Microanalysis of Polymers Using a Windowless Energy-Dispersive

X-Ray Detector

UMAR HAYAT, PHILIP N. BARTLETI', and GEORGE H. DODD, Department of Chemist?y, University of Warwick, Coventry CV4 7AL,

Enghnd, and MICHAEL H. LEWIS, Department of Physics, Uniwrsity of Warwick, Coventry CV4 7AL, England

Synopsis

The surface compositions of various polymeric films, grown electrochemically on platinum foils, have been investigated by energy-dispersive x-ray analysis in conjunction with scanning electron micrmcopy (SEM/EDS). Comparison of the relative mea ratios of peaks for the C and N Kam lines show that the EDS may be used to study the surface composition of polymers. The evidence presented strongly suggests that there is limited structural degradation and the elemen- tal composition is not changed under the electron beam at relatively low accelerating voltages. This technique statistically samples the repeat units of the polymer. For samples grown in both aqueous and nonaqueous solutions. SEM/EDS provides evidence for extensive contamination with oxygen.

INTRODUCTION

Very little work has been reported on the study of surface morphology of electro-active p o l p e r ~ . l - ~ In this paper, we report preliminary x-ray analysis studies of surface composition of various polymers synthesized electrochem- ically on platinum foils from aqueous and nonaqueous solutions. From studies of the relative intensities of x-ray emission lines, excited within a Scanning electron microscope (SEM), we establish the composition of these materials and draw comparison with data pertaining to the bulk. This is obviously of relevance in establishing the incorporation of counter-ions in electroactive polymers as a function of chemical conditions and in the determination of the elemental composition of electrochemically prepared copolymers. For systems in which electron-beam induced degradation is not a severe problem, the composition of polymers can be derived from observation of the relative intensities of core-level peaks. The analytical data is that for a surface layer < 1 pm in thickness, dictated mainly by the excitation depth for the low electron-beam energies (< 5 kev) used in this work. We have shown that there is negligible variation in composition with electron-irradiation time and the stability of SEM images also indicates that irradiation-induced atomic displacements are not accompanied by polymer decomposition. The absence of detectable irradiation damage is probably due to the conductivity of the polymers used in this study.

Journal of Polymer Science: Part A Polymer Chemistry, Vol. 26, 201-206 (1988) 0 1988 John Wiley & Sons, Inc. CCC 0360-3676/88/010201-os$o4.00

202 HAYAT ET AL. TABLE I

Conditions Used on Pt Foils for the Electrochemical Synthesis of Polymers

Et4NBF4 Buffer pH 7.2 Time Potential Monomers (mol dm-3) (mol dm-3) Solvent (min) (V vs. SCE)

Pyridazine, 0.1 mol dm-3 0.1 - CH3CN 40 3.8 2-aminopyridine, 0.1 mol dm-3 0.1 - CH3CN 40 4.4 Pyrrole, 0.05 mol dm- 0.1 0.15 H20 5 0.8 N-methylpyrrole, 0.05 mol dm-3 0.1 0.15 H,O 5 0.75 Aniline, 0.02 mol dm-3 0.1 0.15 H20 5 1.55

Diphenylamine, 0.05 mol dm-3 0.1 - CH3CN 40 1.15 - CH&N - Indole5-carboxylic acid, 0.02 mol dm-3 0.1 -

EXPERIMENTAL

The polymers were synthesized by electrochemical oxidation of the ap- propriate monomers at platinum electrodes. Table I summarizes the polymer- ization conditions used. Tetraethylammonium tetrafluroborate (Et ,NBF,, Aldrich Chemical Co.) was purified by recrystallization from ethanol and dried at 100°C under vacuum. Spectroscopic grade acetonitrile (Fisons PLC) was used as received. Diphenylamine (Fisons PLC) was further purified by recrystallizing twice from f r d y distilled 40-60 petroleum ether. Pyridazine (Fluka AG), 2-aminopyridine, pyrrole, N-methylpyrrole, and aniline (Aldrich Chemical Co.) were distilled under reduced pressure before use. Indole-5- carboxylic acid was used as received. Infrared analysis confirmed the overall purity of the samples and, where comparisons were available, agreed with the literature.

Sample Preparation

Polymeric films for SEM were obtained from solutions containing the appropriate monomer and 0.1 mol dm-3 Et,NBF, either in acetonitrile, or water buffered with di-sodium hydrogen orthophosphate at pH 7.2. The polymerization was performed in a one compartment cell containing a platinum foil (1 cm2) as the working electrode, a platinum gauze counter electrode, and a saturated calomel reference (SCE) electrode. All potentials are reported with respect to SCE (Table I). All films were grown by electro-oxidation of the monomer at a constant potential under a nitrogen atmosphere ,except for indole-5-carboxylic acid. Poly(indo1e-5-carboxylic acid) films were prepared by cycling the potential between -500 and 1200 mV after initially stepping the potential from -500 to 1400 mV and holding at that potential for 30 s. This procedure has been found to give good quality films of poly(indo1e-5-carbox- ylic acid). The adherent polymeric films were washed extensively either with acetonitrile or water and dried under vacuum at room temperature for at least 4 h before use in SEM investigation.

Instrumentation and Analytical Procedure

Potentiostatic measurements were made using a purpose built potentiostat and the cyclic voltammograms were recorded on a Gould, 60000 series, X-Y recorder. Platinum foils (1 cm2) were cleaned in a flame before use.

MICROANALYSIS OF POLYMERS 203

Energy-dispersive x-ray spectra were recorded with a Link Systems LZ5 “ windowless” spectrometer interfaced with a Cambridge, S250MK3, Scanning electron microscope. Typical conditions were: electron accelerating voltage, 5KV; electron beam current, ca. 50 PA; pressure in the chamber, ca. lo-’ torr; and x-ray -+ke-off angle, 45”. The normal x-ray energy-dispersive technique, using a Be-“window” at the detector entry point does not permit the analysis of light elements below sodium. The “windowless” system, used in this research, is capable of detection of x-rays down to boron. Neither technique has access to a reliable “correction” procedure for light element spectra in converting relative peak intensities into quantitative data. However, compari- son of peak-intensities for C and N with the calculated stoichiometric ratios is shown here to be a method for deriving an empirical correction factor for this range of materials under the same analysis conditions.

No evidence was obtained for a change in surface composition from pro- longed exposure to the electron beam (3 h). Overlapping p& were resolved into their individual components using an analogue computer. The line shapes are approximately Gaussian but contain a low-energy “tail” due to incomplete charge collection in the detecting crystal. The deconvolution procedure and comparisons between spectra from different polymers demands a close control over a number of variables such as electron energy, take-off geometry, line- width and so on.

Infrared spectra were obtained on a Perkin-Elmer 457 grating infrared spectrometer.

RESULTS AND DISCUSSION

The SEM has been extensively used to study the surface morphology of wide range of materials,6-’2 but relatively little work has been reported on the use of EDS in microanalysis of polymers and other materials containing light elernent~.’~-’~ As a result of our interest in the characterization and detailed investigation

of electroactive polymers, we have carried out microanalysis on a range of conducting polymer model systems to investigate the use of SEM/EDS for polymer characterization. The x-ray (EDS) spectra for the electrochemically deposited polymeric films on platinum described in Table I are reproduced in Figure 1. In each case, the C, N, 0, and F signals reveal similar FWHM, 120 eV, and are centered at 277, 392, 525, and 677 eV, respectively. The span in energy for a given core level corresponding to substitution in differing electro- negativity is negligible and the shift in energy characteristics of the chemical environment cannot be determined with EDS. The strong signals for oxygen indicate that the polymeric films are extensively oxidized. The presence of high levels of oxygen as seen by EDS in polymers of this type is a general feature. These high levels may arise from the oxidized platinum foils, from water, or from the counter anion, HPOL~. Di-sodium hydrogen orthophos- phate was used as a buffering agent for polymers grown in aqueous solution. Traces of water present in spectroscopic grade acetonitrile, as received, may also contribute to the oxygen signal.

The F signal arises from the counter anion, BF;. The anion accumulates in the film to provide overall electrical neutrality during electrochemical oxida-

204 HAYAT ET AL.

Fig. 1.

I I ' t * I

I ' In I

Poly(pyridazine I

1 I I 1

0.0 5 5 0 1100

Energy ( e v ) SEM spectra for electrochemically deposited polymer films on platinum.

MICROANALYSIS OF POLYMERS 205

Stoichiometric Ratios C / N Fig. 2. Plot of measured C/N peak area ratios vs. stoichiometric ratios for polymers of: (1)

pyridizine; (2) 2-aminopyridine; (3) pyrrole; (4) N-methylpyrrole; (5) aniline; (6) indole-karhxyl- ic acid; (7) diphenylamine. Slope = 0.66.

tion of polymer and is present in significant proportions. The level of incorpo- rated ions in the film depends on the extent of polymer oxidation.

Measurement of the relative area ratios for C and N levels of polymer films, grown electrochemically on platinum foils, plotted against the stoichiometric ratios provide an excellent straight line correlation (Fig. 2), the slope being 0.66 f 0.02. This coefficient was determined as an average of two individually prepared samples, two different areas being scanned for each of the samples. This excellent correlation coefficient for this range of polymer types and the C/N ratios as a function of stoichiometry indicates that the incorporation of NEta and CH,CN in these films under the conditions used to prepare the samples for the EDS is negligible because the additional N signal would have resulted in a systematic shift in a straight line from the origin. However, some difliculty was encountered in determining the base line for very small nitrogen signals during estimation of C/N area ratios as a function of stoichiometry. The slope of Figure 2 provides the required sensitivity factor for the C with respect to N core levels. It was assumed that the incomplete charge collection tail in peak integration is proportional to the main peak height in each case. This fairly simple correction factor takes spectral peaks distorted towards lower energy into account and is only applicable to light elements within this range of materials. The sensitivity factor is of course dependent on the composition and types of elements in the specimen that govern the absorption and fluorescence of x-rays, on the relative cross section for ionization by the electron beam and also on spectrometer factors such as sensitivity of the detector for x-rays of different energies. This technique is thus well suited to the determination of elemental composition of conducting polymer films of unknown polymeric materials produced by the copolymerization of two or more monomers.

206 HAYAT ET AL.

CONCLUSION

We have shown that EDS can be employed to investigate the surface composition of conducting polymers. Direct measurement of relative area ratios for the C and N core levels as a funcFion of stoichiometry of the monomer yields an excellent straight line indicating that NEt: and CH,CN are not significantly incorporated in these polymer films and the films are stable under the electron beam. The correlation factor obtained in this way can be used to derive the stoichiometry from intensity ratios of unknown samples. This approach should also be equally applicable to the study of C/F intensity ratios as a function of stoichiometry and the investigation is in progress.

We wish to thank the Biotechnology Directorate of the SERC for financial support. We are also grateful to R. Whitaker for assistance in preparing polymer films and G. Smith for assistance in operating the SEM/EDS system.

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Received October 29,1986 Accepted February 11, 1987