In the Laboratory
Teaching Polymer Science to Third-Year Undergraduate Chemistry Students
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Alan Jefferson* and David N. Phillips School of Applied Chemistry, Curtin University of Technology, P.O. Box U1987, Perth, Western Australia, Australia 6845
The need to teach polymer chemistry in undergraduate chemistry courses has been stated on several occasions. However, many chemistry departments do not teach a unit of polymer chemistry and completely neglect the broader aspects of polymer science. Wagener and Ford (1) expressed the view that learning about polymers after gaining employment in that industry often results in a lack of fundamental knowledge. An additional unit in polymer chemistry—or better still, the broader aspects of polymer science—at the undergraduate level would pull all the concepts of polymers together into sharp focus. Polymer chemistry is built upon solid principles, and an understanding of macromolecules is invaluable to a future chemical technologist. Marvel (2) has outlined an approach to the teaching of polymers, which points out that macromolecules are not the intractable mystery that they were when Staudinger started his work and that the teaching of polymer chemistry has come of age. The November 1981 issue of the Journal of Chemical Education (3) contained a series of papers on the state of the art in polymer chemistry that were presented at the ACS meeting in Atlanta. An outline is given of some important aspects in the ACS program—which, although labeled Polymer W Supplementary materials for this article are available on JCE Online at http://jchemed.chem.wisc.edu/Journal/issues/1999/ Feb/abs232.html.
*Corresponding author. Email:
[email protected].
Chemistry, also covered a range of useful topics in the wider realm of polymer science, as shown in Table 1. Reports of the ACS Polymers Course Committee recommended the inclusion of polymers in undergraduate physical chemistry courses (4, 5). Hardgrove et al. (6 ) also proposed a physical chemistry laboratory course to include more experiments on the preparation, kinetics and characterization of polymers. Mathias and Storey (7) have reported a polymer course given to gifted children in high school. It is based on the ACS computer course on polymer chemistry and includes the concepts of nomenclature, structure, molecular weight, and the kinetics of both radical chain growth and step-growth polymerization. More recently Maier (8) outlined a Polymer Science Pilot Program designed to encourage undergraduates, especially women, to further their education in the field of materials science and technology. Polymer science in the Applied Chemistry undergraduate course at Curtin University of Technology is presented in the second semester of the final year in the subject entitled Organic Chemistry 302. Background experience on topics such as free-radical mechanisms, addition to double bonds, and reaction kinetics is drawn from units in second-year organic and physical chemistry. The polymer science unit, although biased towards polymer chemistry, also includes physical/mechanical aspects of polymers and consists of 14 hours of lectures with six 4-hour laboratory sessions. The practical work is integrated into the organic chemistry labo-
Table 1. State of the Art: Polymer Chemistry (J. Chem. Educ. 1981, 58 [11]) Topic
Content
Author
Introduction to Polymer Chemistry
Polymer synthesis and the chemical and physical properties of polymers
F. W. Harris (p 837)
Chain Reaction Polymerization
Mechanism, thermodynamics, and kinetics of free-radical chain polymerization; polymerization processes; copolymerization; anionic, cationic, and coordination polymerization
J. E. McGrath (p 844)
Step-Growth Polymerization
Requirements for high molecular weight; kinetics; molecular weight distribution; three-dimensional network systems
J. K. Stille (p 862)
Molecular Weight and Molecular Weight Distribution
A description of molecular weight averages and molecular weight distribution; the characterization of MW and MWD
T. C. Ward (p 867)
Morphology of Crystalline Polymers
Reference to polymer single crystals and spherulitic (melt) crystallization
P. H. Geil (p 879)
Basic Rheological Behavior of Polymer Fluids and Polymer Melts
Basic terminology; effect of external variables of temperature, pressure, and time on viscous behavior; effect of internal variables; elastic effects in polymer melts
G. L. Wilkes (p 880)
Mechanical Properties of Polymers
Modulus temperature behavior of polymers; reference to the influence of time and temperature on modulus, creep, and stress relaxation; mechanical models
J. J. Aklonis (p 892)
Rubber Elasticity
Theories of rubber elasticity; stress–strain relationships, including the dependence of stress on elongation, temperature, and the polymeric network structure
J. E. Mark (p 988)
Evaluation of a Computer-Assisted Introductory Course on Polymer Chemistry
The demand for in-depth polymer chemistry courses from which users can choose specific areas of polymer chemistry to study
K. Chapman & J. Fleming (p 904)
Education Modules for Materials Science & Engineering
Opportunity for the use of a wide range of polymer teaching modules in materials science and engineering courses
P. H. Geil & S. H. Carr (p 908)
Macromolecules in Undergraduate Physical Chemistry
An introduction to the use of matrix methods in rationalizing configurationdependent physical properties of macromolecules
W. L. Mattice (p 911)
Block and Graft Polymers
Synthesis and properties of commercial block and graft copolymers
J. E. McGrath (p 914)
Organometallic Polymers
Unifying factors relating the field of organometallic polymers and small molecules
C. E. Carraher (p 921)
Interpretations of Polymer–Polymer Miscibility
Polymer–polymer phase behavior
O. Olabisi (p 944)
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In the Laboratory Table 2. Summar y of Content of 3rd-Year Organic Chemistr y Tuition Hours
Unit
Content
Physical Organic
Acidity, kinetics, solvent effects, organic mechanism types
Spectroscopic Methods Organic structure determination by UV, IR, NMR, and GC-MS
14 14
Natural Products
14 Mainly terpenes, alkaloids, and compounds related to petroleum geochemistry
Stereochemistry
Conformational analysis, stereochemical effects on reaction rates and equilibria
14
Polymers
See Tables 3 and 4
14 lecture 24 practical
Practical Organic
Synthesis and analysis
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ratory course and is designed to reinforce and even extend the lecture topics. It follows that only prime subject matter can be included in this course, which is intended to give students a grasp of the basic principles of polymer science. This is sufficient to give our graduates an advantage over most other chemistry graduates in Australian universities, where fewer than 10% of chemistry departments teach polymer chemistry or polymer science at the undergraduate level. While most chemistry educators now respond positively to the suggestion that polymer chemistry should be incorporated in their curricula, a key issue that confronts them is what subject matter should be deleted to enable its inclusion. A summary of the content of the third-year undergraduate organic chemistry course is shown in Table 2. It has been necessary to reduce some of the more traditional academic topics such as alicyclic, heterocyclic, flavonoid, and carotenoid chemistry to accommodate macromolecular chemistry. This has scarcely detracted from the organic component of the course and is more than compensated in terms of polymer reaction mechanisms, aspects of polymer stereochemistry, and an extension of structure–property relationships applied to very large molecules. The teaching of polymer science fills a void in the students’ knowledge
about one of the most exciting branches of chemistry. They appreciate what makes polymers so special and can relate to the interdisciplinary nature of polymer science. Only about 20% of the third-year content of the organic course has been deleted to incorporate the polymer chemistry, a goal that other course coordinators could easily achieve. Advanced organic reaction mechanisms, spectroscopy, stereochemistry, and natural products chemistry are given adequate coverage, each being allotted an equivalent time to the polymer section. In addition, a 5-year double-degree program in chemistry and chemical engineering has been established at Curtin University and this course has attracted students with a higher academic standing than those who take the Applied Chemistry degree. The Chemical Engineering Course subcommittee of the ACS has commented that “the matter of integrating polymer topics with standard courses is not a happy situation” (9). We fully integrated these double-degree students into the lecture course and laboratory work in polymer science. If extra time were to become available, then inorganic topics such as polysiloxanes, phosphazenes, and heterogeneous polymerization using Ziegler–Natta catalysts could be considered for inclusion in the course. Although any of the excellent textbooks on polymer science (10) or polymer chemistry (11) can be recommended for this unit, reference 3 is still most appropriate as a backup reference. Our lecture course material is presented in an internal Curtin University publication entitled Organic Chemistry 302—Macromolecular Chemistry, and the practical work has been combined with other organic chemistry experiments in the Organic Chemistry 301/302 Laboratory Manual. Polymer Chemistry Lecture Program An attempt has been made to change the content of an undergraduate degree course to include polymer science and make it more relevant to a technological university. This is the antithesis of many university chemistry courses, whose content has departed from a level of usefulness required for the next millennium. The lecture program for the polymer chemistry is outlined in Table 3. Some extra references have
Table 3. Polymer Chemistr y Lecture Program Week Topic (ref)
Content (ref)
1–2
Introduction to polymer chemistry
Classification of polymers, nomenclature (12), selected thermoplastic (13) and thermoset (14,15) polymers
3–4
Step-growth polymerization
Examples of step-growth polymers, relationship between degree of polymerization and extent of reaction (Carothers equation), effect of impurities and stoichiometric imbalance, kinetics of step-growth polymerization, molecular weight distribution, gelation
5
Molecular weight and molecular weight distribution
Description of molecular weight averages and molecular weight distribution and introduction to their characterization
6–7
Chain-growth Polymerization, free radical addition polymerization
Initiator systems for free-radical chain-growth polymerization, steady-state kinetics, kinetic chain length, autoacceleration, chain transfer reactions, inhibitors and retarders, and bulk, solution, suspension, and emulsion polymerization processes
8–9
Ionic polymerization (16 )
Reactivity of monomers, cationic polymerization (kinetic scheme), effect of temperature, anionic polymerization, “living” polymers
10
Copolymerization
The copolymer equation, monomer reactivity ratios
11
Morphology of crystalline polymers
Polyethylene and polypropylene, factors affecting crystallinity and TM, measurement of crystallinity, microstructure of solid polymers, spherulites
12
The amorphous state
Property changes at the glass transition temperature (17, 18), measurement of Tg , effect of molecular structure on Tg, commercial importance of Tg
13
Mechanical properties of polymers
Modulus temperature behavior of polymers, influence of time and temperature on modulus, creep and stress relaxation, mechanical models, hardness and impact testing
14
Weathering and degradation
Thermal, thermal/oxidative, photolytic, and photolytic/oxidative degradation mechanisms, biodegradable polymers, prevention of UV/oxidative degradation
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In the Laboratory
been included in the table on specific topics for which it is considered appropriate to extend the subject matter. A number of primary goals may be achieved in this concentrated polymer chemistry program. The course aims to introduce students to free-radical and ionic reactions that are used to prepare polymers, and to acquaint them with the structural parameters displayed by macromolecules. Because polymers distinguish themselves from lower-molecular-weight materials by the presence of a repeat or repeating units, an introduction to molecular weight averages and molecular weight distribution is of paramount importance (3). The physical properties of a polymer depend not only upon its general chemical composition but also on subtle effects such as the morphology of semicrystalline polymers. A useful aspect of the course is to relate the mechanical properties of polymers to the modulus-temperature behavior of macromolecules and to describe creep in terms of molecular models. Polymer Chemistry Laboratory Program The polymer chemistry section of the 3rd-year organic chemistry laboratory course is shown in Table 4 and reflects about 20% of the content. The full benefit of teaching this polymer course is realized by posing a series of post-laboratory questions for each exercise, which are included in the overall assessment. The pedagogical goals of the exercises are set out below. Exercise 1 illustrates a method for the determination of the average molecular weight of polymers using viscometry, similar to that described by Collins et al. (19). The exercise can also be used to introduce students to the concepts of chain length, branching, and conformation in polymers. Normally, freedom of rotation will allow each bond to assume a different rotational angle and the polymer will be highly kinked. This causes a tangled shape like a ball of wool and results in a much shorter end-to-end distance. The root-mean-square length of a polymer, representing the distance between the ends of the chain, may be calculated from the number of units and the length of a C–C bond. This exercise is also an opportunity to introduce students to the concept of θ conditions using the analogy of ideal gas behavior (20). Exercise 2 examines the reaction kinetics of the free radical chain-growth polymerization of styrene using azobis(isobutyro)nitrile as an initiator. Dilatometry is used to follow the course of the reaction similar to the method described by McCaffery (21). The calculations are conveniently set out for students on a Microsoft Excel spreadsheet and SI units are used throughout. The results are instructive because they
highlight the values of the various kinetic parameters in a freeradical chain-growth polymerization, and the relationships between them. In Exercise 3, emulsion polymerization is carried out by studying the preparation of polyvinyl acetate. The preparation demonstrates the role of potassium peroxydisulfate as a redox catalyst and sodium lauryl sulfate as a surfactant in emulsion polymerization. The polyvinyl acetate is hydrolyzed to polyvinyl alcohol and infrared analysis is carried out on thin films of both polymers. The students may be quizzed on aspects of emulsion polymerization, such as the reasons why the vinyl acetate monomer needs to be distilled before use. Questions are asked about the effect of surfactant concentration on the degree of polymerization and the locus of initiation. Exercise 4 gives students the opportunity to experience the range of methods and a demonstration of attachments whereby samples of polymers may be studied by infrared spectroscopy. Transmission spectroscopy is suitable for unfilled thermoplastic polymers, such as the polyvinyl acetate and polyvinyl alcohol films from the emulsion polymerization exercise. Interesting extensions to this exercise are to appreciate how infrared spectroscopy can be used to differentiate polymers such as polyethylene, polypropylene, and polystyrene, and how the technique can be used to show the presence of diisooctyl phthalate plasticizer in polyvinyl chloride. Mathias et al. (22) have described the use of Fourier transform infrared spectroscopy for thin films of copolymers. More sophisticated reflectance techniques can be demonstrated using a combined TGA–FTIR experiment for the analysis of ethylene-vinyl acetate copolymers (23). Other useful reference sources for exercise 4 include the books by Hummell (24 ) and Nishikida et al. (25). Exercise 5 demonstrates simple, inexpensive, but effective methods of identifying polymers through combustion and density. Blumberg (26 ) has previously outlined the usefulness of combustion and density techniques in the identification of polymers. These methods reinforce the student’s knowledge of common polymers and rubbers and assist in relating properties to molecular composition. Levy and Wamprei (27 ) have reported the use of pyrolysis–gas chromatography for the identification and differentiation of synthetic polymers. The polymer sample may simply be heated in an ignition tube if a Curie point pyrolysis unit is not available. The volatile pyrolysate is collected and analyzed by capillary column gas chromatography. The characteristic “fingerprint” chromatogram of the pyrolysate provides a wealth of information for the identification of the material and may be related to the mechanism of thermal degradation of the polymer.
Table 4. Polymer Chemistr y Laborator y Program Exercise Topic (ref) 1
Molecular weight by viscometry (19)
Content Determination of the average molecular weight of polystyrene
2
Reaction kinetics by dilatometry (21)
Polymerization of styrene using azobis(isobutyro)nitrile in a dilatometer
3
Emulsion polymerization (19)
The emulsion polymerization of vinyl acetate
4
Analysis of polymers by infrared spectroscopy (22, 23)
Introduction to transmission, cast film, and attenuated total reflectance techniques
5
Identification of polymers by combustion (24) and pyrolysis (25)
Simple, instructive methods to distinguish common polymers
6
Mechanical properties of polymers
Creep behavior of polypropylene and the mechanical properties of rubber
N OTE: Copies of the full laboratory exercises may be obtained by contacting the corresponding author. Email:
[email protected]. An outline of the exercises is available on JCE Online.W
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In the Laboratory
The mechanical properties of polymers examined in exercise 6 are the creep behavior of polypropylene (or polyethylene) and the elasticity of rubber. The tensile creep curve for polypropylene is obtained by monitoring the percentage extension versus time under set stress. Creep recovery can also be demonstrated by removing the weight and continuing the plot of percentage extension versus time. Aklonis (3) has explained how mechanical models, comprising spring and dashpot elements, are used to mimic the viscoelastic properties of the polymer. The high degree of elasticity of the rubberlike state under comparatively small stress may be demonstrated by monitoring stress against the extension ratio and generating the curve for the elastomer. The number of polymer chains per cubic meter and the mean molar mass of the rubber can be calculated as described by Ward and Hadley (28). Conclusions It is the exception, rather than the rule, that polymer chemistry is offered as a specific unit in undergraduate chemistry degree courses. At Curtin University of Technology, a dedicated unit in polymer science forms an integral part of the undergraduate chemistry degree course and is also taken by students who take an Applied Chemistry/Chemical Engineering double degree. The basic principles of polymer chemistry have been incorporated into a 14-week single-semester program that also includes other broader aspects of polymer science, where an attempt is made to relate the physical and mechanical properties of polymeric materials to molecular structure. Six carefully selected polymer science experiments are included in the organic chemistry laboratory sessions. These laboratory exercises also include searching questions, aimed at challenging the student’s ability to carry out a literature search on polymer topics. A knowledge of the principles of polymer science is perceived to be essential for graduates whose future career is certain to bring them into contact with a broad range of polymeric materials. Any undergraduate chemistry course that precludes polymer chemistry and does not attempt to address and explain the principles of polymer science is lacking in relevant educational content as we approach the 21st century. Literature Cited 1. Wagener, K.; Ford, W. T. ChemTech 1984, 14, 721.
2. Marvel, C. S. ChemTech. 1986, 16, 136–137. 3. Various authors. State-of-the-Art: Polymer Chemistry; J. Chem. Educ. 1981, 58, 836–950. 4. Report of the Inorganic Core Course Subcommittee of the ACS Polymer Education Committee. J. Chem. Educ. 1984, 61, 230– 235. 5. Report of the Physical Chemistry Core Course Subcommittee of the ACS Polymer Education Committee. Part 1; J. Chem. Educ. 1985, 62, 780–786. Part 2; ibid., 1030–1036. 6. Hardgrove, G. D.; Tarr, D. A.; Miessler, G. L. J. Chem. Educ. 1990, 67, 979–981. 7. Mathias, L. J.; Storey, R. F. J. Chem. Educ. 1986, 63, 424–426. 8. Maier, M. L. J. Chem. Educ. 1996, 73, 643–646. 9. Report of the Chemical Engineering Core Course Subcommittee of the ACS Polymer Education Committee. J. Chem. Educ. 1985, 62, 1079–1081. 10. Cowie, J. M. G. Polymers: Chemistry & Physics of Modern Materials, 2nd ed.; Blackie: London, 1991. 11. Seymour, R. B.; Carraher, C. B. Polymer Chemistry: An Introduction, 3rd ed.; Dekker: New York, 1992. 12. Carraher, C. E.; Hess, G.; Sperling, L. H. J. Chem. Educ. 1987, 64, 37–39. 13. Chandra, M.; Roy, S. K. Plastics Technology Handbook; Dekker: New York, 1993. 14. Dewprashad, B.; Eisenbraun, E. J. J. Chem. Educ. 1994, 71, 290– 294. 15. Peng, W.; Reidl, B. J. Chem. Educ. 1995, 72, 587–592. 16. Swarc, M. Ionic Polymerisation Fundamentals; Carl Hanser: Munich, 1996. 17. Sperling, L. H. J. Chem. Educ. 1982, 59, 942–943. 18. Beck, K. R.; Korsmeyer, R.; Kunz, R. J. J. Chem. Educ. 1984, 61, 668–670. 19. Collins, E. A.; Bares, J.; Billmeyer, F. W. Experiments in Polymer Science; Wiley: New York, 1973. 20. Florey, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. 21. McCaffery, E. M. Laboratory Preparation for Macromolecular Chemistry; McGraw-Hill: New York, 1970. 22. Mathias, L. J.; Hankins, M. G.; Grubb, T. L.; Bertolucci, C. M.; Muthiah, J. J. Chem. Educ. 1992, 69, A217–A219. 23. Williams, K. R. J. Chem. Educ. 1994, 71, A195–A198. 24. Hummell, D. O. Atlas of Polymers and Plastics Analysis, 3rd ed.; Hanser: Munich–Vienna, 1991. 25. Nishikida, K.; Nishio, E.; Hannah, R. W. Selected Applications of Modern FT–IR Techniques; Harwood Academic: Philadelphia, 1996. 26. Blumberg, A. A. J. Chem. Educ. 1993, 70, 399–403. 27. Levy, E. J.; Wamprei, T. P. J. Chem. Educ. 1986, 63, A64–A68. 28. Ward, I. M.; Hadley, D. W. An Introduction to the Mechanical Properties of Solid Polymers; Wiley: New York, 1993.
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