Relation between phase behaviour and morphology in polyethylene/diphenyl ether systems

The phase behaviour and the related morphology of the system high‐density polyethylene/diphenyl ether was investigated. A liquid‐liquid demixing precedes the crystallization in solutions with polymer weight fractions lower than 0,30. A three‐phase equilibrium is installed, in which two liquid phases with different polymer concentrations and a crystalline phase coexist. This does not lead to an invariant situation, as would be predicted from the phase rule for a strictly binary system. The crystallization and melting temperature decrease in this concentration region with increasing polymer concentration. Transmission electron microscopy observations were carried out on samples with a polymer concentration in the demixing region. A morphology, characteristic of a liquid‐liquid demixing, preceding crystallization, is observed. Globular particles with a variety of surface structures and internal organization are obtained at very low polymer concentration. Solutions with a polymer weight fraction higher than 0,10 are transformed into porous, crystalline structures. An important shrinkage of the concentrated domains takes place. It results in a pronounced solvent synersis. This change in the physical aspect of the samples is ascribed to the nature of the three‐phase equilibrium.