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Related Concept Videos

Anionic Chain-Growth Polymerization: Mechanism01:04

Anionic Chain-Growth Polymerization: Mechanism

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 acceptor.
Cationic Chain-Growth Polymerization: Mechanism00:57

Cationic Chain-Growth Polymerization: Mechanism

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 generated carbocation,...
Radical Chain-Growth Polymerization: Mechanism01:09

Radical Chain-Growth Polymerization: Mechanism

The radical chain-growth polymerization mechanism consists of three steps: initiation, propagation, and termination of polymerization. The polymerization initiates when a free radical generated from the radical initiator adds to the unsaturated bond in the monomer. The unpaired electron of the free radical and one π electron in the unsaturated bond creates a σ bond between the free radical and the monomer. As a result, the other π electron in the unsaturated bond converts this species into the...
Molecular Weight of Step-Growth Polymers01:08

Molecular Weight of Step-Growth Polymers

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.
The extent of the...
Anionic Chain-Growth Polymerization: Overview01:20

Anionic Chain-Growth Polymerization: Overview

The polymerization process that involves carbanion as an intermediate is called anionic polymerization. It is also a type of addition or chain-growth polymerization. Anionic polymerization gets initiated by a strong nucleophile such as an organolithium or a Grignard reagent. The most commonly used initiator for anionic polymerization is butyl lithium. Monomers involved in anionic polymerization must possess a vinyl group bonded to one or two electron-withdrawing groups. For instance,...
Radical Chain-Growth Polymerization: Chain Branching01:17

Radical Chain-Growth Polymerization: Chain Branching

The skeletal structure of polymers synthesized via radical polymerization is always branched. For example, the polymerization of ethylene by radical polymerization results in a low-density grade of polyethylene with a heavily branched skeletal structure. Here, the radical site abstracts hydrogen from the growing chain, and the radical site shifts from the end (a primary carbon center) to anywhere within the growing chain (a secondary carbon center). Consequently, the part of the chain from the...

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Single-chain dynamics in a semidilute polymer solution under steady shear.

Prasanth P Jose1, Grzegorz Szamel

  • 1Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA.

The Journal of Chemical Physics
|June 17, 2008
PubMed
Summary

Brownian dynamics simulations reveal polymer chain tumbling in shear flow, correlating with solution shear thinning. Chain dynamics, including end-to-end vector relaxation, exhibit oscillatory behavior at higher shear rates.

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Area of Science:

  • Polymer Physics
  • Computational Chemistry
  • Rheology

Background:

  • Polymer solutions exhibit complex flow behavior, including shear thinning.
  • Anisotropic structures (butterfly patterns) form in polymer solutions under shear flow.
  • Understanding single-chain dynamics is crucial for explaining macroscopic rheological properties.

Purpose of the Study:

  • Investigate single-chain dynamics in semidilute polymer solutions under shear flow.
  • Elucidate the microscopic origins of shear thinning and butterfly pattern rotation.
  • Compare chain dynamics in semidilute versus dilute solutions.

Main Methods:

  • Brownian dynamics computer simulations.
  • Analysis of end-to-end vector relaxation.
  • Rouse mode relaxation and radius of gyration tensor analysis.

Main Results:

  • Anisotropic structure factors (butterfly patterns) rotate with increasing shear rate.
  • Single-chain dynamics shift from double exponential to oscillatory relaxation with increasing shear rate.
  • Oscillatory relaxation suggests chain tumbling, with frequency showing power-law dependence on shear rate at high rates.
  • Fluctuations in radius of gyration tensor, end-to-end distance, and normal stress synchronize.

Conclusions:

  • Chain tumbling is a key mechanism driving shear thinning in polymer solutions.
  • The observed dynamics provide a microscopic explanation for shear-induced anisotropic structures.
  • Synchronization of fluctuations highlights interconnectedness of chain conformation and stress generation.