Simple and Versatile Protocol for Preparing Self-Healing Poly(vinyl

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Simple and Versatile Protocol for Preparing Self-Healing Poly(vinyl alcohol) Hydrogels Rylie K. Morris,† Abby P. Hilker,† Taylor M. Mattice,‡ Shane M. Donovan,† Michael T. Wentzel,‡ and Patrick H. Willoughby*,† †

Department of Chemistry, Ripon College, Ripon, Wisconsin 54971, United States Department of Chemistry, Augsburg University, Minneapolis, Minnesota 55454, United States

‡

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S Supporting Information *

ABSTRACT: As the role of polymers in undergraduate chemistry curricula continues to expand, opportunities will emerge for adopting experiments involving smart materials (i.e., materials that change properties in response to external stimuli). Slime demonstrations are routinely carried out with poly(vinyl alcohol) (i.e., PVA) hydrogels because the polymer is inexpensive and nontoxic, and the resulting material has interesting physical properties. This report describes an activity where PVA is processed into an autonomous self-healing smart material. Specifically, students prepare rigid PVA hydrogels using a simple freeze/thaw protocol. The resulting material is cut, and the severed edges are pressed together to initiate autonomous self-healing. Healing is observed by measuring sufficiently high (i.e., >40 kPa) uniaxial tensile strengths at the repaired surface. Preparing the hydrogel does not require chemical additives beyond commercially available PVA (i.e., Mw ∼ 145,000 g mol−1) and water. Additionally, the tensile strength can be determined using a spring force gauge and a ruler. The simplicity of the procedure, use of low-cost materials, and ties to green chemistry make the activity suitable for use in high school or introductory college chemistry settings. Furthermore, procedural variation and more rigorous analysis make the activity versatile by allowing the protocol to be used in second- or third-year chemistry courses (e.g., organic and physical chemistry). Overall, the activity provides a straightforward approach to introducing students to modern topics in polymer chemistry and materials science. KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Interdisciplinary/Multidisciplinary, Organic Chemistry, Polymer Chemistry, Hands-On Learning/Manipulatives, Alcohols, Applications of Chemistry, Hydrogen Bonding, Materials Science

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INTRODUCTION Smart materials possess the ability to change their physical and/or chemical properties in response to external stimuli.1 Autonomous self-healing materials are a type of smart material with the ability to repair damage (e.g., cracks or fractures) without external (e.g., human) intervention.2−4 These materials are promising because they have reduced costs and waste associated with repair, disposal, and/or replacement, providing innovative solutions that align with principles of green chemistry and engineering (e.g., Prevent Waste).5 As a result, autonomous self-healing materials serve numerous industries, such as coatings, adhesives, and cements.6,7 Hydrogels represent a large class of autonomous self-healing materials and are often nontoxic and biocompatible, providing © XXXX American Chemical Society and Division of Chemical Education, Inc.

numerous opportunities in biomaterials, tissue engineering, and drug-delivery.8−10 Hydrogels form when cross-linked hydrophilic polymers swell in an aqueous medium as water molecules become “trapped” or imbibed within the polymer network.11 Hydrogel cross-links often form from physical interactions (e.g., hydrogen bonding, aggregation, or crystallization).12 The dynamic nature of these interactions allows cross-links to readily re-form after experiencing stress or damage, resulting in autonomous self-healing of the material.13 Received: February 23, 2019 Revised: May 23, 2019

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DOI: 10.1021/acs.jchemed.9b00161 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 1. Representative PVA (a) structural representations, (b) crystals, and (c) hydrogel material.

Figure 2. Outline and timing for each phase of the activity.

co-workers shows that the freeze/thaw approach can provide rigid PVA hydrogels that exhibit autonomous self-healing.14 Specifically, the authors demonstrated that these gels can be cut in half, pressed back together, and allowed to heal (e.g., 1 h) to give gels that recover 40−70% of their original tensile strength. The authors propose that self-healing arises when surface PVA chains re-form hydrogen bonds after the cut edges are pressed back together. Self-healing studies were performed with hydrogels containing a small amount of dye, suggesting that PVA chains are capable of diffusing across the cut interface to promote interchain hydrogen bonding (cf., Supporting Information). By carefully examining the conditions for hydrogel preparation (i.e., polymer Mw, PVA concentration, and number of freeze/thaw cycles), the authors identified an optimal balance of chain mobility and OH group concentration to produce rigid materials capable of self-healing. Given the simplicity of the freeze/thaw method and an increasing desire to expand the role of polymers within the undergraduate curriculum,32,33 these self-healing studies could be adapted for a classroom experience with the goal of introducing students to concepts related to polymer processing and smart materials. Furthermore, numerous aspects of the procedure adhere to the Anastas and Warner principles of green chemistry (e.g., avoiding chemical cross-linkers to minimize use of hazardous chemicals).5 This report describes a simple, reproducible activity for preparing PVA hydrogels that exhibit autonomous self-healing. The procedure uses practical equipment and inexpensive reagents and can be tailored to numerous instructional settings. In addition to exposing students to concepts related to polymer processing and smart materials, students will be introduced to basic analytical techniques in materials science (i.e., tensile testing).

Recently, Zhao and co-workers demonstrated that poly(vinyl alcohol) (i.e., PVA) hydrogels are readily processed into materials that exhibit autonomous self-healing,14 and we have been studying this report with the intent of adopting the approach for use in a teaching setting. Consisting of only a hydroxylated ethylene repeat unit (cf., Figure 1a), PVA is commercially available,15 a crystalline solid (cf., Figure 1b) accessible in a variety of molecular weights (e.g., Mw ∼ 10,000−200,000 g mol−1) and readily processed into hydrogel materials with interesting properties (cf., Figure 1c).16 Furthermore, PVA is biodegradable,17 biocompatible, and nontoxic, allowing this simple polymer to have several noteworthy applications, such as the material for soft contact lenses,18 a water-soluble temporary packing material, and a surfactant in eye drops.19 PVA hydrogels provide numerous instructional opportunities in the classroom.20−25 In the classic example, PVA hydrogels are used as slime demonstrations where an aqueous solution of low-molecular-weight PVA (i.e., Mw ∼ 20,000 g mol−1) is treated with borax, which serves as a chemical crosslinker to induce gel formation.21 Alternatively, PVA hydrogels can be prepared without a chemical cross-linker by freezing (e.g., −20 °C) and thawing aqueous solutions of largermolecular-weight PVA (e.g., Mw ∼ 145,000 g mol−1).26,27 Subsequent studies28−30 have shown that freezing causes phase separation in homogeneous aqueous solutions of PVA, providing “regions of high PVA [chain] concentration”30 and encouraging the formation of crystallites (i.e., segments highly ordered hydrogen bonds). Because crystallites often form between different polymer chains, they serve as strong physical cross-links. Interestingly, the crystallites and phase-separated regions are maintained as the material thaws, providing a rigid hydrogel capable of holding its molded shape. The extent of crystallite formation is readily controlled (e.g., by changing freezing time and/or temperature) and greatly influences the properties of the resulting hydrogels.31 The report by Zhao and

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EXPERIMENT PROCEDURE Groups of at least two students can complete the following experiment to obtain 3−4 hydrogel samples with tensile B

DOI: 10.1021/acs.jchemed.9b00161 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 3. Representative photos from important steps of the activity: (a) the hydrogel solution during solid addition, (b) transferring the warm PVA solution to the mold, (c) splitting the warm solution into four parts of the mold, (d) the hydrogel solution after cooling to room temperature, (e) the mixture after freezing for 1 h at −20 °C, (f) a PVA hydrogel after thawing overnight, (g) cut hydrogels that have been pressed back together, (h) a hydrogel that has healed along the original fracture line, (i) the tensile tester apparatus with a “healed” PVA hydrogel, (j) pulling downward on healed hydrogel while observing the tension on the spring force gauge, and (k) the apparatus after the material failed along the fracture line.

strengths greater than 40 kPa after fracture healing. Figure 2 shows an overview of the procedure along with average times for student completion. Additional details useful for ensuring that the activity is successful are included in the Supporting Information. Begin by adding 100 mL of reverse osmosis or deionized water to a 150 mL tall-form beaker. Place a thermometer in the beaker so the bulb rests on the bottom of the glassware, and heat the beaker on a standard hot plate until the water reaches 85−95 °C. While the water is warming, obtain 12 g of PVA with Mw ∼ 145,000 g mol−1 (i.e., Mowiol 28−99 from Millipore Sigma). Use of PVA with a larger Mw will not readily form homogeneous solutions in water at the desired concentration, and samples with a smaller Mw may not be sufficiently rigid to assess the ultimate tensile strength (cf., Supporting Information for further discussion). Because the PVA needs to be added portionwise, divide the solid into 8−10 roughly equal fractions. At this point, use a Hot Hand or insulated glove for handling hot glassware. Once the water has reached temperature and the temperature is stable, pour the entire volume of water into a graduated cylinder, quickly transfer 45−50 mL of the hot water back into the beaker, and continue heating on the hot plate. This step will ensure that the appropriate volume of water is present at the start of the experiment. Grasp the warm beaker with a Hot Hand or insulated glove and begin vigorously hand stirring the water with a large spatula while a lab partner slowly adds the first portion of solid PVA. After the complete addition of the first portion of solid, continue to vigorously stir the resulting mixture for 2−3 min before adding the next portion of solid. Continue solid

addition every 2−3 min with constant vigorous stirring and occasional temperature monitoring until all of the PVA has been added. After addition of PVA, the mixture will become viscous and opaque (cf., Figure 3a). After complete addition of the PVA solid, stir the mixture for an additional 2−3 min and transfer the resulting molten hydrogel into a mold (e.g., icecube tray) using a spatula, stirring rod, and/or thermometer (cf., Figure 3b). Attempt to prepare four uniform hydrogels by splitting the sticky and viscous PVA solution into four spaces of the mold (cf., Figure 3c). This approach will increase the likelihood of obtaining at least two hydrogel samples with measurable self-healing properties. After transfer of the material to the mold, cover it with aluminum foil, and, if possible, allow the material to sit in the mold and cool to room temperature for 1−3 h before freezing (cf., Figure 3d). Place the mold and hydrogel mixture in a −20 °C freezer for 1 h to freeze the hydrogels and induce crystallite formation (cf., Figure 3e). Move the materials from the freezer to a room temperature benchtop and allow the material to thaw, covered, in the mold at room temperature. After the material thaws for 12−24 h, carefully remove the thawed hydrogels from the mold and obtain qualitative observations about the appearance and mechanical properties of the virgin (i.e., undamaged) material (cf., Figure 3f). Record the mass of each hydrogel and report the total mass obtained from the experiment. Cut each gel in half with scissors, immediately press the two severed edges back together to initiate self-healing, and rest the resulting material on aluminum foil (cf., Figure 3g). After 1 h, qualitatively assess for the presence of self-healing by carefully picking up the hydrogel. If the gel has healed, the two halves C

DOI: 10.1021/acs.jchemed.9b00161 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Table 1. Representative Student Data from an Organic Chemistry I Laboratory Sectiona Gel Gel Gel Gel

1 (kPa) 2 3 4

Group 1

Group 2

Group 3

Group 4

Group 5

Group 6

Group 7

90 69 140 91

23 22 0

13 21 19 46

53 58 53 32

46 40 41 40

0 0 0 0

0 0 0 0

Values represent ultimate tensile strengths (kPa) of individual “healed” gels for each student group.

a

Table 2. Summary of Data on Assessment of Student Learning Student Resultsa Learning Goals

Does Not Meet

Meets

Exceeds

(1) (A) Determine the ultimate tensile strength of a material in kPa. (n ∼ 300) (B) Describe how this value relates to a material that does not exhibit self-healing.(n = 23)

30%

70%

N/A

5% 42%

87% 37%

9% 21%

16%

58%

26%

5%

68%

26%

(2) Demonstrate understanding of the mechanism by which PVA hydrogels undergo self-healing, including the role of crosslinking. (n = 19) (3) Demonstrate understanding of the role various aspects of the procedure in producing a PVA hydrogel with self-healing properties. (n = 19) (4) Justify the use of self-healing PVA hydrogels in the context of developing sustainable materials and the principles of Green Chemistry. (n = 19) a

Results were obtained from assessing student responses to postlab questions using a standard rubric (cf., Supporting Information). Percentages were rounded, causing some of the total values to not equal 100%.

greater than 5% PVA should disposed of as aqueous hazardous waste. Students should be equipped with eye protection during the entirety of the experiment. There is minimal risk of exposure to hazardous fumes, so a fume hood is likely not required. The primary risk to student safety is handling warm glassware or splashing/spilling hot water. When handling warm glassware, students should use a Hot Hand or insulated glove. Splashing/spilling is minimized if the recommended glassware is used when preparing the aqueous PVA solution (i.e., 150 mL tall-form beaker). Students should work in pairs, especially at the start of the experiment as they slowly add solid PVA to near boiling water with vigorous stirring.

will remain stuck together even while the material is allowed hang downward (cf., Figure 3h). To quantitatively assess the extent of self-healing of the resulting material, determine the ultimate tensile strength (i.e., the load a material can withstand before breaking). There are numerous approaches to measuring the tensile strength of a material,34 and the apparatus described below is one inexpensive example. Using standard laboratory clamps, mount a spring force gauge of the appropriate load capacity (i.e., 20−50 N) to a ring stand. Attach large binder clips to either side of the hydrogel, and attach one of the clips to the mounted spring force gauge (cf., Figure 3i). Position the apparatus so that there is ample space for the gel to be pulled downward (ca. 15−25 cm). One student should slowly pull the gel downward against the force gauge using the opposite binder clip. It is crucial that the gel be pulled at a slow, consistent rate to ensure that the tensile strength of the material is accurately recorded. As the gel stretches, carefully observe the force gauge (cf., Figure 3j). At some force, the ultimate tensile stress will be achieved, the gel will split along the original fracture line, and the spring force gauge will return to zero (cf., Figure 3k). Record the force to failure, which is the maximum value the force gauge read before the material failed. Additionally, determine the cross-sectional area in mm2 along the fracture line of the healed gel by averaging the area of the healing surface on each of the severed halves of the gel. The tensile strength of the healed hydrogel is found by dividing force to failure (N) by the healing surface area (mm2). This will give the tensile strength in pressure units, where a common pressure unit is kilopascal (i.e., kPa) and 1 kPa = 1.0 × 10−3 N/mm2. A similar measurement could be performed on a virgin (i.e., undamaged) hydrogel sample from the same batch, but a force gauge with a much larger loading capacity (i.e., 100−200 N) will be required.

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RESULTS AND DISCUSSION The procedure will allow students to prepare PVA hydrogels capable of achieving an average tensile strength of 25−50 kPa after being cut and healing for 1 h. These results were observed from over 300 students enrolled in General Chemistry II, Organic Chemistry I, Organic Chemistry II, and a high school science camp. To illustrate a possible data set, Table 1 includes data of seven student groups of 2−3 students from a single laboratory section of Organic Chemistry I. The strength of the healed hydrogels ranges from 0 to 140 kPa with a mean of 33 kPa and median of 23 kPa. For comparison, Zhang and coworkers reported a strength of 105 kPa after 1 h of healing.14 The student data also provides a sense of how important it is to use the correct amount of water. For example, student groups 6 and 7 did not observe self-healing because they did not use the correct volume of water at the outset of the experiment, and group 2 obtained less hydrogel solution because they mixed the solid PVA into vigorously boiling water. While all students obtained and analyzed at least three hydrogels, removing systematic errors (i.e., data from Groups 6 and 7) gives a revised mean of 47 kPa and a median of 41 kPa. The deviation of tensile strengths for groups 1 and 3−5 is typical given the constraints of the equipment. Further discussion of the influence of experimental variables on the

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HAZARDS PVA hydrogels can be disposed of in the normal garbage waste stream (i.e., waste basket). Aqueous polymer liquids containing D

DOI: 10.1021/acs.jchemed.9b00161 J. Chem. Educ. XXXX, XXX, XXX−XXX

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self-healing properties of the resulting materials is included in the Supporting Information. A summary of student learning outcomes is provided in Table 2. Using evaluation of student data and responses to postlab questions (i.e., open-ended and survey), student learning was assessed in the context of four learning goals. Differences in the number of total student responses reflect how assessment of student learning evolved as the procedure became more refined. Assuming students had no prior knowledge of self-healing materials and ultimate tensile strengths, the number of students who met or exceeded each of the learning goals was greater than those who did not meet expectations. It was particularly encouraging that a large percentage of students were able to link aspects of the procedure, and self-healing materials in general, to principles of green chemistry. Furthermore, the postlab survey suggested that students generally enjoyed the activity along with being exposed to concepts related to smart materials. Raw data, grading rubrics, and survey questions are included in the Supporting Information.

CONCLUSION This activity provides a straightforward opportunity for students to gain hands-on experience processing polymers into “smart” autonomous self-healing materials. Using a freeze/ thaw approach, rigid PVA hydrogels were prepared without adding chemical cross-linkers. Autonomous self-healing of these materials was observed by measuring the tensile strengths of fractured hydrogels that were allowed to heal for 1 h. The simplicity of the procedure, minimal safety hazards, and use of inexpensive materials will expand the scope of amenable teaching settings and variables to be studied. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.9b00161.

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REFERENCES

(1) Smart Materials, 1st ed.; Schwartz, M., Ed.; CRC Press: Boca Raton, FL, 2008. (2) White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S. Autonomic Healing of Polymer Composites. Nature 2001, 409 (6822), 794−797. (3) Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.; Sheran, K.; Wudl, F. A Thermally Re-Mendable Cross-Linked Polymeric Material. Science 2002, 295 (5560), 1698−1702. (4) Blaiszik, B. J.; Kramer, S. L. B.; Olugebefola, S. C.; Moore, J. S.; Sottos, N. R.; White, S. R. Self-Healing Polymers and Composites. Annu. Rev. Mater. Res. 2010, 40 (1), 179−211. (5) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998; p 30. (6) Self-Healing Materials: Fundamentals, Design Strategies, and Applications.; Ghosh, S. K., Ed.; Wiley-VCH: Weinheim, 2008. (7) Hager, M. D.; Greil, P.; Leyens, C.; van der Zwaag, S.; Schubert, U. S. Self-Healing Materials. Adv. Mater. 2010, 22 (47), 5424−5430. (8) Brochu, A. B. W.; Craig, S. L.; Reichert, W. M. Self-Healing Biomaterials. J. Biomed. Mater. Res., Part A 2011, 96A (2), 492−506. (9) Caló, E.; Khutoryanskiy, V. V. Biomedical Applications of Hydrogels: A Review of Patents and Commercial Products. Eur. Polym. J. 2015, 65, 252−267. (10) Ferreira, N. N.; Ferreira, L. M. B.; Cardoso, V. M. O.; Boni, F. I.; Souza, A. L. R.; Gremião, M. P. D. Recent Advances in Smart Hydrogels for Biomedical Applications: From Self-Assembly to Functional Approaches. Eur. Polym. J. 2018, 99, 117−133. (11) Hiemenz, P. C.; Lodge, T. P. Polymer Chemistry, 2nd ed.; CRC Press: Boca Raton, FL, 2007. (12) Hennink, W. E.; van Nostrum, C. F. Novel Crosslinking Methods to Design Hydrogels. Adv. Drug Delivery Rev. 2002, 54 (1), 13−36. (13) Wojtecki, R. J.; Meador, M. A.; Rowan, S. J. Using the Dynamic Bond to Access Macroscopically Responsive Structurally Dynamic Polymers. Nat. Mater. 2011, 10 (1), 14−27. (14) Zhang, H.; Xia, H.; Zhao, Y. Poly(vinyl alcohol) Hydrogel Can Autonomously Self-Heal. ACS Macro Lett. 2012, 1 (11), 1233−1236. (15) Poly(vinyl alcohol) is marketed under the tradenames Mowiol from Millipore Sigma, Elvanol from DuPont, Celvol/Selvol from Sekisui, and Polyviol from Wacker. (16) Finch, C. A. Some Properties of Polyvinyl Alcohol and their Possible Applications. In Self-Healing Polymers and Composites; Finch, C. A., Ed.; Springer, Boston, MA, 1983; pp 287−306. (17) Solaro, R.; Corti, A.; Chiellini, E. Biodegradation of Poly(vinyl alcohol) with Different Molecular Weights and Degree of Hydrolysis. Polym. Adv. Technol. 2000, 11, 873. (18) Hyon, S.-H.; Cha, W.-I.; Ikada, Y.; Kita, M.; Ogura, Y.; Honda, Y. Poly(vinyl alcohol) Hydrogels as Soft Contact Lens Material. J. Biomater. Sci., Polym. Ed. 1994, 5 (5), 397−406. (19) Jiang, S.; Liu, S.; Feng, W. PVA Hydrogel Properties for Biomedical Application. Journal of the Mechanical Behavior of Biomedical Materials 2011, 4 (7), 1228−1233. (20) McLaughlin, K. W.; Wyffels, N. K.; Jentz, A. B.; Keenan, M. V. The Gelation of Poly(vinyl alcohol) with Na2B4O7•10H2O: Killing Slime. J. Chem. Educ. 1997, 74 (1), 97−3. (21) de Zea Bermudez, V.; de Almeida, P. P.; Seita, J. F. How to Learn and Have Fun with Poly(vinyl alcohol) and White Glue. J. Chem. Educ. 1998, 75 (11), 1410−1418. (22) Hurst, G. A.; Bella, M.; Salzmann, C. G. The Rheological Properties of Poly(vinyl alcohol) Gels from Rotational Viscometry. J. Chem. Educ. 2015, 92 (5), 940−945. (23) Isokawa, N.; Fueda, K.; Miyagawa, K.; Kanno, K. Demonstration of the Coagulation and Diffusion of Homemade Slime Prepared Under Acidic Conditions without Borate. J. Chem. Educ. 2015, 92 (11), 1886−1888. (24) Hurst, G. A. Green and Smart: Hydrogels to Facilitate Independent Practical Learning. J. Chem. Educ. 2017, 94 (11), 1766− 1771.

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Activity

List of materials used, instructor’s notes, and example worksheets (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Patrick H. Willoughby: 0000-0001-5163-0502 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.K.M. and P.H.W. are grateful for financial support from the American Chemical Society Petroleum Research Fund Undergraduate Research Grant Program and the Ripon College Oyster Fund. T.M.M. and M.T.W. are grateful for financial assistant from the Dr. Terry Lindstrom and Augsburg University URGO program. Additionally, we would like to thank Hailey Mattheisen, Shelby Winchell, and Gabriella Mraz for extensively testing the robustness of certain aspects of the activity, Christina M. Othon for helpful discussions, and Denielle Stepka for designing the graphical abstract image. E

DOI: 10.1021/acs.jchemed.9b00161 J. Chem. Educ. XXXX, XXX, XXX−XXX

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(25) Sereda, G.; Hawkins, B. Introducing Students to the Medical Applications of Cross-Linked Hydrogels Using Nontoxic Materials and Experiments Suitable for Many Settings. J. Chem. Educ. 2018, 95 (11), 2068−2070. (26) Yokoyama, F.; Masada, I.; Shimamura, K.; Ikawa, T.; Monobe, K. Morphology and Structure of Highly Elastic Poly(vinyl alcohol) Hydrogel Prepared by Repeated Freezing-and-Melting. Colloid Polym. Sci. 1986, 264 (7), 595−601. (27) Peppas, N. A.; Stauffer, S. R. Reinforced Uncrosslinked Poly(vinyl alcohol) Gels Produced by Cyclic Freezing-Thawing Processes: A Short Review. J. Controlled Release 1991, 16 (3), 305− 310. (28) Hatakeyema, T.; Uno, J.; Yamada, C.; Kishi, A.; Hatakeyama, H. Gel−Sol Transition of Poly(vinyl alcohol) Hydrogels Formed by Freezing and Thawing. Thermochim. Acta 2005, 431 (1−2), 144−148. (29) Lozinsky, V. I.; Damshkaln, L. G.; Shaskol’skii, B. L.; Babushkina, T. A.; Kurochkin, I. N.; Kurochkin, I. I. Study of Cryostructuring of Polymer Systems: 27. Physicochemical Properties of Poly(vinyl alcohol) Cryogels and Specific Features of their Macroporous Morphology. Colloid J. 2007, 69 (6), 747−764. (30) Holloway, J. L.; Lowman, A. M.; Palmese, G. R. The Role of Crystallization and Phase Separation in the Formation of Physically Cross-Linked PVA Hydrogels. Soft Matter 2013, 9 (3), 826−833. (31) Hassan, C. M.; Peppas, N. A. Structure and Applications of Poly(vinyl alcohol) Hydrogels Produced by Conventional Crosslinking or by Freezing/Thawing Methods. Adv. Polym. Sci. 2000, 153, 37−65. (32) In 2015, the ACS Committee on Professional Training implemented updated requirements for ACS-approved programs, which explicitly expanded the role of macromolecules within undergraduate curricula. See: Committee on Professional Training. Undergraduate Professional Education in Chemistry: ACS Guidelines and Evaluation Procedures for Bachelor’s Degree Programs; American Chemical Society: Washington, DC, 2015. (33) Kosbar, L. L.; Wenzel, T. J. Inclusion of Synthetic Polymers within the Curriculum of the ACS Certified Undergraduate Degree. J. Chem. Educ. 2017, 94 (11), 1599−1602. (34) For an example of a sophisticated, inexpensive Arduino tensile tester, see: Arrizabalaga, J. H.; Simmons, A. D.; Nollert, M. U. Fabrication of an Economical Arduino-Based Uniaxial Tensile Tester. J. Chem. Educ. 2017, 94 (4), 530−533.

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DOI: 10.1021/acs.jchemed.9b00161 J. Chem. Educ. XXXX, XXX, XXX−XXX