VISCOELASTICITY AND SELF-DIFFUSION IN MELTS OF ENTANGLED ASYMMETRIC STAR POLYMERS

The crossover from linear to branched polymer dynamics in highly entan gled melts was investigated with a series of asymmetric three-arm star s of poly(ethylene-alt-propylene). Two arms of equal length formed a l inear backbone, kept constant through the series, while branches of va rious length were appended as the third arm. The materials were made b y carbanionic polymerization of isoprene and the judicious application of chlorosilane linking chemistry. Subsequent saturation of the polym eric double bonds with deuterium and hydrogen, followed by fractionati on, led to a set of structurally matched, nearly monodisperse pairs of deuterium-labeled and fully hydrogenous samples. Dynamic moduli were measured over wide ranges of frequency and temperature. With increasin g branch length, the resulting master curves evolve smoothly, but with surprising rapidity, from the relatively narrow terminal spectrum of linear polymers to the much broader spectrum of symmetric stars. The v iscosity eta(o) increases rapidly with branch length, and the diffusio n coefficient D, obtained by forward recoil spectrometry, decreases ev en more rapidly. The product eta(o)D, however, distinguishes the trans ition from linear to branched polymer dynamics most clearly: for a bac kbone with about 38 entanglements, the crossover is already approachin g completion for a single mid-backbone branch with only about three en tanglements. (C) 1997 John Wiley & Sons, Inc.