Photoacoustic Fourier Transform IR Spectroscopy and Its Application

34 Photoacoustic Fourier Transform IR Spectroscopy and Its Application to Polymer Analysis Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch034

D. WARREN VIDRINE and S. R. LOWRY Nicolet Instrument Corporation, Madison, WI 53711 Photoacoustic spectroscopy (PAS) in the UV-visible region, although a promising technique, is limited in its application by the prevalence of PA saturation and by the paucity of structural information in that spectral region (relative to the mid-IR). The recent development of Fourier transform IR photoacoustic spectroscopy (FTIR/PAS) has made mid-IR PA detection practical for solid and liquid samples. Because of the relatively low volume absorptivities characteristic of vibrational absorptions, PA signal saturation is generally not a problem except for some inorganic materials. The applications of FTIR/PAS to polymer analysis have been in three directions so far: (1) obtaining IR spectra of samples that are intractable to most traditional sample preparation methods; (2) identifying or ruling out the existence of sample preparation artifacts in IR spectra obtained by traditional methods; and (3) surface and depth profiling studies of sample surfaces. Applications of FTIR/PAS to the analysis of polymers and composite materials are presented, along with comparisons of PAS with spectra obtained (on the same materials) using other IR measurement techniques.

JL H E P H O T O A C O U S T I C E F F E C T WAS D I S C O V E R E D b y A l e x a n d e r G r a h a m

B e l l (J) w h o found that many materials gave off an a u d i b l e sound w h e n i l l u m i n a t e d b y modulated light. A s s h o w n i n F i g u r e 1, the heating at intervals of the sample surface b y the m o d u l a t e d light causes the air adjacent to the sample to be heated at intervals. T h e 0065-2393/83/0203-0595$06.00/0 © 1983 American Chemical Society

Craver; Polymer Characterization Advances in Chemistry; American Chemical Society: Washington, DC, 1983.

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POLYMER CHARACTERIZATION

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch034

BOUNDARY LAYER

MICROPHONE

SAMPLE

Figure 1. Idealized photoacoustic cell for solid samples.

intermittent expansion of this air causes an alternating pressure w a v e — s o u n d (2). So, photoacoustic detection directly measures the absorption of light energy at the sample surface. I n 1975, U V - v i s i b l e photoacoustic spectroscopy (PAS) became a commercial reality, and immediately found application i n solids analysis. T w o factors have previously l i m i t e d its application: the p h e n o m e n o n of photoacoustic (PA) saturation (analogous to a completely absorbing band) that l i m i t s quantitative application, and the relative paucity of structural information for many polymers i n the U V region. T h e weaker sources and lower photon energy of the m i d - I R region at first p r o h i b i t e d I R - P A S ; however, w h e n P A detection was demonstrated (3) w i t h a M i c h e l s o n interferometer spectrometer (in the v i s i b l e region), the solution of the m i d - I R sensitivity p r o b l e m [by u s i n g interferometric F o u r i e r transform I R ( F T I R ) spectrometers] was inevitable. F T I R spectrometers permit more effective use of measurement time by a l l o w i n g measurement of a l l I R frequencies at a l l times d u r i n g each scan (all o w i n g m u c h signal-averaging of noise), and have characteristically higher light throughputs than dispersive spectrometers (no slits are r e q u i r e d because there is no monochromator). T h e s e sensitivity ad-


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vantages offset the characteristically l o w thermal efficiency of the u n t u n e d air-mediated P A microphone c e l l a n d p e r m i t useful spectra to be obtained i n reasonable measurement times.


The PAS

Experiment

A l t h o u g h the basic P A S experiment is quite s i m p l e , the analog signal measured at the microphone is a result of a complex transfer of energy from the I R source to the microphone. A l t h o u g h P A S is f u n damentally a surface t e c h n i q u e , the actual thermal response is a rather complicated function of m o d u l a t i o n frequency, surface morphology, sample absorptivity, and t h e r m a l diffusivity. Attempts to m o d e l the experiment have b e e n partially successful, but a general theory is not available currently. A major point to be made concerning P A S is the direct relation o f I R absorbance to the signal measured w i t h the microphone. U n l i k e normal transmittance or reflectance techniques w h e r e one measures the loss of signal caused b y the sample absorbing radiation, P A S measures the true magnitude of the interaction. T h i s ability to have m a x i m u m signal w h e n the sample is present reduces the d y n a m i c range problems sometimes encountered i n F T I R . F i g u r e 2 shows the P A S interferogram from a sample of carbon black. Because carbon black is a strong broad range absorber the signal is quite similar to a normal transmitting spectrum, e v e n though the measuring procedure is quite different. F i g u r e 3 shows the P A S i n terferogram from a vapor phase sample of methanol a n d carbon d i s u l fide (CS ). T h i s sample has a few very strong absorption l i n e s , a n d the interferogram shows the beat pattern b e t w e e n these frequencies. A s stated previously, the magnitude of the interferogram is related d i rectly to the sample. If more sample were i n the c e l l , the signal w o u l d be proportionally larger. A n important effect i n P A S is the huge signal enhancement observed w h e n the sample is i n the vapor phase. T h i s is often a p r o b l e m w h e n a sample contains adsorbed water. After the sample is sealed i n the P A S c e l l , a small amount of desorbed water can make a large contribution to the final spectrum. T h e p r o b l e m can be r e d u c e d b y p u r g i n g the sample before sealing the c e l l or by performing a spectral subtraction w i t h a water reference spectrum. 2

Comparison

of Methods

C r u c i a l to understanding the uses of F T I R / P A S is the consideration and comparison of alternative I R methods of measuring p o l y m e r surface properties. T h e most direct methods are p h y s i c a l m i c r o t o m i n g a n d i n c l u d e surface g r i n d i n g , e t c h i n g , a n d s l i c i n g (classical m i crotoming). I n these methods the I R spectra and their properties are


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Figure 2. PAS interferogram from a carbon black sample. not usually the p o i n t of controversy; they are essentially s i m p l e transmission spectra. Rather, the trick of these techniques lies i n the microtoming process itself. T h e f o l l o w i n g section attempts a description of the operational characteristics of the spectroscopic microtoming, or surface methods i n use. Reflectance M e t h o d s . Because most surface methods are reflection methods, it seems appropriate to define some descriptive terms i n order to insert a m o d i c u m of order (and a m i n i m u m of further confusion) into this f i e l d . " P l a n e specular reflection" is the reflection from a p o l i s h e d flat surface, such as a c o m m o n first-surface mirror. "Scattered specular reflection," or " d i f f u s e d specular r e f l e c t i o n " occurs w h e n there are single reflections from a rough surface, such as a sandblasted gold surface. " D i f f u s e reflection" occurs w h e n there are m u l t i p l e reflections and transmissions of each ray w i t h i n a more or less transparent scattering sample, such as p o w d e r e d K B r . A l t h o u g h the latter two often have the same spatial characteristics, i.e., great or total diffusion of reflected light, the nature a n d photometry of these measurements is quite different. T h e term " f / 5 " refers to the common focusing and collection angle found i n most commercial I R spectrometers, and


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" f / 1 " refers to the practical m a x i m u m focus angle and collection angle. T h i s is usually n o m i n a l l y near f/1, but the useful f/number is higher. B y u s i n g these terms, w e may list a n d describe the major reflectance methods i n use.


" N O R M A L " SPECULAR REFLECTANCE.

T h i s t e r m c o m m o n l y refers to

high-angle specular reflection from a plane surface. I n practice, an f/5 beam, w i t h an incidence angle between 45° and 80° to the surface plane, or a Christiansen spectrum of optically thick nonscattering samples, is most often used to measure coatings o n metal surfaces. I n this use, the reflection off the metal surface is not of interest, and w e are measuring actually the double-pass transmission spectrum of the coating. T h i s method suffers i n sensitivity for extremely t h i n films because the electric vector o f the l i g h t approaches zero very near the metal sur face. G R A Z I N G ANGLE REFLECTANCE.

G r a z i n g angle reflectance ( G A R ) ,

or ellipsometry is a true surface technique and refers to the low-angle reflection from a plane surface. I n practice, either a e o l l i m a t e d beam or an f/5 beam is used w i t h the n o m i n a l incidence angle u s u a l l y b e i n g 5° to 10° from the surface. T h e angular dispersion of an f/5 beam is ο œ

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Figure 3. PAS interferogram from a vapor phase sample of methanol and carbon disulfide.



600


significant here and w i l l influence the resulting photometry. N e a r normal i n c i d e n c e , the spectrum is s i m p l y a weak Christiansen, or refractive-index spectrum of the material. A t grazing angles, the spec trum of the p e r p e n d i c u l a r polarization is a reasonable s i m u l a c r u m of the absorption spectrum of the surface. G A R requires a smooth, h i g h l y p o l i s h e d surface of fairly large area as h i g h l y focused l o w f/number optics increase the i n c i d e n c e angle dispersion. T h e method is useful for both p o l i s h e d , optically thick samples and for very t h i n (monolayer) films on metal. Spectral features appear only for l i g h t of polarization, so a polarizer is often used for the measurements. T h e equivalent depth penetration is on the order of 1 μιη. M U L T I P L E E X T E R N A L R E F L E C T A N C E . T h i s method is used most c o m m o n l y as a rough but sensitive version of grazing angle reflec tance. I n practice, an f/1 to f/5 beam is b o u n c e d into the space between t w o p o l i s h e d plates (for background spectra, these are mirrors; other wise one or both are sample), and exits after m u l t i p l e reflections. Because the beam is not c o l l i m a t e d , and the samples are often not completely flat, considerable incidence angle dispersion c o m m o n l y exists, a n d the technique is most often used where sensitivity rather than photometric d e f i n a b i l i t y is paramount, as i n the measurement of submonomoleeular layers on metal. D I F F U S E R E F L E C T A N C E . Diffuse reflectance ( D R F or D R I F T ) is the reflectance of a sample associated w i t h light b e i n g reflectively scattered m u l t i p l e times w i t h i n the optically scattering sample. Photometrically, the sample absorptivity is related to the reflectivity b y the K u b e l k a - M u n k equation. I n practice, f/1 c o l l e c t i o n optics are used to collect as m u c h (20% or so) of the diffusely reflected light as is practical. T w o forms of D R F accessory are available: those that eliminate the central f/5 portion of the collected beam, and those that at tempt to collect the w h o l e beam. T h e former is valuable w h e n a scat tering sample has a smooth p o l i s h e d surface w i t h its specular C h r i s tiansen contribution. F o r most samples, i n c l u d i n g powders, the specular component is also diffused b y surface roughness and cannot be e l i m i n a t e d i n this way. Nonscattering rough-surfaced samples are best measured i n a D R F accessory, but the reflectance is diffused specular reflectance ( D S R , not D R F ) , and does not follow the K u b e l k a - M u n k absorptivity relationship. I n practical D R F spectroscopy, the reflectivity of high-absorptivity spectral regions is often d o m i nated by specular reflectivity, e v e n w h e n the rest of the spectrum is predominantly true diffuse reflectance. T h i s can cause a specious, antinomial, poltergeisterhaft effect k n o w n as b a n d reversal, where the absorptivity peak of a b a n d actually has higher reflectivity than the b a n d wings. I n general, D R F is useful for two classes of p o l y m e r samples: w i t h powders or other h i g h l y scattering samples, a n d w i t h


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nonscattering samples h a v i n g small, scattering blemishes on their surfaces. T h e ease of use o f D R F makes i t a very desirable m e t h o d for repetitive measurements w h e n the sample's optical properties a n d analytical requirements permit.


ATTENUATED TOTAL REFLECTANCE.

Attenuated total

reflectance

( A T R ) , or m u l t i p l e internal reflectance ( M I R ) , is the most c o m m o n l y used technique of surface measurement i n I R spectroscopy. L i g h t traveling inside a h i g h refractive index, transparent crystal is reflected against the A T R crystal surface from the i n s i d e . Because the reflection is b e l o w (more grazing than) the critical angle total dielectric reflection takes place, and the light is not attenuated. H o w e v e r , the electric vector of the light extends b e y o n d the crystal surface, and an optically absorbing, l o w refractive index substance p l a c e d i n contact w i t h the crystal surface w i l l attenuate the reflected light. Because this electric vector, or evanescent wave, is of significant intensity w i t h i n only a few micrometers of the surface, only the surface spectral absorptivity of the sample is measured. I n practice, an f/1 or f/5 beam is introduced into the crystal. T h e h i g h refractive index of the crystal reduces the angular dispersion of the beam w i t h i n the crystal. Crystals of various refractive indices are used. F o r relatively deep penetration, l o w refractive index crystals are used. C o m m o n crystals are K R S - 5 (thallium bromoiodide, refractive index 2.2), a n d z i n c selenide (2.42). F o r shallower penetrations, s i l i con (3.4) or germanium (4.0) is used. Penetration may also be decreased b y reflecting the light at a lower, more grazing angle. Reflection at a h i g h angle increases penetration, but angles near the critical angle (limit of total reflection) produce spectra w i t h serious b a n d asymmetries. Samples must make good, intimate contact w i t h the crystal surface. F o r quantitative work, this requires l i q u i d s or very flat or deformable solids. Fortunately, for qualitative work or relative quantitation less perfect sample contact is permissible a n d e v e n textured surfaces a n d powders may be suitable. Nonreflection Surface M e t h o d s . E M I S S I O N S P E C T R O S C O P Y . T h i s nonreflection method uses the I R light emitted b y the sample itself as the source. T h e sample is p l a c e d on a thermostatted stage at a p r e i n terferometer f/1 beam focus. T h e beam t h e n originates at the sample, is modulated b y the interferometer a n d t h e n detected (the detector must be at a different temperature than the sample, a n d l i q u i d N c o o l e d detectors are u s u a l l y used). T h e spectral zero a n d 100% points are defined b y substituting a gold surface or a carbon black (or p l a t i n u m black) surface i n place of the sample. Results are expressed as percent emissivity or as watts per steradian. T h e r e are three general classes of samples, each analogous to the type of high-angle reflectance spectroscopy. O p t i c a l l y thick, nonscattering samples give fea2


602


tureless gray body e m i s s i o n spectra, w i t h a small C h r i s t i a n s e n c o m ponent similar to that obtained w i t h specular reflectance. T h i n films o n metal give inverted-transmissionlike spectra, again similar to the specular reflectance case. Scattering samples can give spectra similar to inverted diffuse reflectance spectra. I n general, for the majority of sample types i n the m i d - I R , similar results can be obtained w i t h reflectance methods, without p a y i n g the energy and signal-to-noise penalty of a cooler source (reflectance methods use a 1500-K G l o b a r source, w h i l e emission samples are usually 3 0 0 - 6 0 0 K ) . Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1983 | doi: 10.1021/ba-1983-0203.ch034

PHOTOACOUSTIC DETECTION.

Photoacoustic detection ( P A or P A S )

is the subject of this chapter a n d only information for comparison w i l l be given here. A n f/1 or f/5 beam is focused onto the sample stage of the P A microphone c e l l . T h e sample cavity i n c o m m e r c i a l cells is comm o n l y 4 - 1 2 m m i n diameter a n d a few m i l l i m e t e r s deep, but larger samples can sometimes be accommodated. F o r reasons o u t l i n e d elsewhere (4), the detectivity of the P A microphone c e l l is less than other thermal detectors (such as T G S or D T G S ) , and measurement times are, therefore, longer (for equivalent spectral signal-to-noise) than w i t h other methods. T h e spectrum resembles an inverted transm i s s i o n spectrum i n appearance and approximate photometry. Scattering effects that sometimes cause such baseline tilt and curvature i n other methods usually do not v i s i b l y affect P A baselines. H i g h absorptivity organic I R chromophores rarely give P A signals that are more than about 8 0 % of saturation, a n d e v e n strong inorganic bands, e.g., S i - O stretch, are i n c o m p l e t e l y saturated. Sample surface morphology exerts a m i l d effect on spectrum intensity, and a smaller effect on photometry as compared to other methods. General

Applications

F o u r uses for F T I R / P A S suggest themselves. First, P A S offers a way of a v o i d i n g interfering C h r i s t i a n s e n effects on b a n d shape and location. These distortions are particularly noticeable w i t h h i g h l y absorbing samples {see F i g u r e 4). F o r instance, the technique is b e i n g used to obtain spectra of inorganic salts that are free of C h r i s t i a n s e n contributions (5). Diffuse reflectance ( D R F ) spectroscopy of very fine powders and A T R spectroscopy w i t h high-index plates are other techniques capable of m i n i m i z i n g C h r i s t i a n s e n contributions; but both these techniques impose strict limitations on sample morphology. As a second use, P A S offers a way of l o o k i n g at u n d i s t u r b e d surfaces. A T R , the c o m m o n alternative for surface investigations, requires a flat or deformable surface for good sample contact, a n d requires sample contact. I f these requirements can be met, of course, A T R may offer a m u c h less t i m e - c o n s u m i n g way of obtaining the i n formation. S i m i l a r l y , i f the native sample happens to be a fine, scat-


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Figure 4. FTIR/PAS spectra that illustrate interfering Christiansen effects on band shape and location. tering p o w d e r without important bands of very h i g h absorptivity, a n d information about the b u l k composition is desired, D R F may easily be the method of choice. B u t , w h e n spectra that are i n d e p e n d e n t of sample surface morphology are desired, P A S has some intrinsic advantages. F i g u r e 5 illustrates the spectral differences seen w h e n a series



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of nitrile resin samples that differ only i n surface morphology are measured. F o r comparison, note that the same samples measured b y P A S show i d e n t i f i a b l y similar spectra {see F i g u r e 6). O f course, i f the resin samples were f i n e l y ground and m i x e d w i t h K B r , very s i m i l a r spectra w o u l d be obtained b y D R F or K B r p e l l e t i n a shorter time. T o illustrate this, note that the D R F spectra of F i g u r e 5 r e q u i r e d only 10 s of measurement time apiece, w h i l e the P A spectra of F i g u r e 6 req u i r e d 5 m i n apiece. T h i r d l y , m e c h a n i c a l l y intractable or noxious samples may be analyzed without resorting to messy sample preparation techniques. I n this case, one is essentially trading preparation time or effort for a longer measurement time. F i n a l l y , P A S can offer a relatively simple way of calibrating mea-


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Figure 6. FTIR/PAS of a nitrile-containing resin in different surface morphologies. Key is the same as in Figure 5.

surements obtained b y other methods. F o r instance, the curve relating the reflective attenuation of a b a n d b y A T R , and the b a n d absorptivity can make the calibration of an A T R analysis easier. S i m i l a r l y , a refer ence P A spectrum of an u n d i s t u r b e d surface compared w i t h a spec-


606


trum obtained b y a faster technique can make analytical interferences due to the faster technique's artifacts of preparation obvious, a n d thus easily c i r c u m v e n t e d .


Relevant Parameters T h e r e are four interrelated factors that affect F T I R / P A S spectra: sample properties, the p h y s i c a l constants of the gas i n the c e l l , the light modulation frequency, a n d the c e l l characteristics. T h e prime determinants of the effective depth of penetration or measurement depth are the thermal properties of the sample. M o s t organic polymers have similar thermal conductivity and heat capacity; therefore, for b u l k resins a n d composites, the effective measurement depths are reasonably similar. F i n e powders, finely foamed samples, and extremely t h i n samples t e n d to give P A S of higher intensity, a n d the photometric scale of such spectra s h o u l d be v i e w e d w i t h caution. T h e photometric theory of P A S has b e e n presented elsewhere (6) i n some detail. A l t h o u g h there are some problems w i t h the work, the basic concepts are useful for the m i d - I R . A l s o , it is a useful generalization that scattering and C h r i s t i a n s e n contributions to P A spectra are very small, but these contributions do exist and must be considered for critical work. T h e w o r k i n g m e d i u m used i n P A cells designed for s o l i d samples is generally air at atmospheric pressure. H e l i u m has b e e n s h o w n to double the signal obtained, w i t h a corresponding increase i n the signal-to-noise ratio obtained. T h e o r y and some experiments suggest that gases w i t h h i g h molecular weights give poor results. A p p a r e n t l y , no experiments have b e e n done that investigate the effect of pressure, although it w o u l d be an u n l i k e l y coincidence i f atmospheric pressure were o p t i m u m . I n any case, the sensitivity enhancement obtained b y changing the w o r k i n g gas must be j u d g e d against the convenience of using ambient air. T h e m o d u l a t i o n frequency of the light also affects the effective measurement depth. I n F T I R spectrometers, this m o d u l a t i o n is prod u c e d b y the M i c h e l s o n interferometer. T h e m o d u l a t i o n frequency is proportional to the w a v e n u m b e r position i n the spectrum, a n d proportional to interferometer mirror velocity. T h e effective measurement depth is proportional to the square root of the inverse of the m o d u l a t i o n frequency, according to Rosencwaig a n d others, although for practical samples this may not be an exact relationship. Standard gas-microphone photoacoustic cells have a frequency response that varies inversely w i t h frequency at frequencies b e l o w the cell's residual resonance frequency. F i g u r e 7 illustrates the frequency response of a t y p i c a l P A c e l l . B y u s i n g a higher mirror velocity to achieve a smaller effective measurement depth, a severe signal-to-


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Figure 7. Illustration of the frequency response of a typical photo acoustic cell. noise penalty is carried i f m o d u l a t i o n frequencies above the c e l l reso nance frequency are used. T h i s fact imposes a l o w e r l i m i t of a few micrometers on attainable effective penetration depths. W i t h i n these limits, P A S can be used to profile composition as a function of depth. F i g u r e 8 is such a series of P A spectra obtained from a sample of poisoned catalyst. T h e numbers 0 - 2 4 represent increasing mirror velocity (and thus decreasing effective measurement depth) on a logarithmic scale. F o r instance, velocity 10 is twice velocity 0, a n d velocity 20 is twice velocity 10. Several features i n the spectra suggest a change i n composition w i t h depth. I n particular, the changes i n relative heights of the bands around 700 c m " indicate differences i n the aromatic substitutional patterns for the different depths. 1

A Study of a Polymer Surface C o n s i d e r a b l e research has gone into evaluating the performance of perfluorinated sulfonic a c i d polymers as separators i n the elec trolytic cells used i n the production of chlorine and sodium hydroxide.


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T h e sulfonamide groups ( - S 0 N H - ) were created b y reaction of the p o l y m e r i n the sulfonyl fluoride form w i t h the corresponding amine. A significant improvement i n membrane performance was observed w h e n a m o d i f i e d sulfonamide layer was formed w i t h ethylene diamine ( E D A ) . A large n u m b e r of successful A T R experiments were performed on various forms of N a f i o n p r o d u c e d for research investigations. 2



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Figure 9. PAS spectrum of a sample of production membrane placed in the PAS cell with the untreated side face up. H o w e v e r , i n order to improve the mechanical strength o f the m e m branes i n actual production cells the N a f i o n membrane is reinforced w i t h a cross screen o f T e f l o n fibers. T h e T e f l o n fibers create a rough membrane surface where most o f the actual N a f i o n is located. A T R experiments on the production membrane were quite difficult and the resulting spectra were frequently dominated b y features due to the Teflon. Transmission experiments on these samples failed because the C - F stretching mode b l a n k e d the entire fingerprint region. T h e p r o b l e m o f a n a l y z i n g the treated and untreated sides o f a p o l y m e r membrane where good surface contact is impossible seemed w e l l suited for P A S . A sample of production membrane was p l a c e d i n the P A S cell w i t h the untreated side ( - S O " i N a ) up. Figure 9 shows the resulting P A S spectrum. T h i s spectrum has b e e n ratioed to a carbon black spectrum. T h e strong water peaks b e t w e e n 1800 a n d 1400 c m " indicate that water has b e e n desorbed from the h y d r o p h i l i c surface o f the membrane. N o attempt was made to remove the water. T h i s spectrum allows a comparison to be made b e t w e e n the desorption prop+

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Figure 10. PAS spectrum of a sample of production membrane placed in the PAS cell with the treated side face up.

erties of the two sides of the membrane. T h e major peak i n the spectrum at 1200 cm"" is due to the C - F stretch. T h e flat top of this peak indicates clearly that this b a n d is saturating. T h e major peak of interest occurs at about 1050 c m . T h i s peak corresponds to the symmetric S O ^ stretching mode. T h e asymmetric mode occurs u n d e r the large C - F peak and the peak b e l o w 1000 c m seems to be an overlap of a C - S - O vibrational mode a n d a C — F mode. F i g u r e 10 shows the spectrum of the treated side of the membrane. T h e key features here are the almost complete loss of the S O l peak and the formation of a peak at approximately 1375 c m . T h i s can be attributed to the S O N asymmetric stretch. F i g u r e 11 shows the spectral subtraction data for this study. T h e basis of this subtraction was the n u l l i n g of the large C - F peak and is s h o w n i n more detail i n F i g u r e 12. Great care must be taken w h e n interpreting the results of P A S subtractions. C u r r e n t l y , there is not a good function or theory that provides a linear response that can be a p p l i e d generally to P A S data. T h e p r o b l e m becomes particularly difficult w h e n d e a l i n g w i t h saturated peaks; however, i n 1

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this case the subtraction results appear very meaningful. T h e nonsubtracted data clearly show the — S O ^ N a symmetric stretch a n d the - S 0 N - asymmetric stretch. T h e other two bands revealed i n the subtraction are both v i s i b l e as shoulders and the locations correspond w e l l to - S O ^ N a * asymmetric stretch a n d the sulfonamide symmetric stretch observed i n other compounds. A final observation that can be made from this spectrum is the good subtraction of the water vapor. T h i s indicates that no preferential desorption is observed b e t w e e n the two sides of the membrane. T h i s study p r o v i d e d information that w o u l d be extremely difficult or impossible to obtain b y other techniques. T h e subtraction w o r k e d


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Figure 11. Spectral subtraction data for the PAS spectra discussed in Figures 9 and 10.


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Figure 12. Detail of the nulling of the large C-F peak from the sub traction data shown in Figure 11. quite w e l l and revealed the two peaks that correspond to the other two S—Ο stretches. Summary and Prospectus F T I R / P A S is a n e w method of obtaining I R spectra of s o l i d sam ples. It has some u n i q u e advantages i n several s a m p l i n g situations w h e r e : (1) spectra free of C h r i s t i a n s e n contributions are desired, (2) surface measurements of u n d i s t u r b e d surfaces are r e q u i r e d , (3) mechanically intractable or noxious samples must be measured, or (4) sapient analytical design w o u l d be facilitated b y reference spectra w i t h P A S characteristics. T h e application of F T I R / P A S to p o l y m e r problems is still i n its infancy, and therefore, this is not an overview of a w e l l - u s e d method, but rather an introduction to the p r i n c i p l e s of F T I R / P A S , a n d a summary of existing work as it relates to possible p o l y m e r applications. T h e amount of literature o n p o l y m e r a p p l i c a tions is still quite small. Several papers of p o l y m e r interest were pre sented at the 1981 International F T I R Conference (4), and extended


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abstracts are available for these. T h e Literature C i t e d section w i l l hopefully serve as a guide to the groups a n d authors currently active i n this f i e l d . Literature Cited 1. Bell, A. G. Am. J. Sci. 1880, 20, 305. 2. Christiansen, C. Ann. Phys. (Leipzig) 1884, 23, 298. 3. Farrow, M. M.; Burnham, R. K.; Eyring, Ε. M. Appl. Phys. Lett. 1978, 33, 735. 4. Mehicic, M.; Kollar, R.; Grasselli, J. G. Proc. 1981 Intl. Conf. FTIR Spectrosc. Columbia, S.C.; Sakai, H., Ed.; Vol. 289, S.P.I.E., Bellingham, WA. 5. Laufer, G.; Huneke, J. T.; Royce, B. S. H.; Teng, Y. C. Appl. Phys. Lett. 1980, 37, 517. 6. Rosencwaig, A. In "Optoacoustic Spectroscopy and Detection"; Pao, Y.-H., Ed.; Academic: New York, 1977. 7. Rockley, M. G.; Devlin, J. P. Appl. Spectrosc. 1980, 34, 407. 8. Teng, Y. C.; Royce, B. S. H.J.Opt. Soc. Am. 1981, in press. 9. Vidrine, D. W. In "Fourier Transform Infrared Spectroscopy: Application to Chemical Systems"; Ferraro; Basile, Eds.; Academic: New York, 1981; Vol. 3, Chap. 4. 10. Vidrine, D. W. Appl. Spectrosc. 1980, 34, 314. RECEIVED for review December 30, 1981. ACCEPTED October 22, 1982.


Photoacoustic Fourier Transform IR Spectroscopy and Its Application

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