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

Krupski 14

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.

Krupski 15

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>