An Examination of the Relaxation Times of Carboxylated Polystyrene

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An Examination of the Relaxation Times of
Carboxylated Polystyrene

• Nick Ergang
• Edward Hagaman, PhD
• Coe College, Chemistry
• Chemical and Analytical Sciences Division, ORNL

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Abstract

• Solid State 13C CP/MAS NMR Spectroscopy has been
  used to measure the relaxation times of
  carboxylated polystyrene polymers. Through the
  addition of the carboxyl group to the para
  position of the benzene ring, the hydrogen
  bonding between polymer chains increases,
  contributing to the rigidity of the polymer. The
  increase in hydrogen bonding leads to a
  corresponding decrease in the correlation time
  for motion of the polymer chains. The relaxation
  times of the polymer will change due to coupling
  with the intermolecular motion. The measured
  relaxation times were the proton T1 and T1r, as
  well as the carbon T1r.

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Experimental Method

• Experiments were done in a Bruker MSL 100 MHz
  Spectrometer
• Magic Angle Spinning (MAS)
• rotor is spun at 2.2-2.4 kHz at an angle of 54.7o
  from the static magnetic field
• scalar averaging (1-3cos2Q)
• Cross polarization (CP)
• magnetization is transferred from the protons to
  the carbon nuclei when the Larmor frequencies (w)
  are matched

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Polymer Structure

• Carboxylic acid substitution was at 2.3, 4.7, and
  19.3 mole percent
• Hydrogen bonding occurs between the carboxyl
  groups of different chains

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Future Work

• High Temperature Experiments
• analysis of samples at temperatures up to 150 oC
• High and Low Field Strength Experiments
• investigation of the field dependence of the
  signal at a range of 40 kHz to 90 kHz
• Annealed Polymers
• examination of relaxation times over a range of
  annealing times

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Aromatic C
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Aromatic CH
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Aliphatic CH
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Pulse Sequence
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Discussion

• The measured intensities of each of the
  individual peaks were plotted over the variable
  time delay of the carbon spin lock. As the time
  increases there is an exponential decay of the
  signal. The aromatic carbon shows a single mode
  decay while the aromatic CH and aliphatic CH
  demonstrate a bimodal exponential decay. All of
  the polystyrene samples decay at the same rate,
  producing similar relaxation constants. As yet,
  there is no conclusive evidence that there is a
  definite change in the relaxation times as a
  function of carboxyl concentration.

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Acknowledgements

• DOE/ERULF
• GLCA/ACM

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Introduction

• Solid state NMR provides an excellent way to
  analyze the internal processes of materials such
  as polymers. Cross-polarization and magic angle
  spinning are used to achieve clear spectra in a
  reasonable acquisition time. By varying the
  amount of carbon spin lock delay, the relaxation
  times of carboxylated polystyrene are determined
  from the peak intensities. The three peaks that
  appear in the spectra of polystyrene are due to
  the quaternary aromatic carbon, the protonated
  aromatic carbons, and the aliphatic carbons of
  the polymer backbone.

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Cross Polarization