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Anal. Chem. 1904, 56, 1371-1375
Static Secondary Ion Mass Spectrometry for Study of Surface Hydrolysis of Poly(ferf-butyl methacrylate) Joseph A. Gardella, Jr.,l Frank P. Novak, and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Recent research has demonstrated that statlc SIMS provldes an Important complement to ESCA for the analysis of polymer surfaces. There are cases where ESCA samples deep enough In amorphous polymers to yleld compositlonal data equlvalent to bulk values. I n the present study statlc SIMS Is used to analyze a system whlch could not be followed by routlne ESCA experiments: the acldlbase catalyzed hydrolysls of the teff -butyl ester function In poly( teff -butyl methacrylate). Exposure of solvent cast polymer films to aqueous solutlons at varlous pHs shows that for mild acidic and bask condltlons, hydrolysis occurs to a hlgher degree than for exposure to neutral solutions, whlch follows from the catalytlc effect of the nonneutral conditions. At very hlgh and low pHs degradatlon of the polymer occurred too rapldly to monitor. This study demonstrates the feaslbilky of uslng statlc SIMS to study chemlcal reactlons on polymers, especlally for cases In whlch ESCA Is not useful.
Study of mechanisms and products of reactive modification and degradation of polymer surfaces is an active area of research, advanced recently by the use of X-ray photoelectron spectroscopy (XPS or ESCA) (1-4) and, to a lesser extent, complementary surface analytical methods (5)like secondary ion mass spectrometry (SIMS) (6-14), and ion scattering spectroscopy (ISS) (6-10). The use of ESCA for polymer analysis has become almost routine in analyzing contamination problems, degradation, plasma, and chemical reactive modifications and synthesis (1-4). ESCA offers a wide variety of multielement qualitative information from chemical shifts and other seconaary processes (1, 3). Semiquantitative ESCA measurements are usually utilized to monitor composition changes or the extent of reaction by observing changes in integrated peak area ratios or the growth of heteroatom peaks (1-3, 6).
Despite its obvious strengths ESCA is informationally weak for some common polymer applications (1, 3, 5-7). Most polymers contain only a few elements, like carbon, silicon, nitrogen, and oxygen which exhibit relatively small chemical shifts for differing functional groups (1-3, 15). Therefore, under the moderate energy resolution employed in typical ESCA experiments (1.0-1.4 eV fwhm Au 4f ,,2), the results obtained are broad envelopes and peak fitting or deconvolution must be used to distinguish different functionalities. Recently, the use of chemical “labeling” techniques (15-13, for different functionalities has been successful in “separating” the peaks in the envelope by detection of the signal from the heteroatom. Research from our laboratory has emphasized a multitechnique approach ( 5 ) to surface analysis to overcome the weaknesses of each technique alone. Specifically for polymer analysis, several studies have focussed on the development of ISS and SIMS to complment ESCA, emphasizing the need for “static” primary ion beam conditions (6) and to establish Present address: Chemistry Department, State University of New York at Buffalo, Buffalo, NY 14214. 0003-2700/84/0356-1371$01.50/0
the structural sensitivity of SIMS and ISS (6) in comparison with ESCA (7). We have developed a model to interpret SIMS spectra of polymers which views spectra as being combinations of ions from bond scission within the polymer backbone and pendant side chain groups; this model allows one to detect chemical differences in each “region” of the polymer structure (7,8). Delineation of surface sensitivity of ESCA is just being described, and as Briggs (11)has noted, an exact description of the surface sensitivity of static SIMS for polymers has not yet been accomplished. For amorphous homopolymers such definitions may be difficult for the same reasons as in ESCA. Additionally, while the use of ESCA is especially important for monitoring surface chemical modification and degradation, little work has utilized SIMS (6) or ISS (6,18). Our previous preliminary study (6)noted that there are cases in which SIMS can detect effects which ESCA cannot. That work suffered in that the mechanism of chemical modification was not well defined and the extent of reaction was not followed. The present study demonstates the capabilities of static SIMS to detect and quantify the extent of a surface reaction that is sufficiently mild to go undetected by ESCA. The system studied in the present work is the acid or base catalyzed hydrolysis of poly(tert-butyl methacrylate) (PtBMA)
7
CH3
0
H3C-y-&!-O-{-CH3
gH2
+ H20
CH3
IOHl‘or (HI’ ___)
“.t. FI
H3C- -C-OH
Ill
!HZ
The tert-butylcarbonium ion at m/z 57 is the base peak in the static SIMS spectrum of the untreated polymer (7,8) and other peaks are assignable to backbone fragments. This allows one to monitor mild hydrolysis indirectly by the loss of relative intensity of the m / z 57 peak. In comparison, a typical ESCA approach would be to look for changes in the C/O peak area ratio or peak shapes. Since each of the chemical states is not resolvable within the carbon and oxygen envelopes and because there is no heteroatom to monitor, these changes in ESCA spectra are too small to detect.
EXPERIMENTAL SECTION Polymers used in this study were obtained from Scientific Polymer Products, Inc. (Webster,NY). Films of PtBMA samples were supported on stainless steel coupons (1 X 2 cm) by casting thick films from a toluene solution. The solvent was evaporated under a dry argon atmosphere (provided by a Vacuum Atmospheres Dri-Train glovebox system) for 48 h over Pz06.Samples were then ultrasonically extracted in Reagent Grade Hexanes (Fisher) for 20 s to remove any siloxane contaminants. Both ESCA and static SIMS analyses of the resulting films gave spectra which matched the values for unsupported PtBMA (7) (Tables I and 11).
Samples were treated by immersion into stirred aqueous solutions buffered (Fisher) at pH 4.0,7.0, and 10.0. Other solutions (pH 1.4, 2.3, 10.8, 12.6) were prepared by serial dilution of concentrated hydrochloric acid and sodium hydroxide (Fisher); the pH was verified before and after reaction. Exposure time was monitored and fixed at 5 min, the temperature was 23.0 f 0.5 “C. Samples were removed from solution, the excess solution was in the glovebox shaken off, and the coupons were dried over P206 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
Table I. Data from Quantitative ESCA Analysis of PtBMA: Mild Hydrolysis Conditions sample PtBMA PtBMA-supported (blank)
a
C/O integrated peak area ratioa (ratio t re1 std dev)
treatment as received
evaporated film on stainless steel, 20 s ultrasonic hexane wash PtBMA S4 blank + 5 rnin immersion into pH 4.0 solution PtBMA S7 blank + 5 rnin immersion into pH 7.0 solution PtBMA S10 blank + 5 rnin immersion into pH 10.0 solution Ratio of integrated peak area for carbon Is and oxygen Is regions. Cf. ref 7.
for 24-48 h before analysis, to minimize effects from adsorbed
RESULTS AND DISCUSSION Mild Hydrolysis Conditions. Results of the ESCA analysis of PtBMA exposed to mild hydrolysis conditions (pH 4.0, 7.0, 10.0; 5 min; 23.0 "C) are given in Figure 1 and the quantitative data tabulated in Table I. The quantitative data illustrate problems with relying on simple ESCA experiments to analyze this type of problem. Within error limits, all carbonloxygen (C/O) integrated peak area ratios (uncorrected) are equivalent, and visual examination of the core level C 1s and 0 1s envelopes indicates only very small differences (if any) between unreacted PtBMA and the three hyrolyzed samples. All spectra show peaks in the C 1s region a t 285.0 (corrected for charging), 286.8 (within the envelope), and 288.7 eV, assigned to hydrocarbon (CH,), ether (C-01, and carbonyl ( 0 ) type carbons, respectively. Ether (0-C) and carbonyl (O=C) oxygens are convoluted within the 0 1s envelope centered a t 532.7 eV. The ether component is at the higher binding energy side and the carbonyl is at lower binding energy (3). The only change in peak shape detected is a slight decrease in the amounts of C-0 and C-0 carbon relative to the CH, component of the C Is envelope. Peak fitting and curve resolution methods, after deconvolution, would be necessary to establish the exact amount of change and are usually not sufficiently precise (&5-10%) to sense such small changes. These results are similar to previous work (6) where only in cases in which a (nonpolymer) heteroatom from the reaction was monitored could ESCA detect mild hydrolysis of poly(methy1 metacrylate) (PMMA). Furthermore ESCA
1.94 t 0.09 2.04 f 0.24 2.02
c IS
HzO. Static SIMS spectra were recorded with a 3M Model 610 SIMS/525 ISS instrument, operating at a base pressure of 5.0 X torr. Ion beam conditions have been reported previously (7) (%Ne+,2 kV,
