An Investigation of
Polymers
Adi Krupski
February 23rd, 2013
Chemistry 113 M-Experimental Chemistry
Section 101
TA: Nick Dunn
Krupski 2
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 monomer
units in long chains, we have developed technology in order to manipulate their characteristics to
better suit our needs; we are able to modify polymers to make them harder, stronger, more
flexible and more lightweight, to name a few. 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. They also reported that plastic
materials take hundreds of years to break down in a landfill (4). We cannot let these plastics sit
in landfills for hundreds of years while we could be recycling and reusing them for 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
1
http://en.wikipedia.org/wiki/File:Polyethylene_terephthalate.svg
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Polyethylene terephthalate (commonly known as PET) is formed through condensation
polymerization where two molecular species react with each other. During these reactions, the
molecules join together while losing other small molecules as byproducts. The formation and
release of these simple molecules as byproducts is a key component of the condensation
polymerization process. These byproducts are often water or methanol. 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 arrow in the reaction) (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
The monomer of polyethylene is ethylene. 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 http://academic.scranton.edu/faculty/cannm1/industrialche mistry/industrialchemistrymodule.html
3
http://en.wikipedia.org/wiki/File:Polyethylene-repeat-2D-flat.png
Krupski 4
Figure 4-Structure of ethylene4
The mass density of high-density polyethylene can range from
0.95-0.97 g/cm3. This is what differentiates high density polyethylene to
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
5
http://en.wikipedia.org/wiki/Ethylene
http://www.eng.buffalo.edu/Courses/ce435/Polyethylene/CE435Kevin.htm
Krupski 5
#3-Vinyl
Figure 6- Structure of vinyl chloride6
dsdsdsdsd
The structure of vinyl chloride is shown in figure 6. A common application of vinyl is in the
making of chessboards, as well as flooring and siding 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” 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 above), only less dense
6
http://en.wikipedia.org/wiki/File:Vinyl_group.png
7 http://en.wikipedia.org/wiki/File:VinylPolymers.png
Krupski 6
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.
Common uses include protective packaging (for example, CD and
DVD cases), containers, and lids (7). Polystyrene’s structure is hard
and brittle, and is highly flammable yet not very chemically reactive.
The structure is shown in figure 10. It also has strong van der Waals
8 http://en.wikipedia.org/wiki/File:Polypropylen.svg
9
http://plc.cwru.edu/tutorial/enhanced/files/polymers/synth/synth.htm
10
http://en.wikipedia.org/wiki/File:Polystyrene.svg
Krupski 7
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
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 the byproduct
is water (8).
11
http://faculty.uscupstate.edu/llever/Polymer%20Resources/Synthesis.htm
12 http://en.wikipedia.org/wiki/File:Polylactides_Formulae_V.1.svg
Krupski 8
Figure 13- Synthesis Reaction of Polylactic Acid13
Thermosets are classified as polymers with covalent bonds linking the polymer chain
together and are unable to 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.
Four tests will be performed during the following experiment in order to distinguish
different recyclable polymers according to their properties. The tap water buoyancy test will
measure if the density of the recyclable polymer is less than 1. The isopropyl alcohol buoyancy
test will measure the relative differences in buoyancy to the recyclable polymers that floated in
water. The boiling water will test how the recyclable polymer responds to heat and the acetone
test will test 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 properties and use this information in order to identify three different
unknown recyclable polymers by running different tests.
13
http://en.wikipedia.org/wiki/File:PLA_from_lactic_acid_%26_lactide.png
Krupski 9
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. The following steps 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 to 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 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). So,
to see how many squirts of tap water it would take for the plastics to float, I put the three floaters
into the 20ml of 70% isopropyl alcohol and added individual squirts of tap water to see how
many squirts it would take for the plastics to start 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
onto 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 is shown on the next
page.
Table 1.1-Results for the 7 Known Polymers
Sample’s
Appearance
Floats in
Water?
Clear, bumpy,
medium
firmness and
flexibility
Smooth,
medium
flexibility,
translucent,
white
Clear, firm,
smooth
No
Low Density
Polyethylene
(#4)
Polypropylene
(#5)
Polystyrene
(#6)
Soft, flaky, red,
light, very
flexible, flimsy
Clear, smooth,
firm
Blue, bumpy,
firm
Polylactic Acid
(#7)
Clear, smooth,
firm
Plastic Type
(Name and
Number)
Polyethylene
Terephthalate
(#1)
High Density
Polyethylene
(#2)
Vinyl (#3)
Sinkers—
Boiling Water
Results
Curled up a
little, become
more flexible,
and shrunk
NA
Sinkers—
Acetone Test
Results
NA
Floaters—
Alcohol Test
Results
NA
NA
9 squirts
No observable
texture change
NA
Yes
No observable
shape/color
change
NA
NA
5 squirts
Yes
NA
NA
6 squirts
No
No observable
color/shape
change
NA
No
More flexible,
curled up a
little and
became cloudy
“Melts in
acetone”—
curls up into a
ball and some
blue is “melted
off”
NA
Yes
No
NA
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Table 1.2-Results for the 3 Unknown Polymers
Plastic
Type
(Name and
Number)
Unknown 1
Sample’s
Appearance
Floats in
Water?
Smooth,
clear, and
firm
No
Unknown 2
Smooth,
clear,
medium
flexibility
Clear,
bumpy,
medium
firmness and
flexibility
No
Unknown 3
No
Sinkers—
Sinkers—
Floaters— Prediction
Boiling
Acetone
Alcohol
based on
Water
Test Results Test Results
Results
Results
No
No
NA
#3
observable
observable
shape/color
texture
change
change
No
“Melts in
NA
#6
observable
acetone”—
color/shape curls up into
change
a ball
Curled up a
NA
NA
#1
little,
become
more
flexible, and
shrunk
Krupski 12
Flow Chart 1.1—Identifying Unknowns
Does it Float in Water?
Yes
No
How many squirts of
water until it floats in
alcohol?
5
Type 4!
Observable color/shape
change in boiling water?
No
6
9
Type 5!
Type 2!
What occurs when it
is placed in acetone?
Blue dye “melted” off and
turns acetone blue. Plastic
curls up into a ball
Yes
Curls up a
little
shrinks
and
becomes
more
flexible
No observable texture
change
Curls a
little and
becomes
cloudy
Type 7!
Type 6!
Type 3!
Type 1!
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.
Krupski 13
Discussion:
Each of my polymer identifications were 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, which is a measure of mass per unit volume, of the plastics account 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 the LDPE may have caught 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 alcoholwater 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 in acetone and shrinks. 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
Krupski 14
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 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 changes that I would 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. Moreover, the small
beaker had plastics bumping in the way of each other while they together; next time I would
have either used 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 researching more regarding polymers, I was able to link the observed
properties, including the floating, boiling water, and acetone tests to their molecular structures.
Thus, I had successfully accomplished my goal to understand how the chemical structure in a
polymer effect its physical properties and used this information to identify the unknown plastics.
Krupski 15
References for in lab paranthetical 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>
Krupski 16
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>