Inclusion complex formed between poly(ethylene oxide) and thiourea

Study on the structure of PEO–thiourea binary system has been carried out by several groups, but there still remain some questions. In this study, PEO–thiourea complexes with different thiourea fractions were prepared through freeze-drying and characterized by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR) microscopy and Raman scattering. A new PEO–thiourea complex with EO:thiourea molar ratio of 4:1 (form II) was demonstrated for the first time. Then the phase diagram of PEO–thiourea binary system was improved, containing two particular complexes at thiourea molar fraction of 0.4 (form I) and 0.2 (form II), respectively. Further analysis suggested that PEO–thiourea form II complex adopts orthorhombic unit cell, and probably takes channel structure with two side-by-side parallel PEO chains incorporated inside the thiourea clathrate. The form II complex is quite stable in pure state at room temperature; while gradually convert into from I by solid–solid transformation when a few form I complex serves as nucleator.