Effect of Absorber Concentrations on IR Reflection-Absorbance of

9

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Effect of Absorber Concentrations on IR Reflection-Absorbance of Polymer Films on Metallic Substrates JOHN D A Y WEBB, G A R Y JORGENSEN, PAUL SCHISSEL, and A. W. CZANDERNA Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401 A. R. CHUGHTAI and D. M. SMITH University of Denver, Department of Chemistry, Denver, CO 80208

This paper examines the relationships between the concentration of functional groups and observable infrared reflection-absorbance (IR-RA) spectral chan ges and applies the findings to bisphenol-A polycar bonate (BPA-PC) on gold substrates. The paper then notes the importance of studying the mechanisms of photochemically and catalytically enhanced degradation at polymer-metal (oxide) interfaces for solar energy collection and gives the rationale for using the IR-RA technique for these studies. An apparatus is describ ed which applies Fourier transform spectroscopy (FT-IR) for the IR-RA studies. An expression is developed showing the independence of the real and imaginary parts of the complex refractive index (Ν = n - ik) at the fundamental oscillator frequency when the frequency distribution of k can be approxi mated by a Lorentzian distribution. The values of k and n were determined for the polymer between 3200 and 950 cm using an iterative calculation and experimen tally determined reflectance spectra. From the opti cal constants determined for each functional group in BPA-PC, the sensitivity of the IR-RA measurement to changes in the functional group concentration was calculated for polymer thicknesses ranging from 10 nm to 10 μm. The optimum sensitivity is generally found at film thickness between 0.1 and 1 μm. At greater thicknesses, IR-RA spectra may be insensitive to changes in functional group concentration, and at thicknesses below 10 nm, the sensitivity is limited by instrumental signal-to-noise ratios. -1

An understanding of photodegradative reactions of polymers and the extent to which these reactions are influenced by a polymer/ metal (oxide) interface is important for applications involving 0097-6156/83/0220-0143$07.25/0 © 1983 American Chemical Society Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


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SOLAR ENERGY UTILIZATION

the use of polymers to p r o t e c t m e t a l l i c surfaces from d e t e r i o r a t i o n i n outdoor s e r v i c e . Many such a p p l i c a t i o n s e x i s t or are proposed f o r s o l a r energy conversion systems [ 1 ] . For example, polymers have been used i n attempts to p r o t e c t r e f l e c t o r surfaces from exposure to sources of environmental degradation such as u l t r a v i o l e t l i g h t , temperature, atmospheric gases, moisture, and abrasive p a r t i c l e s , w i t h the o b j e c t i v e of maintaining the s o l a r weighted r e f l e c t a n c e of the u n i t at a constant value. A polymer used to encapsulate a p h o t o v o l t a i c device may be i n contact w i t h s e v e r a l d i f f e r e n t surfaces i n c l u d i n g a semiconductor, g r i d metall i z a t i o n , i n t e r c o n n e c t i o n s , a n t i r e f l e c t i o n c o a t i n g , and cover p l a t e c o n s i s t i n g of g l a s s or another polymer. To p r o t e c t the s u b s t r a t e i n a l l cases, the bulk p r o p e r t i e s of the polymeric c o a t i n g must be s t a b l e w i t h respect to u l t r a v i o l e t (UV) r a d i a t i o n , temperature, moisture, and atmospheric gases. However, the p o l y meric c o a t i n g may i t s e l f c o n t r i b u t e to an a c c e l e r a t e d a t t a c k on the protected m a t e r i a l s i f degradative i n t e r f a c i a l reactions change i t s d e s i r a b l e p r o p e r t i e s . Delamination at the polymer/ metal i n t e r f a c e and/or the c o n c e n t r a t i o n of r e a c t i v e gases on the "protected s u r f a c e " could be a l i k e l y r e s u l t of such r e a c t i o n s . I t i s known that thermally induced o x i d a t i v e degradation of cert a i n polymers i s a c c e l e r a t e d when they are placed i n contact w i t h copper (oxide) surfaces [2,3,4]. What i s not known i s whether s i m i l a r c a t a l y t i c or s y n e r g i s t i c e f f e c t s r e s u l t when the polymer i s exposed to u l t r a v i o l e t r a d i a t i o n w h i l e i n contact w i t h s i l v e r , aluminum, or other m e t a l l i c (oxide) s u r f a c e s . R e a l i s t i c s t u d i e s of bulk and i n t e r f a c i a l degradation of polymers f o r outdoor a p p l i c a t i o n s must be concerned w i t h the e f f e c t s of UV r a d i a t i o n , temperature, temperature c y c l i n g , and atmospheric gases [ 5 ] . Experimental methods used to measure these e f f e c t s should be s e n s i t i v e to small chemical changes i n the samples to a l l o w examination of e a r l y degradative events and to perform the experiment i n a reasonably short time. The method should permit these samples to be s t u d i e d i n a c o n f i g u r a t i o n that c l o s e l y resembles t h e i r intended a p p l i c a t i o n . The r e s u l t s o b t a i n ed should be r e p r o d u c i b l e and comparable to those obtained by outdoor exposure of s i m i l a r samples. F i n a l l y , the a n a l y t i c a l procedure should not i t s e l f c o n t r i b u t e to sample d e t e r i o r a t i o n and should be r a p i d compared to the events being observed. Infrared r e f l e c t i o n - a b s o r p t i o n (IR-RA) spectroscopy was the primary technique chosen to o b t a i n a n a l y t i c a l i n f o r m a t i o n on polymer/metallic samples subjected to degradative c o n d i t i o n s s i n c e the method i s s e n s i t i v e and nondestructive [6,7]. The placement of a s p e c i a l l y designed c o n t r o l l e d environment exposure chamber (CEEC) i n t o a F o u r i e r transform i n f r a r e d spectrophotometer (FT-IR) makes p o s s i b l e the technique of IR-RA spectroscopy w i t h c o n t r o l of degradative parameters i n s i t u [8] . Thus, the equipment can c o l l e c t low-noise s p e c t r a r a p i d l y , enabling c o n t i n u a l examination of the polymer/metal sample during exposure to s e v e r a l degradative parameters. The CEEC allows simultaneous c o n t r o l of UV exposure

Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


9.

WEBB ET AL.

IR Reflection-Absorbance of Polymer Films

145

i n t e n s i t y , sample temperature, and the composition o f gases surrounding a p a r t i c u l a r sample. C o l l e c t i o n o f IR-RA s p e c t r a takes p l a c e w i t h the sample i n s i t u . The IR-RA, FT-IR method depends on extreme s e n s i t i v i t y t o s m a l l chemical changes i n a given sample so that the study can be b r i e f , and so t h a t only modestly a m p l i f i e d exposures t o environmental parameters are necessary t o produce detectable degradation. The purpose o f t h i s work i s t o demonstrate the u t i l i t y of the method both through t h e o r e t i c a l c a l c u l a t i o n s o f the r e l a t i o n s h i p between changing polymer f u n c t i o n a l i t y and observable IR-RA spect r a l changes, and through p r e s e n t a t i o n o f experimental r e s u l t s obtained using the apparatus mentioned above. The method has been used t o study s e v e r a l types o f polymer f i l m s on v a r i o u s I R - r e f l e c t i v e s u b s t r a t e s ; however, we s h a l l use only our c a l c u l a t i o n s f o r f i l m s o f bisphenol-A (BPA) polycarbonate on gold and aluminum substrates t o demonstrate the p o t e n t i a l of t h i s experimental approach. BPA polycarbonate (BPA-PC) was chosen f o r i n i t i a l study s i n c e i t s bulk photochemistry i s f a i r l y w e l l known [9,10], thus p e r m i t t i n g the v a l i d i t y o f t h i s i n s i t u s p e c t r o s c o p i c method t o be t e s t e d . Gold was chosen as a s u b s t r a t e on the assumption t h a t i t Is a c h e m i c a l l y i n e r t IR r e f l e c t o r , thus f a c i l i t a t i n g s t u d i e s o f the bulk photochemistry of the m a t e r i a l . Aluminum i s a l i k e l y candidate f o r r e f l e c t o r a p p l i c a t i o n s and i s more l i k e l y than g o l d to show i n t e r f a c i a l a c t i v i t y w i t h respect t o the polymeric coating. The t h e o r e t i c a l p o r t i o n o f t h i s paper provides the b a s i s f o r a d e f i n i t i o n o f optimum sample t h i c k n e s s and p o l a r i z a t i o n s t a t e o f IR r a d i a t i o n f o r the study of degradative changes i n the major f u n c t i o n a l groups of BPA-PC. Experimental r e s u l t s , which have been presented elsewhere [ 8 ] , demonstrate the f e a s i b i l i t y of c o l l e c t i n g IR-RA s p e c t r a on a sample undergoing simultaneous exposure t o UV and a flow o f gases w h i l e m a i n t a i n i n g c o n t r o l o f the sample temperature. This c a p a b i l i t y represents an improvement over p r e v i o u s l y reported methods [11] which depend upon removing the sample from the spectrophotometer f o r i r r a d i a t i o n , r e s u l t i n g i n l o s s of c o n t r o l over the sample temperature, environment, and orientation. P r e s e r v a t i o n o f sample o r i e n t a t i o n i s p a r t i c u l a r l y important i f s p e c t r o s c o p i c accuracy i s t o be maintained i n t h e IR-RA experiment.

Theoretical Basis for the IR-RA Experiment Any technique having u t i l i t y f o r the study of r e a c t i o n k i n e t i c s must i n c l u d e the c a p a b i l i t y of measuring an experimental v a r i a b l e which i s r e l a t e d t o the c o n c e n t r a t i o n o f the species o f i n t e r e s t as a f u n c t i o n of time. The p r o p o r t i o n a l i t y of IR-absorbance t o c o n c e n t r a t i o n o f an absorbing f u n c t i o n a l i t y , which i s summarized i n Beer's law A(v) = e(v)bc, i s g e n e r a l l y accepted. When the measurement i s performed on a sample dispersed a t low



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c o n c e n t r a t i o n i n a nonabsorbing medium, i n t e r f a c i a l r e f l e c t i o n e f f e c t s are low and a l i n e a r r e l a t i o n s h i p between absorbance and species c o n c e n t r a t i o n i s o f t e n obtained. A more complex r e l a t i o n s h i p e x i s t s between IR-RA values and f u n c t i o n a l group concentra t i o n i n an absorbing f i l m on a r e f l e c t i v e s u b s t r a t e . Allara et a l . [12] showed that the frequencies of band maxima observed i n IR-RA spectra do not n e c e s s a r i l y correspond to the fundamental v i b r a t i o n a l frequencies because of the i n f l u e n c e s of anomalous d i s p e r s i o n near the fundamental frequencies on the surface r e f l e c t i o n measurement. T h e o r e t i c a l i n v e s t i g a t i o n s of the dependence of IR-RA i n t e n s i t i e s and frequencies on the o p t i c a l constants d e f i n i n g the r e f r a c t i v e index Ν = η - i k of the m a t e r i a l s being c h a r a c t e r i z e d were reported by Greenler [13,14]. A s i m p l i f i e d ray diagram of the IR-RA experiment f o r a s i n g l e t h i n f i l m on a r e f l e c t i v e s u b s t r a t e i s shown i n F i g . 1 [13]. Greenler [13] obtained a set of four complex equations ( F r e s n e l equations) d e s c r i b i n g the magnetic and e l e c t r i c f i e l d s at each i n t e r f a c e f o r a given angle of i n c i d e n c e , assuming p a r a l l e l or perpendicular p o l a r i z a t i o n . He a l s o pre sented evidence f o r the i n f l u e n c e of s u p e r s t r a t e f i l m t h i c k n e s s and IR incidence angle on the i n t e n s i t i e s of IR-RA s p e c t r a l peaks. A s i m i l a r set of F r e s n e l equations was u t i l i z e d by Toralin [15] f o r a n a l y s i s of the r e f l e c t a n c e of m u l t i l a y e r stacks of absorbing and/or r e f l e c t i v e m a t e r i a l s . We have used a comput e r i z e d s o l u t i o n of the l a t t e r set of F r e s n e l equations to p r e d i c t IR-RA values f o r r a d i a t i o n p o l a r i z e d p a r a l l e l ( p ) , perpendicular ( s ) , or e l l i p t i c a l w i t h respect to the plane of i n c i d e n c e . Calculâtion of Refleetion-Absorbance. Two d i f f e r e n t methods f o r c a l c u l a t i n g the reflectance-absorbance (RA) of a t h i n f i l m on a r e f l e c t i v e s u b s t r a t e have been reported. Greenler [13] used the relationship RA = ( R

-

Q

R)/R

(1)

Q

where R i s the measured r e f l e c t a n c e of the polymer-coated metal i n a i r at a given frequency and R i s the measured r e f l e c t a n c e of the polymer-coated metal c a l c u l a t e d w i t h the assumption that the f i l m i s nonabsorbing (k2 = 0 ) . In p r a c t i c e , experimental values of R are obtained from the r e f l e c t i o n spectrum of an uncoated metal sample s i m i l a r to that used to produce the laminate. The form expressed i n Eq. 1 then becomes analogous to a transmittance measurement. A l l a r a et a l . [12] used the form Q

g

RA = - l o g ( R / R ) 1 0

(2)

o

which i s analogous to a measured absorbance. o u s l y the l e a d i n g term of an expansion of Eq.

Equation 1 i s o b v i 2.



9.

WEBB ET AL.


/" -

1

/

Air

s ,E / V / E 2

\

Figure 1 ·

\

n -ik Polymer Film 2

2

\

y s ^ \ \

2

J Chem. Phys.

Ray Diagram of the IR-RA Experiment for a PolymerCoated Metal* Adapted from work by Greenler [13]. The s u b s c r i p t s 1, 2 and 3 on the o p t i c a l constants correspond t o the electromagnetic wave i n a i r , polymer f i l m , and metal, r e s p e c t i v e l y .

American Chemical Society Library 1155 16th st. N. w. Washington, 0. C. 20036 Gebelein et al.; Polymers in Solar Energy Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


148

POLYMERS IN


Relative Concentration of Absorbing Species* The r e l a t i o n s h i p between η and k i n Ν = n - i k i s of concern i n RA experiments s i n c e some of the IR r a d i a t i o n r e f l e c t e d from the polymer-coated metal may have been r e f l e c t e d from the polymer/air i n t e r f a c e . Thus, IR-RA measurements of r e l a t i v e changes i n c o n c e n t r a t i o n (roughly p r o p o r t i o n a l to changes i n k) w i l l g e n e r a l l y be i n f l u enced by a s s o c i a t e d changes i n η w i t h a r e s u l t a n t l o s s of accu racy. However, there are exceptions to t h i s g e n e r a l i z a t i o n which may be e l u c i d a t e d by an examination of the mathematical r e l a t i o n ship between η and k expressed i n the Kramers-Kronig rela t i o n s [16]. A l l a r a et a l . [12] presented a s i m p l i f i e d v e r s i o n r e l a t i n g η and k, i . e . ,

where n^ represents the c o n t r i b u t i o n of a l l other o s c i l l a t o r s i n the sample to the value of η and i s u s u a l l y evaluated o u t s i d e the band of the o s c i l l a t o r under c o n s i d e r a t i o n d e f i n e d by and V2 but away from other o s c i l l a t o r frequencies and where Ρ denotes the Cauchy p r i n c i p a l v a l u e . Equation 3 i s based on two assumptions: the f i r s t i s t h a t the v i b r a t i o n a l mode having fundamental frequen cy can be modeled as an independent o s c i l l a t o r . The second assumption i s that ν » (ν - V ) i n the range of frequencies i n c l u d e d In the a b s o r p t i o n band (v^ t o 2^» where v i s the f r e quency w i t h i n t h i s band a t which η i s evaluated. The conventions i m p l i c i t i n Eq. 3 are t h a t < v < v and v^^ < v < ν· The i n t e g r a l i n Eq. 3 may be evaluated by assuming t h a t the k frequency spectrum w i t h i n the a b s o r p t i o n band may be approximated by a L o r e n t z i a n d i s t r i b u t i o n : v

m

a

x

q

V

Q

Q

k

(

V

2

) = I π

(v - v

m a x

HI )2

m a x

2

(4) +

(

Γ/2)2

W

v

where Γ i s the f u l l width a t h a l f height ^ ( ) / 2 of the d i s tribution. Equation 3 can be i n t e g r a t e d to y i e l d Eq. 5 by u s i n g the d e f i n i t i o n of the Cauchy p r i n c i p a l value [16] and by i n s e r t i n g Eq. 4 i n t o Eq. 3, i . e . , m a x

v

n( max) =

n

~

+

Γ

I ^

V

(v

+

2

- v

m a x

)2J (5)

\A

2

where the s u b s t i t u t i o n s A = (Γ/2)

+

(V!

and v

-

Q

v )2)\ maK

= v

m a x

were made p r i o r t o


9.

WEBB ET AL.

149

IR Reflection-Absorbance of Polymer Films ν

the i n t e g r a t i o n . I t can be seen from Eq. 5 that η ( ) = η» f o r A « (ν - v ) and A « ( v - ν ) . For an i n t e g r a t i o n i n t e r v a l symmetric about v , i . e . , ( v - v ^ / = (ν - v ^ ) ' , Eq. 6 holds: Μ χ

2

χ

m a x

2

m a x


n(v

m a x

2

) =n

χ

(6)

M

For the i s o l a t e d o s c i l l a t o r that we are c o n s i d e r i n g , the v a l u e of η i s independent of the value of k a t the fundamental o s c i l l a t i o n frequency. This i s shown f o r the i d e a l i z e d k - η s e t s p l o t ted i n F i g . 2. As the a b s o r p t i v i t y k decreases, the values f o r η change everywhere except a t the fundamental o s c i l l a t i o n frequency ν . This i m p l i e s that the f r a c t i o n of the IR beam which i s r e f l e c t e d from the polymer/air i n t e r f a c e w i l l vary as a f u n c t i o n of changes i n both k and the a s s o c i a t e d changes i n η a t every frequency except the fundamental o s c i l l a t i o n frequency. The surface r e f l e c t i o n component can make a s i g n i f i c a n t c o n t r i b u t i o n to the t o t a l r e f l e c t e d IR s i g n a l (vector E j ~ i n F i g . 1 ) , e s p e c i a l l y when unpolarized IR i n c i d e n t r a d i a t i o n i s used. Therefore, s i n c e an unbiased measurement o f changes i n f u n c t i o n a l i t y concen t r a t i o n (changes i n k) i s d e s i r e d , the measurement of absorbance should be performed at a frequency where η i s independent of k. The above treatment shows that f o r an i d e a l i z e d o s c i l l a t o r , t h i s c o n d i t i o n i s met at the fundamental frequency, where s e n s i t i v i t y t o changes i n c o n c e n t r a t i o n (or k) i s a l s o h i g h e s t . The fundamental frequencies may be determined e x p e r i m e n t a l l y e i t h e r by o b t a i n i n g the k spectrum u s i n g p o l a r i m e t r y o r by o b t a i n i n g the transmittance spectrum. Data r e s u l t i n g from a p p l i c a t i o n o f both techniques to BPA-PC w i l l be presented below.

Determination Polycarbonate

of Optical

Constants

i n the IR f o r Blsphenol-A

The purpose of o b t a i n i n g s p e c t r a l data on the complex r e f r a c t i v e index of polycarbonate polymer was to permit d e t a i l e d i n t e r p r e t a t i o n s t o be made o f the IR-RA spectra c o l l e c t e d i n s i t u on metal-backed f i l m s o f t h i s m a t e r i a l . Several o f the p r i n c i p a l methods f o r o b t a i n i n g the o p t i c a l constants η and k of an i s o t r o p i c medium have been reviewed by Humphreys-Owen [18]. A l l o f the methods o u t l i n e d are i n s e n s i t i v e to k when k i s c l o s e t o zero, which i s the case f o r frequencies between absorbance bands. For t h i s study, a p o l a r i m e t r i c technique (method D i n Ref. 18) was chosen t o o b t a i n the o p t i c a l constants o f BPA-PC. To apply t h i s method, the r a t i o o f surface r e f l e c t a n c e s Rp/R a t two l a r g e , but w e l l - s e p a r a t e d , angles o f incidence ( 6 ) was obtained f o r BPA-PC i n the IR. R^ i s defined as the r e f l e c t a n c e measured f o r a sample using r a d i a t i o n p o l a r i z e d p a r a l l e l t o the incidence plane and R s

i

g



150

POLYMERS IN SOLAR ENERGY UTILIZATION

Figure 2.

Plots of η and k for Strong (s) and Weak (w) Absorption for Idealized Oscillators. (Adapted from Nussbaum [17])


9.

WEBB ET AL.

151


i s the r e f l e c t a n c e measured u s i n g l i g h t p o l a r i z e d perpendicular t o t h i s plane. Measurement o f , which i s the r a t i o o f R / R , a t two d i f f e r e n t values o f θ ( i = 1 o r 2) y i e l d s two equations w i t h o n l y η and k unknown. However, the r e s u l t i n g two equations cannot be solved e x p l i c i t l y f o r η and k [12] . To estimate η and k, an i t e r a t i v e c a l c u l a t i o n i s used that proceeds from t r i a l values f o r η and k, values f o r the incidence angles used, and the correspon ding measured values f o r the r e f l e c t a n c e r a t i o s , R0i · Using the i n i t i a l t r i a l values f o r η and k, estimates f o r R Q ^ and RQ? c a l c u l a t e d a t both angles of i n c i d e n c e , and the d i f f e r e n c e s ARQ^ between the c a l c u l a t e d and measured values are computed. Correc t i o n f a c t o r s Δη and Ak are then computed u s i n g G


A R E

where i = 1 or 2. The c o r r e c t i o n f a c t o r s are added to the current t r i a l values f o r η and k, and the r e v i s e d values become the t r i a l values f o r the next i t e r a t i o n . Convergence i s assumed t o have occurred when the sum o f the absolute values o f Δη and Ak f a l l s below 0.005. According t o A l l a r a et a l . [12], the r a t i o I / I o f detectedt o - i n c i d e n t r a d i a t i o n may be r e l a t e d approximately t o the absor bance c o e f f i c i e n t k by assuming that the a b s o r p t i o n band i s w e l l i s o l a t e d . Thus, the r a t i o becomes Q

I/I

0

=£ -

4 7 r k b v

e

(8)

where b i s the o p t i c a l path l e n g t h (cm) f o r a b s o r p t i o n and ν i s the frequency expressed i n wavenumbers. This expression may be combined w i t h the Beer-Lambert absorbance law I/I

£

0

= 10- bc

(9)

to y i e l d an approximate r e l a t i o n s h i p between k, the d e c i d i c molar a b s o r p t i v i t y ε, and the molar c o n c e n t r a t i o n c o f the absorbing species i n the s o l i d polycarbonate matrix. By combining Eqs. 8 and 9, k can be expressed as , * 2.303 sc k 4πν

(10)

Molar a b s o r p t i v i t i e s may be c a l c u l a t e d from a b s o r p t i o n s p e c t r a o f compounds d i s t r i b u t e d u n i f o r m l y at w e l l - d e f i n e d concentrations i n c e l l s of known path l e n g t h , assuming that Beer's Law i s obeyed.


152



Experimental Apparatus and Procedures The major pieces of apparatus used t o perform the abbreviated exposure experiment c o n s i s t of an FT-IR spectrophotometer ( N i c o l e t 7199) w i t h a DTGS d e t e c t o r , a c o n t r o l l e d environmental chamber (CEEC) w i t h monitoring system and gas supply, and a s o l a r simulat o r . The polymer-coated m i r r o r sample i s mounted i n s i d e the CEEC, which i s mounted i n s i d e the sample compartment of the FT-IR. Details of the instrumentation have been presented elsewhere [ 8 ] . A schematic of the CEEC i s shown i n F i g . 3. To o b t a i n the r e f l e c t a n c e measurements necessary t o determine the o p t i c a l constants of BPA-PC, a specular r e f l e c t a n c e accessory ( H a r r i c k S c i e n t i f i c Co., Part No. VRA-RMA) was i n s t a l l e d i n the r e a r IR beam of the FT-IR. This v a r i a b l e - a n g l e accessory was f i t t e d w i t h a kinematic mounting base and a spring-loaded sample holder t o increase r e p r o d u c i b i l i t y of sample p o s i t i o n i n g . The l a t t e r change r e s t r i c t e d the v a r i a b i l i t y o f a c t u a l incidence angle to a range of 77.5 t o 60 degrees i n the high-angle c o n f i g u r a tion. The incidence angles read from the angular s c a l e on t h i s instrument were c a l i b r a t e d t o t r u e values by u s i n g a l a s e r r e f e r ence beam and an aluminum m i r r o r a t the sample l o c a t i o n . A Perkin-Elmer w i r e - g r i d p o l a r i z e r c o n s i s t i n g of gold g r i d elements on an AgBr s u b s t r a t e was used i n c o n j u n c t i o n w i t h the specular r e f l e c t a n c e attachment. For polymers and other substances that are p a r t i a l l y transparent to the i n c i d e n t IR r a d i a t i o n , i n t e r n a l r e f l e c t i o n s may b i a s the measurement of surface r e f l e c t i v i t y . To reduce t h i s e f f e c t while maintaining p l a n a r i t y of the sample s u r f a c e , one surface of a 3-mm t h i c k sheet of BPA-PC was roughened using a 6 0 0 - g r i t emery c l o t h . The other surface of the sheet was prepared f o r IR surface a n a l y s i s by scrubbing w i t h a c o t t o n swab saturated with o p t i c a l - g r a d e e t h a n o l . Any IR r a d i a t i o n t h a t passes through such a sheet without being absorbed i s s c a t t e r e d randomly upon s t r i k i n g the rear surface i n a d d i t i o n t o being d i s p l a c e d l a t e r a l l y from the o p t i c a l path t o the d e t e c t o r by ~3 mm. Thus, r a d i a t i o n undergoing i n t e r n a l r e f l e c t i o n i n the t h i c k sheet has a very low p r o b a b i l i t y of reaching the d e t e c t o r and i n t e r f e r i n g w i t h the measurement o f surface r e f l e c t i o n . A reference r e f l e c t o r c o n s i s t i n g of aluminum, vacuum evaporated onto a 50 x 25 x 3 mm g l a s s o p t i c a l f l a t , was cleaned by c e n t r i f u g a l r i n s i n g w i t h ethanol and chloroform. Single-beam IR r e f l e c t i o n s p e c t r a of t h i s m i r r o r a t two angles of i n c i d e n c e , 76.4° and 61.4° were c o l l e c t e d and s t o r e d , each w i t h the p o l a r i z e r set a t zero and 90° from the v e r t i c a l , or with p o l a r i z a t i o n s perpendicular (s) and p a r a l l e l (p) t o the incidence plane, respectively. This procedure was a l s o used t o c o l l e c t the spectra f o r the BPA-PC sheet. To o b t a i n good s i g n a l - t o - n o i s e r a t i o s f o r the s p e c t r a c o l l e c t e d w i t h p a r a l l e l p o l a r i z a t i o n , data from 1000 scans of the i n t e r f e r o m e t e r were accumulated u s i n g a cooled MCT d e t e c t o r to y i e l d an averaged i n t e r f e r o g r a m which was then transformed i n t o a low-noise spectrum. S p e c t r a l r e s o l u t i o n used was 2 cm between



9.

WEBB ET AL.


UV In Figure 3.

Controlled Environment Chamber (CEEC) Showing IR Transfer Optics


153

POLYMERS IN

154


1

4000 and 400 cm" . The q u a n t i t i e s R and R were determined f o r the two incidence angles as a f u n c t i o n of frequency by d i g i t a l l y d i v i d i n g each of the four r e f l e c t i o n s p e c t r a of the polycarbonate sheet by the appropriate r e f l e c t i o n spectrum of the aluminum reflector. The two s p e c t r a l r a t i o s Rg^ (Eq. 7) were s i m i l a r l y generated from the four r e f l e c t i o n s p e c t r a , and converged values f o r η and k were c a l c u l a t e d . To o b t a i n the a b s o r p t i v i t y c o e f f i c i e n t s of the absorbing polymer f u n c t i o n a l i t i e s f o r use i n p r e d i c t i n g values of k, the IR a b s o r p t i o n spectrum of the s o l i d polymer was a l s o measured. A s o l u t i o n of BPA-PC was prepared at a c o n c e n t r a t i o n of 0.105 M i n reagent-grade chloroform. The a b s o r p t i o n spectrum of t h i s mater i a l was measured over the range 4000 - 400 cm at 2 cm resolu t i o n , using a c e l l having an e f f e c t i v e path l e n g t h of 0.0133 cm as determined by measurement of i n t e r f e r e n c e f r i n g e widths. For q u a l i t a t i v e comparison to the k s p e c t r a obtained by t h i s method, the IR spectrum of BPA-PC was a l s o measured using KBr d i s c s to reduce i n t e r f e r e n c e f r i n g e s and solvent a b s o r p t i o n e f f e c t s . The m a t e r i a l s were dispersed a t ~5% c o n c e n t r a t i o n i n dry KBr by s o l vent evaporation, subsequent g r i n d i n g of the blend, and p r e s s i n g into discs. P l o t s of the η and k s p e c t r a and the absorbance spectrum f o r BPA-PC are shown i n F i g s . 4 and 5, r e s p e c t i v e l y .


p

g

Results and Discussion In t h i s s e c t i o n , the s p e c t r a l o p t i c a l constants measured f o r BPA-PC i n the IR are presented f i r s t . Using these d a t a , the r e l a t i o n s h i p s between IR-RA absorbance values and BPA-PC f u n c t i o n a l i t y c o n c e n t r a t i o n were determined f o r a range of a p p l i c a b l e f i l m thicknesses u s i n g an o p t i c a l model of the IR-RA experiment. Optical Constants of Bisphenol-A Polycarbonate i n the Infrared. The s p e c t r a l values obtained f o r η and k of BPA-PC between 2000 and 950 cm" are p l o t t e d i n F i g . 4. In the 32002800 cm r e g i o n , which encompasses the C-H s t r e t c h i n g r e g i o n of the spectrum (not shown), the expected v a r i a t i o n s i n η and k were too small to be d e t e c t a b l e above the l e v e l of random n o i s e . A constant value of η -1.39 was measured w i t h i n the higher wavenumber r e g i o n . From the data presented i n F i g . 4, the l e v e l of random noise i n the determination was estimated at ±0.01 u n i t s f o r η and ±0.05 u n i t s f o r k. The g e n e r a l l y decreasing slope of the k spectrum between 2000 and 1300 cm may r e s u l t from a minor systematic e r r o r i n the measurement. Such an e r r o r could be caused by i n creased s c a t t e r i n g of IR r a d i a t i o n by the polymer sheet r e l a t i v e to that produced by the m e t a l l i z e d o p t i c a l f l a t used as a r e f l e c tance standard, or by minor misalignment of the sheet r e l a t i v e to the standard. An IR a b s o r p t i o n spectrum of the polycarbonate polymer used i n these determinations i s shown i n F i g . 5. A comparison can be made 1



Figure 4.

Real (η) and Imaginary (k) Parts of the Complex Refractive Index Calculated From Reflectance Measurements for Bisphenol-A Polycarbonate Film


I

*t

s

•S.

§

I

a

S'

S

>

W H

M W W


nnnl 4000

0.80

I 3600

» 2800

Figure 5 .

I 3200 I 2000

I 1600 1

Wave Number (cm )

I 1800

I 1400

I 1200

I 1000

I 800

Absorbance Spectrum of Lexan Polycarbonate Used for Refractive Index Determinations. The polymer was d i s p e r s e d i n a KBr d i s c a t c a . 5 wt %.

I 2400


I 600

I 400

9.

WEBB ET AL.

157


between t h i s spectrum and the k spectrum shown i n F i g . 4. The major a b s o r p t i o n bands a t 1778, 1505, 1230, 1195, 1165, and 1016 cm" correspond i n both shape and p o s i t i o n , but not necessar i l y i n r e l a t i v e i n t e n s i t i e s to the maxima observed i n the k spec trum a t 1775, 1505, 1230, 1195, 1165, and 1015 cm" , respec t i v e l y . In F i g . 5, the l e s s i n t e n s e a b s o r p t i o n bands at 2960 cm" and 1600 cm" do not have d i s t i n g u i s h a b l e counterparts i n e i t h e r the k o r the η spectrum o f F i g . 4, w h i l e the moderately strong a b s o r p t i o n band a t 1080 cm" has an a s s o c i a t e d v a r i a t i o n v i s i b l e i n the η spectrum o n l y . I t i s important to r e c a l l i n t h i s context that the i n t e n s i t i e s of the k s p e c t r a l peaks are f u n c t i o n s of frequency, as w e l l as o f a b s o r p t i v i t y c o e f f i c i e n t and f u n c t i o n a l i t y c o n c e n t r a t i o n , as demonstrated i n the d e r i v a t i o n of Eq. 10. In Table I , a comparison i s made between the k values c a l c u l a t e d u s i n g Eq. 10 and the measured maximum i n t e n s i t i e s ο those peaks i n the k spectrum ( F i g . 4) f o r which s t r u c t u r a l assignments i n the polycarbonate r e p e a t i n g u n i t may be made without ambi g u i t y . The c a l c u l a t e d k values were obtained (Eq. 10) u s i n g molar a b s o r p t i v i t y c o e f f i c i e n t s estimated from the IR absorbance spec trum of a BPA-PC s o l u t i o n i n chloroform. Absorptivity c o e f f i c i e n t s estimated i n t h i s manner are i n good agreement w i t h pub l i s h e d values [19]. S i m i l a r l y , the correspondence between mea sured and c a l c u l a t e d k values i s a l s o f a i r l y good. The value of k c a l c u l a t e d u s i n g IR absorbance f o r the r e l a t i v e l y weak methyl band at 2960 cm i s w e l l below the random noise l e v e l of ±0.05 u n i t s i n the k spectrum determined p o l a r i m e t r i c a l l y ; thus, i t i s not 1


1

Table I .

k

ο ,_,c

Comparison of k Values f o r Various Polycarbonate F u n c t i o n a l Groups Using Two D i f f e r e n t Measurement Methods Spectrum Maximum (cm )

ε, A b s o r p t i v i t y Coefficient (£. m o l cm )

Bulk Concentration (M)

k (from Absorbance Measurement)

k (from Reflectance Measurements)

1230

440

9.45

0.620

0.59

1195

360

9.45

0.521

0.59

1775

380

4.72

0.184

0.18

1505

144

9.45

0.166

0.19

44

9.45

0.025

0

A

-CH,

2960


-


158


s u r p r i s i n g that t h i s band i s not d i s t i n g u i s h a b l e i n the k spec trum. In u t i l i z i n g Eq. 10 to p r e d i c t approximate values f o r k, c o r r e c t assignment of the a b s o r p t i o n bands from which ε i s c a l c u l a t e d to the v a r i o u s polymer f u n c t i o n a l i t i e s i s o b v i o u s l y impor t a n t . For example, the two a b s o r p t i o n bands a t 1230 and 1195 cm were assigned t o the two d i s t i n g u i s h a b l e types of C-0 s i n g l e bonds ( i n s e t , F i g . 5) i n the polycarbonate r e p e a t i n g u n i t , on the b a s i s that the i n t e n s i t i e s of the a b s o r p t i o n peaks and those of the peaks i n the k spectrum ( c o r r e c t e d f o r frequency d i f f e r e n c e ) ( F i g s . 4 and 5) are n e a r l y equal. This should be the case f o r species having s i m i l a r e x t i n c t i o n c o e f f i c i e n t s present i n equal concentrations i n a polymer m a t r i x . The i n t e n s i t y of the peak a t 1165 cm"" i s s i g n i f i c a n t l y lower, e s p e c i a l l y i n the k spectrum, and i t may r e s u l t from a b s o r p t i o n by another f u n c t i o n a l group. Gupta [9] assigns t h i s a b s o r p t i o n to a C-C single-bond v i b r a t i o n . For an independent o s c i l l a t o r , the v a l i d i t y of Eq. 6 as a p p l i e d t o v a r i o u s o s c i l l a t o r s i n the polycarbonate r e p e a t i n g u n i t may be checked w i t h reference to the η spectrum presented i n F i g . 4. Since Eq. 6 was developed w i t h the assumption that the v i b r a t i o n a l frequency was w e l l separated from other s t r o n g a b s o r p t i o n f r e q u e n c i e s , an o s c i l l a t o r whose η spectrum conforms t o Eq. 6 may be considered to be s u f f i c i e n t l y i s o l a t e d to be spec t r a l l y independent of adjacent absorbances. In Table I I , a comparison i s made between t h e values o f η measured at ν ^ χ , w i t h values of n" and n i estimated from measurements of η a t frequencies higher and lower than > such t h a t dn/cLv 0. I t i s apparent from an examination o f Table I I that Eq. 6 holds s t r i c t l y only f o r the carbonyl v i b r a t i o n a t 1775 cm" and the aromatic v i b r a t i o n a t 1505 cm . The η ( ν ) value f o r the e s t e r v i b r a t i o n a t 1230 cm" a l s o f a l l s w i t h i n a range of values d e f i n e d by η»" and n» ; however, the width of t h i s v

m a x

v

m a x

1

χη&χ

1

+

Table I I . Comparison o f η Values Measured a t t h e Fundamental V i b r a t i o n a l Frequencies t o Values Estimated f o r η

V max 1015 1165 1195 1230 *1505 *1775 *n(v

max

n(v v

) max v

1.575 1.694 1.652 1.449 1.388 1.403

7

l

tloo"

950 1110 1110 1110 1450 1650

1.523 1.590 1.590 1.590 1.409 1.415

v

v

2

1055 1450 1450 1450 1660 2000

1.558 1.409 1.409 1.409 1.415 1.402

) = η «



9.

WEBB ET AL.

IR Reftection-Absorbance of Polymer Films

159

range precludes an unambiguous d e f i n i t i o n o f η» ( F i g . 4) · We i n f e r that only the carbonyl and aromatic bands i n the IR-RA spectrum are s u f f i c i e n t l y separated from other a b s o r p t i o n bands t o enable independent c o n c e n t r a t i o n determinations of these polymeric f u n c t i o n a l groups to be made v i a RA spectroscopy. Even measure ments made a t these frequencies may be biased by appearance o f nearby absorbances r e l a t e d t o the buildup o f r e a c t i o n products during the course of i r r a d i a t i o n . The l a r g e v a r i a t i o n s i n η observed a t frequencies near the fundamental frequencies and the r e l a t i o n s h i p between η and k expressed i n Eq. 3, w i l l c l e a r l y i n f l u e n c e attempts t o measure changes i n concentrations o f f u n c t i o n a l groups ( i . e . , changes i n k) using the RA technique. As demonstrated i n the d e r i v a t i o n of Eq. 6, changes In k can be measured independently of the a s s o c i ated changes i n η only a t a fundamental v i b r a t i o n a l frequency. Since the value o f η changes r a p i d l y as a f u n c t i o n of frequency near ^ accurate determinations of are necessary f o r unbiasea measurements o f changes i n k t o be made. Since the i n f l u e n c e of surface r e f l e c t i o n s on p o s i t i o n s of the s p e c t r a l maxima i s low at normal i n c i d e n c e , the frequencies o f the o s c i l l a t o r s (k maxima) may be determined f a i r l y a c c u r a t e l y from an IR a b s o r p t i o n spectrum of the m a t e r i a l to be s t u d i e d . The a s s e r t i o n of A l l a r a et a l . [12] t h a t the p o s i t i o n s of the IR-RA band maxima do not n e c e s s a r i l y d e f i n e the v i b r a t i o n a l frequencies o f the species of i n t e r e s t i s r e i n f o r c e d by an examination of the η spectrum i n F i g . 4. Therefore, i n t e g r a t i n g an a b s o r p t i o n band t o determine changes i n c o n c e n t r a t i o n , a technique commonly used i n transmission-mode experiments, may not be a p p l i c a b l e t o RA bands. Instead, u s e f u l fundamental frequencies of a l l species t o be studied must f i r s t be determined by an a p p r o p r i a t e transmis sion-mode experiment or, more a c c u r a t e l y , by a determination of the complex r e f r a c t i v e i n d i c e s as a f u n c t i o n of frequency. Changes i n k may then be r e l a t e d to the changes i n the a b s o r p t i o n spectrum measured i n the RA mode a t these f r e q u e n c i e s , provided that they do not overlap measurably w i t h the v i b r a t i o n a l frequen c i e s o f other f u n c t i o n a l groups ( i n c l u d i n g those o f r e a c t i o n products) and that the absorbances of the m a t e r i a l s being measured are low enough to permit good s e n s i t i v i t y to changes i n k. ν

v

κ

9

m a x

Most useful Method f o r Calculating Reflection-Absorbance. IR-RA i n t e n s i t i e s c a l c u l a t e d using both Eqs. 1 and 2 show t h a t Eq. 2 y i e l d s good p r o p o r t i o n a l i t y to BPA-PC f u n c t i o n a l i t y concen t r a t i o n over the widest range o f polymer f i l m t h i c k n e s s . There f o r e , t h i s equation was u t i l i z e d i n a l l c a l c u l a t i o n s of IR-RA discussed subsequently. Effect of p- and s-polarized Incident IR Radiation on IR-RA Intensities. The e x i s t e n c e o f an optimum p o l a r i z a t i o n s t a t e f o r the study of t h i n f i l m s on r e f l e c t i v e substrates can be demon s t r a t e d by an examination of the r e l a t i v e magnitudes of the IR-RA



160


s i g n a l s p r e d i c t e d f o r p- and s - p o l a r i z e d IR i n c i d e n t r a d i a t i o n . The IR incidence angle was not t r e a t e d as a v a r i a b l e subject t o o p t i m i z a t i o n ; the dimensions of the spectrophotometer sample compartment and other c o n s t r a i n t s l i m i t e d the p o s s i b l e range o f incidence angles which could be accommodated i n the CEEC. The chosen design value of 75° was used throughout these c a l c u l a t i o n s . The o p t i c a l constants f o r BPA-PC a t the fundamental v i b r a t i o n a l frequencies were determined by a p p l i c a t i o n of the t e c h niques described above. Values of ^ and ^ measured f o r the BPA-PC absorption a t 1775 cm" , as w e l l as the o p t i c a l constants for a i r (n, = 1.0, k = 0.0) and f o r gold (no = 4.2. k = 27.6 [19]), were used to p r e d i c t IR-RA values a t 1/75 cm f o r BPA-PC f i l m s ranging i n thickness from 0.01 t o 10 ]im on gold substrates. C a l c u l a t i o n s were performed f o r both p o l a r i z a t i o n s t a t e s using Eq. 2. E f f e c t s of the substrate p r o p e r t i e s on these c a l c u l a t i o n s are minor. For example, u s i n g the o p t i c a l constants of aluminum ( n ^ = 6.8, k^ = 32.0 [19]) r e s u l t e d i n changes of

Effect of Absorber Concentrations on IR Reflection-Absorbance of

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