Spectroscopic investigation of the photodegradation of polydiacetylene solutions

The degradation of chloroform solutions of nBCMU-polydiacetylenes [nBCMU = (BuO2CCH2NHCO2)-O-n(CH2)(n)], and related polymers with chiral urethane sidegroups, stored in Pyrex glassware under normal laboratory atmosphere and lighting was studied by the measurement of changes in the optical absorption spectrum. Both the magnitude and wavelength of the absorbance maximum decrease as the mean molecular weight is reduced by degradation. Measurements made on a reference sample stored in the dark showed that thermal degradation was negligible. Comparison with the spectra of a sample degraded by UV radiation (lambda < 300 nm) shows that the degradation behaviour of white light exposed samples is distinctly different. While the precise temporal dependence of the degradation varies from polymer to polymer there is a discernible generic behaviour. This is also true for the shift in the energy of the absorption maximum when plotted against the decrease in absorption strength. A model based on the known dependence of the absorption maximum on polymer chain length is developed to describe the observed generic behaviour. Degradation by random scission followed by depolymerization is shown to be consistent with the experimental data The measurements show that polydiacetylene solutions can be stored in the dark for up to four years but that solutions left open to the light can have shelf lives as short as ten days.