INTERMOLECULAR PACKING IN STEREOREGULAR POLYPROPYLENE LIQUIDS - COMPARISON BETWEEN THEORY AND X-RAY-SCATTERING EXPERIMENTS

Atomistically realistic polymer reference interaction site model (PRIS M) calculations on isotactic and syndiotactic polypropylene liquids we re compared with wide-angle X-ray scattering measurements at 180 and 1 83 degrees C. The intramolecular structure factors, required as input to PRISM theory, were obtained from single-chain, Monte Carlo simulati ons using three models with progressively more realism: a rotational i someric state model, a united atom model, and an explicit atom model. Excellent agreement between PRISM theory, employing the united and exp licit atom models. was found with the experimentally determined struct ure 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 s een between theory and experiment for the average intermolecular radia l distribution function in r-space estimated from the Fourier transfor m of the scattering data. The rotational isomeric state model predicte d a structure factor qualitatively inconsistent with the scattering ex periments. This suggests that continuously varying rotations are impor tant in the packing of vinyl polymers. The six individual site-site in termolecular pair correlation functions calculated from PRISM theory s how universal behavior on long length scales in the correlation hole r egime but reveal significant system specific differences in structure on short length scales near contact. At short distances, the pendant m ethyl groups on the polypropylene backbone tend to shield the backbone groups from approaching each other closely in the melt. Subtle differ ences were seen in the pair correlation functions between isotactic an d syndiotactic polypropylene on local length scales.