Chain mobility, secondary relaxation, and oxygen transport in terephthalate copolyesters with rigid and flexible cyclic diols

Publication date: 27 October 2017
Source:Polymer, Volume 129
Author(s): Krishnan A. Iyer
High glass transition temperature (Tg) polyesters based on cycloaliphatic diols such as 1,4-cyclohexanedimethanol (CHDM) and 2,2,4,4-tetramethyl-1,3-cyclobutanediol (TMCD) have attracted significant interest as bisphenol-A replacement. Whereas most literature reports focus on synthetic approaches, the present study investigates in depth various structure-property relationships in CHDM- and TMCD-containing novel copolyesters. Significant enhancements in Tg and increases in fragility are observed with the incorporation of bulky CHDM and rigid TMCD as comonomers. Substantial increase in oxygen permeability is measured with the incorporation of CHDM from 14.3 for poly(ethylene terephthalate) to 52.6 cc-mil/100in².day.atm (SPU) for poly(1,4-cyclohexyldimethylene terephthalate). When 35 mol% CHDM is replaced by TMCD as comonomer, the value further increases to 137.0 SPU. In the case of CHDM-containing polyesters, improvements in both sub-Tg molecular mobility (due to additional chair-to-boat transformations) and fractional free volume (FFV) result in an increase in oxygen diffusion. On the other hand, substituting 35 mol% CHDM with more rigid TMCD yields a suppressed β-relaxation. In particular, CHDM-based copolyesters follow a linear correlation between the strength of β-relaxation and diffusivity whereas the TMCD-based copolyester exhibits a significantly positive deviation from the correlation. Therefore, the much higher FFV in TMCD-containing polyester plays a more important role in determining oxygen transport through such polymer. Finally, we demonstrate that copolyesters based on CHDM and TMCD display significantly enhanced resistance to moisture-induced plasticization and the competing penetrant effect.

Graphical abstract

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