Properties of n-eicosane-filled microcapsules with different morphology. Phase Change Materials studied by positron spectroscopy and complementary methods

Abstract

The Positron Annihilation Lifetime Spectroscopy (PALS) was used to investigate properties of selected Phase Change Materials (PCMs) as a function of temperature from 123 K to 333 K. Three different PCM microcapsules investigated in this work were built of n-eicosane filling and a siloxane polymer as a shell material. It has been found that the properties of ortho-positronium (o-Ps) annihilating in n-eicosane filler depend on the microcapsule morphology. In the samples where the whole interior is composed of n-eicosane and the polymer forms the outer shell only, the n-eicosane behaves like a neat macroscopic sample. In the microcapsules containing the network of polymer threads inside the globule, n-eicosane shows no rise of o-Ps intensity with time which is typical for all pure alkanes. For this morphology, the melting point of n-eicosane is preceded by the 4 K wide temperature range, where the structure resembling the rotator phase is observed (rotator phase appears in neat even-numbered alkanes with carbon chain over 20 atoms). Supplementary data were obtained by the DSC method. SEM images confirmed the morphological differentiation of microcapsules. Low temperature (close to liquid nitrogen) destroys the structure of microcapsules, particularly their outer shell.