Polymers in the Physical Chemistry Laboratory An Integrated Experimental Program George L. Hardgrove, Donald A. Tan, and Gary L. Miessler St. Olaf College, Northfield, MN 55057 We have recently revised our required physical chemistry of a single monomer and is an oversimplification for copolylaboratory course to include more experiments on the prepamerization. However, i t does provide a framework for correration, kinetics, and characterization of polymers. This lating the results of various experiments and is consistent change provides for additional training in an often-neglectwith the actual kinetics of copolymerization. The equation ed area in undereraduate chemical education. At the same below shows the monomers and a oortion of the conolvmer. time this program provides for an integrated experience in a number of techniques such as vibrational and NMR spectroscopy, gel permeation chromaPCH3 tography, and differential scanning calorimeH o=c try, all applied to polymer characterization. \ \ /" n It is important to have students experience a ,c=c /" \ /c=c \ related group of experiments that provide H H CsHs CH3 some real-world situations that always involve both synthesis and characterization. Styrene Methyl Methacrylate An excellent review of laboratory experiments for polymer chemistry has been collected by Mathias ( I ) . Other resources ind u d e books by Collins e t al. (Z), McCaffery (3).and Rabek (4). The synthetic portion of this diverse group of experiments involves the mechanism (5,6) of free radical polymerization given below; Init%represents a free radical initiator and M CsHs H CH3 H the monomer. I Polymer
..
+
J
Init, hit'
InitM,,' InitM,
+M
ki + 2111% k
InitM'
+ M % InitM,,;
Chain Initiation
1
I
Experlmental Methods and Results
Chain Propagation
J
1
+ 1 n i t ~k; A InitM,+,Init k
4 InitM, + InitM,
Chain
~
Usinga steady-state approximation (5,6)on the concentration of all types of radicals [R.] we obtain [R'l = (kJk,)'" [Init,lt"
(1)
rate of propagation = k,W][M] = k,(kj/k,)t/2
chain lensh
nit,]'" [MI
Pwiflcation of ~ o n o h e r and s Kinetic Runs The procedure is adapted with some changes from that of Miller (7)and Collins (2). Samples of 150 mL of styrene (S) and methylmethacrylate (MMA) are each washed five times with an equal volume of 10%NaOH and then five times with water. The samples are dried over CaC12 and distilled under reduced pressure (at about 55 OC for S and 25 OC for MMA). An ice-cold condenser was used for MMA. Different students are assigned various concentrations of S, MMA, and azo-2,2'-bisisobutyronitrile,which initiates the reaction. Reaction mixtures in which the concentration of MMA exceeds that of S should he avoided since nolvmers . , rich in MMA give stick", precrpitates that are difficult to filter and dry.Toluene solutions of the initiator and the mixture of mr,numers are henred separately to 7&8U°C and then mixed lostart thereaction. The three-neck flask used for this reaction is equipped with a condenser to prevent loss of solvent and monomers during the reaction. Samples of 10 mL are removed periodically hy pipet and quenched in 200 mL of methanol and collected on preweighed sintered-glass filter crucibles that are then air-dried and weiehed to determine the extent of reaction. l'heae samples are used';" larer analyre5 We have found nod~fferenee in the amount of polymer produ~edi n a run in which N2was bubbled through the reactlon mixture, SO no spec~alprerautrons have been made to exclude oxygen. For most concentrations of reactants used, the reaction does not proceed to a sufficient extent during a 4-h laboratory period to obtain a definite indication of the order with respect to monomer. Rate constants calculated assuming first order in monomer in agreement with eq 2 are listed in the table. In runs A, B, and C where the ratio [MMA]/[S]was varied, we note an increase in rate constants with higher fraction of MMA in agreement with Miller's (7) expectations based on comparison of the kinetic properties of the monomers. Comparing runs B,D, and Ein which the concentration [Init,]
(2)
rate of propagation rate of initiation
=-
We have chosen to ~olvmerizemixtures of stvrene and methylmethacrylate beciuse the resultingcopol&er is conveniently analvzed for the fraction of each monomer. T h e above kinetic description applies strictly to polymerization
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properties using differential scanning colorimetry. Brief descriptions of these experiments follow. Students may optionally estimate the molecular weight from viscosity measurements.
NMR Analysls Polymer samples (50 mg/mL) are dissolved in CCl+ Proton NMR spectra are obtained with careful integration of the phenyl and nonphenyl resonances. Figure 1shows the proton NMR spectrum of the copolymer with the phenyl resonance of the S residues in the range 7-8 ppm and with the CH3 and CH? groups of both S and MMA residues in the range 1-6 ppm. Assigning x to the male fraction of S in the polymer, the ratio R of areas of the phenyl and nonphenyl regions is R=
51
(1
8R
- r)8 + 3%'and therefore r = 5(1 +R)
(4)
The results show the mole fraction of MMA in the polymer to be a few percent higher than that of the solution from which it was prepared. Theoretical treatments of reactivity ratios is given by Coleman (a),and some experimental observations are described by Mukatis (9).
Figure 1. Prmon NMR spectrum of a copolymer sample from run 8.
lR Analysis The copolymer composition can also he determined (7) conveniently by infrared spectroscopy of thin films deposited on NaCL mull plates. Two drops of a solution of 20 mg1mL of polymer in CHCla are spread over a NaCl plate and allowed to evaporate. Spectra are obtained in the range 4000-500 cm-'. Figures 2aand 2b show portions of these spectra af films of pure polystyrene and ' eharaepolymethylmethacrylate. The carbonyl peak at 1732a-is teristicaf MMA, and the phenylvihration at 698 cm-I isspecifie for S. The ratio of mole fractions can be determined by the ratio of absorbances A compared to the ratios obtained for a standard sample that contains equal concentrations of polymethylmethacrylate and polystyrene. x(MM.4) - A(MMA sample)lA(S sample) -x(S) A(MMAstandard)lA(Sstandard)
(5)
As with the NMR results, the polymer is found to be several percent richer in MMA than the solution from which it was formed.
Gel Permeation Chromatography Gel permeation chromatography provides a method (10)to obtain the molecular weight distribution from which molecular weight averages can be estimated. We chose tetrahydrofuran as solvent because it gives a strong signal with polymers of both S and MMA with a refractive index detector. Ten-milligram samples are dissolved in 10 mL of solvent, and 0.5-mL samples of the solutions are pumped through a Jordi Associates, Inc., mixed-bed gel permeation column at 1.5 mL/miu. The chromatogram is computer recorded from the output of a Spectrophysics 8430 refractive index detector using our own computer programs for data collection and calculations. The apparatus is calibrated with a mixture of four polystyrene standards in the molecular weight range of 1,500-100,000. TheM, and Mu molecular weight averages are shown in the table. For smaller concentrations of the initiator we obtain larger molecular weights as expected (11) from the predictions of eq 3. For the lower concentration of initiator, we find that the molecular weight averages decrease for samples taken at a later time during the kinetic run, but this effect is less evident a t the higher initiator concentrations. The values of M,IM, near 1.4 indicate that chain reactions terminate by combination of radicals rather than disproportionation (6). Figure 2. a. Infrared specb-um (absorbance) of polymethylmethacrylme. b. Infrared spechum of polystyrene.
Exwrlmental Resuns for Various lnnlal Concontratlana Run
was varied, we note that the rate constants kalz in eq 2 sbove are roughly constant. This is expected if the kinetic scheme shove is a correct description of the reaction. Students collect eight polymer samples during their kinetics run. ~ . to~. ~he .. These samoles are saved for characterization exneriments performed during subsequent laburstory periods. In these eaperiments students determine mole fractions of styrene and methylmethaoyl.de by NMR and IR, obtain molecular weights of their polymers by gel permeation chromatography, and study thermal ~
880
Journal of Chemlcal Educatlon
~
~~
~
~
[MMA]
~
[S]
p
[Inlt~l 10Sk,'
~
~
10skk3,.b Mwc MnC M W
Figure 3. DSC curve of lowdensity paiyemylene. Upper trace: Temperalure rising. Lower trace: Temperatme failing.
Figure 4. DSC CUNB of a aampie from run 6. The energy absorption below 50 OC is an artifact.
DSC Analysls
We aIso thank Wilmer Miller for m a n y helpful discussions and for providing t h e laboratory manual for the polymer chemistry course at t h e University of Minnesota.
Differential scanning calorimetry is a very sensitive method to detect thermal changes in glassy and crystalline materials (12).A description of the apparatus and data analysis has been provided by Watson (13).Students make runs with henzoic acid and polyethylene to demonstrate the capabilities of this technique to determine the T, temperatures and AH values of phase changes. The S-MMA copolymer, in contrast, is more disordered and shows less distinct phase changes. Some of the samples do show glass transitions. Samples of 3-5 mg of polymer are analyzed with a Perkin-Elmer DS-I instrument. Figure 3 shows melting and crystallization processes for polyethylene, while Figure 4 illustrates a glassy transition for a noncrvstalliue conolvmer samnle. The -elass transition is a secondorder transition and is very important in determining properties of remicrystalline and amorphouli polymers. ~
~
. .
Acknowledgment W e acknowledge w i t h t h a n k s NSF G r a n t CSI-8750342for partial s u p p o r t for e q u i p m e n t purchases for t h i s program.
Literature Cned 1. Mathias. L. J. J . Chem. Edue. 1983.60.990-993. 2, collina. E. A,:Bares, J.: Billmeyer. F. W.. J~Experiments inPolymo?Seienee:Wiley: N e w York. 1373.
3. McCafle~ey,E.M.LoboroforyPlrpomlion(olMaeromoI~~~I~~Chemiafry;McGrswHill: New York. 1970. 4. Rabek, J. F. Expep.n'manlrrlMethods in Polymer ChemistryPrincipals and ApplVofiona: Wiley: Chiehester,1980. 5. Atkins, P.W. PhysirolChemistry,3rd ed.; Oxford: Oxford, 1986: p 711 6. MeCrath, J. E. J. Chem Edur. 1981.58, 844-861. 7. ~ i l k rW. , PolymerCovrsoLabomforyMonuol: Unirersifyof Minnesota. 8. Coleman, M. M.; Yarnell, W. D. J. Chem. Educ. 1982.59.84-852. 9. Mukstis, W. A,; Ohl, T. J Chom. Educ. 1972,49,367-370. 10. Ref 4, p 72. 11. Ander, P. J Chem. Educ. 1970.17.233-234. lZ Ref 4, p 562. 13. Wsrron,E.S.: O'Neill, M. J.:Justin, J.; Brenncr,N. J. Am. Chem. Soe. 19M,36,12331238.
Improving the Public Perception of Polymers: An Initiative of the IndustrialSponsors Group of the Polymer Chemistry Division, ACS While the average person is probably unaware of it, all known forms of life would not exist if it were not possible to form the large molecules that we call polymers. The range of naturally synthesized polypeptides, polysaccharides, and other polymers have in recent years been supplemented by a vast array of synthetic polymers, designed to make life better, safer or easier. However, a puhlic perception has been created that most of the waste disposal problems are caused by polymeric materials when in fact, synthetic polymers comprise a relatively small percentage of our solid waste and in many cases could be readily recycled. If the general puhlic were more aware of the nature of polymers and the benefits which accrue to society as a result of their use, there would likely he a more rational approach towards finding solutions to the environmental problems. Haw do we as chemists help to get the facts to the general public in a manner that will he truthful and effective? In order to find answers to this question and to help put the answers into action, the Industrial Sponsors of the Polymer Chemistry Division of the American Chemical Society are soliciting proposals for programs designed to create a more positive public perception of polymers. The successful proposals in this competitive process will he given agrant of up to $10,000,generally over a three-year period, The grants will he awarded on the basis of proposals from individuals or groups and may range from individual efforts to programs for group projects. The proposals will be evaluated on the basis of feasibility, impact, and originality. The proposals should be as detailed as is necessary to convey the objective, approaches to he implemented, the action plan, and timetable. While the amount of the award will depend upon the needs and the extent of the program, evidence of cosponsorship, potential for cost sharing or additional funding should he included as this can effect the feasibility of the projects. Project proposals will be evaluated by a panel selected by the Industrial Sponsors coordinating committee. Proposals should be suhmitted on or before April 1, 1991 to: Robert W. Stackman, S. C. Johnson and Son, Inc., 1525 Howe St. MS-117,Racine, WI 53403;(414)631-2108.
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Number 11 November 1990
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