In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
Solvent Swelling and Optical Rotation Demonstrated on the Overhead Projector
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Dean J. Campbell* and Sarah B. Rupe Department of Chemistry, Bradley University, Peoria, IL 61625; *
[email protected] Various methods and precautions for casting Sylgard Elastomer 184 PDMS are discussed in refs 2 and 3. To fashion slabs of cured PDMS, make a smooth-bottomed tray out of aluminum foil to use as a mold. The foil must not have any holes though which uncured PDMS can leak. Fill the tray with the uncured thoroughly mixed base and curing agent to the desired slab thickness. The recommended mass ratio of curing agent/base should be in the range from 1:5 to 1:25. The PDMS might not cure well with mass ratios outside of this range. Curing can take place over two days at room temperature or within an hour at 100 °C (i.e., in a drying oven). The foil tray can be peeled away from the cured PDMS slab. The cured slab can be cut into smaller pieces with a razor blade or scissors. The cross-linked elastomer swells reversibly upon exposure to nonpolar organic solvents such as toluene (2) or cyclohexane (Fig. 1). The swelling of the PDMS in these nonpolar 450
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Toluene (CAS #108-88-3—listed as toxic) and cyclohexane (CAS #110-82-7—listed as an irritant) are available from Aldrich, Milwaukee, WI. (R)-(+)-Limonene (CAS #598927-5) and (S )-(᎑)-limonene (CAS # 5989-54-8) are also available from Aldrich and are listed as irritants. (R)-(+)-Limonene is also listed as a cancer-suspect agent. Therefore, it is recommended that materials containing these organic solvents, such 876
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Safety
as slabs of PDMS (when not being used for demonstrations) and unsealed containers of solvents, be stored in a fume hood. Gloves and goggles should be worn when working with all of these solvents. The Dow Corning Sylgard Elastomer 184 Kit components are relatively harmless, but should not be ingested or allowed to contact the eye. The use of latex gloves is recommended. Contact of the unreacted kit components with strong acids, bases, or oxidizing materials may generate hydrogen gas and should be avoided.
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Polydimethylsiloxane (PDMS) can be produced in the form of a colorless, transparent elastomer that can be used for a variety of applications (1). Elastomers are polymers that exhibit elastic behavior over a relatively wide range of applied forces. That is, they deform upon application of a force and return to their original size and shape when the force is removed. One form of PDMS is marketed as a two-part kit, Sylgard Elastomer 184, by the Dow-Corning Corporation. The two components, referred to as the base and curing agent, contain PDMS oligomers with sites that link together (aided by a platinum-containing catalyst) when the components are combined. The combined liquid components, when poured into a mold and cured, yield a transparent, cross-linked, rubbery solid. The ability of the PDMS to be cured in a mold allows the final product to take on a variety of shapes. The curing process, which involves the addition of Si–H bonds to C=C bonds, does not produce any by-products and results in a transparent polymer. In addition, varying the relative amount of curing agent to base alters the number of crosslinks that form in Sylgard Elastomer 184. More curing agent mixed into the PDMS oligomers before curing yields more cross-linking. This form of PDMS has been used for educational demonstrations and experiments in the form of flexible, transparent diffraction gratings that can demonstrate the reciprocal lattice effect and the copy and compression of surface microstructures (2). The polymer has also been used for educational experiments involving composite materials such as flexible refrigerator magnets (3). The transparency of this form of PDMS allows its use in demonstrations on an overhead projector. The cross-linked elastomer swells reversibly upon exposure to nonpolar organic solvents such as toluene (2), cyclohexane, and limonene. This paper describes how reversible solvent swelling of the PDMS with organic solvents can be shown on an overhead projector. Evaporation of the solvents from a swollen slab of cured PDMS can result in dramatic crack formation throughout the slab. Use of optically active solvents such as the (R)-(+) and (S)-(᎑) enantiomers of limonene results in the solvent-swollen slab having optical activity that can be shown using an overhead projector.
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Figure 1. Percent (left) mass increase and (right) volume increase as a function of time for cured PDMS slabs of various curing agent/ base mass ratios soaked in cyclohexane. Mass ratio of curing agent to base: 䉬 1:05; 䊏 1:10; 䉱 1:15; × 1:20; ×| 1:25; 䊉 1:30.
Journal of Chemical Education • Vol. 77 No. 7 July 2000 • JChemEd.chem.wisc.edu
In the Classroom
Figure 2. (A) Unsoaked cured PDMS slabs with compositions of (left) 1:10 and (right) 1:20 mass ratios of curing agent:base. (B) The same slabs after soaking in cyclohexane for 24 h. (C) The same slabs after being placed in air for 24 h. Cyclohexane leaving the cured PDMS slabs too quickly causes the slabs to crack.
solvents is dramatic; the swollen slabs can be more than triple the mass and triple the volume of the unswollen slabs for a 1:25 curing agent/base mass ratio. PDMS slabs with lower levels of cross-linking are able to swell to a greater extent in the nonpolar solvents (Figs. 2A and 2B) (2). The cross-linked elastomer exhibits little or no swelling in contact with polar solvents such as water, ethanol, and acetone. A demonstration could proceed by placing two similarsized PDMS slabs on an overhead projector, one in air or a transparent container such as beaker with a polar solvent such as water, the other in a transparent container with a nonpolar solvent. The slab must be covered completely with solvent so it absorbs solvent through all its sides. The diameter of the container should be about 50% larger than the largest dimension of the slab to allow the slab to swell without restriction. Covering the container containing the organic solvent with a Petri dish will help minimize solvent vapor
release. The solvent swelling of a thin (~5 mm) cured PDMS slab in direct contact with toluene or cyclohexane is visible in less than an hour. Complete swelling of the PDMS samples takes place within 24 hours. If insufficient time is available for the entire demonstration in class, the soaking and solvent swelling can be done before the class meets. When the samples are left out of direct contact with solvent for a period of time (less than 24 hours), they lose the solvent trapped within the polymer and return to their original size. Care must be taken to use samples with a relatively high surface-to-volume ratio to promote facile solvent loss. This is more readily achieved with slabs that are less than 5 mm thick before swelling. A swollen sample with a relatively low surface-to-volume ratio has a tendency to lose solvent faster and to contract more near its surfaces than the interior of the sample, which is still swollen with solvent. The resulting stresses within the polymer sample are capable of creating large cracks in the polymer (Fig. 2C). In fact, even if thin slabs are completely swollen with a very volatile solvent such as diethyl ether and then removed from the solvent, the solvent trapped within the swollen polymer will immediately begin to evaporate and cause cracking of the PDMS. This process can be shown on an overhead projector over a time period of about 30 minutes. The PDMS slabs also swell reversibly in the presence of limonene enantiomers. These compounds are optically active, and when they are absorbed by the PDMS, the swollen elastomer also becomes optically active. Therefore, the limonene acts as an optical marker of the swelling process. This limonene demonstration can also be made into an overhead projector demonstration (Fig. 3). Place a polarizer sheet on the overhead projector. Next place an unswollen PDMS slab, an (R)-(+)limonene-swollen PDMS slab, and an (S )-(᎑)-limoneneswollen PDMS slab on the polarizer sheet. Finally, hold a second polarizer sheet above the slabs. Rotate the top sheet relative to the bottom sheet until the minimum amount of light passes through the sheets. As the top polarizer is rotated clockwise, one of the limonene-soaked slabs will appear darker than the others. As the top polarizer is rotated counterclockwise, the other limonene-soaked slab will appear darker than the other slabs. This indicates that the swollen slabs are optically active relative to the unswollen slab. Additionally, PDMS soaked in a 1:1 mixture of R,S-limonene does not exhibit optical rotation. Soaking different portions of a single PDMS slab in either the (R)-(+) or (S)-(᎑) enantiomers of
Figure 3. (A) Method for demonstrating optical rotation of solvents trapped within PDMS slabs with a composition of 1:20 curing agent/base mass ratio. The demonstration uses an overhead projector and two polarizers. (B) Photograph of the projection screen showing optical rotation in a slab soaked with (left) ( R )-(+) - limonene; (middle) an unsoaked cured slab; and (right) optical rotation in a slab soaked with (S)-(᎑)-limonene. (C) The same slabs with a different orientation of the top polarizer with respect to the bottom polarizer, resulting in a different shading of the PDMS slabs. (Note the shift in position of the triangle, which is a marking on the top polarizer.)
JChemEd.chem.wisc.edu • Vol. 77 No. 7 July 2000 • Journal of Chemical Education
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school and the college level wherever polymer science and technology are discussed. Acknowledgments We are grateful to the Bradley University Teaching Excellence Committee for generous support of this research. We thank R. G. Neimi and Brian Reno of the Dow-Corning Corporation and Barbara Bruce of Ellsworth Adhesive Systems, Inc., for their advice and generous donation of PDMS elastomer components. More information and PDMS experiments are located on the Education and Outreach Web site for University of Wisconsin–Madison Materials Research Science and Engineering Center (MRSEC) for Nanostructured Materials and Interfaces: http://mrsec.wisc.edu/edetc/educat.htm. W
Figure 4. A slab of cured 1:30 curing agent/base mass ratio is soaked with one end in (R)-(+)-limonene and then the other end is soaked in (S)-(᎑)-limonene. (A) The slab is then placed between two polarizers on an overhead projector. The result is a difference in shading between the ends of the slab. (B) The same slab with a different orientation of the top polarizer with respect to the bottom polarizer, resulting in a different shading of the ends of the slab. (Note the shift in position of the triangle, which is a marking on the top polarizer.)
limonene results in these portions of the slab having different optical rotations (Fig. 4). As in toluene and cyclohexane, PDMS slabs with lower amounts of cross-linking are able to swell to a greater extent in limonene. Conclusions This paper describes a simple, graphic demonstration of solvent swelling of PDMS in nonpolar solvents. Solvent swelling of this transparent PDMS using optically active solvents provides additional evidence that the solvent is penetrating the polymer. This demonstration can be used at both the high
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Supplemental Material
The following supplemental material for this article is available in this issue of JCE Online. Solvents Mass.xls. A Microsoft Excel 97 document describing the absorption of various solvents with PDMS. PDMS slab mass is given as a function of time and PDMS crosslinking for each solvent. Solvents Volume.xls. A Microsoft Excel 97 document describing the absorption of various solvents with PDMS. PDMS slab volume is given as a function of time and PDMS crosslinking for each solvent. Literature Cited 1. Information About High Technology Silicone Materials; Dow Corning Corporation: Midland, MI, 1991. 2. Campbell, D. J.; Beckman, K. J.; Calderon, C. E.; Doolan, P. W.; Ottosen, R. M.; Ellis, A. B.; Lisensky, G. C. J. Chem. Educ. 1999, 75, 537. Campbell, D. J.; Campbell, K. C.; Billmann, J.; Ellis, A. B. University of Wisconsin–Madison MRSEC Education and Outreach; http://mrsec.wisc.edu/edetc/ edetc.html (accessed Mar 2000); specifically, Uses of Polydimethylsiloxane (PDMS) Elastomer; http://mrsec.wisc.edu/ edetc/PDMS.html (accessed Mar 2000). 3. Campbell, D. J.; Olson, J. A.; Calderon, C. E.; Doolan, P. W.; Mengelt, E. A.; Ellis, A. B.; Lisensky, G. C. J. Chem. Educ. 1999, 76, 1205–1211.
Journal of Chemical Education • Vol. 77 No. 7 July 2000 • JChemEd.chem.wisc.edu
