AXIS Applications

The Use of X-Ray Photoelectron Spectroscopy (XPS) Valence Band Spectra for the Determination of Polymer Tacticity

C.J. Blomfield¹, S.C. Page¹, B. Tielsch¹, E. Thomas², A. Ferryman², J.E. Fulghum², and D.J. Surman³

 

¹Kratos Analytical Ltd, Wharfside, Trafford Wharf Road, Manchester, M17 1GP, UK
²Kent State University, Chemistry Dept., Kent, OH 44242, USA
³Kratos Analytical Inc., 535 East Crescent Ave, Ramsey, NJ 07446, USA

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Introduction

XPS analysis is an important method for the characterisation of polymer surfaces. Developments in monochromatic X-ray sources coupled with efficient charge neutralisation systems have played a key role in the further analysis and understanding of polymer surfaces. This study investigates the possibility of distinguishing PMMA tacticities using XPS.

A previous report of XPS analysis of atactic, isotactic and syndiotactic PMMA was unable to differentiate between the isomers using ARXPS of the C 1s and O 1s regions¹. There were, however, no investigations of the valence band region of the isomers, where greater sensitivity to polymer structure than the XPS core levels could be expected.

 

Experimental

XPS analysis was performed on a Kratos Axis Ultra electron spectrometer using a monochromatic Al Ka X-ray source operated at 450 W. Charge neutralisation was provided by the Kratos charge neutralisation system which has the unique ability to provide consistent charge compensation even at near grazing take-off angles. All neutraliser parameters remained constant during analysis. High resolution C 1s, O 1s and valence band regions were acquired using a 20 eV pass energy. Analysis was performed using 90° (normal) and 15° take-off angles.

PMMA isomers (atactic, isotactic and syndiotactic, shown schematically here) were prepared on glass substrates by spin casting from 1% w/w solutions in toluene.

 

Results and Discussion

Survey spectra acquired at a 90 degree (normal) take-off angle from the three PMMA isomers are shown in Figure 1. The spectra reveal C 1s and O 1s photopeaks indicative of PMMA. There was approximately 0.5 % Si contamination on the isotactic PMMA sample.

High resolution C 1s and O 1s spectra were acquired at a 90 degree take-off angle from each sample and are shown overlayed in Figure 2. There are no significant differences in the spectra which can be related to the polymer tacticities.( - although the survey scan backgrounds appear different - is this significant? Ed.)

Valence band spectra acquired at a 90 degree take-off angle from each sample are shown overlayed in Figure 3. Despite the sensitivity of the valence band region to molecular structure, there is no difference in the spectra from the PMMA isomers.

Angle resolved XPS analysis was performed at a 15 degree take-off angle to limit the depth of photoelectron detection from each PMMA isomer. A survey spectrum from the atactic PMMA, along with high resolution C 1s and O 1s spectra using a 15 degree take-off angle are shown in Figure 4. Curve fitting of the C 1s and O 1s show good agreement with results from Beamson and Briggs². This agreement with "standard" spectra is important, and confirms that there is no apparent differential charging during analysis.

An overlay of C 1s spectra from each sample acquired using a 15 degree take-off angle is shown in Figure 5. Once again, there is little difference between spectra from each of the isomers.

High resolution valence band spectra from each sample are shown overlayed in Figure 6. The valence band spectra reveal subtle differences between the PMMA isomers in the C 2s and C 2p regions which may be related to differences in the PMMA tacticities lying in the near surface region.

 

Conclusions

No differences in the high resolution C 1s, O 1s and valence band regions were detected for the PMMA isomers using a 90° take-off angle.

Using a take-off angle of 15 degrees revealed subtle differences in the C 2s and C 2p regions of the valence band spectra from the PMMA isomers.

C 1s spectra before and after valence band acquisition confirmed minimal X-ray degradation of the polymers.

 

References:

¹ T. J. Hook, R. L. Schmitt, J. A. Gardella, Jr., L. Salvati, Jr., and R. L. Chin, Anal. Chem., (1986), 58, 1285-1290.

² G. Beamson and D. Briggs, "High Resolution XPS of Organic Polymers", Wiley and Sons Ltd, London, (1992).

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