Nonequilibrium solidification of poly(ethylene oxide)-sodium thiocyanate mixtures

Mixtures of polyethylene oxide (PEO) and sodium thiocyanate (NaSCN) were isothermally crystallized at temperatures between 7 and 37° C, below the eutectic temperature T _e=63° C. The stable phases are semicrystalline polyethylene oxide, SPEO, and a crystalline complex, CC, with the formula PEO₃ NaSCN; these two solids grow by different mechanisms. Salt mole fraction was varied between 0.067X0.143 for studies by optical microscopy and differential scanning calorimetry. Solidification was observed to proceed by primary growth followed by coupled growth at a nonequilibrium composition X_e much greater than the equilibrium eutectic composition X _e=0.026. The boundaries of this skewed coupled zone could not be determined because of a dependence of X_e on the nature of the primary phase.