Characterization of fatty acids linked to natural rubber - Role of linked fatty acids on crystallization of the rubber

Natural rubber isolated from Hevea brasiliensis contains two types of long chain fatty acids: fatty acids linked to rubber chain at the chain-terminal and those present as a mixture. The composition and role of these fatty acids were characterized by ¹H NMR spectroscopy, gas chromatography and differential scanning calorimetry. The linked fatty acids were composed of saturated and unsaturated C₁₀-C₂₂ fatty acids, the composition of which was similar to that of mixed fatty acids. As models of natural rubber, fatty acid groups, such as decanoyl (C₁₀), myristoyl (C₁₄) and stearoyl (C₁₈), were esterified to synthetic cis-1,4-polyisoprene (IR) selectively at 3,4 isomeric units, by hydroboration followed by esterification with acyl chloride. The acceleration effect of linked fatty acids on the crystallization behavior was analyzed for these models and natural rubber. The IR linked with myristoyl group showed the most rapid crystallization at -25 °C among the samples prepared, as it was mixed with palmitic acid. This was explained to be due to the nucleating effect of saturated fatty acid and plasticizing effect of unsaturated fatty acid for the polymer.