Polymer characterisation - hydrolytic degradation of PET
The degradation of polymers by UV, heat, moisture or γ-irradiation is of significant importance in many different spheres of life. The degradation of polymers can have both a positive and negative impact on our life, for example, polymer degradation is used to control the release rates of drugs in the body but polymer degradation is also responsible for the yellowing of household plastic goods.
Vibrational spectroscopy is very sensitive to changes in the chemistry and morphology of polymers and polymer degradation is often accompanied by significant changes to both. The Polymers, Composites and Spectroscopy Group have looked at the hydrolytic degradation mechanism of polyethylene terephthalate (PET) using RAIRS.
Significant hydrolysis of PET is known to occur in moist, wet or humid conditions at temperatures above the glass transition temperature, Tg. The results include an increase in carboxylic and alcoholic end groups and an increase in smaller chain fragments probably by reverse esterification. The promotion of acidic or basic conditions significantly increases the rate of hydrolysis, although the mechanism is considered to be similar. It is thought that the water diffuses into the amorphous regions of the polymer and it is there that hydrolysis occurs. The rate will obviously depend on the polymer morphology and degree of crystallinity, as well as on the relative humidity and temperature.
After spin coating PET films on inert gold slides, we were able to induce hydrolytic degradation by immersing samples in water at 80 oC for several days. The degree of hydrolysis was measured and conformational changes were also observed. Significant changes in the shapes and intensity of the bands are observed as a function of time.
There is also an increase in the peak area of the hydroxyl stretching, v(OH), band (not shown) associated with an increase in hydroxyl end groups.
A more detailed analysis of the work is given in: AN FTIR-IR study of the effect of hydrolytic degradation on the structure of thin PET films. C Sammon, J Yarwood and N Everall. Journal of Polymer Degradation and Stability. 2000, Vol. 67, No. 1, PP.149-158.