An Analysis Group in an Industrial Research Organization1 i INFRARED SPECTROPHOTOMETRYa D. R. JOHNSON and R. E. MOYNIHAN E. I. du Pont de Nemours & Co.,Inc.,
Polychemicals Dapartment, Wilmington, Dalaware
INAN industrial research organization, the problems submitted to the infrared group parallel the development of a chemist's idea into a commercial product. Thus, while a product is in the scouting or laboratory stage, the infrared group is called on by the research chemist to provide qualitative and quantitative information on starting materials, intermediates, and products. At this stage correlations between spectra and structures must be made with the aid of previously obtained basic information on known compounds, e.g., nitriles and olefins ( 1 , 2). These data are used to characterize the nature and mechanism of the new reaction and to determine optimum reaction conditions. Should a given reaction be taken to semiworks scale, information gained in the previous stage is used by the infrared group to develop and conduct routine analyses, the results of which guide the engineers in process design. It is also a t this stage that the infrared group is called upon to contribute spectroscopic information for use in the design of automatic process control equipment. SPECIAL SAMPLING TECHNIQUES
Sampling for infrared often presents the greatest challenge in obtaining adequate analytical data. While normal mixtures of gases and liquids present little difficulty in this respect, those which have very low concentrations of the sought-for component or those which require very high precision do require special techniques. One of these is "compensation," that is, the use of a pure or synthetic sample as a reference in a double-beam spectrophotometer to accentuate differences between it and the unknown (5'). The technique has been applied to polymer systems for the determination of trace components. One of the experimental obstacles encountered in this application was that of obtaining samples which were adequately matched in thickness to permit use of the compensation technique. To eliminate the time-consuming trial and error pressing of films, an exact thickness match was obtained by pressing the reference film of For previous papers in this Symposium see J. &EM.
E~uc.,
35, 2 (1958).
Presented as part of the Symposium on An Analysis Group in an Industrial Research Organization before the Divisions of Chemical Education and Analytical Chemistry a t the 130th Meeting of the American Chemical Society, Atlantio City, September, 1956.
76
matrix material between nonparallel plates to obtain a film in the form of a shallow medge. This wedge was thicker than the sample film a t one end and thinner a t the other. The spectrophotometer was set to record the transmission of the sample a t an absorption peak contributed by the matrix only, and the wedge was moved parallel to its thickness gradient in the reference beam until the spectrophotometer duplicated a "blank" or "full compensation" reading. Maintaining the respective positions of sample film and reference medge in the instrument, the analytical region was scanned with truly effective compensation. Trace components appeared as discrete bands, free of visible matrix absorption. In this v a y we were able t o determine additives and contaminants directly on samples that would otherwise require time-consuming extraction and concentration. I n order to determine very low concentrations of gases in transparent media, multiple-reflection cells with very long paths are employed. A 10-meter cell can be used to identify contaminants in air to the level of less than one part per million. The value of such sensitivity in the study of air pollution is well known. It is also useful in laboratory studies on volatile degradation products. Illustrative of this is the case in which a chemist found it necessary to identify and determine the volatile products from a polymer a t elevated temperatures. Conventional trapping methods were not applicable because of the transient nature of some of the suspected products. A stream of gas which was first passed over the heated material was passed through a 10-meter cell. Infrared monitoring of this stream not only detected the volatile products a t the parts-permillion level, but also identified them, detected their temperature of appearance, and permitted determination of their rates of formation. Samples received as solids present the most challenging problem for the infrared group. I n the case of polymeric materials, samples can often be pressed or cast into transparent films suitable for infrared examination, but sometimes it is necessary to cut such films from massive sections with a microtome. When the samples be obtained as they be finely divided and examined in oil mulls or in potassium bromide wafers. The latter technique may be used to study samples in the microgram range, Such samples are from a "Ivent the surface of to 5 mg. of finely ground potassium bromide. The salt JOURNAL OF CHEMICAL EDUCATION
acts as a carrier for the sample, and, when pressed into a wafer, provides a transparent matrix for its examination by infrared micro techniques. This sampling procedure has been applied to water-soluble compounds isolated on paper chromatograms (Fig. 1). The spot is cut from the chromatogram and the sample is eluted with a solution of potassium bromide. The solution of KBr and sample is then freeze-dried and the finely divided mixture remaining is compressed to give a small wafer which is suitable for scanning with microsampling equipment. Dramatic illustratiou of the sensitivity of the method is the fact that only metal or polyethylene apparatus may he used in handling the aqueous solutions, since a t equilibrium concentration the horosilicate glass in one milliliter of water is sufficient to obscure completely the useful portion of the spectrum. The infrared "microscope" (microsampling apparatus) is useful in studying inhomogeneities in finished products. One such case involved finding the source of small white particles which appeared in a commercial plastic material. Infrared spectra of these inhomogeneities indicated the presence of a polysiloxane structure. This evidence unerringly pointed to a piece of plant equipment coated with a silicone resin as the source of the particles. In so doing the technique a?quired a reputation as an "industrial sleuth" i? solving production problems. STUDIES OF CHEMICAL STRUCTURE OF POLYMERS
I n its role in long-range research, the analytical infrared group is concerned with the determination of the chemical and physical structures of materials. I n this laboratory these materials are primarily polymers which must he considered in terms of both bulk and incidental structure. The bulk structure stems from the normal reaction of the monomers and may be regarded analytically as the major component or components of the system. The incidental chemical structure arises from such sources as side reactions, impurities in monomer, catalyst residues, and oxidation. These incidental structures may he considered analytically as trace impurities. However, their effect on polymer properties is often far greater than their small concentration would suggest. To illustrate the concept of hulk structure, let us consider a polymer of the type:
I
I
PAPER CHROMATOGRPIN
6
ISOLATED
ELUTION
FREEZE DRYING MICRO WAFER
INFMRED SPECTR0GFU.M Figis...
1.
Prsparation of an Infrared Sssn from a P a p a Chromatogrem spot
react in three ways even if head-to-tail additions are ignored. The three modes of addition are: 1,4 cis, 1,4 trans and 1,2 (Fig. 2). Each type of addition yields a different double-bond type with a t least one characteristic absorption as indicated below the structures. Thus it has been shown that for 1,4-addition, a hand appears a t 13.8 microns or 10.3 microns according to the relative amounts of cis and trans configurations. A band appears a t 11.0 microns, the intensity of ~ ~ h i c h is proportional to the concentration of chain units resulting from 1,2-addition. By treating the polymer as a three-component mixture, the fraction of the monomer present in each of the three different forms can be calculated. This method has been applied satisfactorily
E,-(A),(B)cE,
(A) and (B) represent the chain repeat units and El and
Ezrepresent chain ends. I n an ideal homopolymer only
a single repeat unit exists, i.e., (A) = (B). However, in many polymers different units are present because of deliberate attempts to form a copolymer or because a given monomer can react in more than one way. In a homopolymer of (A), (B) might represent actual or potential cross link sites. I n infrared studies of bulk chemical structure, the object is to determine the nature of (A), (B), etc., to determine the relative magnitudes of m, n, . . . and, perhaps, to determine the distribution of the units (A) and (B) in the polymer chain. This distribution of the repeat units distinguishes between true copolymers, block copolymers, and mixed polymers. The infrared studies of hutadiene polymerizations are an interesting example of the determination of a hulk polymer structure. A hntadiene molecule can VOLUME 35, NO. 2, FEBRUARY, 1958
in studies of butadiene polymerization by several investigators (4, 5). Further studies of this same polymer include the determination of crosslinking by measuring the decrease in the absorption due to vinyl unsaturation during curing processes. The infrared method for the butadiene analysis has been extended to include its copolymer with styrene by using a band a t 14.3 microns as a measure of the styrene included in the copolymer. Minor modifications of chemical structure of a polymer may be thought of as incidental structures. Their characterization and determination by infrared techniques are important contributions to both fundamental and practical knowledge of polymer structures. The work on polyethylene, summarized in the table, serves as a practical illustration of this type of study.
Information About the Structura of Polyethylene Obtained bo Infrared Technimes ( ) Bulk structure
(-CH9-CH2-).
(B) Incidental structure
Unsaturation -CH=CH, (vinyl) -CH=CH-(cis and tmns)
' C = C H ~ (vinylidene) / Oxidation products 'C=O /
(ketone, aldehyde, acid)
-A-O-O-H
(hydroperoxide)
I . Chain branchmg \ /CH-(CH,),CH,
In the spectrum of polyethylene (Fig. 3) these functional groups are, for the most part, readily apparent. Unsaturation is indicated by a band near 6.08 microns. Determination of type and quantity of unsaturated linkages may be made from the bands due to trans olefin, vinyl, and vinylidene groups near 10.37,11.0, and 11.25 microns, respectively. Concentrations of these olefmic groups in commercial polyethylenes are in the range of zero to one group per thousand carbon atoms. A variable intensity band near 5.8 microns in the polyethylene spectrum indicates the presence of carbony1 groups. Rugg, Smith, and Bacon (6) investigated the polyethylene carbonyl bands quite thoroughly a t high resolution and demonstrated that the normal 5.81 micron band is due to ketonic carbonyl. I n the spectra of heavily oxidized samples, a complex series of bands were found in the 5.8 micron region. From the positions of their maxima, the most intense absorptions were assigned to aldehyde, acid, and ketone carbonyl groups. The sensitivity of these determinations is indicated by the fact that in virgin, commercial polymer BULK STRUCTURE CH CH
cn2 I
cn, I
the concentration of carbonyl groups is of the order 1 per 10,000 carbon atoms. Chain branching is indicated in the polyethylene spectrum by the bands due to methyl groups. That the number of methyl groups is too great to be accounted for by two ends per molecule was first pointed out by Fox and Martin (7) on the basis of their studies of the C-H stretching absorption. Later studies revealed that bands a t 7.25 and 11.18 microns were also present because of chain branches. The methyl (terminal group) concentration is computed from the absorption a t 7.25 microns corrected for background absorption by comparison with polymethylene, an unbranched material (8). A further application of infrared spectroscopy is the determination of molecular weight by measuring endgroup concentrations. This requires an end group with a strong, specific absorption, since the concentration of ends is very low in high molecular weight polymers. Therefore, it is applicable only when the end groups are strong absorbers, such as hydroxyls, esters, acids, or characteristic olefins, and when these group absorptions are not masked by absorption of the bulk polymer structure or impurities. STUDIES OF PHYSICAL STRUCTURE OF POLYMERS
In addition to information concerning chemical structure, the spectrum of a polymer may reveal information concerning its physical structure. Although the latter field is not as well explored as-theformer, it is receiving increased attention. Studies of molecular orientation have been made using plane polarized infared radiation. Samples are oriented by drawing or by rolling, and the intensity of a given band is measured as a function of the angle between the plane of the electric vector of the radiation and the direction of draw. Crystallinity, another aspect of physical structure, can be studied through changes in infrared spectra. C-C
,
Cn2 I
I
INCIDENTAL STRUCTURE
c=o.
78
c-C
.
CH3
JOURNAL OF CHEMICAL EDUCATION
% AMORPHOUS
h great many polymers have regular extra-molecular structures, and samples of these can he prepared which have wide ranges of crystallme content. For spectral purposes, a semicrystalline polymer can be considered as a two-component mixture; that is, a mixture of amorphous and crystalline regions with no regard for the boundaries between. The differences between the amorphous and crystalline regions can be summarized from the spectroscopist's viewpoint in the following manner: I n the amorphous regions many configurations or rotational isomers are present. Each configuration is, in a spectroscopic sense, a new molecule with somewhat different frequencies and certainly with different selection rules. As a consequence, hands may appear in the spectrum that are due only to vibrations of the amorphous configuration. Bands peculiar t o the crystalline regions may also appear in the spectrum through intermolecular coupling in the crystal or because of the existence of unique configurations in the crystalline regions. The use of crystalline and amorphous hands in the study of polymers is illustrated in the work of Starkweather and Moynihan on nylons (9). In this work a band a t 10.7 microns was used as a measure of the crystalline content of 66-nylon. The hand a t 8.8 microns was used as a measure of the amorphous content. By plotting the intensities of these absorption hands versus the specimen's density and extrapolating the straight lines obtained to zero absorbance, one may easily determine the densities of the pure crystalline and amorphous materials. Density of the crystalline polymer may then be used with X-ray data, often to select the true unit cell structure from among two or more possible ones. A further application of these bands to polymer characterization was made by plotting the absorbance per unit thickness of one against that of the other for identical samples (Fig. 4). Since these two bands are, respectively, linear measures of the crystalline and amorphous contents of the samples, the intercepts of the straight line plot may be taken as the absorptivity values for the pure materials. A simple superposition of a percentage composition scale then provides a truly ahsolutemeasure of crystallinity in the polymer. By following the changes in the intensity of crystalline or amorphous bands as a function of time, useful crystallization rate data can be obtained. Cobhs and Burton (10) studied crystallization rates for polyethyleneterephthalate by melting a thin film and dropping it quickly into a thermostatted oven which was placed in the sample beam of a spectrophotometer. The change in intensity of a crystalline hand was followed as a function of time by recording the absorbance of the sample with the spectrophotometer set a t the wave length of the hand center. This technique permitted investigation of a wide range of temperatures and crystallization rates. For polymers with regular structures, the methods of erouD theorv can he a ~ n l i e dto the studv of their suectra in much the same way as they are used in the study of the spectra of simple molecules. From changes in density, X-ray patterns, and the spectrum itself, one can extrapolate to obtain the spectrum of the completely crystalline material. This spectrum can then he treated by the mathematical methods reported by -
A
..
VOLUME 35, NO. 2, FEBRUARY, 1938
8 . 8 ~ 1AMORPHOUS BAND, ABSORBANCE/MIL. F5-
4
De-ination
of Clystdlina Content of BB-Nylon
Winston and Halford ( I f ) , Hornig (18),and others. I n this way the number of active vihrations and the orientation of their transition moments can be predicted for a given model. From this information, in turn, choices can he made betveen various models proposed for a polymer's'structure. Two principles in the theory of crystalline spectra reduce the complexity of calculation enormously and demonstrate the power of the method. First, for an infinite crystal, only those vibrations can be active in which corresponding atoms in the different unit cells move in the same way. This reduces the number of vibrations to be considered for a polymer with N atoms in the unit cell to at most 3N-3. Second, the symmetry of the unit cell further reduces the numher of active vihrations and these can be expressed in terms of a small numher of symmetry operations. By means of such methods one can, for example, understand the spectrum of crystalline polymethylene quite thoroughly, even without recourse to a study of simple model compounds. LITERATURE CITED (1) KITBON, R.E., Anal. Chem., 25, 1470 (1953). (2) KITSON,R. E., AND N. E. GRIFFITA, Anal. Chem., 24, 334 (1952). C. F.. AND H. R. ROE, Anal. Chem.,. 25,. 668 (3) . . HAMMER. (1953): (4) HART,E. J., AND A. W. MEYER,J. Am. Chem. Soc., 71, 1980 (1949). (5) HAMPTON, R. R.,Anal. Chem., 21,923 (1949). F. M., J. J. S M I T ~AND , R. C.BACON, J. Polymer Sn'., (6) RWGG, 13, 535 (1954). (7) Fox, J. J., AND A. E. MARTIN,Pmc. Roy. SO.?. Ladan, A175, 208 (1940). W. M. D., AND R. C. VOTER,J . Am. Chem. Soe., (8) BRYANT, 75, 6113 (19.53). H. W., AND R. E. MOYNIKAN, J . Polymer (9) STARKWEATAER, Sei., 22,363 (1956). J. P d y Sn'., ~ 10, (10) COBBS, W. H., JR., AND R. L.BURTON, 275 (1053). H., AND R. S. HALFORD, J . Chem. Phys., 17, 607 (11) WINSTON, (1949). (12) HORNlo, D. F., J . Chem. Phys., 16, 1063 (1948).
