Characterisation by solid phase microextraction–gas chromatography–mass spectrometry of matrix changes of poly( l-lactide) exposed to outdoor soil environment

Abstract

Films of poly( l-lactide), PLLA, were buried in outdoor environment in south Finland during two years. The changes in polylactide matrix were characterised by size exclusion chromatography and differential scanning calorimetry, and formation of volatile oligomers was monitored by solid phase microextraction (SPME)–gas chromatography–mass spectrometry. SPME sample preparation was found suitable for analysing small volatile compounds in solid PLLA samples. The oligomers in polylactide were readily extracted with polar CW/DVB fibre. The oligomers were analysed without any complicated sample handling procedure and no solvent was needed. Lactic acid, lactide and lactoyl lactic acid were extracted from the unaged and aged films. Lactide was observed in all films and its content did not change during the two years of soil burial. No lactic acid was observed in any of the polylactide films during the first year. After 20 months lactic acid and lactoyl lactic acid appeared as a result of hydrolysis. After 24 months the amount of lactoyl lactic acid decreased due to biotic hydrolysis of the film. The molecular weight M̄ n of polylactide increased and the polydispersity decreased during the first year. A slow chemical hydrolysis combined with the migration of small oligomers in soil caused the molecular weight changes for this period. During the second year of soil burial both M̄ n and M̄ w decreased and the polydispersity decreased towards 2. This indicates that a faster hydrolysis took place in the film. It was confirmed by the differential scanning calorimetry analysis of PLLA, which showed that during the second year in soil the thermal properties of the films ( T g and T m) were affected. PLLA was thus degraded by a combination of hydrolysis and biotic activity. In a first stage, after an induction period of 1 year, the film of PLLA undergoes hydrolysis. In a second stage microorganisms assimilate the small products of degradation while hydrolysis takes place.