Rapid Identification of Thermoplastic Polymers Helene Cloutier and Robed E. Pnwl'homrne Laval University. Quebec 10. P.Q., Canada G1K 7P4
Chemistrv students are well aware of the im~ortanceof polymers in everyday life. I t is, thus, the objeitive of the oresent article to Drooose a method of rapid identification of thermoplastic poiymkrs. Before describing this method, let us define some necessary concepts. Polymerlc Maferlals ( 1 ) Most polymers can be classified as being elastomeric, thermosetting, or thermoplastic materials. The elastomers and the thermosets are made of chains that are chemically linked together and that form a three-dimensional network (Fig. la). Individual molecules can hardly be found in these materials and, therefore, the network can he said to he comoosed of a single giant molecule. A direct consequence of this three-dimensional structure is that the elastomers and thermosets are insoluble and infusible, hut at the same time there is a major difference between them: the elastomers are made of molecules that have a high degree of local mobility, whereas the molecules of the thermosets are rieid. The elastomers exhibit rubherlike elasticity and are i s o called rubbers. The thermosets are hardlv deformable. E x a m ~ l e sof thermosets are the eooxy and Bakelite resins. The thermoplastics are made of linear molecules (Fig, lb). They are soluble and they can be deformed under heat and pressure. Polyethylenes and polymethylmethacrylate (e.g., Plexiglas) are well-known examples of thermoplastic polymers. The thermoplastics can be identified by their solubilitv behavior.. bv their thermal behavior. and bv various s~ec: tn)sn~picmethods,inbulkur in sulution, in contrast with the elastomers and the thermosets which are insoluble and infusible. The rest of this article will, therefore, be limited to the identification of thermoplastic polymers which will he often referred to as polymers or linear polymers. Several techniques of identification will be simultaneously proposed: carbon, hydrogen, and nitrogen (CHN) analysis, solubility measurements, differential scanning calorimetry (DSC) or differential thermal analysis (DTA), and spectroscopic analysis (e.g., infrared (IR), ultraviolet (UV), and nuclear magnetic resonance (NMR)). I t will never he necessary t o use each of these techniaues. In some instances. two of them will suifice. In others, t h ; use of three different techniques may bc desiral)le. In all cases. the useofseveral techniuues will be advantageous. In addition, depending upon the &ailability of instrumentation, different techniques could be used in different laboratories.
Figure 1. Schematic representation of a) an elastomeric or a thermosetting polymer, and b) a thermoplastic polymer.
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Crystalllzatlon of Polymers (2) A linear polymer can be amorphous (noncrystalline) or semicrystalline. Polymers without asymmetric carbon atoms eenerallv crvstallize whereas ~ o l v m e r sbearine asvmmetric " carbon &&s can be found in iheamorphous or semicrystalline states deoendine upon their configuration. For exam~le. mono-substituted vinyl polymers of repeat unit C H R A , can be found in three different configurations (3). In the first one, the side groups R lie completely above (or below) the plane of the chain and this configuration is called isotactic (Fig. 2a). The side groups may also be found alternatively above and below the plane of the chain, yielding a
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+CG-
Figure 2. Polymers having an mymetric carbon atom isotactic, b) syndiotactic, or c ) atactic configurations.
can be found in a)
confieuration called svndiotactic (Fie. . .. 2b). Finallv. if the side groups are plnred randomly alwve and helo\\, the plane uithe rhsin. the cuniieuratiun is called atactic (Fie. 2c).'rhe " isotactic and syndiotactic configurations allow a regular arrangement of the chains and lead to semicrystalline polymers. On the other hand, the atactic configuration leads to amorohous ~olvmers. onk shouid note that a polymer is never entirely crystalline since it alwavs contains a fair Dercentaae of its volume in the amorphous -state, often between 40 a n d 60%, due to kinetic restrictions to its crystallization because of its high molecular weight. This is the reason for using the adjective "semicrystalline" instead of "crystalline." The semicrystalline state of polymers leads to their unique morphologies, (4which have been described previously in THIS JOURNAL 6). A semicrystalline polymer has a melting point, T,, which can he described as the temoerature at which the reeular crystalline architecture is destroyed and at which the molecules acquire a high decree of mobilitv. Similarlv, there is a temperaiure wh& d e t k s the trans~iwnof the"morphous frxtion of the ~ u l w n e rirom an immobile ro a mobile state. This temperat&e is called the glass transition temperature, Tr
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As illustrated in Figure 3, the properties of a material change as drastically at T, as they do a t T,. While at temperatures helow T,, amorphous polymers are in a glassy state, at temperatures ahove Tg, they become viscous liquids. Semicrystalline polymers are also glassy materials a t temperatures below Tg. However, between T, and T,, they are soft engineering plastics, and ahove T,, viscous liquids. Both Tg and T, may he used for polymer identification (Tahle 1).They can he detected by DSC or DTA (7,8).In practice, the increase in heat capacity observed at Tgis often too small to he easily detected. Therefore, the measurement of T, can hardly be used as a routine test in contrast to the T, measurement.
Figure 3. Diagram illustrating me different states of linear polymers.
Polymer Solubility (9-11) The free enthalpy of mixing of a polymer solution, AG, is given by the familiar thermodynamics equation
where AH is the enthalpy of mixing, AS, the entropy of mixing, and T, the temperature. For polymer solutions, AS is often small and a negative AG is obtained if AH is small or negative. Hildehrand and Scott (12)have shown that, in first approximation, AH can he written as where ul is the volume fraction of the solvent, u2 that of the nolvmer. of the solvent and 61 . . . 6,... the soluhilitv- Darameter . that of the polymer. 612 and 6z2 are the cohesive energy hv densities of the substances and hT2 can he a~oroximated -. the energy of vaporization per unit volume of the solvent. The 61 and 62 values of several polymers and solvents are reported in Tahle 2. Negative AG values and solubility are is small, or, in other words, if 61 and 62 expected if (61 are close. The solubility parameter theory, then, explains the insolubility of polytetrafluoroethylene in any solvent, the solubility of polyethylene in saturated hydrocarbons, and the solubility of polystyrene in aromatic hydrocarhons, to mention just a few examples (Tahle 2). This theory does not take into account specific interactions between polymers and solvents; it must, therefore, be used with caution when such interactions are present. Polymer l d e n t l f l c a t l o n
The identification of unknown polymer samples can be made following the method described in Figure 4. Several tests must be used subsequently or simultaneously. I I Since the pn,pmed method is d i d only for t h e r m q k w i t r . twu initial dteps are required i n order t o identify and d i m m a t e elmtumrric and thermosetting materials The first step is the follow-
ing: elastomers show rubber-like elasticity upon hand stretching whereas thermosets do not deform and thermoplastics either do not deform (glassy state) or deform but do not recover their initial dimensions after the release of the stress (soft plastics). Table 1.
Table 2.
Solubility Parameters of Several Polymers and Solvents
(14) Meltlng and Glass Transition Temperatures of Common POlvmerS 1131
Polymer
TdK)
T&K)
polyisobutyiene polyethyleneadipate polyethylene oxide polyethylene, low density polyethylene, high density polyvinylidene fluoride i~otacticpolypropylene Nylon 11 Nylon 6 isotactic polystyrene Nylon 66 polyethylene terephthalate polytetrafluoroethylene
317 325 343 393 403 444 459 47 1 493 513 533 538 600
200 210 232
816
Figure 4. Schematic diagram illustrating lhe methbd proposed for polymer identification.
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... ...
233 255 316 319 373 323 434 399
Solvent
6, ( c a l l ~ r n ~ ) " ~
Rhexane isooctane cyclohexane carbon tetrachloride toluene benzene chlorobenzene acetone benzyl alcohol
7.3 7.7 8.2 8.6 8.9 9.2 9.5 10.0 10.8
~y~lohe~anoi elhano1 methanol water
11.4 12.7 14.5 23.4
Polymer
6, ( c a l l ~ m ~ ) " ~
palytetrafluoroelhylene polyethylene polypropylene polybutadiene
6.2 7.9 8.1 6.6
polystyrene polymethyl melhacylate
9.1 9.5
polyefiylene terephthalate
10.7
Nylon 66 polyacrylonitrile
13.6 15.4
Table 3.
Chemical Formulas of Polymers Analyzed In the Solubility Chart (Flg. 6) and Their Theoretical CHN Content
Polymer
Chemical formula
C
CHN Analysis (Wt. %) H N
Others
polyethylene p~lytetraflu~roe~ylene poiyvinylidene fluoride p~lya~ryloniniie polyvinyl alcohol
polypropylene polyvinyl acetate (PVAC)
polystyrene polyvinyl chlaride (PVC)
Nylon 66 Nylon 6 Nylon 11 Ceil~IoSe cellulose acetate celluloSe triacetate polyethylene oxide
polyethylene adipate polyethylene terephthalate polyisoprene, cis or trans polybutadiene, cis or trans
.
The sten is a simnle flame test. Thermosets do not melt - - second ~ ~ or flow when he&d with a match or other simple heat sources, whereas thermoplastics do. In case of doubt, it is advisable t o classify the material temporarily as a thermoplastic. ~~~~~
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2) After classification of the unknown as a thermoplastic material (step (I)),a CHN analysis is most desirable. The carbon, hydrogen, nitrogen, and other elements weight percent of 29 common polymers analyzed in our laboratory is given in Table 3. I t is seen that the CHN analysis allows one to distinguish easily between polymers containing heteroatoms and others and, therefore, restricts the field of investigation to the point where other measurements might appear unnecessary. However, additional measurements are always required for several reasons: Polymers often contain large quantities of additives which change significantly their CHN ratio. For example, polyvinyl
chloride might contain as much as 30% of dihutyl phthalate, a plasticizer, which would increase its carbon content from the theoretical value of 38.4% to 47.6% and add 69% of "other elements." Similarly, CHN analyses performed on four commercial polyethylenes gave carbon contents of 85.5,83.O, 86.0, and 82.0% as compared to the theoretical value of 85.7%. As indicated in Tnhk3, sereralpulgmera hwerqual oraimim
C H N mtios, e g , pdyerhylene, pulypn,pylr~w,uncrosslinkrd pulyisoprene, polybutadiene and polyisohutylene.
A proper identification of polymers cannot be limited to the determination of their CHN content. It must also indicate the configuration of the polymer, as discussed above. For example, the properties of isotactie and atactic polystyrene are drastically different. Similarly, the determination of the cisltrans ratio of polyisoprene is of prime importance: Finally, i t is advisable to Volume 62
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determine the sort of polyethylene analyzed since three grades are available: low, medium, and high density. If s CHN instrument is not readily availahle, step (2) can be eliminated and an adequate polymer identification can nevertheless be achieved. 3) After the CHN analysis, calorimetric, soluhility, and spectroscopic measurements can be selected depending upon the initial results and the instruments available. Each of these steps is not always required, but the suggested analyses are often complementary: The calorimetric analysis (DSC or DTA) Leads to the unambiguous identification of semicrystalline polymers by the measurement of T,. As indicated in Table 1, the value of T, may be sufficient to identifv a uolvmer. For examnle. a samole havine a T., of RS6 K is likelga polyrrhylrne, while that havmi a of 5'33 K is likely a polyrthylene tPrephrhnlate (Fig. 51. However, this methua C d n n O t hrlp for amorphous thermoplastird. Solubility measurements are simple and most useful for polymer identification. A solubility analysis can be carried out following the chart presented in Figure 6. This chart was initially taken from McCaffery ( l o ) ,but we have added numerous polymers which are currently found in commercial products. The chart leads to the identification of 29 polymers, whose chemical formulas are given in Table 3. On looking a t Figure 6, it is clear that the solubility test does not lead to an unambiguous identification of all polymers, even after the CHN analysis. Figure 6 shows six different examples where at least two polymers are found by the same solubility mute. Other similar eramoles will eertainlv be found if this chart i s firrthcr In cases ~. ~eencmlired. .-~~ ~ ~of amhimitv. ~ ~ the - aolubilitv test has to be completed by calorimetric andlor spectroscapic analyses. For example, the different nylons can be distinguished by their T,, and the polyisoprenes by NMR spectroscopy Finally, spectroscopic analyses can be used as complementary tools because bulk polymer samples are not always easily usable for IR analyses in the solid state. If asolvent has to be found, then the solubility test is done and the spectroscopic analyses follow, if required. There are, however, specific examples where spectroscopy is
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.
~~~~~~~~
~
~~
~~
required. For example, polystyrene can he distinguished from polybutadiene and polypropylene by an NMR peak at shout 7 ppm, in carbon tetrachloride. Polyhutadiene can be distinguished from polypropylene by a double-bond resonance IR hand at 1460 em-'. The polyhutadienes can be distinguished from the polyisoprenes in a similar manner. The identification of the cis and trans polyisoprenes can he done by NMR, but it requires an instrument of 100 MHz (15). Conclusions
The identification of commercial thermoplastics is a complex task which cannot he achieved b y a single analysis. Each of t h e four proposed tests, C H N analysis, calorimetric, solubility, and spectroscopic measurements, leads to a very incomplete a n d unsatisfactory identification of thermoplastics when taken alone. However, a combination of two or three of these methods leads t o a clear answer.
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TEYPERATURE
Flgure 5. DSC malting curves of two common semicrystalline polymers, polyethylene (PE), and polyethylene terephthalate (PET).
Flguree. Solubility chart:thesoluble polymersfollowthearmattheright of thesolvent considered while the insoluble ones folloy the arm at the left
818
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The tests chosen are all simple and rapid to use. The most expensive piece of equipment required is a DSC or DTA apparatus, which is nowadays a routine technique in a fair number of Laboratories. Moreover, it has been shown that a simple DTA apparatus can be constructed at a verv low cost (8):~his inexpensive apparatus would certainly ;uffice for the present experiment. The proposed method enables the identification of 29 linear polymers. Of course, this number is not limiting, and it would be interesting to generalize the method, and the flow chart given in Figure 6, to additional samples.
Experlmental Section The solubility measurements were made using 0.5 g of sample in a test tube containing 10 ml of solvent. The mixture was heated to the boiline mint of the solvent. If. a f t e r 5 min o f this treatment. the mixture&ained heteroeeneous..the oolvmer was considered i n s o l u b l e i n t h a t p a r t i c u l a r s o l v r n r . I H m r a s u r r m e n r s were mnde b~ d e p o s i t i n g a d r o p o l a 1% s o l u r i o n o n a KHv w i n d o w . T h e I R a n a l y s i s s,ascarried o u t un t h e r e s u l t i n g C l m ofwr e v a p o r a t i o n uf the solvent. For that purpose a solvent was selected f r o m those shown in Figure 6.
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Acknowledgment The authors thank Sylvia Tnrrell for kindly reviewing the manuscript.
Literature Cited (11 Billmcyer. F. W., Jr.. "Textbaok of Polymer Science.(. 3rd ed.. Wiley-Intmiena, NeaYork, 1981. (21 Meares,P.."Polymers: SVucture and Bu1kProperLiea:'Van Ncdtrand Reinhold Ca., New York, 1871. (3) Harris. F.W.. J. CHEM.EDUC..58,837 (19811. (41 Billmeyer. F. W.. Jr., Ceil, P. H., and van der Wq,K. R., J. CHEM. EDUC.,37,460
,.""",.
,>=m
(5) Stein. R. S.. J. CmM. E~UC..50.748 (19731. ;, J: CHEM.EDUC.,~O,~SI (1973). (6) Bailey,F. E., J ~ . , a n d ~ o l e a k J:v., (71 Wi1hoit.R. C., J. C ~ ~ ~ . E ~ ~ ~ . . 4 4 , A 5 7 1 , A 6 2 9 , A 6 8 5 . s n d A 8 5 3 ( 1 9 6 7 l (81 Earnest. C. M., J. CHEM.EDUC., 55. A331 and A373 (1978). (91 Collins,E. A.,Rerea. J..snd Billmeyer. Jr.,F. W.,"Erperirn&in Polymer Science? Wile"-Interscience. New York. 1973. (10) ~ e ~ a k r E. y . L., ' ~ a b o r a t o r i Preparation for Macromol6cdar Chemistry? McCraw-Hill. New York. 1970. (111 Olabisi, 0..Rohson, L. M., and Shsw, M. T.. "Polymer-Polymer Miscibility: AeademiePress, NewYork. 1979, Chap. 2. (12) Hildehrand, J. H.. and Scott, R. L., "The Solubility of NonelebrolytPs.(. 3rd ed., Reinhold Publishing Co., New York, 1950. "Polvmer Hsndhk!' 2nd d. Wilpv(131 Bandruo. J.. and Immereuf. E. H..' (Editors). ' ~ ~ ~nter;eicA,~ a YO&, w i975. (141 Rdriguez, F.. "Principles of Polymer Systems? McCraw-Hill, New York, 1970. (151 B o v w F. A., "High Resolution NMR of Merrornoleeulos: Academic Press, New York, 1972.
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