raman spectroscopy of some polymers and copolymers of styrene, butadiene, and methylmethacrylate

C. N-Ni-N Bending Mode

This mode is expected to be sensitive to the py-py(ds) substitution and insensitive to the 5SNi-62Ni and halogen substitutions. At least two such bands are found in the 210 to 170 cm -I region (Table I). In the case of Ni- (py)4C12, one of these bands is probably overlapped by the Ni-C1 stretching mode.

D. XNiN Bending Mode

This mode should be sensitive to both py-py(ds) and halogen substitutions. The 176 cm -~ band of Ni(py)~C12 gives a large shift (8.7 cm -1) by the pyridine deuteration and a small shift (0.7 cm -~) by the C1-37C1 substitution. Furthermore, this band shifts progressively to a lower frequency as the halogen is changed from C1 to Br to I.

E. XNiX Bending Mode

This mode is expected to be sensitive to the halogen but insensitive to the py-py(ds) substitution. The 155 cm -1 of Ni(py)4C12 gives a relatively large shift (2.0 cm -I) by the C1-37Cl substitution and a relatively small shift by the pyridine deuteration. Thus, this band must be assigned to the C1NiC1 bending mode. Similar bands are located at 120 and 105 cm -I for the bromo and iodo complexes, respectively.

In order to confirm the above assignments, we have also measured the infrared spectra of the Ni(7-pic)4X2 series. Fig. 4 shows their far-infrared spectra and Table III lists the observed frequencies and band assignments. As expected, the spectra of the ~-pic complexes

are similar to those of the corresponding py complexes. I t is interesting to note that the Ni-N and Ni-X stretching frequencies of the ~-pic complexes are always lower by 30 to 10 cm -1 than those of the corresponding pyridine complexes.

ACKNOWLEDGMENT

The authors wish to express their sincere thanks to Professors T. Takenaka and S. Hayashi of the Chemical Research Institute of Kyoto University for their aid in spectral measurements and to Prof. K. Machida of Kyoto University (Pharmaceutical Sciences) for his valuable comments.

1. R. J. H. Clark and C. S. Williams, Inorg. Chem. 4, 350 (1965).

2. C. W. Frank and L. B. Rogers, Inorg. Chem. 5,615 (1966). 3. M. Goldstein and W. D. Unsworth, Inorg. Chim. Acta 4,

342 (1970). 4. M. Goldstein and W. D. Unsworth, Spectrochim. Acta 28A,

1297 (1972). 5. Y. Saito, M. Cordes, and K. Nakamoto, Spectrochim. Acta

28A, 1459 (1972). 6. C. W. Schl/~pfer, Y. Saito, and K. Nakamoto, Inorg. Chim.

Acta 6, 284 (1972). 7. ~V[. A. Porai-Koshits and M. A. Antishkina, Dokl. Akad.

Nauk. S.S.S.R., 92,333 (1953). 8. K. Thompson and K. Carlson, J. Chem. Phys. 49, 4379

(1968). 9. I~. Shobatake and K. Nakamoto, J. Am. Chem. Soc. 92,

3332 (1970). i0. C. Udovich, J. Takemoto and K. Nakamoto, J. Coord.

Chem. I, 89 (1971). 11. P. M. Boorman and A. J. Carty, Inorg. Nucl. Chem. Lett.

4, 101 (1968).

Raman Spectroscopy of Some Polymers and Copolymers of Styrene, Butadiene, and Methylmethacrylate

Howard J. Sloane* and Randall Bramston-Cookt

Varian Instrument Division, 611 Hansen Way, Palo Alto, California 9~303

(Received 4 October 1972; revision received 27 November 1972)

The application of Raman spectroscopy to polymeric systems is discussed. Experimental factors, including sample preparation, are reviewed. In particular, a three-component polymer is quantitatively analyzed, and lR/aman methods are shown to yield information not available from infrared alone. INDEX HEADINGS : P~aman; Polymers ; Copolymers; Styrene; Butadiene; Methylmethacrylate; Quantitative analysis.

INTRODUCTION

For many years, infrared spectroscopy has been a prime analytical tool in the study of polymeric systems.

* Present address: Beckman Instruments, Inc., 2500 Harbor Blvd., Fullerton, Calif. 92634.

J" To whom requests for reprints should be addressed.

It is only since the availability of powerful laser sources, however, that Raman spectroscopy has been used extensively on polymers. While a few polymer spectra were taken with Toronto are excitation, and a small collection of He-Ne-excited spectra was subsequently published, 1 it has really remMned for the more powerful argon ion laser to establish thoroughly the value of the

Volume 27, Number 3, 1973 APPLIED SPECTROSCOPY 217

technique in the last couple of years. Two recent review papers 2, 3 discussed experimental considerations and gave a number of applications. Spectra have now been published on polyacrylamide, various nylons, polybutadienes and isoprenes, polyethylenes, poly- ethylene sulfide, polyvinyl chloride, polyvinyl fluoride, polymethylmethacrylate, polyacrylonitrile, polyethyl- ene oxide, polyvinylidene chloride and fluoride, poly- butene- l ' s and polytetrafluoroethylene. In addition, a number of spectra of water-soluble polymers have been taken in aqueous solutio~L In terms of structural de- terminations, Koenig 3 has pointed out how the infrared and Raman selection rules operate, for example, with vinyl-type polymers. The combination of Raman polarization experiments and infrared dichroism measurements is sufficient to establish the stereoregularity of helical and open chain structures.

In the area of biologically important polymers, Ra- man seems to be opening new areas of investigation as well. a The work of Peticolas and co-workers 5-7 on natural and synthetic polypeptides, proteins, and polynucleo- tides promises to be especially significant.

This paper discusses some important aspects of sample handling; in addition, results of our analysis of a

commercial graft terpolymer are given, and a method for the quanti tat ive determination of the three components is presented. Limitations of infrared methods are also discussed.

I. EXPERIMENTAL METHOD

A. Sampling Technique

Preparation of polymer samples for examination by Raman spectroscopy offers mixed blessings. In favorable cases, it is possible to obtain excellent spectra with no sample preparation whatever. Samples varying greatly in size and shape from powders, beads, and fibers to films, rods, and plates may be easily accommodated. This is in marked contrast to the sample preparation usually required for infrared analysis where thin films are preferred, if not absolutely required. As an off- setting complication, however, commercial polymers frequently contain low concentrations of impurities tha t produce interfering fluorescence. This can be alleviated either by conventional cleanup techniques (dissolve, add activated charcoal, filter, precipitate), or, more commonly, by using the "drench-quench" technique. Here the sample is simply allowed to sit in the

A

f L i i i i i i K h L i i i i

4000 3600 3200 2800 2400 2000 ] 800 ] 600 1400 1200 I 000 800 600 400 200 0

Acm- '

J

V

i i i i i i i

4000 3600 3200 2800 2400 2000 1800 1600 ] 400 A©m-1

, h 1200 I 000 800 600 400 200

FIG. 1. Raman spectrum of 8% cis-butadiene-modified polystyrene. A, taken immediately after placing sample in the instrument. Broad peak in the background centered near 700 cm -1 is apparently due to low concentrations of fluorescent impurities in the polymer. B, spectrum of same sample obtained after several hours of drench-quench treatment. Fluorescence level has decayed to the point where good quality Raman spectrum is easily obtained. The butadiene C~C stretching modes are observed in the 1650 cm -1 region. Compare to pure polystyrene, Fig. 5A.

218 Volume 27, Number 3, 1973

intense focused laser beam for times ranging from just a few minutes to several hours. Such treatment in- variably produces a dramatically improved background in the Raman spectrum, although the mechanism for fluorescence quenching is not thoroughly understood. Spectra of butadiene-modified polystyrene (Fig. 1) taken before and after the drench-quench treatment illustrate this technique.

Polymer samples which are dark colored or contain highly absorbing fillers are, of course, subject to thermal decomposition from absorption. The rotating sample technique 8, 9 promises considerable relief from this problem, however.

Sometimes colorants produce unusual effects. For example, low concentrations of azo dyes in polythylene terephthalate fibers may be readily observed in the Raman. 1° Very high intensities of the dye bands may have their origin in the resonance Raman effect.

The geometric arrangement of sample relative to exciting beam and collecting lens can be varied so that spectra can be obtained from almost any kind of sample. Numerous types of holders are available or can be devised, based on the nature of the sample and the ingenuity of the investigator. For highly scattering powders or other turbid samples, front surface collection from a sample tamped into the conical cavity of a steel rod is most useful.

With highly transparent plates like polymethylmethacrylate (Fig. 2), it is sometimes possible to illu- minate from the back side of the plate and collect the radiation generated within the transparent sample. In some cases, it is even possible to multipass the laser through the sample to obtain more energy. As an additional benefit, highly transparent samples (and many fiber samples as well) often yield useful polarization information.2.3

B. Apparatus

Raman spectra were obtained on a Cary model 83 Raman spectrometer, with 488.0 nm excitation and 30 to 60 mW laser power at the sample location.

II. INFRARED A N D R A M A N ANALYSES OF S O M E POLYMERS A N D TERPOLYMERS

The utility of Raman spectroscopy was determined recently for analysis of a graft terpolymer, the components being polystyrene (PS), polymethylethacrylate (PMA), and polybutadiene (PBD). Infrared methods for determination of the three components were re- ported to be suspect on two accounts. First, many of these copolymers were cross-linked and were, therefore, not entirely soluble in suitable film-casting solvents. For this reason, samples were simply swelled by the solvent, and the softened polymer was pressed onto alkali halide plates for infrared examination. This method left some questions as to whether homogeneous and representative samples were obtained. Second, the determination of PBD in the infrared is complicated because it has been known for some time that butadiene polymerizes in several ways and at least three structures have been recognized.

- - 1

trans cis

~ CH~--CH 7 and ~H $

vinyl (syndiotaetie or isotaetie)

Fig. 3 shows infrared spectra of pure PS, PS plus 8 % trans-PBD, and PS plus 8 % cis-PBD, respectively. I t can be seen from the characteristic out-of-plane hy- drogen deformation band at 960 cm -1 (Fig. 3B) that infrared is quite sensitive to the presence of the trans- PBD isomer. Although not obvious from this figure, infrared also can detect vinyl bands near 910 and 990 cm -1. The cis structure, however, has its strongest band

,ooo 3,;0 2,;o 2o;o ,~;o ,ogo 8;o & ,;o ~o

; L 32;o 28;o ,,;o ,,;o ,~;o

Acre-1

FIG. 2. Highly transparent plate of polymethylmethacrylate yields a high quality l~aman spectrum. Note that carbonyl band at 1730 cm -1 is weak in Raman in contrast to infrared. Intense band near 810 cm -1 is used in quantitative determination in terpolymer.

APPLIED SPECTROSCOPY 219

at about 730 cm -1 in the infrared, and it is subjected to heavy interference from the PS 750 cm -~ band (Fig. 3C). Thus, low concentrations of cis-PBD are undetectable in PS by infrared (compare Fig. 3C to Fig. 3A). Be- cause of this complication, infrared quant i ta t ive de- terminations of P B D in modified PS are liable to be inaccurate if the butadiene is high in cis content or if the cis content varies significantly from sample to standard.

Fig. 4 shows an infrared spectrum of the graft terpolymer previously mentioned. Bands due to PS (S) and P M A (M) are identified. Also, bands due to the trans (Bt) and vinyl (B~) types of P B D are observed and marked.

Fig. 5A, 5B, and 5C are R a m a n spectra of PS cis- PBD, and trans-PBD, respectively. Low concentrations of either trans- or cis-PBD are easily observed in PS by

Raman (see, for example, the 1650 em -1 region of Fig. 1B). In admixture (Fig. 6), bands for all C ~ C com- pounds may usually be observed, though they may not be entirely resolved (trans 1668, cis 1652, and vinyl 1640 cm-~).

Fig. 7 is the Raman spectrum of the same terpolymer whose infrared spectrum is shown in Fig. 4. Bands for the various components are again identified in Fig. 7 by comparison to the spectra of the individual homopolymers P M A (Fig. 2), PS (Fig. 5A), cis -PBD

(Fig. 5B), and t rans-PBD (Fig. 5C). While no spectrum of the pure vinyl type of P B D is available for comparison, the side band at 1640 cm -1 may be a t t r ibuted to the vinyl isomer, and its intensi ty corresponds reasonably to the vinyl concentration deduced f rom the infrared 910 and 990 cm -1 bands (see Fig. 4).

7

1400

WAVELENGTH (MICRONS)

8 9 10 12 15

i I ,

• 1200 1000 800 600 FREQUENCY (CM")

1400


, 10 . . . . . . . !~ . . . . . . ! , 5 0'o

I

2 1200 1000 800 600

FREQUENCY (CM")


1400 1200 1000 800 600 FREQUENCY (CM")

FZG. 3. A, partial ir spectrum of polystyrene. B, ir spectrum of polystyrene modified with 8% trans-polybutadiene. Band characteristic of trans is marked Bt set 960 cm-L C, ir spectrum of polystyrene modified with 8% cis-polybutadiene. Most intense band characteristic of cis material is near 730 cm -1 and is highly interfered with by polystyrene 750 cm -~ band.

2.5 i , , ? 0.0

0.2 z

i 0 . 4

0.6

0.8 1.0

4000 3500

i i: i

I ,,

I ; i.

WAVELENGTH (MICRONS) 7 8

i i i l i , , - - I l l ' I ! l : L L I l l

I I I ! ! ! ! ! ! ! !

I I I I

t2 15 9 10

~B ~ , I , V

i J

i i i , I ,Ii ! t ;

' I ; i

3000 2500 2000. 1800 1600 1400 1200 1000 800 600 FREQUENCY (CM")

20 30 4050

I

I I

0 2 i

l - I

- - I - - i

400 2 ~

FIG. 4. Infrared spectrum of graft terpolymer of styrene, methylmethacrylate, and butadiene. Some characteristic bands are identified by S, M, and B, respectively. Trans-polybutadiene, B t , yields intense band near 960 cm -1. Vinyl, By , is revealed by 910 and 990 cm -1 bands, but cis type cannot be seen in ir spectrum owing to interference at 730 cin-L


The cis-PBD isomer, which is quite low in concentration in the sample of Fig. 7, is barely observable as a shoulder at 1652 cm -*. However, at higher concentrations (see Fig. 6) cis is more apparent.

A number of other interesting conclusions may be drawn from study of the Raman spectra tha t were not revealed in the infrared spectra. The standard provided

for trans-PBD was a yellow, rubbery material known to be relatively impure. (Other standards were relatively pure white powders, beads, or clear pieces of rod.) Purification of the trans-PBD standard by acetone extraction produced a light colored material. Because of their reduced band intensities in the purified polymer, the emissions at 1530 and 1130 cm -~ in the trans

,4000 3600 3200

: ! iF - ' ,2 I I

T =

4000 3600 3200

2800 2400

RAMAN SHIFT ~ C M - '

2800 2400 i000 800 600 400

200 0

.i: i

r

5"M~:: . . b

22 - : i

200 0

1 6 5 2 c m ~

i i i i i i i

4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 200 Acm-1

C

1 6 6 8 c rn -~

1 1 3 0 c m I

4000 3600 3200 2800 2400 2000 1800

1 5 3 0 c m -;

I I I

1600 1400 120~ I000 800 600 400 200 Acre-1

FIG. 5. A, Raman spectrum of polystyrene. A number of bands free of interference are present which are suitable for quant i ta t ive determinat ions of polystyrene in the terpolymer. B, ~ a m a n spectrum of high cis content polybutadiene. Characterist ic C ~ C s t re tch for cis is intense band at 1652 cm -I. C, I~aman spectrum of high trans content polybutadiene. Characterist ic C- -C s t re tch for trans occurs at 1668 cm -1. Shoulder at 1640 cm -1 is vinyl impurity. Bands at 1530 and 1130 cm -1 are also believed due to impuri ty, possibly of all trans conjugated polyene type (see text) .


material (Fig. 5C) are assigned to an impuri ty. Intense bands in these two regions have been a t t r ibuted to an all trans coniugated polyene structure,

I I I - - C = C - - C = C - - C = C - - ,

such as is found in el-carotene. Even trace concentrations of such an impuri ty would be expected to give intense bands dug to resonance Raman effects? ~, ~2 From a chemical point of view also, this would seem to be a likely impuri ty.

Finally, one surprise feature was observed in the

1800 i ii:~i

RAMAN SHIFT h C M -I

1600 1400 1200 1000

1 6 0 0 1 4 0 0 1 2 0 0 lOOO

B t 1668 cm "~

R a m a n spectrum of the commercial terpolymer shown in Fig. 7 which was not seen in the infrared. This is the weak band at 2235 cm-L A band in this region is likely, of course, to be due to a nitrile, presumably polyacrylonitrile. As we have noted previously, 13 the intensities of nitrile bands vary notoriously in the infrared, and, at low concentrations, the presence of C ~ N may be easily overlooked in the infrared spectrum. This group tends to give a band of more consist- ent intensity in the Raman spectrum, however.

An alternate explanation for the 2235 cm -1 band is to a t t r ibute it to the - - C ~ C - - linkage. Once again, R a m a n is very sensitive to this group, while infrared would be very insensitive if the triple bond even comes close to symmetrical substitution. ]3

III. M E T H O D FOR Q U A N T I T A T I V E T E R P O L Y M E R A N A L Y S I S BY R A M A N S P E C T R O S C O P Y

The R a m a n method for the quant i ta t ive determination of the three components in a terpolymer is based on a relative band ratio method. 14 This type of determinat ion has been widely used for m a n y years by infrared spectroscopists. In the infrared method, the relative concentration ratios may be determined with- out ever measuring the sample's thickness or deter- mining the individual component absorptivities. I t is only necessary to have one (or preferably more than one) s tandard of known concentration.

Similarly, in the R a m a n adaptat ion, no absolute measurement of R a m a n "scat ter ing cross section" (the equivalent to "absorp t iv i ty" in absorption measurements) is required. As with the infrared method, analytical bands are chosen to be as nearly free f rom interference as possible.

A. Choice of Analyt ica l Bands in the Terpo lymer Analys i s

The choice of analytical bands for this part icular terpolymer probably requires some additional experi-

FIG. 6. Mixture of trans, cis, vinyl polybutadienes (estimated 49:39:12%).

S + B t

B t + B c B c 1652 crn ;

Bv1640 -

C=-N

$

L i i t i * i i i i L r *

4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 AGO 200 0 A c r e - '

FIG. 7. l%aman spectrum of same terpolymer whose ir spectrum is shown in Fig. 4. Individual components are again identified by S, M, and B. Evidence of all three types of polybutadiene, trans B t , cis Bc, and vinyl B. are noted, although components are not resolved. In addition, unexpected C_~-N impurity band at 2235 cm -1 is revealed.


O0 A

1800

RAMAN SHIFT ~C.M -n

1600 1400

1600 1400

1200 1000

1200 1000

R A M A N SHIFT ~ C M -I

1800

800 =:

800 1800

1600 1400 1200 1000 8 0 0

1600 1400 1200 1000 800

FIG. 8. A, spectrum of terpolymer utilized as known standard of composition CB~ = 36%, CN0 = 38%, and Cso = 26%. Method of drawing baselines is similar to infrared technique. Peak intensity of trans-PBD band at 1668 cm -~ is taken to be representative of all PBD for this illustrative example. Methylmethacrylate band at 810 cm -~ was preferred because of high sensitivity, although 1730 cm -a carbonyl band is free from interference. B, spectrum of terpolymer of unknown composition.

mental investigation. For example, a number of bands appear to be suitable for PS, including 620, 1000, 1030, and 1603 cm -~. In the analysis which follows, the 1603 cm -1 band was used, since it is relatively intense and totally free from interference.

For PBD, certainly the greatest sensitivity is given by the C ~ C stretching bands in the 1650 to 1670 cm -~ region. This is complicated, however, by the presence of the three isomers (cis, trans, and vinyl) as discussed earlier.

If the band structure were resolvable either instru- mentally or by computer t reatment of the data, it might be possible to determine all three isomers sepa- rately with suitable known standards. In this case, the " terpolymer" would simply be treated as having five components instead of three.

Treating the PBD as a single component is also possible. If it is assumed that all three components have approximately equal scattering coefficients, a measurement of the total band area from 1625 to 1700 cm -~ would be representative of total PBD's. Alterna- tively, measurement of the peak intensity of just the one major component (in this case, the 1670 cm -1 trans material) is suitable, provided it is known that this peak will always be representative of total PBD, i.e., tha t the distribution among the three types of PBD will remain constant for all samples and standards. Unfortunately, this condition does not always hold, which is one reason that the infrared method sometimes fails. The infrared technique normally utilizes a band characteristic of the t rans isomer only, at 960 cm -~, or the vinyl isomer only, at 1640 cm-~, 15 to represent all PBD. For the example shown here, the distribution problem is ignored, and the 1670 cm -~ peak intensity is used to represent all PBD. [Note that

there actually is evidence of variable distribution be- tween standard (Fig. 8A) and sample (Fig. 8B) in the ratio of vinyl band intensity I8 , at 1640 cm -1 to t rans

band intensity IB, at 1668 cm-1.] For PMA, the greatest sensitivity is given by the

intense 810 cm -1 band (see Fig. 2), but there is slight interference by a PS band at 790 cm -~ (see Fig. 5A) requiring considerable judgment in constructing the baseline. The carbonyl band at 1730 cm -~ is entirely free from interference but is considerably weaker and therefore inherently less sensitive. Considerable gain in readabili ty of this band intensity would, of course, be obtained by operating the instrument at higher sensi- t ivi ty over this region, utilizing longer periods and slower scan speeds if required. For the example worked out here, the 810 cm -~ band was used for PMA.

B. The Relat ive Band Ratio M e t h o d

For each component at some characteristic frequency, the intensity (I) of a Raman emission band is linearly related to the component's concentration (C) in a mixture. Thus, for PBD (B) we write the equivalent of the Beer's Law expression:

IB = K1CB (1)

where K1 is a constant for given instrument parameters of slit width and sensitivity. Similarly, for PS (S) and PMA (M):

I~ = K2C~ (2)

and

[M • K3CM (3)

Then for the standard of known composition, denoted


by subscript 0, dividing Eq. (1) by Eq. (2) :

I,__~ = K_2 . C,__2 ISo K2 Cso

and

where K' = K1/K2

Similarly,

(4)

Ieo _ K' • C.o (5) Iso Cso

K' = I.o Cs0 (6) ISo C,o

K" = __K2 = Iso Cmo (7) K3 [Mo Cso

A third and redundant expression may also be derived:

K " = K--2 = I,o Coo (8) K3 IMo C~0

The spectrum of a known standard comprised of butadiene (36 %), methylmethaerylate (38%), and styrene (26 %) is shown in Fig. 8A along with the analytical bands selected, method of drawing baselines, and measured intensities, I ,o , Iso, and I~o. From this standard,

IBo . Cso = 59.__=26~ 1.0 _ K ~ (9) Iso CBo 42 36 K2

Ci0 =__42 .__38 = 2.9 _ K2 (10) Cs0 21 26 K3

K ! _ ~ - -

and

K" = I So .l'M o

Note that if several standards of known composition are avMlable, a considerable increase in precision in the determination of K ' and K " are possible by graphical methods? 4 I t may be seen from Eq. (5) that by plot- ting the ratio IBo/Iso as a function of CBo/Cso for the several standards, the slope of the resulting linear plot will be K' .

For the unknown sample, from Eqs. (1) and (2) :

or

1_2 = K__~. C2 = K ' C-2 (11 ) Is K2 Cs Cs

c_~ = ~ . _1 (12) Cs Is K'

Cs Is 1 C ~ = I--~ " K " (13)

Similarly,

From the spectrum of the unknown, Fig. 8B, IB = 78, Is = 41, and IM = 9. Then, as a sample calculation, from Eqs. (12) and (9):

CB I . 1 78 1 . . . . . . . . . . 1.9 (14) Cs Is K' 41 1

o r

C8 = 1.9 Cs (15)

From Eqs. (13) and (10):

Cs Is 1 41 1 . . . . . . . . . . 1.6 (16) CM IM K" 9 2.9

or

C~ = 0.63 Cs (17)

Assuming that only the three components B, S, and M constitute all of the sample:

C B + C s + CM = 1.0 (18)

Substituting Eqs. (15) and (17) into Eq. (18):

1.9 Cs + Cs + 0.63 Cs = 1.0 (19)

Cs = 0.28 or 28%

and from Eqs. (15) and (17)

C, = 53%

CM = 18%

IV. S U M M A R Y OF R E S U L T S

From the preceding discussion, the following conclusions may be drawn about the applicability of Ra- man spectroscopy to polymeric materials.

1. With argon ion excitation, good quality Raman spectra may be taken quite readily on most polymers.

2. A variety of shapes, forms, and sizes may be accommodated, often with no extensive sample preparation. For the terpolymer system studied here, the samples' insolubility made the infrared preparation subject to suspicion.

3. Samples which exhibit high fluorescence back- grounds usually yield markedly improved spectra with the drench-quench technique.

4. In the terpolymer system studied, C = C stretching bands are readily observed for the three types of PBD in the 1650 cm -1 region in the Raman. While there is no interference from PS or PMA bands, the three PBD components are not entirely resolved from one another. In the infrared, it is virtually impossible to detect low concentrations of cis-PBD in the presence of PS.

5. Raman examination sometimes may reveal un- suspected components or impurities which are not readily seen by infrared. The suspected trans polyene impurity in trans-PBD and the suspected - - C ~ N or - - C ~ C - - in the "terpolymer" are cases in point.

6. A quantitative band ratio method may be used for determination of the individual components.

A C K N O W L E D G M E N T

The cooperation of Mr. Jack Avery of the General Electric Company, Plastics Department, Selkirk, New York, in providing known standards and infrared results is gratefully acknowledged.


1. D. S. Cain and A. B. Harvey, Naval Res. Lab., Report 6792 (1968).

2. M.J . Gall, P. J. Hendra, D. S. Watson, and C. J. Peacock, Appl. Spectrosc. 25,423 (1971).

3. J. L. Koenig, Appl. Spectrosc. Rev. 4,233 (1971). 4. J. L. Koenig, "Raman Spectroscopy of Biological Mole-

cules--A Review," T. 1~. No. 225, to be published in J. Polymer Sci., Part D.

5. B. Fanconi, B. Tomlinson, L. Nafie, W. Small, and W. L. Peticolas, J. Chem. Phys. 51, 3993 (1969).

6. E. W. SmM1 and W. L. Peticolas, Biopolymers 10, 69 (1971). 7. E. W. Small and W. L. Peticolas, Biopolymers 10, 1377

(1971). 8. W. Kiefer and H. J. Bernstein, Appl. Spectrosc. 25, 500

(1971).

9. W. Kiefer and I-I. J. Bernstein, Appl. Spectrosc. 25, 609 (1971).

10. G. E. McGraw, Tennessee Eastman Company, private communication.

11. D. Gill, 1~. G. Kilponen, and L. Rinlai, Nature 227, 743 (1970).

12. L. Rimai, R. G. Kilponen, and D. Gill, J. Am. Chem. Soc. 92, 12 (1970).

13. H. J. Sloane, in Polymer Characterization, C. D. Craver, Ed. (Plenum Press, New York, 1971), p. 15.

14. See, for example, "Infrared Spectroscopy--Its Use in the Coatings Industry," Infrared Spectroscopy Comnfittee of the Chicago Society for Paint Technology, Federation of Societies for Paint Technology (pub.) Philadelphia, 1969.

15. A. S. Wexler, Anal. Chem. 36, 1829 (1964).

Improved Discriminant Training and Feature Extraction for the Generation of Simulated Mass Spectra of Small Organic Molecules

Joseph Schechter and Peter C. Jurs

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 25 September 1972)

An empirical method employing computerized pattern recognition techniques has been applied previously to the generation of simulated mass spectra of small organic molecules. The techniques have been improved in two ways to yield superior performance. First, a method for training adaptive binary pattern classifiers using an iterative least squares approach is used. Second, a feature extraction technique known as an attribute inclusion algorithm is used to investigate the importance of multiple features in the molecular descriptions. INDEX HEADINGS: Pattern recognition; Computer applications; Mass spectrometry.

INTRODUCTION

Previous work has shown that computerized learning machines can be applied to the generation of low reso- lution mass spectra of organic molecules? By using only a molecule's structure, adaptive binary pattern classifiers can directly calculate a mass spectrum.

The method of implementing a binary pattern classi- tier (BPC) is dependent on representing the patterns, in this case molecular structures, as points in a pattern space, or equivalently as pattern vectors. Decision surfaces (hyperplanes) are developed to separate the points into binary subsets.

To classify a pattern, it is necessary to determine on which side of the decision surface the point lies. A convenient way of doing this is to develop a normal vector to the decision surface, here called the weight vector and denoted by W. The dot product of the weight vector, W, and the pattern vector, X, yields a seMar s:W.X = s. The sign of this scalar determines on which side of the surface the point falls and thus classifies the pattern into one of two categories: s > 0 corresponds to one category, and s < 0 to the other. One method, known as error correction feedback

(ECF) training, has been employed to develop the weight vectors in previous studies. 2

In the first section of this work, a pattern recognition technique known as iterative least squares is used to determine W. Pattern classifiers employing this technique are examined for their ability to predict the presence or absence of a mass spectral peak in 10 m/e positions selected from the original 60 m/e positions examined previously. 1 In addition, correla- tions of structural fragments vdth m/e positions as chosen by the pattern classifiers using ECF training are verified by the application of this technique.

In the second section of this work, a feature extraction technique known as an attribute inclusion algorithm is tested for its ability to generate new descriptors of molecular structures. By using adaptive binary pattern classifiers that utilize ECF training, these descriptors are examined as a means of increas- ing the predictive ability and the rate of training for each of eight m/e positions.

I. DATA SET

The present work uses a data set taken from a collection of mass spectra on magnetic tape available from

Volume 27, Number 3, 1973 APPLIED SPECTROSCOPY 225

raman spectroscopy of some polymers and copolymers of styrene, butadiene, and methylmethacrylate

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