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Polymers: Molecular Weight Distribution01:10

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For any given polymer, the weight average molecular weight (Mw) is higher than, if not equal to, the number average molecular weight (Mn). The only situation in which the weight average molecular weight and the number average molecular weight are equal is when a polymer consists only of chains with equal molecular weight. However, this never happens in a synthetic polymer, since it is difficult to control the polymerization process up to a molecular level with accuracy to a hundred percent.
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Anionic Chain-Growth Polymerization: Overview01:20

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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,...
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Ziegler–Natta Chain-Growth Polymerization: Overview01:17

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Ziegler–Natta polymerization is another form of addition or chain‐growth polymerization used for synthesizing linear polymers over branched polymers. The catalyst used for polymerization is the Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, who developed it in 1953. This catalyst is an organometallic complex of titanium tetrachloride and triethyl aluminum, with the active form of the catalyst being an alkyl titanium compound. Using the Ziegler–Natta...
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Anionic Chain-Growth Polymerization: Mechanism01:04

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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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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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Polymers

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The word polymer is derived from the Greek words “poly” which means “many” and “mer” which means “parts”. Polymers are long chains of molecules composed of repeating units of smaller molecules, known as monomers. They either occur naturally, such as DNA and proteins, or can be constructed synthetically, like plastics. They have varied structural characteristics, such as linear chains, branched chains, or complex networks, that contribute to the...
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A Ribbon Model for Nematic Polymer Networks.

Harmeet Singh1, Epifanio G Virga2

  • 1Laboratory for Computation and Visualization in Mathematics and Mechanics, Institute of Mathematics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.

Journal of Elasticity
|June 9, 2023
PubMed
Summary
This summary is machine-generated.

We developed a theory for nematic polymer network (NPN) ribbons, predicting serpentine deformations in response to heat or light. This work simplifies complex material behavior for ribbon applications.

Keywords:
Nematic elastomersNematic polymer networksPhotoactivable elastic materialsRibbon theorySoft matter elasticity

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Area of Science:

  • Materials Science
  • Polymer Physics
  • Soft Matter Physics

Background:

  • Nematic polymer networks (NPNs) combine rubber elasticity with liquid crystal properties.
  • NPNs respond to external stimuli like heat and light.
  • Existing models describe NPN sheets but not ribbons.

Purpose of the Study:

  • To derive a theoretical model for the deformation of NPN ribbons.
  • To simplify a 2D sheet energy model to a ribbon energy model.
  • To demonstrate ribbon behavior with a specific example.

Main Methods:

  • Utilized a dimension reduction method.
  • Derived ribbon energy from a pre-existing 2D sheet energy model.
  • Applied boundary conditions to a rectangular NPN ribbon model.

Main Results:

  • Developed a theoretical framework for NPN ribbon deformation.
  • Successfully predicted in-plane serpentine deformations in a model ribbon.
  • The derived energy model is suitable for NPN ribbons.

Conclusions:

  • The dimension reduction approach effectively models NPN ribbon mechanics.
  • NPN ribbons can exhibit complex, stimulus-responsive deformations.
  • This theory provides a basis for designing novel NPN ribbon-based devices.