Thermal and kinetic studies of epoxidized natural rubber in lithium salts-epoxidized natural rubber polymer electrolytes

Abstract

The thermal and kinetic studies of epoxidized natural rubber (ENR) and its polymer electrolytes, LiX/ENR PEs, (where X = ClO 4 − , CF3SO 3 − , COOCF 3 − , I−, and BF 4 − ) were carried out using thermogravimetric analysis at different heating rates. The thermal behaviors for LiX/ENR PEs are closely related to the morphology and interactions between the LiX and ENR chains. The LiCF3SO3, LiCOOCF3, and LiI form pseudo-crosslinking within the ENR; their thermal behavior resembled purified ENR. The LiClO4 tends to form aggregates within the ENR. This phenomenon has promoted a much earlier decomposition of epoxide in the ENR. The occurrence of ring-opening and complexation or cross-linking reactions in and between the ENR chains in the LiBF4/ENR has produced a thermally stable macrostructure. The activation energy for the thermal degradation (E d) of purified ENR was 239.8 and 239.9 kJ mol−1 using Kissinger and FWO methods, respectively. According to the Coats–Redfern method, the degradation mechanism of purified ENR follows the F1 type model, while the Criado method revealed that the degradation starts with F1 followed by D3 type models. The E d for LiX/ENR (X = COOCF 3 − , CF3SO 3 − , I−, and BF 4 − ) PE’s obtained via the Kissinger method are 258.5, 257.0, 251.0, and 198.9 kJ mol−1, respectively, and the corresponding E d values obtained by FWO are 236.0, 223.6, 349.7, and 206.6 kJ mol−1, respectively. The degradation of ENR in these PEs followed the D3 type model. However, for LiClO4/ENR, the presence of two distinct degradations of ENR gave two E d values. These are 174.5 and 234.7 kJ mol−1 using Kissinger and 117.8 and 293.6 kJ mol−1 using FWO method. The degradation mechanism of ENR in the LiClO4/ENR PE was similar to purified ENR that is F1 followed by D3 type models.