Intermolecular packing in stereoregular polypropylene liquids: Comparison between theory and X-ray scattering experiments

Atomistically realistic polymer reference interaction site model (PRISM) calculations on isotactic and syndiotactic polypropylene liquids were compared with wide-angle X-ray scattering measurements at 180 and 183 degrees C. The intramolecular structure factors, required as input to PRISM theory, were obtained from single-chain, Monte Carlo simulations using three models with progressively more realism: a rotational isomeric state model, a united atom model, and an explicit atom model. Excellent agreement between PRISM theory, employing the united and explicit atom models. was found with the experimentally determined structure factor in the wavevector range 0.3 Angstrom(-1) less than or equal to k less than or equal to 16 Angstrom(-1). Good agreement was also seen between theory and experiment for the average intermolecular radial distribution function in r-space estimated from the Fourier transform of the scattering data. The rotational isomeric state model predicted a structure factor qualitatively inconsistent with the scattering experiments. This suggests that continuously varying rotations are important in the packing of vinyl polymers. The six individual site-site intermolecular pair correlation functions calculated from PRISM theory show universal behavior on long length scales in the correlation hole regime but reveal significant system specific differences in structure on short length scales near contact. At short distances, the pendant methyl groups on the polypropylene backbone tend to shield the backbone groups from approaching each other closely in the melt. Subtle differences were seen in the pair correlation functions between isotactic and syndiotactic polypropylene on local length scales.