The Preparation and Simple Analysis of a Clay ... - ACS Publications

Laboratory Experiment pubs.acs.org/jchemeduc

The Preparation and Simple Analysis of a Clay Nanoparticle Composite Hydrogel David S. Warren,*,† Sam P. H. Sutherland,† Jacqueline Y. Kao,‡ Geoffrey R. Weal,† and Sean M. Mackay† †

Chemistry Department, University of Otago, Dunedin, New Zealand Science Knowledge Ltd., Taipei 11444, Taiwan, R.O.C.

‡

S Supporting Information *

ABSTRACT: Samples of a composite hydrogel incorporating clay (Laponite XLG and S-482) nanoparticles were prepared using N-isopropylacrylamide. The hydrogels were formed via a radical-initiated addition polymerization using potassium persulfate and N,N,N′,N′-tetramethylethylenediamine. Students then measured the force required to stretch the gels and obtained Young’s modulus for each system, finding the S-482-based hydrogels to be more elastic. The exercise was developed by students in the chemistry department for use in the Otago University Advanced Sciences Academy (OUASSA), a program run by the University for final-year high school students from rural and low-decile (lowsocioeconomic) schools. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Laboratory Instruction, Organic Chemistry, Polymer Chemistry, Public Understanding/Outreach, Hands-On Learning/Manipulatives, Addition Reactions, Alkenes, Materials Science

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popularize science.12 However, there are relatively few that look at their other applications, such as drug delivery1,5 or electrophoresis.3 Hydrogels can be made from a wide range of materials, such as poly(ethylene glycol) (PEG), poly(Nisopropylacrylamide) (PNIPAM), or even materials from natural sources such as collagen or alginate.9,10

INTRODUCTION Although polymers are almost ubiquitous in the modern world, a comprehensive review of the literature1 found that they are underrepresented in many books and courses. Relatively few of the laboratory exercises that use polymers1−4 are aimed at senior high school/first year students.5,6 This laboratory was designed to be a straightforward hands-on experience of preparing a modern, topical material. The intention is to show students how, using simple tests, the physical properties of tough hydrogels (toughgels) can be modified in a manageable and inexpensive manner. Hydrogels are materials made up of hydrophilic polymer chains held together via cross-links to form a solid, three-dimensional network and may contain large amounts of water. For example, baby diapers (British: nappies), which are cross-linked poly(acrylic acid-co-sodium acrylate), can retain much more than 90% water by mass. A hydrogel network retains the hydrophilic nature of the polymer chains, but the cross-links that connect the entire hydrogel network restrain the hydrogel from dissolving. This allows them to absorb large amounts of water and has led to their use in a variety of commercial applications, for example in waste management and agriculture,7,8 where their ability to absorb and release large amounts of water is a useful property. Their water-absorbing properties combined with their low stiffness and highly deformable nature means that they are also widely used in biomedical devices, implants, and drug delivery systems.4,9 Indeed, one of the first commercial uses of hydrogels was in the manufacture of soft contact lenses10 and they are also used in wound dressings and scaffolding for tissue implants.7,9,11 The water-absorbing properties of hydrogels are often the underlying principle used for lab work in high schools2,6 and are a favorite experiment for those seeking to © XXXX American Chemical Society and Division of Chemical Education, Inc.

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BACKGROUND Swelling of hydrogels as they absorb water can cause problems, in particular a loss of mechanical strength, and some gels will even fracture under their own weight as they absorb increasing amounts of water.7 This fragility limits the potential application of hydrogels. In the last 10 years or so there have been new approaches to cross-linking the gels that have produced socalled “tough” hydrogels (or “toughgels”) with much better mechanical properties, broadening the applicability of hydrogels.11,13 Mechanical testing of materials can be relatively simple,14 and the requirements for the testing used in this exercise was that it should be easily understood by a cohort with no background in the area and that the students should be able to compare generated data with literature values. Techniques to produce toughgels involve introducing a mechanism that prevents crack propagation and can be summarized in three broad fields: topological gels, doublenetwork gels, and nanocomposite (NC) gels. These are described below. Special Issue: Polymer Concepts across the Curriculum Received: June 1, 2016 Revised: March 21, 2017

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DOI: 10.1021/acs.jchemed.6b00389 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Topological Gels

These materials have polymer chains that are joined by mobile cross-links. Unlike the traditional image of polymers, having short side chains bonded to the polymer chains, these mobile cross-links are able to slide along the polymer chains (Figure 1).13,15 This allows strain to be evenly spread among the

Figure 3. Schematic illustrating how loosely cross-linked gels can interpenetrate a more rigid sample and prevent crack growth. Reprinted from ref 16. Copyright 2004 American Chemical Society. Figure 1. Schematics showing the principles behind topological gels: the mobile cross-links allow the chains to move and spread the applied load, and the cross-linking is maintained.

polymer chains (Figure 1), as opposed to a classic hydrogel, where the uneven length of the cross-links means strain is not evenly distributed, leading to breakage of the polymer chains (Figure 2).

Figure 4. Schematic showing how poly(NIPAM) chains can attach to small (nanometer-scale) clay discs and then spread the strain as a force is applied. Reprinted from ref 19. Copyright 2002 American Chemical Society.

hydrogels;21−23 those that do are usually based on acrylamide,22 a widely available material used extensively for electrophoresis.24 One of the attractions of NC gels as the basis of a teaching lab is the simplicity of the preparation,21 which occurs around room temperature and without stirring. In addition to the fact that they are relatively simple to prepare, the basic chemistry of these materials is at a level accessible to senior high school students around the world, who should be familiar with the theory of addition polymers. The preparation of toughgels will reinforce this knowledge and understanding and increase their appreciation of the applications of these polymers.21 The ACS Polymers across the Curriculum document25 identifies the need to bring macromolecular chemistry into foundation courses to enhance student interest. It then goes on to specifically mention alkene addition reactions, the importance of radicalinitiated polymerization, and the use of hydrogels. This project was used for a final-year high school group who are part of Otago University Advanced School Sciences Academy (OUASSA), an extension program run by the University of Otago that is intended to inspire and support students from rural and low-decile schools during their final year at school. As such, there were no specific learning outcomes associated with the activity. Rather, the two-day

Figure 2. Schematic showing how the short cross-linker chains in a conventional hydrogel break under an applied force, with a resulting loss of strength and elasticity.

Double-Network Gels

As their name suggests, these gels are composed of two networks that interlink with each other. A loose network with very low levels of cross-linking is spread through a second network that is rigid and highly cross-linked. The loose network fills the voids within the more rigid network and absorbs the elastic energy around cracks as they form (Figure 3). Nanocomposite Gels

In these gels, the polymer chains are cross-linked by interactions with inorganic clay nanoparticles rather than organic cross-linkers (Figure 4). As the gel is stretched, the polymer chains untangle and spread the strain through the clay particles into other chains.17−20

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ABOUT THE ACTIVITY It is surprising, given the widespread use of these materials, that there are few examples of laboratory preparations involving B




Table 1. Selected Monomers Used To Prepare Hydrogels

from two different clay samples. Since the students were still in high school, the collected data were plotted “in reverse” to how it would be done in a tertiary material science course, i.e., force was plotted on the x axis and extension on the y axis. This is in line with the high school science approach of plotting the controlled variable on the x axis and the dependent variable on the y axis. This approach was employed to allow students to compare gel samples more easily, since a steeper slope indicated more extension, i.e., a weaker gel sample. In view of the academic level of the students involved in this project, some of the general outcomes expected were (i) a simple appreciation of the behavior of materials under an applied force, (ii) a simple explanation of radical-initiated addition polymerization, and (iii) an understanding of how clay− NIPAM interactions could result in a stronger gel. However, for use in higher-level courses such as those reported by Schueneman and Chen,21 more in-depth investigations of each aspect could be carried out as extension projects. They used the thermal sensitivity of a NIPAM-based system to develop skills and awareness in an upper-level chemistry course. Other extensions could include variation of the initiator system, which has been shown to affect the mechanical properties of NC-type hydrogels,33,34 or more complete coverage of the mechanical properties of these hydrogels in line with published data.14,18,19,35−37 Higher-level courses would also, of course, plot data in the more conventional manner instead of that used here.

activity as a whole was assessed to measure the students’ engagement. The lab was developed by a small group of chemistry students as part of their involvement in the Chemistry Outreach Program running at the University of Otago. As can be seen in Table 1, many of the monomers used for hydrogels contain CC bonds that open to form the backbone of the polymer, i.e., in a wide range of instances hydrogels are formed by addition polymerization, making this an appropriate and fascinating project for the target group.10 Projects used in the OUASSA camps are often based on current research from within the department,26−30 and this exercise was based around work carried out by one of the authors during his M.Sc. studies.31 These projects are intended to stimulate interest in chemistry through insights into this research rather than to deliver course content. This project used NIPAM (Figure 5) as the monomer. The poly(NIPAM) was cross-linked using synthetic hectorite clays

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EXPERIMENTAL SECTION

Recrystallization of the NIPAM Monomer

Figure 5. Structure of N-isopropylacrylamide (NIPAM).

This is optional for students; it is required for the production of reproducible results but can always be carried out prior to the experiment. However, in the work described here, the recrystallization was carried out by the students as part of their two-day chemistry lab. Full details of the methods and background provided to the students may be found in the Supporting Information. Typically, 5 g of NIPAM was dissolved in 20 mL of toluene. The toluene solution was then added slowly to a beaker of petroleum ether (50 mL), and the beaker was placed in an ice/salt/water bath for 20 min. The resulting white crystals were filtered through a sintered glass funnel and air-dried for 30 min. Yields were usually in the region of 80%.

(Laponite XLG and S-482).32 It was successfully used with three school groups in 2015 and 2016, a total of about 70 finalyear high school students. The clay samples are slightly different in their properties: S-482 can be used at higher concentrations and produces more robust samples that are easier to handle. By measuring the change in length of the polymer samples as force was applied, students could obtain an approximate value of Young’s modulus for the samples produced by the class, allowing a comparison to be made between the gels prepared C




(Figure 7). The two clips were lined up with the marks, and the distance between the clips was measured. Masses were added incrementally, and after each addition the students recorded the length and mass in a spreadsheet provided by us with all of the relevant calculations inserted. This continued until the cylinder broke. For samples prepared from XLG, short pieces of preweighed copper wire were used to increase the mass (Figure 7), whereas the more robust S-482 samples were stretched using 1 mL aliquots of water added to a preweighed “ziplock” bag (Figure 8).

This step is essential for the production of good-quality gels; impurities are common, and using material straight from the bottle may result in a slightly yellow NIPAM sample.24 Sample Preparation

The following stock solutions were used to prepare the hydrogel samples: potassium persulfate (KPS) (0.02 g per 1 mL of distilled water; 7.4 mol L−1) and N,N,N′,N′-tetramethylethylenediamine (TEMED) (72 μL per 468 μL of distilled water; 0.1 mol L−1). Clay dispersions (Laponite XLG, 1.5% w/ w, and Laponite S482, 10% w/w; BYK Additives and Instruments, Wesel, Germany) were prepared by degassing distilled water for 30 min using a nitrogen flow and then adding the clay to the water over a period of 30 s with rapid stirring. The dispersion must continue to be degassed and stirred while in use for the preparation of the samples since oxygen acts as an inhibitor of the initiation process and needs to be excluded as much as possible.24 The composition of the initiator has been shown to affect the mechanical properties of hydrogels.1,2 NIPAM (0.5625 g, 5 × 10−3 mol) was weighed into a sample vial with 4.5 mL of the clay dispersion with stirring. After the NIPAM had dissolved, 100 μL of the TEMED solution was added. After brief stirring, the solution was degassed by bubbling nitrogen through it for a minimum of 15 s. Then 0.5 mL of the KPS solution was added using a plastic 5 mL syringe. The whole solution was gently drawn back up into the syringe body to ensure proper mixing without introducing air bubbles. A balloon filled with nitrogen was placed over the open end of the syringe to prevent oxygen exposure, and the sample was left overnight at 20 °C. The next day the balloon was removed, and the end of the syringe was cut off near the tip using a sharp blade. The contents of the syringe were removed by gently compressing the plunger. The system of acrylamide and TEMED/persulfate initiator is well-known in the world of electrophoresis gels, and troubleshooting sheets can be found on the Web if difficulties are encountered.24

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HAZARDS Because of the nature of the chemicals involved in this reaction, it is important that instructors carry out a full MSDS search before using the procedures described here and in the Supporting Information. Toluene, petroleum ether, and TEMED are all highly flammable liquids (and vapors). They should be kept away from sources of ignition. Their harmfulness and toxicity are also issues, so it is highly recommended that they be used in a fume hood and that the students carrying out the procedures wear gloves. Waste products should not be released into the environment. Students should only handle the dilute TEMED solution used for the experiment and should wear gloves when doing so. NIPAM is harmful and a skin irritant. Students should wear gloves and not handle the monomer. The polymer will have a minimal amount of the monomer left in it, but it is advised that that students wear gloves when handling the hydrogel. Potassium persulfate is an oxidizing substance, and the dry powder must not come into contact with the organic substances used in this experiment. Students should wear gloves when using the solution. The waste products must not be released into the environment. A full list of hazard statements can be found in the Supporting Information along with CAS numbers for all of the chemicals used.

Stress Testing the Samples

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The main focus of the lab was the toughgel synthesis, requiring the testing protocol to be simple and easy to follow. The gel sample was placed on a plastic ruler (Figure 6), two marks were made approximately 0.5 cm from each end, and the distance between them was measured. This value represents the unstretched (i.e., zero-force) length. The hydrogel was then suspended from a small bulldog clip mounted on a lab stand, and preweighed masses were added

RESULTS Typical results from the students’ 1.5% XLG hydrogel samples are shown in Figure 9. In general, there is a long period of gradual stretch from the initial length, which is approximately linear as increasing force is applied. The small steps in some of the data are taken to be the gel slipping in the clips since the slope after the step is approximately the same as before. The gels usually break before reaching their elastic limit, i.e., the point at which the polymer chains start to stretch past each other and cannot then return to the original length when the force is removed. They usually break at the stress point that is caused by the clip used to hold them. Since the slope of the plot is linear, this presents no problem for the calculation of Young’s modulus, but it does mean the students cannot see the behavior after the elastic limit is reached. The preparation of extra samples that the students could deform by pulling with their hands allowed them see and feel this effect even if they could not measure the force required. Adding force past this point introduces permanent damage to the polymer; the slope of the plot steepens, and the sample eventually snaps. There are hints of this behavior in some of the data in Figure 9. Occasionally one of the samples would exhibit it in a more obvious manner. This can also be seen in Figure 10, where the data show an initial linear trend up to a force of ∼0.2 N and then an

Figure 6. Typical NC hydrogel sample after removal from the syringe. D




Figure 7. Measurement setup for the stretch test for XLG hydrogels. The distance between the bulldog clips was measured. The copper wires were preweighed and used as masses. The photograph on the right shows a nice transparent sample and illustrates the necking that occurs as the sample force is applied.

Figure 9. Typical student results for NC hydrogels based on a 1.5% (w/w) XLG clay suspension. All of the samples had a cross-sectional area of 1.04 × 10−4 m2 (see Table 2).

Figure 8. Students measuring the stretch of S-482 samples. The samples were more robust, and greater mass could be used than for the XLG samples. The ziplock bag was preweighed, and the mass was increased by adding 1 mL aliquots of water.

increasingly steep gradient until the gel finally breaks under an applied force of approximately 0.55 N. The linear data can be used to calculate Young’s modulus (E) for each sample: E=

Figure 10. Results for a sample prepared from a 1.5% (w/w) XLG clay suspension with a cross-sectional area of 1.04 × 10−4 m2, illustrating behavior past the elastic limit.

which can be expressed in terms of the force (F) applied over the area (A) of the sample cross section, the change in length (ΔL), and the original length (L) as

tensile stress tensile strain E


Journal of Chemical Education E=


F /A FL = ΔL /L AΔL

Therefore, from a plot of ΔL versus F (conceptually an easier graph than a stress/strain curve for the students at this level), the Young’s modulus can be found by L E= A(gradient) The calculation assumes that the cross-sectional area remains constant throughout the extension. In the case of the samples tested here this would not be true, but the assumption is that any change would be negligible within the bounds of the experimental procedure used for the testing. Applying the equation to the samples shown above (Figure 8) gives values (Table 2) that compare favorably with literature data but Figure 11. Typical student results for NC hydrogels based on a 10% (w/w) S-482 clay suspension. All of the samples had a cross-sectional area of 1.04 × 10−4 m2.

Table 2. Typical Student Results for Samples Prepared from 1.5% (w/w) Laponite XLG Suspensions Sample 1 2 3 4

L/m 0.041 0.037 0.066 0.035

A/m2 1.04 1.04 1.04 1.04

× × × ×

−4

10 10−4 10−4 10−4

Gradient/m·N−1

E/Pa

0.2641 0.2444 0.1852 0.3845

1493 1417 1981 876

Table 3. Student Results for Samples Prepared from 10% (w/w) Laponite S-482 Suspensions Sample 1 2 3

cannot be compared directly since the cross-sectional area for each gel was based on the interior diameter of the syringe rather than the actual area of the gel sample. Each value represents one sample prepared by a group, and since the testing procedure usually damaged the sample, it was not possible to retest samples. These results are typical from the classes that prepared samples using this clay (70 students total). Variations in the results, such as the low values for sample 4, allow such outriders to be identified and possible causes discussed. Comparison of the slopes also allowed students to see that the change in extension in relation to force applied was the important factor in calculating the modulus, as opposed to the total extension. Most samples snapped when they were 550−800% longer than the original sample, with little evidence of the elastic limit. Considering the level of the students carrying out the work, this compares favorably with the 1000% increase reported in published data.17 In view of the level of the students involved in this exercise, these simple plots were considered satisfactory, as they allowed the students to see the direct link between the extension and the force applied. However, with more advanced classes it would be possible to introduce the concept of strain and plot stress/strain curves for the samples. This would allow more direct comparisons with the literature, which are normally reported as stress/strain. Upon comparison of the XLG-based hydrogels with those prepared from S-482, it becomes apparent from the data that the slope of the graph is much lower for S-482 (Figure 11), indicating less stretch for a given applied force and thus a much tougher hydrogel. This is confirmed by calculation of the Young’s modulus for these gels (Table 3). Again, these results represent data from single samples and are typical of those prepared by the classes who prepared samples from this clay (70 students total). The behavior is a result of the greater clay concentration that is possible with S-482 because of the

L/m 0.01 0.025 0.04

A/m2 −4

1.04 × 10 1.04 × 10−4 1.04 × 10−4

Gradient/m·N−1

E/Pa

0.0148 0.1354 0.0536

6503 6797 7182

presence of dispersing agents, allowing the formation of a stable sol when dispersed in water.

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DISCUSSION It is important to note that the aim of this laboratory, as it was delivered in our program, was not to teach the calculation of Young’s modulus. Rather, it was to demonstrate the preparation of an addition polymer and explore how its mechanical properties can be tuned through the application of published research. The stress/strain testing of the gels was intended to match the intuitive understanding that the students have regarding the stretching of, for example, rubber bands, with an insight into how scientific measurements can be used to quantify these everyday experiences. The students collected the data and filled in a preformatted spreadsheet. The plots produced from their data provided a visual representation of their measurements, which then generated discussions around the class results. The overall project was received favorably by the students, with 91% indicating a strong interest in the subject and the same proportion indicating that it had made them more enthusiastic about chemistry as a subject. There were many passionate comments about the hands-on practical approach to chemistry, something a lot of these students noticed was lacking in school. The project proved to be a useful teaching exercise in terms of simple addition polymerization and supported the students’ learning; the majority of the students (73%) reported the level was just right for them, with the remainder reporting that it was a little easy and no students classifying the level as too difficult. Student feedback on the preparation and discussion of everyday applications of the materials, or “real-life chemistry”, was highly positive in both years the project has been run. The added bonus of the tactile experience of “playing” with the samples added to the students’ experience and involvement, where they could stretch them by F




High School Chemistry and Biology. J. Chem. Educ. 2013, 90 (7), 918−921. (6) Chen, Y.-H.; He, Y.-C.; Yaung, J.-F. Exploring pH-Sensitive Hydrogels Using an Ionic Soft Contact Lens: An Activity Using Common Household Materials. J. Chem. Educ. 2014, 91 (10), 1671− 1674. (7) Naficy, S.; Brown, H. R.; Razal, J. M.; Spinks, G. M.; Whitten, P. G. Progress Toward Robust Polymer Hydrogels. Aust. J. Chem. 2011, 64 (8), 1007. (8) Ahmed, E. M. Hydrogel: Preparation, Characterization, and Applications: A Review. J. Adv. Res. 2015, 6 (2), 105−121. (9) Peppas, N. A.; Hilt, J. Z.; Khademhosseini, A.; Langer, R. Hydrogels in Biology and Medicine: From Molecular Principles to Bionanotechnology. Adv. Mater. 2006, 18 (11), 1345−1360. (10) Kopeček, J.; Yang, J. Hydrogels as Smart Biomaterials. Polym. Int. 2007, 56 (9), 1078−1098. (11) Rennerfeldt, D. A.; Renth, A. N.; Talata, Z.; Gehrke, S. H.; Detamore, M. S. Tuning Mechanical Performance of Poly(ethylene Glycol) and Agarose Interpenetrating Network Hydrogels for Cartilage Tissue Engineering. Biomaterials 2013, 34 (33), 8241−8257. (12) Helpful Hydrogels. https://www.stevespanglerscience.com/lab/ experiments/helpful-hydrogels/ (accessed March 21, 2017). (13) Tanaka, Y.; Gong, J. P.; Osada, Y. Novel hydrogels with excellent mechanical performance. Prog. Polym. Sci. 2005, 30 (1), 1−9. (14) Rodriguez, F. Classroom Demonstrations of Polymer Principles: IV. Mechanical Properties. J. Chem. Educ. 1990, 67 (9), 784. (15) Okumura, Y.; Ito, K. The Polyrotaxane Gel: A Topological Gel by Figure-of-Eight Cross-Links. Adv. Mater. 2001, 13 (7), 485−487. (16) Na, Y.-H.; Kurokawa, T.; Katsuyama, Y.; Tsukeshiba, H.; Gong, J. P.; Osada, Y.; Okabe, S.; Karino, T.; Shibayama, M. Structural Characteristics of Double Network Gels with Extremely High Mechanical Strength. Macromolecules 2004, 37 (14), 5370−5374. (17) Haraguchi, K. Synthesis and Properties of Soft Nanocomposite Materials with Novel Organic/Inorganic Network Structures. Polym. J. 2011, 43 (3), 223−241. (18) Haraguchi, K. Nanocomposite Hydrogels. Curr. Opin. Solid State Mater. Sci. 2007, 11 (3−4), 47−54. (19) Haraguchi, K.; Takehisa, T.; Fan, S. Effects of Clay Content on the Properties of Nanocomposite Hydrogels Composed of Poly(N -Isopropylacrylamide) and Clay. Macromolecules 2002, 35 (27), 10162−10171. (20) Haraguchi, K.; Li, H.-J.; Matsuda, K.; Takehisa, T.; Elliott, E. Mechanism of Forming Organic/Inorganic Network Structures during In-Situ Free-Radical Polymerization in PNIPA−Clay Nanocomposite Hydrogels. Macromolecules 2005, 38 (8), 3482−3490. (21) Schueneman, S. M.; Chen, W. Environmentally Responsive Hydrogels. J. Chem. Educ. 2002, 79 (7), 860. (22) Silversmith, E. F. Free-Radical Polmerization of Acrylamide. J. Chem. Educ. 1992, 69 (9), 763. (23) Paddock, J. R.; Maghasi, A. T.; Heineman, W. R.; Seliskar, C. J. Making and Using a Sensing Polymeric Material for Cu2+: An Introduction to Polymers and Chemical Sensing. J. Chem. Educ. 2005, 82 (9), 1370. (24) Menter, P. Acrylamide PolymerizationA Practical Approach. http://www.bio-rad.com/LifeScience/pdf/Bulletin_1156.pdf (accessed May 30, 2016). (25) Polymers across the Curriculum: Macromolecules as a Unifying Theme Across Foundational Courses in Chemistry. https://www.acs. org/content/dam/acsorg/about/governance/committees/training/ acsapproved/degreeprogram/polymers-across-the-curriculumsupplement.pdf (accessed May 24, 2016). (26) Goswami, S. K.; McAdam, C. J.; Lee, A. M.; Hanton, L. R.; Moratti, S. C. Linear Electrochemical Actuators with Very Large Strains Using Carbon Nanotube-Redox Gel Composites. J. Mater. Chem. A 2013, 1 (10), 3415−3420. (27) Goswami, S. K.; Ghosh, S.; Mathias, L. J. Thermally Stable Organically Modified Layered Silicates Based on Alkyl Imidazolium Salts. J. Colloid Interface Sci. 2012, 368 (1), 366−371.

hand and watch them return to the original size. Extra samples can be prepared to allow the students to play with gel samples; the S-482-based gel was especially good for this given its greater strength. The stronger gel produced using S-482 makes it a more suitable material for use in the laboratory, whereas the XLG-based hydrogel requires careful handling when it is removed from the syringe. Thus, we have used S-482 clay far more often than XLG. To illustrate the role of the clay in the NC toughgels, a simple “classic” example of cross-linking was used. This was based on the poly(vinyl alcohol) (PVA)/borax system, widely used in schools around the world. By adding corn starch to the PVA before addition of the borax solution, the properties of the resulting gel can be altered. By preparing samples with differing amounts of corn starch, the students could optimize the amount of corn starch to produce the best “toughgel”. This allowed them to create a simple model of the NIPAM toughgels, and they were encouraged to freely explore the limits of this system by varying the amounts of each component. Through the combination of the NC toughgel and PVA/ borax/starch toughgel activities, the students are able to get a glimpse of how a research group would approach the development of a new material without enduring the timeconsuming process of academic research. It also affords them immediate feedback on how the properties of these materials depend on the composition of the starting reagents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00389. Student handouts, instructor notes, CAS registry numbers, and safety notes (PDF, DOCX)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

David S. Warren: 0000-0002-8080-6615 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the OUASSA classes of 2015 and 2016, the students attending the 2016 Madame Curie Chemistry Camp, and Lisa Bucke for graphic design.

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REFERENCES

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