Dynamics in renewable sourced random poly(trimethylene 2,5-furanoate-co-trimethylene suberate) copolymers: ambient-pressure fragility and compensation law in secondary relaxations

Abstract

This article presents an experimental study on the relaxation dynamics in a series of random copolymers with optimum gas barrier properties based on bio-friendly monomers. The relaxation response of poly(trimethylene 2,5-furanoate-co-trimethylene suberate) copolymers is examined in the glassy and viscoelastic regime via frequency-domain spectroscopies (dielectric and mechanical). We report lower values of dynamic fragility, a dimensionless index introduced in 1985 [C. A. Angell, Relaxations in Complex Systems, ed. K. Ngai and G. B. Wright, National Technical Information Service, US Department of Commerce, Springield, VA, 1985, p. 3; C. A. Angell, Formation of glasses from liquids and biopolymers, Science, 1995, 267(5206), 1924–1935], in comparison to conventional polyesters used in industry, such as poly(ethylene terephthalate). We propose that this is consistent with their low permeability to gases. The sub-glass dynamics is complex and can be described by two local modes, β₁ and β₂, whereas for poly(trimethylene 2,5-furanoate) and the copolymer with the lowest suberate unit content we detect a single β relaxation. Our study reveals that sub-T_g relaxations obey the so-called compensation law, which is interpreted with respect to the multiexcitation entropy model by Yelon and Movaghar, and the more recent collective small displacements model.