Creation of Tortuosity in Unfilled Rubber via Heterogeneous Cross-Linking toward Improved Barrier Property

High-performance rubbers with low gas permeability have attracted extensive attention due to their wide application. In this work, for the first time, the tortuosity effect is created in unfilled rubbers via heterogeneous cross-linking. The heterogeneously cross-linked rubbers exhibit a sea–island morphology with excellent interfacial interactions in which the dispersed rubber granules (RGs) are densely cross-linked, while the matrix phase is normally cross-linked. The hard RG phases showed low gas permeability, which can not only serve as a rigid yet elastic reinforcement for rubbers but also inhibit the gas permeation in rubbers. The excellent interfacial interactions between the two phases have a pronounced effect on the ultimate performance of rubbers. Surprisingly, the simultaneously improved mechanical performance and the gas barrier property achieved in the heterogeneously cross-linked rubber are competitive with the rubber nanocomposites filled with two-dimensional fillers. It is revealed that the ingenuity of such a heterogeneous design is the tortuosity effect imposed by the dispersed RG phases with low permeability, which makes an additional contribution to the reduction of gas permeability. Accordingly, a modified Nielsen equation was developed to model the gas barrier property of heterogeneously cross-linked rubbers, offering reliable prediction and rational mechanism elucidation of gas permeation for the heterogeneous design. In view of the scale-up capability and universality of this method, we envisage that this work offers a promising avenue, alternative to nanofilling, for preparing the rubbers with excellent gas barrier properties.