To explain the characteristics of X-ray scattering by isotactic polystyrene dry gels, a coiled-coil molecular model is proposed instead of the previously reported almost fully extended 12/1 helix having sixfold helical symmetry (per dimer). The coiled-coil chain is assumed to have basically the same 3/1 helical structure of (-TG-)3 conformation as in the crystalline phase and the 3/1 helix axis itself again makes a large helix having a long pitch. The observed chain repeat length of 30.6 ,~ comprises six turns of the 3/1 helix (a total of 18 monomer units) in one turn of the large helix: the 3/1 helix axis itself makes a large helix of 4.08 A radius and tilts by about 40 ° from the direction of stretch. The coiled-coil can be expressed again in terms of a single 6/1 large helix in which the asymmetric unit comprises three monomer units in one turn of the original 3/1 helix. Since the 3/1 helix axis tilts by about 40 °, the axial length of the asymmetric unit becomes 5.1 ~ (6.65 ~ x cos 40 ): the strong 5.1 A meridional reflection comes from the asymmetric unit. Although the detailed structure cannot be defined at this stage, because of uncertainty about the unit cell and the solvent content which contributes coherently with the polymer to the X-ray scattering, the molecular transforms of several coiled-coil models possessing different helical senses seem to explain the characteristics of the X-ray scattering.
Посилання на статтю:
Coiled-coil molecular model for isotactic polystyrene gels / Yozo Chatani* and Norio Nakamura // Polymer. – 1993. – Vol 34. – P. 1644-1648.