Diffusion of Aromatic Solutes in Aliphatic Polymers above Glass Transition Temperature
2013
Fang, Xiaoyi | Domenek, Sandra | Ducruet, Violette | Réfrégiers, Matthieu | Vitrac, Olivier
The paper presents a harmonized description of the diffusion of solutes with repeated aromatic jumping units (JU) in entangled aliphatic polymers above their Tg. It is shown that the trace diffusion coefficients, D, are scaled with the number of JU or equivalently with solute molecular mass, M, as M–¹M–ᴷᵅ/⁽ᵀ–ᵀᵍ ⁺ ᴷᵝ⁾, where Kα and Kᵦ are temperature-equivalent parameters related to Williams–Landel–Ferry (WLF) ones. The scaling of diffusion behaviors of linear aliphatic and aromatic solutes appear separated by a temperature shift, Kᵦ, of ca. 91 K. The effects of the number of JU and the distance between two JU were specifically probed in several aliphatic polymers (polypropylene, polylactide and polycaprolactone) at different temperatures above Tg with two homologous solute series: short oligophenyls and diphenylalkanes. An extended free-volume theory for many JU was accordingly inferred to account for the observed statistical independence between the fluctuations of the free volumes probed by each JU and the probability of the collective displacement of the center-of-mass of the solute. Outstanding properties of short oligophenyls series provided further insight on the underlying molecular mechanism of translation. Their activation energies grow differently according to the number of phenyl rings, NPₕ, being odd or even. Constrained molecular dynamics demonstrated that such a parity effect could be remarkably reproduced when the translation of each JU (i.e., phenyl ring) was randomly controlled by a combination of short and long-lived contacts.
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