Quantitative Study of Methyl Methacrylate-Poly(Ethylene Glycol) Methacrylate Copolymer Films using High Mass Resolution ToF-SIMS

SURFACE AND INTERFACE ANALYSIS, VOL. 25, 725È733 (1997)

Quantitative Study of MethylMethacrylate–Poly(Ethylene Glycol) MethacrylateCopolymer Films using High Mass ResolutionToF-SIMSs

D. Briggs1,* and M. C. Davies21 ICI Plc, Wilton Research & Technology Centre, PO Box 90, Middlesbrough, Cleveland TS90 8JE, UK2 Laboratory of Biophysics and Surface Analysis, Department of Pharmaceutical Science, University of Nottingham,University Park, Nottingham NG7 2RD, UK

A previous XPS and low mass resolution (quadrupole) SIMS study of methyl methacrylate–poly(ethylene glycol)methacrylate random copolymers indicated (by XPS) a surface composition close to that of the bulk, however thetrends in SIMS intensity ratios for peaks chosen to represent the individual monomers showed departures fromlinearity that were not understood. High mass resolution time-of-Ñight (ToF) SIMS data has shed light on theearlier results and, as a result, linear correlations have been established. In addition it has been possible to studyhigh mass fragments derived from the poly(ethylene glycol) side chain. 1997 by John Wiley & Sons, Ltd.(

Surf. Interface Anal. 25, 725È733 (1997)No. of Figures : 14 No. of Tables : 1 No. of Refs : 11

KEYWORDS: ToF-SIMS; high mass resolution ; quantiÐcation ; methacrylate copolymers

INTRODUCTION

At a fairly early stage in the development of static SIMSfor polymer surface studies it was shown that intensityratios could be used to establish compositional trends,provided that the surface composition was the same asthat of the bulk.1,2 This criterion is generally satisÐedby random copolymers, and numerous studies of suchsystems have conÐrmed the early prognostication. Mostcorrelations have been based on intensity ratios of thetype A/(A ] B), where A and B are peaks representing(ideally, uniquely) the two monomers. It is well recog-nized, however, that these peaks need to be chosen withcare.3 A better approach, in principle, is to use chemo-metrics to either make use of the entire spectrum or toidentify the principal components. Early results arepromising but a number of problems have been encoun-tered, not least the best way to weight the intensityinformation. The recent introduction of high massresolution time-of-Ñight (ToF) SIMS instrumentationhas markedly increased the information content ofSIMS spectra but has also added complexity to theseanalytical approaches.

Recently, Shard et al.4 published a static SIMS studyof the random copolymers of methyl methacrylate(MMA) and poly(ethylene glycol) methacrylate(PEGMA) using a quadrupole mass spectrometer, com-plemented by XPS data. The latter gave quantitative

* Correspondence to : D. Briggs, ICI Plc, Wilton Research & Tech-nology Centre, PO Box 90, Middlesbrough, Cleveland TS90 8JE, UK.

¤ Part 21 of the series “Analysis of Polymer Surfaces by SIMSÏ, pre-sented at the QSA-9 Conference, 15È19 July 1996, Guildford, UK.

conÐrmation that the surface composition was the sameas the bulk across the series. Although the static SIMSresults, based on correlations between two sets ofA/(A] B) intensity ratios and bulk composition, gavereasonable trends, the data showed signiÐcant depar-tures from linearity in mid-range. This study is uniquein that one of the monomers is itself a macromolecule(the average molecular weight of the PEG side-chain inPEGMA being D1000), which presents the possibilityof segregationÈin fact this was one reason for thestudy. Reinvestigating the same sample set with a highmass resolution ToF-SIMS instrument presented anopportunity to compare directly the data from the twotypes of instruments and to probe the cause(s) of thenon-linearities in the quadrupole data. The copolymerstructure is shown below:

EXPERIMENTAL

Materials

The synthesis of the methyl (MMA)Èpoly(ethyleneglycol)(MW\ 1000)methacrylate (PEGMA) randomcopolymers has been described previously4 and thesamples studied in this work were from the same batch.The Ðve samples contained 12.5, 30, 40, 50 and 80%

CCC 0142È2421/97/090725È09 $17.50 Received 3 December 1996( 1997 by John Wiley & Sons, Ltd. Accepted 21 March 1997

726 D. BRIGGS AND M. C. DAVIES

PEGMA by weight. The pure homopolymer PMMAwas from Polysciences Inc., Warrington, PA(MW\ 25 000). Homopolymer PEGMA was synthe-sized from the monomer (ICI Paints, Slough, UK) usingbenzoyl peroxide initiator.

Polymer Ðlms were spun-cast onto pieces of siliconwafer that had previously been ultrasonically cleaned inchloroform. Two drops of a 0.1% w/w solution of thepolymer in chloroform were successively placed ontothe wafer, which was then spun for a few seconds at5600 rpm. This procedure led to Ðlms that were suffi-ciently thin (\10 nm) to be conducting under ion bom-bardment, so that surface potential control (chargeneutralization) was not required. An Si-speciÐc com-ponent at m/z 28, from the substrate, was just detect-able.

Time-of-Ñight SIMS analysis

Spectra were obtained using a Physical Electronics 7200instrument equipped with an 8 keV Cs` ion source anda two-stage reÑectron mass analyser.5 The ion beam ofD50 lm diameter was rastered over an area of 100lm ] 100 lm (measured between the beam centres)with a time-averaged pulsed primary ion current ofD0.4 pA). The dose for spectral acquisition was D1012ions cm~2. The time-per-step was 625 ps and under thespectrometer conditions employed the mass resolution(m/*m) was typically [8000 at m/z[ 40. Secondaryions were postaccelerated into the detector with avoltage of 10 kV. Only positive ion spectra are dis-cussed.

As described more fully below, some of the prominentpeaks in the spectra exhibit distinct asymmetry withtailing on the high m/z side. This peak shape reÑectsthe secondary ion energy distribution (which mayinclude components resulting from direct ejection andfrom fragmentation processes occurring during theinitial acceleration (extraction) period6,7) and the energycompensation achieved by the mass spectrometer. Twosets of data were acquired for all the polymers underslightly di†erent instrumental conditions, to achieve dif-ferent degrees of tailing (by variation of the voltage thatcontrols the penetration of the ions into the ionmirrorÈthe “decelerationÏ voltage). SpeciÐcally, this wasreduced from 508 V to 502 V between datasets 1 and 2(see below). For each data set, all conditions were keptconstant.

Peak intensities were obtained by measuring the totalpeak area or, for highly tailed peaks, a constant majorfraction of the total peak area (i.e. integrating over Ðxedmass ranges).

Mass calibration was achieved in two stages. First,the default set of three low mass (single component)peaks at m/z 15 27 and 41 was(CH3), (C2H3) (C3H5)employed. Second, peaks of increasing mass were sys-tematically assigned (based on agreement between cal-culated and measured masses to within 20 ppm) andthen added into the calibration set. Only peaks that hadsimilar shapes (i.e. the secondary ions have similarenergy distributions) to the peak requiring high accu-racy mass measurement were used in this procedure ;this is because the software takes the peak centroid todeÐne the ion mass.

RESULTS AND DISCUSSION

Spectral interpretation

“SurveyÏ spectra of the two homopolymers are shown inFig. 1. (In the following : PEGMA refers to thepoly(ethylene glycol) methacrylate monomer andPPEGMA to the homopolymer made from it.) Thesee†ectively emulate the unit mass resolution spectraobtained from quadrupole instruments, such as used inthe earlier study.4 Despite the potential di†erences intransmission function, the agreement is extremely good.The only di†erence is in the m/z 113 peak in thePPEGMA spectrum; this is signiÐcantly more intensein the ToF spectrum. Accurate mass assignment sug-gests that its identity is Although theC6H9O2`.PPEGMA was of a di†erent batch in the present study,this is not thought to be a contributing factor becausethe m/z 113 peaks is also more intense in the copolymerspectra. This may be an example of an ion-lifetimee†ect, a reÑection of the di†erent ways in whichunimolecular decomposition events between secondaryion emission and detection manifest themselves in quad-rupole and ToF mass spectra. A striking example forPVC has been reported recently.8

At high mass resolution the major PPEGMA peaksare seen to be single component and accurate massmeasurement conÐrms the previous assignments.4 Thesituation for PMMA is di†erent ; most peaks have twoor more components and whereas the majority havepreviously been assigned to accurate massC

xH

y`,

analysis shows that components are moreCxH

yO

z`

likely. A full analysis will be presented elsewhere ; thefollowing comments are limited to assignments dis-cussed previously4 (relative component intensities areonly qualitative) : m/z 29 is CHO`, (1 : 2) ; m/z 43C2H5`is (2 : 1) ; m/z 55 isC2H3O`, C3H7` C3H3O`, C4H7`(1 : 3) ; m/z 59 is (3 : 1) ; m/z 69 isC2H3O2`, C3H7O`

(5 : 1) ; m/z 109 is (weak),C4H5O`, C5H9` C6H5O2`(3 : 1) ; m/z 121 isC7H9O`, C8H13` C8H9O`, C9H13`(5 : 1), m/z 126 is and m/z 128 isC7H10O2` C6H8O3` ,(1 : 1).C7H12O2`In the Shard et al. study4 two pairs of peaks were

chosen, on the basis of their relative uniqueness inspectra from copolymers containing 12.5 and 80 wt.%PEGMA, to represent the individual monomers : m/z 89and 103 for PEGMA and m/z 91 and 107 for MMA.The intensity ratios 89/(89 ] 91) and 103/(103 ] 107)were then plotted against copolymer composition (wt.%PEGMA). The high mass resolution spectra in thesemass regions for the two homopolymers are shown inFigs 2 and 3. Several features are observed. Firstly,none of the peaks are unique to either homopolymer.Relative ion yields (estimated from the absolute inten-sity ratios) for the chosen species between the twohomopolymers are of order 10È15 except for m/z 91(D3) (see Table 1). Secondly, all the peaks contain morethan one component in at least one of the two spectra(PMMA or PPEGMA). Thirdly, there are distinct dif-ferences in peak shape ; the two characteristic peaksfrom PPEGMA (m/z 89 and 103) are very asymmetricand tailed to high m/z.

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 725È733 (1997)

HIGH MASS RESOLUTION ToF-SIMS OF MMAÈPEGMA COPOLYMERS 727

Figure 1. Positive ion survey spectra of PMMA and PPEGMA up to m /z 200.



Figure 2. High mass resolution spectra at chosen nominal masses for PMMA.

Figure 3. High mass resolution spectra at chosen nominal masses for PPEGMA.



Table 1. Ratios of absolute intensities of peaks in the spectraof PMMA and PPEGMA

m /z Main component IPMMA

/IPPEGMA

IPPEGMA

/IPMMA

89 C4H

9O

2½ 0.07 14.5

91 C7H

7½ 3.12 0.32

103 C5H

11O

2½ 0.09 11.4

107 C8H

11½ 11.4 0.09

126 C7H

10O

2½ 31.2 0.03

QuantiÐcation

Figures 4 and 5 are plots based on the same intensityratios used in the Shard et al. study,4 for the two datasets employing di†erent spectrometer settings. Here thetotal peak areas (i.e. including all components) at thegiven m/z value have been calculated so as to emulatethe quadrupole data as closely as possible. The corre-spondence between these plots and those in Ðg. 5 of Ref.1 is rather remarkable. Especially noteworthy is themarked divergence from linearity of dataset 2 in the40È70 wt.% PEGMA region, which closely mirrors thequadrupole data, and the fact that this is also the mostobvious di†erence between the two ToF datasets.

The discussion in the previous section leads to a clearstrategy to improve the quantitative correlationbetween peak intensity ratios and bulk composition.Peaks involving more than one component should beavoided. If this is not possible the peaks should have thelargest possible intensity di†erence between the homo-

polymers and the components should be uniquelyassociated with the two monomers so that a correlationcan be made. Accordingly, m/z 91 fails because it ismulticomponent in PMMA and has a high relativeintensity (Table 1) in PPEGMA (it is also a fragmentthat is not closely related in structure to the polymerrepeat unit) ; m/z 107 fails because its two compon-ents are the same for both homopolymers

m/z 103 is a better choice than m/z(C7H7O`/C8H11` ) ;89 for PEGMA because it is largely due to inC8H7`PMMA and to in PPEGMA, whereas m/zC5H11O2`89 is predominantly due to in both cases. AC4H9O2`good choice for PMMA is m/z 126. Not only does thisrepresent the repeat unit, it is also single component

and has a higher relative ion yield between(C7H10O2`)PMMA and PPEGMA (D30) than any of the otherpeaks considered (Fig. 6).

QuantiÐcation was therefore carried out using m/z126 representing MMA and m/z 103 (C5H10O2`component) representing PEGMA. The structures ofthese two ions are shown below, m/z 103 being derivedfrom the methyl-terminated end of the PEG side chain :

Figure 4. Relative intensity ratios (integrating all the intensity at the nominal mass) for m /z 89 and 91 as a function of copolymer composi-tion, for two sets of instrument parameters.



Figure 5. Relative intensity ratios (integrating all the intensity at the nominal mass) for m /z 103 and 107 as a function of copolymercomposition, for two sets of instrument parameters.

Figure 6. High mass resolution spectra at m /z 126 for PMMAand PPEGMA.

The component intensity at m/z 103non-C5H11O2`could be measured directly by peak Ðtting but this wasnot possible with the available software. Instead, theintensity ratio of m/z 103/126 was measured from thePMMA spectrum (in which m/z 103 is initially due to

and this was used to estimate the inten-C8H7`) C8H7`sity in all the other spectra on the assumption that theratio is a constant. The results forC8H7`/C5H11O2`both datasets are plotted in Fig. 7 ; the di†erences

between the datasets in Figs 4 and 5 have now almostbeen eliminated and the plot is linear. The regressionline shown is for dataset 1, giving a correlation coeffi-cient of 0.99 ; that for dataset 2 is almost identical(correlation coefficient\ 0.98).

In the Shard et al. study4 XPS data from the samesamples gave a very good correlation between quanti-Ðed surface composition (based on both the O :C atomicratio and, more signiÐcantly, the di†erent functionalitieswithin the C 1s spectrum) and bulk composition. This,in itself, suggested that the deviations in the SIMSintensity ratio plots were unlikely to be due to surfacecompositional e†ects. The present results stronglysupport this interpretation and, instead, point to theimportance of spectrometer characteristics. Thereappears to be two factors involved, which may not beindependent. The unit mass resolution of the quadru-pole mass spectrometer obscures the possible multi-component nature of a peak at a given nominal mass.



Figure 7. Relative intensity plot based on m /z 126 and m /z 103 component only) as a function of wt.% copolymer(C7H

10O

2½) (C

5H

11O

2½

composition for both sets of instrument parameters used in Figs 4 and 5. The regression line is for dataset 1 only.

Additionally, the rather narrow acceptance energy ofquadrupole optics (a few electron-volts, typically) cancause severe discrimination between secondary ionshaving di†erent energy distributions.9 The MMAÈPEGMA polymer system seems to exemplify this e†ect.The di†erent shapes observed in the ToF spectrabetween characteristic PEG chain peaks and the rest is,almost certainly, a result of these ions having widerenergy distributions. The ToF spectrometer with itsgreater acceptance energy (D100 eV in this case) shouldbe less susceptible to discrimination than the quadru-pole, but as these results show, quadrupole-like behavi-our can be obtained under certain set-up conditions.The Ðlms studied herein were conducting, so any contri-bution to peak broadening from surface potential non-uniformity can be safely discounted. The reason for thebroader energy distributions of the PEG-derived ions isnot known at this time. Kinetic energy distributionmeasurements, of the type recently reported by Ber-trand and co-workers,6,7 would certainly shed light onthis question.

So far we have only discussed the ToF-SIMS data interms of its direct comparison with the Shard et al.4approach to the interpretation of SIMS data from thesecopolymers. Their quantiÐcation, based on the bulkcomposition expressed as wt.% monomer, is untypical.The A/(A] B) type of peak relative intensity plotshould be versus the mol.% composition (as exempli-

Ðed, for instance, in Ref. 2), because the peaks arechosen on the basis of their uniqueness in the homo-polymer spectra (and therefore represent the individualmonomers). The result of this type of plot is given inFig. 8, which appears to suggest a very rapid build-upof PEGMA at the surface. In fact, the correlation isclose to a logarithmic dependence, as shown in Fig. 9.

Figure 8. Relative intensity plot as for Fig. 7 but as a function ofmol.% composition.



Figure 9. Relative intensity plot for the copolymers as a functionof log (mol.% PEGMA).

However, this may be a misleading impression.Although the PEG side-chain is an integral part of thePEGMA monomer, the m/z 103 ion does not representthe whole monomer in the same way that m/z 126 rep-resents MMA. The emission of m/z 103 does not requirebackbone scission and it can be formed both by a singlebond cleavage (retaining the chain end) or by emissionfrom within the PEG chain.10 Moreover, as discussedbelow, there is the complication of another route for theformation of PEG-derived ionsÈvia cationization.Figure 8 may, therefore, imply a matrix e†ect on the ionyield for m/z 103 (and m/z 89). At the present time, weconclude that the linear plot based on wt.% PEGMAmust be fortuitous. However, the smooth trends in therelative intensity plots still tend to support the view thatsurface segregation of PEG chains is not occurring.

High mass spectra

The previous discussion has focused on the mass rangein which the “lowÏ mass fragments that characterizepolymers are observedÈthe so-called “ÐngerprintregionÏ. Beyond this region (typically m/z 0È250), quad-rupole systems generally lack adequate sensitivity tolarger fragments with their inherently lower yields(assuming they can access higher masses). The beneÐtsof ToF-SIMS are nicely illustrated by Fig. 10, whichreveals an extensive series of peaks beyond m/z 250 withintensities 103È105 times lower than the “ÐngerprintÏpeaks. This region is shown in more detail in Figs11 and 12. The most intense components form twoseries that accurate mass measurements assign to thegeneric formulae and[CH2(OCH2CH2)nNa]`

These are, almost certainly,[CH2(OCH2CH2)n]K`.fragments of the methyl-terminated PEG side-chaincationized by either Na` or K`. The most likelygeneric structure for the neutral (non-cationized) frag-ment is :

CH3(OCH2CH2)n~1OCHxCH2Alkali metal cationization of molecules containing eth-ylene glycol units is well known in mass spectrometrygenerally. It is an extremely efficient process and, as inthis case, where the peaks due to Na` and K` have lowrelative intensities in the spectra, only traces of these

Figure 10. Positive ion spectrum of the 80 wt.% PEGMA copoly-mer displayed on a log scale on intensity (acquired with a 10 nstime per step).

Figure 11. Poly(ethylene glycol) side-chain fragmentation regionof Fig. 10 displayed on a linear intensity scale.

Figure 12. Portion of Fig. 10 indicating the repeating pattern ofpeaks at higher mass resolution.



elements need to be present (residues from the PEGpolymerization provide sufficient concentrations).

The PEG side-chain in the PEGMA monomer has anominal average molecular weight of 1000 and thereforean average number of ethylene glycol units of D22. Thehighest mass peak in Fig. 11 corresponds to a fragmentcontaining 31 units, so ToF-SIMS is capable of detect-ing a major fraction of the original PEG oligomer dis-tribution.

The form of the oligomeric envelope in Fig. 11 isexpected to be complex. It is a convolution of severalrelationships : the original PEG molecular weight dis-tribution ; the probability of fragment formation ; andthe probability of cationization by either Na` or K`.(In this mass range, variation of detector response is notexpected to be signiÐcant.) Although the peaks are veryweak, there is evidence in Fig. 11 for a typical oligo-meric intensity distribution11 centred around m/z 1000,consistent with the original PEG distribution. This isobserved on the high mass side of m/z 900. Below m/z900 the fragments increase in intensity more or less lin-early with decreasing mass, suggesting that (n [ l)merfragments are formed from n-mer fragments. It is inter-esting to note the change in relative intensities of theNa`/K` cationized fragments containing the samenumber of ethylene glycol units across the envelope.

These oligomeric fragments appear in all the copoly-mer spectra, with systematically increasing intensitiescorresponding to the increasing PEGMA content. Theirrelative intensity in the homopolymer PPEGMA is,however, lower than would be anticipated on the basis

of the copolymer spectra. Because the homopolymerwas synthesized di†erently, this may simply reÑect alower probability of fragment cationization.

CONCLUSIONS

High mass resolution ToF-SIMS has been used toinvestigate the deviations from linearity observed in aprevious quadrupole SIMS study of the quantitativerelationship between relative peak intensities and com-position for random copolymers of MMA andPEGMA. Two factors that would have a†ected theearlier results were identiÐed : the multicomponentnature of the chosen characteristic peaks used for quan-tiÐcation (leading to non-uniqueness) ; and signiÐcantdi†erences in the energy distributions of these peaks. Bycareful choice of peaks from the ToF spectra, linearcorrelations of relative intensities with compositionwere established. The high mass sensitivity of the ToF-SIMS allowed the distribution of fragments derivedfrom the oligomeric PEG side-chain in the PEGMAmonomer to be detected and investigated.

Acknowledgement

We gratefully acknowledge the contribution of Dr Alex Shard in syn-thesizing the polymers and providing advice during this study.

REFERENCES

1. D. Briggs,Org.Mass Spectrom. 22, 91 (1987).2. D. Briggs and B. D. Ratner, Polym. Commun. 29, 7 (1988).3. L. T. Weng, P. Bertrand, W. Lauer, R. Zimmer and S. Busetti,

Surf . Interface Anal . 23, 879 (1995).4. A. G. Shard, M. C. Davies, S. J. B. Tendlar, C. V. Nicholas,

M. D. Purbrick and J. F. Watts, Macromolecules 28, 7855(1995).

5. S. Reichlmaier, J. S. Hammond, M. J. Hearn and D. Briggs,Surf . Interface Anal . 21, 739 (1994).

6. A. Delcorte and P. Bertrand, Nucl . Instrum. Methods Phys. B117, 235 (1996).

7. A. Delcorte, B. G. Segda and P. Bertrand, in press.8. D. Briggs and I. W. Fletcher, Surf . Interface Anal . 25, 167

(1997).9. D. Briggs, in ‘Practical Surface Analysis ’, 2nd edn, Vol. 2, ed.

by D. Briggs and M. P. Seah, p. 369. Wiley, Chichester(1992).

10. A. G. Shard, M. C. Davies and E. Schacht, Surf . Interface Anal .24, 787 (1996).

11. See, for example, R. W. Linton, M. P. Mann, A. M. Belu, J. M.DeSimone, M. O. Hunt, Jr., Y. Z. Menceloglu, H. G. Cramerand A. Benninghoven, Surf . Interface Anal . 20, 991 (1993).


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Quantitative Study of Methyl Methacrylate-Poly(Ethylene Glycol) Methacrylate Copolymer Films using High Mass Resolution ToF-SIMS