Bio-based poly(butylene terephthalate) copolyesters containing bicyclic diacetalized galactitol and galactaric acid: Influence of composition on properties

Copolyesters from 1,4-butanediol, dimethyl terephthalate and either 2,3:4,5-di-O-methylene-galactitol or dimethyl 2,3:4,5-di-O-methylene-galactarate with compositions of 10, 20, 30, 40 and 50% of either galactitol or galactarate units were prepared by melt-polycondensation. The copolyesters had Mw in the 32,000–41,000 g mol−1 range and had a random microstructure. They displayed Tg from 20 to 70 °C with values steadily decreasing with the content in galactarate units but increasing with the content in galactitol units. All the copolyesters were semicrystalline with Tm between 115 and 210 °C and they adopted the crystal structure of PBT. Copolyesters containing up to 20% of galactitol units as well as all galactarate copolyesters were able to crystallize from the melt, at a crystallization rate that decreased with the content in carbohydrate-based units. Stress-strain essays revealed an increment in the tensile strength and elastic modulus with increasing contents in galactitol units whereas these parameters decreased when galactarate units were the replacing ones.