HHMI-BRIDGES Research Experiences for STEM Teachers
Furman University - 2010
Ray Tedder
NBC Chemistry Teacher, Paul M Dorman High School Science Department
Determining if propylene carbonate can be used as a safer solvent for
finding the size of molecules in polyesters
The reason for this research
O
PET is a form a plastic in a class of polymers called polyesters. They are
called polyesters because they are long molecules with lots of structures
that organic chemists call esters. The prefix poly means many. So, poly +
ester = many esters, or polyester. The letters in PET come from its formal
name which is polyethylene terephthalate.
PET is used for lots of things from clothing to soft drink bottles.
Just how long the PET molecules are is important to people in companies
who make PET and the people and other companies that use it. The length
of the molecules changes the properties of the plastic, so it’s important to
be able to test the product to make sure that it has the molecule length that
is wanted.
Determining the average length of these PET molecules is usually done by
mixing samples is a suitable liquid and testing the viscosity of that mixture.
The viscosity is then compared to a correlation curve which is a kind of
graph or math equation that tells the average length of the chains in the
sample.
CH2 O C
Ester group
Ester group
O
Our hypothesis is: If the concentration of PET dissolved in PCA is
increased, then—because of increased attractions between molecules—
efflux time of the solutions/suspensions will also increase and will do so in
a way that can be used to determine the average molecular mass of PET
samples.
Figure 7. This is what
the timing marks on the
viscometer look like
when it’s in the constant
temperature water bath.
[η]
Intrinsic Viscosity of PET in PCA solvent
0.8
y = 0.0006x0.7295
0.6
0.4
0.3
0.2
0.1
0
Future work:
0
5000
10000
15000
20000
Molecular Mass (in g) of PET dissolved or suspended in PCA
25000
Figure 6. In this photo Ray Tedder is
using vacuum to pull PET/PCA liquid
up into the viscometer to measure the
time it takes for the top of the liquid to
fall by gravity from one timing mark to
the other. The viscometer is inside the
purple container which is a constant
temperature bath filled with water. The
temperature inside must be kept at
25.00°C.
Figure 1. This is an illustration of a correlation curve and equation that would
allow a manufacturer to determine the average molecule size of a sample of
PET.
Viscosity is often defined as
“resistance
to
flow.”
Basically, that means that the
more viscous a fluid is the
slower it flows as long as all
other conditions are the same.
Viscosity is tested in a
viscometer. There are lots of
different kinds of viscometers
but most of them do not
determine viscosity directly.
The viscometer is used to
measure how long it takes for
a certain volume of the liquid
to flow through a very small
glass tube called a capillary.
The viscosity of the liquid is determined through one of a
number of different mathematical formulas.
Figure 2. This is a
Ubbelohde viscometer.
Solutions, colloids, or
suspensions are
placed in the bottom.
A vacuum is used to
pull the liquid up into
the tube with the 2
bulbs near the top.
The time it takes for
the top of the liquid to
drop from one of 2
timing marks to
another is used to The
timing marks are
located here.
O
O
O
Figure 3.
Chemical
structure
of PCA.
The reason that this process of determining the size of
molecules from the viscosity works is that as the average
molecule length (and the average molecular mass) grows, the
attractions between the molecules is greater.
Those
attractions slow the solution or suspension down as if flows
through the viscometer’s capillary.
Most solvents used for all of this are potentially dangerous to personnel,
danger to the environment, and they cost a lot. The purpose of our inquiry
was to determine if propylene carbonate (PCA) (see figure 2) could be
used as a solvent. PCA is safer than most of the solvents normally used
and it costs a lot less.
C
Ester group
O
CH2 CH2 O
Figure 9. This photo shows a series of
solutions/suspensions that have been
prepared for testing in the viscometer.
C
O
C
Ester group
This section is one of the repeat units in the PET structure.
O
Hypothesis
0.9
0.5
C
C
O
Figure 5. Repeat unit of PET .
C
O
Equation for the
0.7
O
Figure 4. PET is a group of really long molecules built from a lot of smaller molecules. PET has to be
built in a tank under very high temperatures and very lower pressures. The structure above shows a
very small section of one of those molecules. The ester functional groups have been highlighted for you.
Molecular mass vs. intrinsic viscosity [η] of PET in PCA
solvent
1
CH2 CH2 O
O
O
Ester group
O
C
Determining whether sodium dimethylsulfonate can be used as a
substitute for heavy metal catalysts in synthesizing polyesters
CH2
CH2
O
n
*
Figure 15. Dr. Robert (Bob) Posey
standing in front of the lab apparatus
used for synthesizing PET. Dr. Posey
is holding a reagent bottle containing
his organic catalyst substitute.
The reason for this research
PET is typically manufactured with one of 2 catalysts: a titanium salt or an
antimony salt. A catalyst can be defined as a substance that changes how
fast a reaction proceeds but is not consumed by the reaction itself. The
problem is that minute particles of the catalyst remain stuck in the PET
structure. Since PET is used as food and drink packaging, there is concern
about the presence of these materials.
Titanium poses little risk, but antimony is a heavy metal with a serious
health effects. Antimony is used as a catalyst because PET produced with
it is almost clear while titanium catalysts produce PET with an undesirable
yellow or gold color. The antimony exists in the PET packaging but in
extremely small amounts and usually remains trapped in the polymer
structure so it poses little risk.
Figure 17. Liquid nitrogen boils at
196°C around the flasks that is
capturing side products of the PET
synthesis reaction.
Never-the-less, eliminating the need for any metallic catalysts—such as
titanium and antimony—would be a valuable development to the PET
industry and consumers.
O
O
Hypothesis
Figure 8. The solute (PET)
and the solvent (PCA) must be
heated to about 210°C to get
them to mix together into a
solution. Once they cool, the
mixture forms a suspension if
the PET molecules are very
large. They can form a
solution when the PET
molecules are small enough.
Conclusions
Even though we were faced with a number of technological challenges
(some of which were unavoidable) the data and analysis of our
experimental, scientific research were sufficient to conclude that propylene
carbonate (PCA) is a suitable solvent for use in determining the average
molecular mass of polyethylene terephthalate (PET).
Our hypothesis is: If the organic
chemical salt, sodium dimethylsulfate—which becomes dimethylsulfonic acid (DMSA) in
solution inside the reaction
chamber—is added to the
reaction in place of titanium or
antimony salt catalysts—it will
act as a suitable replacement for
the those catalysts. We do expect
that the reaction conditions will
have to be optimized for this
synthesis.
H3C
O
C
Figure 11.
Chemical
structure
of sodium
dimethylsulfonate.
H3C
O
C
O S O
O
CH3
O
O
O
C
C
O S O
Figure 13. Reaction vessel
(called an autoclave) in
which we synthesized PET.
The autoclave is inside a
large heating mantle and
insulation.
Now that the feasibility of using PCA as a solvent
for the average molecular mass determination of
PET has been shown, more accurate measurements
of viscosity of PET in PCA should be done to
determine more accurate viscosities
and
correlations between these viscosities and the
average molecular mass of the PET samples
produced.
Figure 18. Ray Tedder tightens one
of the fittings around the stirrer where
it is inserted into the reaction vessel
to eliminate a leak. Leaks must be
minimized because the pressure
inside the vessel most be kept below
6/100ths of normal atmospheric
pressure.
Na+
O
CH3
Figure 12.
Chemical
structure of
dimethylsulfonic acid.
OH
A reasonable Mark-Houwink equation, which we
determined to be [η] = 0.000624 × mw0.7295, can now
be used to predict the molecular mass of PET
samples. A reasonable correlation equation,
manufacturer’s [η] = our [η] × 0.5657, also exists to
further verify or refute our analysis and conclusions.
Future work:
Figure 16. Ray Tedder adds liquid
nitrogen (which has a temperature of
196°C) to a special container called
a Dewar. The liquid nitrogen to keeps
a flask in the vacuum system
extremely cold to collect side
products of the PET synthesis.
Figure 19. Dr. Posey is using a heat
gun to melt some side product solids
that have collected at the end of the
condenser while Mr. Tedder watches.
These low molecular mass materials
are boiled off in the reaction vessel
but they tend to clog the vacuum
system and must be removed.
Conclusions
Figure 20. Dr. Posey (left) and Zach
Svec (right) remove PET product from
the reaction vessel after the synthesis
process has run it’s course.
Early tests with the sodium dimethylsulfate catalyst substitute in the
synthesis of polyethylene terephthalate (PET) shows some success and
much promise. The products from our synthesis process do not yet
demonstrate the desired behaviors. Analysis of the product material
indicates that it contains a high concentration of side products—particularly
diethylene glycol (DEG).
Future work:
The reactions will continue to be modified to search for the right set of
conditions for this catalysts replacement to work.
Acknowledgments
Figure 10. Teachers in the HHMI-BRIDGES
Research Experiences for STEM Teachers program
meet with program director Dr. John Kaup (2nd from
right) to discuss research progress, development of
lesson plans tied to the research, and
presentations. Meetings were held about once
every 1½ weeks during the 6 week program.
For further information
Please contact Ray Tedder at [email protected], or (864)578-9396.
Thanks to Dr. Bob Posey for his tireless mentorship.
Figure 14.
Reaction vessel
without the
heating mantle
and insulation.
Figure 15. First PET product
removed from the reaction vessel
using DMSA.
Thanks to Dr. John Kaup for his assistance in preparing this presentation.
Thanks to the Howard Hughes Medical Institute for providing the funding
for a stipend and their support in the development of research skills.
Thanks to Furman University for providing the facilities for this research
and for the university’s work to develop research skills.
Thanks to all the people at a local industry who—because of proprietary
considerations—cannot be named here.