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| - Expansion of Polystyrene Using Supercritical Carbon Dioxide: Effectsof Molecular Weight, Polydispersity, and Low Molecular WeightComponents
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| - Closed cell foams of broad molecular weight distribution commercial polystyrene samples,prepared by expansion of supercritical-CO2-swollen specimens, exhibit cell diameters that are ∼3−10times (19−24 μm) larger than those of foams prepared from polystyrene samples with narrow molecularweight (NMW) distributions (∼2−6 μm). Cell diameters for NMW samples are independent of molecularweight from 147K to 1050K. Simulated polydisperse samples prepared by blending NMW distributionsamples ranging from 560 to 1050K and a polydisperse sample prepared by radical polymerizationproduced foams with cells of the same size as in foams prepared from the NMW distribution samples.These observations suggest that molecular weight and polydispersity are not important factors indetermining cell size and are not responsible for the disparity in cell sizes described above. This disparityis due to the presence of a very low molecular weight component (∼270) in the commercial samples.Extraction of this component reduced the cell diameter of resulting foams to that of the NMW distributionsamples. Addition of a styrene oligomer (285) to a NMW distribution sample resulted in foams with largercell diameters. Varying the concentration of this oligomer allows control of cell size in foams. Classicalnucleation theory cannot explain these observations, suggesting that an alternative mechanism of cellformation is active.
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