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Related Concept Videos

Atomic Nuclei: Types of Nuclear Relaxation01:28

Atomic Nuclei: Types of Nuclear Relaxation

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Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.
In spin–lattice or longitudinal relaxation, the excited spins exchange energy with the surrounding lattice as they return to the lower energy level. Among several mechanisms that contribute to spin–lattice relaxation, magnetic dipolar interactions are significant. Here, the excited nucleus transfers...
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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

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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...
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¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

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The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
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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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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.
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Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level
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Long-Time Diffusion in Polymer Melts Revealed by 1H NMR Relaxometry.

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Field-cycling 1H NMR relaxometry offers a simple way to measure polymer diffusion. This method reveals how polymer diffusion coefficients depend on molecular mass, aligning with reptation theory.

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Area of Science:

  • Polymer Physics
  • Nuclear Magnetic Resonance (NMR) Spectroscopy
  • Materials Science

Background:

  • Understanding polymer dynamics, particularly translational diffusion, is crucial for predicting material properties.
  • Traditional methods for measuring diffusion coefficients can be complex and time-consuming.
  • Nuclear Magnetic Resonance (NMR) relaxometry is a powerful tool for probing molecular motion.

Purpose of the Study:

  • To establish field-cycling 1H NMR relaxometry as a straightforward method for determining translational diffusion coefficients in polymers.
  • To investigate the molecular mass dependence of the diffusion coefficient (D) in polybutadiene.
  • To compare NMR relaxometry results with established techniques like field gradient NMR.

Main Methods:

  • Utilized field-cycling 1H NMR relaxometry to measure spin-lattice relaxation dispersion (R1(ω)).
  • Studied polybutadiene samples with varying molecular masses (M) across a wide temperature range.
  • Analyzed the intermolecular contribution to relaxation rates in the low-frequency regime.

Main Results:

  • Determined translational diffusion coefficients (D) from the frequency dependence of relaxation rates.
  • Observed a molecular mass dependence of D following two distinct power laws: D ∝ M-1.3±0.1 and D ∝ M-2.3±0.1.
  • Identified a crossover in the D(M) dependence near the entanglement molecular mass (Me) of polybutadiene.

Conclusions:

  • Field-cycling 1H NMR relaxometry is a viable and simple technique for measuring polymer diffusion.
  • The observed molecular mass dependence of diffusion aligns with predictions from the tube-reptation model.
  • This method provides accurate diffusion coefficients comparable to field gradient NMR.