Application of 129Xe NMR to the Study of the behaviour of Polymers in Supercritical Carbon Dioxide

Kylie Varcoe (2009). Application of 129Xe NMR to the Study of the behaviour of Polymers in Supercritical Carbon Dioxide PhD Thesis, Australian Inst of Bioengineering & Nanotechnology.

       
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Author Kylie Varcoe
Thesis Title Application of 129Xe NMR to the Study of the behaviour of Polymers in Supercritical Carbon Dioxide
School, Centre or Institute Australian Inst of Bioengineering & Nanotechnology
Publication date 2009-03
Thesis type PhD Thesis
Supervisor Professor Andrew Whittaker
Dr Idriss Blakey
Total pages 187
Total colour pages 20
Total black and white pages 167
Subjects 09 Engineering
Formatted abstract This thesis comprises the first study of the use of 129Xe nuclear magnetic resonance (NMR) as a
characterisation tool for polymer systems in supercritical carbon dioxide (scCO2). The use of scCO2
is an important technique for the processing of polymers as well as for polymer synthesis. To
maximise the performance of polymers using such techniques requires polymer characterisation
tools with the ability to monitor changes down to the nanometre scale. 129Xe NMR is one technique
that is increasingly being used as a characterisation tool for the study of polymer structure and
morphology, as the 129Xe chemical shift is known to be sensitive to changes on the nanometre scale.
This sensitivity is due to the large polarisability of the electron cloud of xenon.
The major aim of this thesis is thus to gain a fundamental understanding of the use of 129Xe NMR
for the study of polymers in scCO2. Kalrez was chosen as a model polymer for these investigations
because it is a fluorinated polymer and hence well suited to processing in scCO2. In addition, it is
used in high pressure applications as o-rings, hence knowledge of its swelling behaviour is of
industrial importance.
To gain this fundamental understanding, the ability of 129Xe NMR to monitor polymer swelling and
dynamics was investigated as a function of: (i) time, (ii) CO2 pressure, and (iii) system temperature.
To provide supporting information to the 129Xe NMR experiments, 19F NMR relaxation
measurements were made to provide information on the chain mobility, dynamics and solubility.
Interaction parameters were also calculated to provide information of the degree of interaction
between the polymer and scCO2.
In these investigations it was found that the 129Xe NMR signal showed up to a three-fold increase in
signal intensity after the addition of scCO2 to the system. This was determined to be due to the large
increase in density of the system which caused a decrease in the spin-lattice (T1) relaxation time and
consequently an increase in signal intensity. Since one of the difficulties of 129Xe NMR is the poor
signal-to-noise that is often observed, this increase in signal intensity with the addition of scCO2
makes 129Xe NMR a promising technique for understanding the behaviour of polymers in scCO2
and other high pressure fluids.
By monitoring the 129Xe chemical shift as a function of time for the Kalrez/scCO2 system at a set
temperature (313 K) and pressure (10 MPa), it was demonstrated that the chemical shift provides a
reliable method of monitoring the rate of swelling of the polymer. Without the dramatic increase inthe signal intensity, this measurement would not be possible due to the signal averaging normally
required. It was found that the degree of swelling of Kalrez increased with time until approximately
400 mins, at which time equilibrium was observed and the polymer had reached its maximum
degree of swelling. Alternatively, the 129Xe chemical shift resulting from the xenon in the scCO2
fluid surrounding the polymer (δfluid) as well as the signal intensity of this peak provided other
methods for the real time monitoring of the swelling of the polymer in scCO2. These alternative
methods could be useful if a poor 129Xe signal is obtained for the polymer.
In order to use the 129Xe chemical shift to determine the effect of pressure and temperature on the
polymer, it was found that the 129Xe chemical shift in the fluid (δfluid) needed to be used as a
reference peak for the 129Xe chemical shift in the polymer (δpoly) to remove the effect of Xe-CO2
interactions. By the use of the corrected 129Xe chemical shift (δpoly - δfluid) it was found that the
degree of swelling experienced by the polymer increased with pressure and decreased with
temperature. In both these cases this was determined to be a function of density with the degree of
swelling increasing with an increase in density. Both the 19F relaxation measurements and
interaction parameters provided supporting results to the corrected 129Xe chemical shift.
By measurement of 129Xe diffusion coefficients in Kalrez it was determined that the rate of
diffusion increased with increasing scCO2 pressure as well as with increasing temperature of the
system. This was determined to be a result of both the density of the system as well as chain
mobility. Therefore, to maximise the rate of diffusion through the polymer an increase in either the
pressure or temperature of the system is needed. In addition, it was found that the rate of diffusion
in the scCO2 swollen polymer was approximately two to three orders of magnitude faster than the
diffusion through non-swollen polymers.
In summary, this work provides the first study of polymers in scCO2 using 129Xe NMR. This work
will therefore provide a basis to the future use of 129Xe NMR for the study of scCO2 swollen
polymers. Future studies of more complex systems are now possible, for example the study of
swelling of the different components of polymer blends.
Keyword xenon, nuclear magnetic resonance, NMR, supercritical carbon dioxide, polymers, Kalrez, interaction parameters
Additional Notes 1, 50, 64, 65, 84, 91, 92, 123, 161, 162, 163, 164, 165, 166, 167, 168, 171, 173, 174, 176

 
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Created: Thu, 15 Oct 2009, 14:55:57 EST by Ms Kylie Varcoe on behalf of Library - Information Access Service