Measurement of Chain Dimensions in Dilute Polymer Solutions. A Light Scattering and Viscometric Study of Linear Polyisoprene in Cyclohexane

Several methods for obtaining the size of macromolecules in dilute solution were applied to a series of linear polyisoprenes in the thermodynamically good solvent cyclohexane. The polymers, prepared by anionic polymerization, cover a wide range in molecular weight (1.5 X 104< M < 3.4 X 106) and have narrow molecular weight distributions (Mw/Mn< 1.1). The viscometric radius RVfthe radius of gyration Rg, the hydrodynamic radius RH,and the thermodynamic radius RTwere determined by viscometry and by static and dynamic light scattering. The molecular weight dependences of all four size measures are well-described by power laws (R ∞ Ma). The exponents obtained for RH, RV and RTagree fairly well with the predicted value of 0.588 for good solvents. The exponent obtained for RGis smaller, although RG/RH in fact changes rather little over the range of available data. The values ofRG/RH,RV/RH andRT/RHare in reasonable accord with the theory for self-avoiding coils and with results for polystyrene and poly (α-methylstyrene) in good solvents. © 1987, American Chemical Society. All rights reserved

Viscoelasticity and self-diffusion in melts of entangled asymmetric star polymers

The crossover from linear to branched polymer dynamics in highly entangled melts was investigated with a series of asymmetric three-arm stars of poly(ethylene-alt-propylene). Two arms of equal length formed a linear backbone, kept constant through the series, while branches of various length were appended as the third arm. The materials were made by carbanionic polymerization of isoprene and the judicious application of chlorosilane linking chemistry. Subsequent saturation of the polymeric double bonds with deuterium and hydrogen, followed by fractionation, 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 increasing 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 viscosity η0 increases rapidly with branch length, and the diffusion coefficient D, obtained by forward recoil spectrometry, decreases even more rapidly. The product η0, however, distinguishes the transition from linear to branched polymer dynamics most clearly: for a backbone with about 38 entanglements, the crossover is already approaching completion for a single mid-backbone branch with only about three entanglements. © 1997 John Wiley & Sons, Inc

Measurement of Chain Dimensions in Dilute Polymer Solutions. A Light Scattering and Viscometric Study of Linear Polyisoprene in Cyclohexane

Several methods for obtaining the size of macromolecules in dilute solution were applied to a series of linear polyisoprenes in the thermodynamically good solvent cyclohexane. The polymers, prepared by anionic polymerization, cover a wide range in molecular weight (1.5 X 104< M < 3.4 X 106) and have narrow molecular weight distributions (Mw/Mn< 1.1). The viscometric radius RVfthe radius of gyration Rg, the hydrodynamic radius RH,and the thermodynamic radius RTwere determined by viscometry and by static and dynamic light scattering. The molecular weight dependences of all four size measures are well-described by power laws (R ∞ Ma). The exponents obtained for RH, RV and RTagree fairly well with the predicted value of 0.588 for good solvents. The exponent obtained for RGis smaller, although RG/RH in fact changes rather little over the range of available data. The values ofRG/RH,RV/RH andRT/RHare in reasonable accord with the theory for self-avoiding coils and with results for polystyrene and poly (α-methylstyrene) in good solvents. © 1987, American Chemical Society. All rights reserved

Thermodynamic Interactions in Model Polyolefin Blends Obtained by Small-Angle Neutron Scattering

The dependence of Flory-Huggins interaction parameter χ on temperature, composition, and chain length was investigated for binary blends of amorphous model polyolefins, materials which are structurally analogous to copolymers of ethylene and butene-1. The components were prepared by saturating the double bonds of nearly monodisperse polybutadienes (78 %, 88 %, and 97 % vinyl content) with H2 and D2, the latter to provide contrast for small-angle neutron scattering (SANS) experiments. Values of χ were extracted from SANS data in the single-phase region for two series of blends, H97/D88 and H88/D78, using the randomphase approximation and the Flory-Huggins expression for free energy of mixing. These values were found to be insensitive to chain length (one test only) and to the component volume fractions for ϕ= 0.25, 0.50, and 0.75. Their temperature dependence (27-170 °C) obeys the form χ(T) =A/T + B with coefficients that connote upper critical solution behavior, yielding Tc ∼ 40 °C for one blend series (H97A/D88) and Tc ∼ 60 °C for the other (H88/D78). These estimates are consistent with SANS pattern changes and supplemental light scattering results that indicate two-phase morphologies at lower temperatures. The χ(T) coefficients for the two series are also consistent with the random copolymer equation, although the interaction parameter obtained for branch C4‑linear C4 chain units is much larger than that found by Crist and co-workers for saturated polybutadienes with lower ethyl branch contents. © 1992, American Chemical Society. All rights reserved