On the nature of morphological features in phase-separated (PEO)nNaSCN mixtures: a SAXS investigation

The SAXS technique is used to throw light on the morphology of phase-separated poly(ethylene oxide)n–sodium thiocyanate [(PEO)nNaSCN] mixtures. A temperature-dependent investigation is reported for (PEO)4.5NaSCN. (PEO)nNaSCN mixtures show a complex phase behaviour. At the investigated composition, a crystalline complex (CC) is formed between PEO and NaSCN. The considered composition is representative of thermodynamic states where three different phases coexist simultaneously: crystalline PEO (CPEO), amorphous PEO (APEO) and crystalline complex (CC). Various SAXS data analysis approaches are presented to understand the nature of these coexisting phases. Invariant analysis shows a bimodal behaviour which seems to be related to the melting of CPEO and a different process occurring well before the melting of the CC. Electron density correlation function analysis shows a well defined border between a complex morphology with bimodal lamellae (when both CPEO and CC coexist) and a simple pseudo-two-phase model (when CPEO is melted and only CC lamellae persist). A detailed knowledge of the system morphological details is then obtained as a function of temperature.