Vibrational spectroscopic studies of degradation and diffusion processes in poly(ethylene terephthalate)
The interaction of polyethylene terephthalate (PET) with water at both ambient and elevated temperatures has been studied. The diffusion of water, at ambient temperatures, into PET films, ofthe order of 10 um thick, has been followed using Founer transform infrared attenuated total reflectance, FT-IR ATR, spectroscopy. Films of differing degrees ofcrystallinity were prepared using two different methods. One method involved the annealing ofthe cast films at 85-90° C for different lengths oftime, to obtain a range of crystallinities. The other method involved the incorporation of different amounts ofan isophthalate group, to obtain a range of crystallinities. The rate of water diffusion with time was then measured as a function of crystallinity. The diffusion was shown to be classically Fickian in nature and the diffusion coefficients decreased with increased polymer crystallinity for both sets offilms. The perturbation of the v(OH) band of the water in the polymer matrix was studied as both a function oftime (i.e. concentration) and crystallinity. The water band was shown to be decoupled at low concentrations within the polymer matrix, indicating a breaking up of the water hydrogen bonding network. At higher concentrations, longer times, the v(OH) band gained more 'pure water' like character, but remained at higher frequency than pure water even at equilibrium water content, suggesting clustering of the water molecules, but an overall weakening ofthe hydrogen bonding network relative to the pure water spectrum. The study ofthe interactions ofwater at elevated temperatures including the degradative hydrolysis of PET at 90° C was undertaken using reflection absorption infrared spectroscopy (RAlRS). Films of <150 nm were immersed in pure water at 52,62, 70, 80 and 90° C and the effect on the polymeric structure was examined. At temperatures below 90° C the effects noted, on the time scales studied, were annealing effects, resulting in an increase in crystallinity. Estimations ofthe apparent activation energy ofthe gauche to trans isomerisation, for different degrees ofcrystallinity, were calculated and were found to be lower than those reported in the literature in air. This difference was thought to be a result ofthe plasticisation effects of water. At 90° C, during several days of immersion, the polymer was found to undergo hydrolysis. Complex changes in the RAIRS spectrum were related to changes in the polymeric structure, resulting from degradation. The autocatalytic nature ofthe degradation was highlighted, as was the loss of(small) mobile species from the polymer matrix. Amechanism involving the preferred site ofhydrolysis being a terminal ester group was proposed. Comparisons with hot alkaline hydrolysis were made. This occurred much faster and with more random chain scission. The diffusion of two organic liquids, methanol and ethylene glycol, into PET was studied. The diffusion was shown to be non-Fickian in nature due to the swelling and crystallisation that accompanied the diffusion. For the amorphous PET films, diffusion was accompanied by swelling and crystallisation for both molecules and was fitted to a dual sorption model. There was spectroscopic evidence for both a 'bound' alcohol- PET moiety and a 'free' alcohol species within the polymer matrix. For methanol in PET, the proportion of the sorbed alcohol which was 'bound' was found to increase with crystallinity, but for ethylene glycol the reverse was true. For methanol diffusion, increasing the crystallinity was shown to have a drastic effect on both the rate of diffusion and degree of swelling. For ethylene glycol diffusion, the degree of crystallinity appeared to affect the rate of swelling and the initial rate of sorption of penetrant, but the rate of the subsequent diffusion seemed to be unaffected by morphology. The interface between two layers of 20 and 30 um co-extruded PET laminates of PET and PET with an isophthalate comonomer, were examined using confocal Raman microscopy. The methods of confocal depth profiling through the polymer laminate and scanning, step-wise, along a cut edge were compared. The interface was examined using the carbonyl band width ofPET as an indicator of crystallinity. The interface was shown to be 2-3 um thick, independent of film thickness and contain a gradient of v(C=O) band width, indicating either interdiffusion or a transesterification reaction between the two polymer layers during co-extrusion.