Weakening of a Polyethylene Chain by Methyl Side Groups

The effect of methyl side groups on the strength of a single polyethylene chain is calculated with Density Functional Theory. Using transition state theory, the scission rate is calculated as function of stress for bonds next to a methyl side group. The increase in the scission rate due to the presence of the side group is maximal at zero stress/strain, where it is enhanced by a factor of 5,000. This increase results from a hyperconjugation interaction between the methyl side group and the radical that is formed during the scission process. The calculated scission rate is used as input for a model in which a fiber consists of independent chains. With the help of the model we estimate how much the tensile strength of polyethylene fiber can deteriorate due to the presence of methyl side groups. The strength of the fiber is reduced with 6% for a concentration of one side group in 10 main chain carbon atoms.