Environmental degradation of biodegradable polyesters 1. Poly(ε-caprolactone), poly[(R)-3-hydroxybutyrate], and poly(L-lactide) films in controlled static seawater

Abstract

Films of biodegradable aliphatic polyesters, poly(ε-caprolactone) (PCL), poly[(R)-3-hydroxybutyrate] (R-PHB), and poly(L-lactide) (PLLA) were prepared by solution-casting and annealing from the melt. Their biodegradation in static seawater controlled at 25 °C was investigated using polarizing optical microscopy, gravimetry, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and tensile testing. The change in weight loss, tensile strength, and Young's modulus revealed that the biodegradabilities of the aliphatic polyesters in the controlled seawater decreased in the order: PCL>R-PHB>PLLA. The results of gravimetry, GPC, and DSC showed that the biodegradation of PCL and R-PHB films proceeds via surface erosion mechanisms. The polarizing optical microscopy indicated that the PCL and R-PHB films were biodegraded inhomogeneously on the film surface where the marine microbes attached, resulting in pore formation. The crystalline residues of PCL and R-PHB films could not be traced by GPC even when large weight losses occurred. Polarizing optical microscopy and GPC indicated that the decreased tensile strength and Young's modulus of the PCL and R-PHB films are attributed to the formation of pores and cracks during biodegradation. The biodegradation of the PLLA films was insignificant even after immersion in the controlled seawater for 10 weeks and the initial crystallinity had no significant effects on the biodegradability of the PLLA films, excluding the tensile properties change. The biodegradation of these aliphatic polyester films in the controlled static seawater could not be traced by GPC and DSC measurements.