Dynamic analysis, measurement, and control of cell growth in solid state polymeric foams.

摘要

This dissertation addresses critical manufacturing issues associated with the automatic control of solid state microcellular foam processes and the beneficial attributes such control can provide to the accuracy and repeatability of the foams produced. The research addresses the topics of empirical process characterization, dynamic modeling, sensor design, and non-invasive structure inference.; Process characterization and dynamic modeling are applied to the solid state process for amorphous gas-polymer systems. The analytical and experimental techniques used are specifically demonstrated to be valid for polycarbonate-carbon dioxide system.; Four significant contributions were attained through the process characterization and modeling effort. First, the "nucleation phenomenon" was found to be s triaxial tensile failure process. Second, a self regulating mechanism is identified describing foam growth dynamics as a function of the interrelationship between the balance of gas in the cells and the cell walls, and the strength of the polymer in the cell walls as a function of dissolved gas concentration. The mechanism clearly identifies the strength transition at the effective glass transition temperature of the gas-polymer mixture as the dominant factor determining both the rate of foam expansion and the steady state structure of the foam produced in the solid state. Third, a simulation model for a polycarbonate sheet subject to time varying thermal and mass transport boundary conditions and was shown to accurately predict dynamic bulk void fraction response with a maximum dynamic error of 13% and maximum steady state error of 7%. Finally, the temperature of the gas-polymer mixture must never exceed the effective glass transition temperature of the clean base polymer otherwise the self regulating characteristics of the mixture will vanish and the foam cells will explode, coalesce, and collapse.; Methods of non-invasive structure inference are of significant interest as a means of monitoring the solid state process for the purpose of real-time control of foam structure. Spatially periodic interdigitated capacitive probes arrays with a discrete set of spatial wavelengths were examined in the research. A spatially periodic interdigitated probe array is constructed using a set of co-planar electrically conducting strips arranged parallel to each other. Alternating successive strips are connected to one of two electrode terminals. When an electrical potential is applied across the terminals the electrode strips produce an electric field that penetrates the specimen to a depth proportional to the spatial periodicity of the probe array. Two significant results were determined. First, the optimal sensor probe sensitivity to changes in material structure for a spatialty periodic capacitive probe of spatial period {dollar}lambda{dollar} is achieved when the electrode plate width for the high and low potential electrodes are equal and the interelectrode gap separating the two electrodes is as small as possible. Second, the probe array technology can be used to accurately estimate foam void fraction as a function of position normal to the probe-foam interface plane.