Polymers, Giant Molecules with Properties: An Entertaining Activity

Jun 4, 2019 - In this activity, polymer materials are introduced to 13–16 year old students. The activity is aimed at students with no or little kno...
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Polymers, Giant Molecules with Properties: An Entertaining Activity Introducing Polymers to Young Students Nejla B. Erdal,*,† Minna Hakkarainen,† and Anders G. Blomqvist‡ †

Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden Stockholm Vetenskapens Hus, SE-106 91 Stockholm, Sweden

‡

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S Supporting Information *

ABSTRACT: In this activity, polymer materials are introduced to 13−16 year old students. The activity is aimed at students with no or little knowledge of polymers. An engaging lecture covering the basics of polymer technology and sustainable development in the plastics field is presented. Important polymers such as polyethylene (PE), cellulose, and polylactide (PLA) are presented, and examples of their everyday use are shown. Quiz questions are employed in the introductory lecture to engage the students, to start discussions, and to evaluate the learning progress. The students are then engaged in two entertaining activities involving a natural polymer alginate and superabsorbent polymers. Alginate spaghetti is produced using different salt solutions enabling the students to create and destroy materials just by playing around with the chemistry, which helps them understand the polymeric material. The second activity has an application-based approach where the ability of superabsorbent polymers in diapers to retain water is investigated. The overall quiz results and discussions after the activities show an improved understanding of polymers and their applications and properties, making this activity useful for teaching polymers to young students. KEYWORDS: Polymer Chemistry, Materials Science, High School/Introductory Chemistry, Applications of Chemistry, Public Understanding/Outreach, Green Chemistry, Problem Solving/Decision Making, Hands-On Learning/Manipulatives, Humor/Puzzles/Games, Testing/Assessment

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associated problems for living organisms.3 It is therefore important to also raise a discussion regarding the environmental concerns and benefits of plastics and to unravel common misconceptions. A key concept that is emphasized is the awareness of the environmental issues associated with materials in general as well as polymers in particular, to engage the students in their choice of materials in their everyday life. This sustainability perspective is introduced at the beginning of the activity after the students learn about materials and their applications. Common materials such as aluminum cans, apples, glass, toilet paper, poly(ethylene terephthalate) (PET) bottles, and plastic bags are featured, and their degradation times in nature are discussed to address the issue of littering. Often, students are surprised to know how long it takes for a material to degrade in nature. However, these values are only rough estimations, and the degradation times are strongly dependent on the specific environmental conditions. An important biodegradable polymer polylactide (PLA) is then introduced as an example of a sustainable polymer. In contrast to the most common polymers (e.g., polyethylene, PE, and polypropylene, PP), PLA is biodegradable and made of

lastics have, since their discovery, emerged to play an essential part in our society. Polymers together with additives are what we in daily life call plastics, the most versatile materials for diverse applications.1 In our activity, the concept of polymers and different polymeric materials are introduced to 13−16 year old students. The lecture takes place in an outreach facility called Vetenskapens Hus (House of Science) located in Stockholm, Sweden, and is limited to 1 h and 30 min. Vetenskapens Hus hosts an average of 7400 unique visitors each year, who participate in activities related to chemistry and other science, technology, engineering, and math (STEM) fields, and the students only attend a certain activity once.2 Generally, students participate in a lecture to cover part of their learning goals in school or just to raise inspiration about a subject. In the lecture described herein, most students are unfamiliar with the term polymer, but they do know what a plastic is. The aim of this exercise is to give an understanding of polymers by doing two activities involving common polymeric materials. Engaging and inspiring hands-on activities are conducted and combined with an interactive slideshow to introduce the fundamental concepts and definitions. The materials are presented, and the differences in the molecular structures are explained. The plastic industry has in recent years earned a bad reputation as a result of increased littering of oceans and © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: November 8, 2018 Revised: May 21, 2019

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DOI: 10.1021/acs.jchemed.8b00918 J. Chem. Educ. XXXX, XXX, XXX−XXX

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renewable resources.4 A lot of research has been dedicated to replace the commodity plastics in short-term applications by biodegradable alternatives in order to eliminate plastic waste and environmental pollution.5−8 Green and renewable resources are explored and biodegradation and biorecycling are discussed, as well as the fact that it is the chemical structure and not the origin that determines the susceptibility of the material to degradation in different environments. Furthermore, even polymers that have an inherent potential for (bio)degradation do not rapidly degrade in all natural environments and it is important to realize that throwing plastics into the environment is not a viable economic or environmental solution.9 It is therefore important to raise a discussion regarding the most environmentally benign solutions in each case. As a first exercise, the students examine the properties of alginate, an environmentally friendly and renewable material.10,11 Alginate is a well-known natural polymer derived from the cell walls of brown algae, and it is often referred to as a cousin of cellulose, the most abundant natural polymer on earth. It is used as a thickening agent, in dentistry, and in some antacids. One interesting property of alginate polymers is their ability to form gels when calcium is added to a solution of sodium alginate.12 At this educational level, the students are learning about different chemical compounds and how atoms are assembled into molecular and ionic compounds by chemical reactions in their classroom science courses, according to the learning guidelines launched by the Swedish National Agency for Education.13 Knowledge about precipitations, ions, and the existence of monovalent and divalent ions is thus desirable. During the exercise, the monovalent sodium ions in sodium alginate are exchanged with divalent calcium ions, which serve as physical cross-linkers binding the long alginate chains together to form a gel.12 Hence, changed material properties can be obtained simply by adding or depleting ions. This first activity assigned to the students is referred to as Alginate Spaghetti, in which the students get to know this natural polymer through its behavior in different salt solutions. As a second exercise, the students investigate superabsorbent polymers (SAP), which are charged cross-linked polymers.14 They have the fascinating feature of retaining moisture many times their own weight without dissolving because of their hydrophilicity, ionic nature, and chemical configuration. This makes them ideal materials for personal care products, diapers, and agricultural applications. There are several kinds of superabsorbent hydrogels that are made from synthetic or natural polymers and used for agricultural purposes, such as cross-linked polyacrylates, cross-linked polyacrylamides, and cross-linked acrylamide− acrylate copolymers containing a major percentage of acrylamide units. They are, for example, used to improve the physical properties of soil by increasing the water-holding capacity and nutrient retention of sandy soil to enhance the growth and performance of plants.15 A diaper is something all the students have most likely encountered previously and can be found in many households. Disposable diapers in most cases consist of several layers of different materials, such as cellulose, polyethylene, and SAP that is generally cross-linked sodium polyacrylate or poly(acrylic acid).14,16,17 The repeating units of the SAP comprise of charged carboxylic groups and neutral carboxylic acids which create a hydrophilic backbone able to induce hydrogen

bonding with water. In the swollen state, the coiled polymer expands as water penetrates into the 3D network and interacts with the charged polymer backbone. As the water molecules push the chains apart, the cross-links between the chains maintain a 3D network and prevent the polymer from dissolving.14 The second activity is to assess the ability of sodium polyacrylate (from diapers) to absorb water. Although similar activities and demonstrations involving alginate cross-linking and diapers have been reported before, different approaches and learning goals have been employed.12,16,18−20 Ting et al. presents valuable polymerchemistry topics for high-school students, such as advanced polymer synthesis, block polymer micellization, and polymer swelling and rheology.16 The students are instructed to use diapers to investigate the absorption capacity of SAP. However, in contrast to the exercise presented here, no extraction of the superabsorbent polymer is made. The extraction adds a valuable aspect as the students can visually examine and touch the SAP, which gives them a deeper understanding. Criswell shows in a report how the superabsorbent polymers can be used as an activity followed by an alginate lab using Gaviscon as a source of alginate.18 The learning goals were, however, directed to the understanding of the relationship between the positions of different elements in the periodic table and the observed outcomes, rather than the polymeric material itself. Moreover, previous reports have not touched upon the sustainability concerns while presenting the concept of polymers. Considering the environmental impact of polymers and all the materials we are using today, the authors find it important to combine three key points: sustainability, entertainment, and general polymer facts. In addition, it is possible to expand or shorten the whole program easily with the provided slideshow. A quiz is employed as an assessment method during the activity. The application used for this purpose is Turning Point (ver. 8.2.3.1), in which the students either compete in defined groups or respond anonymously. Applications such as Kahoot and Quizizz can also be used. The quiz contains questions about the definitions and fundamental concepts elucidated throughout the whole activity. We perceive that this activity generates enthusiasm when the student groups compete with each other. At the same time, valuable information about the knowledge that the students gained during the exercises is obtained. Such feedback also allows the teacher to instantly fill in the knowledge gaps during the lecture.

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ACTIVITY The activity is divided into three parts: Introduction, Alginate Spaghetti, and Superabsorbent Polymers. The students are divided into groups of two or three members. The goals of these experiments are to • introduce basic polymer technology and familiarize students with different polymers and their properties, • train students in laboratory work, • perform hands-on activities with polymers to get a deeper understanding in an entertaining way, and • reflect upon and answer questions about polymers and their properties throughout the activity. Alginate Spaghetti

Alginic acid sodium salt (low viscosity) was purchased from Alfa Aesar (Heysham, England). Calcium chloride dihydrate (CaCl 2 ·2H 2 O, 100.6%), sodium carbonate anhydrous B

DOI: 10.1021/acs.jchemed.8b00918 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 1. Illustration of alginate-gel formation with calcium ions that enable physical cross-links between the alginate chains.

diaper where the SAP is extractable is acceptable. The students weigh the polymer and note how much polymer they obtained before water is added. Small volumes of water are recommended in the beginning. Water molecules penetrate into the polymer matrix, pushing the polymer chains apart and cause the swelling of the polymer (see Figure 3). Since the polymer chains are entangled, cross-linked, and consist of acrylate repeating units with stabilizing sodium ions, the water is trapped in the 3D network. The water interacts through hydrogen bonding. The amount of water added is noted, and the ratio is calculated according to eq 2:

(Na2CO3, 99.8%), and Pasteur pipettes (5 mL, nonsterile, graduated up to 1 mL) were purchased from VWR Chemicals (Leuven, Belgium). Sodium chloride (NaCl, 99.5%) was bought from Merck (Darmstadt, Germany). Each student pair is provided the following materials: 20 mL of 2 wt % sodium alginate solution, 20 mL of 10 wt % NaCl, 20 mL of 10 wt % CaCl2, 20 mL of 20 wt % Na2CO3, a pipette, and a spoon. The experiment is divided into two parts. Initially, sodium alginate is poured into the calcium chloride and sodium chloride solutions, and the outcome is observed. A pipette or similar tool is useful for this purpose. The behavior of sodium alginate in the different salt solutions is studied and discussed within the groups. The divalent calcium ions, Ca2+, bind to two alginate polymer chains creating a cage-like structure and forming a gel (alginate spaghetti), whereas monovalent sodium ions, Na+, only can interact with one alginate chain and thus are not able to create physical cross-links (see Figure 1). The alginate spaghetti is then poured into a sodium carbonate solution, and the students study the outcome (see Figure 2). Once again, the characteristics of the ions in the salt solutions play an important role in determining the fate of the spaghetti. At this point, the students are already more familiar with the cause of the different behaviors, and the focus is directed to the ions in the solution. In this case, the students initially observe small white precipitates in the solution and a weakening of the spaghetti. The spaghetti slowly disappears as a result of the competition between the two ions as the solution gets cloudier. The Ca2+ ions in the alginate are replaced by Na+ ions. This causes the free Ca2+ ions to interact with carbonate ions in the solution, resulting in a white precipitate of calcium carbonate (CaCO3) and loss of the structural integrity of the gels (see eq 1). Ca 2 + + Na 2CO3(aq) → CaCO3(s) + 2Na +

R=

water polymer

(2)

where R stands for the ratio between the weight of water and the weight of polymer, water (g) stands for the maximum amount of water the SAP could retain, and polymer (g) represents the amount of SAP extracted from the diaper. All the groups are instructed to calculate their water/ polymer ratios, and a discussion about the ability of the polymer to absorb water is carried out. Generally, the superabsorbent polymer is found to absorb water 200 times its own weight. This phenomenon often surprises the students, and the instructors follow up their curiosity with possible ideas of other application areas or further testing in solutions other than water. The experiment provides an understanding of the superabsorbent polymers and how their properties are related to their applications.

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HAZARDS Sodium alginate is not classified as a hazardous substance, although normal lab practice should be followed. Calcium chloride and sodium carbonate cause eye irritation; therefore, upon contamination, the eye should be thoroughly and cautiously rinsed for several minutes. Sodium polyacrylate is not regulated as hazardous under 29 CFR and 49 CFR. However, the manufacturer recognizes the potential for respiratory-tract irritation as a result of inhalation of this material as a respirable dust. For safety, students should wear safety goggles and a long-sleeved lab coat throughout the experiments. Safety gloves are offered but are not mandatory.

(1)

Superabsorbent Polymers

To expand the understanding of polymer properties, a second exercise is initiated. Sodium polyacrylate is obtained from a diaper that is cut along the edges, allowing the granules of sodium polyacrylate to be extracted by hand. Pampers Baby Dry (S3, S4, and S5) have been utilized for this purpose. Any C

DOI: 10.1021/acs.jchemed.8b00918 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Facemasks may be offered to cover mouth and nose but are not mandatory.

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DISCUSSION The quiz results show a general understanding of the very basic concepts of polymers. This is concluded from the scores of the quizzes performed and evaluated. More quiz questions are available in the Student Handout found in the Supporting Information. Student knowledge in this area, however, is not assessed prior to the activity. One question in the quiz is used to check whether the students remember an important synthetic polymer, PE. According to the evaluation, after the introduction, 75% of the students remember that PE is a polymer, whereas water and lactic acid are not (see Table 1). Table 1. Comparative Results from Three Quiz Questions during the Introductory Lecture Statements for Participant Response Polymers consist of monomers. One example of a polymer is Are these statements correct? If an alginate solution is mixed with a divalent ion solution, a gel is created. A superabsorbent polymer can absorb water many times its own weight. a

Response Options

Responses, % (N = 180)

Water Polyethylenea Lactic acid

10 75 15

Yesa I do not know No Yesa I do not know No

61 27 12 77 0 23

Correct answer.

The exercises are also followed by questions from which it can be elucidated that more than half of the students understand that divalent ions such as calcium can be used to create alginate gels as a result of physical cross-linking and that superabsorbent polymers have the ability to absorb water several times their own weight (see Table 1). It should be noted that the questions are only asked once. When the correct answer is revealed to each question many students that first gave the wrong answer learn the correct answer. Therefore, these results should be used as indicative, and the learning might be further improved during the follow up discussion. However, it should also be noted that the learning outcomes are short-term because the questions are asked directly after the activities. The learning guidelines of the Swedish National Agency for Education refer to different aspects and fundamentals of

Figure 2. Images of (a) sodium alginate in sodium chloride (NaCl), calcium chloride (CaCl2), and sodium carbonate (Na2CO3) solutions; (b) alginate spaghetti taken from the calcium chloride (CaCl2) solution and transferred to the sodium carbonate (Na2CO3) solution; and (c) precipitation of CaCO3 after 0 min, when small white precipitates can be observed, and after 10 min, when the solution is much cloudier and the gel starts to dissolve.

Figure 3. Illustration of a superabsorbent polymer in water. Water is sequestered in the entangled polymeric network and interacts with the hydrophilic polymer chains. D

DOI: 10.1021/acs.jchemed.8b00918 J. Chem. Educ. XXXX, XXX, XXX−XXX

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chemistry (e.g., chemistry in nature, in everyday life and society, and in the conception of the world).13 The methods and work procedures in chemistry are also highlighted. This activity is in alignment with most of the mentioned aspects but more particularly engaging in the sustainability questions, which are stressed in the category of chemistry in our everyday life and in the society. It also trains students in laboratory work. Moreover, this lecture aims at serving as a package with a useful yet entertaining learning strategy for younger students with no knowledge about polymers. Pratt et al. show in a study that some of the main goals of chemistry outreach events are learning and having fun as a result of attending an event. The authors emphasize the importance of enjoyment combined with learning.21 In addition, they evaluated the success of the outreach event by observing the audience. This activity was found to generate enthusiasm among the students, which in combination with the quiz results suggests a useful activity.

CONCLUSIONS Overall, this activity accomplished the goal of introducing polymers to middle-school and junior-high-school students with little or no basic knowledge about polymers. Quiz results showed that the activity generated an increased understanding of introductory polymer chemistry. This activity also demonstrates how the learning outcomes can be improved by combining a lecture with inspiring lab activities and that it is possible to teach scientific concepts in an entertaining way. The activity can and has been carried out by undergraduate and graduate students who possess the basic knowledge that is discussed here. Future ideas on how to expand the study would involve expanding the experiments with, for example, several other divalent ions for the alginate lab and trying out other solvents (simulated urine, saline solutions, and basic vs acidic solutions) or different cross-linking densities for the superabsorbent polymer activity. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00918. Student handouts (ZIP)

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REFERENCES

(1) Vert, M.; Doi, Y.; Hellwich, K.-H.; Hess, M.; Hodge, P.; Kubisa, P.; Rinaudo, M.; Schué, F. Terminology for biorelated polymers and applications (IUPAC Recommendations 2012). Pure Appl. Chem. 2012, 84, 377−410. (2) Vetenskapens Hus. http://www.vetenskapenshus.se/?language= en (accessed May 2019). (3) Hillmyer, M. A. The promise of plastics from plants. Science 2017, 358, 868−870. (4) Fukushima, K.; Abbate, C.; Tabuani, D.; Gennari, M.; Camino, G. Biodegradation of poly(lactic acid) and its nanocomposites. Polym. Degrad. Stab. 2009, 94, 1646−1655. (5) Tokiwa, Y.; Calabia, B. P.; Ugwu, C. U.; Aiba, S. Biodegradability of plastics. Int. J. Mol. Sci. 2009, 10, 3722−3742. (6) Gross, R. A.; Kalra, B. Biodegradable polymers for the environment. Science (Washington, DC, U. S.) 2002, 297, 803−807. (7) Babu, P. R.; O’Connor, K.; Seeram, R. Current progress on biobased polymers and their future trends. Prog. Biomater. 2013, 2, 8. (8) Rujnić-Sokele, M.; Pilipović, A. Challenges and opportunities of biodegradable plastics: A mini review. Waste Manage. Res. 2017, 35, 132−140. (9) Hakkarainen, M.; Albertsson, A.-C. Designed to degrade. Science 2017, 358, 872−873. (10) Sirviö, J. A.; Kolehmainen, A.; Liimatainen, H.; Niinimäki, J.; Hormi, O. E. O. Biocomposite cellulose-alginate films: Promising packaging materials. Food Chem. 2014, 151, 343−351. (11) Rinaudo, M. Biomaterials based on a natural polysaccharide: alginate. TIP, Rev. Espec. Cienc. Quim.-Biol. 2014, 17, 92−96. (12) Waldman, A. S.; Schechinger, L.; Govindarajoo, G.; Nowick, J. S.; Pignolet, L. H.; Labuza, T. The alginate demonstration: Polymers, food Science, and ion exchange. J. Chem. Educ. 1998, 75, 1430−1431. (13) This is the Swedish National Agency for Education. Skolverket. https://www.skolverket.se/andra-sprak-other-languages/englishengelska (accessed May 2019). (14) Buchholz, F. L. Superabsorbent polymers. J. Chem. Educ. 1996, 73, 512. (15) Abd El-Rehim, H. A.; Hegazy, E-S. A.; Abd El Mohdy, H. L. Radiation synthesis of hydrogels to enhance sandy soils water retention and increase plant performance. J. Appl. Polym. Sci. 2004, 93, 1360−1371. (16) Ting, J. M.; Ricarte, R. G.; Schneiderman, D. K.; Saba, S. A.; Jiang, Y.; Hillmyer, M. A.; Bates, F. S.; Reineke, T. M.; Macosko, C. W.; Lodge, T. P. Polymer Day: Outreach Experiments for High School Students. J. Chem. Educ. 2017, 94 (11), 1629−1638. (17) Zohuriaan-Mehr, J. M.; Kabiri, K. Superabsorbent polymer materials: A review. Iran. Polym. J. 2008, 17, 451−47. (18) Criswell, B. A Diaper a Day and What’s Going on with Gaviscon? Two Lab Activities Focusing on Chemical Bonding Concepts. J. Chem. Educ. 2006, 83, 574−576. (19) Meyer, L. S.; Panee, D.; Schmidt, S.; Nozawa, F. Using demonstrations to promote student comprehension in chemistry. J. Chem. Educ. 2003, 80, 431−435. (20) Cleary, J. Diapers and polymers. J. Chem. Educ. 1986, 63, 422− 423. (21) Pratt, M. J.; Yezierski, J. E. Characterizing the landscape: Collegiate organizations’ Chemistry Outreach Practices. J. Chem. Educ. 2018, 95, 7−16.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nejla B. Erdal: 0000-0002-3046-7547 Minna Hakkarainen: 0000-0002-7790-8987 Anders G. Blomqvist: 0000-0002-8250-8152 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The School of Engineering Sciences in Chemistry, Biotechnology and Health at KTH Royal Institute of Technology is acknowledged for financial support. Marie Danielsson is thanked for help during the development of the activity. E

DOI: 10.1021/acs.jchemed.8b00918 J. Chem. Educ. XXXX, XXX, XXX−XXX