SURFACE AND INTERFACE ANALYSIS, VOL. 24, 419-421 (1996)

SHORT COMMUNICATION

Surface Morphology of a PVC/PMMA Blend Studied by ToF-SIMSt

D. Briggs,’.* I. W. Fletcher,’ S. Reichlmaier,’ J. L. Agulo-Sanchez3 and R. D. Short3 ICI Research and Technology Centre, Wilton, PO Box 90, Cleveland TS90 8JE, UK Physical Electronics GmbH, Fraunhoferstrasse 4, D-85737 Ismaning, Germany University of Shegeld, Department of Engineering Materials, PO Box 600, Shegeld S14DU, UK

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) studies, utilizing both laterial imaging and shallow depth profiling, of a 50 : 50 poly(viny1 chloride)/poly(methyl methacrylate) (PVC : PMMA) blend have confirmed the presence of a thin overlayer of PMMA on the phase-separated bulk morphology.

INTRODUCTION

Polymer blends are technologically extremely important’ and the thermodynamic and bulk physico- mechanical properties have been extensively studied. Most binary polymer pairs are immisible and produce bulk phase-separated materials on mixing; however the surface morphology has received relatively little atten- tion. For such a blend three extreme alternatives are possible : (1) bulk morphology extending to the surface; (2) molecular mixing of the two components in the

(3) preferential adsorption of one component at the

Distinguishing between these alternative surface mor- phologies may not be easy: implied is the simultaneous requirement for chemical speciation at both high lateral and depth resolution within the surface region.

The problems are illustrated by previous work on the poly(viny1 chloride)/poly(methyl methacrylate) (PVC/ PMMA) blend system (a relatively popular choice for study) prepared by casting from tetrahydrofuran (THF) solution. Schmidt et al.’ showed, by XPS, that for spun- cast blends the surface was PMMA-enriched across the entire blend composition range, with, furthermore, a constant composition over the uppermost 9 nm (i.e. no concentration gradient within this depth). However, for conventional solution-cast films Short and co-workers found only a slight, and irreproducible, surface enrich- ment of PMMA by XPS,3 whilst imaging time-of-flight secondary ion mass spectrometry (ToF-SIMS) studies seemed not to distinguish between the surface (1 nm) and ‘bulk’ ( N lo00 nm) seen by SEM.3,4 X-ray photo- electron spectroscopy imaging of identically prepared films, however, revealed much lower contrast in 0

surface region;

surface.

* Author to whom correspondence should be addressed. t Part 18 of the series ‘SIMS Applied to Polymer Surfaces’ (for Part

17, see Ref. 7).

images (representative of PMMA in the absence of any contaminating species) compared to the complementary C1 images (representing PVC).’ This result has been confirmed by high-resolution XPS line-scans, using a different instrument, suggesting a uniform PMMA over- layer with a thickness less than the 0 1s sampling depth under the conditions used.6

In the earlier ToF-SIMS studies, imaging with molec- ular fragments was unsuccessful because of inadequate charge compensation. This would have allowed any possible uncertainty about the origin of surface oxygen- containing species to be resolved. In the present work this difficulty has been overcome. Furthermore, shallow depth profiling has been carried out to further investi- gate compositional variation within the uppermost molecular layers.

EXPERIMENTAL

The ToF-SIMS experiments were carried out on a Physical Electronics 7200 instrument utilizing a 15 keV 69Ga+ beam (FEI single isotope source) and a reflec- tron ToF analyser. The performance of the instrument and the principles of ‘retrospective imaging’ (based on the storage of the complete spectrum at each image pixel) have been discussed in previous Images in the static mode were acquired with a beam of -0.5 pm and a pulsed current of 0.2 PA. Each 40 ns pulse delivered about 125 ions. The imaged area was 400 x 400 pm (256 x 256 pixels). The complete image was acquired from 100 frame scans using one primary ion pulse per pixel, i.e. 5 x 10l1 ions cm-’. The mass range covered was rn/z = 10-200. Charge compensation was by 20 eV flooding electrons pulsed out of phase with the ion beam. The ion beam raster pattern was more complicated than the normal TV pattern, with the aim of minimizing the possibility for local charge build- up; this proved to be a most important route to the imaging of negatively charged cluster ions.

Received 8 February 1996 Accepted 8 March 1996

CCC 0142-2421/96/060419-03 0 1996 by John Wiley & Sons, Ltd.

420 D. BRIGGS ET AL.

10'

10'

g 10'-

8 10s:

10'

The sample studied was a 50: 50 blend of PVC and PMMA cast from THF as previously de~cribed.~

before spmer

RESULTS

Figure 1 shows the static negative ion spectra A ,i, da a0 100 120 140 160 180 200 (m/z = 10-200) from the same 400 x 400 pm area ~. ~ ..

before and after the continuous current exposure. Because of the dynamic range involved, these are dis- played with a log scale of intensity. The well-known PMMA molecular fragments at m/z 85, 87, 141 and 185 are prominent in the original surface spectrum. As expected after a primary ion dose in excess of loi2 ions cm- ', these are absent in the subsequent static spec- trum (the peaks at m/z 139-143 and 174-180 are due to 69GaC12- and 69GaC1,-, respectively).

The same areas are imaged in Plate 1. The three 20 40 60 80 100 120 140 160 180 200 images of the original surface, generated retrospectively,

involve the sum of: all the ions in the negative spectrum

m l Z

&er sputter

mlZ

Figure 1. Negative ion spectra acquired in the static mode (tl 0l2 ions before and after sputtering (- 1 023 ions cm-') the same area of 400 pm x 400 pm imaged in Plate 1.

To investigate subsurface composition (shallow depth profiling), the ion beam was rastered over 500 x 500 pm in continuous current mode (- 1 nA) for 30 s. The total dose was 7.5 x loz2 ions cm-2. The amount of material removed is not known. However, from depth profiling experiments on a Ta,O,/Ta standard under non- identical conditions (25 keV instead of 15 keV Ga'), a removal depth for Ta'O, of 15-30 A can be estimated. The larger raster area was to accommodate any image shift resulting from residual charging when returning to static conditions for subsequent analysis.

(Total 1); PMMA-specific ions of m/z 31, 85,-87, 141 and 185 (PMMA 1); and ions of m/z 35 and 37 (C1 1). After sputtering, the retrospective image of ions of m/z 35 and 37 was again constructed (C12).

In the Total 1 image, evidence of the blend domain structure is evident and this is partly reflected in the C1 1 image. The PMMA 1 image, however, is featureless. Two regions in the Total 1 image are highlighted and the spectra formed from the summation of the spectra of all pixels inside the boxes are presented in Fig. 2. These two regions are of overall low intensity (upper left) and higher intensity (lower right), respectively, in the C1 1 image. Both spectra feature the characteristic PMMA fragments, confirming the impression given by the PMMA 1 image. After sputtering, the C1 image is strikingly different (C12). The features originally present in C1 1 are intensified, but a new circular feature has appeared (upper left) corresponding to a domain evident in the Total 1 image (residual charging has led to an overall downward shift in the image of -50 pm and possibly some image contraction).

20 40 60 80 100 120 140 160 180 200 WZ

Lower RloM

m/z

Figure 2. Negative ion spectra corresponding to the two boxed areas highlighted in the 'Total 1' image of Plate 1.

__

DISCUSSION

Because the number of counts per pixel are of the same order of magnitude in images PMMA 1 and C1 1, the lack of complementarity strongly suggests that in those regions where PVC is detected, the outer surface is actually PMMA. The spectra give no evidence for oxygen-containing species other than PMMA at any significant concentration. Therefore, these results are entirely consistent with previous XPS evidence for surface segregation of PMMA, i.e. an example of prefer- ential adsorption of one component at the surface.

We believe the C1 1 image is evidence for a greater escape depth for the elemental (Cl-) ion compared with the molecular fragments used to form the PMMA 1 image. This is supported by three observations : after sputtering to remove material from the outer surface (the appearance of a previously undetected PVC domain in C1 2 clearly shows that this has occurred), all

Plate 1. Images of a 400 Fm x 400 pm area acquired in the static mode (<lo’’ ions cm~’) before and after sputtering ions cm~*): see text for explanation of titles. To the right of each image is displayed the thermographic intensity scale,

together with the maximum number of counts per pixel (e.g. 131 in Total 1).

PVCPMMA STUDIED BY ToF-SIMS 421

the original weak features in C1 1 are intensified; before sputtering, the image based on 0- is a lower intensity version of Total 1, whereas after sputtering the intensity is roughly halved and complementary features to C1 2 are emerging; a careful comparison of the spectra in Fig. 2 using difference techniques shows that whilst C1- is the dominant contributor to the spectrum from the lower right region, it is not accompanied by the expected C1,- pattern (m/z 70, 72, 74). Spectra from pure PVC run on the same instrument give a 35C1-/70C1, - intensity ratio of N 40. Any contribution from C1, - in Fig. 2 (lower right) is at least an order of magnitude lower than this. The larger size of this ion might be expected to lower its escape depth, but it is also possible that it is formed in the selvedge region (i.e. closer to the surface than the elemental ion). More work

is needed, however, to fully substantiate this last observ- ation.

CONCLUSION

Static SIMS imaging/microanalysis and shallow depth profiling provide strong support for the view that a thin (of order N 1 nm) layer of PMMA uniformly covers the phase-separated bulk in 50 : 50 PVC/PMMA blends when cast from THF solution. The results also suggest that elemental ions (e.g. 0-, C1-) can have an escape depth in organic matrices which is significantly greater than that of polyatomic fragment ions.

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