Long-chain Branching In Polymers Research Articles

Effect of long‐chain branching molar fraction on scratch behavior of polypropylene

AbstractPolymers containing a certain amount of long‐chain‐branching (LCB) structure are expected to possess improved mechanical properties over those of the linear structure counterpart. However, fundamental knowledge on the structure–property relationship in LCB containing polypropylene (PP) is still illusive. In the present study, a set of model PP systems containing an increasing molar fraction of LCB (5–19 mol%) were prepared by reactive extrusion to determine how LCB content may influence the scratch behavior of PP. It is shown that with only 5 mol% of LCB content in PP can improve resistance against scratch‐induced fish‐scale formation by over 25%. The improvement of scratch resistance is attributed to the increases in entanglement density in LCB‐containing PPs, which is evidenced by their creep‐recovery behavior. The present study demonstrates that the incorporation of LCB in PP leads to higher viscoelastic recovery and increased tensile strength, which account for the observed improvement in scratch performance. The usefulness of LCB in polymers for improving scratch performance is discussed.

A rheology theory and method on polydispersity and polymer long-chain branching

Model calculations were performed to investigate the sensitivity of zero-shear melt viscosity (η0 or Eta0) on the molecular weight (MW) polydispersity of linear polymers. Simulated MW distributions (MWD) were generated with the generalized exponential (GEX) distribution function for various levels of polydispersity Mw/Mn and Mz/Mw. For linear entangled polymeric chains in the melt, the linear viscoelastic properties were predicted by using the double reptation blending rule and the so-called BSW relaxation time spectrum, named after the authors: Baumgaertel, Schausberger and Winter [Baumgaertel M, Schausberger A, Winter HH. Rheol Acta 1990;29:400–8]. Published rheological parameters appropriate for polyethylene were used in the calculations. It was found that Eta0 depended mostly on Mw, but it also significantly depended on the extent of high-MW polydispersity Mz/Mw. A revision to the fundamental MW dependency of Eta0 was proposed to compensate for this polydispersity effect. To offset the polymer polydispersity differences, we propose a new MW average (MHV or Mx with x=1.5) to replace Mw in the historical rheological power-law equation of Eta0∝Mwa, where the literature value of exponent “a” ranges from 3.2 to 3.6. The use of MHV instead of Mw in the power-law equation made the calculated Eta0 independent of the sample high-MW polydispersity. With the removal of the complication from polydispersity effect, the new Eta0 power law can now provide a more robust base for studying polymer long-chain branching (LCB). A new LCB index is thus proposed based on this new melt-viscosity power law. The values of MHV in the new power law can be calculated for polymer samples from the conventional gel permeation chromatographic (GPC) slice data.

Separation of Polymers by Molecular Topology Using Monolithic Columns

Long chain branching (LCB) is most often incorporated into polymeric materials to enhance rheology and processing. Presently, the characterization of LCB in polymers is limited to the determination...

A new model for rapid evaluation of the degree of long-chain branching in polymers

Abstract A new model is proposed for rapid evaluation of the degree of long-chain branching in polymers, which correlates the intrinsic viscosity and the molecular weight. Intrinsic viscosity is expressed as a simple inverse tangent function of molecular weight. The model is based on the observation of rheological behaviour of polymer melts and solutions. It can describe the intrinsic viscosity behaviour over a wider range than the Mark-Houwink equation. This model does not use trial and error procedure to decide the threshold molecular weight where the intrinsic viscosity starts to deviate from the Mark-Houwink relation. Calculation by the proposed model of the long-chain branching frequency of low density polyethylene shows good agreement with experimental results in the literature.

Estimation of Long-Chain Branching in Ethylene-Propylene Terpolymers from Infinite-Dilution Viscoelastic Properties

Abstract A method is outlined for estimation of small degrees of long-chain branching in polymers with moderately narrow molecular weight distribution (Mw/Mn<1.4). The storage and loss shear moduli, G′ and G″, are measured in dilute solution by the Birnboim-Schrag multiple-lumped resonator and extrapolated to infinite dilution, choosing a suitable solvent viscosity and frequency range such that the data lie in the terminal zone where G′ and G″ are proportional to the second and first powers of frequency, respectively. The intrinsic reduced steady-state shear compliance is determined from these data and corrected for moderate molecular weight heterogeneity (assuming a Gaussian distribution) from knowledge ofMw/Mn and the Mark-Houwink exponent a. The resulting value of S2/S12 (where S1=∑ τp/τ1, S2=∑ (τp/τ1)2, the τp's being the relaxation times and τ1 the longest one) is compared with values calculated by the Zimm-Kilb theory as evaluated by Osaki for comb polymers of regular geometry and different numbers of branch points. The method has been illustrated by measurements on four ethylene-propylene copolymers. One containing no termonomer and one containing a saturated termonomer appeared to be linear; two containing unsaturated termonomers showed small degrees of branching. The method appears to be promising for detecting from one to four branch points per molecule.

Estimation of long-chain branching in ethylene–propylene terpolymers from infinite-dilution viscoelastic properties

A method is outlined for estimation of small degrees of long-chain branching in polymers with moderately narrow molecular weight distribution (Mw/Mn <1.4). The storage and loss shear moduli, G′ and G″, are measured in dilute solution by the Birnboim-Schrag multiple-lumped resonator and extrapolated to infinite dilution, choosing a suitable solvent viscosity and frequency range such that the data lie in the terminal zone where G′ and G″ are proportional to the second and first powers of frequency, respectively. The intrinsic reduced steady-state shear compliance is determined from these data and corrected for moderate molecular weight heterogeneity (assuming a Gaussian distribution) from knowledge of Mw/Mn and the Mark-Houwink exponent a. The resulting value of S2/S (where S1 = Στp/τ1, S2 = Σ(τp/τ1)2, the τp's being the relaxation times and τ1 the longest one) is compared with values calculated by the Zimm-Kilb theory as evaluated by Osaki for comb polymers of regular geometry and different numbers of branch points. The method has been illustrated by measurements on four ethylene–propylene copolymers. One containing no termonomer and one containing a saturated termonomer appeared to be linear; two containing unsaturated termonomers showed small degrees of branching. The method appears to be promising for detecting from one to four branch points per molecule.