Investigating the influence of long chain branching and compositional changes of aliphatic-aromatic copolyesters on their rheological properties under shear and elongational flows

The processability and properties of poly (butylene succinate-co-ethylene terephthalate) (PBSET) are dictated more than everything by the content of monomers, namely succinic to terephthalic acid ratio, and by the presence of branched structures. With this in mind, in the present study, the changes of rheological properties under shear and elongational flows as well as the thermal and mechanical properties of the PBSET copolymers were evaluated with the variation of succinic to terephthalic ratio and with the inclusion of long chain branched (LCB) structure. Copolymers with different succinic to terephthalic ratios (i.e. 10:90, 20:80, 80:20 and 90:10 mol%) in linear and branched forms were synthesized and tested. Shear rheology analyses corroborated that with the slight increase of SA ratio from 10 to 20% and from 80 to 90%, viscosity and modulus values were decreased and shear thinning became less pronounced. On the other hand, the considerable increase of SA ratio from 20 to 80 not only promoted the melt elasticity and viscosity but also intensified the shear thinning. Branching in general induced an increasing effect on elastic behavior of the copolymer melt. Only in copolymer with 80% SA, the elastic behavior of the linear form was better than that of the branched counterpart. Elongational viscosity results were in line with those of shear viscosity. Small increase of SA ratio decreased, whereas branching increased the elongational viscosity values. Only in copolymer with 80% SA, elongational viscosity values of the linear copolymer were higher than those of branched counterpart. Strain hardening of polymer melt was observed for LCB copolymers with high SA ratio. DSC thermograms corroborated that with the increase of SA and TA in respective terephthalic and succinic rich copolymers Tc and crystallinity were decreased. Moreover, with branching Tc and crystallinity were decreased and cold crystallization peak disappeared. Tensile testing implied that with the increase of SA and TA in respective terephthalic and succinic rich copolymers, tensile strength and modulus were decreased whereas tensile strain was increased. And branching increased elasticity, whereas decreased flexibility of the copolymers.