Bond scission in a perfect polyethylene chain and the consequences for theultimate strength

We present ab initio calculations concerning the bond scission rate in a po lyethylene chain. The energy barrier for scission is calculated both by usi ng an effective potential scheme based on ab initio data and from the ab in itio transition state itself. In the latter case the prefactor for scission is calculated. In both methods the relaxation of the polymer chain is take n into account, leading to a strong strain dependence of the energy barrier . The two schemes agree quantitatively. The barrier is reduced from 3.9 eV at zero strain to 1.7 eV at a strain of 5%. The calculated scission rate is used to estimate the strength of a polyethylene fiber consisting of perfec tly aligned, independent chains in a constant strain rate experiment result ing in 18 GPa at a strain of 8%. This value of the ultimate strength is a f actor 2 larger than found in experiments. This contrasts previously reporte d quantum chemical calculations, which reported values a factor 5-10 larger than the experimental values.