Spectroscopic Methods in Research and Analysis ... - ACS Publications

29 Spectroscopic Methods in Research and Analysis of Coatings and Plastics CLARA D. CRAVER Chemir Laboratories, 761 West Kirkham, Glendale, MO 63122

E l e c t r o n i c Absorption and Emission Spectroscopy V i s i b l e Spectroscopy U l t r a v i o l e t Spectroscopy Luminescence Spectroscopy Electron Spectroscopy Atomic I d e n t i f i c a t i o n and Analysis Infrared and Raman Spectroscopy Infrared Spectroscopy Infrared Sample Preparation Internal Reflection Spectroscopy (IRS, ATR) Reflection Absorption (RAIR or IRRAS) Diffuse Reflectance (DRIFT) Pyrolysis Computer-Assisted Infrared Raman Spectroscopy Nuclear Magnetic Resonance Mass Spectrometry (MS) Data Banks and Computer R e t r i e v a l

During the past f i v e decades, spectroscopy has moved out from the laboratories of physicists and theoretical chemists into every area of analysis and chemical research. Applications vary from routine, s i n g l e data point measurements for control of plant streams to structural analysis of complex molecules and conformational analysis of polymers. Enhancement of the sensitivity of all spectroscopic methods has been achieved through computer-assisted data handling. It is possible to find s t r u c t u r a l differences i n polymers under different degrees of stress and to analyze for very low l e v e l s of chemical structures at surfaces and interfaces. For difficult s t r u c t u r e d e t e r m i n a t i o n s , data from s e v e r a l s p e c t r o s c o p i c disciplines may be combined and require months or years of research. Each subcomponent or energy l e v e l that makes one kind of atom or molecule different from another gives r i s e to physical phenomena 0097 6156/85/02854)703$10.00/0 © 1985 American Chemical Society

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around which a measuring system can be devised to provide detection and characterization. From X-rays to microwaves the interaction of electromagnetic energy with matter provides chemists with powerful investigative tools. Thus, excitation of orbital electrons can cause u l t r a v i o l e t and v i s i b l e absorption or fluorescence that characterizes the energy l e v e l s of atoms in specific bonding situations. Still more vigorous s t i m u l a t i o n can cause atomic emission, which serves to i d e n t i f y metallic elements. Mass spectroscopy permits precise measurements of the mass, and therefore, the empirical formula of a molecule or its fragments. The magnetic moment from the spin of atomic nuclei permits the measurement of nuclear magnetic resonance and of the influence on it of c l o s e l y associated atoms. The energy i n the infrared (IR) region of the electromagnetic spectrum corresponds to the v i b r a t i o n a l and r o t a t i o n a l energy of atomic groups within molecules. IR i s commonly observed as an absorption phenomenon, but analytical applications for IR emission are occasionally found. The Raman effect i s an energy emission that corresponds to the same range of vibrational energies as observed by infrared spectroscopy. Workers in the f i e l d of applied polymer science may require only a few or a l l of these techniques f o r q u a l i t y c o n t r o l , p o l l u t i o n monitoring, trouble-shooting, or research on new products. Only the l a r g e s t of research centers can j u s t i f y maintaining a complete l i n e of the most s e n s i t i v e spectroscopic equipment. Even more important and c o s t l y i s the maintenance of a s t a f f of s p e c i a l i s t s i n the many s p e c t r o s c o p i c d i s c i p l i n e s . These s p e c i a l i s t s need e x p e r t i s e i n t h e i r own and r e l a t e d d i s c i p l i n e s i n c l u d i n g instrumentation. They need to be f a m i l i a r w i t h the chemistry of the m a t e r i a l s they work on, and they need a good a n a l y t i c a l sense of r e p r e s e n t a t i v e sampling, standardization, r e p e a t a b i l i t y , and possible i n t e r f e r i n g m a t e r i a l s . They need judgment about how complete an a n a l y s i s i s r e q u i r e d f o r a g i v e n problem. They need to m a i n t a i n u p - t o - d a t e s p e c t r a l reference f i l e s r e l e v a n t to the company's products and be f a m i l i a r with data d i g i t i z a t i o n and computer technology. Oftentimes t h e y must have the e n g i n e e r i n g a b i l i t y t o h e l p i n t e r f a c e spectroscopic equipment with research experiments or plant c o n t r o l devices. I t i s apparent t h a t such i d e a l l y t r a i n e d s p e c t r o s c o p i s t s are r a r e . I t i s even r a r e r to f i n d l a b o r a t o r y or company management p o l i c i e s t h a t b r i n g equipment, s p e c t r o s c o p i s t , and a n a l y t i c a l problems together at the r i g h t time to gain maximum benefit from the p o t e n t i a l t h a t the s c i e n c e of spectroscopy o f f e r s . I t i s the purpose of t h i s chapter to provide applied polymer s p e c i a l i s t s with an understanding of what the different spectroscopic methods can do f o r them. The a u t h o r hopes t o p r o v i d e enough p e r s p e c t i v e to h e l p management i n both l a r g e and s m a l l o r g a n i z a t i o n s decide which spectroscopic a n a l y t i c a l t o o l s i t needs in-house and which ones i t s h o u l d o b t a i n from s p e c i a l i z e d l a b o r a t o r i e s and r e s e a r c h institutions. Abundant textbook r e f e r e n c e s and an e x t e n s i v e a p p l i c a t i o n s b i b l i o g r a p h y are i n c l u d e d to h e l p the a n a l y t i c a l chemist or polymer chemist who f i n d s h i m s e l f i n the p o s i t i o n of a d o - i t - y o u r s e l f spectroscopist. Myers and Long (_1) devoted a chapter to each of the p r i n c i p a l s p e c t r o s c o p i c techniques a p p l i c a b l e to c o a t i n g s , and a " P l a s t i c s

29.

Table I .

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Spectroscopic Methods in Research and Analysis

GRAVER

P o s i t i o n and Intensity of Typical E l e c t r o n i c Absorption Bands

Compound

λ/max

e/max

a

A c y c l i c Structures Monoalkyl ethylenes D i a l k y l ethylenes Ketones Esters Carboxylic acids Butadiene c=c-c=o Hexatriene Decapentaene

173-178 185-205 195 205 208 217 217 258 335

Aromatic Structures Benzene Styrene Diphenyl trans-Stilbene Azobenzene

198 244 246 295 319

a

5,000 5,000 1,000 50 60 21,000 16,000 35,000 118,000 8,000 13,000 20,000 27,000 20,000

a

X i s expressed as nanometers (nm), and the band i n t e n s i t y , ε , i s expressed as the molar absorption c o e f f i c i e n t , which i s the product of the a b s o r p t i v i t y and molecular weight of a substance (22).

Table I I .

Useful Transparency Limits of Common Solvents

Solvent Pyridine Tetrachloroethylene Benzene N,N-Dimethylformamide Carbon tetrachloride

Cutoff (nm) 305 290 280 270 265

Solvent

Cutoff (nm)

Chloroform Dichloromethane Ethyl ether Acetonitrile Alcohols, hydrocarbons

245 235 220 215 210

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factor not only for q u a l i t a t i v e a n a l y s i s but a l s o for q u a n t i t a t i v e a n a l y s i s of mixtures. The most useful feature of e l e c t r o n i c spectroscopy i s the high s e n s i t i v i t y i t offers for the h i g h l y conjugated s t r u c t u r e s , which have s t r o n g bands i n the u l t r a v i o l e t and v i s i b l e r e g i o n s , i n the presence of more saturated structures. Luminescence Spectroscopy. Luminescence spectroscopy i s based on the energy d i s s i p a t i n g b e h a v i o r of e l e c t r o n i c systems of atoms, i o n s , and m o l e c u l e s . Two major s u b d i v i s i o n s of luminescense are fluorescence and phosphorescence. The r e l a t i o n s h i p between these phenomena i s d e s c r i b e d by Smith (28) and by Fox and P r i c e (29). A fluorescence spectrum can be produced as an apparent "mirror image" of the l o n g e s t w a v e l e n g t h band system of the a b s o r p t i o n spectrum. An a d d i t i o n a l energy t r a n s i t i o n may f o l l o w , i . e . , at a lower r a t e , and produce phosphorescence. The diagram i n F i g u r e 1 shows the r e l a t i o n s h i p between absorption, fluorescence, and phosphorescence. Instrumentally, time d i s c r i m i n a t i o n i s provided by l i g h t choppers to separate fluorescence and phosphorescence s i g n a l s . An advantage of luminescence spectroscopy, as with a l l emission systems, i s f l e x i b i l i t y of geometry of the sample that permits front surface viewing. That i s , opaque or g e o m e t r i c a l l y i r r e g u l a r specimens may be sampled d i r e c t l y (28). Applications of luminescence spectroscopy include very s e n s i t i v e measurements for a d d i t i v e s i n synthetic rubber (30), brighteners on synthetic f i b e r s (31, 32), and monitoring of photooxidation products i n h i g h l y pure p r o d u c t s (33^). I n v e s t i g a t i o n of m o l e c u l a r o r i e n t a t i o n and s m a l l molecular segments i n polymer networks (34-36) i s r e c e i v i n g increased attention. When excimer trap fluorescence i s studied i n aromatic systems, conformational changes can be followed as a function of environmental parameters such as temperature (37) or pressure (38) and the k i n e t i c s of phase s e p a r a t i o n i n polymer blends can be evaluated i n terms of s o l v e n t s , mixing temperature and m o l e c u l a r weight (39). P o l y m e r i z a t i o n reactions can be monitored with molecular probes that e x h i b i t viscosity-dependent fluorescence (40, 41). A method f o r d e t e r m i n i n g the number average m o l e c u l a r weight by f l u o r e s c e n c e procedures has been reported (42). An e x t e n s i v e b i b l i o g r a p h y through 1980 i s i n c l u d e d i n a book on fluorescent probes (43). In contrast to the u l t r a v i o l e t and v i s i b l e absorption methods d e s c r i b e d e a r l i e r , d e t a i l s of the methodology of luminescence spectroscopy are not w i d e l y known and few standard methods have e v o l v e d . Books on theory and techniques are h e l p f u l (44) as are memoranda on a p p l i c a t i o n s from instrument manufacturers. ASTM Committee E-13.06 on M o l e c u l a r Luminescence has had l a r g e task f o r c e s working f o r s e v e r a l years on p r a c t i c e s f o r instrument t e s t i n g , nomenclature, and a n a l y t i c a l procedures. Recent symposia sponsored by t h a t committee are the b a s i s f o r two new books (45, 46). E l e c t r o n Spectroscopy. E l e c t r o n spectroscopy i s a r a p i d l y d e v e l oping f i e l d i n v o l v i n g measurement of e l e c t r o n s e j e c t e d from a bombarded sample. I t may be d i v i d e d i n t o c a t e g o r i e s a c c o r d i n g to the bombarding source, u l t r a v i o l e t e x c i t a t i o n (UPS), X - r a y photoelectron spectroscopy (XPS), e l e c t r o n spectroscopy for chemical

29.

CRAVER


709

a n a l y s i s (ESCA), and e l e c t r o n - g u n e x c i t a t i o n (Auger spectroscopy). P r a c t i c a l a p p l i c a t i o n s are being reported at a r a p i d l y increasing rate for ESCA and Auger spectroscopy. They provide a powerful t o o l for s t u d y i n g s u r f a c e s up to about 20-Λ depth and are t h e r e f o r e f i n d i n g r e s e a r c h a p p l i c a t i o n s i n surface chemistry areas such as f r i c t i o n and wear, adhesion, and c a t a l y s t research. There i s a J o u r n a l of E l e c t r o n S p e c t r o s c o p y and R e l a t e d Phenomena, and the Nobel P r i z e Award address by Siegbahn i s widely a v a i l a b l e (47). Basic background i n both theory and a p p l i c a t i o n s i s a v a i l a b l e i n a book e d i t e d by B r u n d l e and B a k e r ( 4 8 ) . An i n t r o d u c t o r y monograph by Barker and B e t t e r i d g e i s focused on c h e m i c a l and a n a l y t i c a l a s p e c t s of e l e c t r o n spectroscopy (49) and broader a s p e c t s are d i s c u s s e d by S e v i e r (50). G e n e r a l a r t i c l e s r e l a t i n g to surface a n a l y s i s (51-53), corrosion research (54), and c a t a l y s t s (55-57) have been reported. G l a s s - f i b e r r e s i n composites were studied (58) and adhesive transfer of polytetrafluoroethylene to m e t a l s was measured (59). A study of adhesion at the r u b b e r m e t a l i n t e r f a c e (60) used XPS as d i d s t u d i e s of s u r f a c e g r a f t s of polypropylene, polyethylene and polyester (61, 62), polyimide f i l m s (63), and surface functional groups on polyethylene f i l m s (64). While most of the a p p l i c a t i o n s important to polymer science have been on s u r f a c e s of s o l i d s , papers have been p u b l i s h e d on l i q u i d phase XPS (65) and s u l f a t e groups on polystyrene latexes have been a n a l y z e d (66). Atomic I d e n t i f i c a t i o n and A n a l y s i s . Atomic emission and absorption spectroscopy and X-ray f l u o r e s c e n c e and a b s o r p t i o n are used f o r e l e m e n t a l a n a l y s e s . These methods vary i n t h e i r s e n s i t i v i t y and q u a n t i t a t i v e a p p l i c a b i l i t y . A summary of the u s u a l l y accepted v i r t u e s and l i m i t a t i o n s of these methods i s given i n Table I I I . A two-part s p e c i a l i s s u e of S p e c t r o c h i m i c a A c t a (67, 68) i s d e d i c a t e d to atomic a b s o r p t i o n s p e c t r o s c o p y . A book by Van Loon (69) and a chapter by Robinson (70) are good b a s i c t e x t s . An educational audiocassette/slide course by SAVANT (71) covers basic p r i n c i p l e s of atomic, emission, and fluorescence spectroscopy, and SAVANT has a l s o produced a course on i n d u c t i v e l y c o u p l e d plasma (ICP). There are now abundant references on ICP i n c l u d i n g various a s p e c t s of i n d u c t i v e l y c o u p l e d plasmas to e m i s s i o n spectroscopy (72). C o m p i l a t i o n s o f s p e c t r a l t a b l e s (73) and s p e c t r a l interferences (74) are v a l u a b l e contributions to t h i s technique. An easy to read and e n l i g h t e n i n g comparison of v a r i o u s methods of atomic spectroscopy has r e c e n t l y been published (75). A n a l y s e s of pigments and f i l l e r s are the major c o a t i n g s a p p l i c a t i o n s of atomic s p e c t r o s c o p y . L i s t i n g s of the standards organizations, which recommend s p e c i f i c methods, are d e t a i l e d i n the 1983 review by Anderson and Vandeberg (76). Research a p p l i c a t i o n s are apt to be i n areas of complexing or c h e l a t i o n c h a r a c t e r i s t i c s (77) or adsorption, contamination or migration studies. An example i s the use of X-ray a n a l y s i s i n e s t a b l i s h i n g a method for p r e d i c t i n g d u r a b i l i t y of paints under marine exposure. I t was found that s m a l l changes i n the l e v e l of t o x i n a f t e r s h o r t exposures c o u l d be extrapolated to predict l i f e t i m e persistence (78).

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APPLIED POLYMER SCIENCE VIBRATIONAL

S'

SINGLET,

EXCITED

I N T E R S Y S T E M

CROSSING

DEACTIVATION

( RADIATION L E S S

10-13

10-8

SEC.

TO

I0~7

)

S E C .

3

S T A T E

CD

or LU

ζ LU

S °

SINGLET,

G R O U N D

S T A T E

A B S O R P T I O N

F L U O R E S C E N C E

10-9

TO

10-7

P H O S P H O R E S C E N C E SEC.

10-4

TO

10

S E C .

WAVELENGTH

Figure 1.

Table I I I .

Energy r e l a t i o n s h i p s of e l e c t r i c e l e c t r o n i c t r a n s i t i o n s of a m o l e c u l e . Reproduced w i t h p e r m i s s i o n from Ref. 28. C o p y r i g h t 1971 Plenum Press.

Spectroscopic Methods of Elemental Analysis

Method

Atomic Phenomenon

Atomic emission

Light emission from excited e l e c t r o n i c states of atoms

General for q u a l i t a t i v e i d e n t i f i c a t i o n of metals; simultaneous determinations

Atomic absorption

Absorption of atomic resonance l i n e

Metals analyzed i n d i v i d u a l l y ; for q u a n t i t a t i v e analysis

X-ray fluorescence

Reemission of X-rays from excited atoms

General for a l l elements above atomic no. 10

X-ray absorption

X-ray e x c i t a t i o n of Κ and L s h e l l electrons

Good quantitative method for heavier elements i n presence of l i g h t elements

Use

29.

GRAVER


711

Infrared and Raman Spectroscopy I n f r a r e d and Raman spectroscopy correspond to s i m i l a r m o l e c u l a r energy phenomena—the v i b r a t i o n a l e n e r g i e s of atoms or groups of atoms within molecules, and r o t a t i o n a l energies. I n f r a r e d spectroscopy may be observed as a b s o r p t i o n o r , l e s s commonly, emission spectra for which the frequencies that i n t e r a c t with electromagnetic r a d i a t i o n are those that i n v o l v e a change i n the d i p o l e moment of the m o l e c u l e as the v i b r a t i o n takes p l a c e . This spectrum i s observed i n the region past the red end of v i s i b l e r a d i a t i o n to the microwave r e g i o n . The common " f i n g e r p r i n t i n g " r e g i o n i s from 5000 wavenumbers ( c m * ) t o 200 cm""-*-. This corresponds to 2-50 m i c r o n s , or i n SI u n i t s , 2-50 ym. I n f r a r e d spectroscopy i s the most broadly a p p l i c a b l e molecular spectroscopic t o o l and r e l a t e s so i n t i m a t e l y to the nature of the c h e m i c a l bond that i t i s predicted that i t w i l l be a leading i n v e s t i g a t i v e t o o l for another 50 years (79). Raman spectroscopy i s the observation of an emitted pattern of frequency displacements from an e x c i t i n g l i n e caused by v i b r a t i o n s within a molecule that r e s u l t i n a change i n p o l a r i z a b i l i t y . I t can be observed by s t i m u l a t i o n w i t h e l e c t r o m a g n e t i c r a d i a t i o n f a r removed from IR, e.g., UV or v i s i b l e , a l t h o u g h the observed frequency d i s p l a c e m e n t s correspond to the frequency of i n f r a r e d radiation. The Raman e f f e c t i s weak, and i t s a p p l i c a t i o n was s e v e r e l y l i m i t e d u n t i l an extremely strong e x c i t i n g source, the l a s e r , became a v a i l a b l e . The p r i n c i p a l l i m i t a t i o n s of Raman spectroscopy now a r i s e from the simultaneous phenomenon of fluorescence, which i s a s t r o n g i n t e r f e r i n g s i g n a l i n the spectrum of many commercial materials. Infrared and Raman spectroscopy are complementary i n s t r u c t u r a l determinations because some molecular v i b r a t i o n s that are i n a c t i v e i n the infrared (that i s , do not r e s u l t i n a change i n d i p o l e moment and t h e r e f o r e do not cause an a b s o r p t i o n band) do have a s t r o n g Raman l i n e . The reverse i s a l s o true. Some bands that are weak or forbidden i n the Raman spectrum are strong i n the infrared spectrum. With the combined use of these techniques, the v i b r a t i o n a l energies of a molecule can be f u l l y described. -

I n f r a r e d Spectroscopy. Infrared spectroscopy was f i r s t recognized to be a nearly u n i v e r s a l t o o l for characterizing chemical structure after the monumental research of W. W. Coblentz, reported i n a major p u b l i c a t i o n i n 1905 (80). Research by many i n v e s t i g a t o r s over the next 30 years developed the w e l l - k n o w n group f r e q u e n c i e s which c o r r e l a t e w i t h c h e m i c a l structure for common groups such as OH, CH, and C=0. I t was learned t h a t the 7-15 ym r e g i o n of the spectrum p r o v i d e d a f i n g e r p r i n t of i n d i v i d u a l molecules even for c l o s e l y s i m i l a r isomers. Commercial spectrometers became a v a i l a b l e i n the l a t e 1930s. World War I I spurred p r o d u c t i o n of improved spectrometers and development of a n a l y t i c a l methods to s o l v e the isomer a n a l y s i s needs of the petroleum, rubber, and chemical i n d u s t r i e s . Fortunately, the rock s a l t prism data obtained during those productive years i n the a p p l i c a t i o n of i n f r a r e d spectroscopy to commercial products and complex mixtures are as e f f e c t i v e i n the f i n g e r p r i n t i n g region for

712


most of these m a t e r i a l s as data obtained on newer g r a t i n g and interferometric instruments, so that many of the methods developed during that period are s t i l l i n use today. The t e c h n i c a l l i t e r a t u r e of the 1940s and 1950s r e f l e c t s the rapid development of methods and t h e i r extension to polymers (81-83), crude o i l i d e n t i f i c a t i o n (84), a n a l y s i s of inorganics (85), and characterization of c o a l and c o a l h y d r o g é n a t i o n products (86). In the polymer and o r g a n i c c o a t i n g s f i e l d the wide range of a p p l i c a t i o n s i s i n d i c a t e d by s t u d i e s on epoxy compounds (87), novolak and resole resins (88), polyurethanes (89), p o l y e t h y l e n e t e r e p h t h a l a t e (90), p o l y a c r y l o n i t r i l e (91), polybutadiene and butadiene-styrene (92), c u r i n g s t u d i e s on epoxy resins (93) and e l u c i d a t i o n of the effect of d r i e r s on the o x i d a t i v e drying of o i l s and varnishes (94). I n f r a r e d spectroscopy c o n t i n u e s to be one of the p r i n c i p a l techniques for s t r u c t u r a l a n a l y s i s of polymers and for i d e n t i f y i n g components of complex f o r m u l a t i o n s . The d i s t i n c t i v e n e s s of important v i n y l , a l k y l , and a r y l chemical structures i n the infrared such as e s t e r , amide, n i t r i l e , i s o c y a n a t e , h y d r o x y l s , amine, and s u l f o n e makes i t i d e a l f o r the f i r s t gross c h a r a c t e r i z a t i o n of c h e m i c a l types present and f o r f o l l o w i n g the r e a c t i o n s of these functional groups i n curing or degradation studies. Up-to-date compendiuras on a p p l i c a t i o n s of infrared spectroscopy i n a p p l i e d p o l y m e r s c i e n c e a r e as f o l l o w s . "An I n f r a r e d Spectroscopy A t l a s f o r the C o a t i n g s I n d u s t r y " (95) d e s c r i b e s techniques, has l i b e r a l references to s p e c i f i c methods, and contains h i g h - q u a l i t y g r a t i n g r e f e r e n c e s p e c t r a on p a i n t components and blended compositions. " A t l a s of Polymer and P l a s t i c s A n a l y s i s , " 2nd ed., by Hummel and S c h o l l (96), has i s s u e d two volumes: V o l . 1, P o l y m e r s ; V o l . 3, on A d d i t i v e s and P r o c e s s i n g A i d s ; V o l . 2, on P l a s t i c s , Fibers, Rubbers, Resins, i s i n press. "Infrared Spectra of P l a s t i c i z e r s and Other A d d i t i v e s , " 2nd ed., p u b l i s h e d by The C o b l e n t z S o c i e t y , I n c . , i s a h i g h - q u a l i t y IR r e f e r e n c e spectrum c o l l e c t i o n (97). The i d e n t i f i c a t i o n of polymers i s l a r g e l y done by f i n g e r p r i n t matching. Representative spectra of the most important commercial polymers are i n c l u d e d here f o r the convenience of the reader ( F i g u r e s 2-10). They are grouped by s t r u c t u r e to emphasize the f e a t u r e s t h a t c h e m i c a l l y r e l a t e d polymers have i n common and to demonstrate the d i s t i n c t i v e i d e n t i f y i n g c h a r a c t e r i s t i c s of each polymer c l a s s . A recent book on theory and a p p l i c a t i o n s of v i b r a t i o n a l spectroscopy to polymers (98) by Painter, Coleman, and Koenig i s a major reference book, as i s the new e d i t i o n of the f a m i l i a r Haslam and W i l l i s book (99) c o v e r i n g t h e combined c h e m i c a l and spectroscopic a n a l y s i s of p l a s t i c s and resins. An e x c e l l e n t chapter i n which G. Z e r b i d e s c r i b e s the b a s i s f o r " P r o b i n g the R e a l Structure of Chain Molecules by V i b r a t i o n a l Spectroscopy" i s i n an ACS Advances i n Chemistry book (100), along with polymer degradation s t u d i e s by FTIR ( 1 0 1 , 102). H e n n i k e r has w r i t t e n on IR o f i n d u s t r i a l polymers (103), P e r k i n - E l m e r C o r p o r a t i o n d i s t r i b u t e s a t e c h n i c a l a p p l i c a t i o n s sheet on infrared grating spectra of polymers (104), and two e x c e l l e n t p u b l i c a t i o n s on i n f r a r e d spectroscopy of rubber and e l a s t o m e r s have been w r i t t e n by R. Hampton (105) and W. C. Wake (106). P r a c t i c a l approaches to coatings a n a l y s i s (I) and

29. CRAVER

4000

3500


3000

2500

2000

1800

1600

1400

1200

1000

800

600

400

713

200

FREQUENCY (CM")

Figure 2.

Spectrum of h i g h - d e n s i t y p o l y e t h y l e n e , low-density polyethylene, l i n e a r l o w - d e n s i t y p o l y e t h y l e n e , and i s o t a c t i c polypropylene.

714

APPLIED POLYMER SCIENCE WAVELENGTH ( M O O N S )

cis-POLYBUTADIENE

9

10

1?

15

20

30

4050

FT

3500

3000

2500

2000

1800

1600

1400

1200

1000

800

600

400

200

FREQUENCY ( C M ' )

POLYBUTADIENE

ETHYLENE/PROPYLENE/DIENE

TERPOLYMER

ETHYLENE/PROPYLENE/DIENE

TERPOLYMER

Figure 3.

(50%,

(70%,

46%,

26%,

4%)

4%)

Spectrum of c i s - p o l y b u t a d i e n e , ethylene/propylene/diene terpolymer.

1 5

15

20

20

30

30

4050

4050

polybutadiene,

29.


CRAVER HYCAR

POLYBUTADIENE

1.9% CARBOXYL

HYCAR

POLYBUTADIENE

2.5%

4000

HYCAR

3500

3000

2500

POLYBUTADIENE

Figure 4.

CARBOXYL

9

K)

12

10% ACRYLONITRILE

15

715 20

30 4050

20

30 4050

20

30 4050

2000

CARBOXYLATED

27% ACRYLONITRILE

Spectra of Hycar polybutadiene/1.9% carboxyl, Hycar polybutadiene/2.5% c a r b o x y l / 1 0 % a e r y 1 o n i t r i 1 e, c a r b o x y l a t e d Hycar polybutadiene/27% a c r y l o n i t r i l e , and poly(ethyl acrylate).

716

APPLIED POLYMER SCIENCE POLY(METHYL

Figure 5.

METHACRYLATE)

S p e c t r a of p o l y ( m e t h y l m e t h a c r y l a t e ) , p o l y ( e t h y l m e t h a c r y l a t e ) , p o l y ( b u t y l m e t h a c r y l a t e ) , and p o l y ( i s o b u t y l methacrylate).

29. GRAVER


ETHYLENE/VINYL

ACETATE

COPOLYMER

(14%

PVAc)

ro

„

717

20

15

600

ETHYLENE/VINYL

rr

ACETATE

mm.

COPOLYMER

(25%

Figure 6.

w

12

15

20

200

30

4050

h 55

ETHYLENE/VINYL

3500

9

400

MM

++

POLY(VINYL

PVAc)

30 4050

ACETATE

ACETATE)

3000

2500

COPOLYMER

(40%

WAVtUNGTH (MOONS) 6 7

~l

! M

PVAc)

,

8

9

1

0

0

12

12

800

,

20

5

15

30 4050

2 0 3 0

600

400

4050

200

Spectra of e t h y l e n e / v i n y 1 a c e t a t e copolymer (14%, 25%, and 40% PVAc) and polyvinyl acetate).

718


METHYL

CELLULOSE

ETHYL CELLULOSE

Figure 7.

W A V E L E N G T HM C R IO N S

9

10

12

15

20

30

4050

Spectra of m e t h y l c e l l u l o s e , e t h y l c e l l u l o s e , c e l l u l o s e acetate butyrate, and sodium a l g i n a t e .

29. CRAVER


POLYETHYLENE

GLYCOL

POLYETHYLENE

GLYCOL

MW 600

MW 1500

EPICHLOROHYDRI N/ETHYLENE

OXIDE

W A

*

E L E N G T H

^

, C R O N S

;

WAVELENGTH (MICRONS) 7 8

9

COPOLYMER

9

719

10

12

15

20

30

10

12

15

20

30 4050

10

12

15

20

30

4050

4050

3 0.2 ---:-~ r

Figure 8.

S p e c t r a of p o l y e t h y l e n e g l y c o l (MW 600, 1500, and 6800) and epichlorohydrin/ethylene oxide copolymer.

720

APPLIED POLYMER SCIENCE POLYVINYL

CHLORIDE

POLYVINYL

CHLORIDE,

CHLORINATED

POLYETHYLENE

ETHYLENE/ACRYLIC

Figure 9.

CARBOXYLATED

ACID

36%

COPOLYMER

CHLORINE

20%

9

K)

12

15

20

30

4050

9

10

12

15

20

30

4050

ACRYLIC

ACID

15

20

30

4050

S p e c t r a of p o l y ( v i n y 1 c h l o r i d e ) , c a r b o x y l a t e d p o l y ( v i n y l c h l o r i d e ) , chlorinated polyethylene (36% c h l o r i n e ) , and e t h y l e n e / a c r y l i c acid copolymer (20% a c r y l i c acid).

29.

721


CRAVER POLYSTYRENE

POLYPHENOXY

RESIN

BAKELITE

PKHH

7

8

9

10

12

15

20

30 4050

τη

m

m 4000

3500

P0LY(OIALLYL

3000

m

t Τ Τ f~

M "

2500

PHTHALATE)

2000

5

1600

1400

WAVtUNGTH (MOONS) 6 7

1200

8

1000

9

10

p -

800

12

II Figure 10.

600

15

400

20

200

30 4050

frzx

S p e c t r a of p o l y s t y r e n e , poly(a-methyIstyrene), polyphenoxy r e s i n , B a k é l i t e PKHH, and p o l y ( d i a l l y l phthalate).

722


a p p l i c a t i o n s of infrared and Raman spectroscopy to characterization and a n a l y s i s of surfaces (107) have been reported. The c l a s s i c a l book on techniques by W. J . P o t t s (108) i s s t i l l the book of choice for sample preparation and q u a n t i t a t i v e methods. P r a c t i c a l aspects of theory, sample handling, and a p p l i c a t i o n are w e l l - d e s c r i b e d by A. L . Smith (109). ASTM E-13 i s expected to r e l e a s e a new p r a c t i c e on q u a l i t a t i v e a n a l y s i s i n 1985, and i t s "Manual on P r a c t i c e s i n M o l e c u l a r Spectroscopy" (27) c o v e r s nomenclature, instrument t e s t i n g , microanalysis, internal r e f l e c t i o n , and q u a n t i t a t i v e a n a l y s i s . Bellamy (110) and Colthup (111) are the best recognized and most extensive references on group frequency i n t e r p r e t a t i o n , and an easyto-use i n t r o d u c t i o n to group f r e q u e n c i e s and a summary of major compound classes are i n the Coblentz Society's "Desk Book" (112). I n f r a r e d Sample P r e p a r a t i o n . D e t a i l s of sample p r e p a r a t i o n f o r o b t a i n i n g i n f r a r e d s p e c t r a are d i s c u s s e d f o r f i l m s , c a p i l l a r y l a y e r s , demountable c e l l s , f i x e d thickness c e l l s , pressed h a l i d e p e l l e t s , and o i l m u l l s i n the above r e f e r e n c e s by P o t t s and Smith and i n ASTM methods (27). Other more s p e c i a l i z e d techniques are described b r i e f l y below. I n t e r n a l R e f l e c t i o n Spectroscopy (1RS, ATR). The spectrum of the sample i s determined at the interface of the specimen and a c r y s t a l in o p t i c a l contact. I t has found widespread use i n the f i e l d of c o a t i n g s and s u r f a c e s because of the ease of s a m p l i n g and the opportunity i t provides to i n v e s t i g a t e successive s t r a t a through a coating. R e p o r t s on i n t e r n a l r e f l e c t i o n d e t e r m i n a t i o n o f contaminants on a d h e s i v e s u r f a c e s , exuded p l a s t i c i z e r , f i b e r surfaces, i n s i t u tissue, and sensors i n viscous plant streams are w e l l referenced ( I , "27, 107, 113, 114). A p p l i c a t i o n l i t e r a t u r e i s r e a d i l y a v a i l a b l e from instrument companies: Foxboro A n a l a b s , Norwalk, Conn.; P e r k i n - E l m e r C o r p o r a t i o n , Norwalk, Conn.; Beckman Instruments, I r v i n e , C a l i f . ; Barnes Engineering Company, Stamford, Conn.; H a r r i c k S c i e n t i f i c , O s s i n i n g , N.Y. An a r t i c l e focused on a n a l y s i s of a d h e s i v e s (113) p o i n t s up some of the e x p e r i m e n t a l d i f f i c u l t i e s i n obtaining reproducible data. R e f l e c t i o n Absorption (RAIR or IRRAS). A s i g n i f i c a n t development i n infrared spectroscopy during the 1960s has been reported i n j o u r n a l s not commonly read by either polymer chemists or a n a l y t i c a l chemists. I t has developed i n the f i e l d s of electrochemistry and c a t a l y s i s and i s c a l l e d r e f l e c t i o n - a b s o r p t i o n spectroscopy. Papers by G r e e n l e r (115-118), Yates (119), and Hansen (120, 121) d e s c r i b e the theory and some e x p e r i m e n t a l data, and B o e r i o and G o s s e l i n r e p o r t on i t s a p p l i c a t i o n s to polymers (122). E s s e n t i a l l y the t e c h n i q u e i n v o l v e s t r a n s m i s s i o n through u l t r a t h i n sample layers on metal plates and r e f l e c t i o n at a glancing angle as diagrammed i n Figure 11. Actual experimental layouts are h i g h l y v a r i e d as reference to the a b o v e - c i t e d a r t i c l e s w i l l delineate. The o b s e r v a t i o n t h a t i n f r a r e d s p e c t r a of u l t r a t h i n f i l m s on metal surfaces measured at a high angle of incidence g i v e absorption i n t e n s i t i e s greater than that c a l c u l a t e d from the sample thickness and number of r e f l e c t i o n s was observed and d i s c u s s e d i n the l a t e

29.

CRAVER


723

1950s (123). G r e e n l e r has demonstrated t h e o r e t i c a l l y t h a t the s p e c t r a l s e n s i t i v i t y at an interface can be enhanced a 1000-fold or more by choosing an a n g l e of i n c i d e n c e i n the range 8 0 - 8 8 ° . Some workers appear to prefer lower angles of incidence, but experimental work i s i n e a r l y s t a g e s . The advance of theory r e l a t i n g to ATR (114) and i t s importance i n measuring electrochemical reactions at electrodes l e d to the present day understanding that promises that r e f l e c t i o n - a b s o r p t i o n may p r o v i d e a powerful t o o l f o r s t u d y i n g molecules or a few bond lengths of a coating at metal interfaces. Diffuse Reflectance (DRIFT). T h i s technique has been so s e v e r e l y energy l i m i t e d that rare use was made of i t u n t i l computer-averaging c a p a b i l i t i e s prompted d e s i g n work on improved s a m p l e - h a n d l i n g attachments. S i m p l e attachments are now a v a i l a b l e f o r most new spectrometers, both F o u r i e r transform and d i s p e r s i v e . I r r e g u l a r sample shapes can be used, but better r e s u l t s are g e n e r a l l y obtained on powders. E x t e n s i v e coverage i s g i v e n by G r i f f i t h s and F u l l e r (124), and i t s a p p l i c a t i o n w i t h d i s p e r s i v e spectrophotomoters i s described by Hannah and Anacreon (125). P y r o l y s i s . T h i s i s best regarded as a h i g h l y v a l u a b l e but " l a s t r e s o r t " technique. I t i s v a l u a b l e because i t can s u p p l y s p e c t r a when other e f f o r t s to o b t a i n them have f a i l e d , f o r example, on i n s o l u b l e thermoset p l a s t i c s and on c a r b o n - f i l l e d c r o s s - l i n k e d elastomers. I t i s used as a l a s t r e s o r t because the c o l l e c t e d pyrolyzate may not represent a l l of the components of the sample and the same sample may not give the same spectra i n consecutive runs; t h u s , p y r o l y s i s i s u s e f u l , but some components of a b l e n d or copolymer may be missed e n t i r e l y and comonoraer proportions on even a s e m i q u a n t i t a t i v e b a s i s are d i f f i c u l t to a c h i e v e . As w i t h other t e c h n i q u e - s e n s i t i v e a n a l y t i c a l methods, i t i s p o s s i b l e to s t a n d a r d i z e c a r e f u l l y i n a h i g h l y r e p e a t a b l e system f o r c o n s i s t e n t a n a l y s i s i f good c a l i b r a t i o n procedures have been used. A p y r o l y s i s u n i t f o r maximum e f f e c t i v e n e s s s h o u l d be e v a c u a b l e , c o n t r o l temperature and t i m e , and p r o v i d e f o r c o l l e c t i o n of vapor-phase pyrolyzate and of condensed pyrolyzate. The "Pyro-Chem" u n i t , which was designed by Wilks S c i e n t i f i c and i s now s o l d by Foxboro Analabs, f i t s these requirements. A p y r o l y s i s unit by Chemical Data Systems, I n c . , Oxford, Pa., o f f e r s these f e a t u r e s as w e l l as programmed temperature and c o n t r o l l e d sweeping of the pyrolyzate for stepwise sampling. There are many useful references a v a i l a b l e on p y r o l y s i s techniques (82, 126-128). Computer-Assisted Infrared. The impact of computers on spectroscopy has been high. In v i b r a t i o n a l spectroscopy, i t was f e l t strongest i n the 1970s when the r e v o l u t i o n i n computer t e c h n o l o g y brought p r i c e s down i n t o the range where i t was p r a c t i c a l to have a dedicated computer as an i n t e g r a l part of a spectrometer. I n i t i a l l y t h i s development was most important i n the f i e l d of Fourier transform spectroscopy. The F o u r i e r transform (FT) spectrometer i s e s s e n t i a l l y an i n t e r f e r o m e t e r , which measures the e n t i r e spectrum s i m u l t a n e o u s l y i n s t e a d of measuring s e q u e n t i a l f r e q u e n c i e s as i s the case w i t h d i s p e r s i v e spectrometers. The spectrum i s computed from the i n t e r f e r o g r a m and, i n order to be p r a c t i c a l , t h i s computation requires a computer. Once the spectrum

724


i s a v a i l a b l e i n d i g i t a l form, computerized data handling advantages are so great that newer d i s p e r s i v e spectrometers are a l s o computer assisted. Repeated scans, computer averaging, and curve smoothing r o u t i n e s y i e l d a h i g h s i g n a l / n o i s e r a t i o t h a t has p e r m i t t e d extension of infrared spectroscopy i n t o ultramicrosampling, diffuse r e f l e c t i o n , and photoacoustic spectroscopy (PAS). Spectrum s u b t r a c t i o n techniques are among the most p o p u l a r features of computerized IR and are proving e s p e c i a l l y a p p l i c a b l e to comparing c l o s e l y s i m i l a r polymers (129-132), and e x c e l l e n t accounts of data p r o c e s s i n g of polymer s p e c t r a have been w r i t t e n by Koenig (133, 134). In the opposite d i r e c t i o n from new t h e o r e t i c a l developments and e l a b o r a t e instruments i s the i n t e r e s t i n g e n g i n e e r i n g by W i l k s S c i e n t i f i c (now The Foxboro Co.) to produce portable spectrometers containing v a r i a b l e long-path c e l l s for s e n s i t i v e monitoring of a i r p o l l u t a n t s (Figure 12). The spectrometer i s a s m a l l unit that may be used as a single-beam point reader or connected to a recorder to provide spectra s i m i l a r to the commercial instruments of 1940. The long-path c e l l i s the b u l k i e s t part of the apparatus as i s shown i n Figure 12, and i t i s capable of measuring most gases at the current OSHA l i m i t s (0.1-1000 ppm) or lower. I t offers the big advantage of g i v i n g instantaneous samplings around any g i v e n work area w h i l e changes i n processing v a r i a b l e s or exhaust systems are being made. Thus, i t can be a v a l u a b l e t o o l to the plant engineer i n the process of planning for or making c l e a n - a i r i n s t a l l a t i o n s . Raman Spectroscopy. As mentioned e a r l i e r , the Raman e f f e c t i s an e m i s s i o n phenomenon, which means t h a t f r o n t - s u r f a c e s a m p l i n g i s possible. An i r r e g u l a r l y shaped s o l i d may be used i n t h e spectrometer without processing i t to make i t f l a t , as required for i n t e r n a l r e f l e c t i o n , or w i t h o u t p r o c e s s i n g i t to a f i l m or powder for transmission IR. Another sampling convenience i s that Raman i s more e a s i l y a p p l i c a b l e to water s o l u t i o n s than IR. Specific circumstances i n which Raman spectra may be more useful than IR on an "as received" sample are as f o l l o w s . (1)

(2)

(3)

F i l l e d polymers or composites c o n t a i n i n g s i l i c a , c l a y , or s i m i l a r materials may have l e s s interference from the f i l l e r i n the Raman spectrum than i n the IR spectrum of the polymer because most f i l l e r s are poor Raman scatterers but g i v e strong infrared bands that i n t e r f e r e with polymer i d e n t i f i c a t i o n . I t may not be necessary to remove the f i l l e r i n order to obtain a good Raman spectrum, but such a step i s u s u a l l y necessary for IR. Chunks or pieces of polymers can be examined d i r e c t l y by Raman, which i s an advantage f o r thermosets and tough rubbery materials. V a r i a t i o n s i n c r y s t a l l i n i t y and amorphous regions or o r i e n t a t i o n produced by processing may be observed because sample preparation i s not required. The i n t e n s i t y advantage of Raman for the C=C band that i n IR i s h i g h l y v a r i a b l e i n i n t e n s i t y , i n f a c t , being absent i n m o l e c u l e s t h a t are s y m m e t r i c a l around the C=C band, has been used by G r a s s e l l i e t a l . t o s t u d y c r o s s - l i n k i n g i n a polystyrene polymer (135). A s i m i l a r curing study i n styrenebased p o l y e s t e r r e s i n s u s i n g the v i n y l C=C i n the s t y r e n e

29.

GRAVER


725

Figure 11.

O p t i c a l path and s p e c t r a l e f f e c t s absorption spectroscopy.

in reflection-

Figure 12.

Gas analyzer consisting of a v a r i a b l e path length gas c e l l and i n f r a r e d spectrometer can be used f o r the a n a l y s i s o f any gas or v a p o r h a v i n g i n f r a r e d absorption i n concentrations ranging from a few parts per m i l l i o n to s e v e r a l percent.

726

(4)


monomer r e l a t i v e to the aromatic r i n g band at 1590 cm~l was r e p o r t e d by Koenig (136). Q u a n t i t a t i v e a n a l y s i s of t h r e e component systems i s reported (137) and determination of t h i o l groups i n s u l f u r polymers showed high s e n s i t i v i t y (138). F i f t y - n i n e references on the a p p l i c a t i o n of Raman spectroscopy to polymers are c i t e d i n a recent popular book on a p p l i c a t i o n s of Raman (139). Polyacetylene and r e l a t e d polymers are being widely investigated at t h i s time (140-143).

S l o a n e , B o e r i o and Koenig, and McGraw have d e s c r i b e d the sampling and other instrumental considerations for Raman spectra of polymers (144). Other reports on Raman i n v e s t i g a t i o n s of polymers include molecular o r i e n t a t i o n i n b u l k p o l y e t h y l e n e t e r e p h t h a l a t e (145), c r y s t a l l i n i t y of e t h y l e n e - p r o p y l e n e rubber (146), and the structure of unsaturated polyester resins c r o s s - l i n k e d with styrene

(Ml)Orientation of polymers has been i n c r e a s i n g l y studied by Raman. PMMA o r i e n t a t i o n (148), oriented PP and PE (149), q u a n t i t a t i v e data on c h a i n o r i e n t a t i o n of PP (150), and v i b r a t i o n a l s p e c t r o s c o p i c studies of polymer chain order have been reviewed (151). The Raman spectrum of most compounds i s not s i m i l a r enough t o the i n f r a r e d spectrum f o r d i r e c t f i n g e r p r i n t i n g comparisons. However, a s p e c t r o s c o p i s t competent i n infrared group frequencies can r e a d i l y i n t e r p r e t Raman data. Sloane (144) g i v e s examples of the most important group frequencies for which Raman spectra offer advantages over infrared. Nyquist and Kagel (107) compare infrared and Raman s p e c t r a of t y p i c a l c l a s s e s of o r g a n i c compounds, and D o l l i s h et a l . (152) have compiled comprehensive data on Raman group frequencies. C o l t h u p (111) t r e a t s i n f r a r e d and Raman bands simultaneously. General s p e c t r a l comparisons between infrared and Raman are made i n T a b l e I V . I t s h o u l d be r e c o g n i z e d t h a t the frequencies l i s t e d are i n round numbers and that both band positions and i n t e n s i t i e s may vary s i g n i f i c a n t l y for i n d i v i d u a l compounds. An e x c e l l e n t a p p l i c a t i o n of Raman to inorganics i n the coatings f i e l d i s the a n a l y s i s of T1O2 (153). An accurate q u a n t i t a t i v e Raman a n a l y s i s for anatase i n r u t i l e was developed for quantities i n the range of 0.03 and 10 wt%. The d e t a i l s of the a n a l y s i s are shown i n F i g u r e s 13-15. F i g u r e 13 shows the Raman spectrum of anatase natural c r y s t a l . The Raman l i n e at 143 cm~l i s so intense and sharp that i t i s proposed as an i n t e r n a l standard for Raman spectroscopy. In Figure 14 the s p e c t r a l comparison i s between a synthetic r u t i l e c r y s t a l cube and a powder sample containing 99.34% r u t i l e and 0.66% anatase. The l i n e a r i t y of the c a l i b r a t i o n curve for anatase below 1% i s attested to by the data i n Figure 15. There i s a tendency i n d i s c u s s i n g a new technique to p o i n t out i t s advantages and that i s the approach taken here for l a s e r Raman spectroscopy. I t i s worth emphasizing t h a t f l u o r e s c e n c e s t i l l presents a major sampling problem for most commercial materials i n the Raman and t h a t at t h i s time i n f r a r e d i s much more w i d e l y a p p l i c a b l e to applied polymer science. Infrared i s g e n e r a l l y the more e f f e c t i v e t o o l for trace analyses and for q u a n t i t a t i v e data. However, a phenomenon broadly described as the resonance Raman effect has been investigated with great interest i n the past decade and may be expected to g r e a t l y i n c r e a s e s e n s i t i v i t i e s f o r some analyses. In t h i s resonance Raman effect, the i n t e n s i t y of a Raman

29. GRAVER

Table IV.


727

Comparison of Group Frequencies i n Infrared and Raman Spectra

Structure

Frequency

Comments

OH

3300

Strong i n IR; weak i n Raman

NH, a l i p h a t i c

3300

Weak i n IR; stronger i n Raman

SH

2600

Weak i n IR; strong i n Raman

R-CEC-R, R ' S equal

2200

Forbidden i n IR; strong i n Raman

-C=N

2200

Variable i n IR; stronger i n Raman

-C=0

1700

Strong i n IR; medium i n Raman

-C=C-

1640

Medium to absent i n IR; strong i n Raman

-P=0

1270

Strong i n IR; weak i n Raman

C-S

800-570


S-S

550-500


728


CM

Figure 13.

Raman spectrum of anatase n a t u r a l c r y s t a l . Laser power was 450 mW at 4880 A. Spectral s l i t width was 4 cm~l. Reproduced w i t h p e r m i s s i o n from Ref. 153. Copyright 1972 Appl. Spectrosc.

CM"

Figure 14.

1

Raman s p e c t r a of T1O2. A, s y n t h e t i c r u t i l e s i n g l e c r y s t a l cube; B, powder sample, 99.34% r u t i l e and 0.66% anatase. Reproduced w i t h p e r m i s s i o n from Ref. 153. Copyright 1972 Appl. Spectrosc.

29. CRAVER


729

s i g n a l may be i n c r e a s e d o r d e r s of magnitude by e x c i t i n g a sample w i t h a l a s e r h a v i n g an e m i s s i o n w a v e l e n g t h w i t h i n the e l e c t r o n i c absorption band envelope of the compound being excited. A general d i s c u s s i o n o f t h e s p e c i a l i z e d t e c h n i q u e was r e p o r t e d i n European Spectroscopy News (154), and the many v a r i a t i o n s that have been developed on enhancement of Raman s p e c t r a are d e s c r i b e d i n a book by Harvey (155). Nuclear Magnetic Resonance Some atomic n u c l e i behave l i k e spinning magnets. I f these n u c l e i are p l a c e d i n a s t r o n g magnetic f i e l d , they may absorb i m p i n g i n g r a d i o - f r e q u e n c y s i g n a l as a f u n c t i o n of the f i e l d s t r e n g t h . At a p a r t i c u l a r radio frequency, the magnetic f i e l d strength* at which a nucleus absorbs i s a function not only of that p a r t i c u l a r i s o t o p i c nucleus but a l s o of i t s immediate e l e c t r o n i c environment, i.e., the nature of the chemical bonding of the atom. Thus, for a hydrogen atom (proton resonance), the f i e l d strength at which the resonance occurs for a CH2 group i s different from the f i e l d strength at which a CH3 group resonates. A nuclear magnetic resonance spectrum of ethanol shows different bands for the H of the OH group and f o r the CH2 and the C H 3 . Much more s u b t l e c h e m i c a l d i s t i n c t i o n s are, of course, possible with h i g h - r e s o l u t i o n NMR. The nature of the nucleus i s the most important consideration i n whether a useful l e v e l of nuclear magnetic resonance occurs. The most s e n s i t i v e n u c l e i are those t h a t behave l i k e s t r o n g magnets, have a h i g h r e l a t i v e i s o t o p e abundance, and have n u c l e a r charge d i s t r i b u t i o n approaching s p h e r i c a l . In T a b l e V , the n u c l e a r resonance s u s c e p t i b i l i t y of some common atoms i s summarized. As with infrared spectroscopy, Fourier transform techniques are used to p r o v i d e i n s t a n t a n e o u s data or to accumulate scans f o r increased s e n s i t i v i t y on microsamples. Spectra have been obtained on as l i t t l e as 10 yg w i t h F o u r i e r transform NMR. However, i t i s not s e n s i t i v e to minor components i n a mixture. Recent fundamental books are A k i t t ' s on F o u r i e r transform and multinuclear NMR (156), Mehring's on h i g h - r e s o l u t i o n NMR i n s o l i d s (157), and Levy's on methods and a p p l i c a t i o n s (158). Books directed more toward polymers are by Kaufman (159), Bovey (160, 161), and Iven (162), which contains useful reviews. The greatest advantage NMR brings to polymer characterization i s i t s s p e c i f i c i t y for i d e n t i f y i n g the chemical structures adjacent to a given group i n a molecule. Thus, i t can be used to d i f f e r e n t i a t e between b l o c k and random copolymers and measure the t a c t i c i t y of polymer chains. As examples of application, NMR characterizes polymers as to stereoregularity (163,164), monomer sequences (165-167), and microstructure (168-170) and is used to analyze monomers (171-174) and elucidate polymerization mechanisms (175-178). Of considerable interest is the use of pulsed NMR to measure cure in UV-irradiated coat ings (179). The importance of NMR in polymer characterization is described in the proceeding of the 1983 Phillips Award Symposium honoring F. A. Bovey (180).

730


1—

τ

/

ο

Figure 15.

C a l i b r a t i o n c u r v e f o r Raman spectroscopic determination of low l e v e l s of anatase i n T1O2. R = ( 143/ 61θ) ~ ( 143/ 610)rutileReproduced w i t h p e r m i s s i o n from Ref. 153. C o p y r i g h t 1972 A p p l . Spectrosc. Ι

Table V.

Ι

I n t r i n s i c A p p l i c a b i l i t y of NMR

Hi F l 9 p31

Most useful

l l fll4 Si29

Useful Not abundant, but s p e c i a l instrumentation has made i t a v a i l a b l e and very useful

Cl3 2

H Ο C

1 2

I

Usefulness of Common Nuclei i n Natural Abundance for NMR Studies

Nucleus

B

I

1 7

Ο

S 1 6

3 3

S

Ν 3 2

1 5

Usable 2 8

3

Si * 0

Not usable

29.

CRAVER


731

Mass Spectrometry (MS) Mass spectrometry requires that the sample under i n v e s t i g a t i o n be v o l a t i l e or produce v o l a t i l e fragments under high vacuum. In the polymer f i e l d , i t i s used p r i n c i p a l l y to a n a l y z e raw m a t e r i a l s , r e s i d u a l monomers or s o l v e n t s , d e g r a d a t i o n p r o d u c t s , and low molecular weight oligomers. Instrument c a p a b i l i t i e s are increasing with the use of h i g h - f i e l d magnets and s p e c i a l i o n i z a t i o n techniques so that increasing a p p l i c a t i o n s to molecular weight determinations are being reported. Mass s p e c t r a l data were observed on polybutadiene samples with m o l e c u l a r weights i n the range of 1000-3000 (181), on PEG and PPG o l i g o m e r s (182), and on p o l y s t y r e n e o l i g o m e r s (183) by f i e l d desorption MS. Reviews on the a n a l y s i s of polymers and polymer products have recently appeared (184-187). T y p i c a l a p p l i c a t i o n s are covered for a n a l y s i s of raw materials for coatings (188-191) and determination of v o l a t i l e components of commercial polymers (192). Mass spectrometry of t h e r m a l l y t r e a t e d polymers (193) and of s t r e s s e d polymers are t r e a t e d (194) w i t h i n s t r u m e n t a l and e x p e r i m e n t a l d e t a i l s g i v e n i n more d e t a i l than i n c o r r e s p o n d i n g j o u r n a l references. P y r o l y s i s , combined w i t h gas or l i q u i d chromatography and i n f r a r e d and mass s p e c t r o s c o p y , i s a powerful t o o l f o r s t u d y i n g polymer fragments. A 14-page bibliography covering selected reports from 1973 to 1980 i s a v a i l a b l e at no c o s t (195), and the method i s reported on by Liebman (5, 128). Methods of e x t e n d i n g MS to s o l i d s t a t e are summarized by G a r d e l l a , Graham, and Hercules i n a comprehensive report on l a s e r desorption mass spectrometry (5). T h i s i s a n o t h e r f i e l d i n w h i c h t h e u n i n i t i a t e d can be overwhelmed by acronyms, so some of the most w i d e l y used ones are explained here: HRMS = h i g h - r e s o l u t i o n mass spectrometry; HRGC/HRMS = h i g h - r e s o l u t i o n gas c h r o m a t o g r a p h y / H R M S ; LC/MS - l i q u i d chromatography/mass spectrometry; CI = c h e m i c a l i o n i z a t i o n ; FD = f i e l d d e s o r p t i o n ; FAB = f a s t atom bombardment; LDMS = l a s e r desorption mass spectrometry. Data Banks and Computer R e t r i e v a l Both i n f r a r e d and mass spectroscopy produce complex band patterns t h a t r e q u i r e comparison to r e f e r e n c e s p e c t r a f o r i d e n t i f i c a t i o n . Peak p o s i t i o n data have been abstracted from published spectra i n both d i s c i p l i n e s , and computer programs to r e t r i e v e the data i n the f i l e are h i g h l y e f f e c t i v e . As Raman spectroscopy becomes more prevalent the same approach may be expected for i t . The p r a c t i c a l l i m i t of these search systems i s now only one of having adequate q u a l i t y data bases a v a i l a b l e . S e v e r a l t e c h n i c a l groups address themselves to t h i s problem. Among the leaders are the J o i n t Committee on Atomic and M o l e c u l a r P h y s i c a l Data, whose secretary i s from the National Standard Reference Data Center of the N a t i o n a l Bureau of Standards, Washington, D.C., and The C o b l e n t z S o c i e t y , I n c . , whose s e c r e t a r y i s at P e r k i n - E l m e r C o r p o r a t i o n , Norwalk, Conn.

732


Instrument companies have added s p e c t r a l search systems as a spectrometer accessory f o r modest-sized f i l e s . The l a r g e s t mass s p e c t r a l f i l e s are a c c e s s i b l e through the NIH-EPA Chemical Information System (CIS), and the largest IR o n - l i n e search system i s a v a i l a b l e on Tymshare (196).

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20.

Myers, R. R.; Long, J . S. "Treatise on Coatings, V o l . 2, Characterization of Coatings, Physical Techniques"; Marcel Dekker: New York, 1970. Krause, Α.; Lange, Α.; E z r i n , M. " P l a s t i c s Analysis Guide"; Verlag: Munich, 1983. U r b a n s k i , J.; C z e r w i n s k i , W.; J a n i c k a , K.; Majewska, F. "Handbook of Analysis of Synthetic Polymers and P l a s t i c s " ; Wiley: New York, 1977. Crompton, T. R. "Chemical Analysis of Additives i n P l a s t i c s " ; International Series i n A n a l y t i c a l Chemistry; Pergamon: Oxford, England, 1978; V o l . 46. Craver, C. D. "Polymer Characterization: Spectroscopic, Chromatographic and Physical Instrumental Methods"; American Chemical Society: Washington, D.C., 1983. Bauman, R. P. "Absorption Spectroscopy"; Wiley: New York, 1962. Meehan, E. J . "Treatise i n A n a l y t i c a l Chemistry," 2nd ed.; E l v i n g , P. J.; Meehan, E. J.; Kolthoff, I. M., Eds.; Wiley: New York, 1981. F l e e t , M. St. C. "Physical Aids to the Organic Chemist"; Elsevier: Amsterdam and New York, 1962. Cahill, J . E. "Fundamentals of UV/Visible Spectrophotometry: An Outline"; Applications Data B u l l e t i n , ADS 123, Perkin-Elmer Corp.: Norwalk, Conn., 1980. Silverstein, R. M.; B a s s l e r , G. C.; Morrill, T. C. "Spectrometric Identification of Organic Compounds," 4th ed.; Wiley: New York, 1981. Esko, K.; K a r l l s o n , S.; Porath, J . Acta Chem. Scand. 1968, 22(10), 3342. B e l i s l e , J. Anal. Chim. Acta 1968, 43, 515. Eliassaf, J . J . Polym. Sci., Part Β 1972, 10, 697. Schmitz, F. P.; M u e l l e r , H.; Rossback, V. Kunststoffe 1979, 69(6), 321. Rotzsche, H.; P r i e t z , U.; Diedrich, H.; Clauss, H.; Hahnewald, H. Plaste Kautsch 1978, 25(7), 390. Hargis, L. G.; Howell, J . A. Anal. Chem. 1980, 52, 306R. Howell, J . Α.; Hargis, L. G. Anal. Chem. 1982, 54, 171R. S n e l l , F. D.; Snell, C. T. "Colorimetric Methods of Analysis Including Some Turbidimetric and Nephelometric Methods," 3rd ed.; Van Nostrand: Princeton, N.J., 1971; V o l . 4AAA. Mavrodinean, R.; Schultz, J . K.; Menis, O. "Accuracy i n Spectrophotometry and Luminescence Measurements"; NBS Special Publication No. 378, U.S. Government P r i n t i n g O f f i c e : Washington, D.C., 1973. Burgess, C.; Knowles, A. "Techniques in Visible and U l t r a v i o l e t Spectrometry, V o l . 1: Standards i n Absorption Spectrometry"; Chapman and H a l l : London, 1981.

29. GRAVER

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733

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