The hierarchical relationships among the configuration, conformation, solution properties, crystal structure, thermodynamic characteristics, and mechanical properties of polystyrene (PS) were investigated by NMR analyses of model compounds and by periodic density functional theory (DFT) calculations on isotactic (iso) and syndiotactic (syn) PS crystals. At the configurational level, the meso linkage exhibits nearly equal populations of the trans (∼50%) and one of the two gauche (∼45%) conformations, whereas the racemo linkage strongly favors the trans state (∼70–90%). These conformational populations remain essentially unchanged in cyclohexane over the good-to-poor solvent range (55–15 °C) and show no anomaly at the Θ temperature (34.5 °C). Periodic DFT calculations indicate that the syn-β″ polymorph is thermodynamically the most stable from 0 K up to the melting point. However, the free-energy difference between the syn-α″ and syn-β″ forms decreases with increasing temperature. Residual entropies associated with orientational disorder (syn-α′) and stacking faults (syn-β′) lower the Gibbs free energies, narrowing the free-energy differences among the α′, α″, β′, and β″ forms to within a few tenths of a kilocalorie per mole. The calculated Young’s moduli along the chain axis are 33.06 GPa (iso-α), 88.08 GPa (syn-α″), 94.80 GPa (syn-β″), and 11.65 GPa (syn-δe), identifying the syn-β″ polymorph as the crystal structure providing the highest mechanical stiffness together with the greatest thermodynamic stability.
Saito et al. (Thu,) studied this question.