Zone fractionation of cyclohexane‐rich polystyrene–cyclohexane solid solutions

Abstract

AbstractExperimental results on the application of zone melting techniques to dilute solutions of polystyrene in cyclohexane under various operating conditions are reported together with observations on static solidification tests on the solutions. The variables investigated were: polymer concentration, number of zones passed, zone length, and zone travel rate. The results showed that at slow zone travel rates (ca. 4.01 cm./hr.) redistribution of solute species occurs in conformity with thermodynamic predictions. In this case, a cyclohexane‐rich liquid and a polymer‐rich liquid, which coexist at temperatures just above the solidification range, react to form a cyclohexane‐rich solid solution upon solidification. The solid solution contained a higher concentration of all polymer species than the cyclohexane‐rich liquid, with the ratio of the concentration in the solid to the concentration in the liquid increasing exponentially with molecular weight of the solute. The net result was the accumulation of all solute species toward the head (i.e., first melted and first frozen) end of the zone melted ingot, with the higher molecular weight species enjoying the greatest redistribution in this respect. At moderately faster speeds of zone travel (ca. 4.83 cm./hr.), a reversal in the accumulation trend with molecular weight was found; the higher molecular weight species were carried increasingly to the tail (i.e., the last frozen) end of the sample. Increases in total polymer concentration enhanced this effect. These facts have been interpreted as suggesting that resorption of some of the higher molecular weight species from the polymer‐rich phase was not sufficiently fast compared to the rate of freezing, and part of the polymer‐rich phase was carried in the moving liquid zone. At significantly faster speeds of zone travel (ca. 9.66 cm./hr.) a decreased redistribution of solute species was found. Increased total polymer concentration reduced the selectivity of fractionation. This effect was interpreted as being the result of the lesser extent to which the resorption reaction in these experiments occurred in relation to the rate of movement of the interphase. Qualitative considerations on the thermodynamic characteristics of the polymer–solvent system most suitable for effective zone fractionation are discussed. The effect of zone fractionation upon the distribution of molecular weight for some of the samples is reported.