Strain induced crystallization and reinforcement were studied for poly(isoprene)s from the following sources: Ziegler–Natta catalysis, hevea brasiliensis (HNR), taraxacum kok-saghyz (TKS), known as the russian dandelion, partenium argentatum (GR), known as guayule. Two HNR samples were studied, with high (HNR-H) and low (HNR-L) molar mass. Investigated guayule samples were: as isolated from the latex (GR-R) and after extraction with acetone (GR-P). All of the samples had weight average molar mass higher than 1.5 × 10⁶ Da. TKS was found to be the most stereoregular sample, with undetectable amounts of stereoerrors. Wide angle X-Ray diffraction patterns were collected on samples, unstretched after processing and during stretching, up to 5 as the strain ratio. Quasi-static tensile measurements were performed. HNR and GR-P exhibited rubber crystallinity already in the undeformed state and the orientation of their crystalline phase remained low also for the highest strain ratios. GR-R and TKS were amorphous at low strain and developed highly oriented crystalline phases under stretching. TKS developed extraordinary mechanical reinforcement under stretching: stresses at large elongations were much higher than those obtained with HNR. It is thus shown that the formation of highly oriented crystalline phases brings large mechanical reinforcement.

In conclusion, an amorphous NR sample from a natural source, such as TKS, which has high molar mass and does not contain non rubber components which could act as plastifiers, is able, under stretching, to develop crystallinity and a high degree of axial orientation. Crystallization occurs at high strain ratio, when the chains are prevailingly aligned. The biosynthesis of TKS is likely to play a strategic role, as it promotes the chain end crosslinking of the polymer chains.

Ultrasonic extrusion of BR was conducted and processing characteristics were evaluated. Ultrasonic power consumption increased with the increase of ultrasonic amplitude. Die pressure decreased with the increase of ultrasonic amplitude indicating that the imposition of ultrasound can be used to increase extrusion output rate. The effect of ultrasonic amplitude on molecular structure of BR, including molecular weight and gel formation behavior were studied. Depending on the ultrasonic amplitude, structure changes occurred in BR, including a degradation, formation of long-chain branching and gel. Untreated and treated BR were mixed with CB, silica and silica/silane. Bound rubber content and flocculation behavior were studied. Formation of the long-chain branching was shown to increase the bound rubber content and rubber-filler interaction and to reduce the filler-filler interaction and flocculation of BR/silica compound. Both the untreated and treated BR, BR/CB, BR/silica and BR/silica/silane were vulcanized and their crosslink density, gel fraction and mechanical properties were measured. Based on DMA temperature sweep of BR/silica vulcanizate, it was shown that formation of the long-chain branching during ultrasonic treatment reduced the loss tangent at 60 °C, predicting a lower rolling resistance if the rubber used in tires.

The excellent mechanical properties and fatigue-resistance of natural rubber (NR) are closely related to the strain-induced crystallization (SIC) capability of NR, which is originated from the unique network structure of NR. The synthetic counterpart of NR, cis-1,4-polyisoprene (IR) generally possesses inferior mechanical performance due to insufficient SIC capability. In this contribution, amino-functionalized carbon nanodots (CDs) were introduced as high-functionality cross-linkers into sulfur-cured sulfonated IR, aiming to improve the SIC capability of IR. The amino groups on CD surfaces are connected with the sulfonic acid groups on the IR backbones to form ionic bonds, and a covalent crosslinking is concurrently obtained by using sulfur vulcanization, thereby a dually crosslinked IR network is resulted. When the rubber is deformed, the ionic bond breaks preferentially prior to the rupture of covalent bond, leading to efficient energy dissipation. The preferential rupture of the ionic bonds also promotes the orientation process of IR chains and hence SIC capability, as evidenced by lowered onset strain for crystallization and increased crystallinity. The promoted SIC leads to remarkably improved tensile properties.

Graphene has exceptionally high surface area, mechanical properties, electrical conductivity, thermal conductivity, and gas-barrier performance, thus it is considered as an ideal multifunctional filler for rubbers. However, harnessing these properties in rubber nanocomposites requires us to carefully tailor the dispersion state of graphene, the vulcanization kinetics, the interfacial interaction and so on. This review summarized our recent works on how to disperse graphene homogeneously in rubber matrix, what influence of graphene or graphene oxide on the vulcanization behavior of the rubber nanocomposites, how to design a compact filler network in the rubber matrix, and how to engineer a strong interfacial interaction between graphene and rubber. These fundamental researches give us some thumb rules to develop graphene/rubber nanocomposites with significantly improved mechanical properties, gas-barrier performance, thermal stability, electric conductivity, antioxidation ability as well as some functionalities.