Microcellular Foaming And Properties Of Multiphase/ Multicomponent High-Performance Thermoplastic Polymers

Microcellular polymers typically refer to novel polymeric foams having average cell sizes of around 10 μm, cell densities from 109 to 1012 cells/cm3 and a uniform cell size distribution. Compared with solid polymers and conventional polymeric foams, microcellular polymers usually exhibit reduced weight and high strength, improved impact toughness and fatigue life, elevated thermal stability, as well as low dielectric constant and thermal conductivity. Owing to these superior properties, microcellular foams have attracted extensive attention for its potential applications in various high-tech fields such as lightweight structural materials in aerospace, microelectronic packaging, automotive parts and insulation materials. Thus microcellular polymers are claimed as ―novel materials for the 21 st century‖. In recent years, microcellular polymers and processing technology, mechanism for cell nucleation and growth, as well as microstructure and macroscopic properties have become popular research topics in the area of polymer materials and processing. However, most of the previous studies focused on commodity polymers such as polystyrene, polyethylene, polypropylene, poly（methyl methacrylate）, et al. Microcellular foaming of high-performance thermoplastic polymers and their reinforced composites, polymer blends is still lacking.This dissertation focuses on the studies of amorphous polycarbonate（PC）, crystalline poly（phenylene sulfide）（PPS） and glass-fiber reinforced PPS（PPS/GF） composites, crystalline poly（phenylene sulfide）/amorphous poly（ether sulfones）（PPS/PES） and crystalline poly（phenylene sulfide）/crystalline poly（ether ether ketone）（PPS/PEEK）. Pure polymers, PPS/GF composites, PPS/PES and PPS/PEEK blends were firstly prepared through the twin-screw melt compounding and extrusion method. Subsequently, high-performance microcellular polymers were fabricated using the solid-state batch foaming technique with supercritical CO₂（SC-CO₂） as a physical-blowing agent. The effects of fiber content and blend ratio on the rheological properties of composites and blends were discussed using the rotational rheological measurements. The thermal properties and crystallization behaviors were studied by DSC and XRD analyses. The effects of processing conditions and microscopic morphology on the microcellular foaming behaviors and final cellular structures were detailedly investigated by SEM. The comparative analysis of tensile, compression, impact, dynamic mechanical and dielectric properties between the solid and mocrocellular polymers were also carried out. The dissertation has important theoretical and practical significance for the microcellular foaming of high-performance multiphase/multicomponentthermoplastic polymers, as well as their microstructure and properties. The results are summarized as follows:（1） The dissolved CO₂ gas in PC exhibits significant plasticization effect on the polymer, leading to the decreased glass transition temperature and foaming temperature. Cha-Yoon model can effectively predict the actual glass transition temperature of the PC/CO₂. Microcellular PC foams with unimodal or bimodal cell-size distributions were prepared using the single and double depressurization processes during the saturation procedure, respectively. The mechanism for the fabrication of bimodal PC foams were elaborated using the critical radius concept. Trimodal PC foams containing dense nanocellular structures can be obtained when the foaming temperature is 160°C. The generation of nanocellular structures can be attributed to the local stress-induced nucleation mechanism. Specifically, the expansion of larger cells causes local extensional stress in the surrounding regions, resulting in the reduced critical radius and free energy barrier for cell nucleation. Relative density and cell-size distribution have significant influences on the macroscopic properties of microcellular PC foams. With increasing relative density, the compressive strength, compressive modulus, storage modulus and loss modulus of the microcellular foams are improved. Compared with the unimodal PC foams, the bimodal PC foams show significantly elevated tensile, compressive and dynamic mechanical properties. The Gibson-Ashby model is accurate in predicting the compressive properties of microcellular PC foams with a unimodal cell-size distribution at low relative densities（<0.45）. The dielectric properties of microcellular foams are primarily determined by the total porosity, but not the microstructures such as cell size, cell-size distribution and cell density. The relationship between the dielectric constant and porosity of microcellular foams can be simulated by the Maxwell-Garnett-spheres model very well.（2） The presence of crystals and GF alters the diffusion path in the PPS matrix, leading to reduced gas diffusion rate and dissolvability. By adjusting the fiber content and processing conditions, a controllable preparation of lightweight microcellular PPS/GF composites with various microstructures and relative densities can be obtained. At elevated foaming temperatures above the cold crystallization temperature of PPS（125°C）, the transcrystals formed on the surface of glass fibers induce the heterogeneous cell nucleation, generating numerous small cells around the fibers. Therefore, microcellular composite foams with a trimodal cell-size distribution are obtained due to heterogeneous cell nucleation caused by theoriginal crystals in the PPS matrix, spherulites and transcrystals formed during the foaming process. After a heat treatment on the press at 300°C for 10 min and then a rapid quenching process, nanocellular PPS foams with an ultra-low dielectric constant can be prepared by the foaming technique from the melt-extruded and naturally cooled pure PPS. The dielectric constant of nanocellular foams can be decreased to as low as 1.33 when the porosity is 0.79. The microcellular foamed PPS and PPS/GF composites possess higher specific tensile strength, strain at break and impact strength over the solid ones. Compared with the microcellular PPS, the microcellular PPS/GF composites show elevated storage modulus, loss modulus and wider glass transition temperature region.（3） The crystalline/amorphous PPS/PES sheets with different weight ratios of 10:0, 8:2, 6:4, 5:5, 4:6, 2:8 and 0:10 were prepared through the melt compounding and extrusion method. The PPS component is benefitial to improve the melt fluidity, while the amorphous PES component is benefitial to improve the tensile strength, strain at break and impact strength of the blends. As the PES content increases, the phase morphology of PPS/PES blends transforms from the single continuous structure into the bicontinuous structure. The increased PES content also leads to the higher diffusion rate and equilibrium concentration of CO₂. Blend ratio, phase structure, crystallinity, gas concentration and foaming conditions are key factors affecting the microcellular foaming behaviors of PPS/PES blends. Microcellular PPS/PES foams with tailored microstructures can be prepared by adjusting these factors. The specific tensile strength, strain at break and impact strength all increases, and the dielectric constant decreases after the microcellular foaming process. The blend ratio has significant influences on the storage modulus and loss factor for both the solid and microcellular PPS/PES blends. The storage modulus of microcellular PPS foams is higher than that of the solid PPS, while the other microcellular foamed PPS/PES blends show lower storage modulus and loss factor than the solid ones.（4） The crystalline/crystalline PPS/PEEK sheets with different weight ratios of 10:0, 8:2, 5:5, 2:8 and 0:10 were prepared through the melt compounding and extrusion method. Compounding PPS with PEEK provides the blends combined properties such as excellent melt fluidity of PPS, and outstanding thermal stability and mechanical properties of PEEK. The blends also exhibit higher crystallinity than the pure polymers. The blend ratio and crystallinity show great influences on the diffusion rate and equilibrium concentration of CO₂ in the blends. With increasing PEEK content and saturation pressure, the equilibriumconcentration of CO₂ gradually increases. The diffusion rate of CO₂ in PPS/PEEK（5:5） is relatively lower due to the higher crystallinity. The supercritical CO₂ saturated PPS/PEEK blends present a double cold crystallization phenomenon in the heating process. The lower cold crystallization temperature is caused by the gas-induced crystallization, while the higher cold crystallization temperature is caused by the thermally induced crystallization. Blending broadens the foaming temperature range of the foaming systems. At the same time, the combined factors including crystal structures, phase interfaces, different matrix strength and gas concentration in the PPS and PEEK phases lead to the formation of hierarchical cellular structures with much smaller cell size and larger cell density. Processing conditions such as saturation pressure, foaming temperature and foaming time also affect the cell size, cell-size distribution and cell density of the microcellualr foams significantly. Microcellular foaming improves the crystallinity, specific tensile strength and impact strength of the PPS/PEEK blends, as well as decreases the storage modulus, loss factor and dielectric constant.