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Effect of Divalent Metal Ions of the Viscosity of Natural Rubber from Manihot Glaziovii
The effect of divalent metal ions on the intrinsic viscosity of natural rubber (NR) from Manihot glaziovii has been investigated. The molar mass of cis-1, 4-polyisoprene from Manihot rubber was found to be M_v = 1.0 x 10⁶ and the intrinsic viscosity [η] = 5.66 dL/g, for toluene solution at 30°C. Some metal ions such as Ca²⁺ and Sr²⁺ increase the NR viscosity in toluene solution (6.00 dL/g and 6.26 dL/g for 15.0 µmol of metal ion/g of rubber, respectively). Others such as Mg²⁺, Zn²⁺ and Cu²⁺, decrease the NR viscosity to 4.82 dL/g, 4.73 dL/g and 4.96 dL/g respectively, for the same addition of metal ion/g of rubber. A correlation between relative variation of intrinsic viscosity ( [η]NR+M/[η]NR) and cation radius has been proposed. A relative variation greater than unity for large cations has been observed, while for small metals such variation was less than 1. Cations that decrease the intrinsic viscosity do not lead to alteration in the activation energy of flow (E_a) for the 0.5% (w/v)(E_a = 7.3 kJ/mol) solution. Cations that increase [η] > also cause an increase of E_a to 7.8 kL/mol and 7.6 kJ/mol for NR solutions containing Sr²⁺ and Ca²⁺, respectively. Intramolecular crosslinks may be formed with small and large cations. No evident interaction has been detected between metal ions and synthetic cis-1-4-polyisoprene. The interaction between NR and metal ions may be due to naturally occurring non-rubber constituents, probably those containing carboxylic groups.