Broadband dielectric spectroscopy of the inter- and intramolecular dynamics of a series of random polyester copolymers

Broadband dielectric spectroscopy (1-10⁶ Hz, 183-423 K) and differential scanning calorimetry are employed to analyze the inter- and intramolecular dynamics of a series of random copolymers based on poly(ethylene terephthalate) and poly(1,4-cyclohexylene dimethylene terephthalate). In addition to an interfacial relaxation (α*-process), three dielectric relaxation processes are observed: The α-relaxation ("dynamic glass transition") and two secondary relaxations (β- and β*-relaxations). The α-relaxation depends sensitively on the composition of the copolymer and shows a rapid slowing down with increasing content of cyclohexylene dimethylene (CHDM) linkages. Besides the β-relaxation, attributed to local motion of the ester group, an additional process (β*-relaxation) is observed on introducing the CHDM linkages. Increasing the content of the latter reduces the strength of the β-relaxation strongly and increases its activation energy by more than 30%. This proves that owing to interactions between the cylohexylene rings and the ester group the β-relaxation no longer has local character only.