Demonstration of Solvent Differences by Visible Polymer Joseph H. Ross Indiana University at South Bend, South Bend, IN 46634 The experiment shows the effect of the "polarity" of lowpolarity solvents on the amount of swelling produced in a solid polymer. I t has heen used here as a demonstration during an organic chemistry laboratory lecture, hut it could also he used as a laboratory experiment, with each student assigned to a small group of solvents from a list. Although the emphasis here has been to show differences among solvents, the experiment could be used as a starting point for the discussion of polymer properties. In several parts of the curriculum the effects of hydrogen bonding and dipole moment are considered, most often in terms of ionic solutes and more polar solvents. On the other hand, when solvents are chosen for use in crystallization or chromatography, the differences between solvents of low polarity can be quite striking. It is not easy to set up a demonstration of differences in solubility. By using polymer swelling, which in a sense amounts to dissolving the solvent in the polymer, the experiment descrihed here produces a large visible effect within one hour's time. Distinctly polar aprotic and protic solvents produce only small amounts of swelling. Among the nonpolar solvents, differences hetween alkanes and cycloalkanes are clearly seen. The polymer used in the experiment is Norsorexa brand of polynorbornene, a synthetic elastomer (1-3) recently developed by CdF Chimie of Paris and available in North America from REG Associates. The polymerization involves a ring opening of norbornene (bicyclo[2.2.1] hept-2-ene) and produces a linear polymer with one five-membered ring and one double bond per repeat unit. The polymer is manufactured
as a oowder made un. of . oarticles with an exoanded structure and pours much like a granulated soap or detergent. It swells as it absorbs oreanic solvents and eventuallv dissolves in hetter solvents (solubility parameter -9) to give viscous solutions. (If the soluhility parameter is not familiar to you, see the explanation in the Discussion section.) Norsorexa has been used a t least once in marine oil spill recovery ( I ). Procedure In the demonstration 10 ml of each solvent was placed in a labeled 16 X 150-mm test tube ahead of time; hexane, cyclohexane, toluene, chlorobenzene, acetone, 2-propanol, and acetonitrile were used here. The polymer, 0.30 g, was measured into a separate small vial for each solvent, for ease of carrying to the classroom. At the beginning of the experiment the ~ o l v m e rwas added to each tube of solvent. followed bv until near the knd of the class, when the height of the polymer
-
Height of Polymer in Test Tube Experiments After One Hour
Height mm
Solvent Perfluorotributylamine 2,2,4-Trimethylpentane 2.3-Dimethylbutane Hexamethyldisiloxane n-Pentana n-Hexane 1.1.2-Trichloro-1.2.2.-trifluoroethane n-Heptane Diethyl ether BDecane Methylcyclohexane 2-Chloropropane Cyclopentane Cyclohexane Trifluoromethylbenrene Cyclooctane Ucane Alkylate 11 t-Butylbenrene Benmnitriie (fkLimonene (dipentene) 1.1.1-Trichloroethane 4-Methyl-Bpentanone Toluene Decahydronaphthalene Chloroform 1.2.3.4-Tetrahydronaphthalene Tetrahydrofuran Acetone Chlorobenzene 1-Methylnaphthalene Dichloromethane Nitrobenzene l-Decanol Butyronitrile l-Heptanol 2-Propanol l-Butanol Acetonitrile Nitromethane Acetic Acid Adiponitrile Methanol Water
Solubility Parameter ~a1"~
Reference
Comment a
a
a
a a
C
a a. c a a
a
a
a
The polymer floated. A long-chain alkylbenrene (see text). The height was more difficult to estimate because of the neartransparency of the ~"spension.
layer was measured with a ruler. The results could he observed qualitatively. One larger-scale experiment was conducted in a 100-ml graduated cylinder, for the benefit of those considering demonstrations to larger classes. The height a t different time intervals was also observed. A 3.00-g sample of polymer was added to 100 ml of toluene and stirred with a stirring rod. About 5 min standing was needed for the polymer to settle Volume GO
Number 2
February 1983
169
enough for a distinct line to be seen a t the 35-ml mark of the graduated cylinder. The total volume was 102.5 ml. At the end of 20 min the unner nart of the liauid was nearlv clear and the line had hecokerat'her sharp, a t t h e 47-ml mark. The height of the nolvmer. in terms of the milliliter markings. was 49.5 after lh hi, 52.5 after 1 hr, 54 after 2 hr, and 55 after 4 hr. In order to prevent the polymer settling rate Crom affecting the heights, the mixtures should probably he allowed to stand for a t least M hr. Although the polymer layer became more and more translucent, and later dissolved, on long standing in better solvents, the layer was clearly visible for the first hour except in tetrahydronaphthalene and l-methylnaphthalene. The tahle shows the heights reached by the polymer layer after one hour, when 0.30 g was added to various solvents in test tubes. The overall heieht of the combined liauid and solid " was around 65 mm. Heights were measured with a millimeter rule. outside of the test tuhes. from the bottom to the too of
a
tom or a t the meniscus will likely make difference in the apparent height, and for this reason the solvents in which the polymer floated are noted in the tahle. The values in the table are intended as a basis for examining qualitative trends and as a guide to readers wishing to select solvents for experiments, rather than for making quantitative calculations. Solvents were chosen so as to include straight-chain, branched, and cyclic hydrocarbons, alcohols and nitriles of different chain length, halides, and a few other functional groups. The polymer looked somewhat different in acetone and 4-methyl-2-pentanone (MIBK); the particles seemed to he clumped together. With butanone, cyclohexanone, methyl the clumnine acetate.. ethvl acetate.. dielvme. -. and .nwidine . . . was so pronounced that it was not useful to try to estimate the heights of the irregular piles of clumps. Solubility parameters of the clumping solvents range from 9.1 to 10.6. The disposal of water-insoluble polymers can he a problem. Because of the viscosity of solutions of polynorbornene, it is somewhat tedious to try to wash the polymer from tuhes with a "good" solvent. Stirring the contents of a tuhe with acetone r extracts or a similar " ~ o o r "solvent shrinks the ~ o l v m eand most of the good solvent. The polymir l&ps can then be scraped into a container with more acetone. After standing, the solvent can he decanted into a liquid-waste container, and the polymer, after further treatment with acetone, can be dried under a hood and disposed of with solid wastes. Most of the material remaining in the tuhes can he removed by brushing with soap and water. Materials
The sample of Norsorexa polynorbornene was kindly supd i e d hv American Cvanamid Co.. who handled the nolvmer at thattime. The poiymer is now available from REG ~ s s o ciates, and samples may be requested from Bob Gleason, REG Associated, P.O. Box 106, Bound Brook, NJ 08805. Some reported properties ( 2 , 3 ) are molecular weight, greater than 2 X lo6; softening point (glass transition temperature), about 35°C.; density when compounded, 0.96 g1cmVthis appears to apply approximately to the suspended polymer). R. F. Ohm, of R. T . Vanderhilt Co., Inc., Norwalk, CT, (private communication) has calculated a solubility parameter of 9.2 for the monomer. The polymer can be vulcanized, but the experiments were performed with unvulcanized polymer. Commercial or reagent grades of solvents were used. An attemnt was made to choose less toxic ones (toluene rather The diethyl ether was anhydrous reagent grade (not further dried), and the chloroform was reagent grade (which contains a small amount of ethanol as a stabilizer). Perfluorotributylamine, which is similar to a fluorocarbon, was included to show the small interaction of such compounds 170
Journal of Chemical Education
with other materials of low polarity. Because of the high price of perfluorotributylamine and the small swelling anticipated, only 2 ml was used. Hexamethyldisiloxane is the first member of the silicones. A dron of detereent solution was added to the tuhe of water in ordeito wet t h i polymer. Although the results with water can, of course, he nredicted aualitativelv, the exneriment provides an indicaGon of the bolymer height in an incompatible solvent. Ucane Alkylate 11 is a trademark of Union Carbide Corporation for a linear alkylbenzene used in the manufacture of detereents. The material was included here to show the " effect of an aromatic hydrocarbon with a long alkyl chain. The average alkyl chain length is 11.5 carbon atoms, and the phenyl group is attached near the end (13). A sample was kindly furnished by Union Carbide Corporation. Discussion
Values of the Hildebrand solubility parameter are included in the tahle, and the solvents are arranced in order of in-
thek.3 position, on the hasis of trend; for other hydrocarbons. For those unfamiliar with the Hildebrand solubility parameter (7, 91, it is a measure of intermolecular attractive forces based on heat of vaporization of a solvent. I t has been widely used for solvent classification (and polymer classification on the basis of solubility and swelling) by polymer chemists and has more recently been given attention in partition chromatoeranhv (141. The general
