William D. Joseph, Robert Romuald, and John P. Droske*
University of Wisconsin-Stevens Point
Department of Chemistry
Stevens Point, Wisconsin 54481
The "Nylon Rope Trick"1 is among the most popular chemical demonstrations. It involves formation of a nylon fiber during interfacial polycondensation of a diacid chloride monomer with a diamine monomer. Since fiber forms at the interface between two clear, immiscible liquids, the origin of the fiber is almost mystifying to the observer, allowing the demonstration to be both intriguing and educational. However, the "Nylon Rope Trick" generally has employed chlorinated solvents, such as carbon tetrachloride and tetrachloroethylene, which exhibit high toxicity (see Table I). This restricts the use of this demonstration, especially in pre-college classrooms.
Hexane offers lower toxicity than these chlorinated solvents and it is a useful solvent for this demonstration2. However, fibers formed with hexane as the organic solvent exhibit significant tackiness which makes removal of the fiber impossible in an interesting self-propelled demonstration3.
We have examined 1,1,2-trichloro-1,2,2-trifluoroethane (also known as Freon 113, a DuPont trademark) as an alternative solvent for this demonstration4. This solvent gives excellent fibers and offers much lower toxicity than any of the other solvents reported previously. A serious drawback to the use of this solvent is the phasing-out of the Freon solvents due to environmental concerns. Since there is not yet an "ideal" solvent for this demonstration, we report here a comparison of the solvents commonly used for the "Nylon Rope Trick"5.
| Table I Solvent Toxicity | ||
| Solvent | Safe Exposure Levels | |
| Carbon Tetrachloride | 10 ppm | |
| Tetrachloroethy lene | 100 ppm | |
| Cyclohexane | 300 ppm | |
| Hexane | 500 ppm | |
| Methylene Chloride | 500 ppm | |
| 1,1,2-Trichloro-1,2,2-trifluoroethane | 1000 ppm |
Source, NIOSHA/OSHA Pocket Guide to Chemical Hazards; Mackinson, F. W., Ed. U.S.G.P.O: Washington D.C., 1978
EXPERIMENTAL
Known experimental conditions were followed for the chlorinated solvents1 and hexane2. Clear, colorless sebacoyl chloride was used as received from Aldrich Chemical Company. Hexamethylenediamine was fractionally crystallized before use and kept under nitrogen atmosphere to prevent oxidation. Solvents were distilled under reduced pressure prior to use.4,6 Fibers were wound on a drum at a constant removal rate of 0.9 ft/sec. The resultant fiber was washed with 50% acetone, ground in a blender with 150 mL of distilled water for 10 seconds and then suction filtered. The fiber was washed with acetone for 48 h and then with distilled water for 48h in a Soxhlet extractor. The fiber was air dried for 24 h and further dried for 48 h at 50-60 0C under reduced pressure. The inherent viscosity of the polymers was determined at 25 0C in m-cresol at a concentration of 0.5 g/dL.
RESULTS AND DISCUSSION
Carbon tetrachloride, tetrachloroethylene, hexane, and 1,1,2-trichloro-1,2,2-trifluoroethane all afforded fibers quickly and in sufficient quantity for an effective demonstration. Inherent viscosities of solutions of the resultant polymers were very similar (see Table II) indicating that polymer molecular weight was relatively insensitive to changes in solvent or monomer concentration. Thus, with advanced students, it should be emphasized that interfacial polymerization deviates significantly from other step-growth polymerizations especially with regards to the rate of the reaction and the insensitivity of the degree of polymerization to monomer imbalance.
The demonstration generally is quite reliable, although starting of the fiber pull out from the interface proved difficult at times. It was found that inadequate mixing of the organic phase prior to combining with the aqueous phase was the major factor in poor fiber "startability." Increasing the concentration of hxamethylenediamine improved the reliability of the demonstration. However, this improvement also may be attributed to inadequate mixing of the organic phase since inadequate mixing previously has been shown to result in increased acid chloride concentrations at the interface6. Solutions of the organic phase that were magnetically stirred prior to combining with the aqueous phase gave the best reliability and consistently afforded the highest yields.
CONCLUSIONS
All of the tested solvents can be used for the Nylon Rope Trick. Hexane afforded the poorest fibers and was not suitable for a self-propelled demonstration. The chlorinated solvents gave good fibers, but have high toxicity. In the short term, 1,1,2-trichloro-1,2,2-trifluoroethane is the solvent of choice because it forms a superior fiber and has a low toxicity. However, environmental concerns limit the long-term use of this solvent.
Table II. Polymerization conditions, percent yields, and polymer viscosities.
reagents % yield inherent viscosity
1.0g NaOH
3.0g H2N-(CH2)6-NH2
1.75mL ClOC-(CH2)8-COCl
50mL Hexane (ref 2) 47%b 0.90
3.0g Na2CO3
2.8g H2N-(CH2)6-NH2
1.5mL ClOC-(CH2)8-COCl
50mL Cl2C3DCCl2 (ref 1) 28% 0.99
3.0g Na2CO3
2.8g H2N-(CH2)6-NH2
1.5mL ClOC-(CH2)8-COCl
50mL Freon113 24-27% 0.97-1.06,
4.0g Na2CO3,
2.8g H2N-(CH2)6-NH2
1.5mL ClOC-(CH2)8-COCl
50mL Freon113 51-57%c 1.00
4.0g Na2CO3
2.2g H2N-(CH2)6-NH2
1.5mL ClOC-(CH2)8-COCl
50mL Freon 113 43-46%c 0.99
NOTES
a. Sufficient polymer fiber for an effective demonstration was obtained in all cases.
b. Gave a tacky fiber which was not suitable for self-propelled demonstration (ref 3).
c. Organic phase magnetically stirred prior to combining with the aqueous layer.
REFERENCES
1. Morgan, P.W.; Kwolek, L.L. J.Chem.Ed 1959,36,182-184.
2. Shakhashiri, B.Z. Chemical Demonstrations; Univ. of Wisconsin, Madison, 1983; Vol. 1, p213.
3. Morgan, P.W.; Kwolek, S.L. J.Chem.Ed. 1959,36,530.
4. Joseph, W.; Droske, J. Presented at the Wisconsin ACS Undergraduate Research Symposium, April 1988.
5. Note: Some school districts also may express concern about the toxicity of the monomers. In this case, materials should be handled only by the teacher.
6. Bieber, T.I. J.Chem.Ed. 1979,56,409.