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Related Concept Videos

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Thermal Sigmatropic Reactions: Overview01:16

Thermal Sigmatropic Reactions: Overview

Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred to as...
Polymer Classification: Crystallinity01:21

Polymer Classification: Crystallinity

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.
Crystalline domains are the regions where polymer chains are aligned in an orderly manner and held together in proximity by intermolecular forces. For example, chains in the crystalline domains of polyethylene and nylon are bound together by van der Waals...
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...
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...

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High-Contrast and Fast Photorheological Switching of a Twist-Bend Nematic Liquid Crystal
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Multiple shear-banding transitions in a supramolecular polymer solution.

J van der Gucht1, M Lemmers, W Knoben

  • 1Laboratory of Physical Chemistry and Colloid Science, Wageningen University, PO Box 8038, 6700 EK Wageningen, The Netherlands. jasper.vandergucht@wur.nl

Physical Review Letters
|October 10, 2006
PubMed
Summary

This study reveals unique flow behaviors in reversible supramolecular polymers due to hydrogen bonding. Three distinct shear-banding regimes emerge with increasing shear rate, driven by chain alignment and bond dynamics.

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Area of Science:

  • Polymer Science
  • Rheology
  • Materials Science

Background:

  • Supramolecular polymers offer tunable properties through reversible bonding.
  • Understanding their rheological response under flow is crucial for material design.

Purpose of the Study:

  • To investigate the nonlinear rheology of a hydrogen-bonded supramolecular polymer.
  • To elucidate the relationship between flow, chain dynamics, and macroscopic flow behavior.

Main Methods:

  • Nonlinear rheological measurements.
  • Analysis of velocity profiles under varying shear rates.
  • Observation of flow-induced structural changes.

Main Results:

  • Observed three distinct shear-banding regimes with increasing shear rate.
  • First regime shows mechanical instability.
  • Second regime exhibits shear-induced phase separation and birefringence.
  • Third regime arises from instability of the shear-induced phase at high shear rates.

Conclusions:

  • The coupling of chain alignment and bond dynamics dictates complex flow behavior.
  • Shear-induced phase separation is a key mechanism in one regime.
  • Multiple instabilities govern the rheology of these polymers across different shear rates.