Effects of frequency, molecular weight and thermal oxidation on the dynamic mechanical response of poly (ethylene oxide)

The technique of dynamic mechanical thermal analysis (DMTA), operated in the dual cantilever mode, was used to characterize the effects of frequency, crystallinity, molecular weight (MW) and the extent of thermal oxidation on the dynamic mechanical response of poly(ethylene oxide) (PEO). The glass transition temperature (Tg) of PEO (MW = 9 × 105 Dalton) was found to be −44°C. For PEO (MW = 1 × 105 Dalton) the Tg is −39°C and this value increases by 2–9°C for every decade increase in the measuring frequency. Two minor, second‐order transitions of PEO are also discernible at −33 and 32°C. An inverse dependence of Tg on molecular weight was found in the molecular weight range studied and this is contrary to the Fox‐Flory theory. It was also found that a partially crystalline sample is obtained despite very rapid quenching of PEO from the melt into liquid nitrogen. Thermal oxidation of PEO before processing leads to an increase in the amplitude of the loss tangent peak. This reflects the effect of oxidation products in restricting polymer crystallization and the subsequent increase in the amorphous fraction of the polymer. The position of the Tg peak in PEO remains reasonably fixed with progressive ageing and this was attributed to crosslinking having occurred in addition to chain scission during thermal oxidation.