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Step growth polymerization involves bi or multifunctional monomers. Bifunctional monomers react to form linear step growth polymers, whereas multifunctional monomers react to form non-linear or branched polymers.
As the step-growth polymerization involves step-wise condensation of monomers, the molecular weight also builds up eventually. Consequently, high molecular weight polymers are obtained at the late stages of the polymerization, where 99% of monomers have been consumed.
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The mechanism for anionic chain-growth polymerization involves initiation, propagation, and termination steps. In the initiation step, a nucleophilic anion, such as butyl lithium, initiates the polymerization process by attacking the π bond of the vinylic monomer. As a result, a carbanion, stabilized by the electron‐withdrawing group, is generated. The resulting carbanion acts as a Michael donor in the propagation step and attacks the second vinylic monomer, which acts as a Michael...
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Stress is a quantity that describes the magnitude of a force that causes deformation, generally defined as internal force per unit area. When forces pull on an object and cause its elongation, like the stretching of an elastic band, it is called tensile stress. When forces cause the compression of an object, it is known as compressive stress. When an object is being squeezed uniformly from all sides, like a submarine in the depths of the ocean, we call this kind of stress bulk stress (or volume...
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Unlike ionic or small covalent molecules, polymers do not form crystalline solids due to the diffusion limitations of their long-chain structures. However, polymers contain microscopic crystalline domains separated by amorphous domains.
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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the...
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Molecular Entanglement and Electrospinnability of Biopolymers
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Molecular Processes Leading to Shear Banding in Entangled Polymeric Solutions.

Mahdi Boudaghi1, Brian J Edwards1, Bamin Khomami1

  • 1Materials Research and Innovation Laboratory, Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA.

Polymers
|August 12, 2023
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Summary

Transient shear banding in polymer solutions was observed during startup shear flow. Shear bands showed evolving polymer concentrations and dissipated at high strains, leading to steady-state flow.

Keywords:
complex fluidsdissipative particle dynamicsflow instabilitynonequilibrium molecular dynamicsnonlinear dynamicsrheologyshear banding

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Area of Science:

  • Polymer physics
  • Rheology
  • Computational fluid dynamics

Background:

  • Shear banding is a complex flow phenomenon in entangled polymer solutions.
  • Understanding its temporal and spatial evolution is crucial for predicting material behavior.

Purpose of the Study:

  • Investigate shear banding dynamics during startup and steady-state shear flow.
  • Analyze the influence of solvent on shear banding.
  • Characterize the evolution of polymer concentration within shear bands.

Main Methods:

  • Developed a high-fidelity coarse-grained dissipative particle dynamics (DPD) method.
  • Evaluated the DPD method against Non-Equilibrium Molecular Dynamics (NEMD) simulations.
  • Simulated entangled, linear, monodisperse polyethylene C3000H6002 in hexadecane and benzene.

Main Results:

  • Observed transient shear banding at specific shear rates during startup flow.
  • Shear bands formed at strain values corresponding to the maximum first normal stress difference.
  • Inhomogeneous polymer concentrations within shear bands were evident, varying with shear rate.
  • A reverse flow phenomenon was noted in hexadecane solutions at low shear rates.
  • Shear bands dissipated at high strains, leading to homogeneous flow and concentration.

Conclusions:

  • Transient shear banding is a key feature of entangled polymer solutions under shear.
  • The solvent plays a role in phenomena like reverse flow.
  • DPD simulations provide a valuable tool for studying complex polymer dynamics.