Polymer degradation on reflecting metal films: Fourier transform infrared (FTIR) reflection-absorbance studies

Solar Energy Materials 11 (1984) 163-175 North-Holland, Amsterdam

163

POLYMER DEGRADATION ON REFLECTING METAL FILMS: F O U R I E R T R A N S F O R M I N F R A R E D (FTIR) REFLECTION-ABSORBANCE STUDIES J.D. W E B B , P. S C H I S S E L , T.M. T H O M A S , J.R. P I T T S a n d A . W . CZANDERNA Materials Research Branch, Solar Energy Research Institute, 1617 Cole Boulevard, Golden, 80401, USA

The technique of Fourier transform infrared reflection-absorption (FTIR-RA) spectroscopy has been successfully adapted to the study of bulk and interfacially activated photodegradation of several types of polymers on various metallic substrates. The technique enables qualitative and quantitative study of photochemical reaction mechanisms and rates. A controlled Environmental Exposure Chamber (CEEC), which permits collection of IR-RA spectra of the polymer/metal samples during their exposure to controlled spectral distributions of UV, temperatures, and gas mixtures, was built into the sample compartment of a Nicolet 7199 FTIR spectrophotometer. Surface analysis, gel permeation chromatography (GPC), UV spectroscopy, and UV spectroradiometry were used to complement the FTIR-RA results.

1. Introduction P o l y m e r s have been p r o p o s e d o r utilized as p r o t e c t i v e coatings for metallic solar reflectors a n d p h o t o v o l t a i c devices [1]. F r e e - s t a n d i n g p o l y m e r i c films have also been p r o p o s e d o r utilized as c o m p o n e n t s of active o r passive solar collectors. T h e metallic surface layer of c o m p o s i t e structures m a y c o n t a i n significant c o n c e n t r a t i o n s of oxides or salts as a result of d e p o s i t i o n of the m e t a l layer or p o l y m e r coating, or s u b s e q u e n t o u t d o o r e x p o s u r e of the p o l y m e r / m e t a l c o m p o s i t e [2]. K n o w l e d g e o f the m e c h a n i s m s of p h o t o d e g r a d a t i o n of b u l k p o l y m e r s a n d at p o l y m e r / m e t a l or p o l y m e r / ( m e t a l oxide o r salt) interfaces is i m p o r t a n t for assessing the p o t e n t i a l d u r a b i l ity of technological systems.

2. Experimental 2.1. A p p a r a t u s

F T I R - R A s p e c t r o s c o p y was chosen as the p r i m a r y a n a l y t i c a l t e c h n i q u e b e c a u s e it is fast, sensitive ( p a r t i c u l a r l y to thin organic films on metals) a n d nondestructive. A N i c o l e t 7199 d u a l - b e a m F T I R s p e c t r o p h o t o m e t e r with 0.06 c m - 1 m a x i m u m resolution was used. I n s t a l l a t i o n of the C E E C in the front b e a m of the F T I R s p e c t r o p h o t o m e t e r p e r m i t t e d the s a m p l e s to be e x p o s e d to s i m u l a t e d e n v i r o n m e n t a l conditions, 0 1 6 5 - 1 6 3 3 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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J.D. Webb et al. / Polymer degradation on reflecting metal films

including UV radiation, while IR-RA spectra were collected. A prototype CEEC has been described [3,4]; an improved version is sketched in fig. 1. The latter instrument utilizes two off-axis parabolic reflectors to achieve an optical throughput of 75% (with the chamber salt windows removed). The reflector focal length was chosen to minimize the area of the IR image at the sample and detector using a software model of the FTIR optical bench. The IR image on the sample is approximately 1 × 0.3 cm 2, and the incidence angle (q,) is 77.5 °. The CEEC accepts a collimated UV beam from an Oriel Model 6732 solar simulator with a 100 W, high-pressure xenon arc lamp. The simulator is equipped with a dichroic reflector that essentially limits the spectral output of the lamp to a band between 240 and 500 nm and greatly reduces the thermal loading of the samples. Dichroic and absorption filters are available to attenuate further the short wavelength portion of this band. The spectral output distributions of the simulator created by using a series of Schott glass absorption filters are presented in fig. 2. The number assigned to each filter indicates the wavelength (rim) of 50% transmittance. The distribution produced by the WG-305 filter is a fair match to the outdoor UV spectrum measured at the equator in December 1975 [5]. The measured terrestrial UV flux at 295 nm was about 0,025 ~W c m - 2 n m - 1 and UV fluxes were detected at wavelengths as short as 290 nm [5]. The simulator intensity is about 16 times the reported terrestrial irradiance from 310-320 nm. The simulator spectrum for WG-305 filtering is greater than 16 times more intense than their spectrum in the region from 295 to 310 nm. Measurable UV flux was observed at wavelengths as short as 287 nm in the simulator output spectrum for WG-305 filtering. The non-zero baselines in the simulator spectra in fig. 2 are artifacts of stray light in the G a m m a Model DR-2 spectroradiometer used to measure these spectra. To assess the chemical composition of the reflective metallic substrates, a

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Fig. 1. Controlled Environmental Exposure Chamber (CEEC) and IR path incident onto a polymeric coating on a reflecting substrate.

J.D. Webb et al. / Polymer degradation on reflecting metal films

165

L e y b o l d - H e r a e u s LHS-10 surface analysis system with capabilities for AES, XPS, SIMS, ISS and ion b o m b a r d m e n t depth profiling was employed. The molecular weight distribution of the polymeric samples was determined using a Varian Model 5030 liquid chromatograph equipped with a Varian Model TSK-4000H gel permeation column and a Varian Model VUV-10 detector with variable wavelength. The thickness of the polymeric films was determined at six points around the periphery of each sample using an Alpha-Step surface profiler. 2.2. S a m p l e s

The substrates used were float glass, 1.4 × 1.0 × 0.1 cm 3. Silver reflective surfaces (100 nm) were prepared by r f / d c sputter deposition onto the cleaned substrates in vacuum. Gold reflectors were prepared by vacuum evaporating silicon (20 nm) onto the substrates to promote adhesion, followed by evaporating a 170 nm gold film. Bisphenol-A polycarbonate (BPA-PC) and polymethylmethacrylate (PMMA) films were cast onto those reflectors by dipping them into 2.4 g/1 tetrahydrofuran (THF) solutions of the polymer. After annealing in vacuum at 100°C, the polymer film thickness d was measured. 2.3. Technique

To obtain the I R - R A spectrum of a sample, an uncoated reflector was placed in the C E E C and its spectrum (reference spectrum) was collected and stored. The spectrum of the sample was recorded, divided by the reference spectrum, and

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166

J.D. Webb et al. / Polymer degradation on reflecting metal films

converted to absorbance units. Similar IR-RA spectra were generated as the sample was exposed. Difference spectra showing only IR-RA changes were obtained by subtracting the original IR-RA spectrum of the sample from those subsequently collected.

3. Quantitative IR-RA 3.1. Optical model

An optical model of the polymer/metal samples was developed to enable quantitative information on polymer functional group depletion a n d / o r reaction product accumulation to be obtained from IR-RA spectra of degrading samples. The model required knowledge of the optical constants N = n - ik for the polymeric functional groups and reaction products at their fundamental vibrational frequencies, as well as the optical constants of the metallic substrates. Sufficiently accurate optical constants of metals in the IR are generally available in the literature [6], but the optical constants must be measured for the polymers. The procedure used for determining the optical constants of a 3 mm thick polymer sheet by using a polarimetric surface reflectance technique has been described [7]. The absorptivity k may also be determined from the absorbance of a dilute solution of the polymer, enabling n to be determined using the Kramers-Kroning relationship. For the latter technique, a polarimetric measurement of n~ [7] is needed, but this can be done at a single frequency sufficiently distinct from the fundamental frequency [8]. Polarimetrically determined optical constants for BPA-PC are compared with those determined from absorption measurements and a polarimetric n~ in fig. 3. The iterative software for converting the polarized surface reflection spectra of a polymer sheet to n and k as a function of frequency was compiled into a Fortran 77 program compatible with the Nicolet 1180 minicomputer, enabling direct access to the FTIR reflectance spectra used as input data. The authors have made this program available through the Nicolet User's Society. 3.2. IR-RA of 15 nm thick P M M A

The utility of using 0.1-1.0 p~m thick polymeric films on metals for studying rapid bulk photoprocesses was discussed [7]. Thinner films may be utilized to study interfacial reactions. The carbonyl IR-RA band of a 15 nm thick PMMA film on gold is shown in fig. 4; the fundamental vibrational frequency is at 1731 c m - ~ [8], in contrast to the absorption of the highly conjugated BPA-PC carbonyl at 1775 cm-1 The spectrum exhibits a S / N ratio of about 30/1 although there is some interference by water vapor at these low levels of absorption. The high IR throughput of the CEEC, achieved by using antireflective coatings on the salt windows and polarizer, is responsible for the good S / N ratio. Ten thousand scans were made during the 2 h required to collect the data shown in fig. 4. A cooled MCT IR detector was used for collecting data for both the sample and reference spectra.

167

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3.3. Calculated and measured IR-RA spectra for B P A - P C The optical model [7] of the p o l y m e r / m e t a l samples enables the IR-RA spectra of polymer functional groups to be predicted (fig. 5) and enables correlation of changes in the measured spectra with changes in the concentration of the functional groups in the polymeric bulk. The model assumes random orientation of the functional groups, but a model for oriented polymeric films is being developed. Fig. 6 is a plot of the concentration of the first product of the photo-Fries rearrangement [3,4,9] in two BPA-PC/gold samples as a function of UV exposure time in air at 25 °C. To calculate these concentrations, a value of 0.224 for k ..... for photo-Fries (I) was estimated from the carbonyl absorption (1685 cm -1) of phenyl salicylate (a monomeric analog to photo-Fries (I)), with the assumption that 100 mol% (4.72 M) of the BPA-PC carbonyl groups were rearranged. Using this value for k .... and a constant value of n = n ~ = 1.410 (n is invariant with k at the fundamental oscillator frequency of an isolated k band [7]), a plot of IR-RA (1685 cm -1) vs. k was developed. For values of k above approximately 0.022, corresponding to 10% rearrangement of BPC-PC carbonyl to photo-Fries (I), the plot is non-linear, indicating that standard techniques based upon measurement of absorbance band intensity or area would give inaccurate results in I R - R A characterization of strongly absorbing samples. This point is presented in quantitative detail in ref. [7] and ref. [9]. p. 72.

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J.D. PVebb et a L / Polymer degradation on reflecting metal films

169

3.4. Quantum yields of BPA-PC photoproducts From the plot of RA vs. k derived from each sample, the extent of rearrangement was calculated as a function of exposure time (fig. 6) from the IR-RA measurements made during exposure of the samples. The accuracy of the IR-RA concentration measurements was confirmed using UV-RA spectroscopy [9]. From the initial photodegradation rate, the initial absorption spectrum of the polymeric film, the measured film thickness, and the UV output spectra of the solar simulator (AM-0 spectra not shown in fig. 2), initial quantum yields for photo-Fries (I) were calculated. The initial quantum yield for AM-0 filtering was 0.016, and 0.023 without filtering, indicating that the quantum yield of this rearrangement product increases with decreasing wavelength. Thickness reductions, of 10-15% were also noted after exposure of the BPA-PC films.

3.5. Influence of UV wavelength distribution on photodegration of PMMA PMMA films ( = 100 nm thick) were cast onto substrates coated with pure Ag, Au and A1 films. Each sample was exposed in air at 25°C to WG-305, WG-295, WG-280, and unfiltered UV radiation (fig. 2) for 3.75 h each. The sensitivity of the

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170

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F T I R - C E E C measurements to change in the P M M A carbonyl functional group concentration is about 1% for the 250 scans used to collect the sample spectra. Only when samples were exposed to WG-280 or unfiltered UV were reductions in carbonyl functionality detected. A typical set of P M M A I R - R A spectra, collected before and after exposure of P M M A / g o l d Sample 67, is shown in fig. 7. The subtraction spectrum generated from these spectra (curve (A) fig. 8) when compared with a subtraction spectrum generated after exposure of the sample to WG-305 filtered UV (curve (B)) shows that there is a loss in carbonyl (curve (A)) but none in curve (B). There was no difference noted in the behavior of the samples cast on A1, Ag or Au. Therefore, for greater than a 1% reduction in the P M M A carbonyl group concentration to be observed in 3.75 h, UV wavelengths shorter than 260 nm (WG-295 filter cut-off) must be utilized. All of the P M M A absorption bands decreased in intensity under exposure to short wavelength UV, suggesting that the photoproducts are volatile and are carried away by the air flow maintained in the CEEC. In support for this hypothesis, G P C analysis of P M M A films that were solvent-stripped from the substrates showed reductions in the weight average molecular weight and increases in dispersivity for the samples exposed to unfiltered and WG-280-filtered UV.

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J.D. Webb et a L / Polymer degradation on reflecting metal films

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J.D. Webb et aL / Polymer degradation on reflecting metal films

172

3.6. Photodegradation of P M M A when chloride is present on silver Although PMMA films cast on pure silver surfaces do not exhibit rapid photodegradation when exposed to WG-305 filtered UV, contamination of the silver surface with a UV-absorbing chloride can lead to interracial degradation. In fig. 9, curve (C) is a subtraction spectrum showing substantial change in the IR-RA spectrum of P M M A / A g Sample 74, which was exposed to WG-305-filtered UV for 80 min at 25°C in moist air containing 104 ppm NO z. The sample substrate (evaporated Ag) appeared contaminated (milky yellow-white) before coating with PMMA. The sample darkened after UV exposure. The IR-RA spectral changes following exposure of this sample to long-wavelength UV are similar to those observed for P M M A / m e t a l samples exposed to short-wavelength UV (unfiltered or WG-295 filtered). However, sample 74 exhibited a dramatic loss of IR specularity, as exhibited by the sloping baseline of curve (C). Exposure under identical conditions of two similar samples (nos. 73 and 76) of PMMA on pure silver, produced no IR-RA spectral changes (no curves are plotted in fig. 9 for these two samples). This indicates that substrate contamination rather then the U V / N O 2 / H 2 0 exposure caused the degradation of sample 74. Surface analysis and depth profiling of sample 74 after exposure and removal of 0.4 0 = 77.5 ° 0.3 c¢-

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J.D. Webb et al. / Polymer degradation on reflecting metal films

173

the PMMA film revealed 14 at% of chloride contamination in a layer extending at least 10 nm into the silver substrate. A contaminated sputtering target was a likely source of AgCI. Similar analysis of sample 76, which was insensitive to long-wavelength UV, revealed only 1.4 at% of chloride contamination. This led us to hypothesize that photo-induced decomposition of the silver chloride interfacial layer of sample 74 was responsible for the degradation of the PMMA film. The reaction sequence: UV()~ > 287 nm)_ kT AgC1 [ A g ° . . . CI'] , Ag + CI" RH + CI'--, R'+

HCI

explains both the degradation of the PMMA film (which itself is insensitive to WG-305-filtered UV) as well as the darkening and loss of IR specularity of sample 74 (scattering by finely-divided Ag). A similar reaction sequence was proposed by Zaitseva et al. [10] to explain the sensitization of PMMA to long-wavelength UV by ferric chlorides. The photo-activity of the silver halides is well-known. To confirm our hypothesis, sample 73, which previously exhibited no detectable degradation, was exposed to C12 (92 kPa, 25°C) for 45 min. The sample visibly whitened and lost reflectivity, presumably because of AgC1 formation a n d / o r C12 attack on the PMMA film. A subtraction spectrum generated between IR-RA spectra of sample 73 before and after C12 exposure is given in fig. 9, curve (A). The spectral changes are similar to those reproduced in curve (C) for sample 74, except that specularity was retained and an (unassigned) broad band appeared between 1000-750 cm-a. Sample 73 was then exposed to WG-305-filtered UV for 45 min in

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2800

2600

2400

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1800

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174

J.D. Webb et al, / Polymer degradation on reflecting metal films

dry air at 25 ° C. Further losses in PMMA functionality and a decrease in specularity were noted (curve (B)). The loss in specularity is similar to that evident in curve (C). Evidently, both the PMMA and Ag were attacked by the C12 to yield volatile fragments and AgCI. Upon UV exposure, the remaining PMMA was further degraded by AgC1. Dispersions of PMMA in solid AgCI exhibited similar spectral changes upon UV exposure. When the latter experiment was carried out in a sealed cell filled with dry, CO2-free air, the IR spectra of the gaseous reaction products could be recorded (fig. 10). The presence of HCI confirms the proposed reaction sequence. The presence of CO 2, H20, and CO indicates that further reactions of the polymer radical occur. The absorption at 1770 cm-1 indicates that volatile degradation products containing carbonyl functional groups are also present. The results in this section have important implications for attempts to protect silver surfaces by a polymeric coating, since chlorides are common environmental pollutants [11] and react readily with silver. The polymeric coating must be chloride-free and must resist permeation by rainwater (containing chlorides) in order to prevent this degradative reaction, since it would be difficult to stabilize the coating against attack by chloride radical.

4. Conclusions The technique of in-situ FTIR-RA spectroscopy has been used to assess the environmental stability of PMMA and BPA-PC polymer on various metallic substrates. Both bulk and interfacial reactions have been observed, and the capability of obtaining quantitative as well as qualitative information about these reactions has been demonstrated. Coupling the FTIR-RA technique with other analytical methods provides a more detailed understanding of the degradative reactions and confirms the quantitative results obtained.

Acknowledgements The authors are pleased to acknowledge support for this work by the Division of Materials Sciences, Office of Energy Research, through DOE Prime Contract DE-AC02-83CH10093. The assistance of Daryl Myers (SERI) in collecting the spectra presented in fig. 2 is also gratefully acknowledged.

Glossary d k max

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J.D. Webb et al. / Polymer degradation on reflecting metal films

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175

real portion of refractive index of polymer (2) or substrate (3), polarization parallel to incidence plane, polarization ratio of incident IR radiation, polarization perpendicular to incidence plane, fundamental vibrational frequency, incidence angle, standard deviation of Gaussian distribution, roughly equal to a band half-width at 2//3 maximum.

References [1] W. Carroll and P. Schissel, Polymers in Solar Energy Utilization, eds. C.G. Gebelein et al., ACS Symp. Ser. vol. 220 (Am. Chem. Soc., Washington, DC, 1983) p. 3. A.W. Czanderna, Solar Energy Mater. 5 (1981) 349. [2] T.M. Thomas, J.R. Pitts and A.W. Czanderna, Appl. Surface Sci. 15 (1983) 75. [3] J.D. Webb, D.M. Smith, A.R. Chughtai, P. Schissel and A.W. Czanderna, Appl. Spectr. 35 (1981) 598. [4] J.D. Webb, P. Schissel, A.W. Czanderna, A.R. Chughtai and D.M. Smith, SPIE 289 (1981) 190. [5] W.H. Klein and B. Goidberg, Proc. ISES, vol. 1, eds. F. de Winter and M. Cox (Pergamon Press, New York, Jan. 1978) p. 414. [6] M.A. Ortal et al., Appl. Opt. 22 (1983) 1099. [7] J.D. Webb, G.J. Jorgensen, P. Schissel, A.W. Czanderna, D.M. Smith and A.R. Chughtai, Polymers in Solar Energy Utilization, eds. C.G. Gebelein et al., ACS Symp. Ser. vol. 220 (Am. Chem. Soc., Washington, DC, 1983) p. 143. [8] D.L. Allara, A. Baca and C.A. Pryde, Macromolecules 11 (Dec. 1978) 1215. [9] J.D. Webb, SERI/TR 255-1602, Solar Energy Research Institute, Golden, CO (Nov. 1983) (avail. NTIS). [10] N.I. Zaitseva, T.V. Pokholok, G.B. Pariiskii and D.Ya. Toptygin, Dok. Akad. Nauk. SSR 229 (1976) 906. [11] Corrosion: Metal/Environmental Reactions, vol. 1, ed. L.L. Shreir (Newnes-Butterworths, London, 1976) p. 2 : 27.