Raman spectroscopic study of reinforcement mechanisms of electron beam radiation crosslinking of natural rubber composites filled with graphene and silica/graphene mixture prepared by latex mixing

Mechanisms of reinforcement of graphene (GE) in natural rubber (NR) composites casted by latex mixing method remain an area for further investigation. NR composites were produced by mixing either GE or treated silica (SiO2)/20%GE mixture (1, 1.5, 2, and 2.5 phr) into latex and curing by electron beam (EB) irradiation at 150 kGy. The morphology of the composites obtained from scanning electron microscopy (SEM) indicated good dispersion of both GE and SiO2/20%GE in NR matrix, with small degrees of agglomeration from many layers of graphene sheets. It was found that adding only GE to NR improved tensile properties of the composites more than adding a mixture of SiO2/GE. Among the tested composites, the maximum tensile strength was obtained for that filled with 2 phr filler(s). It was also found that applying a single dose of 250 kGy of EB irradiation to latex-state composites could affect the crosslink density as much as applying 150 kGy of EB irradiation once time to latex and the same dose again at the same dose to the solid. To investigate reinforcement mechanisms, Polarized Raman spectroscopy results revealed that there was random orientation of graphene in NR composites. The data of stress-induced Raman band shifts showed many slippages of graphene, indicating spatial variation of stress transfer from NR to GE. The effective moduli of graphene in NR composites were found to be in the range of 51-378 MPa, pointing to an acceptable level of stress transfer between graphene and NR in the composites.