Happy Polymer Party! M. Pilar Tarazona and Enrique Saiz Universidad de Alcala de Henares, 28871 Alcala de Henares, Spain Many high school students realize that, if they couldmake all the nolvmers thev encounter in a normal day disappear, a lot of things would be gone, including a great part of-themselves! However, students do not often realize that the importance of polymers is based on the special properties of the long chains that form the macromolecules (1,2). With the aim of showing some polymer properties to high school students, we planned a few experiments with the three prerequisites that thev had to be easv to c a m out. use c h e a ~ materials, and be able to be done in only o k class period. ~k present three of these experiments that illustrate some typical properties of polymers such as temperature dependence of stress-strain behavior, transitions between glassy . . and amorphous states occurring at the glass transition temperature T,,how this temperature can be lowered using solvents, and the processes of polymer swelling and dissolution. In addition, we thought that the experiments would be more appealing to students if they were presented as a "Polymer Party". Decorating for the Party Hang the streamers, and blow UD the balloons. The rubber balloo& are made of polymers. careful, putting the balloon too close to the heat will cause i t to expand. The air is a pas and expands a lot when heated. The pressure exerted by the air against the walls of the balloon increases one of rubber's properties, elasticity, and i t is capable of withstanding large deformations without bursting (and of spontaneously recovering almost to its initial dimensions after the stress is released) (3). The balloon keeps inflating, until it cannolonger hold the increase in Dressure and oow! Well let us pickupthe strips of the rubber balloon and use them to tie un wooden welcome signs we have made. This unusualdeco~atingfashioncatches ;.our fancy,and you blow up a few balloons yourself to keep in style. Again you hang one too close to a heat source, but you are surprised by the results. Ruhber does not behave like a metal spring, which becomes longer with increasing temperature, but behaves just the opposite. Stretched rubber actually becomes shorter when heated and longer when cooled. This behavior was observed by Gough as early as 1805 and can be easily tested (3). Materials Rubber band Hair dryer Piece of wood with graph paper Weights Fasten one end of the rubber band to the wood, and fix a weight at the other extremity (see the figure). Ply the rubber hand with successively hot and cold air with the hair dryer, and the rubber band will shrink or stretch as it gets hotter or colder. Tbis is easy to measure against the graph paper. The differences between our setup and more sophisticated eauipment used in research labs for these measurements are onlyihose required for accurate determinations of the forces applied t o the rubber, lengths of the sample, and careful control of temperature. An intermediate degree of sophistication is obtained with the apparatus described by Bader 238
Journal of Chemical Education
(4),which allows for quantitative measurements a t two fixed temperatures. But, even with this very simnle exneriment, two import.& features of the stress-strain behavior of polymers can be shown. (1) The elastic force produced by the sample is mainly due to changes in entropy (or disorder) rather than in energy. The unstretcbed state of the sample is a highly disordered "random coil" in which each macromolecule is changing among many different states or "conformations" (a bead necklace can be used to demonstrate this situation). Upon stretching the sample, t h e macromolecules a r e forced t o straighten, a n d many of the "compact" conformations are thus eliminated. Therefore the number of allowed states and the disSehrp In measuring polymer order or entropy of the sample ewnsion. decrease with little or no variation of energy. The syst e m onnoses t h i s entroov decrease by means of an elastic forc$tknding to recover tb; unperturbed state. As temperature increases, the macromolecules find i t easier to change from one conformation to another in the random coil state. I t might be said that they "feel" more comfortable when disorder increases with increasing temperature; therefore, they will try harder not to lose any of the allowed conformations. In other words, the elastic force opposing the stretching of the sample will grow stronger with increasing temperature. (2) The elastic force depends on the degree of crosslinking. Up to now, we have avoided the possibility that the macromolecules may iust separate without changina their random coil state as-aiesponse to the external f 0 G e . t the macroscopic level this will mean that the sample is not elastic but instead undergoes a permanent deformation and simply flows. Tbis kind of behavior can be observed if the experiment is run again replacing the rubber band by a strip cut from a plastic bag. The main performance difference between the strip of plastic bag and the rubber band is that the macromolecules of rubber have been "nailed" to each other bv means of chemical bonds to ensure that they will not separate when the sample is stretched: we say thai they are crosrr-linked. It is intuitive that the strenflh of the sample will depend strongly on the degree of cr&-linking. For example, as cross-linking increases in going from rubber bands to car tires to cured epoxy glue, the force required to stretch a sample of these materials increases.
At the Party
We are now a t the party, and the first thing we pick up is a disposable polystyrene plate for food. But the food is not here yet,so why not study the behavior of this polymer? How about the good old heating-a-plate-in-the-oven trick?
fore noticeably lowering the value of T,without causing the polymer todissolve. This converts a glassy and rigid polymer intoa flexible s a m ~ l that e can be shaoed and returned to the glass state by elimination of the solvent without changing the temperature, as we did in the second part of the experiment.
A Poivmer Game You need some flexible plastic clips for a game a friend have the stack of oolvstvrene wants to show off,. hut vouonlv plates.
We Run Out of Dishes At last the f w d is here, hut we have cut all the disposable plates into strips! What can we do? Since there are still some dispasable cups left, why not change a cup into a dish?
Materials Polystyrene plates Hair dryer
Materials Polystyrenc disposable cups Beakers Solvents ethyl acetate, acetone, ethyl alcohol, toluene, hexane (Caution must be used when working with these solvents, they are verv flammable. A fire extineuisher should be resdilv available. Good ventilation is required; they have a strong odor.) Watch glasses
Heating the plate with a hsrr dryer (or in a oven at a low temperature fur 5-10 mm) will cause it to revert t o s s m d saucer of disklike shape (5). We notice that it is not elastic like rubber but is thin enough to be flexible. Although its flexibility can be compared to that of a very thin strip of steel, we can find some differences between the behavior of the two materials. Materials Polystyrene disposable plates Scissors Watch glasses Ethyl acetate or toluene (Caution must be used when working with these solvents; both are very flammable. A fire extinguisher should be readily available. Good ventilation is required; they have a strong odor.) Hair dryer Cut strips of about 15 X 100 mm from the center of the dish (disposable cups can be used too, but then cut the strips from the walls). We can easily bend one of these strips by joining the two ends between our fingers, and it will recover its original form as soon as we let go. But, if, while keeping the ends together, we heat the middle of the strio with the hair drver for 1or 2 min and then let it cool off, the strip will stay folded whln the ends are released. The same result can be achieved with a solvent. (Caution-no sparks or open flame should be present.) While pressing the two ends together, dip the middle of the strip into a solvent such as ethyl acetate or toluene in a watch glass for a few seconds, and then let it evaporate. The strip will stay folded. Steel does not do this unless heated uery hot. We have explained that segments of the macromolecules of rubber are rapidly moving among many different conformations in random coils, and that the macroscopic properties of the amorphous state are characterized by elasticity and deformahility. However, if the temperature is lowered, these macromolecules find i t harder to move. At a low enoueh temnerature chanees in conformations are no loneer possilble. ~ i w e v e rthe , ma&ornolecules are not ordered, and the s a m ~ l eis not crvstalline. Nonetheless. thev cannot change firm, and the material is rigid. This is'easi6 demonstrated by dipping a rubber band into liquid nitrogen and seeing how it becomes rigid. We call this rigid form the glassy state of the polvmer, similar to the behavior of window glass. The glass tian;ition temperature, T,,is the temperat&e a t which the conformation of the macromolecules "freezes." This behavior is not exclusive of ruhher. and all oolvmen and many nonpolymeric materials have this kind of cansition. Polvstvrene has a T, of 360 K. and is rieid a t room temperat;rd, although it can be reshaped a t temperatures above T,and then hardened in the new shaoe . bv. lowerine the temperature as we did in the experiment. This behavior, although interesting, is not very exciting. After all, we know t h a t window glass and met& can bk reshaped by heating although the physics of the process is different. What is unusual for polymers is that a solvent may enter a glassy sample separating the macromolecules, thus allowing more room for conformational changes and there-
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First, we have to find a suitable solvent for this process. We cut cups into small pieces, put them into six beakers, and pour in the following solvents ethyl alcohol, acetone, toluene, ethyl acetate, hexane, and water (other solvents can be used too). After stirring and waiting 1 or 2 min, we notice that the pieces are not soluble in hexane. water. or ethvl alcohol: we alreadv know that these CUDS can be used for both ''aokt"and "hard" drinks. However.. the" a;e dissolving in ethyl acetate and toluene,while inaretone they only swell. Thus, aretone is our solvent of choice. Using tweeren o r a spatula, we remove the swollen polystyrene, put it between two watch glasses, and press. With a spatula or knife, we cut the excess polymer around the glass edge, then remove the upper glass to let the acetone evaporate. We have made s "dish." If we want to recover the cup pieces dissolved in toluene or ethyl acetate, we can precipitate the polymer by adding hexane to the toluene solution and ethvl alcohol or a mixture of ethvl alcohol and water to the ethyl acetate solution. The reason for not using just water for the latter is that water is only partially soluble in ethyl acetate. ~~
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The solubility of polymers is in many aspects similar to that of "small molecules", but it also presents some peculiarities. In the case of polystyrene in water, the interactions between molecules of oolvmer and water are weaker than those of polymer and wit& molecules each with themselves. The solid does not dissolve. In the cases of ethvl acetate. acetone, or toluene, the polymer-solvent interactions are r solvated. However, the stroneer, and the ~ o l y m e becomes large macromolecules so that each polymeris formedby of them will require many (on the order of thousands) molecules of solvent to be solvated. In addition, the macromolecules a t the surface may have parts of themselves buried deen in the solid so that thev cannot easilv he removed from the surface as can small molecules such as sugar dissolving in water. Instead, the molecules of solvent have to enter into the solid to loosen up the entangled chains in order tosolvate the macromolecules. This process is called swelling and can be easily observed as an increase of volume of the solid. If the solvent is "good" (the polymer-solvent interactions are strong), the swollen polymer dissolves as we observed in the cases of ethvl acetate or toluene. However, the solvent may be "poor", meaning that the interactions, although strong enough for swelling, cannot completely separate the macromolecules. If the polymer has heen cross-linked so that the chains cannot he separated by a solvent, only swelling can occur even for "good" solvents. In both cases, the polymers stay swollen but do not dissolve, as we observed with aceinteractone. In the swollen samole. the ~olvmer-~olvmer tions are weakened by solvation- (the T,bf the system is lowered), so that i t can be reshaped as we did bv oressine i t between the watch glasses. As the solvent is thenkGaporaGd, the sample hardens in its new form.
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Volume 67
Number 3 March 1990
239
If we add a had solvent to a solution of the polymer (for instance waterlethyl alcohol to the solution in ethyl acetate), the quality of the solvent decreases, and the polymer precipitates as a swollen solid that can he filtered and dried to recover the original polymer.
So Your Hands Are Getting Cold You have been holding your can of cold soda too long, and your hands are getting cold-polymer chemistry to the rescue! You decide to make a n insulated can holder out of polyurethane foam.
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Materials Polyurethane foammixture (two components needed) Plastic gloves A large paper cup or an empty milk carton Empty pop can that has been greased Stirring rod In a well-venrilared area, using gloves, pour about 25 ml. of each component into the large cup, and stir until the contents are well mired and the foam begins co expand. Place the rmpry. greased pup
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Journal of Chemical Education
can into the exoandine " foam. Do not handle the freshlv ..oreoared . foam as it may contain unreacted isocyanates. After several hours of curing, slide the can out of the cup. This insulated can holder is made by the additional polymerization of the polyester polyol, HO-R-OH, and polyfunctional isocyanate, OCN-R1-NCO. The heat of the reaction is enough to expand the blowing agent, causing the polyurethane foam to expand to 20-30 times its original volume (6). The party's over and acknowledgments are due. We are grateful to Carol F. Warren of the I.C.E. (U.A.H.) for helping with the preparation of this manuscript. Our students have found it a most enjoyable way to learn chemistry.
Llterature Cited 1. "State of the Art Symposium: Polymer Chemistry." J o u r ~ ofl ChemicolEducotion.
1981.58.836-955. 2. HorLl,A. J. J. ChemEdue. 1985.62.286292.
3. Flory. P. J. PrinnplesoiPo~ ! h e r Chemirtrv: Cornell Univ.: Ithaes. NY. 1953 4. Bader, M. J. Chem. td;e. 1981.58.285. 5. Rodrigun. F.: Mathil13, L. J.: Kroschwitz, J.;Carraher, C. E., Jr. J. Chem.Educ. 1988,
65,352-355. 6. Shakhanhiri, B. Chemical Damonslrnfions 1983; p 216.
Vol. I ; Univ, of Wisconsin: Msdison, WI,
