Preparation of random poly(butylene alkylate-co-terephthalate)s with different methylene group contents: crystallization and degradation kinetics

Random copolyesters having 1,4-butanediol units were synthesized from a transesterification process between homopolymers constituted by aliphatic dicarboxylates (i.e. succinate, adipate or sebacate) and the aromatic therephthalate derivative, as verified by NMR spectroscopy. Biodegradability of resulting copolyesters was studied via enzymatic hydrolysis using Pseudomonas cepacia lipase at pH = 7.2 and 37 degrees C. Kinetics of degradation showed that in all cases the degradation rate decreased after 19 days of exposure. The observed glass transition temperatures, T-g, of the random copolyesters showed a non-linear dependence on composition, a feature that was explained in terms of the internal stiffening effect of butylene terephthalate units. Copolymers with higher aliphatic (i.e. 50 and 70 mol-%) and methylene (i.e. adipate and sebacate units) contents showed double melting peaks in DSC thermograms. These copolyesters resulted in two different crystalline rich phases after melt-crystallization and subsequent cooling. The ratio between these phases logically depended on the predominant aliphatic or aromatic dicarboxylate content. The copolymers initially crystallized via the aromatic units through a heterogeneous nucleation and a spherulitic growth. The presence of aliphatic dicarboxylate units hindered the beginning of the crystallization process, but the overall growth kinetic constant was similar for all samples. The secondary nucleation constants were determined and showed higher values for samples with higher adipate and sebacate contents.