\Y. C., “Fusion Methods in Chemical Microscopy,” Interscience, New York, 1957. (29) Miller, H. L., Kielsen, L. E., J . Polynier Sci. 44, 391-5 (1960). (30) Moora, L. D., Peck, V., Ibid., 36, 141-53 (1959). (31) Newman, S.,Cox, W. P., Ibid., 46, 29-49 (1960). (32) Reisner, J. H., RCA Sci. Inst?. 6 , KO. 1, 14-21 (1961). (33) Ries, H. E., Jr., Sci. Bmerican 204, March, 1961, pp. 152-64. (34) Rochow, T. G., Am. SOC. Testing Materials, Spec. Tech. Publ. No. 143, 81-93 (1952). (35) Rochow, T. G., IXD. E N A CHEY., AXAL.En. 11!,629-34 (1938). (36) Rochow, 1. G., Resinography in “Encyclo edia of Microscopy,” ed. by G. L. d a r k , pp. 525-37, Reinhold, New York, 1961. i ) Rochom, T. G., Gilbert, R. L., in (3”Protective and Decorative Coatings,” ed. by J. J. Mattiello, chap. 5, T’ol. 5 Wilcy, S e w York, 1946. (38) Rochow, T. G., Rochow, E. G., Science 111, 271-5 (1950). (39) Rochow, T. G., Home, F. G., ASAL. CHEM.21, 461-6 (1949). (40) Rochow, T. G., Rowe, F. G., electron micrograph exhibited at 9th meeting, Electron Microscope Society of America, Philadelphia, Pa., Sov. 9-10, 1951. (41) Siegel, B. M., Johnson, D. H., Mark, H., J . Polymer Sci. 5, 111-20 (1949). (42) Statton, W. O., Am. SOC. Testing
(28) AIcCrone, Organization of Polymers into Materials
LWQ~ Description Composition-Decomposition Properties I Macromolecular domain Atoms in stoichiometric Chemical reactivity, inproportion and stereoherent solubility of influence arrangement I1 Habit of particles in a Kinds, yo molecules con- Specific surfaces; rates of tacting same or different reactions; etc. point, edge, or surface kind 111 Each phase: liquid rub- Kinds, yo components in Bulk properties at each space: random or order phase: absolute soluber, glass, crystalline bility IV Material: laminate. lstex, Kinds, 5’0 and distribution Properties as manufacof phases; changes tured, tested, or used foam, fabric
The classification is meant to he flexible enough to encompass additional information aiid n m knosvledge so as to increasr understanding of present materials and point the nay toward improvement. ACKNOWLEDGMENT
The author gratefully acknowledges the experinicntal and interpretive skills of his colleague, Ann hl. Thomab, in electron microecopical experiments illustrated by Figures 3 and 4. He also acknowledges the advice of Donald 1)‘. Davis and the Microscopical Group. Donald L. Swanson, Ivor H. Updegraff, Henry P. Wohnsiedler, and William G. Deichwt. LiTERATURE CITED
(1) d S l ’ X Bull., July 1952, p. 22. (2) Bacskai, R., Pohl, H. A,, J , Polymer Sci. 42, 151-7 (1960). (3) Beredjick, S . , Ahlbeck, R. A,, Kmei, T. K., Iiies, H. E., Jr., Ibid., 46, 268-70 (1960). (4) Bdlmeyer, F. W., Jr., unpublished data. (5) Botty, M. C., ASTM Photogra hic Exhibit. 50th Meeting, New York &ty, June 23-2T, 1952. (6) Botty, hl. C., Felton, C. D., Anderson, R. E., Textile Research J . 30, 959-65, (1960). (7) Chamot, E. 321.. Mason, C. W., “Handbook of Chemical Microscopy,” Vol. I, 3rd ed., Wiley, New York. 1958. (8) Chem. Eng. .Vews, Kov. 2, 1959, p. 39
(9) Critchfield, F. E., Johnson, D. P., ANAL. CHEX 33, 1834 (1961). ( l o ) Eirich, F., private communication, hov. 6, 1957. (11) Eirich, F., Mark, H., CongzBs Intern. Nicroscopie Electronipe Paris, Sept. 14-22, 1950. (12) Erath, E. H., Spurr, It. A., J . Polymer Sei. 35, 391-9 (1959). (13) Flory, P. J., “Principles of Polymer Chemist*ry,” p. 424, Cornel1 University Press, Ithaca, N. Y., 1953. (14) Freudenthal, A. M . j Phus. Today 12, 16-19 (1959). (15) Frey-Wyssling, A,, “Submicroscopic Mor hology of Protoplasm,” 2nd. Englsh ed., Elsevier, New York, 1953. (16) Gailey, J. A., h . 4 ~ CHEM. . 33, 1831 (1961). (17) ,Gibson, &I. E., Jr., IIeidner, R. H., Ibzd., 33, 1825 (1961). (18) Gloor, W. E., Modern Plastics 38, 111-14, 212 214 (1960). (19) Hall, C. h., Doty, P., J . Am. Chem. SOC.80, 1269-74 (1958). (20) Hock, C. W Abbott,, A. ii., Rubber Age 82, Dee. 1957, pp. 471-5. (21) Ke, B., Division of Analytical Ghemistry, 139t,h Meeting, ACS, St. Louis, Mo., March 1961. (22) Lindsay, R. B., Rhode Island College J . 1, 7-12 (1959). (23) Loveland, R. P., Am. Soc. Testing Materials Bull. Yo. 143, 95 (1952). (24) Luongo, J. P., ASAL. CHEM. 33, 1816 (1961).
(25) Mark, H., Chem. W e e k 76, April 2, 1958, p . 36-40. (26) Mar!. , H.., Sci. American 197. 81-9 (1957). (27) Mark, H. F., Tobolsky, A. V., “Physical Chemistry of High Polymeric Systems,” p. 27, Interscience, New York, 1950.
-
&laterials Spec. Tech. Pub. No. 247,
242-56 (1958). (43) Thompson, M. S.,“Gum Plastics,” p. 1, Reinhold, New York, 1958, (44) Tobolsky, 9. V., “Properties and Structure of Polymers,” pp. 81-2, Wiley, New Yorlr, 1960. (45) Tsvetkov, V. N., Boitsova, N. N., Vvsokomolekulyarnye Soedineniya 2, ii76-88 (1960). (46) Vassallo, D. A,, A N ~ L CHEM. . 33, 1823 (1961). (47) Weissberger, A., ed., “Physical Methods in Organic Chemistry,” pp. 1620-3, 1701. I, Part 11, 3rd ed., Interscience, New York, 1960 (Cha . 32 on Electron IJIicroscopy by F. Hamm) .
1.
RECEIVEDfor review July 17, 1961. Accepted September 19, 1961. Division
of Analytical Chemistry, 139th Meeting, ACS. St. Louis, %Io., March 1961.
9. P. LUONGB Bell Telephone laboraforie;, Inc., Murray Hill,
b Absorption spectroscopy has been utilized to characterize the chemical structure (and structural abnormalities) of polymers. Among the examples described are the determination of the various olefinic groups, identification of substituted aromatks, crystallinity measurements, pyrolysis techniques, olyethylene branching studies, quantitative determinakions, and following induced structural changes. Ultraviolet s p e c ~ ~ o 5 c o ean ~ y also contribute to
N. 1.
polymer studies. Examples such as the quantitative determination of polymer additives-Le., antioxidants, expanding agents, slip agents, etc.--are described.
large number of publications on the subject indicate that the use of absorption spectroscopy for the structural analysis of polymers has grown rapidly during the past 10 to 16 HE
years. This widespread use of infrared has coincided with the rapid growth of the synthetic and natural resins industries since about 1946. During this time an advanced polymeriaation technology has produced polymers whose structures as well as structural abnormalities necefisitated a new and more suitable technique for their identification. The unique capabilities of infrared spectroscopy became recognized as a means of fulfilling the analytical
--TEREPHTHALIC
-1SOPHThAL1C
bClU I P o l 0 1
AC!D
;me101
Figure 1. The 625- to 825-cm.-' region of terephthalic acid (para-substituted) and isophthalic acid (metasubstituted)
some meta-substituted rings, probably from the use of the meta-substituted isophthalic acid as one of the starting materials. The introduction of the isophthalic unit reduces the chain symmetry as compared to the polyethylene terephthalate chain. This is believed to have some effect on the laminating and solubility properties of this product. Unsaturation. I n the study of many polymers, knowledge of the type and extent of olefinic unsaturation is often required because of thcir effects on certain properties. Fortunately, infrared can measure the unsaturation and also distinguish between the different types (4, 16, 20). I n Figure 3 some of the more common types are shown. The top spectrum / H
1
is that of the trans -C=C-
('a)
group
and research requirements of the rapidly growing polymer industries. Today, infrared spectroscopy has become one of the largest contributors to our knowledge of polymer structure. This paper illustrates some of the more practical and useful applications of absorption spectroscopy currently used in polymer analysis. To describe a reasonable number of diverse examples to cover the many applications, i t is necessary to omit some of the experimental details. However, where possible, references have been made to which the reader may refer. APPLICATIONS
Aromatic Bands. The use of the bands in the 650- to 95O-cm.-' region to detect aromatic substitution is well established (2, 6, 26). To illustrate, the spectra in Figure 1 show the 650- to 800-cm.-' region of para-substituted terephthalic acid (top) and meta-substituted isophthalic acid (lower). Although the assignment of the strong 730-cm.-' band is not entirely settled ( I I ) , the band a t 785 em.-' is characteristic of a para-substituted ring. In the lower trace, the band a t 685 em-' is attributed to a meta-substituted ring. Figure 2 shons how this information can be used. The top trace is the characteristic spectrum of polyethylene terephthalate, a commercially available product. The chain of aromatic rings in this polymer is connected in the para positions. The lower spectrum is that of a modified polyethylene terephthalate. I n this spectrum there is an additional band a t 680 em.-' which shows that this polymer contains, in addition to the para-substituted terephthalate rings,
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FREQUENCY, CM.'
thetic polyisoprenes is the most similar to natural rubber from a structural viewpoint. The top spectrum is natural rubber (Hevea) with the strong band a t 8-10 cm.-1 due to the cis-l,4 adduct, which is the repeating unit (-98%) in. natural rubber (21). The high frequency shift of the cis-1,4 band is due to the substitution of the methyl group a t the alpha carbon (16). The band a t $88 em.-' is attributed to the 3,4 addition type unsaturation (actually a pendant type, R1R2C=CH2). This rcyresents approximately 2 or 3% of the natural rubber structure. The -OH, -C=O, and COO- absorptions often found in natural rubber are probably due to the proteins and amino acids of the latex impurities. The lower three spectra of Figure 4 are the synthetic "natural" rubbers. The gross features of these spectra are similar to those of natural rubber, except for the intensity of the band a t 888 em.-' I n regard to this band, sample GW islmost nearly like natural rubber, perhaps containing only 1 or 27, more of the 3,4 unit, whereas the others have more. The splitting of the 1660-cm.-' C=C band is found in many synthetic polyisoprenes. It is attributed to the large amounts of both internal and external unsaturation in a sample-Le., C=C in the chain; or externally as in a pendant or isopropenyl group. Katural rubber, consisting almost entirely of cis-1,4, has a high concentration of internal unsaturation; hence the single band a t 1660 em.-' The singlet nature of the 1 6 6 0 - ~ n i . - ~ band in sample GW supports the finding that most of the unsaturation is in the chain, as it is in natural rubber. Thus, from a structural viewpoint, sample
Figure 2. The 625- to 825-cm.-' region of polyethylene terephthalate and a modified polyethylene terephthalate
4TRANS
nith its strong sharp band a t 964 em.-' The middle trace shows the two characteristic bands due to the vinyl group (CH=CH,) a t 990 and 910 em.-' I n the lower curie are the unsaturation bands as they appear in nonlinear polyethylene, which also includes the $88em.-' band due to RIR2C=CH2 (pendant type). There is another band near 740 em.-' ~ h i c hhas been as-
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signed to the cis structure This band is broad and its profile T aries \\ ith cis content. Consequently, the mea of the band is generally used for quantitative measurements (20). How these unsaturation bands are used in a structural analysis is shown in Figure 4. In this case, the problem n a s to detrrmine which of thrce syn-
4 RIRpC=CnZ
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Figure 3. Types of olefinic groups found in polymers VOL. 33,
NO. 13, DECEMBER 1961
a
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RUBBER
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Figure 4. Infrared spectra of natural and synthetic polyisoprenet
GW appears to be a desirable substitute for natural rubber. When the methyl group on the cis-1,4 group is replaced by a heavy chlorine atom, the 840-cm.-l band shifts to about 825 cm.-l (15). This is shown in Figure 5, where the spectrum of natural rubber is compared to that of neoprene. Note also the lack of the 1370-cm.-l -CHa band in neoprene. Morphology. Infrared can also be used in many cases for morphology studies of polymers. For example, in Figure 6, the two top spectra are: A , isotactic (considered t o be mostly crystalline), and B, atactic (considered to be mostly amorphous) polypropylene. The bands for each phase are characteristic, and i t is possible t o use them for distinguishing between the atactic and isotactic phase. The lower curve in Figure 6 is of commercial atactic material and the bands a t 995 cm.+ and in the 800- to 900-cm.-’ region show that some isotactic material is present. I n Figure 7 the 800- to 1000-cm.-1 region of samples containing known amounts of atactic and isotactic material is shown. It is possible to use the ratio of the 995- to 974-cm.-‘ bands as a measure of the atactic-isotactic content. By measuring the absorbance ratios of these bands from suitably prepared standards and plotting a working curve, the atactic-isotactic content can be measured with reasonable accuracy (17). B y periodically recording the spectra of a cooling sample (from the melt),
one can follow the onset of crystallinity by observing the decreasing ratio of the 995- to 974-cm.-l bands. The similarity of the periodically run “cooling” spectra with the isotactic-atactic standards shows the very close relationship between isotacticity and crystallinity (I?‘). Determination of Plasticizer CORcentration. The next example is a quantitative determination by infrared. I n a mixture (such as a copolymer or a polymer containing compounding agents), the intensity of a n absorption band due to a constituent is proportional (in accordance with Beer’s law) to the concentration of the constituent. Hence, the concentration of that constituent can be determined by comparing the intensity of the band with that in a known (or standard) sample Butyl oleate is used as a low temperature plasticizer in a neoprene base compound known as NHB-7. Since it is possible for butyl oleate concentration to vary in different shipments, a rapid and reasonably accui Ate control method was required in our laboratory to determine the butyl oleate content. Among the absorption bands in the spectrum of the NHB-7 resin, two well resolved bands, the 1 7 3 5 - ~ m . - ~ester carbonyl band of the butyl oleate and the 1658-cm.-l C=C band of the neoprene-base material, were selerted as the analytical bands. The absorbance ratio, R-Le., the ratio of the intensity of the butyl oleate
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Spectra of natural rubber and neoprene
band to the intensity of the neoprene band-of standard samples (Figure 8) varied directly as the per cent butyl oleate concentration, C, such that C = 6.822. From an analytical curve, the butyl oleate concentration can be determined from the absorbance ratio values of the bands in the unknown samples. The use of two well resolved bands eliminates the necessity of precisely measuring the film thickness for each sample. The precise measuring of the film thickness (for quantitative comparison to known samples) is required when only one analytical band is avaiiable because of masking or interference among the other bands. In this euample, i t is necessary to adjust only the film thickness to obtain optimum spectral presentation-Le., the intensity of the bands to be measured should be between 0.2 and 0.8 absorbance for maximum accuracy. Pyrolysis Technique. Another infrared technique, particularly suitable for “insoluble, infusible, cured, and resilient” resins, is the highly usefui pyrolyzate method reported by Harms (IS). He found that the spectra of the dry distillation products-Le., the pyrolyzate-of certain “intractable” polymers were characteristic of the original material. The pyrolyzates are actually the decomposition products or, in some cases where depolymerization occurs, they are the monomeric or low molecular weight constituents. Harms illustrates that these pyrolyzates yield reasonably defined spectra which serve for qualitatively identifying “intractable” polymers. Figure 9 shows the spectrum of nylon 66 as it appears in a molded film. The lover spectrum is the pyrolyzate of nylon 66. One can see the similarity of the two spectra, particularly the
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Figure 6. Types of polypropylene VOb. 33,
NO. 13, DEC€MBER I961
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20 % ISOTACTIC
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Figure 7.
80 % lSOTA,CTlC
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The 1050- to 750-cm.-' region of four laboratory-prepared polypropylene standards of known isotatic content
N-H bands in the 3.0- and 6.0micron regions which appear in both. In Figure 10 the spectrum of a cast film of buna N is compared with its pyrolyzate spectrum. I n this example, with the exception of the telltale C=N band a t 4.5 microns, the pyrolyzate spectrum bears less resemblance to the original material. However, the pyrolyzate spectrum is characteristic of buna N and is reproducible, even though it is probably the decomposition products. Compensation Spectra. One of the many advantages of a double-beam spectrophotometer is in the recording of compensated spectra (7, 12). In this technique Material 1, composed of known component A and unknown component B, is placed in the sample beam. Material 2, composed of only component A and of variable thickness, is placed in the reference beam. Since Material 2 is of variable thickness. it is possible to adjust its position in the reference beam so that one can balance out (or compensate) component A's absorption bands which are common in both samples. This would leave only the absorption bands due to component B to be recorded.
One of the best known current applications of the compensation technique is its use in polyethylene branching studies (26,l 7 ) . I n the infrared, there is a band due to the -CH8 group (terminal group of each branch) at 1378 cm.-l and this is shown in Figure 11. At the top is the spectrum of polymethylene, which has essentially no -CHS groups and only bands at 1355 and 1370 cm.-l due to -C& groups. Below is the spectrum of branched polyethylene with the additional band a t 1378 cm.-l due to -CHs groups. There are considerable overlapping and interference between the -CHI band and neighboring -CHz bands. However, through the use of compensation spectral recording (sometimes called differential or ratio recording), the problem of interference from neighboring --CHz bands is resolved satisfactorily. By proper positioning of a wedgeshaped sample of polymethylene in the reference beam and with the branched polyethylene in the sample beam, the -CH2 absorption common to both will be compensated out and only the -CH3 absorption will be observed.
The ratio spectrum of polymethylene us. branched polyethylene is shown in
Figure 12. The intensity of the absorption of 1378 cm.-l gives a measure of the -CH3 content when compared t o the compensated spectra of polyethylenes having known -CHS concentrations (96). Based on reported applications of this technique (6, 14, IO), a task group within ASTM Committee D-20 is considering methods for a standard determination of methyl content by.infrared spectroscopy. The compensation technique can also be used for determining additives in polymer. For example, if the major components of a polymer are known, the known polymer bands can be compensated out and the recorded bands used to identify or a t least classify the additive. Another frequent use of compensation spectroscopy is with solution techniques. For this, the unknown sample is dissolved in the appropriate solvent and the cell placed in the sample beam of the spectrometer. It is then run against a variable space cell (in the reference beam) containing the solvent. The path length of the variable space cell
WAVE LENGTH, MICRONS FREQUENCY, CM-'
Figure 8.
1826
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Butyl oleate-neoprene standards
ANALYTICAL CHEMISTRY
Figure 9. nylon 66
Spectra of cast film and pyrolyzate of
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DYNK V.S. POLYMETHYLENE
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Spectra of cast film and pyrolyzafe
is adjusted so that the solvent bands are balanced out and only the sample bands are recorded (19, 93). Special Applications. Two other applications of infrared which are not, perhaps, as widely used as those previously described are useful in special cases. The first method is that of reflectance measurements. I n this technique, a thin film (-0.5 mil) of the resin is deposited on a reflective surface such as polished brass, aluminum, steel, etc. By using a commercially available reflectance wcessory, the spectrum of the deposited film can be run. This method is suitable for the analysis of organic surface coatings in solvent evaporation studies, curing rates, thermal and photostability studies, and identification of thermoset resins on metallic containers. There are a limited number of reports in the literature regarding this technique; however, recently Dannenberg and coworkers have covered this method amply (9). The second method is the examination of oriented samples with polarized infrared radiation. In an oriented polymer, changes in dipole moments (the cause of absorption bands) of certain molecular groups may be restricted to definite directions in relation to the chain axis. When a beam of radiation polarized in one direction passes through a film, a selected absorption band will be of maximum intensity, if the direction of the dipole moment change accompanying the vibration is in the same direction as the electric vector of the polarized radiation. l f the electric vector of the polarized radiation is rotated 90" to the vector of the libration, the band will be of minimum intensity, since the two vectors do not coincide. The ratio of these band intensities is called the "dichroic ratio" and is ased to determine the orientation of various molecular groups as well as crystalline-amorphous regions in poly-
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14
.
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16
Figure 12. ethylene
of buna N
Determination of methyl group content in p o l y
Figure 1 1. infrared spectra of polymethylene and polyethylene in the 1370cm.-l region
mers, since various configurations and various kinds of chain folding produce characteristic dichroic ratios. Polarized infrared radiation has been applied to configuration studies in nylon 66 ( I ) , rubber (99),and polyethylene (1). Ultraviolet Spectroscopy. Ultraviolet spectroscopy can also provide additional information in polymer analysis, particularly in the determination of additives. For example, an analytical method was required in our laboratory to determine the amount of phenyl salicylate in cellulose propionate samples which also contained a plasticizer. The ultraviolet spectra (from 200 to 400 mfi) of the individual components in solution with chloroform showed that only the phenyl salicylate had a strong absorption at 312 mp (Figure 13). The absorbance of this band in a series of standards (Figure 14), was plotted us. concentration to form an analytical curve in which per cent I
concentration (Salol) = 1.7 (Awz). From the absorbance of the 312-mp band in the unknown sample (prepared in similar manner) one can rapidly and accurately determine the phenyl salicylate content in the sample using an analytical curve for comparison. Another application of ultraviolet is in the quantitative analysis of additives in hot molded films of polymers such as polyethylene and polypropylene. Fortunately, both polyethylene and polypropylene have relatively flat curves whereas almost all antioxidants, expanding agents, and slip agents contain phenyl rings, conjugated groups, aromatic amines, phenolics, etc., which exhibit strong ultraviolet absorption bands. In Figures 15 and 16 are shown the ultraviolet spectra of some commonly used antioxidants in different film thicknesses ( 5 to 15 mils) of polyethylene. These antioxidants have ultraviolet bands which are applicable to quantitative analysis. For example, in the quantitative determination of (390-5, standards were prepared from a masterbatch containing a known amount of the antioxidant. The ultraviolet spectra are shown in Figure 17. Then, in the usual manner, the absorbance per centimeter of the 282-mp band was plotted us. concentration, resulting in an analytical curve in which per cent concentration CAO-5 = 0.0063(abs./~m.~~). Although the ultraviolet spectra of many additives are somewhat characteristic, it is better to verify their identification by other means such as PXtraction and infrared analysis. Generally the light scattering of a 10-rnil film of polyethylene or polypropylene is not too significant, and it is poseible to use somewhat thicker films t o determine lower concentrations. The method is limited, however, by the presence of heavy pigmentation or unknown impurities with ultraviolet absorptions similar to the additive. The use of the base line technique is VOL. 33, NO. 13, DECEMBER 1961
e
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Ultraviolet spectra of Salol, cellulose propionate,
icizer in chloroform
generally sufficient for measuring the intensities of the bands. Using the base line technique, one can obtain results with an accuracy of about =k0.02% of the additive, which is well within the accuracy that is obtained during the commercial milling in of the additive. ther Spectral Regions. Two other spectral regions in the field of absorption spectroscopy can also be used in structural analysis. The first is the near-infrared region, which extends from 0.8 to about 2.5 microns. The strongly absorbing combination and overtone bands can provide a considerable information regarding the structure of amines, phenolics, olefins, GH groups, etc. (8, 10, 24). At the other end of the 2- to I5micron sodium chloride region is the far-infrared, extending from 15 microns to beyond 35 microns. I n this spectral
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ANALYTICAL CHEMISTRY
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