Some Properties of Poly(Methy1 Methacrylate] Studied An Interdisciplinary Student Experiment D. J. T. Hill a n d J. H. O'Donnell University of Queensland, St. Lucia, Brisbane, 4067 Australia A great deal of research work has been carried out over the past few decades into the effects of high energy radiation on polymers. This has led to a better understanding of the nature of the resultant scission and crosslinkina.. . orocesses which occur, of the changes in the hulk physical and mechanical properties, and of t h e dependence of these properties on t h e structure of t h e repeat units in t h e polymer chain. As a consequence of this work numerous commercial applications for radiation processing have been found ( I 1, for example in t h e manufacture of plastic medical goods which require sterilization (2). A student experiment based upon the radiation effects on polymers therefore allows a n opportunity not only t o teach some of t h e fundamentals of radiation chemistry, polymer chemistry and material science, b u t also t o demonstrate how the information gained in such a n experiment has a direct commercial application in product manufacture or pro-
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Poly(methy1 methacrylate) and copolymers of methyl methacrylate with other vinyl monomers are widely used industrial polymers, so a n undergraduate experiment hased on poly(methy1 methacrylate) is seen by students to have clear practical importance. In addition, t h e radiation chemistry of ihese polym>rs has been well documented ( 3 , 4 ) . In t h e experiment described here t h e effects of ionizing radiation on t h e molar mass and t h e mechanical strength of poly(methy1 methacrylate) are considered, and t h e wellknown radiation "yellowing" of t h e polymer is also examined. The Experiment In this experiment we have used commercially available molding grade poly(methy1 methacrylate), PMMA, granules. The " eranules are first dried in a vacuum oven for 18 hr a t 370 K and then pressed into sheets under a nitrogen atmosphere usine a Selson hvdraulic Dress. T h e ~ r e s s i n is e carried out a t ~ ~ 460 and a t a pressure of about 2 5 . mZ2. T h e pressed sheets are c u t into bars (75 m m X 13 m m X 3 m m ) a n d t h e edges of t h e hars polished. T h e y are then stress annealed by heating a t 360 K for about 2 hr, placed in glass tubes, and evacuated t o a pressure of less than N m-2for 48 hr. T h e evacuated bars are subsequently irradiated a t ambient temperature t o a series of five doses in the range 0-0.5 MGy' using 6"Co y-radiation. After irradiation the hars are kept sealed for about one month a t room temperature t o allow most of the trapped radicals t o decay. Students are asked t o oerform a series of exoeriments on unirradiated and irradiated bars. T h e variety and nature of t h e exoeriments thev can verform deoends on the amount of labora&ry time av.kahle, h u t t h e kxperiments descrihed below can all he easily completed by a pair of students in a three-hour laboratory session.
k
Molar Mass Determinations High energy radiation causes a variety of reactions to take place in a polymer which can result in either chain scission, chain crosslinking, or bathsimultaneous chain scission and erosslinking. Chain scission leads toadiminution in the molar mass of the polymer, while chain crosslinking leads ta an increase. The rates of radiation-induced chain scission end crasslinking are expressed by their radiation chemical yields. C(S) and G(X) respectively. These yields, G values, are expressed in terms of the yield per lfi.02 aJ of absorbed energy (5L2 In poly(methy1methacrylate)scission is k n o w to predominate over crosslinking; thus the molar mass of the polymer decreases with increasing dose. Ifwe assume that all the polymer chains are linear and that random sciesion takes place, then the scission yield is given by the expression ( 6 ) G(S) =
9.65 X lo9 i
M,
(1)
where MI is the molar mass of a structural monomer unit and i is the average number ol' chain scissions per structural unit per unit dose (Gy) of radiation. For a oolvmer onlv random scission. the varialion . . which undergoes ,, 2 in ih, numher average molar ,n:w I$/,,, c m Iwts~,rr;sed in terms of the r;ldiat~ondtm through the rqu:ttl.m ( 1 ; ) 1 1 iD
V"
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+-
(2)
W"(0) M I whereWn(0)is the numher averare molar mass of the initial polymer, and D is the radiation dose. Thus a plot of llR. versus D will be a straight line, and the slope of this line will yield the value of i and hence the net scission yield. The numher average molar mass of a polymer can he determined by osmometry, but such measurements are frequently time consuming. An alternative procedure is to determine the viscosity average molar mass (M,), from which the number average molar mass can be calculated. The limiting viscosity nu_mber 1771 for solutions of a polymer are related to the molar mass M, through the Mark-Houwink equation where K and a are constants which depend upon the solvent. The numher average molar mass can he calculated from Mu using therehtionshio (7)
where o is the Mark-Houwink constant, and r is the mathematical gamma function. For these experiments students are provided with one percent weight per volume solutions of a sample of polymer from each bar. The solvent chosen is either toluene or chloroform. They are asked to either perform a series of viscosity measurements an these solutims using a Uhhelohde dilution viscometer, or measure their osmotic pressure using a Knauer membrane osmometer. In each case measurements should be made at a minimum of two concentrationsso that extrapolationscan be made to infinite dilution, thus accounting for any deviations from ideality (8).If the viscosity method is being used to obtain the molar mass, values far the appmoriate Mark-Houwink confitantscan beobtained from the literature
is). Editor's Note: This paper was not presented at the Stateof-the-Art Symposium on Radiation Chemistry hut was included in this issue because it offers a laboratory experience in one of the areas covered by the Symposium.
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Journal of Chemical Education
Some typical students' results are shown in Figures 1 and 2 for the
The Gray (Gy) is the S.I. unit of absorbed dose and corresponds to energy absorption of one joule per kilogram of material irradiated. 1atta Joule (aJ) is equal to 10-'8.7.
0
0.10
0.20 0.30 DOSE/ MGRAV
0.50
0.40
Figure 1.The dependence of lhe viscosity average molar mass (G")on lhe absorbed radiation dose for poly(methy1methacrylate)irradiated at ambient tern peratwe. X Diakon. 0 Shinkolite.
F@re 2. The dependence of the reciprocal of lhe number average molar mass (MJ on lhe absorbed radiation dose for poly(memy1mehcrylate) irradiated at ambient temperature. X Diakon, 0 Shinkolite.
two commercisl polymers (Diakon produced by Imperial Chemical Industries and Shinkolite orodueed bv Mitsubinhi). The variation in the viscosity average m h r ma+ with dose shown in F~gure1 reveals a rapld fall in mass for hw dmes radiation, followed hy a flattening of the curve towards higher doses. This clearly demonstrates the predominance of scission over crosslinking for PMMA. In Figure 2 the reciprocal of the number average molar mass has heen plotted against dose. The near linear regmn far doses up to about 0.20 kCy conforms to the predictions of equatmn (Z),and allows the net scission vields to be calculated. The slleht curvature which becomes more pronounced at high d o j r 1. I. uotl in most studlei of thi. type, and it can he e~plainedI,) f r , t u r i s m I1 as an incred~ei n the prohabilitv ofchainrad~calrecomhi~~at~c.~~ rmcti8,nsin polymersirradiated 1,)high ~
~
Both Figures 1 and 2 show a clear differen* between the radiation sensitivity of the two commercial polymers. This is undoubtedly due to their different chemical comvosition: for exam~leantioxidants and lubricants are frequently added to eommerical bolymers to prevent deeradation and assist in fabrication. Small amounts of methvl , acrylaie are also generally inwrpurnted in poly(meth)l methacrylate) to reduce thermal depmpngat~mdwmg molding. Smce poly(methyl acrylate) is known to ondcrgt~rnmlinking (10. 11). added methyl acrylate could account for the difference between the two polymer samples. The values of the net scission yield calculated from the initial slopes of the curves in Figure 2 were 1.7 for Shinkolite and 1.0 for Diakon. These values comoare favorablv with values in the literature far poly methyl meth&rylatr. irr.rd/ntd under vacuum, uhere s~ission itlds in the range 1 2-2 ? n m v I m n found, depend in^ on the nature mers t~,IIunthe same curve for the dependence ot ilr wrnl rtrenprh on molar mass (Fig. 4). The dependence curves for the flexural strength (Fig. 4) or tensile strength (15) on molar mass allow students to deduce why, for example, moldinggrade poly(methy1 methacrylate) is generally produced with an M, of approximately 1.5 X 105g malF1. Here students can be asked to consider other aspects of polymer chemistry such as the effect of molar mass and molar mass distribution on the melt viscosity ofapalymer. An upper limit on the molar massof many industrial polymers is set by the fabrication processing requirements on the melt viscosity. For many polymers, including poly(methy1 methacrylate), the melt viscosity increases in proportion to the third
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Volume 58
Number 2
February 1981
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550
WAVELENGTtIInm Figwe 4. The dependence of flexural strength on molar mass (%Ifor poly(methy1 methacrylate). X Diakon. 0 Shinkolite.
Figure 5. A plot of absorbance versus wavelength for irradiated poly(methy1 methacrylate)(path length 3 mm, inadiation dose 0.5 MGy). Measurements were made using Shinkolite and are relative to an unirradiated polymer sample.
to the fourth power of the molar mass if the average chain length is greater than about six hundred units (16).This necessitates a eom~ r o m i s ebetween the demands set hv fahricstion and the need for polymer strength Polymer Yellowing The ultraviolet and visihle spectra of many polymers and capolymers are ohserved to change following y-irradiation, particularly in the ultraviolet region. An increased absorption in the ultraviolet with a tail into the visible can result in the polymer displaying a distinct yellow coloration. Although it has not been explained in detail, this yellowing is believed to result from the production of free radicals, traooed eleclrons. and other ehromoohores Ie.e.. . . .coniueated double bonds) in the polymvr, w h d c:m rcmajn long after the radintwn treatment Studies l f t l ~ t ;dswl,ticm > charncter~rticsof>.irrad~ated pd>lmerhyl naethawyl.m~,slwx wnsiderahle variation from author to author, which suggests that in this polymer the spectral changes are somewhat dependent upon either the purity of the particular polymer sample, or its pre- and post-irradiation treatment. After y-irradiation the commercial poly(methy1 methanylate)~ showed an ahsorotion maximum about 385 nm (Fie. 5 ) . which contributes to the veilow coloration. On standine. this v&$ color fades
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absorption maximum a t 385 nm increases with dose as shown in (Fig. 6), which has led to suggestions that poly(rnethy1methacrylate) may he used as a method of dosimetry (17, 18). The color of cnmmercial products can he an important consideration in product marketing. For example, if medical syringes made from poly(methyl methacrylate) were to be sterilized using gamma radiation, coloration and polymer strength, may both be important considerations from the consumer viewpoint. The fact that poly(methyl methacrylate) yellows, and that this yellow color remains in the polymer for long periods of time, has lead to the use of copolymers of methyl methacrylate and styrene for some commercial applications (2). because here the vellow color fades fairlv auicklv. . These eooolv. mer; also h a w the xMrd ndwwtny*.!hat they are more rerlstmt to hssuf mechan~cnlsrrrtlytl~or. ~rr..d:t.tion,141.
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Conclusions T h e experiment described here allows s t u d e n t s t o gain a n insight into a number of aspects of polymer science. In a single experiment they can investigate: (a) t h e molar mass of a series of polymers, (h) t h e scission yield on irradiation, (c) t h e mechanical strength of t h e polymers, (d) t h e spectral changes which result from radiation damage, a n d (e) t h e importance of the nature of t h e relationship between mechanical strength a n d molecular weight. I n addition t o these features s t u d e n t s a r e also able to see t h e relevance of molecular characterization to t h e material o r o ~ e r t i e s .fahrication. a n d marketine of polymer products. T h e experiment is therefore suitable for s t u d e n t s in h o t h general a n d applied chemistry courses.
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Figure 6. A plat of absorbance at 385 nm versus irradiation dme of poly(methy1 methacrylate).Measurements were made using Shinkolite and are relative to an unirradiated polymer sample. (path length = 3mm)
Acknowledgments W e wish t o t h a n k t h e Australian I n s t i t u t e of Nuclear Science a n d Engineering for their assistance in t h e developm e n t of t h i s experiment. Literature Cited (11 Resddy, A F.. "Applieaiiona of Ionizing Radiations in
Plastin and Polymer Technology:.Pl.,tff, 1971 (21 Alrbrg, H., Radial. Phys. Chem.. 16.65 (1980). I31 Chapiro, A,. '"RadiationChemistry of Polymeric Systems;' Intoracicnee,New Ymk. 1962.Voi. 15. p. 509. (41 Dole, M.,'"The Rsdiarinn Chemistry orMaemmoledos:AcaddmiiPmsp, New Yurk.
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1973. Vo1.2.0 P 7 ~ ~
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O'Donneil,d. H.. and Sbngster, D. F.,"Principles of Radiation Chemistry: Arnold. i.ondon, 1970,p. 180. (61 O'Oonnelt,J. H., Rshmsn, N. P.. Smith, C. A , and Winzor, D. J., Mnciomol~cvles.12, (5)
113 (19791. (71 Chaitenhy, A. (Editor),"Atomic Radiation and Polymers,"Pergarnun Press. oxford, l960, p. 385. (8) Billrneyer,F. W.,"TeithwkofPolymer Seionce:znd ed., Wiiey, NewYork, 19BZ.p.
