Commentary pubs.acs.org/jchemeduc
Inclusion of Synthetic Polymers within the Curriculum of the ACS Certified Undergraduate Degree Laura L. Kosbar† and Thomas J. Wenzel*,‡ †
IBM T. J. Watson Research Center, Yorktown Heights, New York 10547, United States Department of Chemistry, Bates College, Lewiston, Maine 04240, United States
‡
ABSTRACT: The 2015 Guidelines approved by the Committee on Professional Training of the American Chemical Society require that the curriculum for the certified undergraduate degree include the principles that govern macromolecular, supramolecular, mesoscale, and nanoscale systems. The rationale for this new requirement recognizes that the synthesis, analysis, and physical properties of small molecules give an incomplete picture of the higher-order interactions that occur in these systems. Programs can satisfy this requirement through an in-depth course on these systems or distributed coverage over two or more required courses. It is anticipated that the majority of undergraduate degree programs will include at least some coverage of synthetic polymers, but there may be concerns in many departments on how best to implement such coverage. Suggested topics that illustrate important aspects of the chemistry and properties of synthetic polymers in the curriculum are described. KEYWORDS: Curriculum, Polymer Chemistry, Second-Year Undergraduate, Upper-Division Undergraduate, Polymerization
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Programs have considerable flexibility in the manner in which they fulfill the MSN requirement. One possibility is to offer an in-depth course on materials and/or polymers. If such a course were the only option for a student to meet the MSN requirement, then that in-depth course would be required for the ACS certified degree. A second approach to meeting the MSN requirement is to distribute coverage among two or more courses required for the certified degree. If the MSN requirement is met through a distributed model, coverage should constitute the equivalent of approximately one-fourth of a standard semester course, and the coverage of any one largescale system can be counted for up to half of the required content. It is anticipated that for many if not most departments, the distributed model may be a preferable way to meet the MSN requirement. It avoids the addition of a new required course from the perspective of both student and faculty schedules. In fact, CPT’s review of the first periodic report forms since the implementation of the 2015 guidelines indicates that the majority of schools are implementing a distributed approach. Whether departments have a dedicated course or a distributed approach, coverage of polymers is generally one of the core components. One obvious reason for this is the ability to easily incorporate aspects of synthetic polymers into organic, analytical, physical, and even inorganic chemistry class and laboratory along with the coverage of biological macromolecules in the required biochemistry course(s). While MSN content may easily be incorporated into any of the standard subdisciplines, it is not required that the exposure span every subdiscipline. CPT anticipates that coverage of synthetic
INTRODUCTION Traditional undergraduate chemistry curricula have historically focused overwhelmingly on the synthesis, characterization, and properties of small discrete molecules. However, a large portion of naturally occurring as well as human-made materials include large-scale chemical systemssuch as synthetic polymers, biological macromolecules, and supramolecular, nano-, and mesoscale materialsthat are not adequately described from the perspective of small molecules. In some of these materials, the boundaries between discrete molecular and bulk properties are blurred. This may impact the ways that we conceptualize and characterize their structure as well as having real-world implications for their chemical and physical properties. In otherssuch as macromoleculesthough the molecular boundaries may be clear, the increased influence of noncovalent and intramolecular interactions and the physical and entropic constraints inherent in long molecular chains produce properties that are not effectively predicted from the more ideal behavior of small molecules. Recognizing these differences between small molecules and larger systems is crucial to understanding and predicting the behavior of materials that are pervasive in our society, the biosphere, and modern chemistry. Because of the importance of these large-scale chemical systems and the observation of the Committee on Professional Training (CPT) that many programs had limited coverage of this area, the macromolecular, supramolecular, mesoscale, and nanoscale (MSN) requirement was added to the 2015 ACS Guidelines.1 The new requirement states that the curriculum for certified undergraduates must include coverage of the principles that govern MSN systems. Coverage of at least two of the following is required: synthetic polymers, biological macromolecules, supramolecular aggregates, and meso- or nanoscale materials. This instruction must include content on the unique properties of large-scale systems compared with small molecules, including the preparation, characterization, and physical properties of large systems. © XXXX American Chemical Society and Division of Chemical Education, Inc.
Special Issue: Polymer Concepts across the Curriculum Received: November 28, 2016 Revised: March 7, 2017
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drimer, or ladder polymers) as well as how these variations can significantly impact the properties and function of the polymer. Unlike small molecules, which exist in solid, liquid, and gaseous states, polymers can exist in a variety of additional states, including glass, rubber, and gel, and may have multiple states (such as glassy and microcrystalline) occurring in the same sample. For example, thermoplastic polymers, such as the commonly recycled polymers poly(ethylene terephthalate) (PETE), high- and low-density polyethylene (HDPE and LDPE, respectively), poly(vinyl chloride) (PVC), polypropylene (PP), and polystyrene (PS), can be molded by heating the glassy or microcrystalline polymer to achieve a viscous liquid a process that can be repeated when recycling polymers. Crosslinking may convert monomers, oligomers, or high-molecularmass polymers into glassy or rubbery solids that will not melt and cannot be reformed but may potentially be swollen into a gel with an appropriate solvent. Finally, it is useful for students to learn some of the nomenclature conventions of polymers.
polymers will continue to be a core component of many departments’ approach to meeting the MSN requirement. Complying with the MSN requirement has ramifications for the way in which topics such as synthetic polymers are incorporated into the curriculum. For example, it is not enough to merely mention that a given organic reaction that can be used to synthesize a low-molecular-mass amide can similarly be used to synthesize a poly(amide). Discussion of polymer synthesis should address aspects of chemical synthesis that differ between small molecules and polymers (e.g., the necessity of very high yields and associated modifications of the reaction conditions for polymer synthesis). Similarly, a lab in which students use size-exclusion chromatography to separate polymers by molar mass should be enhanced with information about how the quality of solvent−polymer interactions influences the swollen size (molecular volume) of chains relative to their molecular mass and how this can influence the size-based chromatographic separations of polymers in ways that are unique compared with small molecules.
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Molecular Weight (MW) and Molecular Weight Distribution (MWD)
EXAMPLES OF POLYMER TOPICS FOR THE CURRICULUM Incorporating new content into an existing or new curriculum offering, especially for topics outside one’s area of expertise, can be a challenging task. Therefore, a list of overarching topics including concepts related to synthetic polymers that are beneficial for all chemists to know are included in Table 1 along
The molecular masses of small molecules are easy to define and generally constant, excluding the influence of isotopes. That is not the case for synthetic polymers. Any polymer sample contains a collection of polymer chains with varying lengths. The properties of that sample are impacted by both the average length and the distribution of lengths of the polymer chains. Thus, the molecular mass of polymers (traditionally termed the molecular weight) refers to the average MW of the sample, and there are multiple ways in which that average is defined, including the number-average (Mn), weight-average (Mw), and viscosity-average (Mv) molecular masses. The distribution of molecular masses is generally defined as Mw/Mn. Physical properties such as the viscosity or light-scattering properties of a polymer are more impacted by the higher-MW chains (emphasized by Mw), while colligative properties are more influenced by the number of chains (emphasized by Mn). Both the type of polymerization reaction and the reaction conditions may impact the distribution of molecular masses in a polymer sample, which in turn impacts the properties of the material. Polymers are one of the best examples of the adage “length matters”, as the consequences of intramolecular interactions and intermolecular entanglements are among the key things that distinguish polymers from small molecules. These characteristics influence the strength (especially the strengthto-weight ratio), toughness, viscosity (whether pure polymer or in solution), and solubility of polymers. Polymers also have a “critical entanglement length”, as short polymers that do not get physically entangled may not exhibit some of the unique rheological and physical properties typical of long, entangled polymer chains.
Table 1. Topics That Are Beneficial for Chemists To Know about Polymers and Subdisciplinary Areas Where These Topics Are Readily Covered (A = Analytical, I = Inorganic, O = Organic, P = Physical) 1 2 3 4 5 6 7 8 9
Anatomy of a polymer (A, I, O, P) Molecular weight (MW) and molecular weight distribution (MWD) (A, I, O, P) Polymer synthesis and kinetics (O, I, P) Cross-linking and its implications (O, P) Polymer “phases” and phase transitions (A, P) Instrumental techniques used to evaluate polymer properties (A, P) Rheology and non-Newtonian polymer properties (P) Phase separation with polymers (P) Interactions with small molecules: dissolution, diffusion, swelling, and plasticizing (A, P)
with the subdisciplinary fields where aspects of these topics can be readily covered. While an in-depth course in polymer chemistry might cover all of these topics, the coverage needed to meet the distributed MSN requirement could involve a much smaller subset of topics. For all MSN topics, it is important to remember that emphasis should be placed on highlighting the distinctions between large- and small-scale molecular systems. A few examples of the polymer topics suggested in Table 1 are included below.
Polymer Synthesis and Kinetics
While many reactions used to synthesize polymers are similar or identical to those used to synthesize small molecules, there are unique aspects of polymerization reactions. An important consideration in all synthetic chemistry is product yield. While a 90−95% yield might be quite acceptable for the synthesis of a small molecule, for condensation polymerization reactions a 90% yield would result in an average chain length of only about 10 monomersmolecules that would not have any of the characteristics of a polymer. A 99% yield would average about 100 monomers and 99.9% about 1000 monomers. Aspects of how condensation reactions can be pushed to their limits to
Anatomy of a Polymer
Any new topic requires a definition of terms and how they will be used. For synthetic polymers, this generally starts with the idea that a polymer is a large chain of covalently bonded repeating monomeric units as well as a definition of “large” most polymers have molecular masses between 103 and 108. Important variations include incorporation of more than one type of monomer (to form co- or terpolymers) and different structures (linear, branched, cross-linked/network, star, denB
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occur over a range of temperatures. The glass transition temperature (Tg) is the center of the temperature range over which segments of polymer chains achieve enough mobility to move, such that the polymer transitions from a hard, glassy material to a soft, highly viscous liquid or rubbery solid. Linear, stereoregular polymers may have a (micro)crystalline phasesmall crystalline intra- or intermolecular regions that act as “cross-links” within larger amorphous regions. The degree of crystallinity is highly dependent on the synthesis conditions. The degree of crystallinity within the polymer is responsible for the differences in both density and physical properties of HDPE versus LDPE. Physical manifestations of crystalline polymers relate to properties such as the increased opacity and toughness of HDPE.
achieve high-MW polymers (e.g., high temperatures and low pressures to remove small-molecule reaction products) or how the reactions can be controlled to produce a specific MW (by using a smalloften less than 1%stoichiometric imbalance of the reactants) provide students with a basis for better understanding both polymer and small-molecule synthesis. Condensation reactions inherently produce polymers with relatively large MWD. Addition polymerizations (e.g., radical, cationic, and anionic reactions) can be used to discuss other ways in which slight differences in reaction mechanisms can produce significant impacts on the MW, MWD, and properties of the resulting polymers. For example, high-MW polyethylene can be synthesized using radical, ionic, and coordination polymerizations. Radical polymerization will produce branched, lowdensity polymers that lack stereoregularity and have much higher MWD as a result of the various chain termination options. By comparison, ionic and coordination polymerization can produce largely unbranched, high-density, extremely stereoregular polymers with narrow MWD, resulting in the different properties of HDPE and LDPE. The concept of a “living” polymerization, where the ability to terminate the polymer chain is removed, also allows discussion of block copolymers (with long sections grown from potentially very different monomers within each chain) and the associated phase separation of the different blocks within a single chain. Derivation of the reaction kinetics for step-growth polymerizations can be used to predict both the extent of reaction and broad MWD typical of these reactions. The kinetics of chaingrowth polymerizations allow discussion of the influence of the rates of chain initiation, growth, and termination on polymer size and MWD.
Instrumental Techniques Used To Evaluate Polymer Properties
A variety of common chemical techniques exist to measure the molecular masses and physical properties of polymers. Sizeexclusion chromatography (SEC) (e.g., gel-permeation chromatography (GPC)) can be used to measure both Mn and Mw. Light scattering, viscometry, and end-group analysis are other common techniques used to measure molecular mass in the laboratory. Techniques for measuring glass and crystalline transition temperatures (differential scanning calorimetry (DSC)), chemical degradation temperatures (thermogravimetric analysis (TGA)), the coefficient of thermal expansion (thermomechanical analysis (TMA)), viscosity and flow properties (rheometers), and properties such as stress−strain and fracture toughness (dynamic mechanical analysis (DMA)) are also commonly available at many institutions. Interesting experiments can be performed with common materials such as nylon, HDPE and LDPE, polystyrene, and poly(vinyl chloride) to determine the degree of crystallinity, glass transition temperature, plasticizer content, etc. Many of these techniques are well within the price range of equipment commonly found in the undergraduate chemistry curriculum and amenable for use in a variety of laboratory experiences.
Cross-Linking and Its Implications
The properties of polymers are highly influenced by both covalent and non-covalent links between chains. Branched or cross-linked polymers can be achieved by including a monomer with at least three reactive sites or by reacting di- or polyfunctional molecules with existing unsaturated chains (such as vulcanization of rubber). Short, stiff cross-links generally produce resins that are hard, strong, inflexible, and possibly brittle, such as epoxy materials. Longer, flexible chains with greater distances between cross-links lead to rubbers or gels that are soft and pliable while also being tough, strong, and resilient. Cross-linked polymers cannot be dissolved, but they can often “swell” by incorporating appropriate solvents, and polymer swelling, such as rubber bands in toluene, can be used in the laboratory to determine the chain length between crosslinks. An important attribute of cross-linked polymers is that they can have huge molecular masses (e.g., a rubber ball can be one giant molecule). They cannot be easily recycled into new products but may only be physically modified, such as shredded rubber tires used on running tracks or in asphalt. Non-covalent interactions, such as microcrystallization, can effectively act as cross-links, allowing a polymer that should be a liquid at room temperature to be a tough, flexible solid (e.g., HDPE).
Rheology and Non-Newtonian Polymer Properties
Rheology is the study of deformation and flow. Polymers have flow properties that are very different from those of small molecules because of their extreme length and physical entanglements. Polymers can exhibit pseudoplastic behavior, otherwise known as shear thinning. In shear thinning, the polymer chains extend in the direction of flow, which reduces entanglements and viscosity. Alternatively, polymers can exhibit dilatant behavior, termed shear thickening. In shear thickening, the polymer has transient “cross-links”, often from orderinduced crystallinity or physical entanglements. Some of the more unique non-Newtonian flow properties of polymers such as rod climbing (due to entanglements), postextrusion swelling (due to recovery after deformation of the thermodynamically favored random coil), and “tubeless” siphon effects (due to entanglement, elongation, and drag effects on surrounding small molecules) that are used to increase flow in oil pipelines are easily demonstrated to students. Common polymeric materials (such as aqueous solutions of high-MW poly(vinyl alcohol) and “silly putty”-like materials) can be used to demonstrate the impact of various stress regimes from viscous flow to dilatant fracture.
Polymer “Phases” and Phase Transitions
Whereas small molecules have sharply definable transitions between solid, liquid, and gaseous phases, polymeric phase transitions can be much more complex. Polymers also have “additional” phases that are less common in small molecules for example, most polymers largely exist in an amorphous, “solid” glassy phase. Phase transitions for polymers generally C
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Phase Separation with Polymers
govern small molecules may be adapted to apply to polymers. Sources that instructors can use for more ideas of polymer topics to include in the curriculum and more indepth information on the examples provided herein are available.2−13 Also, CPT has developed two supplements, Polymers Across the Curriculum14 and Macromolecular, Supramolecular, and Nanoscale (MSN) Systems in the Curriculum,15 that provide additional examples of topics that can be incorporated into the curriculum to meet the new MSN requirement.
As a result of the enforced proximity of monomer segments in a chain due to the covalent bonds between them, polymers are entropy-restricted systems, and this has a significant impact on their phase separation and dissolution behavior. The covalent bonding within the polymer dramatically reduces the number of possible configurations, and thus entropic states, available to the chain. One consequence of this is that it is often thermodynamically unfavorable to form homogeneous mixtures of even chemically similar polymersenthalpy trumps entropy. A very interesting case that highlights this effect is block copolymers, where the phase-separated ends of the polymers form various structuresspheres (body-centered cubic), cylinders (hexagonal lattice) alternating sheets (lamellae), and dual labyrinths (double gyroid)based on the lengths of the polymer segments and the degree of incompatibility of the polymer segments. Many engineered nanostructured materials make use of the highly reproducible phase separation of block copolymers.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Thomas J. Wenzel: 0000-0001-9058-051X Notes
The authors declare no competing financial interest. T.J.W. and L.L.K. are the Chair and former Vice Chair of the Committee on Professional Training, respectively.
Interactions with Small Molecules: Dissolution, Diffusion, Swelling, and Plasticizing
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Entropy and enthalpy are important considerations when examining aspects such as dissolution, diffusion, swelling, and plasticizing of polymers. Defining an “ideal” solution for a dissolved polymer is more complex than for solutions involving small molecules, as polymer connectivity radically impacts how Raoult’s law is applied to polymer/solvent systemsthe volume ratio of components becomes more important than the mole ratio. The influence of entropy is reduced in solutions of polymers compared with solutions of small molecules. Polymers often have limited or no solubility in solvents that would be highly compatible with the polymer precursors (monomers). “Good” and “bad” solvents are defined in terms of the effects of different solvents on the size of the solvated polymer coil relative to its size in the pure polymer. The compatibility of polymeric gloves with different chemicals, for example, is based on the ability of the chemical to diffuse through or swell the polymer matrix. Thermodynamically “good” solvents are more likely to swell a polymer than thermodynamically “bad” solvents. The process of adding plasticizers such as phthalate esters to poly(vinyl chloride) (PVC) to change the properties of polymers (such as Tg) can be used to explain the difference between PVC plumbing pipes and PVC ponchos.
REFERENCES
(1) 2015 Guidelines for Undergraduate Professional Education in Chemistry. https://www.acs.org/content/dam/acsorg/about/ governance/committees/training/2015-acs-guidelines-for-bachelorsdegree-programs.pdf (accessed October 2016). (2) Tyrell, J. A. Fundamentals of Industrial Chemistry; John Wiley and Sons: Hoboken, NJ, 2014; Chapters 7−9. (3) Introduction of Macromolecular Science/Polymeric Materials into the Foundational Course in Organic Chemistry; Howell, B. A., Ed.; ACS Symposium Series, Vol. 1151; American Chemical Society: Washington, DC, 2013. (4) Wittcoff, A. A.; Reuben, B. E.; Plotkin, J. S. Industrial Organic Chemicals, 3rd ed.; John Wiley and Sons: Hoboken, NJ, 2013. (5) Brazel, C. S.; Rosen, S. L. Fundamental Principles of Polymeric Materials, 3rd ed.; John Wiley and Sons: Hoboken, NJ, 2012. (6) Young, R. J.; Lovell, P. A. Introduction to Polymers, 3rd ed.; CRC Press: Boca Raton, FL, 2011. (7) Hiemenz, P. C.; Lodge, T. P. Polymer Chemistry, 2nd ed.; CRC Press: Boca Raton, FL, 2007. (8) Campo, E. A. Industrial Polymers; Hanser Publications: Cincinnati, OH, 2007. (9) Odian, G. Principles of Polymerization, 4th ed.; John Wiley and Sons: Hoboken, NJ, 2004. (10) Allcock, H. R.; Lampe, F. W.; Mark, J. E. Contemporary Polymer Chemistry, 3rd ed.; Prentice Hall: Upper Saddle River, NJ, 2003. (11) Sandler, S. R.; Karo, W.; Bonesteel, J.; Pearce, E. M. Polymer Synthesis and Characterization: A Laboratory Manual; Academic Press: San Diego, CA, 1998. (12) Stevens, M. P. Polymer Chemistry: An Introduction, 3rd ed.; Oxford University Press: Oxford, U.K., 1998. (13) Coleman, M. M.; Painter, P. C. Fundamentals of Polymer Science: An Introductory Text, 2nd ed.; Technomic Publishing/CRC Press: Boca Raton, FL, 1997. (14) Polymers Across the Curriculum. https://www.acs.org/content/ dam/acsorg/about/governance/committees/training/acsapproved/ degreeprogram/polymers-across-the-curriculum-supplement.pdf (accessed October 2016). (15) Macromolecular, Supramolecular, and Nanoscale (MSN) Systems in the Curriculum. https://www.acs.org/content/dam/ acsorg/about/governance/committees/training/acsapproved/ degreeprogram/macromolecular-supramolecular-nanoscalesupplement.pdf (accessed October 2016).
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CONCLUDING REMARKS The requirement for inclusion of the principles that govern macromolecular, supramolecular, mesoscale, and nanoscale (MSN) systems into the undergraduate chemistry curriculum for an ACS certified degree recognizes that the properties of these systems have important differences from those of small molecules. Preliminary observations indicate that many chemistry programs are including these topics in the curriculum through distributed coverage over two or more courses and that aspects of synthetic polymers are generally covered to some extent, although what material is covered and whether it focuses on the unique properties of macromolecules varies greatly. When discussing aspects of synthetic polymers in the curriculum, it is important to focus on properties and behaviors that differentiate them from small molecules. The previous section gives examples of the types of polymer topics that are of benefit to undergraduates and demonstrate how the “rules” that D
DOI: 10.1021/acs.jchemed.6b00922 J. Chem. Educ. XXXX, XXX, XXX−XXX
