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Abstract
In this work, isothermal and nonisothermal crystallization kinetics of poly(ethylene oxide) (PEO) and PEO in PEO/fatty acid (lauric and stearic acid) blends, that are used as thermal energy storage materials, was studied using differential scanning calorimetry (DSC) data. The Avrami equation was adopted to describe isothermal crystallization of PEO and nonisothermal crystallization was analyzed using both the modified Avrami approach and Ozawa method. Avrami exponent (n) for PEO crystallization was in the range 1.08–1.32 (10–90% relative crystallinity), despite of spherulites formation, while for PEO in PEO/fatty acid blends n was between 1.61 and 2.13. Hoffman and Lauritzen theory was applied to calculate the activation energy of nucleation (Kg) – the lowest value of Kg was observed for pure PEO, despite of heterogeneous nucleation of fatty acid crystals in PEO/fatty acid blends. For nonisothermal crystallization of PEO in PEO/lauric acid (1:1 w/w) and PEO/stearic acid (1:3 w/w) blends, secondary crystallization occurred and values of the Avrami exponent were 2.8 and 2.0, respectively. The crystallization activation energies of PEO were determined to be −260 kJ/mol for pure PEO, −538 kJ/mol for PEO/lauric acid blend, and −387 kJ/mol for PEO/stearic acid blend for isothermal crystallization and −135,6 kJ/mol, −114,5 kJ/mol, and −92,8 kJ/mol, respectively, for nonisothermal crystallization.
Acknowledgments
The authors are grateful to Prof. Phillip H. Geil for his valuable comments and suggestions to improve the quality of this paper.