Linear 1,2-polybutadiene is crosslinked at 0° by γ-irradiation while strained in simple extension, with extension ratios from 1.3 to 2.0. During irradiation times up to several hours, entanglement slippage is slight, since the temperature is only slightly above the glass transition. Subsequently, samples are released and reach their equilibrium states of ease at room temperature. From the extension ratio at state of ease, the ratio of ν x (effective network strands terminated by crosslinks introduced) to ν N (effective network strands terminated by entanglements) is calculated by composite network theories of Flory and others; and from the extension ratios together with the modulus, measured at small extensions, ν N is calculated explicitly. It appears that ν x increases approximately proportional to irradiation time; ν N is approximately independent of irradiation, and it corresponds to a molecular weight between effective entanglement loci of about 13,000. This figure, however, which is larger than that deduced from rheological properties of the uncrosslinked polymer, is subject to future downward correction for partial entrapment of the entanglements and other refinements.

Kramer, O., Ty, V., & Ferry, J. D. (1972). Entanglement Coupling in Linear Polymers Demonstrated by Networks Crosslinked in States of Strain. Proceedings of the National Academy of Sciences, 69(8), 2216–2218.