Langmuir 1990, 6, 420-424
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The Contact Angle of Poly(methy1 methacrylate) Cast against Glass D. Briggs,*lt H. Chan,* M. J. Hearn,+D. I. McBriar,§ and H. S. Munro**§ ZCZ plc, Wilton Materials Research Centre, P.O. Box 90, Middlesbrough, Cleveland, TS6 8 J E , U.K., Department of Chemistry, National University of Singapore, Kent Ridge Road, Singapore, and Department of Chemistry, University of Durham, South Road, Durham, D H l 3LE, U.K. Received February 3, 1989. I n Final Form: J u n e 26, 1989 Films of poly(methy1 methacrylate) (PMMA) of both medium and high molecular weight have been prepared by casting onto clean glass. The difference in water contact angle of the surfaces originally in contact with glass and air and the variation over time of this parameter have been studied. By use of the surface analytical techniques X-ray photoelectron spectroscopy (XPS) and, particularly, static secondary ion mass spectrometry (SSIMS), it has been shown that migration of low molecular weight impurities from the bulk of the film to the film/air interface is responsible for the contact angle behavior.
Introduction The preferential orientation of polar groups in polymer surfaces has been the subject of some discussion in the literature over the last few years. In studies on poly(methyl methacrylate) (PMMA)l and poly(buty1 methacrylate) (PBMA),' it has been shown that the contact angle with water, at the substrate/film interface, is significantly lower on casting a film against a high surface energy substrate (e.g., glass, Hg) than for the corresponding interface on a low surface energy substrate. The contact angle for the latter interface is similar to that for the air/film interface. The contact angle data have been interpreted in terms of preferential orientation of the polar groups in the polymer. For PBMA, the contact angle of the film/glass interface can be converted to that of the film/air interface by heating the polymer at 50 "C (Le., 22 "C above the T,).' Elevation of the polymer above the T, was considered to be necessary to provide the required mobility for reorientation. As far as we are aware, no direct surface spectroscopic evidence has been reported to support the contact angle data. The ability of surface polar groups to reorientate has also been alluded to in the interpretation of contact angle hysteresis3 and has a contribution to the aging of polymer surface modification^.^^ XPS studies on corona and plasma oxidized polymers have shown correlations between changes in surface composition and contact angles on storage. However, the interpretation of the data in terms of preferential orientation in these cases is complicated by the migration of low molecular weight species away from
* To whom correspondence should be addressed. IC1 plc.
* National University of Singapore. University of Durham. Current address. D. I. McBriar: IC1 Chemicals and Polymers Ltd., P.O.Box 8, The Heath, Runcorn, Cheshire WA7 4QD, U.K. H. S. Munro: Courtaulds Research, P.O. Box 111,Lockhurst Lane, Coventry CV6 5RS, U.K. (1) Carre, A.; Schreiber, H. P. J . Coat. Technol. 1982, 54, 31. (2) Haq, Z.; Muigins, J. Polym. Commun. 1984,25, 269. ( 3 ) See, for example: Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J . D., Ed.; Plenum: New York, 1985; Chapters 2 and I. 3, H.; Sharma, A. K.; Yasuda, T. J . Polym. Sci., Polym. iem. Ed. 1981, 19, 1285. (5) Brigga, D.; Rance, D. G.; Kendall, C. R.; Blythe, A. R. Polymer 180, 21, 895.
(6) Munro,
H. S.; McBriar, D. I. Proc. ACS Diu., Poly. Mat. Sci. 0743-7463/90/2406-0420$02.50/0
the In these studies, the use of a surface analytical technique, namely, X-ray photoelectron spectroscopy (XPS or ESCA), to complement contact angle measurements has proven extremely valuable. In recent years, however, static secondary ion mass spectrometry (SSIMS) has emerged as a powerful additional tool, providing greater surface sensitivity and a much higher degree of molecular ~peciation.'.~Specific reviews on the application of XPS and SSIMS to polymer surface characterization have recently been p~blished.'.'~ In this paper, we demonstrate by the characterization (contact angle, XPS, and SSIMS) of PMMA films that differences in contact angle observed between glass/ film and air/film interfaces can be attributed to the presence of trace contaminants at the air/film interface. In order to avoid any misunderstanding, it is important to define the measurements made and the relationship of the surface probed to the original interface. The cast film has two interfaces: air/film (AF) and glass/film (GF). Once peeled from the glass, the film has two surfaces, referred to as AF and GF, respectively, to identify the contact medium during casting. Because all contact angles are measured with either surface in contact with air, the examination of the GF surface was carried out immediately after peeling and subsequently at intervals to study any reequilibration. All surface analysis measurements are carried out under ultrahigh vacuum, but again GF surfaces were examined immediately after peeling.
Experimental Section Film Preparation. Medium and high molecular weight atactic PMMAs (Cellomer Associates Inc.) were found by GPC (referenced to polystyrene standards) to have molecular weights of 79 000 and 301 000, respectively. The films were prepared from 10% solutions of bulk polymer dissolved in chloroform and cast onto glass Petri dishes. Prior to casting, the dishes were cleaned with fuming nitric acid, rinsed thoroughly with distilled water, and allowed to dry. The dried films had a thickness of around 125 Fm. The films could be removed from the glass fairly e a ily by scoring through with a clean razor blade and using the (7) Briggs, D. Polymer 1984,25, 1379. (8) Briggs, D. Br. Polym. J . 1989, 21, 3. (9) Briggs, D. Surface Analysis In Encyclopedia of Polymer Science and Technology; Kroschwitz, J . I., Ed.; Wiley: New York, 1989; Vol. 16, p 399. (10) Briggs, D. Characterization of Surfaces In Comprehensive Polymer Science; Booth, C., Price, C., Eds.; Pergamon: Oxford, 1989 Vol. 1, Chapter 24, p 543.
0 1990 American Chemical Society
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Contact Angle of PMMA Cast against Glass Table I. Contact Angle Data and Corresponding XPS Data for PMMA Films % of contact angle, total C Is 0 Is/C 1s sample surface deg (0-C=O) area ratio Medium Mol Wt 17 0.57 air/film 72 glass/film glass/film (after 2-day storage) air/film scraped air/film scraped (after 2-day
64 72
65 72
19 18 19 18
0.68 0.63 0.67 0.63
15 17 18 18 18
0.51 0.61 0.64
18
0.64
storage)
High Mol Wt air/film glass/film
glass/film (after 2-day storage) air/film scraped air/film scraped (after 2-day storage) powder (as received)
82 73 71 75 73
0.64 0.64
blade to lift an edge, allowing peeling or delamination of the film. After aging experiments, the films were sometimes scraped to remove material from the upper surface. This was done by gently passing a clean scalpel blade over the film surface once, under hand pressure. Although this is bound to introduce some roughness, the scale of roughness is throught unlikely to affect the contact angle measurements appreciably. Contact Angle Measurements. These were performed by using the sessile drop method, with triply distilled water having a surface tension of -72 mN m-l. A 3-pL drop was observed by using a homemade apparatus involving an optical microscope fitted with a graticule. This allowed measurement of the height (h) and base (w)of the drop. The equilibrium contact angle (0) was computed from tan (Of21 = 2h/w The measurement temperature was 18 O C . Quoted values of 0 are the mean of at least five measurements, and the reproducibility was f 2 O . XPS. Core level spectra were recorded in the fixed retardation ratio mode on a Kratos Scientific Instruments ES300 spectrometer using Mg Ka,,, X-rays. Samples were attached to the probe tip by using double-sided Scotch tape. Measurements were made at an electron “take-off angle” of 60’ with respect to the surface. Peak areas and C 1s component analyses were obtained by using a Kratos OS300 data station. The C-H peak at a binding energy of 285.0 eV was used for energy calibration. SSIMS. Spectra were obtained on a SIMS/XPS instrument based on the VG ESCALAB Mk 1 vacuum system described elsewhere.l’ A 4-keV Xe+ primary ion beam, focused to a 100pm diameter, was rastered over an area 6 mm X 6 mm with an average current density of 1nA cm-’. Samples were mounted on double-sided adhesive tape. Sample charging was neutralized by a flood of 700-eV electrons,and the electron beam parameters and bias potential applied to the sample holder were separately optimized to obtain positive and negative ion spectra.’ The total ion dose accumulated during setting up and spectral acquisition was -5 X 10” ion cm-2, well within “static”conditions for this polymer.12
-
-
Results and Discussion Contact Angle. The contact angle data of water on PMMA (medium and high molecular weight) for the AF surface and the freshly exposed GF surface are shown in Table I. The data for these surfaces are qualitatively consistent with the data previously reported for PMMA’ and PBMA2 in that the GF surface has a lower contact angle than the AF. These data could suggest preferential orientation of the polar groups toward the interface for the GF samples. After storing the medium molec(11) Brigge, D.; Wootton, A. B. Surf. Interface Anal. 1982, 14, 109. (12) Brigge, D.; Heam, M. J. Vacuum 1986,36, 1005.
290
BE,eV
285
280
Figure 1. High-resolution C 1s spectrum of medium molecular weight PMMA film: (a) GF surface; (b) GF surface after aging for 2 days; (c) AF surface. The main peak at 285.0 eV is due to C-C/C-H, the shoulder at -287 eV is due to the 0CH, carbon (unresolved peak at 286.5 eV), and the carboxyl (0-C=O) carbon gives rise to the peak at 289 eV. ular weight PMMA GF surface for 2 days in air, the contact angle had increased significantly. Further, after both the medium and high molecular weight AF surfaces were scraped, contact angles similar to those from the freshly peeled GF surfaces were obtained. These latter data indicate that a contaminant has been removed from the surface. If this is the case, then the increase in contact angle for the stored medium molecular weight GF and scraped AF samples could arise from migration of the contaminant from the bulk of the sample to the surface. This migration would appear to be inhibited in the high molecular weight sample. Surface Analysis. 1. XPS. The C 1s core level spectra for the medium molecular weight AF, GF, and stored GF surfaces are shown in Figure 1. The spectra have been normalized to the peak at 285.0 eV arising from the C-C and C-H environments in the polymer. The most hydrophilic surface as determined by the contact angle measurements (the freshly prepared GF interface) also corresponds to the surface with the highest relative intensity of the carboxylate functionality (0-C=O) and 0 ls/ C 1s area ratio, as shown in Table I. For the stored GF surface, the intensity is intermediate between the values of the freshly prepared GF and AF samples. The theoretical value for the percentage contribution of the carboxylate peak to the total C 1s area is 20. This value has been previously reported for chloroform-cast PMMA films. In this study, the GF surface is the most representative of the theoretical case. The AF and GF stored surfaces have smaller carboxylate intensities and smaller 0 ls/C 1s area ratios, as might be anticipated from the contact angle data. The corresponding data for the high molecular weight samples displayed the same trends. Scraping the AF surface of either the high or medium molecular weight polymer films gave rise to C 1s spectra that were superimposable on those of the freshly peeled GF samples. A plausible explanation of the XPS data would be that the AF and the stored medium molecular weight GF surfaces are contaminated (as was also suggested from the contact angle data), perhaps by a hydrocarbon-containing species. The nature of the latter cannot be determined by XPS. 2. SIMS. Positive and negative ion spectra of the GF and AF surfaces of the high molecular weight PMMA are shown in Figures 2 and 3. Original spectra are reproduced to illustrate the signaknoise achieved. The GF spectra correspond quite closely to “standardn spectra of
422 Langmuir, Vol. 6, No. 2, 1990
Briggs et al.
6.8 --
0 2.2
1
50
---
some of the prominent peaks in the negative ion spectrum to give some indication of the relationship between polymer structure and the secondary ions observed (values in parentheses are m/z values):
100
I
(185)
100 14
150
ml z
-
I
i
200
The m/z = 87 peak is typically -20% the intensity of the m / z = 85 peak, and its structure has not previously been assigned. It has been suggested" that end group fragments could be at least partly responsible for this ion:
i
C - H
CH3
I II C
4
bCH3
1 L
85
The AF spectra differ considerably from the GF spectra. The negative ion spectrum, Figure 3b, contains additional peaks at m/z = 71, 73, 129, and 183. These are characteristic of a butyl methacrylate p01ymer.l~
t
t 0 o'82-
(87)
i
100
50
-I
dl
I
100
150
m/
200
Figure 2. (a, Top) Positive SSIMS of high molecular weight PMMA, GF surface. (b, Bottom) Negative SSIMS of high molecular weight PMMA, GF surface.
PMMA obtained from freshly spun-cast thin (-1 pm) films under the same experimental ~0nditions.l~ This is particularly true of the positive ion spectrum (Figure 2a), which deviates only in the relative intensity of the m / z = 41 peak (-20% greater than the standard) and in the presence of a Na+ (mlz = 23) peak. In the negative ion spectrum (Figure 2b), the C2-, C2H- (mlz = 24,251 doublet is about twice the standard, as is the m / z = 87 peak (relative to other peaks), and a C1- (mlz = 35,37) peak is prominent. A detailed interpretation of the PMMA spectra has been presented previ~usly.'~*'~ For the purposes of this discussion, it is only necessary to identify
Clearly, the fragments at m / z = 73,129, and 183 are the equivalent fragments to those at m / z = 31, 87, and 141 from PMMA (or a polymer containing MMA units). The pattern of peaks in the m / z = 0-70 region of the positive ion spectrum, Figure 3a, suggests either n-butyl or sec-but 1 as the isomeric variant, based on previous studies1'*''of the spectra of the pure isomeric variations of poly(buty1 methacrylate). Although the SIMS intensity data cannot be directly quantified, relative intensity data indicate a significant concentration of butyl methacrylate units within the upper two monolayers, possibly up to a complete monolayer. Results from the medium molecular weight polymer are shown in Figures 4 and 5. All the significant spectral differences occur in the m / z = 0-100 range, and for the sake of brevity and clarity, these regions are collated as line spectra. All the comments made above concerning the high molecular weight GF spectra can be similarly applied to the GF spectra from the medium molecular weight polymer. The AF spectra differ from the GF ~~
(13) Briggs, D.; Brown, A.; Vickerman, J. C. Handbook of Static Secondary I o n Mass Spectrometry; Wiley: Chichester, 1989. (14) Hearn, M.J.; Briggs, D. Surf. Interface Anal. 1988,11, 198.
~~
(15) Lub,J.; Benninghoven, A. Org. Mass.Spectrom. 1989,24,164. (16) Briggs, D.;Hearn, M. J.; Ratner, B. D. Surf. Interface Anal. 1984,6,184.
Contact Angle of PMMA Cast against Glass 189
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Langmuir, Vol. 6, No. 2, 1990 423
1
0
50
100
150
100
m/ z
200
Figure 4. Positive SSIMS of medium molecular weight PMMA: (a) GF surface; (b) GF surface after aging for 2 days; (c) AF
surface.
0 0.81i
i/
100
1
100
50
i
j
I 150
m/ z
I1 I
b
I/
C
200
Figure 3. (a, Top) Positive SSIMS of high molecular weight PMMA, AF surface. (b, Bottom) Negative SSIMS of high molec-
ular weight PMMA, AF surface.
(and the "standard" PMMA) spectra in rather subtle ways. In the positive ion spectrum, the mlz = 15 and 59 peaks are of much reduced intensity, while the m / z = 57 peak is enhanced. In the negative ion spectrum, there is a significant increase in the relative intensities of the H( m / z = 1)peak and the C, (mlz = 12-14) and C, ( m / z = 24, 25) clusters. Aging the GF surface for 2 days, in a protected environment, leads to changes in the spectra consistent with a transformation from GF- to AF-like composition. These data suggest that the AF surface and the aged GF surface are contaminated with hydrocarbon species. Most of the peaks in the positive ion spectrum of PMMA are either hydrocarbon fragments derived from the polymer backbone or oxygen-containing clusters having the
0
mi z
100
Figure 5. Negative SSIMS of medium molecular weight PMMA: (a) GF surface; (b) GF surface after aging for 2 days; (c) AF
surface.
same nominal mass as hydrocarbon fragmental4 However, the m / z = 59 ion, CH30-C=O+ from PMMA, is of negligible intensity in hydrocarbon spectra,13 whereas the m l z = 57 ion, C4HB+,is an intense peak in hydrocarbon spectra but has low intensity in the PMMA spec-
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Langmuir, Vol. 6, No. 2, 1990
trum. Also, CH,+ ( m / z = 15) is relatively much more intense in the PMMA spectrum than in a hydrocarbon spe~trum.'~ Given the negative ion data and the absence of any distinctly different fragments (as in the high molecular weight polymer case), the presence of surface hydrocarbon is the only plausible explanation of the spectral differences between GF and AF surfaces. General Discussion The SIMS data confirm the interpretation of the XPS results, namely, that high contact angles are associated with additional hydrocarbon entities at the PMMA surface. They add the additional detail that the causes are different for the two polymers used. In the case of the high molecular weight PMMA, the surface (AF) contaminant appears to be a poly(buty1methacrylate) (or a polymer containing butyl methacrylate units), whereas in the case of the medium molecular weight PMMA it is likely to be a simple hydrocarbon. In either case, the effect on the XPS C 1s spectrum would be the same: an increase in the contribution of the C-C/C-H component to the total envelope. The high molecular weight PMMA was subjected to a thorough bulk analysis by using 13CNMR and pyrolysis GC/MS. Although some ethyl acrylate was detected, no butyl methacrylate could be found. The difference in the actual contaminants helps explain the fact that whereas the XPS and contact angle data correlate within the data sets for each polymer, there is no overall correlation. However, the difference in the GF contact angle values between the two polymers is surprising since these surfaces are, from XPS and SIMS, most representative of pure PMMA. There are two possible indications from the SIMS data. Firstly, the GF surfaces are contaminated with inorganic ions (Na+, C1-) which may affect the contact angle measurements. Secondly, there is a marked increase in the m / z = 87 peak intensity in the negative ion spectrum, which may indicate segregation of end groups at the polymer/glass interface. There exists the possibility that the C1- signal could derive from trapped chloroform. We discount this for two reasons. Firstly, XPS, which samples a greater depth of polymer ( 50 A) than SSIMS ( N 10 A)," did not detect C1 in any of the film surfaces. Secondly, when SSIMS N
(17) Hearn, M. J.; Briggs, D.;Yoon, S. C.; Ratner, B. D.Surf. Interface Anal. 1987, 10,384.
Briggs et al. did detect C1, this was accompanied by Na, and both ions were only seen on GF surfaces.
Conclusions Contact angle measurements are a popular means of characterizing polymer surfaces, being quick and relatively inexpensive. They are also sensitive to the composition of the uppermost molecular layer. The drawback of contact angle measurements is that they can only be indirectly interpreted in terms of surface structure and are therefore very prone to artifacts. The danger of using contact angles to follow surface chemical changes in the absence of supporting direct surface analysis has previously been highlighted in studies of polymer surface functionality modification.18 The results from this study reinforce this view, showing that polymers purchased as pure materials (and analyzed by conventional, but sensitive, bulk analysis techniques as such) may, nevertheless, contain migratory trace organic contaminants which can dramatically affect surface composition. The different PMMAs showed similar behavior in terms of contact angle measurements, but SSIMS identified different causes. This alone rules out any systematic effects due to external contamination during film preparation. The results demonstrate the superior surface sensitivity and molecular specificity of SSIMS relative to XPS. Besides the effects of segregating impurities, two other processes may be affecting the contact angles. Firstly, there is evidence (admittedly tentative at this stage) that polymer end groups may preferentially adsorb at the film/ glass interface. Secondly, transfer of inorganic ions (Na+, C1-) to the polymer from the glass surface occurs, and these ions may also migrate when the GF surface is exposed to air. These two effects, in contrast to the organic contamination of the bulk polymer, are independent of the source (and molecular weight) of the PMMA.
Acknowledgment. We thank Cambridge Contact Lens Technology for the provision of a research studentship to D.I.M. and the British Council for the provision of a Visiting Fellowship to H.C. We are grateful to A. Bunn and G. Manton for NMR and pyrolysis GC/MS measurements, respectively. Registry No. PMMA, 9011-14-7; water, 7732-18-5. (18) Chew, A.; Dahm,R. H.; Brewis, D. M.; Briggs, D.; Rance, D.G. J . Colloid Interface Sci. 1986,110,88.
