Structure-Property Relations in Polymers - American Chemical Society

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16 Fourier Transform Infrared and Raman Studies of Coatings Michael Claybourn and Paul H . T u r n e r 1

2

Research Department, ICI Paints, Wexham Road, Slough SL2 5DS, United Kingdom B r u k e r Spectrospin Ltd., Banner Lane, Coventry CV4 9 G H , United Kingdom 1

2

Fourier transform (FT) IR reflectance measurements were used to monitor the top surface and back-interface curing processes of a polyurethane system. Problems associated with the reflectance technique are discussed. Additionally, the fundamental kinetic parameters for bulk film cure, obtained by nonisothermal FTIR measurements, were combined with a model for polymer network growth to predict the buildup in modulus of the curing film. The results were in excellent agreement with experimental data. Processing conditions for polyester coatings play a significant role in the behavior of the final film. FT Raman measurements were performed on coatings based on polyethylene terephthalate-isophthalate to demonstrate structural reordering of polymer chains during heat treatments.

SURFACE

COATINGS U N D E R G O A VARIETY

o f structural a n d c h e m i c a l changes

d u r i n g initial processing a n d d u r i n g their lifetimes. C u r i n g , solvent behavior, heat treatments, deformation, a n d weathering can influence such factors as c h e m i c a l a n d structural integrity, adhesion, hardness, a n d flexibility ( 1 - 3 ) . T h e c h e m i c a l a n d physical behavior o f these systems must b e understood so that predictions c a n b e made c o n c e r n i n g the subsequent properties o f the protective coating. I n addition, chemical, physical, a n d t h e r m a l stress w i l l influence the integrity o f the coating. V i b r a t i o n a l spectroscopic techniques c a n give functional group i n f o r m a t i o n that can b e l i n k e d to c h e m i c a l a n d structural changes. Infrared measure ments

allow b u l k a n d surface

processes to b e followed, although

0065-2393/93/0236-0407$10.00/0 © 1993 American Chemical Society

Urban and Craver; Structure-Property Relations in Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

some

408

STRUCTURE-PROPERTY RELATIONS IN POLYMERS

thought must be given to the physics o f the measurement (discussed later). F o r the conventional R a m a n technique using visible laser excitation, many industrial materials are subject to fluorescence, w h i c h tends to obscure the R a m a n scattered radiation. T h i s p r o b l e m has largely b e e n overcome b y F o u r i e r transform ( F T ) R a m a n spectroscopy using near-IR ( N I R ) excitation ( 4 - 6 ) ; this c o m b i n e d technique can be used for m o n i t o r i n g b u l k processes i n coatings. A l t h o u g h the c o m m e r c i a l implications are significant, comparatively little literature discusses the application o f vibrational spectroscopic techniques to these problems. C l e a r l y there are difficulties i n sensitivity a n d discrimination for studying surface a n d interface effects, even w i t h supposed surface tech niques such as attenuated total reflectance ( A T R ) spectroscopy. H o w e v e r , characterization o f processes at surfaces a n d interfaces o f coatings is feasible. F o r example, surfactant enrichment at latex film interfaces has b e e n investi gated b y A T R a n d photoacoustic F T I R spectroscopy (7); adhesion a n d interfacial failure between polymers have b e e n characterized w i t h rheophotoacoustic spectroscopy (8); a n d adhesion failure at the p o l y m e r - m e t a l interface has b e e n characterized b y A T R spectroscopy (9). H o w e v e r , factors such as species enrichment or contamination, w h i c h govern surface a n d interface properties, are generally outside the sensitivity limits o f these techniques. Information about the b u l k c h e m i c a l a n d structural properties o f coatings is easier to obtain b y F T I R a n d R a m a n techniques because sensitivity is less o f a p r o b l e m . T h i s i n f o r m a t i o n can help i n understanding, for instance, h o w p o l y m e r cross-linking behavior or structural changes u n d e r processing c o n d i tions affect final coating properties. F o r these types o f measurements p u r e l y e m p i r i c a l kinetic data may be sufficient. H o w e v e r , the c u r i n g kinetics o f a p o l y m e r or resin system can be strongly dependent u p o n such factors as the glass-transition temperature a n d the degree o f cross-linking at any time d u r i n g the cure. T h i s dependence means that determination o f the u n d e r l y i n g kinetic parameters, such as rate constant, order o f reaction, frequency factor, a n d activation energy, becomes difficult (10). These problems can b e c i r c u m v e n t e d to some degree b y using nonisothermal kinetic measurements so that the system is maintained above its glass-transition temperature (10). Investigation o f surface coatings b y vibrational spectroscopic techniques clearly requires a b r o a d range o f approaches to give an understanding o f the c h e m i c a l a n d physical behavior i n the b u l k a n d at interfaces. F o r this investigation F T I R a n d F T R a m a n measurements were used for the analysis o f c u r i n g a n d structural changes i n p o l y m e r coatings, i n particular, epoxy-polyester blends a n d polyurethanes. T h e optical properties o f the coating i n relation to F T I R reflectance measurements is discussed. I n the analysis of these data, allowance for the optical interactions must be made, otherwise misleading conclusions can be d r a w n .


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FTIR and Raman Studies of Coatings

FTIR Reflectance Measurements on Polymer Coatings F T I R reflectance techniques are w i d e l y u s e d f o r characterizing coating systems. H o w e v e r , care must b e taken w i t h these types o f measurements because they are susceptible to various optical effects that c a n make interpre tation o f the spectral data difficult. A basic understanding o f the physics o f these interactions is useful so that misleading conclusions are n o t drawn. F i g u r e 1 shows t h e possible optical p h e n o m e n a f o r light incident u p o n a protective coating o n a reflecting substrate. T h e reflected light w i l l consist o f several components, namely, front-surface specular reflection (R ), backsurface specular reflection ( R ) , scattering f r o m t h e t o p ( a n d to a m u c h lesser degree t h e bottom) surface ( R ) , a n d diffuse scatter f r o m scattering centers w i t h i n the film (R ). T h e measured reflectance, R , may b e w r i t t e n as a s u m o f all o f these: s

b

t

d

m

R =R m

s

+ R

d

+ R + R t

b

(1)

A n y c o m b i n a t i o n o f these reflectance components c a n occur d e p e n d i n g u p o n the optical state o f the sample. I f all o f these p h e n o m e n a are superimposed, then t h e spectrum is highly distorted a n d difficult to interpret. A s a conse quence, i n any reflectance measurement a s a m p l i n g m e t h o d that is o p t i m i z e d for one o f these reflectance components must b e e m p l o y e d . F i g u r e 2 shows spectra f o r different types o f coatings that give p r e d o m i n a n t l y one type o f reflected fight. Clearly, the type o f coating is significant f o r discrimination o f an i n d i v i d u a l reflectance component.

The Diffuse Component. T h e diffuse reflectance c o m p o n e n t arises w h e n scattering centers are present i n the matrix o f the coating. T h i s system Transflectance Diffuse Specular

Figure 1. Reflectance of light from a coating on a reflecting substrate.


410


100 MULTIPASS TRANSMISSION

DIFFUSE *S10 Ο C D

"υ

£

5 20 Η SPECULAR

10

3500

3000

2500 2000 1500 Wavenumbers (cm—1)

1000

Figure 2. FTIR reflectance spectra from different types of coatings showing the possible spectral types.

w i l l b e inhomogeneous a n d have large discontinuities i n refractive index at the interface o f the host material a n d scattering centers. T h e s e diseontinuitities m a y b e inorganic pigments, inclusions, extenders, crystalhtes, voids, o r phase-separated components. A s the scattered light passes t h r o u g h the sam ple, i t is absorbed at characteristic frequencies f o r the scattering centers a n d for the host material. Eventually, t h e light escapes t h e front surface a n d is collected at the detector. T h e geometry o f this system is described as a diffuse reflectance measurement a n d w i l l b e o p t i m i z e d w i t h a diffuse r e flectance

accessory c o m p r i s i n g a h e m i s p h e r i c a l collecting lens. I f the mea

surement is p e r f o r m e d w i t h a specular reflectance

accessory o r a n F T I R

microscope operating i n reflectance mode, t h e n the specular c o m p o n e n t w i l l be e n h a n c e d relative to t h e scattered component. I n addition, the path length through the sample w i l l b e several micrometers. I n effect this reflected light


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w i l l comprise reflectance a n d transmittance at the p o l y m e r - i n h o m o g e n e i t y interface a n d multipass transmission t h r o u g h the matrix after scattering at this interface. I n some cases the inhomogeneity has a similar refractive index to the matrix, so that little or no scattering occurs. T h e degree o f refractive index mismatch as w e l l as the concentration o f scattering centers w i l l strongly influence the intensity o f the diffusely scattered component. T h i s approach is useful o n l y for qualitative analysis o f highly p i g m e n t e d paint films. F i g u r e 3 shows an example o f a urethane-modified alkyd paint o n a metallic substrate c o m p a r e d w i t h the transmission spectrum o f the same material. T h e reflectance spectrum is highly distorted a n d displays peak shifts o f 2 0 - 3 0 c m " to higher wave numbers. I n addition, the reflectance spec t r u m has interference fringes i n spectral regions w i t h h i g h transmission. T h e distortion w i l l be associated w i t h superpositioning o f the specular reflectance f r o m t h e front surface. T h e distortion can cause ambiguities, a n d so t h e technique is used only for qualitative analysis at best. 1

Back-Surface Reflectance. A back-surface reflection is essentially a double-pass transmission experiment, a n d subsequently a so-called transflectance spectrum is obtained. Interference between the front-and backsurface-reflected fight gives rise t o interference fringes; the fringe spacing is related to film thickness, d ( I I ) :

3500

3000

2500

2000

1500

Wavenumbers (cm-1) Figure 3. Reflectance (a) and transmission (b) spectra for a urethane-modified alkyd paint film.


412


w h e r e m is the n u m b e r o f fringes b e t w e e n t h e initial a n d final fringe, σ is the wave n u m b e r difference between the initial a n d final fringe, φ is t h e angle o f incidence, a n d η is the refractive index o f the sample. D e p e n d i n g u p o n their magnitude, these fringes c a n obscure spectral detail, a n d it is best to avoid t h e m . T h e y c a n b e r e m o v e d , for example, b y w e d g i n g the sample. A n additional p r o b l e m w i t h these types o f measurements

o n metallic

substrates is that dramatic changes i n relative b a n d heights c a n b e observed (12, 13). T h e s e changes are d u e to a n interference effect; the standing waves originating at the metal surface at a l l the different frequencies give rise to combinations o f nodes a n d antinodes corresponding to m i n i m a a n d m a x i m a i n the reflectivity. F o r a given thickness o f sample, nodes a n d antinodes ( a n d hence, m i n i m a a n d maxima) w i l l occur across the w h o l e range o f t h e reflectance spectrum. C o n s e q u e n t l y , f o r spectra f r o m a material at different thicknesses, nodes a n d antinodes w i l l occur at different wavelengths, a n d so spectral comparison c a n b e misleading even f o r qualitative w o r k because b a n d ratios w i l l change. C o r r e c t i o n f o r this effect c a n b e made as l o n g as t h e optical properties o f the material a n d substrate are k n o w n . F i g u r e 4 shows the geometry o f a measurement. T h e amplitude o f the reflected light ( r ) is given b y (14)

r

12

+ r

1 + r

1 2

r

1 2

e^

2 3

e

2 i p

(3)

w h e r e i is r andr are the amplitudes o f t h e reflected light f r o m the a i r - f i l m a n d film-metal interfaces, respectively, a n d for n o r m a l incidence are 1 2

2 3

Light at normal incidence

AIR

n

POLYMER

n* = n + ik

1 =

1

2

2

\

METAL

Figure 4. Optical geometry for a polymer coating of thickness ά on a metallic substrate.


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given b y n n where

n

2

and n

3

are the

substrate, respectively (see

2

— 1

n

x

+ 1

n

3

3

— n +

2

(4)

%

complex refractive indices for the

film

and

F i g u r e 4). A t n o r m a l incidence, β is given b y 2TT

dn

9

«9

l » — ^

w h e r e d is the film thickness a n d λ is the wavelength o f light, β describes the degree o f attenuation o f the light i n the system. T h e measured re flectance R can be d e t e r m i n e d f r o m R = r · r*

(6)

w h e r e r * is the complex conjugate o f r . Therefore, f r o m a knowledge o f the optical properties o f the p o l y m e r a n d substrate the reflectance spectrum can b e m o d e l e d to give quantitative data. T h i s approach was e m p l o y e d b y Packansky a n d co-workers ( J 5 ) to m o d e l the composition a n d thickness o f organic layered photoconductors. T h e y also showed h o w this m o d e l i n g ap p r o a c h can be u s e d for a range o f polymers o n various m e t a l substrates (13). T o show the result o f this optical effect, F i g u r e 5 displays spectra across a polyester film o n a metal substrate. T h e film thickness was n o m i n a l l y 7 μ π ι , b u t showed variations f r o m 5 to 8 μ ι η . T h e spectra are n o r m a l i z e d to the intensity o f the C = 0 stretching b a n d at about 1730 c m . C l e a r variations i n b a n d intensities are seen, and, without any knowledge o f the physics o f this optical interaction, misleading conclusions can be d r a w n . - 1

U s i n g this analysis, the variation i n b a n d ratios can b e m o d e l e d . F i g u r e 6 shows the variation i n reflectance w i t h film thickness f o r three bands at 1720, 1200, a n d 925 c m ; these bands are taken as arbitrary at different points i n the spectrum to demonstrate the effect. W h a t may be surprising is that w i t h a comparatively small change i n film thickness, the ratio can change signifi cantly. C l e a r l y , i f this technique is u s e d to obtain structural o r c h e m i c a l i n f o r m a t i o n about a p o l y m e r coating o n a metallic substrate, t h e n correction o f the reflectance spectra must be p e r f o r m e d to obtain m e a n i n g f u l data (15). - 1

Top-Surface Specular Reflectance. T h e specular component (front-surface reflection) is often considered as a source o f spectral distortion, particularly i n diffuse reflectance measurements (16, 1 7 ) . H o w e v e r , the technique can b e made use o f i n cases w h e r e this reflected component is p r e d o m i n a n t . T h i s situation arises w h e n the light that penetrates the sample is either strongly scattered or strongly absorbed. E x a m p l e s are very thick


414


2000

1800

1600

1400

1200

1000

WflVENUMBER CM-1 Figure 5. Reflectance spectra taken across a polyester coating on a metal substrate to show the variation in relative band intensities (spectra are normalized to C=0 maximum).


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415

1720cm-1 1200cm-1 925cm-1

2 4 6 8 Film thickness (microns)

10

Figure 6. Variation in reflectance intensity with film thickness at 1720, 1200, and 925 cm ~ 1

samples so that back-surface-reflected light is absorbed, a n optically rough back surface so that the light is scattered off the specular axis, a n d strongly absorbing components w i t h i n the sample. T h e technique can easily provide analytical information, a n d additionally, i f the measurement is p e r f o r m e d w i t h t h e I R microscope, t h e n microscopic structural i n f o r m a t i o n c a n b e obtained ( 1 7 ) . T h e technique has b e e n u s e d to study carbon-filled polymers ( I S , 1 9 ) because any fight penetrating the sample is absorbed a n d only the top-surface-reflected c o m p o n e n t is observed. H o w e v e r , one o f the problems w i t h this type o f sampling is that the spectrum takes o n a derivativelike shape. T h e shape arises f r o m dispersion i n t h e refractive index about resonant species i n t h e material ( 2 0 ) . H o w e v e r , t h e absorption index (related to absorption coefficient) c a n b e recovered mathematically f r o m the specular reflectance spectrum. F o r n o r m a l incidence, the measured specular reflectance is given b y

R =

(n-1)

2

(n + 1)

(7) 2

where η is the complex refractive index of the material a n d takes into account b o t h refraction (real) a n d absorption (imaginary). T h e imaginary part o f the complex refractive index, k, is t h e absorption index a n d is related to t h e absorption coefficient α b y

α

=

2TT/CO


(8)

416


w h e r e σ is the frequency i n wave numbers. A n additional effect to be considered is that light undergoes a phase change u p o n reflection at an interface; the reflection coefficient, r * , is related to the reflection a m p l i t u d e , r , a n d phase angle φ , b y

r * = \r\e~^

(9)

a n d the measured reflectance, R, is given b y R = |r|

(10)

2

Separate expressions for η a n d k at wave n u m b e r σ can be obtained f r o m these equations:

1 -

k =

2)fR

COS

2)fR sin

φ +

R

φ

1 + 2\/fi COS φ +

R

(11)

(12)

Therefore, i f φ was d e t e r m i n e d at a l l frequencies, t h e n η a n d k c o u l d b e fully d e t e r m i n e d . T h i s c o n d i t i o n is possible because R a n d φ are related b y the K r a m e r s - K r o n i g ( K K ) integral, w h i c h enables φ to be d e t e r m i n e d f r o m the measured reflectance (21):

* =

f

H

R

\ < U r

(13)

where Ρ is the p r i n c i p a l part o f the integral a n d means that the singularity at σ = σ is calculated as a C a u c h y p r i n c i p a l value. T h e integral is p e r f o r m e d at each frequency σ over all frequencies, σ . F o r any causal f u n c t i o n , the K K integral describes the relationship between its real a n d imaginary parts (22), a n d f r o m equation 9: 0

0

l n ( r * ) = 0.51n(H) - ίφ

(14)

Details o f the origin o f this integral have b e e n discussed i n the literature ( 2 0 , 21).

T h e K r a m e r s - K r o n i g transform is n o w readily i m p l e m e n t e d b y fast,

c o m m e r c i a l l y available software that uses established methods.

FTIR Reflectance Measurements. A l t h o u g h reflectance mea surements have inherent problems, valuable data can be obtained. F o r these measurements F T I R specular reflectance was p e r f o r m e d w i t h a B r u k e r F T I R microscope attached to a B r u k e r I F S 48 spectrometer. T h i s sampling geome-


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try discriminates i n favor o f the specular reflected component. F o r the K r a m e r s - K r o n i g transform, the data were transferred to a personal c o m p u t e r for processing using L a b C a l c (23). I n a d d i t i o n A T R measurements f o r the polyurethane reaction at the substrate-coating interface were made w i t h a Specac horizontal A T R accessory w i t h a Z n S e A T R crystal.

FT Raman Measurements on Coatings Clearly, the difficulties must b e carefully considered i n a n F T I R reflectance measurement o n a p o l y m e r coating. R a m a n spectroscopy offers a n alternative approach a n d does not suffer f r o m the problems associated w i t h the I R reflectance m e t h o d . H o w e v e r , for the conventional R a m a n technique using visible laser excitation, the types o f materials that c a n b e examined are l i m i t e d because o f sample fluorescence. T h i s p r o b l e m has b e e n overcome b y the use of N I R laser excitation, so that fluorescence has been, to a large extent, eradicated. T h e disadvantage i n going to lower energy excitation giving weaker R a m a n intensity has b e e n overcome b y c o m b i n i n g w i t h F T instru mentation ( 5 , 6), w h i c h gives h i g h throughput. A d d i t i o n a l problems are associated w i t h R a m a n spectroscopy f o r studying processes i n coatings, namely, film thickness a n d optical properties. F o r this w o r k o n u n p i g m e n t e d p o l y m e r films, the major factor affecting the signal-to-noise ratio is the film thickness because it significantly affects the R a m a n scattering v o l u m e . T h e instrument used f o r these measurements was a B r u k e r I F S 66 F T I R w i t h the F R A 106 R a m a n m o d u l e attached. T h i s instrument uses a N d : Y A G laser f o r excitation operating at 1064 n m . F o r the measurements samples w e r e s i m p l y p l a c e d into the sample chamber o f the instrument without any sample preparation ( 2 4 ) .

Polyureihanes Cross-Linking i n Polyurethane Coatings. T h e characterization o f the cure processes o f coating materials provides an essential base f o r developing models to describe film formation ( 2 5 ) . T h i s base gives a means for p r e d i c t i n g the cure behavior as a f u n c t i o n o f certain variables such as formulation a n d temperature. F u r t h e r m o r e , the resulting properties o f the c u r e d film c a n b e related to the cure behavior. M a n y coatings are based o n a cross-linking reaction between isocyanate a n d hydroxyl functional resins ( 2 6 , 27) to f o r m a durable polyurethane film. I n real coating systems, b o t h reactants are usually multifunctional, a n d the alcohol m a y have b o t h p r i m a r y and secondary - O H groups. T h e resulting film is often highly cross-linked to give a high-performance protective coating. T h e cure chemistry o f isocyanate w i t h hydroxy functional polymers to f o r m polyurethane films is w e l l characterized, a n d the kinetic parameters


418


have b e e n d e t e r m i n e d (28). F r o m a technological point o f v i e w other parameters, such as p o l y m e r functionality, type o f solvent, the rate o f solvent loss, the c u r i n g profile t h r o u g h the film, a n d h o w these affect the properties o f the subsequent coating, are o f significance. T o obtain realistic measure ments that represent the behavior o f the coating as it is used, film thicknesses prevent the use o f I R transmission measurements. Reflectance methods are left. H o w e v e r , this situation does not l i m i t us i n sensitivity n o r i n b e i n g able to m o n i t o r the chemistry a n d kinetics o f the reactions. H o w e v e r , the optical interactions d e s c r i b e d must b e considered i n any analysis.

Surface and Interface Curing Reactions. A m e t h o d for obtain ing surface c u r i n g i n f o r m a t i o n has b e e n developed w i t h a n o r m a l incidence specular reflectance measurement. T h e material for investigation is spread as a 2 0 0 - μ m film onto a black p o l y m e r substrate, i n this case polypropylene; this step prevents any back-surface reflectance (17). T h e subsequent surface-re flectance measurement gives spectra w i t h derivativelike line shapes that require K r a m e r s - K r o n i g transformation t o obtain absorption index spectra. T w o isocyanate-hydroxy systems w e r e investigated w i t h d i f f e r i n g cure rates (the functionalities d i f f e r e d b y a factor o f 3). F i g u r e 7 shows the behavior o f the isocyanate functionality w i t h cure time at r o o m temperature as deter m i n e d f r o m the N = C = 0 stretching b a n d at 2275 c m " . C l e a r l y the rate o f reaction is not dependent u p o n the functionality o f the polymers because the rates at t h e surface were almost identical. T h e probable reason for this property is that the surface is a dynamic system w i t h very m o b i l e species (e.g., solvent). I n addition, the m o b i l i t y o f the polymers at the surface w i l l be higher than i n the b u l k , so that d u r i n g the p e r i o d o f the cure that was m o n i t o r e d , the cross-fink density h a d little influence. A n additional factor i n this reaction w i l l b e atmospheric water, w h i c h w i l l react w i t h the surface 1

0 20 40 60 80 100120140160180200 time (mins) Figure 7. Surface cure of two isocyanate-hydroxy functional polymers with different functionalities.


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isocyanate functionality. T h e solid line i n F i g u r e 7 is a r e d u c e d time-fit to the data giving the o r d e r o f reaction ( 2 9 , 3 0 ) ; this was f o u n d to b e 2 as expected for this b i m o l e c u l a r reaction. T h e surface cure plays a significant part i n the properties o f a coating once f o r m e d . I f too m u c h stress is b u i l t u p because o f extremes i n cure rate at the surface, b u l k , a n d back interface, t h e n the adhesion properties o f the coating w i l l be poor. Therefore direct i n f o r m a t i o n o n the cure profile is needed. T o a large extent solvent loss plays an important role because it provides an environment that aids diffusion o f p o l y m e r chains, i n particular those w i t h cross-linking functional groups. T h e process u s e d to obtain coat ings w i t h good properties is generally e m p i r i c a l , without direct measurements on fundamental processes. F T I R spectroscopy can give i n f o r m a t i o n about the reactions at the top surface a n d coating-substrate interface. Subsequently, the system can be m o d e l e d a n d the physical properties o f the coating can be o p t i m i z e d . T h e reactions for a r a p i d c u r i n g system at the top surface w e r e m o n i t o r e d as already described. T h e reaction mixture i n b u t y l acetate solvent was spread onto a carbon-filled polypropylene p a n e l at a thickness o f 200 μ υ ι , a n d the processes were m o n i t o r e d w i t h time b y a n o r m a l incidence specular re flectance measurement i n the F T I R microscope. F o r the reaction at the coating-substrate interface the sample was spread onto a horizontal A T R accessory at the same thickness as for the specular measurement. T h e reaction was t h e n m o n i t o r e d w i t h t i m e u n d e r identical conditions to the top-surface measurement. F i g u r e 8 shows the specular reflectance a n d calcu lated absorption index spectra for the top-surface cure measurement. T h e b a n d at 1242 c m due to the ester C - O o f the solvent decays rapidly over 20 m i n . T h e appearance o f the b a n d at about 1520 c m is due to the urethane linkage. F o r m o n i t o r i n g the solvent loss, the 1 2 4 2 - c m b a n d was used a n d not the C = 0 ester b a n d because it was obscured b y the C = 0 b a n d o f the polymer, a n d i n addition, the urethane C = 0 increased d u r i n g the process. - 1

- 1

_ 1

F i g u r e 9 shows the spectra f r o m the horizontal A T R measurement for the cure at the back surface. T h e spectra are n o r m a l i z e d against the maxi m u m b a n d , w h i c h at t = 0 is the solvent C - O changing rapidly to a p o l y m e r b a n d as solvent is lost. A p r o b l e m w i t h solvent loss for this measurement is that the average refractive index changes. T h i s factor means that the intensity o f the evanescent wave changes a n d t h e n so does the absolute intensity o f the A T R spectral bands. Consequently, the isocyanate b a n d (2273 c m ) was n o r m a l i z e d against the C - O - C b a n d (1147 c m ) o f the p o l y m e r . F i g u r e 10 shows plots o f the behavior o f solvent a n d isocyanate d u r i n g the c u r i n g reaction for the top surface a n d the coating-substrate interface. T h e solvent loss for the top a n d b o t t o m surfaces was at a similar rate w i t h almost complete loss after 20 m i n . T h e isocyanate decay over the p e r i o d o f solvent loss was r a p i d r e d u c i n g b y 7 0 % at the top surface a n d b y 4 0 % at the b o t t o m - 1

- 1


420


1500 Wave n u m b e r s ( c m - 1 )

1000


16.

CLAYBOURN & TURNER


421

surface. Subsequent isocyanate decay was slow as a result o f b u i l d u p i n cross-link density o f the coating, w h i c h prevented interaction of reactive functional groups. T h e difference i n the rates o f decay o f the top a n d b o t t o m surfaces are due to physical effects such as solvent loss, w h i c h create a dynamic system. A t the top surface, w h e r e all the solvent i n the film w i l l eventually migrate a n d evaporate, the reaction w i l l be enhanced c o m p a r e d to the b o t t o m surface, where solvent w i l l s i m p l y diffuse to the surface. T h i s type o f i n f o r m a t i o n c o n c e r n i n g the cure profile helps i n understanding the b u i l d u p o f stress i n the c u r i n g film. I n this particular case the coating h a d very good adhesive and protective properties.

Isothermal versus Nonisothermal Kinetic Measurements. T h e c u r i n g m e c h a n i s m is an important consideration i n understanding a n d hence, p r e d i c t i n g the behavior o f coatings. T h e approach taken here was to obtain kinetic parameters w i t h F T I R spectroscopy a n d to attempt to relate this i n f o r m a t i o n to changes i n modulus b y using a m o d e l for network growth. T o obtain the u n d e r l y i n g kinetics for the reaction to b u i l d this m o d e l , a comparison o f isothermal a n d nonisothermal measurements was initially made to assess the applicability o f b o t h . A B r u k e r I F S 48 F T I R spectrometer was used to f o l l o w the c u r i n g reaction b o t h isothermally a n d nonisothermally. T h e - N C O a n d - O H components were m i x e d i n the appropriate ratio a n d coated onto a K B r disk. T h e disk was m o u n t e d onto a Specac variable-tem perature c e l l that has a linear temperature p r o g r a m m e r for c o n t r o l l e d sample heating a n d thus allows b o t h isothermal a n d controlled-temperature r a m p i n g experiments. T h e cure process was m o n i t o r e d b y the decay o f the isocyanate b a n d at 2275 c m " b o t h isothermally a n d at different heating rates. 1

F o r the isothermal measurements, w h e r e b y the extent o f reaction, a , was m o n i t o r e d w i t h t i m e (t) at constant temperature, the kinetic behavior for an n t h - o r d e r reaction can be described b y the relation

—

= *(l-a)"

(15)

w h e r e k is the rate constant a n d is thermally activated, following an A r r h e nius expression:

k=Ae-^ ^ E

(16)

w h e r e R is the gas constant; Τ is absolute temperature; a n d the kinetic


422


3500

3000

2500

2000

Woven umbers

1500 1000 (cm—1)

Figure 9. ATR spectra for the polyurethane curing system. The * denotes the main solvent bands.

parameters, namely, the activation energy ( E ) , preexponential factor ( A ) , a n d the o r d e r o f reaction ( n ) , c a n b e d e t e r m i n e d b y a series o f isothermal measurements at different temperatures. Isothermal measurements w e r e p e r f o r m e d at temperatures o f 20, 80, 100, a n d 120 °C; the behavior o f the b a n d due to - N C O at each temperature is given i n F i g u r e 11. A t 20 °C the reaction was very slow, a n d after 15 h the process h a d gone only to 1 5 % c o m p l e t i o n . F o r each temperature,

except

20 °C where the extent o f reaction was too l o w , the apparent order o f reaction o f reaction was calculated b y u s i n g the reduced-time m e t h o d ( 2 9 , 30). T h e orders o f reaction f o r temperatures o f 80, 100, a n d 120 °C w e r e 5.7, 3.6, a n d 4, respectively. T h e s e values are anomalously h i g h b u t are typical f o r diffusion-controlled reactions. I n this case the controlling parameter f o r the cure process was the increasing glass-transition temperature ( T ) . T h e results g

demonstrate the shortcoming o f the isothermal m e t h o d a p p l i e d to p o l y m e r systems w i t h final T values that are above ambient conditions. T h e different g

behavior at the different temperatures means that the activation energy for the reaction cannot b e d e t e r m i n e d f r o m these isothermal results.


16.

CLAYBOURN & TURNER


0

20

40 60 time (mins)

80

423

100

Time (mins) Figure 10. Behavior of the isocyanate and solvent for top surface (top) and coating-substrate interface (bottom). N o n i s o t h e r m a l kinetic measurements o n film c u r i n g processes c a n over come these experimental problems. T e c h n i q u e s such as d y n a m i c mechanical analysis ( D M A ) , differential scanning ealorimetry ( D S C ) , a n d F T I R spec troscopy can provide a means for obtaining i n f o r m a t i o n f o r the kinetics o f the cure using temperature scanning methods ( 2 5 , 31-34). T h e effect o f the T is avoided as l o n g as the experimental temperature is always h e l d above the T . T o obtain t h e kinetic parameters f r o m t h e rising-temperature F T I R results, w e m o d i f i e d t h e approach originally devised b y O z a w a ( 3 5 ) . T h i s analysis is based o n a series o f experiments p e r f o r m e d at different heating rates. F o r this technique, t h e kinetic behavior is m o d e l e d b y using a n Arrhenius-type expression based o n equations 15 a n d 16: g

g

da Έ

(1 — a )

-

n

£ — Αβ ~^* (

Γ>


(17)

424


200

20

400

Time (min)

40

Time (min)

Figure 11. Fraction of isocyanate lost with cure time for isothermal measurements. (Reproduced with permission from reference 30. Copyright 1992 John Wiley.) w h e r e b = dT/dt, ment ( 3 6 )

that is, the heating rate. F o r a rising-temperature experi

AR

(18)

g(«)=-rV χ > 0.35)

studied, the

g a u c h e - t r a n s conformational change i n the glycol units o c c u r r e d w i t h the 110 °C heat treatment. T h e major contribution to the structural o r d e r i n g i n the

p o l y m e r occurs

across the

ethylene

terephthalate

(ET)

units;

the

g a u c h e - t r a n s conformational change over a section of p o l y m e r chain w i t h a significant n u m b e r of E T units w i l l enhance the linearity over that region. I n the ethylene isophthalate ( E I ) units, the g a u c h e - t r a n s isomerism w i l l not have a significant effect o n the linearity o f the p o l y m e r chains because o f the 1,3-siibstitution o n the aromatic ring. Increasing the amount o f E I i n the p o l y m e r increases the

disorder i n the p o l y m e r . H o w e v e r , for the

heat

treatment used i n this case, for l o w E I compositions (up to 10%), the degree a n d rate o f crystallization r e m a i n constant. F o r the 1 8 % E I composition, the rate drops significantly, although the final degree of crystallization (equi l i b r i u m after p r o l o n g e d heating at 110 °C) is only slightly less than for P E T . F o r higher E I concentrations, the disordering effect o f the E I units plays a significant role, a n d i n fact n o detectable change i n crystallinity was f o u n d .

Structural Properties of Polyester-Epoxy Films by F T Ra man Spectroscopy. T h e p r o b l e m of reflectance measurements for films on reflective substrates was discussed earlier. T o obtain structural information f r o m F T I R measurements w i t h the reflectance approach, a clear knowledge of the optical properties of the p o l y m e r must b e k n o w n . It is a complex process to obtain accurate values for the optical properties over the spectral range of interest. Consequently, the p r e f e r r e d m e t h o d for characterizing the films was F T R a m a n spectroscopy. F o r this w o r k w e were interested i n following the change i n structural ordering o f P E T I (I = 18%)

i n a m o d e l film composed of a b l e n d o f the

polyester a n d an epoxy resin. Changes i n crystallinity can affect the protective properties o f the film. F i g u r e 22 shows spectra for the raw materials. T h e films w e r e a p p l i e d to metal substrates at a thickness of about 10 μ m a n d were heat-treated at 110 °C for periods u p to 3 h . T h e spectra for the untreated a n d the 3 - h treated samples are shown i n F i g u r e 23. T h e signal-tonoise ratio was not sufficient to allow accurate measurement b a n d w i d t h . I n addition the 1 0 9 6 - c m

- 1

o f the

C=0

b a n d c o u l d not be m o n i t o r e d because

it was obscured b y a b a n d f r o m the epoxy resin; although a change was just detectable (see

F i g u r e 23 bottom). Therefore, the b a n d at 855 c m


- 1

was

16.

CLAYBOURN & TURNER


437

m o n i t o r e d to determine o r d e r i n g effects i n the polyester. F i g u r e 24 shows the behavior o f this b a n d w i t h time o f annealing. T h e absolute value for the b a n d ratio (855/633) d i d not correlate w i t h the previous w o r k o n the pellets because each overlapped w i t h other bands f r o m the epoxy resin. T h i s condition meant that the integrated intensities h a d some c o n t r i b u t i o n f r o m neighboring bands. T h e rate o f change appeared to occur at a similar rate, reaching a m a x i m u m after about 2 h for b o t h the P E T I pellets a n d the p o l y m e r b l e n d films. T h e potential f o r R a m a n spectroscopy to m o n i t o r structural changes, even i n b l e n d e d p o l y m e r films, is evident f r o m these results. T h e physical a n d c h e m i c a l properties o f the film can t h e n b e related to the molecular


438


3000

2000 Raman Shift ( c m - 1 )

1000

'amorphous' PET

'crystalline' PET

Epoxy

1140

1120 1100 Raman Shift ( c m - 1 )

1080

Figure 22. FT Raman spectra for PET and the epoxy.


16.

CLAYBOURN & TURNER


1200

1140

1000 Raman shift

1120 1100 Raman shift

800

439

600

1080

1060

Figure 23. Top: PET-epoxy blend film (about 8 μ m thick) showing the increase in the 857'-cm band. Key: —, untreated; and + , treated at 110 °C for 3 h. Bottom: expanded region around the 1096-cm ~ band. -1

1


440


i n f o r m a t i o n obtained b y R a m a n spectroscopy. T h i s ability is particularly important for o p t i m i z i n g the processing o f the p o l y m e r material to give the desired properties.

Acknowledgments W e thank I C I Paints for their support a n d encouragement i n this w o r k , i n particular D . M . D i c k a n d M . R e a d i n g .

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Ellis, W. H . J. Coatings Technol. 1983, 55, 63. Provder, T. J. CoatingsTechnol.1989, 61, 33. Sullivan, W. F. Prog. Org. Coatings 1972, 1, 157. Chantry, G. W.; Gebbie, Η. Α.; Hilsum, C. Nature 1964, 203, 1052. Chase, B. J. Am. Chem. Soc. 1986, 108, 7485. Hirschfeld, T.; Chase, B. Appl. Spectrosc. 1986, 40, 133. Urban, M . W.; Evanson, K. W. Polymer Commun. 1990, 31, 279. McDonald, W. F.; Urban, M . W. J. Adhesion Sci. Technol. 1990, 4, 751. Spadafora, S. J.; Leidheiser, H . J. Oil Colour Chem. Assoc. 1988, 71, 276. Claybourn, M.; Reading, M . J. Appl. Polym. Sci. 1992, 44, 565. Harrick, N . J. Appl. Optics 1971, 10, 2344. Briggs, L. M.; Bauer, D. R.; Carter, R. O. Ind. Eng. Chem. Res. 1987, 26, 667. Packansky, J.; Waltman, R. J.; Grygier, R. Appl. Spectrosc. 1989, 43, 1233. Born, M.; Wolf. E. Principles of Optics; Pergamon Press: New York, 1964.


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15. Packansky, J.; England, C.; Waltman, R. J. J. Polym. Sci. B: Polym. Phys. 1987, 25, 901. 16. Hembree, D. M.; Smyrl, H . R. Appl. Spectrosc. 1989, 43, 267. 17. Claybourn, M . ; Colombel, P. Bruker Report; Bruker Analytische Messtechnik G M B H : Karlsruhe, Germany, 1990; Vol. 1, pp 8-12. 18. Claybourn, M.; Colombel, P.; Chalmers, J. Proc. Int. Workshop Fourier Trans form Infrared Spectrosc.; Vansant, E. F., Ed.; University of Antwerp Press: Antwerp, Belgium, 1990. 19. Claybourn, M.; Colombel, P.; Chalmers, J. Appl. Spectrosc. 1991, 45, 279. 20. Lipson, S. G.; Lipson, H . Optical Physics; Cambridge University Press: Cam bridge, England, 1981. 21. Harbecke, B. Appl. Phys. A 1986, 40, 151. 22. Roth, J.; Rao, B.; Dignam, M . J. Trans. Faraday Soc. 1975, 71(1), 86. 23. LabCalc software, Galactic Industries Inc., 395 Main Street, Salem, N H 03079. 24. Claybourn, M.; Agbenyega, J.; Hendra, P. J., Chapter 17 in this book. 25. Provder, T.; J. Coatings Technol. 1989, 61, 32. 26. Saunders, J. H.; Frisch, K. C. Polyurethanes: Chemistry and Technology; Vol. I, Robert E. Krieger Publishing: Malabar, FL, 1962. 27. Polyurethane Handbook: Chemistry, Raw Materials, Processing, Applications, and Properties; Oertel, G., Ed.; Hanser Publishers: Munich, Germany, 1985. 28. Hernandez-Sanchez, F.; Vazquez-Torres, H . J. Polym. Sci. A: Polym. Chem. 1990, 28, 1579. 29. Sharp, J. H.; Brindley, G. W.; Achar, Β. Ν. N . J. Am. Ceram. Soc. 1966, 49, 379. 30. Claybourn, M.; Reading, M . J. Appl. Polym. Sci. 1992, 44, 565. 31. Snyder, R. W.; Wade Sheen, C. Appl. Spectrosc. 1988, 42, 655. 32. Carlson, G. M.; Provder, T. Proc. Water-Borne High Solids Coating Symp. 1985, 12, 44. 33. Carlson, G. M.; Neag, C. M.; Kuo, C.; Provder, T. Polym. Prepr. 1984, 25, 171. 34. Provder, T.; Neag, C. M.; Carlson, G. M . ; Kuo, C.; Holsworth, R. M . Anal. Calorim. 1984, 5, 377. 35. Ozawa, T. J. Ther. Anal. 1970, 2, 301. 36. Whitehead, R.; Dollimore, D.; Price, D.; Fatemi, N . S. Proc. 2nd Europ. Symp. Therm. Anal.; Dollimore, D., Ed.; Heyden: Aberdeen, Scotland, 1981; p 51. 37. Gupta, V. B.; Kumar, S. J. Appl. Polym. Sci. 1981, 26, 1885. 38. Ward, I. M . Chem. Ind. (London) 1956, 905. 39. Manley, T. R.; Williams, D. A. Polymer 1969, 10, 339. 40. McGraw, G. E. Polym. Prepr. 1970, 11, 1122. 41. Melveger, A. J. J. Polym. Sci.: Polym. Phys. Ed. 1972, 10, 317. 42. Cunningham, Α.; Ward, I. M.; Willis, Η. Α.; Zichy, V. Polymer 1974, 15, 749. 43. Boerio, F. J.; Bahl, S. K.; McGraw, G. E. J. Polym. Sci.: Polym. Phys. Ed. 1976, 14, 1029. 44. Purvis, J.; Bower, D. I. J. Polym. Sci.: Polym. Phys. Ed. 1976, 14, 1461. 45. Ward, I. M.; Wilding, M. A. Polymer 1977, 18, 327. 46. Jarvis, D. Α.; Hutchinson, I. J.; Bower, D. I.; Ward, I. M . Polymer 1980, 21, 41. 47. Hutchinson, I. J.; Ward, I. M.; Willis, Η. Α.; Zichy, V. Polymer 1980, 21, 55. 48. Stokr, J.; Schneider, B.; Doskocilova, D.; Lovy, P.; Sedalek, P. Polymer 1982, 23, 714. 49. Lin, S-B.; Koenig, J. L. J. Polym. Sci.: Polym. Phys. Ed. 1982, 20, 2277. 50. Lin, S-B.; Koenig, J. L. J. Polym. Sci.: Polym. Phys. Ed. 1983, 21, 1539. 51. Lin, S-B.; Koenig, J. L. J. Polym. Sci.: Polym. Phys. Ed. 1983, 21, 2365. 52. Yazdanian, M.; Ward, I. M.; Brody, H . Polymer 1985, 26, 1779.


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53. Adar, F.; Noether, H . Polymer 1985, 26, 1935. 54. Daubeny, R.; Bunn, C. W.; Brown, C. J. Proc. Roy. Soc. 1954, 226, 531. 55. McGraw, G. E. In Polymer Characterization: Interdisciplinary Approaches; Craver, C. D., Ed.; Plenum Press: New York, 1972. 56. Melveger, A. J.; Polym. Prepr. 1972, 13(1), 180. 57. Bulkin, B. J.; Lewin, M.; DeBlase, F. J. Polym. Mater. Sci. Eng. 1986, 54, 397. 58. Purvis, J.; Bower, D. I. J. Polym. Sci.: Polym. Phys. Ed. 1976, 14, 1461. 59. Hsiue, G.; Yeh, T.; Chang, S. J. Appl. Polym. Sci. 1989, 37, 2803. RECEIVED for review July 15, 1991. ACCEPTED revised manuscript June 25, 1992.


Structure-Property Relations in Polymers - American Chemical Society

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