Polymer materials for industrial, medical, and household use can be degraded by sun light, especially by the UV portion of the sun light. In this research, we applied a nonlinear optical spectroscopic method, sum frequency generation (SFG) vibrational spectroscopy, to study molecular surface structural changes of various polymer materials after exposed to UV irradiation. SFG is a second order nonlinear optical spectroscopic method, the selection rule of which indicates that it has sub-monolayer surface specificity. Here SFG was applied to investigate surface structural changes of plasticized poly(vinyl chloride) (PVC) as a function of plasticizer bulk concentration, UV irradiation wavelength, and UV irradiation exposure time. In addition, SFG was applied to study the surface degradation under UV irradiation with H<sub>2</sub>O<sub>2</sub>. With the help of supplemental analytical tools such as secondary ion mass spectrometry, FTIR, and chromatography, detailed molecular degradation mechanisms of plasticized PVC surfaces were deduced by SFG. SFG has also been applied to study surface degradation mechanisms of polypropylene, nylon, polyurethane, and poly (methyl methacrylate) under UV irradiation. SFG spectra collected from the polymer surfaces in the C-H, N-H and C=O stretching frequency regions provided in-depth understanding on surface structural changes of various polymers under UV irradiation, demonstrating that SFG is a powerful technique to elucidate molecular degradation mechanisms of polymer surfaces after UV irradiation.

Molecular degradation mechanisms of polymer surfaces under UV irradiation investigated by nonlinear optical spectroscopy

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