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Measuring the Coexistence Curve of 8-Arm Star Polystyrene in Methylcyclohexane

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Mark A. Lightfoot

Advances were made on an existing experiment that will enable the measuring of the coexistence curve of eight-arm star polystyrene in methylcyclohexane to determine the coexistence curve amplitude B. An automated measurement of the minimum deviated angle in each phase provids the refractive index and thus the composition in each phase. By exploring temperatures from a few milliKelvins to ten Kelvin below the critical temperature, the shape of the coexistence curve will be determined and compared to a simple power law of amplitude B and exponent β. The exponent should be independent of molecular weight while the amplitude should vary as a power-law in molecular weight. Preliminary data were obtained using refractive index as the order parameter. Strange behavior near the critical temperature prevented an accurate analysis. Still the data were fit to a power law equation where the β exponent was held constant at its value for the 3D Ising model (0.326) to show how much the data agree with universality theory. Eventually, results for one molecular weight (228,000) can be compared to published values for a smaller molecular weight (74,000) and to the results for a linear polystyrene in the same solvent.