STIFFER UNDER STRAIN - MECHANICAL-PROPERTIES OF POLY-P-PHENYLENE

The mechanical properties of poly-p-phenylene and its simplest oligome r, biphenyl, have been explored by ab initio and semiempirical molecul ar orbital methods. The contribution of zero-point energy to the torsi onal barrier is negligible, but the variation of vibrational entropy w ith torsion increases the height of the free energy barriers to rotati on through the D-2h and D-2d conformations. The computed Hessians argu e for a backbone that becomes 14% stiffer when strained to planarity. Cluster calculations of the polymer using periodic boundary conditions illuminate the role of bowing in compression and the importance of in termolecular forces to compressive mechanical performance of rigid-rod polymers.