Characterization of polyolefin melts using the polymer reference interaction site model integral equation theory with a single-site united atom model

A general approach, based on the polymer reference interaction site model ( PRISM) integral equation theory, suitable for characterizing arbitrarily co mplex polyolefin melts is described. We tested the method by calculating th e melt structures of linear polyethylene (PE) and isotactic polypropylene ( iPP) and the spinodal decomposition temperatures for PE/iPP blends. The com putational expense of the PRISM calculation was reduced with a single-site united atom model in which the polyolefin CH, CH2, and CH3 groups were appr oximated as chemically equivalent sites with spherically symmetric energeti c interactions. The site-site interactions were defined by a potential func tion comprising a hard core with an attractive Lennard-Jones term. These en ergetic parameters were optimized with a central composite design strategy that enabled a simultaneous fit of experimental melt density and structure factor data. Values were obtained for PE and iPP individually and for commo n universal parameters that could potentially be used for all polyolefins. The rotational isomeric state-metropolis Monte Carlo (RMMC) technique was u sed to generate sets of conformers at specified temperatures covering the m elt-temperature range of the polymers. The characteristic ratio was used to assess the quality of the conformers and the RMMC method. Values of 9.68 f or PE and 9.27 for iPP were obtained. The single-chain structure factors ca lculated by the RMMC method were used to calculate the total structure fact or for each melt. These were validated against published X-ray diffraction results. (C) 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39 : 1803-1814, 2001.