1. Reinforcement of natural rubber by nanofiller
2. In situ silica generated into natural rubber matrix by the sol-gel process
3. Strain-induced crystallization in natural and synthetic rubbers

Two phenomena, which can explain the reinforcement effect in rubber vulcanizates, are the addition of reinforcing filler and the strain-induced crystallization. Rubber composites are physical mixtures of rubber (as the matrix) and reinforcing filler (as the dispersed phase) that serves to improve some mechanical property such as modulus, tensile strength or abrasion resistance. Fillers may be organic (e.g., carbon black) or inorganic (e.g., silica or calcium carbonate) compounds.
Crystallization is of great importance with regard to the mechanical properties of elastomers since the generated crystallites are thought to provide the reinforcement in the materials. Some elastomeric networks show a large and rather abrupt increase in the modulus at high elongation. This increase is very important since it corresponds to a significant toughening of the elastomer. It had been widely attributed to either the “limited extensibility” of network chains or to strain-induced crystallization. However, many of the synthetic rubbers, particularly those formed by the random co-polymerization of two different monomer units, such as SBR and NBR, do not have a regular sequence of chain atoms and hence are incapable of forming a crystal lattice. Such rubbers, after vulcanization, generally have much lower strength than NR vulcanizates, though from the practical standpoint this difference can be largely overcome by suitable compounding techniques, e.g. by the incorporation of reinforcing filler.
Also, silica generated in situ, the new composite material, is the most interesting nanofiller to study the effect of reinforcement. It is produced in the rubbery matrix by the sol-gel reaction. A typical reaction is base-hydrolysis and condensation reactions of tetraethoxysilane (TEOS) as a precursor of silica. The mechanism of sol-gel reaction is

This reaction is concurrent and can be reversible depending on the reaction conditions such as pH, concentration, kind of catalysts, and temperature. This technique can produce the silica particles dispersed homogeneously in rubbery matrix both before and after curing.

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