Relaxation behaviour and free volume of bio-based Poly(trimethylene terephthalate)-block-poly(caprolactone) copolymers as revealed by Broadband Dielectric and Positron Annihilation Lifetime Spectroscopies

Abstract

Abstract Broadband Dielectric (BDS) and Positron Annihilation Lifetime Spectroscopies (PALS) have been used to investigate the relaxation behaviour of novel bio-based aliphatic-aromatic block copolymers based on poly (trimethylene terephthalate) (PTT) and poly (caprolactone) (PCL). Copolymerization increases the ability to crystallize of the PTT-block-PCL copolymers in relation to PTT due to a decrease of the glass transition (Tg) temperatures as the amount PCL increases since PCL acts as plasticizer. The dielectric relaxation of the PTT-block -PCL copolymers is dominated by that of PTT being characterized by a α relaxation process above Tg and a bimodal β relaxation below Tg. The faster mode of the β relaxation has been assigned to the bond between the ester oxygen and the aliphatic carbon of the PTT block and the slower β mode has been attributed to the bond between the aromatic ring carbon to the ester carbon also in the PTT block. Both modes for the copolymers appear to be faster than those of PTT at comparable temperatures while for the copolymer series a nearly similar subglass dynamics is observed. Since free volume values for PTT and PTT-block-PCL (75/25) as measured by PALS are very similar and those for the copolymers increase with PCL content this effect is likely to be attributed to change in the flexibility of the polymeric chain upon going from PTT to the copolymers rather than to a free volume dependence confirming the local nature, i.e. non cooperative, of the β relaxation. The weak dependence of the β relaxation dynamics with PCL content for the copolymers suggests that this relaxation is mainly related to the PTT block. The α relaxation becomes faster the higher the PCL content is following the decrease of the Tg with increasing PCL content. This effect is supported by the free volume and o-Ps annihilation life time for the block copolymers measured by PALS at room temperature revealing an increase of both magnitudes with PCL content. By scaling free volume and o-Ps annihilation life time with Troom-Tg, being Tg the corresponding glass transition temperature for every copolymer, a characteristic sigmoidal variation with temperature is observed.