Ab initio calculations of the deformation of polyethylene

Ab initio H-F SCF quantum-mechanical calculations have been done to evaluate the energy of covalent bond deformation of a -CH₂CH ₂- ethylene repeat unit at strains up to ε=0.6. The computational scheme involves subtracting energies of axially strained normal paraffins (n-C₃H₈, n-C₅H₁₂, and n-C₇H₁₆) differing in length by one ethylene unit. At small strains it is found that the deformation is contributed to equally by C-C bond stretch and by CCC bond angle opening. At higher strains the majority of the deformation is accomplished by C-C stretch. The calculated elastic modulus is 405 GPa and the tensile strength of the polymer in the chain direction is 66 GPa. While both of these values are higher than previous estimates, it is believed that these quantities are the most reliable which have been calculated.