Schematic of the GO crosslinking mechanism. (a) Two types of free radicals are produced, one of which diffuses away from GO and leads to crosslinking of the rubber matrix, while the other is localized on the GO where it results in crosslinking between the GO and rubber. (b) In addition to the chemical crosslinks induced by the free radicals (green dots), there are also physical crosslinks (red circles) that arise from absorption of the polymers onto GO.Rubber in its natural form is a sticky liquid, but add crosslinking agents and filler particles and a solid elastic material can be produced. The process, however, is time and energy consuming. Now researchers from Sichuan and Harvard Universities have found that graphene oxide (GO) can both crosslink and reinforce rubber in a single easy step [Xing et al., Composites Science & Technology 144 (2017) 223].
“Crosslinking and reinforcement are two most important strategies of improving the mechanical properties of rubbers,” explains Jinrong Wu of Sichuan University. “The rubber industry uses very complex crosslinking recipes and reinforcing nanoparticles, which is tedious, energy-consuming, and even polluting to the environment.”
Conventional fillers such as carbon black and silica are inert, so cannot perform any crosslinking function. Conversely, functional nanoparticles that provide crosslinking tend to be specific to certain polymers. Rubber, by contrast, is typically crosslinked via free radical reactions. An additive that could provide both reinforcement and generate free radicals to induce crosslinking would be highly useful to the rubber industry.
Wu and his colleagues believe that flakes of GO, just 1 nm thick and ~1 micron wide, could be the answer, simultaneously crosslinking and reinforcing rubber.
“We use graphene oxide to crosslink rubbers by generating free radicals at high temperatures and simultaneously reinforce rubbers because of its high mechanical property and high surface area,” says Wu.
The process is very simple, green, and energy saving, say the researchers. An aqueous solution of GO is mixed with rubber latex, which coagulates and is dried. The mixture is then hot pressed into a composite material – with no need for organic solvents or mechanical mixing apparatus.
The researchers’ findings indicate that heating generates OH and COOH radicals, which both diffuse into the rubber matrix and congregate at the GO/rubber interface. As well as generating free radicals, which crosslink rubber chemically, GO also interacts with the rubber physically. The physical interfacial interactions are not permanent, but dissociate – faster at high temperatures – to allow the rubber to relax.
The result is a GO/rubber composite with tensile strength four times that of conventionally crosslinked rubbers and a noticeably larger tensile strain at breaking. The researchers believe that the mechanical properties can be further improved by fine-tuning the GO fraction and hot pressing conditions.
“We have only explored the crosslinking and reinforcement effects of GO in one type of rubber (styrene-butadiene),” says Wu. “We will now extend the study to other elastomers, as well as looking at the influence of oxidation degree and GO particle size on the crosslinking and reinforcement effects.”