A Simple, Discovery-Based Laboratory Exercise The Molecular Mass Determination of Polystyrene Greg A. Slough The College of Wooster, Wooster, OH 44691 Polymer science is a n important discipline in modern science and technology Because of this special position in science, it is important that introductory chemistry students, both a t the high school and college levels, gain some knowledze of oolvmers and their ~hvsical ~rooerties. Students .. ." . and others often assume that the high molecular mass and structural complexity of polymers preclude any systematic analysis. In some regards they are justified because sophisticated techniques such a s gel permeation chromatography, intrinsic viscosity, and low-angle X-ray scattering measurements are used tvoicallv to determine the molecular mass of polymers. oneo of these analytical methods are prsrt~rali n ~ntrodurtoryteachmg laboratories. In 1985 an amcle in lhrs Journal reported a aun11tatn.e thin-layer chromatographic analysis-of polys~yrene(I). This orocedure is elenant and accessible to most introdncu tory students. A major drawback of this experiment is the relatively expensive reverse-phase TLC sheets that are used. In large teaching laboratories this cost becomes prohibitive. This problem motivated us to devise a n alternative analyticaf method that uses less expensive silica gel TLC sheets. Asecond coal included analvsis of oolvstvrene " " standards up to several million mass units. A wider range of mass standards would extend the limits of the mentioned above and orovide a means bv which ordinarv samples of polystyrene, such a s packing.. peanuts and coffee . . . cups, can he examined in a discovery-based experiment. We wish to r e ~ o r at s i m ~ l eoolvstvrene analvsis usine inexpensive, plastic-backed, silica gel - TLC sheets that *ves reliable and semi-quantitative separate polymeric samoles havine molecular masses between 8.0 x lo3 and 6.0 x A
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Because the discovery component of this experiment utilizes unknown ~olvmers.it is necessarv to establish the G best accompolymeric c o n t e k df each sample. T ~ is plished using infrared spectroscopy. Thin films of polystyrene display a characteristic four-hand overtone pattern hetween 2000 and 1667 em-' and a sharp stretching frequency a t 1601 cm-l. One may assume that samples such a s packing peanuts and coffee cups contain primarily polystyrene and the infrared analysis need not be done. Discussion Development of a simple, reliable molecular mass analvsis of pol?mwr.: fullills one ufthe educat~onalgonli imtlined 111 the report ofthc p d \ ~ mcore t ~ course committee 3 , . The simplicity of this method and the inexpensive materials (80 cents for two silica gel TLC sheets) and chemicals needed for this experiment make it ideally suited for high school and introductory college chemistry classes. The linear dependence of R f to molecular mass over nearly three orders of magnitude allows the analysis of almost all commercial polystyrene samples. The 6.0 x lo6 g mol-' standard need not be used in most cases because commercial samples of polystyrene have molecular masses between 1.0 x lo5 and 4.0 x lo5 g mol-'. The linear log-log relationship found for the molecular mass and retardation factor data is not obvious. However, this relationship is similar to the equation derived by Martin for the paper chromatographic analysis of homologous amino acids (4). The original Martin equation related log [ ( l / R c b l lversus the homologous chain number. The Martin equation applies to only a few systems. Therefore, partly empirical and partly theorical corrections have been applied to this relationship. For example, Franc and Jokl
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(51,and Niederwieser (6) both found linear log-log relations between the quantity (11Rf)-1 and the number of units in the homologous chain. The latter studv also observed a non-linear relationship in the analysis of homolowus 2.4-nitroohenvl . " amino acids depending on the solvent system used (6). Asimil a r s i t u a t i o n exists i n t h e analysis of polystyrene by the method reported herein. For instance, a small change in the developing solvent to hexane:pdioxane::500 mL: 417 mL was found to produce a non-linear relation. However, if the developing solvent is prepared accurately and stored properly a s to avoid concentrating the less volatile p-dioxane, the difficultv associated w i t h nonlinearity can be minimized. Plot of log[(l/R,)-I] versus log (molecular mass) x lo4 for a series of polystyrene standards. Rfvalues are the average of 16 trials, and the error bars represent 95% confidence intervals. Also presented is a stylized drawing of a TLC sheet after development with 500:433::hexane:p-dioxane.Molecular mass Methods and Materials standards on the TLC sheet are expressed in lo3 g mol-I. Thin-layer chromatography was conducted on ~ a k k r - f l e x rial may be observed due to a distribution of molecular IGF-2 silica gel sheets (J. T. Baker). The developing solmasses. vent was prepared by mixing 500 mL of hexane (Fisher The discovery-based experiment starts by having stuHPLC made) and 433 mL of D-dioxane (Fisher). The soldents prepare a solution of their unknown polystyrene vent mikn-e (5 mL) was added to a 180-mL tall-form Bersample. An infrared spectrum is taken on samples that zelius beaker. without a snout.. which contained a filter pahave a suspicious polymeric content. Each student then per wick. ~ o l k i o n of s standards (8.0 x lo3, 9.0 x lo4, 6.k x orenares TLC sheets with three molecular mass standards lo5, and 6.0 x lo6 Polyscience; 4.5 x lo4, 1.8x 1 0 ~ ~ l d r i c h ) Hnd the unknown sample on each plate. A range of standwere prepared by dissolving 25 mg of each standard in 3 ards. for example. 8.0 x lo3. 4.5 x lo4. and 6.0 x lo5. can be inclided along' with the unknown on'one sheet. ?l;o 2.5 x mL of dichloromethane. One spot of each standard solution 7.5 cm Baker-flex sheets suffice for eight analytes. Aplot of was placed on the TLC sheet, approximately 1cm above log (molecular mass x lo3) versus log [(llRfkll of standthe bottom. Three mass standards and one unknown samards gives a linear plot that can be used as a semi-quantiple can be placed easily on a 2.5 x 7.5 cm sheet. The solvent tative estimate ofthe molecular mass of the unknown samwas allowed to migrate to the top of the sheet, and the solple. This exercise exposes introductory s t u d e n t s to vent front was marked accordingly Standards were visualinfrared and thin-layer chromatographic analyses and ized with a W lamp, and the center of each W-active eives practice in m a ~ h i n and e m a ~ h i c a analvsis. l band was marked and used to determine the Rf value. In situations ofiimitediaborsor$ time, a s in high school Unknown samples of polystyrenes were collected from lab periods. or limited budget. analvsis of onlv the unvarious sources. includine ~ a c k i n apeanuts and coffee known can be conducted. he retardation factor of the mocups. To verify the nature i f ;he p o l s e r in the unknown, lecular mass standards are very repeatable with small exa thin film of the samnle was ~ r e n a r e dand a n infrared perimental error (see figure). heref fore, retention data for spectrum was taken. A thin film of each sample was prethe unknown can be substituted into the linear equation pared by dissolving 25 m g of t h e polymer i n dichloand the molecular mass of the sample can be calcuiated. mmethane (3 mL), adding three drops of the solution to a NaCl disk, and allowing the solvent to evaporate slowly. Literature Cited Some samples, particularly those from coffee cups, gave 1. Armetrong,D. W.;Ma-, J. N.: Ky1e.D.: Atsk, A. J. Chsm. Educ 1985.62.705 hazy suspensions; however, the insoluble material did not interfere-with either the infrared or molecular mass analyses. The molecular mass of each unknown was determined by thin-layer chromatographic analysis. The most intense W-active band or bands were located and measured. This 4. Martin, A. J. E Bioehem. Soe. Symp. (Cambridge, England) 1950,3,4. analytical method yields only average molecular weight 5. Franc.J.; Jokl, J. J. Chmmolog 1959.2. 423. 6. Niederwieser. A. Dissertation. Universitst Gieseen. 1962. information. Several hands or a streak of W-active mate-
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