Crystallization of poly(ethylene oxide) from stirred solutions

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Abstract
Poly(ethylene oxide) fibrillar crystals were prepared from ethyl alcohol solutions by stirred crystallization in the temperature range 31°-37°C. Electron microscopy and calorimetric measurements show that the crystals consist of a central thread with extended chains and lamellar overgrowths with folded chains. The melting behavior of crystals prepared at the same temperature with stirring (fibrillar) and without stirring (lamellar) was compared and the results discussed in relation to their respective morphologies. The fibrillar crystals exhibited superheating which increased with the temperature of crystallization. This is related to the amount of material which crystallizes in the extended-chain central threads. The relatively narrow temperature range in which the fibrillar crystals form is related to the greater tendency of the highly flexible poly(ethylene oxide) chains to crystallize in the folded conformation. Molecular fractionation occurs during crystallization, and the plot of molecular weight of precipitated polymer against temperature of crystallization shows a minimum. This behavior is explained by postulating that in the formation of the fibrillar crystals there is a superposition of two crystallization processes with different fractionation effects.

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