Pulsed n.m.r. of cis-polyisoprene: 2

Pulsed n.m.r. spin-spin relaxation time measurements are reported for a commercial whole polymer sample of cis-polyisoprene which has been crosslinked to varying extents by γ-irradiation. The spin-spin relaxation decays at 150°C consist of two components, T_2S and T_2L, which are attributed to protons in a network structure (entangled or crosslinked) and in non-network molecules respectively. Comparison of the n.m.r. data with solubility measurements, which only detect the presence of a crosslinked network, provides a value for the average molecular weight for entanglements which is in the region of 40 000 to 50 000.