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Polyethylene glycol (PEG) is a common polymer that is widely used in food and pharmaceutical industries, whereas urea is an organic molecule that is abundant in human body. As PEG can easily enter the body by food digestions, there is a possibility of PEG/urea complexation resulting in stable crystal formation, which might raise some health concerns such as kidney or urinary stones. Therefore, it is of paramount importance to understand the phase behaviors and the mechanisms of formation of stable compounds of the PEG/urea complexes.In this study, detailed investigations on phase diagrams of various molecular weights PEG/urea mixtures have been presented. Different characterization techniques (Differential Scanning Calorimetry, Polarized Optical Microscope, and Fourier Transform Infrared Spectroscopy) were utilized to determine the phase transitions such as the crystal-liquid and crystal-crystal transition. Various coexistence regions involving induced crystalline phases were investigated by Wide angle X-ray diffraction, FTIR and POM. Phase diagrams of each PEG/urea system were calculated by self-consistently solving the combined free energy of Flory-Huggins for liquid-liquid separation and the phase field free energy for crystal solidification. It was found that the blends of varying molecular weight PEG (8k,1k and 400) with urea show different types of phase diagrams. The phase diagram of PEG8k/urea is a combination of azeotrope and eutectoid reactions, covering the metastable submerged phases such as induced ¿ phase and metastable ¿ phase. However, the phase diagram of PEG1k/urea is of peritectic type, whereas PEG400/urea exhibits eutectic character. Of particular interest is that only very small urea amount (2 wt%) is needed to form stable ¿ inclusion crystal via complexation with PEG. If such crystals form in blood stream, it may be of health concern, especially diabetes patients. However, such a risk may be alleviated by drinking a lot of water, because the ¿ crystal is water soluble and is expected to dissolve in body serum. |
