The elastic (Young’s) moduli of trans-polyethylene (PE), trans-polyacetylene (PA), and two types (cis-transoid and trans-cisoid) of cis-PA were studied using three theoretical schemes. The analytical method derived in this work not only predicts fairly good values for the moduli when the spectroscopic force constants are used but also gives information on geometrical changes upon longitudinal deformation. With this method, the moduli of trans-PE and trans-PA were calculated from the spectroscopic force-constant matrices to be 286 and 372 GPa, respectively. The moduli of two types of cis-PA are similar in value and about half of the value for trans-PA. Although the semiempirical modified neglect of diatomic overlap solid-state theory predicted higher moduli of the polymers compared with experimentally determined values, it provides reasonable values when combined with force-constant scaling. The moduli of the polymers were analyzed and discussed in terms of geometrical changes of the carbon chain and branched hydrogens. The following qualitative conclusions emerged from this study: (a) the Young’s modulus is strongly influenced by not only the force constants but also the conformation of polymers, (b) side groups may have large contributions, and (c) bond-angle deformations do not always dominate Young’s modulus.

• Received 4 December 1989

DOI:https://doi.org/10.1103/PhysRevB.41.11368