Influence of the degradation mechanism on the molecular weight changes for some degradable polymers
Abstract: The influence of degradation mechanism on molecular weightchanges was investigated for certain degradable polymers. Thepolymers investigated were modified low density polyethylene(LDPE), poly(ethylene-co-vinyl alcohol) (EVOH) and aliphatic polyesters.Thermo- and photo-oxidation was studied for LDPE mixed withstarch and a pro-oxidant system, LDPE mixed with starch andLDPE alone. The chemical hydrolysis ofpoly(?-caprolactone) (PCL) was also studied. The abioticdegradation of these materials was compared with thebiodegradation of LDPE mixed with starch and pro-oxidant, LDPEmixed with starch and LDPE inoculated withArthrobacter paraffineusorFusarium redolensandPenicillium simplicissimum.It was also compared withpoly(lactic acid) (PLA), poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) and PCL degraded in compost.The degradation of PCL was also investigated in anaerobicsludge at two temperatures (37 and 55 ¡C).During the thermo-oxidation of the LDPE samples a decreasein molecular weight was seen. The samples with pro-oxidantshowed the fastest decrease in molecular weight. Starch had astabilising effect on LDPE during thermo-oxidation. During thephoto-oxidation an initial increase in molecular weight wasseen and was explained by a fast decomposition ofhydroperoxides during UV irradiation which lead to chainelongating reactions. Samples degraded by hydrolysis did sofaster at a higher temperature, while pH did not effect therate of degradation.When the modified LDPE samples were exposed tobiodegradation a decrease in the amount of carbonyls was seentogether with an increase in the amount of double bonds. At thesame time there was a small increase in molecular weight forthe biodegraded samples together with assimilation of thedegradation products or a larger amount of evolved14CO2. EVOH samples also displayed a slightly highermolecular weight after biodegradation together with a higheramount of14CO2. The micro-organisms assimilate faster the lowmolecular weight part of the samples, which explains theobserved increase in molecular weight. PLA and PHBV did notdisplay changes in molecular weight during composting. PCLdecreased its molecular weight during biodegradation, with afaster reduction in number average molecular weight (Mn) than weight average molecular weight (Mw). The same observations were made for the samplesexposed to thermophilic anaerobic sludge. This is indicative ofchain cleavage near the chain ends, which is associated withthe readier degradation of the amorphous regions where thechain ends are concentrated. During abiotic degradation thechain is cleaved mainly in a random fashion. PLA and PHBV didnot show any changes in molecular weight which is typical fordegradation byexo-enzymes. PCL showed a reduction in molecular weightas would be seen for polymers degraded byendo-enzymes. During the degradation of LDPE the polymerchanges in a way that affects the determination of molecularweight with size exclusion chromatography (SEC). The degradedpolymer will have a different relationship between molecularweight and hydrodynamic volume as compared to the undegradedpolymer. The observed molecular weight changes will, therefore,be incorrect if they are determined relative to a standard ofdifferent chemical type.Keywords:LDPE, polyethylene, starch, EVOH,poly(ethylene-co-vinyl alcohol), polyesters,poly(?-caprolactone), PCL,14C-labelled, biodegradation, composting, anaerobicsludge, thermo-oxidation, photo-oxidation, chemical hydrolysis,molecular weight changes, size exclusion chromatography.
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