Characterization of the Ageing of Plasma-deposited Polymer Films: Global Analysis of X-ray...

SURFACE AND INTERFACE ANALYSIS, VOL. 24, 271-281 (1996)

Characterization of the Ageing of Plasma-deposited Polymer Films: Global Analysis of X-ray Photoelectron Spectroscopy Data

Thomas R. Gengenbach,* Ronald C. Cbatelier and H a n s J. Griesser CSIRO, Division of Chemicals and Polymers, Private Bag 10, Rosebank MDC, Clayton, Victoria 3169, Australia

A protocol for global analysis of x-ray photoelectron spectra of plasma-deposited polymer films is presented. These materials are difficult to analyse because of the multitude of different chemical groups present. The combination of different primary and secondary binding energy shifts results in relatively broad, featureless photoelectron peaks. The protocol is based on fitting a series of spectra obtained by monitoring the surface composition of a plasma polymer film over extended periods of time after deposition. Information obtained from this first round of curve- fitting is used in the form of additional constraints for a second round of fitting. This leads to a self-consistent procedure which is akin to a global approach to curve-fitting. To illustrate application of this method, results of a study of an n-heptylamine plasma polymer are described. The spectral changes on ageing provided clear evidence for radical-initiated oxidation reactions. These reactions generated additional radicals, which sustained the oxidation process for many months and not only led to substantial incorporation of oxygen into the material (forming a variety of carbon-oxygen functionalities) but also to conversion of most of the existing carbownitrogen functionalities to an oxidized form.

INTRODUCTION

Surface modification based on gas plasma treatments is showing increasing importance for the controlled optimization of polymer surfaces in many different techno- logical fields such as biomedical application, adhesion, composite materials, printing and corrosion inhibi- tion.’” However, plasma-deposited thin polymeric coatings (‘plasma polymers’) are known to be suscep- tible to oxidation reactions after fabr i~at ion.~ These post-deposition reactions are insufficiently understood.

We have previously investigated the long-term stabil- ity of plasma polymer films deposited from an n-hexane p l a ~ m a . ~ Based on detailed surface analysis using a multi-technique approach (x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and water contact-angle measurements), oxidation reactions were shown to be the major mecha- nism causing the observed compositional changes. These oxidative processes were initiated by free radicals which were present in the freshly prepared material as, in our experience, is the case for all plasma-deposited films. They resulted in incorporation of substantial amounts of a range of carbon-oxygen functionalities into the plasma polymer, accompanied by an increase in surface hydr~philicity.~

The characterization of these processes was facilitated by the comparatively simple structure of the n-hexane plasma polymer, which is a hydrocarbon-based polymer network with oxygen as the only heteroatom. However, most plasma-deposited coatings of interest for potential applications are more complex and thus present a for-

* Author to whom correspondence should be addressed.

CCC 0142-2421/96/040271 11 0 1996 by John Wiley & Sons, Ltd

midable challenge to the surface analyst. In order to optimize a surface for a particular application, detailed characterization of the surface structure of these materials is crucial; the amount of relevant information that can be obtained from surface analysis data has to be maximized by using sophisticated data processing methods following current best practice.

In the present report we will describe a self-consistent XPS curve-fitting procedure designed to extract detailed information from the spectra of polymers of complex compositions. As an example we will use the analysis of a coating deposited from an n-heptylamine gas plasma. Compared to the n-hexane plasma polymer, additional complexity is introduced by the presence of nitrogen in the polymer.

Nitrogen-containing, plasma-deposited coatings have shown promising performance in biomedical applica- t ion~. ’ -~ It was shown that the mechanisms of the initial attachment of human vein endothelial cells and fibroblasts are different on nitrogen-containing surfaces compared to purely oxygen-containing materials. It was suggested that surface amide groups are the main pro- moters of cell attachment to nitrogen-containing plasma polymer films,’ but it is necessary to improve compositional analysis and to quantify the amounts of specific chemical surface functionalities. Also, the long-term evolution of the surface chemistry needs clarification.

E X P E R I M E N T A L

Mater ia ls

The substrate for plasma deposition was a polyimide tape (Kapton 100HN, 25 pm thick, 12.7 mm wide)

Received 2N July 1995 Accepted 15 December 1995

212 T. R. GENGENBACH, R. C. CHATELIER AND H. J. GRIESSER

which had been coated with -0.1 pm of aluminium by electron beam evaporation. The highly reflective aluminium surface not only provides a good substrate for FTIR analysis in the specular reflectance mode but also facilitates estimation of the thickness of plasma polymer films by observing the interference fringes.

The monomer n-heptylamine (99% purity, Aldrich Chemical Company, Inc., Milwaukee, WI, USA) was used as received. A fresh batch of the monomer liquid was placed in a round-bottom flask and connected to the reactor chamber by a stainless-steel line and a manual flow control valve. Dissolved gas and volatile impurities were removed by pumping on the liquid for a few minutes prior to ignition of the plasma.

Plasma-deposition

Plasma deposition was carried out in a custom-built reactor which has been previously described in more detail8 It contains two capacitively coupled parallel electrodes which are of rectangular shape (dimension: 18 mm x 90 mm) and spaced by 16 mm. The electrodes form part of a flow channel with a rectangular cross- section of dimensions 16 mm x 18 mm. The gas stream is confined in this channel, which, by guiding all the incoming gas through the discharge region, maximizes usage of the gas as well as providing a defined gas flow path. The other major feature of this reactor is a tape conveyancing system. Several metres of tape (up to - 100 m, depending on the thickness of the tape) can be plasma-treated under stationary plasma conditions. The oscillator used in this study was a commercial plasma generator (EN1 HPG-2) equipped with a matching network and operating in the range 125-375 kHz.

The parameters for the plasma deposition were : monomer pressure 0.17 mbar, frequency 200 kHz, load power 40 W and effective deposition time 15 s. The effective deposition time is the time that the moving substrate tape spent in the plasma glow. The estimated film thickness was 70-90 nm.

The uniformity of the deposition was assessed by XPS analysis of specimens from different locations of the freshly coated substrate and by visual examination of the interference fringes. The tape was cut into short sections, providing a large number of identical specimens. These were stored in clean tissue-culture polystyrene (TCP) dishes (exposed to conditioned laboratory air) at room temperature (20 f 1 "C) and analysed periodically. The TCP containers do not contain any polymer additives which could diffuse to the surface and provide a potential source of adventi- tious contamination. The analysis of the first specimen took place within 30 min of venting the plasma reactor with N, and the subsequent exposure to air. The avail- ability of a large number of identical specimens is crucial for a long-term study because it prevents the accumulation of radiation damage due to repeated XPS analyses of the same specimen over longer periods, as outlined below.

X-ray Photoelectron spectroscopy

X-ray photoelectron spectroscopy analysis was per- formed in a Vacuum Generators Escalab V unit with a

non-monochromatic A1 Ka source at a power of 200 W (10 kV, 20 mA) and a hemispherical analyser operating in the fixed analyser transmission mode. The total pressure in the main vacuum chamber during analysis was typically 2 x lo-' mbar. Calibration procedures which are detailed in our previous publication4 were based on the work by Anthony and Seah.' With a pass energy of 30 eV, the full width at half-maximum (FWHM) of the Ag 3d,,, peak was 1.5 eV. A value of 285.0 eV for the binding energy of the main C 1s component (aliphatic hydrocarbon) was used to correct for charging of specimens under irradiation." Several specimens were also analysed on a Surface Science Instruments SSX-100 spectrometer equipped with a monochromatic A1 Ka source.

Elements present were identified from survey spectra. For further analysis, high-resolution spectra were recorded from individual peaks at 30 eV pass energy. All results presented in this report were obtained at photoelectron emission normal to the surface (90° take- off angle). The elemental composition of the surface was determined based on a first-principles approach;' atomic number ratios were calculated with integral peak intensities (using a non-linear Shirley-type background) and published values for photoionization cross-sections.'2 The inelastic mean free path of the photoelectrons was assumed to be proportional to Eo ', where E is the kinetic energy.13 The transmission function of the analyser had previously been determined to be proportional to E-0 .5 .

The random error associated with quantification was determined following the method suggested by Harrison and H a ~ e l 1 . l ~ The largest error of -8% was associated with measuring the area of the comparatively weak N 1s peak. The contribution of systematic errors is difficult to estimate, but was assumed to be small because corresponding data obtained using another instrument agreed to within a few per cent. Also, in this study we are more interested in relative changes of surface properties over time rather than in absolute values; a thorough estimation of systematic errors was therefore not attempted.

Sample decomposition under x-radiation, particularly with non-monochromatized radiation, has been con- sidered. The combined influence of x-radiation, heat and secondary electrons impinging on the sample surface causes a slow degradation of the material being analysed." This effect has been quantified by measuring atomic ratios repeatedly while the specimen was under constant irradiation. Figure 1 displays the results as a relative change (the initial levels of oxygen and nitrogen corresponding to loo%, respectively). While there was no measurable change of the nitrogen content, the oxygen content decreased exponentially over a 5-h period. Specimens were therefore exposed to x-radiation <1 min before the start of the actual data acquisition. Spectra were recorded in the order: 0 Is, C Is, N 1s. Total exposure time never exceeded 30 min; changes to the elemental composition due to sample degradation were thus kept at or below 5%.

Individual components of the photoelectron spectra arising from different chemical species or bonding con- figurations were assumed to have a Gaussian/ Lorentzian lineshape and were quantified using damped non-linear least-squares regression (curve-fitting). The

GLOBAL XPS ANALYSIS OF AGEING PLASMA-DEPOSITED FILMS

I

; I H A . . , / : Oxygen / Carbon ' ii 0 Nitrogen I Carbon

>** - --+ ---- _ .~.~ -4 -,. . .. .~. . . -&.. ..+

213

I ' 0 0 0 1

i

0 6 0 120 1 a0 240 300 Irradiation Time (min.)

Figure 1. Relative change of the XPS atomic ratios as a function of irradiation time in the x-ray photoelectron spectrometer. The data were obtained from an aged sample (after 4 years of storage)

custom-designed data processing software for efficient processing of large numbers of spectra (>50) is based on the implementation of a curve-fitting algorithm by Hughes and Sexton' using a Gaussian/Lorentzian product function as described by Sherwood.16 The software provides the flexibility to apply any combination of constraints (either absolute or relative to a reference component) on any number of fit parameters.

RESULTS AND DISCUSSION

Characterization of freshly deposited n-heptylamine plasma polymer

The only elements detected by XPS were carbon, nitrogen and oxygen. The initial nitrogen content of the plasma polymer was slightly more than half that of n- heptylamine monomer (N/C = 0.086 os. 0.143). This is ascribed to fragmentation of the monomer in the r.f. glow discharge followed by a loss of nitrogen during the polymerization of the fragments onto the substrate. The composition of the plasma polymer depends on the many experimental parameters controlling the plasma. These correlations will not be discussed here.

The initial level of oxygen was very low (O/ C = 0.028). The presence of oxygen in plasma polymers deposited from monomers which do not contain oxygen has been reported previously (Ref. 4 and references therein) and is due to the quenching of trapped carbon- centred radicals with oxygen following exposure to air.

(smoothed data) are included solely to facilitate visualization of changes over time.

The oxygen/carbon ratio increased substantially with time. Several distinct steps were observed in the rate of oxygen incorporation. After each step the rate slowed down noticeably, i.e. a distinctive discontinuity could be observed :

0.20 0 B .- -

0.15 .$

0.10 2 X

0.05

0.3

0 .- 4-

E 0 0.2

5 2

b 0.1

.-

v)

I ,

1 Oxygen I Carbon : ! Nitrogen I Carbon

J

. Oxygen I Carbon : 6 Nitrogen I Carbon

i

Evolution of the surface properties with time

Figure 2 displays the XPS atomic ratios as a function of storage time. Experimental data are displayed using individual plot symbols; the solid or dashed lines

0 5 0 0 1000 Storage Time (days)

Figure 2. The XPS atomic ratios as a function of storage time. Error bars represent one standard deviation. The three stages of oxygen incorporation described in the text are indicated by roman numerals and are separated by the vertical dashed lines. (a) Data over first 3 months. (b) Complete data set.

T. R. GENGENBACH, R. C. CHATELIER AND H. J. GRIESSER

A very rapid increase of the oxygen content (by -0.08) occurred during the first few days of exposure to air after deposition. After 4-6 weeks the O/C ratio had increased by an additional 0.08. The incorporation of oxygen continued thereafter at a very low rate. Saturation of the oxygen content has not been observed vet.

These stages have been observed with other plasma polymers also (to be published).

The nitrogen/carbon ratio slightly decreased with time, the total nitrogen loss after several years of storage amounting to nearly 20%. The major drop occurred during the first few days, simultaneously with the rapid incorporation of oxygen. Several reasons for the observed drop of the N/C ratio are possible: attenu- ation of the N 1s photoelectrons by a hydrocarbon (contamination) overlayer, which might build up over time; an energetically unfavourable situation at the polymer/air interface could lead to a rearrangement of the polymer, thereby burying the polar amine groups below the surface; and actual loss (sublimation) of nitrogen-containing fragments from the plasma polymer could occur, e.g. by hydrolysis of imines as proposed by Gerenser.” Attempts to remove possible contamination using ultrasonication in a range of solvents resulted in no significant change in the measured surface compositions. Experimental evidence indicates some involve- ment of surface rearrangement in the decrease of the N/C ratio.’ *

Based on the knowledge gained from a study of the model system n-hexane plasma p ~ l y m e r , ~ we can sum- marize and interpret the results so far: the freshly deposited n-heptylamine plasma polymer film consists of a hydrophobic hydrocarbon network and a low level of incorporated nitrogen (partially in the form of amines”). It contains a substantial concentration of trapped carbon-centred radicals which are rapidly quenched by oxygen on exposure to air. A long-term oxidation process follows, passing through several distinct stages and resulting in the formation of a range of carbon-oxygen (carbon-oxygen-nitrogen) functionalities on the surface of the plasma polymer. Fourier transform infrared spectroscopy showed the ‘bulk’ of the material to be affected in a similar way (data not shown).”

Up to this stage the analysis has been relatively straightforward. However, more detailed information concerning the chemical changes over time is required in order to sufficiently characterize the mechanisms of oxidative ageing and the changing surface chemistry. This information is essential in evaluating the performance of the material in biological or other applications.

Chemical derivatization combined with XPS should, in principle, permit quantification of specific functional groups at polymer surfaces.” This method is, however, not without problems, particularly in the case of plasma polymer surfaces where a wide variety of functional groups co-exist, raising doubts about the selectivity of a reaction.” Also, the possibility of non-specific binding effects have to be ~onsidered.’~ At this stage derivatization-XPS is not mature enough to allow reli- able quantification of various chemical species possibly present on our samples.

Curve-fitting XPS spectra

Curve-fitting can be a powerful method of extracting additional information from XPS data, if used with caution. It is usually carried out for two reasons: (1) Contributions from different chemical species can be

quantified. (2) Peak positions of the individual components can be

determined. Once the position of a reference signal such as the hydrocarbon C 1s signal is known, one can then correct peak positions of photoelectron peaks other than the C 1s peak for the inevitable charging of the sample and thus determine their binding energy.

Carbon 1s difference spectra obtained from the n- heptylamine plasma polymer at various times of storage are displayed in Fig. 3. The difference spectra (straight subtraction of the spectrum of the fresh sample from the spectra of the aged sample), which enhance visualization of the spectral changes occurring over time, provided clear evidence for the formation of a range of carbon-- oxygen-nitrogen functionalities as the material aged. At least two additional peaks appeared between 285 eV and 290 eV, and their position relative to the main peak seemed to changed. Difference spectra are, however, difficult to interpret quantitatively because they are very sensitive to the correct alignment of the individual spectra before substraction. Curve-fitting is therefore necessary to quantify position as well as fractional contribution of the individual components as a function of storage time.

Plasma polymers present particular difficulties in this respect for two reasons: firstly, photoelectron peaks are usually broadened due to differential charging; secondly, a wide range of chemical functionalities is introduced on the surface of a plasma-treated material. Each of these groups might cause the observed binding energy of a particular peak (e.g. C 1s) to shift to varying degrees (due to primary as well as secondary shifts). The resulting overall peak shape is a superposition of many components which are not clearly resolved. For

1000

800

600

400 2 v,

2

s u h

- 200 p

0 s - t 1-200 I , 1

Binding Energy (eV)

Figure 3. Carbon 1 s spectra at different times of storage: full spectrum of the freshly deposited n-heptylamine plasma polymer (dotted line) and difference spectra obtained after (a) 1 day, (b) 1 week, (c) 1 month, (d) 6 months, (e) 1 year and (f) 2 years (solid lines). All spectra were normalized to the intensity at 285.0 eV prior to subtraction.

292 290 288 286 284 282

GLOBAL XPS ANALYSIS OF AGEING PLASMA-DEPOSITED FILMS 215

carbon-oxygen functionalities the situation is somewhat simplified because the chemical shifts due to different groups are well known." In this case the shift of curve- fit components with respect to the main (CH,) component can be fixed to known values. In a previous publication we used 1.5 eV for C-0-based groups such as hydroxyls or ethers, 2.9 eV for carbonyls and 4.3 eV for carboxylic acid or ester group^.^ If, in addition to oxygen, nitrogen is present in these materials, the situation becomes more complicated. Firstly, there is considerable scatter of experimental literature data on the C 1s binding energy of different carbon-nitrogen bonds. In a recent review, Petrat et al. have listed the following ranges of binding energy values: 0.8-1.5 eV for amines, 1.5-2.8 eV for imines, 1.3-3.3 eV for nitriles, 3.0-3.6 eV for amides and 3.8-4.5 eV for rea as.'^ Secondly, the difference in binding energy between C 1s contributions arising from C-0 and C-N groups (or C=O and N-C=O groups) is small compared to the spectral resolution of the spectrometer. This is demonstrated in Fig. 4 where even monochromated XPS is unable to resolve the individual C 1s components of n- heptylamine plasma-deposited material. In contrast, the corresponding spectrum of an n-hexane plasma polymer is barely resolved into individual components by monochromated XPS.

When studying the ageing of a plasma-deposited polymer containing both oxygen and nitrogen, it is necessary to interpret spectra in a consistent manner. In view of the above complications, a novel curve-fitting protocol was developed.

First, the minimum number of spectral components needed to obtain an acceptable fit was determined: a series of weighted least-squares fits were calculated for one spectrum (aged sample), increasing the number of peak components successively. The 'quality' of each curve-fit was measured using the value of reduced x 2 and the value of the Q-function. The use of these sta- tistical criteria is based on a discussion in Ref. 25. Reduced x 2 is simply x 2 (sum of the squared residuals)

with respect to the number of degrees of

290 285 280

Binding Energy (eV)

Figure 4. Carbon 1s spectra of aged plasma polymers: (a) n-hexane plasma polymer (monochromatic Al Ka radiation) ; (b) n -heptylamine plasma polymer (monochromatic Al Ka radiation) ; (c) n-heptylamine plasma polymer (non-monochromatic Al Ka radiation).

freedom v (number of independently adjustable parameters subtracted from the number of channels in the spectrum minus 1). Assumingf(z/v) dt to be the probability density function which defines the x 2 distribution, then

Qb lv) = 6;(~ I v) dt

where Q(z 1 v) is the probability of obtaining a fit with a value of xkin (value of xz after minimization) greater than z purely by chance. In our case v is large (100 < v < 150) and therefore the x 2 distribution closely resembles a normal distribution with the most probable value of reduced xHi, being unity. Q(T I v) then resembles the complementary error function where ideally one expects a value of 0.5 for a perfect fit. One standard deviation of the x 2 distribution corresponds to Q values of 0.159 and 0.841, and two standard deviations to Q values of 0.005 and 0.995. A Q value outside this range (0.005-0.995) obtained after a particular curve-fit therefore means that the model used for the fit is highly improbable (< 1%) if only random uncertainties domi- nate. In this case the model would have to be rejected.

In Fig. 5(a) the values for x$in and Q(&,Iv) are plotted vs. the number of components used for the fit. At least four components were required to achieve a Q-value above 0.005, whereas more than five components did not lead to any further improvement. Taking into account physical considerations also, each

........_... a = 0.005

1 2 3 4 5 6 7 0 Number of Peak Fit Components

c1

290 285 Binding Energy (eV)

Figure 5. Carbon 1 s curve-fit. (a) Reduced kin (filled diamonds) and a($,,. Iv) (empty diamonds) plotted vs. the number of components used (see text for details). (b) Carbon 1 s spectrum, including a curve-fit (using five spectral components) and the weighted residuals (vertical scale expanded by a factor of 100 compared to the actual spectrum).


spectrum was therefore fitted with five components (C, , C,, C,, C, and C,), using a single fit parameter to describe the peak width and Gaussian/Lorentzian mixing ratio of all components. These components were assumed to represent CH, at 285.0 eV (C,), C-N (possibly C=N) and C - 0 with a shift of 1.4 eV with respect to C, (CJ, C=O and N-C=O with a shift of 3.0 eV (C,) and acid, ester and urea groups shifted by 4.3 eV (C,). These shifts represented an initial guess of the respective fit parameters. Figure 5(b) shows an actual C 1s curve-fit using five components.

The component C, at -2 eV below the hydrocarbon signal (C,) was found to be necessary to account for the peak asymmetry which we observe in the case of almost all spectra recorded on polymer samples. Adding C, to the three-component fit (C,, C, , C,) increased the value of Q(x,?$, I v ) from zero to 0.091, whereas adding C, to the same three-component fit increased Q(x$, 1 v) to just 0.013 (both points included in Fig. 5(a). Petrat et al. mention the appearance of a C 1s signal at - 1.5 eV below the main CH, peak after nitrogen plasma treat- ment or Ar ion bombardment of polystyrene surfaces.24 This signal, which disappeared on exposure to air, was tentatively described as a graphite-like species. In our case, this additional contribution to the C 1s spectrum did not change on storage. The most likely explanation for the observed asymmetry is differential charging of the specimens. This assumption is supported by the fact that the peak asymmetry is not observed in the spectra obtained with monochromated XPS (see Fig. 4). By using an electron flood gun for charge neutralization, the effects of differential charging are minimized, if not eliminated, in monochromated instruments. In our case, (non-monochromated XPS) differential charging would affect all components and should therefore, in principle, be taken into account by either using asymmetric peak shapes or by adding one extra component for each of the four components C,, C,, C, and C, . This, however, would not only complicate the curve-fit protocol con- siderably but it would also introduce more uncertainties. We therefore prefer to work with the simplified protocol (adding only C,), and note that this will result in a slight systematic error. In the case of one spectrum of the aged sample, which represents the worst case, a five-component fit was compared with an eight- component fit (the latter being equivalent to using four asymmetric peaks): using only five spectral contributions, CH, was overestimated by -1% (C, + Cl); the other components were underestimated by - 1.5% (C,) and by between 2% and 3% (C, and C,, respectively). Because the fraction of C, (associated with the hydrocarbon component) was observed to be approximately constant over extended periods of storage, we believe that the interpretation of the relative changes of the various fractions is not affected. For the remainder of this report we will refer to the combined component (C, + C,) as C,. Secondary chemical shifts, i.e. shifts induced on a neighbouring carbon atom, were not taken into account. These are only significant in the case of, for example, acid and ester functionalities (- 0.7 eV lo) whose contribution to the C 1s spectra were found to be relatively small.

The shifts of C, and C, with respect to C, were not fixed. This allows for the fact that these components represent the superposition of signals from more than

one functional group at slightly different positions. If the chemical structure of the material changes with time, the relative contributions from different functionalities to one particular C 1s component would change. This would result in a change of the peak position of the corresponding peak component with time. Com- ponent C, was fixed at a separation of 4.3 eV. Initial attempts at curve-fitting did not yield a consistent peak position for this particular component because its low relative abundance made a negligible contribution to the sum of the squared residuals.

After optimization, a peak width (FWHM) of 2.07 0.05 eV was obtained, taking into account all fitted spectra. The mean mixing ratio was 0.24 & 0.14, where 0 corresponds to a purely Gaussian peak and 1 to a purely Lorentzian peak. No systematic variation of these parameters was observed as a function of storage time. The shifts of C, and C, determined by this first round of curve-fitting where plotted against storage time. These data are displayed in Fig. 6. Despite considerable scatter, the data indicate that for both components the shift increases with time, which was already suggested by the C 1s difference spectra (Fig. 3). In order to derive a consistent curve-fit protocol, these data were now fitted with an exponential function as follows

Chemical shift (eV) = a, + al[l - exp(-t/a,)] (1)

where t = storage time (days). a, = initial shift (ev), a, + a1 = final shift (eV) and a, = characteristic time constant (days).

Table 1 lists the values and their standard deviations obtained after least-squares minimization. The shift a, was hardly significant in the case of component C, (and the uncertainty in the time constant a, large) but quite substantial in the case of C, .

Each C 1s curve-fit was then recalculated using the separations of the individual components calculated by Eqn (1). Because the large number of spectra in this series (- 50) provide clear evidence for the described long-term trends, this protocol seemed justified. The second round of curve-fitting is equivalent to a global approach to curve-fitting where the information on all available spectra is used to apply an internally consistent procedure to every individual spectrum. This step leads to a reduction in the uncertainty of the final fit parameters, as illustrated in Fig. 7 for the case of the C, component. The global analysis also resulted in a com-

Table 1. Results of non-linear least- squares fitting Eqn (1) to the separations of C 1s components C, and C, with respect to C,. The fit parameters describe the time dependence of the separations (see text for details)

C 1 s component

Fit parameter' c2 c,

a, (ev) 1.56 f 0.06 2.90 * 0.07 a, (ev) 0.1 3 * 0.07 0 37 0.08 az (days) 48 + 7 5 2 5 * 1 4

a For use in Eqn (1).

GLOBAL XPS ANALYSIS OF AGEING PLASMA-DEPOSITED FILMS

3.5

5 v

j 3.0-

i7j

6 2.5- e 3 .o 2 .0-

m . X

c

5 m Q B

1.5-

211

-

0

..___

i " " ' ' " " ' ' ' ' " " " " ' 1

. 4 _ I . _ -

4 1

0 0 0

1.01 0 2 . . . I

10-1 100 1 o1 1 o2 1 o3 Storage Time (days)

Figure 6. Results of curve-fitting XPS C 1 s spectra. Displayed are the separations of the two components C, and C, with respect to the main component C, as a function of storage time, derived from the first round of curve-fitting (see text for details). Data are plotted using a logarithmic time scale to emphasize the changes during the first few months of storage. Error bars represent approximately one standard deviation and were estimated using a method described by Evans."

parable reduction in the uncertainties of the percentages of the other C 1s components (data not shown).

Results of C 1s curve-fitting

Within 3-6 weeks [characteristic time constant a2 of Eqn (l)] the two major C 1s components (apart from the CH, peak) had assumed their final position with respect to C1. This characteristic time is consistent with the evolution of the elemental composition, which changed most dramatically up to about the sixth week of storage. It corresponds to the first two stages of oxygen incorporation mentioned above. In particular C , had shifted by almost 0.4 eV during this period of

time. In the case of the freshly deposited film, where the concentration of nitrogen was about three times higher than the oxygen concentration, the separation of Cz and C, was mainly determined by carbon-nitrogen functionalities. The initial value of 1.6 eV indicated a combination of amines and imines, taking into account the ranges of possible chemical shifts for these groups mentioned earlier.24 Nitriles can be excluded because the N 1s binding energy was -0.5 eV lower than expected for these functionalities (399.1 eV us. 399.6 eV;" see discussion of the N 1s peak below). According to Beamson and Briggs, the C-0 bond causes a shift between 1.1 eV and 2.0 eV, with 1.6 eV being the mean value of over 40 oxygen-containing polymeric compounds." This is also consistent with the observed value. The nitrogen/carbon ratio hardly changed over

10-1 100 1 o1 1 o2 1 o3 Storage Time (days)

Figure 7. Results of curve-fitting XPS C 1s spectra. The fraction determined for the C, component of the C 1s signal is plotted vs. time of storage (the logarithmic time scale facilitates visualization of the data both after short and after long periods of time). Open squares represent results of the first round of curve-fitting (variable separation of C, and C, with respect to C,); filled circles represent results of the second round of curve-fitting (fixed separations), clearly showing the reduction in scatter.


-8 5 Y &?

time ; the oxygen/carbon ratio, however, increased within 6 weeks to a level 2.5 times the N/C level, and therefore C, clearly represents mainly C-0-based functional groups at this stage.

Regarding the separation of C, and C,, the initial and the final values determined were 2.9 and 3.3 eV, respectively. The former value is characteristic for carbonyls, whereas the latter is typical for amides." We conclude that initially the C=O peak originated mainly from carbonyls ; after several months, the increased shift of C, suggests that amides seemed to constitute the majority of C=O functional groups (further evidence supporting this interpretation will be presented during the discussion of the N 1s data). As oxygen is incorporated into the material, there is an increase in the probability that secondary shifts will affect the binding energy of C 1s photoelectrons, which might also con- tribute to the observed changes.

Figure 8 shows the evolution of the functional contributions of the individual components to the C 1s signal intensity over time. The uncertainties associated with quantifying these fractions were estimated following the method proposed by Cumpson and SeahZ5 and are included in the figure as error bars. They are most likely to be an underestimation of the real uncertainties because of the additional constraints applied during the second round of curve-fitting. Therefore, an upper limit for the uncertainty associated with the intensities of the individual C 1s components was estimated as follows: first, 5% confidence limits with respect to position, width and height of each component were determined

i 0 500 1000

I

I C t* - 0 component C3

U.

5 -

0 500 1000 Storage Time (days)

Figure 8. Results of curve-fitting XPS C 1 s spectra. The fraction determined for each C 1 s component is plotted vs. time of storage. (a) Hydrocarbon species. (b) Components representing carbon bonded to nitrogen and/or oxygen. Error bars are equivalent to one standard deviation (see text for details).

for selected spectra using the method suggested by Evans.25 This yielded upper (lower) limits for the peak parameters, above (below) which the value of reduced x2 increased by more than 5%. Intensities of each component were then calculated with the parameter values at the 5% limits obtained during the first step. The following rough estimates for the relative uncertainties were determined: +one-tenth in the case of C,, +one- quarter in the case of C, and +one-third in the case of C, and C,. This assumes a worst case scenario, where the final error in the intensity is a sum of all possible errors due to uncertainties associated with the three individual peak parameters.

Immediately after deposition of the plasma polymer only C, and C, contributed significantly to the overall C 1s signal (-goo/, and -8%, respectively). The fraction of the hydrocarbon signal (the C, component), which is by far the major fraction of the total C 1s signal at all times, continuously decreased. It mirrored the distinct stages in the increase of the O/C ratio. For the other components (C2, C, and C,), the relative intensities were C, > C, > C, throughout. The contribution of C, to the C 1s signal increased rapidly until it reached a maximum after -6 weeks of storage. It then dropped slightly, followed eventually (after more than 1 year) by a slow but steady increase. The intensity of the C, component increased in the same manner as the O/C ratio as a function of storage time, i.e. displaying a rapid rise initially followed by a substantially slower increase. The C, component also increased but remained much less intense than the other contributions to the C 1s spectrum.

Overall, the evolution of the various C 1s fractions of the n-heptylamine plasma polymer resembled the results obtained with an n-hexane plasma polymer., This leads us to assume that the same basic oxidation reactions participated in the observed ageing process. Even the behaviour of C, (C-N/C-0 groups), displaying a maximum after around 1-2 months, was similar. This maximum is believed to be due to metastable species such as hydroperoxides which are formed during the early stages of oxidation and decay over time to form more stable reaction products such as alcohols or carbonyls. We conclude that the additional presence of nitrogen in n-heptylamine plasma polymer compared with the n-hexane plasma polymer did not have a significant influence on the observed ageing process.

In Fig. 9 the fraction of carbon atoms bound to nitrogen and/or oxygen is plotted us. the total content of heteroatoms (O/C in the case of the n-hexane plasma polymer and (N + O)/C in the case of the n- heptylamine plasma polymer). If every carbon atom, contributing to C 1s components C, , C, or C,, were to be associated with just one heteroatom (e.g. as amines, hydroxyls, carbonyls) then the data would follow the straight line x = y. The n-hexane plasma polymer followed this relationship quite closely whereas the n- heptylamine plasma polymer, which had been aged for the same period of time, did so only up to a value (N + O)/C of between 0.25 and 0.30, after which the data clearly levelled off. As Fig. 2(a) shows, a value of (N + O)/C = 0.25 was reached during the second stage of oxygen incorporation, up to about the sixth week. The observed deviation from the linear correlation is due to the formation of chemical groups with a ratio of

GLOBAL XPS ANALYSIS OF AGEING PLASMA-DEPOSITED FILMS 219

533.0

$ 532.8-

2 532.6

m 532.4- m

0 532.2

- % UI

C w

.- r

X 5 0.35: f 0.30- z 5 I

0.25:

;i 0.20:

C m

!i E 0 D

0 a

0.15- r

-

-

-

-

- 1

-

-

- -

-

0.05 0.10 0.15 0.20 0.25 0.30 0.35 Total (N + 0) I C

Figure 9. Correlation between results of curve-fitting XPS C 1 s spectra and elemental composition. See text for details.

-400.0 z -. 0

-399.8 g.

399.4 5

5 399.6 2

m

Y - 399.2 3

heteroatoms to carbon greater than unity. These could be either carbon-oxygen functionalities such as carboxylic acids (O/C = 2) or carbon-nitrogen-oxygen functionalities such as amides ((N + 0) jC = 2). The fact that the oxidation of the n-hexane plasma polymer only resulted in a weak deviation (due probably to the formation of carboxylic acids during the later stages of ageing) leads us to the conclusion that in the case of the n-heptylamine plasma polymer the data indicate the increasing association of oxygen and nitrogen as the material ages (second stage). This confirms the increasing presence of, for example, amides in the aged material, as proposed earlier.

So far we have concentrated on the interpretation of the C 1s peak; both the N 1s and the 0 1s peak were featureless and therefore were less accessible to data processing techniques such as curve-fitting. In both cases, however, we have to assume that they are composite peaks, i.e. a superposition of individual components representing chemically different species. Based on the position of the hydrocarbon component of the C 1s spectrum which had been determined via curve- fitting and then set to 285.0 eV, all N 1s and 0 1s spectra were corrected for charging. The resulting binding energy values are shown in Fig. lO(a).

The N 1s binding energy displayed a clear trend, increasing from an initial value of just over 399 eV to -399.8 eV within the first two stages of oxidation, after which the rate of increase slowed down markedly. It further shifted to higher values (>400.0 eV) as oxidation proceeded during exposure to air. The N 1s data are consistent with the initial presence of C-N-based functional groups such as amines and/or imines. A value of 399.8 eV is characteristic for N-C=O (N-C(0)-N)-based compounds [e.g. poly(acrylamide), nylon, poly(urea)] for which Beamson and Briggs quote a mean value of 399.85 eV." Higher values are observed for urethanes or imides (400-401 eV). This again is clear evidence for the conversion of amines to amides and probably to even higher oxidation states after extended periods of storage.

In an attempt to further quantify the effects of oxidation, we assumed that only two categories of functional

I 1; Oxygen i s 0 Nitrogen 1s (a)



Figure 10. (a) Binding energy values of the N I s and 0 Is photoelectron peaks plotted vs. time of storage. Error bars represent one standard deviation, which includes contributions from determining the positions of the actual peaks (0 Is, N Is) as well as the position of the reference peak (C 1s signal of hydrocarbons). These individual contributions were determined using the method described by Evans." (b) Fraction of the N 1 s peak originating from carbon-nitrogen groups associated with oxygen (component N2) plotted vs. time of storage. See text for details.

groups contributed to the N 1s line: component N, representing amineslimines (N 1s at 399.1 eV) and component N, representing amides/urethanes/ureas/imides (N 1s at 400.2 eV), respectively. The N 1s spectra were fitted using these two components. Figure 10(b) shows the results: after the first two stages of oxidation [see Fig. 2(a)] nearly two-thirds of the nitrogen present is associated with oxygen (component N,). This indicates preferential rather than random oxidation of the carbon-nitrogen functionalities over time. Figure 11 illustrates this point : the nitrogen component N, plotted us. the oxygen concentration for all data points reveals a linear relationship with a slope of -4. The following qualitative explanation for this observation is proposed: one of the key steps of oxidative degradation is the abstraction of hydrogen, whereby new carbon- centred radicals are generated. Hydroxyl radicals, rel- eased by the dissociation of hydroperoxides, are known to be highly reactive toward hydrogen abstraction and have, via diffusion, access to any site within the polymer. The probability of a hydrogen atom being abstracted, however, depends on its immediate chemical environment. Wicks et al. recently ranked amines first in a general ordering of functional groups, which pro- motes the abstraction of hydrogen from adjacent carbon atoms.,' It can therefore be envisaged that


100-

I / V .

5 10 15 2 0 Oxygen Concentration ("6.)

Figure 11. Percentage of nitrogen atoms associated with oxygen (N Is component N2) plotted vs. the atomic concentration of oxygen a t various times of storage. The solid line represents a linear fit through the origin with a slope of 4.27.

hydroxyl radicals preferentially abstract hydrogen atoms adjacent to amines, leading to the observed fast conversion of nitrogen to nitrogen-oxygen functionalities. Possible reaction pathways responsible for this conversion will be discussed in a separate publication.20

The 0 Is binding energy remained constant within experimental uncertainty at between 532.4 and 532.5 eV. Most carbon-oxygen functionalities display an 0 1s binding energy of - 532.5 eV, with the exception of two categories : the oxygen singly bound to the carbon in an ester or acid group ( - 533.5 eV) and the oxygen of an amide group (-531.5 eV).1° However, throughout the ageing process, both of these functional groups represented a minor fraction of all oxygen-containing groups. Because they both increased during ageing, their contributions would not result in a measurable shift of the composite 0 Is peak but merely a slight broadening.

Figure 12 summarizes the XPS results, on a logarithmic time scale, for direct quantitative comparison : clearly the major change of the n-heptylamine plasma polymer during storage was the incorporation of oxygen (labelled 0 in the figure), up to more than 20% of the total composition. The main products of oxidation were C-0- and C=O-based groups (C, and CJ. The concentration of other species, which would cause an even larger chemical shift of the C Is binding energy (e.g. carboxylic acids or urethanes), was negligible during the first year and remained a minor fraction thereafter (CJ. Initially, C, consisted mainly of amines and/or imines. These were either lost (slight decrease of

C 0

1 5 - t! * C Q 0

0 E 1 0 - 0 .- z 5 l

0

NZ

N1

0 .-

0 T - . ..:.; . . . .a .v . I , - . . I . .

10-1 loo l o 1 lo2 lo3 Storage Time (days)

Figure 12. Overview of the time dependence of the surface composition. A logarithmic time scale was used, as in Fig. 7. All data were plotted as atomic concentrations of the total composition of the analysed material. (a) Total oxygen (0) and the two nitrogen components (N,, N2). (b) The carbon components associated with heteroatoms (C2, C,, C.,).

N, + N,) or were oxidized to form carbon-oxygen- nitrogen functionalities such as amides (N, converted to N,). The actual increase of C-0-based groups was therefore greater than the observed increase (5-6%) of C,; over the same period of time C, was reduced by -6% due to the 'loss' of C-N and/or C=N groups (N,). Hence, the overall increase of C-0 species was 11-12%. C , increased by almost 8%, nearly two-thirds of which were nitrogen-containing groups such as amides, and the remainder were present as carbonyls.

CONCLUSION

The surface composition of a polymer coating deposited from an n-heptylamine gas plasma was assessed by XPS over long periods following fabrication. A rapid increase in the oxygen content was observed, followed by a slower oxygen uptake which continued for several years. The level of nitrogen decreased only to a small extent. The resulting large number of spectra enabled us to develop a self-consistent protocol for processing XPS spectra of plasma polymers. The protocol is based on a two-step global non-linear least-squares curve-fitting procedure. When no constraints were applied to fit parameters, such as the position of individual peak components, the uncertainty associated with their value after optimization was considerable due to excessive overlap of components and inevitable spectral noise. By taking into account the information from all spectra we

GLOBAL XPS ANALYSIS OF AGEING PLASMA-DEPOSITED FILMS 28 1

were able to define additional constraints with respect to the separation of different photoelectron peak components. This enabled us to extract more specific information from individual spectra in a second round of curve-fitting. The evolution of various types of chemical functionalities could thus be followed as the oxidation of the plasma polymer proceeded. Our findings are consistent with a radical-initiated oxidation scheme similar to the scheme that we proposed for an n-hexane plasma p ~ l y m e r . ~ In addition, the effect of oxidation on the distribution of nitrogen-containing chemical groups could be characterized: after several months of storage most of the carbon-nitrogen functionalities are associated with at least one oxygen atom (e.g. amides). This eluci- dation of the changes in the chemical composition upon ageing is of direct relevance to the interpretation of interfacial interactions in intended applications of n- heptylamine plasma polymers, where one is interested in using amines either to directly control interactions with the material contacting the plasma coating (e.g.

adhesives, human tissue) or for covalent coupling reactions.

Other techniques (FTIR, contact angle measurements, static SIMS) have dso been used for the characterization of the surface (and bulk) properties of the n-heptylamine plasma polymer. These additional data, which are in agreement with the XPS results, will be presented elsewhere.20

Acknowledgements

We thank Professor €3. D. Ratner for access to the SSX-100 spectrometer at the National ESCA and Surface Analysis Centre for Bio- medical Problems, University of Washington, Seattle, USA (supported by the Division of Research Resources, NIH Grant RR01296).

We acknowledge Mr 2. R. Vasic for the preparatioa of the plasma polymer specimens and Drs A. E. Hughes and H. A. W. St John for helpful suggestions and comments.

The aluminized Kapton was a generous gift from Mr R. Spahn, Eastman-Kodak Research Laboratories, Rochester NY, USA.

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