Synthesis and characterization of hydroxyl terminated liquid epoxidized natural rubber grafted graphene oxide

Abstract

Hydroxyl terminated liquid epoxidized natural rubber (HTLENR) was synthesized via an oxidative degradation of liquid epoxidized natural rubber (LENR) in the presence of cobalt acetylacetonate as an oxidizing agent. The subsequent treatment was conducted using sodium borohydride as a reducing agent for the formation of hydroxyl group. In order to obtain a low molecular weight of HTLENR, the reaction times were varied for 1 h, 4 h, 7 h, 10 h, and 24 h. Gel permeation chromatography was used to determine the molecular weight of HTLENR. The lowest molecular weight was achieved after 24 h reaction where Mw and Mn were 21869 g/mol and 3233g/mol, respectively. However, HTLENR prepared for 10 h reaction time with Mw and Mn of 37545 g/mol and 3879 g/mol, respectively was chosen as an optimum parameter for further reaction due to the economic factor as well as based on the highest rate of chain scission. The molecular structure of HTLENR was analyzed using Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR). The appearance of broad peak around 3250 - 3550 cm-1 in FTIR spectrum after the reaction confirmed the existence of OH group in the prepared HTLENR and the peak intensity increased as the reaction time increased. The NMR analysis confirmed the highest hydroxyl content was achieved at 10 h reaction with the amount of approximately 7.56%. HTLENRs with different graphene oxide (GO) loadings (1%, 5% and 10% w/w) were produced by grafting reaction. The FTIR analysis showed a broad peak of OH in HTLENR-g-GO at higher amount of GO loading. Meanwhile, the differential scanning calorimetric analysis revealed the increase of glass transition temperature as the amount of GO in the HTLENR-g-GO increased. Thermal stabilities of HTLENRs were analyzed by thermogravimetric analysis which showed that at 1 wt% and 5 wt% of GO loadings give a higher decomposition temperature compared to the 10 wt% GO loading. Degree of grafting was calculated to be 40.56% at 5 wt% of GO loading and remained almost constant at 10% of GO loading. Based on the result obtained, it can be concluded that HTLENR was successfully produced with the optimum degree of grafting at 5 wt% GO loading.