Differential scanning calorimetry studies on poly(ethylene glycol) with different molecular weights for thermal energy storage materials

Melting and crystallization behaviour of poly(ethylene glycol) (PEG) with different molecular weights (from 1000 to 35 000) and chosen blends of PEG is investigated by means of differential scanning calorimetry (DSC) operating in dynamic mode at different heating rates. The influence of the molecular weight of PEG on its melting point and enthalpy of fusion is evaluated; from the DSC data the degree of crystallinity is calculated–it is found that there is an increased tendency of higher-molecular-weight PEGs towards the formation of crystalline phase owing to their lower segmental mobility and more convenient geometrical alignment. During the freezing cycle, an increase in the molecular weight of PEG causes an increase in the solidification temperature and heat of crystallization. Thermal transition data are supplemented by optical microscopy–numerous small sphere-shaped crystalline structures are observed to join together and impinge on their neighbours, forming eventually a multilayered lamellar texture. By implementing second polymeric component the polydispersity of the system increases, thus lowering the crystallization degree during preparation phase. It influences also the course of solidification by lowering the crystallization temperature, Tc. An additional effect observed in the case of the blend's freezing is associated with larger supercooling, probably due to morphological constraints and entanglements in interlamellar regions. The possible advantage of using PEG blends to replace pure components is connected with the possibility of changing the temperature range and heat associated with melting/freezing. Copyright © 2003 John Wiley & Sons, Ltd.