Hydrolytic degradation behaviour and microstructural changes of poly(ester-co-amide)s

Abstract

Poly(ester-co-amides), PBSe random copolymers, were prepared by polycondensation of sebacic acid and 1,4-butanediol, introducing 1,4-butanediamine as comonomer. The homopolymer was semi-crystalline with T_m of 65 °C. When the amide content was over 10%, two melting endotherms were observed and their melting enthalpies decreased significantly. WAXD results also showed a decrease of crystallinity with increasing amide content. Poly(ester-co-amides) had intramolecular and intermolecular hydrogen bonding, which was conformed by FT-IR. The degradability of the PBSe series increased with the amide content. Quantitative FT-IR analysis shows that the areas of the amide-amide hydrogen bonded NH band increased but the areas of amide-ester hydrogen bonded NH band decreased during hydrolysis.

Introduction

Biodegradable polymers have been widely investigated as a part of the growing concern about environmental problems [1]. Among synthetic biodegradable polymers, aliphatic polyesters are recognized to be one of the most important materials. But, they are not easily substituted for generally used polymers because they have poor thermal and mechanical properties.

Aliphatic polyamides have better thermal and mechanical properties than aliphatic polyesters, but their hydrolytic degradation rate is too low to classify them as biodegradable polymers. A high frequency of hydrogen bonding (H-bonding) and the high crystallinity resulting from structural regularity are the reason for the low hydrolytic degradation rate.

Incorporating amide groups into an ester main chain leads to the formation of intramolecular and intermolecular H-bonds between ester and amide groups, which are expected to complement the defect of aliphatic polyesters. However, if the content of amide groups is over 30%, the poly(ester-co-amide)s will loose the biodegradability due to the low degradability of polyamide segments [2]. If the amide segments in the copolymer are randomly distributed, the biodegradability of the copolymers would be improved because of reduced crystallinity.

In the poly (ester-co-amide)s, it is possible to form two types of H-bonds; those between the hydrogen of the amide group and the oxygen of the ester group (amide-ester H-bond) and those between hydrogen and oxygen of the amide groups(amide-amide H-bond) [3], [4].

Goodman et al. [5], [6], [7], [8] have reported the crystalline and general properties of the copolyesteramides of ϵ-caprolactam (6A) with ε-caprolactone (6E) and of hexamethylene diamine with hexamethylene adipate. Gonsalves et al. [9] have studied the hydrolytic degradation in buffer solution and the biodegradation by fungi of random poly(ester-co-amide)s. Several researchers [10], [11] have reported on the synthesis, characterization, and in vitro degradation mechanism in buffer solutions of random poly(ester-co-amide)s made from tartaric and succinic acids. Tokiwa et al. [12] studied the biodegradation of a series of copolyamide-esters by enzyme and lipase. They proposed that biodegradability is influenced by the distribution of the hydrogen bonds on the amide groups; increased hydrogen bonding leads to lower biodegradability. Kaczmarczyk et al. [3], [4] assigned the amide–amide and amide–ester hydrogen bonds of aliphatic and aromatic poly(ester-co-amide)s in the FT-IR spectra.

In this research, we prepared poly(ester-co-amide)s from sebacic acid, 1.4-butanediol and 1.4–butanediamine as a comonomer with various ester/amide ratios ranging from 100/0 to 70/30 and investigated their characterization, degradability and microstructural changes during biodegradation.