Polymers R Us Instructor’s Notes

Goal:  To investigate how chemical composition and morphology affect the properties of polymers.

Objectives:

1. Collect and analyze data on mechanical properties of polymers (stress, strain, tensile strength, Young’s modulus, toughness and elongation to break.).
2. Connect physical properties with chemical structure of polymers.
3. Critically evaluate the reasonableness of experimental and calculated results.
4. Use experimental data to choose a polymer for a particular application.
5. Use the Polymer Chemistry Resource Pages to discover additional characterization techniques for differentiating polymers.
6. Use statistical methods to analyze data collections.
7. Extension 1:  Collect and analyze density and percent crystallinity data.
8. Extension 2:  Collect and/or analyze data on thermal properties of polymers.

Key concepts and vocabulary:

Stress

Strain

Tensile strength

Stress-Strain curves

Mechanical properties

Young’s modulus

Elongation to break

Cross-linked or network polymers

Linear/non-linear or branched polymers

Isomerism

Tacticity (isotactic, syndiotactic, atactic)

Chain packing

Intermolecular forces

Plasticizers

Morphology

Strength

Stiffness

Toughness

Elasticity

Flexibility

Elastomer

Fiber

Plastic

Degree of Polymerization

Number and Weight-Average Molecular Weight

Chain entanglement

Threshold molecular weight

Extensions:

Differential scanning calorimetry

Density

Percent crystallinity

Transitions (glass transition, crystal melting transition)

Crystalline, semi-crystalline, amorphous

Glassy state and rubbery state

Week 1

During Week 1 students answer the demographic and assessment questions on-line and log on to the discussion board.  Each group is responsible for mailing a sample of a polymer film (Ziploc bag, grocery bag, or Saran wrap) to another group so that groups testing a particular polymer film will be using samples from the same source.  Students are presented previous experimental results from this project to critique on the discussion board.  They will need to consult the Polymers Resource Web Pages to get an understanding of the measured properties as well as how the properties are measured and/or calculated.  Students should note some of the following:

• toughness data is missing for some polymers, was there a problem determining the area under the stress-strain curve?
• there are large differences in % elongation for the same polymer, did they forget to multiply by 100?
• there are large differences in tensile strength and Young’s modulus for some polymers, were the same samples of the polymers used by different groups?  Did they do the experiment the same way?
• measuring the initial slope of the stress-strain curve will depend on the number of data points used, so measurement of Young’s modulus is subjective.
• only two measurements of some properties are given which makes it hard to get an average or sense of confidence in the data
• too many significant figures for the degree of accuracy of the measurements and the span of student results
• it is possible to have a % elongation greater than 100%

Also during this week, students are given a MathCAD document from a previous group that was used to determine the tensile properties in the results table.  Students are asked to annotate the document and check for errors in calculations, significant figures, and units.  Students should find some of the following errors or omissions:

• labeling of formulas, explanations of calculations
• incorrect number of significant figures
• the weight of the bucket was omitted from the calculation of stress
• details on how the thickness of the film was determined
• units of stress in stress-strain curve should be Pa
• in the calculation of Young’s modulus, the wrong values of stress were used for the chosen values of strain.  There are 10 values of stress but only 9 values of strain since the last screws added broke the strip.
• toughness calculation should include the initial point on the graph at 0 stress and 0 strain

Week 2

This week students do tensile testing on both crosswise and lengthwise strips of their plastic film.  The tensile testing can take several hours to complete.  When the strip begins to neck, the strip will stretch very slowly and it can take 20-30 minutes to obtain a stable reading for a given amount of stress.  Since the tensile tester is simple (JCE 1996, 73, 1062-1065), students can set up two testers and simultaneously test both strips.  It is important to carefully tape the ends of the strip at the marks to keep the strip from stretching outside the marks.  Place the tape perpendicular to the strip just at the mark and reinforce with more layers of tape.  Make sure the strip is securely taped to the long bolt.

One of the instructors obtained the following results:

Week 3

Students collect results from the tensile testing from all groups and obtain averages and standard deviations for measured properties.  Q-tests are applied to eliminate possible outliers.  Each group is assigned specific questions to post initial answers to on the discussion board.  Groups respond and critique each other’s answers on the discussion board.  Students are also asked to read a research paper on polyethylenes designed for use in prostheses.  The paper discusses the tensile properties introduced in this project and looks at how changing polymer morphology affects these properties.  Students will recognize the stress-strain curves in the paper and can use these to select the best polymer for this particular application.  The paper also presents % crystallinities of the polymers obtained by DSC.  Students can use the Polymer Resource web pages to help understand this technique.  Instructors could select different papers from the literature to illustrate other applied uses of polymers.