Effect of molecular structure on ethylene-vinyl alcohol copolymer thermal degradation and stability

To study the effect of molecular structure on ethylene-vinyl alcohol copolymer (EVOH) thermal stability during processing, EVOH samples with different molecular weights and comonomer distributions were prepared by solubility fractionation from two different commercial materials, and then, the structure features of samples were characterized by 1H-NMR, viscometry, differential scanning calorimetry, and, especially, successive self-nucleation and annealing. Their thermal stabilities were further analyzed by isothermal oxygen treatment and thermogravimetry. Attenuated total internal reflectance Fourier transform infrared spectroscopy and UV were used to obverse the structural changes in samples (the formation of chromogenic groups: CO and -CC-). According to the results of isothermal oxygen treatment at 200 degrees C, at the same ethylene content, low-molecular-weight samples have worse thermal stability, which were extremely easy to yellowing and form insoluble gel. To investigate the structure differences between the low-molecular-weight samples, it was found that long vinyl alcohol segments may not worsen the thermal stability in the conditions of the same ethylene content. But samples with a uniform distribution of ethylene segments and a certain length of ethylene segments will have better thermal stability, which makes it more difficult to form carbonyl even diketene conjugation structure. Ethylene-vinyl alcohol copolymer's different degradation degrees under 200 degrees C.image