13 - Oriented Polymers

Abstract

Polymeric chain molecules contain atomic groups, covalently bonded in the chain direction. In most polymers the covalent bonds have approximately the same strength as the C–C bond in diamond, which is the strongest material in nature. However, the forces perpendicular to the chain direction are relatively weak intermolecular forces such as van der Waals forces and hydrogen bonding. The large difference in strength between the intramolecular and intermolecular forces generates a large mechanical anisotropy. The elastic modulus of a polyethylene (PE) crystal is approximately 240GPa in the chain direction due to stretching of bonds and distortion of valence angles, and about 4GPa perpendicular to the chain direction due to van der Waals interactions.1 Even in the case of poly(vinyl alcohol), which is intermolecularly hydrogen bonded, the moduli are 255GPa and 9GPa, parallel and perpendicular to the chain direction respectively.1 The mechanical anisotropy of polymer chains is well utilized in synthetic polymeric fibers. The highest elastic modulus reported for a PE fiber is about 150GPa obtained for a sample produced by drawing single-crystal mats of ultrahigh molecular weight PE.2