gas cavities that can greatly decrease the nucleation energy barrier might be formed between polymer matrix and pores of particles, which in turn will also enhance the heterogeneous nucleation efficiency.A sol-gel method was used to obtain sphere ordered mesoporous silica (s-OMS) particles. To improve the compatibility between polystyrene (PS) and particles, a layer of PS brushes was grafted onto the surface of s-OMS particles before template agent extracted, from which the porous structure was preserved. Brunauer-Emmett-Teller surface area analysis (BET) and Thermogravimetric Analysis (TGA) showed the brushes impede the permeation of atmospheric CO2into internal of particles, which decreased the CO2absorption capacity. However, higher solubility of scCO2in PS/s-OMS composites as compared to pure PS was observed because the porous structure can serve as reservoirs for blowing agent. The modified particles can be well dispersed in the PS matrix, and exhibited an excellent heterogeneous effect in PS foaming, which is represented by the decreased average cell diameter, increased the cell density, and even lower bulk density of the composite foams. This dissertation also investigated the heterogeneous effect at different foaming conditions including lower foaming temperature and lower saturated pressure, which showed that the nucleation effect was more significant under these conditions.To further understand the underlying mechanism of the particles with porous structure, solid silica particles with similar particle size were used for comparison. All the particles were modified with same silane coupling agent. Transmission electron microscopy (TEM), X-ray diffraction (XRD), BET certified that coupling agent was grafted on both outside and inside of s-OMS particles, and these channels were partially reserved. It was found that all these particles could be well dispersed in the polymethyl methacrylate (PMMA) matrix, and exhibited excellent heterogeneous nucleation performance during the foaming process. Compared to solid silica particles, s-OMS particles showed higher nucleation efficiency. The pre-existing gas cavities resulted in a lower nucleation energy barrier, and then enhanced nucleation efficiency. The superior nucleation effect of the s-OMS particles became more obvious when the foaming process was conducted at low foaming temperature, low saturation pressure, and/or high pressure drop rate.