Sound and Vibration Damping with Polymers - American Chemical

Chapter 9

Emission of Acoustic Waves from Polymers under Stress Rheo-Photoacoustic Fourier Transform—IR Spectroscopy Marek W. Urban and William F. McDonald Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105

A new rheo-photoacoustic F o u r i e r transform i n f r a r e d c e l l has been developed to perform s t r e s s - s t r a i n s t u d i e s on polymeric m a t e r i a l s . The rheo-photoacoustic measurements lead to the enhancement of the photoacoustic s i g n a l and allow one to monitor the effect of e l o n g a t i o n a l f o r c e s on the molecular s t r u c t u r e of polymers. Propagating a c o u s t i c waves are detected as a r e s u l t of i n f r a r e d r e a b s o r p t i o n and the deformational and thermal property changes upon the a p p l i e d s t r e s s .

Several years ago, r h e o - o p t i c a l F o u r i e r transform i n f r a r e d spectroscopy emerged as a method f o r studying the deformation phenomena during mechanical s t r e t c h i n g of polymeric m a t e r i a l s . This approach has allowed the determination of s t r u c t u r a l changes w i t h i n the polymer network. In essence, two s p e c t r o s c o p i c observations were reported. Experiments performed by S i e s l e r revealed that the i n f r a r e d band i n t e n s i t i e s change upon a p p l i c a t i o n o£ mechanical s t r e s s to polymeric m a t e r i a l s . Wool et a l . , on the other hand, detected s h i f t s of i n f r a r e d bands and f u r t h e r supported these observations with the t h e o r e t i c a l approach based on conformational energy minimization methods. Both harmonic and anharmonic (Morse) p o t e n t i a l energy f u n c t i o n s were a p p l i e d to the C-C s t r e t c h i n g modes i n the valence f o r c e f i e l d . In s p i t e of the f a c t that the detected wavenumber s h i f t s were very s m a l l , the energy minimization c a l c u l a t i o n s y i e l d e d good agreement with the frequency s h i f t i n g c o e f f i c i e n t s obtained experimentally. While

the wavenumber s h i f t s were p r i m a r i l y 0097-6156/90/0424-0151$06.00/0 © 1990 American Chemical Society

observed

152

SOUND AND VIBRATION DAMPING WITH POLYMERS

i n u n c r o s s l i n k e d thermoplastic systems , the i n t e n s i t y changes observed i n the spectra of c r o s s l i n k e d y a t u r a l rubber as w e l l as p o l y ( v i n y l i d e n e fluoride) were a t t r i b u t e d to the s t r e s s induced phase changes and s t r e s s induced c r y s t a l l i z a t i o n . The s t r e s s induced phase changes were suppoijte^d by wide angle x-ray diffraction experiments. ' Although the experiments of S i e s l e r and Wool seem to provide c o n t r o v e r s i a l observations, i t should be remembered that the above s t u d i e s have been conducted on d i f f e r e n t systems. For example, thg mechanical deformation of polyethylene and polypropylene i s quite, d i f f e r e n t from that of c r o s s l i n k e d n a t u r a l rubbers. In s p i t e of the f a c t that both studies have reported important spectral features associated with the s t r u c t u r e - p r o p e r t y r e l a t i o n s h i p , an A c h i l l e s heel of both approaches r e s u l t s from the n e c e s s i t y of using o p t i c a l l y transparent f i l m s to allow i n f r a r e d l i g h t to pass through the sample. New m a t e r i a l s , such as f i b e r s and composites, cannot be studied by transmission FT-IR techniques because they are o f t e n o p t i c a l l y opaque. Thus, i n order to monitor s t r u c t u r a l changes induced by e x t e r n a l f o r c e s , it i s necessary to u t i l i z e a method p e r m i t t i n g the d e t e c t i o n of i n f r a r e d spectra on any m a t e r i a l , regardless of i t s o p t i c a l p r o p e r t i e s , shape or t h i c k n e s s . Although the a p p l i c a t i o n s of photoacoustic FT-IR spectroscopy have shown s e v e r a l pr^m^s^in^ features i n various sampling situations, ' ' the rheo-photoacoustic measurements have been reported only recently. In t h i s work, a novel approach u t i l i z i n g photoacoustic FT-IR spectroscopy to monitor the elongation processes i n f i b e r s i s described. In an e f f o r t to monitor the events on a molecular l e v e l o c c u r r i n g upon static load on virtually all materials, rheo-photoacoustic FT-IR c e l l was designed. Although the applications and theory of photoacougt^c^FT-IR spectroscopy have been recently reviewed, ' ' here, we w i l l only b r i e f l y mention that the d e t e c t i o n of the photoacoustic s i g n a l i s a two stage process. This i s depicted i n Figure 1. F i r s t , i n f r a r e d l i g h t i s absorbed by the sample and the reabsorption process produces heat generating a c o u s t i c waves which are detected by a s e n s i t i v e microphone. The theory that governs photoacoustic^ detection was described by Rosencwaig and Gersho. However, the second stage, that i s the production of heat and subsequently a c o u s t i c waves, can be induced by e x t e r n a l f o r c e s leading to deformational and conformational changes w i t h i n the polymer. A simple example of s i m i l a r phenomenon i s the c r a c k i n g of i c e on a pond producing sounds audible to the human ear. Thus, i s i t our hypothesis that the molecular l e v e l movements w i t h i n polymer w i l l a l s o a f f e c t the i n t e n s i t y of a c o u s t i c waves generated ^ ^ r e s u l t of the l i g h t absorption-reabsorption process. I f stress i s induced i n a polymeric m a t e r i a l and photoacoustic FT-IR 1

a s

9. URBAN AND MCDONALD

Acoustic Waves Emitted under Stress 153

measurements are performed, i n a d d i t i o n to a "normal" photoacoustic i n f r a r e d spectrum obtained as a r e s u l t of the r e a b s o r p t i o n process, an a c o u s t i c s i g n a l due to deformations w i t h i n the polymer w i l l occur. With t h i s i n mind, we will monitor the deformations of poly(p-phenylene terephthalamide) (PPTA) fibers and polyethylene f i l m s , and analyze the s p e c t r a l changes occurring i n i n f r a r e d when the e x t e r n a l f o r c e s are applied. EXPERIMENTAL The poly(p-phenylene terephthalamide) (PPTA) f i b e r s were used as r e c e i v e d from Du Pont de Nemours. The mechanical p r o p e r t i e s of the f i b e r s were analyzed on an Instron Model TM mechanical t e s t e r f o l l o w i n g a common procedure to determine t h e i r e l o n g a t i o n to break. Each t e s t e d sample c o n s i s t e d of 30 strands of f i b e r . The same procedure was employed to monitor fiber and film elongations, followed by simultaneous c o l l e c t i o n £ i n f r a r e d s p e c t r a . The c e l l design was reported elsewhere and allows measurements of v a r i o u s samples such as f i b e r s , composites, and f i l m s . The procedure of loading the sample c o n s i s t s of removing the c e l l top, clamping the sample i n the clamping b l o c k s , t u r n i n g the lead screw the d e s i r e d amount to elongate the sample, purging the c e l l with helium f o r s e v e r a l minutes and p r e s s u r i z a t i o n . In a t y p i c a l experiment, each i n f r a r e d spectrum consists^ of an average of 400 scans recorded at a 4 cm" r e s o l u t i o n with a mirror v e l o c i t y of 0.3 cm/sec. A l l PA FT-IR s p e c t r a were recorded on a D i g i l a b FTS-10M FT-IR spectrometer. The coadded sample scans were ratioed against a carbon black r e f e r e n c e . A l l s p e c t r a were t r a n s f e r r e d to an AT compatible computer f o r f u r t h e r s p e c t r a l manipulations u t i l i z i n g Spectra C a l c . software (Galactic Industries). RESULTS AND

DISCUSSION

Before we begin the a n a l y s i s of the photoacoustic FT-IR s p e c t r a of PPTA f i b e r s and f i l m s recorded as a f u n c t i o n of the sample e l o n g a t i o n , i t i s f i r s t necessary to set the stage and d e f i n e our approach. In monitoring the photoacoustic e f f e c t , infrared light strikes the sample surface and, due to r e a b s o r p t i o n processes, heat i s r e l e a s e d generating a c o u s t i c waves on the s u r f a c e . When the f i b e r s are elongated, conformational changes as w e l l as the molecular deformations occur. Although these deformational processes a l s o produce a c o u s t i c waves which may c o n t r i b u t e to the i n t e n s i t y of the photoacoustic spectrum, these are two independent a c o u s t i c processes. Thus, i n s i g h t s i n t o the molecular changes that occur as a r e s u l t of e l o n g a t i o n a l or shear f o r c e s i n polymeric m a t e r i a l s can be gained. With t h i s i n mind l e t us examine the s p e c t r a l changes

154


detected i n PPTA f i b e r s when the f i b e r s are subjected to external forces. Figure 2 illustrates the rheo-photoacoustic^ FT-IR s p e c t r a of the PPTA f i b e r s i n the 4000-2500 cm" region. Traces A through F represent the s p e c t r a of the PPTA f i b e r s elongated at 0.0 %, 0.42%, 0.83%, 1.25%, 1.67%, and 2.08%, r e s p e c t i v e l y . As seen, s t e a d i l y i n c r e a s i n g i n t e n s i t i e s of the bands i n t h i s s p e c t r a ^ region, incluc^jn^g the N-H s t r e t c h i n g band at 3327 cm" , are observed. ' Upon 2.08% elongation (trace F), however, the 3327 cm" band becomes weaker. The i n t e n s i t y of t h i s band i s of p a r t i c u l a r i n t e r e s t because of extensive N-H 0=C a s s o c i a t i o n s between neighboring chains. Thus, one would ^ a l s o expect that the C=0 s t r e t c h i n g band at 1656 cm" w i l l respond s i m i l a r l y under the a p p l i e d shear f o r c e s . Indeed, t h i s band shows trje same trend i n the i n t e n s i t y changes as that at 3327 cm" . This i s i l l u s t r a t e d i n Figure 3, A, with the Y-scale from 0 to 250 to the f a r l e f t . In an effort to further relate the molecular deformations of PPTA f i b e r s upon t h e i r elongation, the i n t e g r a t e d i n t e n s i t i e s of both bands were p l o t t e d as a f u n c t i o n of percent f i b e r e l o n g a t i o n . As seen i n Figure 3, B, (the corresponding Y-scale from 0 to 7000 i s on the r i g h t ) , the i n t e n s i t i e s of both bands increase as the f i b e r i s s t r e t c h e d . The i n t e n s i t y decreases, however, when the sample breaks, i n d i c a t i n g that t h i s band i s s e n s i t i v e to the shear f o r c e s i n v o l v e d when the f i b e r i s elongated. I t i s a l s o i n t e r e s t i n g to note that the shapes of both curves depicted i n Figure 2, resemble a load versus percent e l o n g a t i o n curve obtained from an Instron mechanical analyzer (Figure 3, C, with the Y-scale to the inside l e f t ) . The i n t e n s i t y changes of the N-H and C=0 bands are mostly r e s p o n s i b l e f o r the i n t e r m o l e c u l a r f o r c e s between the polymer chains. Let us now determine how the polymer backbone i s a f f e c t e d by the e x t e r n a l s t r e t c h i n g f o r c e s . In order to do so, tjjie band due to the C-C aromatic s t r e t c h i n g at 1408 cm" and the C-N in-plane bending at 1261 cm modes w i l l be examined. Figure 4, traces A through F, d e p i c t s the rheo-photoacoustic s p e c t r a i n the 1450-1200 cm" region. S i m i l a r to the N-H and C=0 s t r e t c h i n g modes, the i n t e n s i t i e s of both bands increase as the f i b e r i s elongated. However, the maximum i n t e n s i t y i s reached at 1.25% elongation (trace D). This i s i l l u s t r a t e d i n Figure 5, t r a c e s A and B, which presents p l o t s of the i n t e g r a t e d i n t e n s i t i e s of both bands as a f u n c t i o n of e l o n g a t i o n . The behavior depicted i n Figure 5 i n d i c a t e s that the aromatic and C-N groups of the polymer backbone are a f f e c t e d i n the elongation region from 0.83% to 1.25 %. In c o n t r a s t , the N-H and C=0 groups are s e n s i t i v e virtually throughout the e n t i r e elongation process. Although the breakage region i s the same f o r a l l bands, the molecular processes leading up to breakage are detected l a t e r f o r the aromatic C-C and the C-N bands than f o r the N-H and C=0 bands. 1

9.

URBAN AND McDONALD


Figure 1. Schematic diagram of photoacoustic

detection.

3900 3700 3500 3300 3100 2900 2700 2500 1

Wavenumbers (cm"" ) Figure 2. Rheo-photoacousti^ FT-IR spectra of the PPTA f i b e r s i n the 4000-2500 cm" region a t various stages of e l o n g a t i o n : A - 0.0%; B - 0.42%; C -0.83%; D 1.25%; E - 1.67%; F - 2.08%.

156


0.0

0.5

1.0

1.5

2.0

2.5

3.0

Z ELONGATION Figure 3. Integrated i n t e n s i t i e s p l o t t e d as a f u n c t i o n of percent PPTA f i b e r e l o n g a t i o n : A - carbonyl band at 1656 cm" (Y-scale to the f a r l e f t ) ; B - N-H band at 3327 cm" (Y-scale to the f a r r i g h t ) ; C - load p l o t t e d as a f u n c t i o n of percent PPTA f i b e r elongation from the s t r e s s - s t r a i n mechanical t e s t e r (Y-scale to the inside l e f t ) .

35

— I

1440

1

1

1

1380

1

1

1

1320

1

1

1

1260 1

1

I

I

1200

Wavenumbers (cm- ) Figure 4. Rheo-photoacoustic FT-IR spectra of the PPTA f i b e r s i n the 1450-1200 cm" region at various stages of e l o n g a t i o n : A - 0.0%; B - 0.42%; C -0.83%; D 1.25%; E - 1.67%; and F - 2.08%.

9.

URBAN AND MCDONALD


A comparison of the N-H and C=0 bands with the aromatic C-C and the C-N bands with respect to d e t e c t i n g the onset of f i b e r breakage i s important because of the d i f f e r e n t c o n t r i b u t i o n s of each group to the chemical s t r u c t u r e of the PPTA f i b e r . Figure 6 shows the s t r u c t u r e of the PPTA f i b e r and i n d i c a t e s that the N-H and C=0 groups are side groups and, t h e r e f o r e , may p a r t i c i p a t e i n hydrogen bonding with the amide carbonyl and amide N-H groups of neighboring chains. On the other hand, the C-C aromatic s t r e t c h i n g band a r i s e s from 1:he aromatic r i n g of the polyimide backbone. The aromatic r i n g i s not capable of p a r t i c i p a t i n g i n the intermolecular i n t e r a c t i o n s such as hydrogen bonding, although i t does i n t e r a c t with the neighboring chains through 11-11 i n t e r a c t i o n s . Like the aromatic C-C bonds, the C-N bond i s an i n t e g r a l part of the backbone and should not be a p p r e c i a b l y a f f e c t e d by the hydrogen bonding. In view of the above c o n s i d e r a t i o n s , the a n a l y s i s of the i n t e g r a t e d i n t e n s i t i e s of the N-H and C-C normal v i b r a t i o n s i n d i c a t e s that while the i n t e n s i t i e s of the N-H band (Figure 3, B) continuously change with the a p p l i e d load, the C-C aromatic band (Figure 5, A) i s only s e n s i t i v e when the elongation reaches 0.83%. Above 1.25% elongation, the i n t e n s i t y of t h i s band remains v i r t u a l l y unchanged. This behavior may suggest the separation of II o r b i t a l s of the two neighboring r i n g s or conformational changes due to s t r e s s e s imposed on the f i b e r . Although one could propose other r e l a t e d phenomena, such as a movement of c r y s t a l l i t e s with respect to each other, or deformations w i t h i n the c r y s t a l l i n e regions, at this point there i s no evidence as to which part of the f i b e r c o n t r i b u t e s to the observed phenomenon. ^ As was reported in earlier studies, extensive hydrogen bonding between N-H and C=0 groups of neighboring chains contributes s i g n i f i c a n t l y to the mechanical i n t e g r i t y of the polyaramid (Kevlar ) f i b e r . Thus, during the s t r e t c h i n g process, an equilibrium between H-bonded and non-H-bonded groups w i l l s h i f t i n the d i r e c t i o n of the non-bonded s p e c i e s . The d i s s o c i a t i o n of hydrogen bonds w i l l enhance a c o u s t i c waves generated as a r e s u l t of i n f r a r e d reabsorption at the energy l e v e l s required to d i s r u p t the bonding. As a result, the i n t e n s i t y of the N-H and C=0 bands w i l l i n c r e a s e . As i l l u s t r a t e d i n Figure 3, the i n t e n s i t y changes i n the elongation p r o f i l e reach a maximum and f u r t h e r elongation r e s u l t s i n the f i b e r breakage. Upon breakage, however, the intensities of the N-H and C=0 bands slightly decrease i n d i c a t i n g that a new H-bonded-non-H-bonded e q u i l i b r i u m has been e s t a b l i s h e d and upon load r e l e a s e , a fraction of the original H-bonded species has been reformed. Such behavior i s not observed f o r the C-C aromatic and the C-N bands i n the e l o n g a t i o n a l region from 1.25% to 2.08%; t h e i r i n t e n s i t i e s remain v i r t u a l l y unchanged. R

158


600

z 100T i—i

QI

0.0

'

0.5

'

1.0

1

1.5

'

3.0

1

2.5

I FIBER EXTENSION

Figure 5. Integrated i n t e n s i t i e s p l o t t e d as a f u n c t i o n of percent PPTA f i b e r e l o n g a t i o n : A - C-C aromatic bancl at 1408 cm" ; B - the C-N-C amide band at 1261 cm" .

Figure 6. S t r u c t u r e of the PPTA f i b e r with hydrogen bonding between amide groups of neighboring PPTA chains.


9. URBAN AND MCDONALD

In order to f u r t h e r c o r r e l a t e the response of various f u n c t i o n a l groups to the a p p l i e d s t r e s s , the bond d i s s o c i a t i o n energies of the f i b e r f u n c t i o n a l groups were compared. One would expect the weakest bonds to break f i r s t , followed by the next weakest and so on, u n t i l the f i b e r sample i s s t r e s s e d to the breakpoint. The hydrogen bonds are the weakest bonds present i n the molecule and thus, d i s s o c i a t e f i r s t . The d i s s o c i a t i o n e n e r p ^ g AH, of the hydrogen bonds range from 17 - 21 kJ/mole. ' Once a f r a c t i o n of the hydrogen-bonded amide groups p a r t i a l l y dissociate, the N-H and C=0 groups will not be a p p r e c i a b l y a f f e c t e d by the e l o n g a t i o n a l f o r c e s . However, the C ( )-N and C ( )-C(^(where " r " i n d i c a t e s the aromatic r i n g carbon and "o" the carbonyl carbon) bonds of the polymer backbone e x h i b i t d i s s o c i a t i o n energies of 300 and 27 5 kJ/mole, r e s p e c t i v e l y . The N-C^ ; bond of the amide groups i s r e l a t i v e l y strong with a AH = 305 kJ/mole. The C-C bonds of the aromatic r i n g s are stronger s t i l l , with a d i s s o c i a t i o n energy of 720 kJ/mol. Thus, a comparison of the dissociation energies further supports the a n a l y s i s of the rheo-photoacoustic data i n d i c a t i n g that the rate of d i s s o c i a t i o n of hydrogen bonds at the i n i t i a l stages of s t r e t c h i n g i s high, followed by the deformation of the backbone components. At t h i s point i t i s necessary to r a i s e the question as to why the i n t e n s i t y increases as the f i b e r i s elongated followed by the decrease when the f i b e r breaks. If upon f i b e r deformation only s e l e c t e d bands changed i n t e n s i t i e s , t h i s e f f e c t would have been a t t r i b u t e d only to the s t r u c t u r a l changes w i t h i n the sample. However, i n the case of rheo-photoacoustic measurements, the m a j o r i t y of the i n f r a r e d bands are being enhanced, but the changes appear at d i f f e r e n t r a t e s . This behavior i n d i c a t e s that each v i b r a t i o n a l band has d i f f e r e n t s e n s i t i v i t y to the deformation process. In an e f f o r t to understand f u r t h e r what p h y s i c a l p r o p e r t i e s are r e s p o n s i b l e f o r the observed changes, we w i l l i s o l a t e two processes that occur during the photoacoustic d e t e c t i o n and simultaneous e l o n g a t i o n of f i b e r s . F i r s t , we w i l l consider "an o r d i n a r y " PA FT-IR spectrum r e s u l t i n g from absorption of l i g h t and heat r e l e a s e generating a c o u s t i c waves, and the second, caused by the s t r e t c h i n g process, which r e s u l t s i n the bulk modulus and p o s s i b l y thermal property changes caused by the e l o n g a t i o n process. Let us represent the unstretched f i b e r spectrum as P', and designate P" to d e s c r i b e the c o n t r i b u t i o n to the o v e r a l l spectrum, P, a r i s i n g from the s t r e t c h i n g process. Therefore, r

r

0

P = P' + P"

(1)

While P* depends only on the o p t i c a l and thermal p r o p e r t i e s of the m a t e r i a l , P" w i l l be a f f e c t e d by emission of a c o u s t i c waves due to bond breakage which, i n

160


turn, w i l l a f f e c t the thermal property changes as a result of e l o n g a t i o n . Since a l l measurements were performed s t a t i c a l l y , the emission of a c o u s t i c waves i s relatively fast and i s not as r e a d i l y detectable. However, as a r e s u l t of e l o n g a t i o n , thermal p r o p e r t i e s of the polymer change. Therefore, we w i l l take advantage of t h i s pheonomenon and des^c^ribe the amount of heat t r a n s f e r r e d to the surface by:

H(x)=

P

exp

I

Q

(

1

-

[

Sound and Vibration Damping with Polymers - American Chemical

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