Molecular orbital studies of polyethylene deformation

✍ Scribed by Buckley Crist; Peter G. Hereña

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 749 KB
• Volume: 34
• Category: Article
• ISSN: 0887-6266
• DOI: 10.1002/(sici)1099-0488(199602)34:3<449::aid-polb5>3.0.co;2-o

✦ Synopsis

Young's modulus E for polyethylene in the chain direction is calculated with molecular orbital theory applied to n-alkanes C3Hs through n-Cl3HZ8 and analyzed with the clusterdifference method. Semiempirical CNDO, 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 extrapolated to infinite chain length, give E in good agreement with moduli evaluated with molecular cluster or crystal orbital methods, provided 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 with established behaviors of the various models. Our calculations do not reproduce earlier molecular cluster or crystal orbital results, which gave E < 330 GPa. The most rigorous MP2/6-31G* model is known to overestimate force constants by -11%; the scaled modulus E = 299 GPa is in good accord with E = 306 GPa from recent calculations based on experimental vibration frequencies.