From Fast to Slow Degradation : Different Strategies to Characterise Polymer Degradation by Chromatographic Techniques

by Gallet, Guillaume

This thesis presents different analytical strategies for the study of the degradation of synthetic polymers. Particular attention was given to the mechanisms involved at the beginning of the polymer breakdown. Three model polymers were used in order to assess different degradation rates. The first polymer studied was a stabilised polyether: Poly(ethylene oxide-propylene oxideethylene oxide) triblock copolymer (poloxamer) containing 2,6-di-tert-butyl-4-methylphenol (BHT). The oxidative thermal degradation of the poloxamer at 50°C and 80°C in air (starved conditions) was monitored by Solid phase microextraction/gas chromatography-mass spectrometry (SPME/GC-MS), size exclusion chromatography (SEC), matrix assisted laser desorption ionisation-time of flight mass spectrometry (MALDI-TOF MS). At 80°C degradation was initiated on the PPO block of the copolymer by three mechanisms involving hydroperoxyl formation and depropagation. 1,2-propanediol,1-acetate;1,2-propanediol,2-formate; 1,2propanediol,1-acetate,2-formate and 2-propanone,1-hydroxy were the first degradation products produced. Random chain scissions and a sharp decrease in the molecular weight of the material followed the initiation period. Formic acid and acetic acid, formed upon degradation, participated in esterification reactions leading to the formation of the formate and acetate forms of 1,2propanediol and ethanediol. The second polymer studied was semicrystalline poly(L-lactide) (PLLA). Films of PLLA were buried in soil in south Finland during two years. Degradation of the polymer was monitored by SEC and differential scanning calorimetry. Low molecular weight degradation products were characterised by SPME/GC-MS. Lactic acid, lactide and lactoyl lactic acid were extracted from the unaged and aged films. In a first stage, after an induction period of one year, the ester bonds of PLLA underwent hydrolysis. In a second stage, microorganisms assimilated the small products of degradation created by hydrolysis. It is during this stage that the thermal properties of the films were significantly affected. The use of SPME/GC-MS for analysis of low molar mass products, in parallel with molecular weight determination of larger polymer chains by SEC or MALDI-TOF MS, was a promising method for better understanding degradation mechanisms in polymers. The third polymer studied was a glassfibre reinforced polyester composite which was subjected to accelerated ageing in air at 40°C and 60°C and 80 % relative humidity for periods up to 6 years. Before the accelerated ageing the materials were stored for 20 years at ambient temperature. Low molecular weight products in the materials were identified with GC-MS and Headspace/GC-MS. Multivariate data analysis (MDA) was then used to interpret the results. Alcohols, phthalates and other aromatic compounds were identified. Principal component analysis showed that temperature had a large influence on the degradation of phthalates and the formation of alcohols. At 40°C hydrolysis of phthalates was too slow to be correlated with ageing. At 60oC we built partial least square regression models able to predict the age of the samples from the amount of 13 low molecular weight products. The combination of MDA with chromatography techniques is a promising tool for analysis of polymer degradation.