Mixing of isotactic and syndiotactic polypropylenes in the melt

The miscibility of polypropylene (PP) melts in which the chains differ only in stereochemical composition has been investigated by two different proce dures. One approach used detailed local information from a Monte Carlo simu lation of a single chain, and the other approach takes this information fro m a rotational isomeric state model devised decades ago, for another purpos e. The first approach uses PRISM theory to deduce the intermolecular packin g in the polymer blend, while the second approach uses a Monte Carlo simula tion of a coarse-grained representation of independent chains, expressed on a high-coordination lattice. Both approaches find a positive energy change upon mixing isotactic PP (iPP) and syndiotactic PP (sPP) chains in the mel t. This conclusion is qualitatively consistent with observations published recently by Mulhaupt and co-workers. The size of the energy change on mixin g is smaller in the MC/PRISM approach than in the RIS/MC simulation, with t he smaller energy change being in better agreement with the experiment. The RIS/MC simulation finds no demixing for iPP and atactic polypropylene (aPP ) in the melt, consistent with several experimental observations in the lit erature. The demixing of the iPP/sPP blend may arise from attractive intera ctions in the sPP melt that are disrupted when the sPP chains are diluted w ith aPP or iPP chains.