Molecular mechanism of photolysis and photooxidation of poly(neopentyl isophthalate)

Study on aging behavior of polyethylene glycol under three wavelengths of ultraviolet light irradiation

PEG2000 (polyethylene glycol, molecular weight: 2000) is commonly used for the dehydration and reinforcement of waterlogged wooden cultural relics, but its photo-aging degradation will seriously affect the long-term conservation of the wooden cultural relics. In this study, the photo-aging characteristics and mechanisms of PEG2000 under UV (ultraviolet) irradiations of three wavelengths were comprehensively investigated, and the surface morphology, crystal structure, and relative molecular weight of PEG2000 were systematically characterized. The results showed that PEG2000 showed a higher gloss loss rate, carbonyl index and crystallinity, and a wider molecular weight distribution with increasing aging time, especially under the irradiation of 313 nm ultraviolet light. The evolution of the PEG2000 from surface to interior during photoaging was elucidated by SEM (scanning electron microscopy) and FTIR (Fourier transform infrared spectroscopy), and it was determined that photodegradation not only occurs on the surface of PEG2000 but also gradually extends to the interior of the sample with the prolongation of irradiation time, resulting in the transformation of the basic component unit of spherical crystals in PEG2000 from fibrous crystals to spherical particles. Based on 1H-NMR (nuclear magnetic resonance spectroscopy), the photochemical reactions for the generation of degradation products were proposed, and it was found that the degradation occurred at the C-H and C-O-C bonds on the main chain, forming a large number of ester and ethoxy structures. The aging degree of PEG2000 was evaluated from the perspective of surface morphology and chemical structure by gloss and FTIR spectroscopy, and it was found that the combination of gloss loss rate and carbonyl index was more suitable to evaluate the aging degree of the sample. The relevant theoretical research will provide reliable guidance for the preservation of polyethylene glycol in waterlogged wooden cultural relics.

Unexpected Susceptibility of Poly(ethylene furanoate) to UV Irradiation: A Warning Light for Furandicarboxylic Acid?

Poly(ethylene furanoate) (PEF) is widely advocated as a renewable alternative to the fossil-based polyester poly(ethylene terephthalate) (PET). Whereas the UV stability of PET is well-studied, little is known for PEF. Here, we compare the UV stability of both polyesters after 500 h of UV irradiation in a Q-SUN xenon arc chamber. Both the virgin and irradiated polyesters were characterized by FTIR, SEC, DSC, NMR, TGA, and MALDI-TOF MS. PET showed only minor signs of degradation under the applied test conditions, while PEF showed significant discoloration as well as evidence of both cross linking/chain extension and chain scission. Also, the thermal properties and the ability to crystallize of PEF were severely impacted by UV irradiation. Although a detailed study on the degradation mechanism is out of the scope of this work, we found indications that Norrish type I and II degradation reactions play an important role in the UV degradation of PEF.

Kinetic Monte Carlo simulation of the photodegradation process of polyester-urethane coatings

A numerical method to simulate reactions in a cross-linked polymer is developed and applied to the photodegradation process of polyester-urethane clearcoats during artificial exposure in a Weather-Ometer. This coarse-grained simulation method, which is based on a kinetic Monte Carlo scheme, is verified with experimental data on the depth-resolved changes in optical properties and chemical composition that have been previously determined. By modelling the depth-dependency of physical processes that occur in the coating, such as the absorption of photons and the diffusion of oxygen, the experimentally observed evolution of depth gradients in chemical composition can be well described by the simulation. A sensitivity analysis of individual simulation input parameters with respect to a set of resulting observables is performed and the results provide insight into the influence of specific reaction mechanisms on the overall degradation process and help to distinguish essential from less important processes. The values of input parameters that result in the most accurate simulation of the experimental data are determined with an optimisation procedure. In this way, the numerical values of several kinetic and physical parameters that are difficult to determine directly in an experimental way, such as various reaction rate constants, can be obtained from the simulations.