MOLECULAR-ORBITAL STUDIES OF POLYETHYLENE DEFORMATION

Young's modulus E for polyethylene in the chain direction is calculate d with molecular orbital theory applied to n-alkanes C3H8, through n-C 13H28 and analyzed with the cluster difference method. Semiempirical C NDO, MNDO, and AM1 models and ab initio HF/STO-3G, HF/6-31G, HF/6-31G , and MP2/6-31G models are used. Cluster-difference results, when ext rapolated to infinite chain length, give E in good agreement with modu li evaluated with molecular cluster or crystal orbital methods, provid ed minimal basis sets are employed. E decreases from 495 GPa (CNDO) to 336 GPa (MP2/6-31G) as the level of theory is improved, consistent w ith established behaviors of the various models. Our calculations do n ot reproduce earlier molecular cluster or crystal orbital results, whi ch gave E < 330 GPa. The most rigorous MP2/6-31G model is known to ov erestimate force constants by similar to 11%; the scaled modulus E = 2 99 GPa is in good accord with E = 306 GPa from recent calculations bas ed on experimental vibration frequencies. (C) 1996 John Wiley & Sons, Inc.