An Investigation of Polymers - Adi Krupski's E-Portfolio...The mass density of high-density...
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An Investigation of
Polymers
Adi Krupski
March 20th, 2013
Chemistry 113 M-Experimental Chemistry
Section 101
TA: Nick Dunn
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Introduction:
A polymer is a large organic molecule assembled from multiple repeating chains of many
smaller molecules, known as monomers (1). Polymers have been studied since 1832 and perform an
extremely important role in our daily lives in applications such as coatings, foams, biomedical
devices, and optical devices (2). As polymers consist of many repeating monomers in long chains,
we have developed technology in order to manipulate their characteristics to better suit our needs; we
are now able to modify polymers to make them harder, stronger, more flexible and lighter. Polymers
are also capable of exhibiting a wide range of thermal, electrical, and optical properties making them
even more useful in a broad range of settings (3).
Polymer recycling is extremely important for the upkeep of our planet. Plastics (which are all
polymers) are versatile recyclables and can be recycled to make items such as clothes, containers,
films, bags, and garden products. In 2007, The Environmental Protection Agency reported that there
was more than 30 million tons of plastic waste. As plastic materials take hundreds of years to break
down in a landfill (4),we cannot let these plastics sit in landfills while we could be recycling and
reusing them to create a more sustainable world.
Below are the name, chemical structure, synthesis reaction, and special properties/primary
applications of the 7 recyclable polymers (13):
#1- Polyethylene terephthalate. The chemical structure can be seen in figure 1. Note that n is
an integer called the degree of polymerization.
Figure 1- Structure of polyethylene terephthalate1
Polyethylene terephthalate (commonly known as PET) is formed through condensation
1 Wikipedia The Free Encyclopedia. Rohieb. 4 March 2007 <http://en.wikipedia.org/wiki/Polyethylene_terephthalate>
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polymerization when two molecular species react with each other. In condensation polymerization,
the molecules join together while losing other small molecules as byproducts during the process. The
formation and release of these simple molecules as byproducts is a key component of the
condensation polymerization process. In these reactions water and methanol are common
byproducts. In Figure 2, the formation process of PET is shown. Terephthalic acid reacts with
ethylene glycol in an esterification reaction with water as a byproduct (shown below the reaction
arrow) (6).
Figure 2- Esterification to produce PET2
Polyethylene terephthalate can appear transparent or opaquely white due to its semicrystalline
structure. The density of Polyethylene terephthalate is 1.38 g/cm3 (20 °C) and soft drink bottles
comprise of the majority of the world's PET production. (11).
#2- High Density Polyethylene. The chemical structure is shown in Figure 3.
Figure 3- Structure of high density polyethylene3
Polyethylene is composed of ethylene monomers. Ethylene has the
chemical formula C2H4 and can also be viewed as a pair of
methylene groups (which can be written as =CH2 with “=”
denoting a double bond). Ethylene is a gaseous hydrocarbon. The
structure of ethylene is shown in Figure 4.
2 A Green Chemistry Module. Trudy A. Dickneider. Greening Across The Chemistry Curriculum. 21 February 2013.
<http://academic.scranton.edu/faculty/cannm1/industrialche mistry/industrialchemistrymodule.html>
3 Wikipedia The Free Encyclopedia. Pngbot . 24 January 2007. <http://en.wikipedia.org/wiki/File:Polyethylene-repeat-2D-flat.png>
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Figure 4-Structure of ethylene4
The mass density of high-density polyethylene can range from 0.95-
0.97 g/cm3. This difference in density differentiates high density
polyethylene from low density polyethylene. The density of the following
polymers play an extremely important role in their properties (especially in the
tap water buoyancy test explained later). The primary use of high-density polyethylene is for
packaging such as plastic bags, plastic films, and containers including bottles.
The synthesis of polyethylene is demonstrated in Figure 5. Polyethylene is an addition
polymer; in this process many ethylene’s (the monomer of polyethylene) bond together by
rearranging their bonds and do not lose any atoms of molecules (unlike condensation polymerization,
where there is a byproduct).
Figure 5- Synthesis of polyethylene5
4 Wikipedia The Free Encyclopedia. Mills, Ben . 2 February 2009 <http://en.wikipedia.org/wiki/Ethylene>
5 University of Buffalo. Todtenhagen, Kevin.2007. <http://www.eng.buffalo.edu/Courses/ce435/Polyethylene/CE435Kevin.htm>
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#3-Vinyl
Figure 6- Structure of vinyl chloride6
The structure of vinyl chloride is shown in Figure 6. Vinyl is used to make chessboards as well as
flooring as vinyl is extremely resistant to moisture and humidity. Vinyl polymers are produced by
addition polymerization, similar to the synthesis reaction of polyethylene. An example of a vinyl
monomer is styrene (a small molecule containing carbon-carbon double bonds). The synthesis
reaction of vinyl is shown in Figure 7. Note that for vinyl chloride the “R” in the diagram would be
chloride (Cl).
Figure 7- Synthesis Reaction of Vinyl7
#4-Low Density Polyethylene
LDPE is defined by a density range of 0.920–0.940 g/cm3. It has a similar chemical structure
and synthesis reaction to that of high density Polyethylene (see Figure 6 and 7), only it is less dense
6 Wikipedia The Free Encyclopedia. Edgar181. 15 November 2007 <http://en.wikipedia.org/wiki/File:Vinyl_group.png>
7 Wikipedia The Free Encyclopedia. V8rik . 4 February 2007 http://en.wikipedia.org/wiki/File:VinylPolymers.png
dsdsdsdsd
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due to its chemical structure. Thus, LDPE is more flexible than the rigid HDPE. Common
applications of LDPE are plastic wraps, six pack rings, and trays.
#5-Polypropylene
Figure 8- Structure of Polypropylene8
The structure of polypropylene is shown in Figure 8. Its density is around 0.946 g/cm3 and
polypropylene has a crystalline structure. Polypropylene is commonly used in packaging and
labeling, textiles, loudspeakers, and laboratory equipment (12). The synthesis reaction is shown in
Figure 9. The monomer of polypropylene is propylene.
Figure 9- Synthesis Reaction of Polypropylene9
#6-Polystyrene
Figure 10—Structure of Polystyrene10
The density of polystyrene is approximately 0.96-1.04 g/cm3. Polystryene
is common used for protective packaging (for example, CD and DVD cases),
containers, and lids (7). Polystyrene’s structure is hard and brittle, and is also
highly flammable yet not very chemically reactive. The structure is shown in
8 Wikipedia The Free Encyclopedia. NEUROtiker . 27 March 2008 <http://en.wikipedia.org/wiki/File:Polypropylen.svg>
9 Polymers and Liquid Crystals - Case Western Reserve University. 2009. <http://plc.cwru.edu/tutorial/enhanced/files/polymers/synth/synth.htm> 10 Wikipedia The Free Encyclopedia. Yikrazuul. 21 May 2008 <http://en.wikipedia.org/wiki/File:Polystyrene.svg>
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Figure 10. It also has strong van der Waals forces that hold the hydrocarbon chains together. The
synthesis reaction is shown in Figure 11; the formation of polystyrene is an addition polymerization
and the monomer is styrene (1).
Figure 11- Synthesis reaction of Polystyrene11
#7-Polylactic Acid
Figure 12—Structure of polylactic acid12
11 University of South Carolina. 11 July 2000 . <http://faculty.uscupstate.edu/llever/Polymer%20Resources/Synthesis.htm>
12 Wikipedia The Free Encyclopedia. Jü . 13 November 2012. <http://en.wikipedia.org/wiki/File:Polylactides_Formulae_V.1.svg>
Some common uses for polylactic
acid are tea bags and medical
implants in the form of screws,
pins, rods, and as a mesh. The
condensation polymerization
reaction is shown in Figure 13.
Polylactic acid is formed through
the condensation of these lactic
acid monomers and water is
formed as a byproduct (8).
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Figure 13- Synthesis Reaction of Polylactic Acid13
Thermosets are classified as polymers with covalent bonds linking the polymer chain together
and are unable to be re-processed if they are heated; however, thermoplastics are linear and branched
polymers which can be re-processed upon heating (1). All recyclable plastics are thermoplastics as
they must be able to be re-processed into different shapes if they are to be recycled.
In this experiment, four tests were performed in order to distinguish different recyclable
polymers according to their properties. The tap water buoyancy test will determine if the density of
the recyclable polymer is less than 1. The isopropyl alcohol buoyancy test will measure the relative
differences in buoyancy of the recyclable polymers that floated in water. The boiling water test will
demonstrate how the recyclable polymer responds to heat, and finally, the acetone test will show how
the recyclable polymer responds when dropped in acetone (CH3)2CO).
The goal of my project was to understand how the chemical structure of the polymers
contributes to their unique properties and use this information in order to identify three different
unknown recyclable polymers by running these four different tests.
13 Wikipedia The Free Encyclopedia. Rifleman 82. 21 September 2012. <http://en.wikipedia.org/wiki/File:PLA_from_lactic_acid_%26_lactide.png>
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Procedure:
The procedure to classify the known polymers and identify the unknown was taken from the
“Journal of Chemical Education” (5). I ran a series of tests on seven different kinds of plastic in
order to learn more about their physical properties in order to create a flow chart that would help me
identify my unknown plastics. This procedure required the seven kinds of plastics, scissors, two
beakers, two stirring rods, room temperature water, 70% isopropyl alcohol, a graduated cylinder,
acetone, 2 plastic pipets, and boiling water. Note that 6 of the 7 different plastics were already precut
for use. I cut a piece around 1 in2 of the #2 plastic from a milk jug provided.
The first test that I performed was the tap water test. I put the seven plastics in a beaker and
stirred vigorously to dislodge any bubbles, as bubbles tend to adhere to plastics. These bubbles
would change the apparent density as they would capture air making the plastic float (making it seem
less dense).
Next, I took the plastics that floated, “the floaters,” and put them in a solution of 20 ml of
70% isopropyl alcohol. The density of alcohol is less than the density of water and none of the
floaters floated in the alcohol solution (logically we knew that the other four plastics that did not float
in water would not float in the alcohol solution since alcohol is less dense than water). Then I
proceed to add tap water to the 70% isopropyl alcohol to determine at what point the three floaters
would float in the alcohol water mixture This was done by adding individual squirts of tap water
until the plastics started floating. These observations were recorded.
Next, I put the four plastics that did not float originally in the water into boiling water for
around 30 seconds and observed if there were any color/shape changes. I used tongs to remove the
four pieces one at a time in order to test their flexibility, size, and color. I recorded these observations
in my data table. Lastly, I put the two remaining plastics that had no observable shape/color change
in the boiling water into a small amount of acetone for one minute and recorded my results observing
any changes while in the solution.
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Results:
I recorded the following data during the tests in order to distinguish differences in the
properties of the different plastic types. Table 1.1 shows the results of the 7 known polymers and
Table 1.2 shows the results for the 3 unknown polymers. In order to correctly identify the 3 unknown
polymers I used Flowchart 1.1 that was created using Table 1.1 by examining the differences of the
plastic types through each step of the process.
Table 1.1-Results for the 7 Known Polymers
Plastic Type
(Name and
Number)
Sample’s
Appearance
Floats in
Water?
Sinkers—
Boiling Water
Results
Sinkers—
Acetone Test
Results
Floaters—
Alcohol Test
Results
Polyethylene
Terephthalate
(#1)
Clear, bumpy,
medium
firmness and
flexibility
No Curled up a
little, become
more flexible,
and shrunk
NA NA
High Density
Polyethylene
(#2)
Smooth,
medium
flexibility,
translucent,
white
Yes NA NA 9 squirts
Vinyl (#3) Clear, firm,
smooth
No No observable
shape/color
change
No observable
texture change
NA
Low Density
Polyethylene
(#4)
Soft, flaky, red,
light, very
flexible, flimsy
Yes NA NA 5 squirts
Polypropylene
(#5)
Clear, smooth,
firm
Yes NA NA 6 squirts
Polystyrene
(#6)
Blue, bumpy,
firm
No No observable
color/shape
change
“Melts in
acetone”—
curls up into a
ball and some
blue is “melted
off”
NA
Polylactic Acid
(#7)
Clear, smooth,
firm
No More flexible,
curled up a
little and
became cloudy
NA NA
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Table 1.2-Results for the 3 Unknown Polymers
Plastic
Type
(Name and
Number)
Sample’s
Appearance
Floats in
Water?
Sinkers—
Boiling
Water
Results
Sinkers—
Acetone
Test Results
Floaters—
Alcohol
Test Results
Prediction
based on
Results
Unknown 1 Smooth,
clear, and
firm
No No
observable
shape/color
change
No
observable
texture
change
NA #3
Unknown 2 Smooth,
clear,
medium
flexibility
No No
observable
color/shape
change
“Melts in
acetone”—
curls up into
a ball
NA #6
Unknown 3 Clear,
bumpy,
medium
firmness and
flexibility
No Curled up a
little,
become
more
flexible, and
shrunk
NA NA #1
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Flow Chart 1.1—Identifying Unknowns
After comparing the physical properties of my three unknowns to my data table of the known
plastics, I correctly identified my unknowns as plastic types 3, 6, and 1 (labeled in the graph) using
Flow Chart 1.1.
Does it Float in Water?
Yes No
How many squirts of
water until it floats in
alcohol?
5 6 9
Type 4! Type 5! Type 2!
Observable color/shape
change in boiling water?
No Yes
What occurs when it
is placed in acetone?
Blue dye “melted” off and
turns acetone blue. Plastic
curls up into a ball
Type 6!
No observable texture
change
Type 3!
Curls up a
little
shrinks
and
becomes
more
flexible
Type 1!
Curls a
little and
becomes
cloudy
Type 7!
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Discussion:
Each of my polymer identifications was quite simple based on my test results. I created a
flow chart in my lab notebook (Flow Chart 1.1) based on my table of test results (Table 1.1). I was
able to follow this flow chart and compare the observations of the known and unknowns to correctly
identify the polymers as polystyrene, vinyl, and polyethylene terephthalate.
The density of the plastics, which is a measure of mass per unit volume, accounts for their
floating behavior in water. The molecular structure of the polymers affects the observed floating
properties as a more compact (dense) polymer would have a higher density thus having a lower
tendency to float. For example, the polypropylene is less dense than HDPE and LDPE so it should
float before the HDPE and LDPE in the alcohol-water solution. In our experiment, the PP floated
after 6 squirts and the LDPE floated after 5 squirts (10). However, I believe this slight error was
caused due to a bubble that formed on the LDPE during the experiment causing it to appear less
dense than it actually is. Since the density of LDPE is 0.92-0.94 g/mL and the density of HDPE is
between 0.95-0.97 g/Ml, the approximate density of the alcohol-water solution would have to be
between 0.92-0.97 g/mL in order to identify LDPE from HDPE (5).
Moreover, none of the four plastics melted in the boiling water; the metals indeed softened but
they remained solids—they did not melt into liquids. The polyethylene terephthalate shrunk a little
and the polylactic acid became a little cloudy but their chemical structures remained the same (10).
The point of the boiling water test was to measure the glass transition of the polymers. The glass
transition temperature for polymers indicates the activation energy required for the molecular chains
to slide past each other, causing the polymer to become softer and more flexible (14). Although it
was a rough estimate, if the plastics were to soften in the boiling water, it would indicate the glass
transition (or the start of one) occurred at approximately 100 degrees Celsius.
The point of the acetone test was to measure the solubility of the polymers. As witnessed in
the experiment, polystyrene dissolves and shrinks in acetone. Polystyrene has strong van der Waals
forces caused by strong intermolecular polarities within the polymer, which define the solubility of
the polymers (15). These forces are what cause the polystyrene to dissolve in acetone. Also, any dye
on the polystyrene (in the case of our experiment, blue dye) “melts” off the plastic. It is also
interesting to note that when the polystyrene reacts with acetone it is a physical change, not a
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chemical one. The acetone does not melt the polystyrene but actually dissolves it. The polystyrene
shrinks but the chemical composition of the polystyrene does not change. This is a dissolution
reaction and dissolutions reactions are physical.
There are a couple of changes that I can now propose that would have made the identification
of the unknown plastics more definitive. The first change I would propose involves the squirt test.
When I added squirts of tap water to the 70% isopropyl solution, these squirts did not have a specific
volume. It bothered me that each squirt was different and there was no consistent method for
applying equal volume squirts to the solution. I would propose squirting into a beaker first and
making sure that the squirts have consistent volumes or using a form of pipette in which the volume
added could be identified. Moreover, the small beaker had plastics bumping together throughout the
experiment; next time I would use a larger beaker or smaller plastics so that this does not happen
again.
Conclusion:
As shown above, I was able to correctly identify the three unknown plastics using my
observations from the four different tests. I would not have been able to identify them only on
appearance alone. By using a flowchart and tables listing the observed phenomena of the polymers in
different environments, I was able to identify each of my unknown polymers using logical reasoning.
After more research regarding polymers, I was able to link the observed properties, including the
floating, boiling water, and acetone tests to their molecular structures. Thus, I have successfully
accomplished my original goal to understand how the chemical structure of a polymer affects its
physical properties and used this information to identify the unknown plastics.
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References for in lab parenthetical citations:
1-Chem 113 M Laboratory Manual. 2012-2013 by Joseph T. Keiser. Published by Hayden McNeil
pgs. 9-1-9-28.
2- What are Polymers?. Department of Materials Science and Engineering. University of Illinois
Urbana-Champaign. 22 February 2013. <http://matse1.matse.illinois.edu/polymers/ware.html>
3- Polymers. Virtual Textbook of Organic Chemistry. 1999 William Reusch. 21 February 2013.
<http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/polymers.htm>
4- Plastic Recycling Facts. Complete Recycling. 19 February 2013.
<http://www.completerecycling.com/resources/plastic-recycling>
5- Journal of Chemical Education” (JCE Classroom Activity #104) February 2010 pgs. 1-5
6- A Green Chemistry Module. Trudy A. Dickneider. Greening Across The Chemistry Curriculum.
21 February 2013. <http://academic.scranton.edu/faculty/cannm1/industrialche
mistry/industrialchemistrymodule.html>
7- Introduction to Plastics Science Teaching Resources. American Chemistry Council, Inc.. Retrieved
24 December 2012.
8- ‘Synthesis, Structures, Properties, Processing, and Applications’ by Rafael Auras, Loong-Tak Lim,
Susan E. M. Selke, Hideto Tsuji, ed. Poly(Lactic Acid). 1st edition, Wiley, 9 May 2011.
9- What Is Vinyl? Geno Jezek . 18 February 2013. <http://www.whatisvinyl.com>
10-Krupski, Adi. Chemistry 113M Notebook, pp. 18-20.
11- Polyethylene Terephthalate (PET, #1). CalRecycle.19 October 2009. 22 February 2013.
<http://www.calrecycle.ca.gov/Plastics/markets/PETEProfile.htm>
12- Polypropylene . Lenntech. 22 February 2013. <http://www.lenntech.com/polypropylene.htm>
13-‘Standard Practice for Coding Plastic Manufactures Articles for Resin Identification.’ ASTM
International. 24 January 2013.
14- Cowie, J. M. G. and Arrighi, V., Polymers: Chemistry and Physics of Modern Materials, 3rd Edn.
2007.
15- Van der Waals. Chaney, Allison. Princeton University. 19 February 2013.
<http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Van_der_Waals_force.html>
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References for foot noted pictures:
1- Wikipedia The Free Encyclopedia. Rohieb. 4 March 2007
<http://en.wikipedia.org/wiki/Polyethylene_terephthalate>
2- A Green Chemistry Module. Trudy A. Dickneider. Greening Across The Chemistry
Curriculum. 21 February 2013. <http://academic.scranton.edu/faculty/cannm1/industrialche
mistry/industrialchemistrymodule.html>
3- Wikipedia The Free Encyclopedia. Pngbot . 24 January 2007.
<http://en.wikipedia.org/wiki/File:Polyethylene-repeat-2D-flat.png>
4- Wikipedia The Free Encyclopedia. Mills, Ben . 2 February 2009
<http://en.wikipedia.org/wiki/Ethylene>
5- University of Buffalo. Todtenhagen, Kevin.2007.
<http://www.eng.buffalo.edu/Courses/ce435/Polyethylene/CE435Kevin.htm>
6- Wikipedia The Free Encyclopedia. Edgar181. 15 November 2007
<http://en.wikipedia.org/wiki/File:Vinyl_group.png>
7- Wikipedia The Free Encyclopedia. V8rik . 4 February 2007
http://en.wikipedia.org/wiki/File:VinylPolymers.png
8- Wikipedia The Free Encyclopedia. NEUROtiker . 27 March 2008
<http://en.wikipedia.org/wiki/File:Polypropylen.svg>
9- Polymers and Liquid Crystals - Case Western Reserve University. 2009.
<http://plc.cwru.edu/tutorial/enhanced/files/polymers/synth/synth.htm>
10- Wikipedia The Free Encyclopedia. Yikrazuul. 21 May 2008
<http://en.wikipedia.org/wiki/File:Polystyrene.svg>
11- University of South Carolina. 11 July 2000 .
<http://faculty.uscupstate.edu/llever/Polymer%20Resources/Synthesis.htm>
12- Wikipedia The Free Encyclopedia. Jü . 13 November 2012.
<http://en.wikipedia.org/wiki/File:Polylactides_Formulae_V.1.svg>
13- Wikipedia The Free Encyclopedia. Rifleman 82. 21 September 2012.
<http://en.wikipedia.org/wiki/File:PLA_from_lactic_acid_%26_lactide.png>