Anal. Chem. 1992, 64, 043-847
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Analysis of Polystyrene/PVME Blends by Coincidence Counting Time-of-Flight Mass Spectrometry B.Dwain Cox,* Melvin A. Park, R.Gene Kaercher, and Emile A. Schweikert Center for Chemical Characterization and Analvsis, Chemistry Department, Texas A&M University, College-Station, Texas 77840
MeV/amu 262Cfflrrrlon fragments were used to probe the
surface of mlrdMe and phase-separatedpolymer blend films. A glven fldon fragment Is capable of causing the emlsslon of several crecondary kns. Colncklence counting tlmeof4Ight mass spectrometry (CC-TOF-MS) was used to study such codosorptlon of secondary Ions and thus determlne thelr relatlonrhlps wHh one another. Using this method, we were able to obtaln lnformatlon about chmnlcal and spatial relatlonshlps between secondary Ions. For the case of mlsclble and phase-separated polyotyrene/poly(vinyl methyl ether) Mends, characterlstk fragmentatlon patterns and secondary Ion ylelds were obtalned. Spatial lnformatlon such as sub mlcrometer chemlcal heterogeneltles were also revealed.
INTRODUCTION Several studies have described the use of coincidence counting combined with time-of-flight mass spectrometry (CC-TOF-MS) for the analysis of chemical relations between secondaqy ions. One interesting approach to CC-TOF-MS was recently discussed by Fransinski et al.’ They were able to identify the structure and charge state of molecular parent ions by observing temporal correlations in the production of atomic daughter ions. We have shown in previous research how coincidence chemical information can also be useful in determining the origin of secondary ions.2 Coincidence counting TOF-MS exploits the fact that analyte ions produced as a result of a given primary excitation contain both chemical and spatial information related to their specific origin on the analyte surface. In this experiment, plasma desorption mass spectrometry (PDMS)3is combined with single-ion counting in the coincidence mode to determine the surface structure and microhomogeneity of an amorphous polymer blend. When a sample is exposed to a source of excitation (such as high-energy primary ions), the result is a high density of translational energy at or near the surface. Deexcitation of the surface is often accomplished by the desorption of secondary ions (SI) into the vacuum. During our experiments, the secondary ions from one excitation event are analyzed and recorded before the next event is considered. The results of an event may be recorded in a “conventional” and a set of “coincidence” TOF mass spectra depending on the type of secondary ions observed. This type of event-by-event counting ensures that the chemical and spatial information from each primary excitation is retained in the final spectra. The chemical information obtained includes molecular structure and secondary ion yields, while molecular environment and sample homogeneity are examples of the spatial information. For the case of PDMS, where 252Cf fission fragments are used as primary ions, chemical information and surface homogeneity are revealed at the submicrometer level with perhaps an ultimate resolution of 10-20 nm? The study of secondary ion relationships and their exploitation for testing the composition and microhomogeneity of multicomponent solids is the theme of this research.
Time-of-flight MS allows for the simulheous mass analysis of the ions desorbed during one event. Coincidence counting is used to collect and analyze the data obtained from each desorption event. During a CC-TOF-MS experiment, the data are collected event-by-event and stored in both a conventional and set of coincidence mass spectra. The data stored in the conventional spectrum represent the cumulative results of all the desorption events which occurred during the acquisition. A coincidence spectrum is composed only of counts due to analyte ions detected from events which contain an ion of interest. Comparison of ion intensities in the conventional and coincidence mass spectra allows one to determine which ions are related. Through these relationships, both chemical (identification and quantification) and spatial (sample microhomogeneity) information is obtained. The first successful application of CC-TOF-MS to a test of chemical microhomogeneity has been described elsewhere? Briefly, submicron-sized NaF crystals were dispersed a t approximately a 3% surface coverage on a polystyrene backing. Each 252Cffission fragment that hit the sample addressed a region of approximately 10-20 nm in diameter! By analyzing the desorbed SI’S from each fission fragment event independently, Park et al. were able to determine sample homogeneity down to the submicrometer level. In this report we describe a similar procedure for determining the surface homogeneity of an amorphous polymer blend. A polymer blend can be defined as a combination of two or more polymers resulting from common processing steps, e.g., mixing of polymers in the molten state or casting from common solvent. The polymers exist in a completely homogeneous state where their segments are mixed at the most intimate level (50-100A), in which case the polymers are said to be “miscible”,or segregate into distinct p M . 6 One system which has received substantial attention is the binary mixture of polystyrene (PS) and poly(viny1 methyl ether) PVME.7v8 Under dilute solution conditions, the PS/PVME blend is miscible when cast from toluene but is phase separated when cast from trichloroethylene (TCE).8 Pans and Prest were the first to investigate the compmition of the solid surface of an amorphous miscible polymer blend.7 Their XPS results demonstrated that there are significant differences between the composition of the surfaces (e.g., outmost 6 nm) and the bulk of the PS/PVME blends. They attributed these differences to an enrichment of PVME at the surface. The results of our research corroborate those of Pans and Prest and show the f i t application of mass spectrometry to the study of miscible and immiscible polymer blend surface phenomenon. EXPERIMENTAL SECTION The method used for preparing the polymer blend solutions has been described elsewhere.’ Polystyrene (M,= 114200, M,, = 109900 narrow distribution, Scientific Polymer Products, Inc.) was used as received. Poly(viny1methyl ether) (50% solids, M, = 130000;Scientific Polymer Products, Inc.) was reprecipitated twice from toluene into hexane and vacuum-dried before use. Miscible PS/PVME blends were prepared by dissolving various weight-to-weight ratios (95% PS:5% PVME to 5% PS:95%
0003-2700/92/0364-0843$03.00100 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64,
NO. 8. APRIL 15, 1992
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Mass Flgw. 2. %f POMS conventional spectra of the homopolymers pdy(vinyi methyl ether)(A) and polystyrene (B). cheracteristic peaks f r o m t h @ ~ s p e c t n r ( P V M mess E 7 5 , s . and 101; PS mess 91 and 193) are used as representative “flngerprint” peaks during a coincidence experiment.
PVME) in 2% (w/w) toluene solutions. Phase-separated PS/ PVME blends were prepared similarly from 3% (w/w) solutions in trichloroethylene. Thin polymer films were made by spincasting approximately 100 p L of each solution onto a smooth stainless steel Kimball Physics plate. The films were dried overnight under vacuum and then cured for 4 days in an oven at 50 OC. The films were analyzed consecutively using a 252Cfplasma desorption maas spectrometer,which has been described in detail e1sewhere;O the setup is shown schematically in Figure 1. The vacuum chamber used for this set of experiments achieved a minimum pressure of lo4 Torr. In this setup (Figure 1)262Cfis used as a source of primary ions. Nuclei of a2Cf spontaneously fission to produce pairs of MeV/amu fission fragments. These fission fragments are emitted simultaneously and in opposite
directions; therefore, as one fission fragment strikes the sample the other strikes a ‘start” detector. The fission fragment that strikes the polymer film sample can cause the emission of secondary ions. These ions are accelerated by an electric field (+5 kV)into a field-free flight tube. The ions that enter this region have equal kinetic energy and reach the ‘stop” detector at flight times proportionalto the square root of their mass-to-charge ratios. individual ions Because microchannel plate detectors were d, were detected. The start and stop detectors signal a timatdigital converter (TDC) when the desorption event begins and when a secondary ion arrives at the stop detector. The TDC used in this work was built at IPN in Orsay, The secondary ion flight times measured by the TDC were sent to a personal computer where the data were stored on an event-by-event basis. In this arrangement,the personal computer produced a conventional TOF spectrum and a set of coincidence spectra A coincidence spectrum is made up of only events which contain an ion of interest. During a coincidence counting experiment, coincidence spectra are collected simultaneously with the conventional spectrum.
RESULTS AND DISCUSSION Figure 2 shows the homopolymer mass spectra of PVME and PS. Both spectra contain peaks characteristic of the individual homopolymer and can therefore be used as a ’fingerprint” for identifying their presence. Figure 3A is a conventional spectrum of a dry blend mixture of PS and PVME. Present in the conventional spectrum are peaks characteristic of PS (e.g., maw 91 and 1931, and PVME (e.g., mass 75,85, and 101). The mass 101 coincidence spectrum, Figure 3B and the mass 91 coincidence spectrum, Figure 3C were collected simultaneously with the conventional mass spectrum. Figure 3B contains the results of all those desorption events from which mass 101 was detected. Notice that the mass 101 coincidence spectrum and the mass 91
ANALYTICAL CHEMISTRY, VOL. 64, NO. 8, APRIL 15, 1992
coincidence spectrum strongly resemble the individual homopolymer maw spectra. The greater the separation between the two polymer phases, the higher the probability of desorbing ions characteristic of only one component per event. If the polymer pairs were mixed completely homogeneously (at a molecular level) the coincidence spectra would be expected to resemble the conventional spectrum. By analysis of the results of each event on an evenbby-event basis, subtle differences between the conventional and coincidence mass spectra can be used to ascertain the degree of surface homogeneity. The relationships between the peaks in the spectra can be seen more readily by calculating either their correlation coefficients or percent coincidences. The calculation of the correlation coefficient is based on the probabilitiea of observing the ions with one another. If the probability, P, of observing two ions in coincidence is greater than would be expected by chance, then the ions are said to be correlated with one another. The experimental probability of observing an ion from a given desorption event is taken to be the number of counts in the ion peak, N(A+),divided by the number of primary ions striking the sample during the analysis periods, S, (P(A+) = N(A+)/S. The percent yield of ion A+ is defined as 100(N(A+)/S). Similarly, the experimental probability of observing two ions in coincidence is given by P(A+, B+) = N(A+,B+)/S where N(A+,B+)is the number of B+ ions observed in coincidence with A+. If the two ions are unrelated, then the P(A+,B+)should equal P(A+)P(B+). The correlation coefficient, Q, is therefore defined as P(A+,B+) - N(A+,B+)S
-
(1)
= P(A+)P(B+) N(A+)N(B+) If Q > 1, the ions are said to be correlated; if Q = 1, independent, and if Q < 1, anticorrelated. The term anticorrelated refers to a situation in which the desorption of ion A+ adversely effects the desorption of ion B+. Alternatively, one can calculate the percent coincidence of observing two ions simultaneously. Percent coincidence is defined literally as the percentage of a peak observed in coincidence with an ion of interest (i.e., %(A+,B+) = 100N(A+,B+)/N(B+))where % (A+,B+)is the percent coincidence of ion B+ with ion A+. This is related to Q as
Thus, percent coincidence is proportional to Q and S/N(A+) acta as a proportionality constant. For a given acquisition and secondaryion of intereat, S/N(A+), is ked. Therefore percent coincidence is a measure of the relative correlation (QR) of secondary ions with the ion of interest:
(3) where M = secondary ion of mass M. The percent coincidence or QR of secondary ions with an ion of interest can be plotted as a function of the mass of the secondary ion, as shown in Figure 4. In this figure the ions which are more related with mass 101 have larger values of percent coincidence and QR than do unrelated ions. The data points are separated into two groups. Those points which are highly correlated with mass 101 are associated with PVME, and those points which are uncorrelated with mass 101 are associated with PS. Because these sets of points are highly separated, we conclude that the sample is indeed heterogeneous, as was suggested by the spectra of Figure 3. The data shown in Figures 3 and 4 resulted from the analysis of a dry blended sample of PS/PVME; thus there
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was no mixing of different polymer chains on a molecular level, Although equal amounts of both polymers were used, the average distance between two different chains should seldom have been less than 100 nm. Therefore, when a fiision fragment addressed a region which contained PS, the only secondary ions detectad from such an event should be associated with PS. To tegt the feasibility of the technique for polymer samples which were mixed at the molecular level, both phase-separated and miscible PS/PVME blends were analyzed. An example of the results of these analyses is shown in Figure 5. Figure 5 is a plot of QR, with mass 101, versus secondary ion mass. Both sets of data shown are for the 7525 ratio of PS:PVME. The square data points represent the phase-separated blend. The miscible 7525 PS/PVME is represented by the circular data points. Due to differences in percent coverage of the blend samples on the stainless steel plates, it is difficult to make accurate quantitative comparisons between the miscible and phase-separated blends. However, it is apparent that segregations do exist. The data shown for the common solvent phase-separated blend (Figure 5) are qualitatively similar to the data shown in Figure 4,representative of the dry blended sample. This indicates that the phase-separated blend is segregated on at least a macromolecular level. The results of the miscible blend, shown in Figure 5, were somewhat unexpected. If the sample were completely homogeneous, one would expect a random distribution of the points centered around and closer to QR = 1.0. From in-
846
ANALYTICAL CHEMISTRY, VOL. 64,
NO. 8, APRIL
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%-PVME (bulk) Figure 8. Plot of the percent coincldence of mass 75 (m), 101 (A), and 91 (e),vs percent PVME in the bulk of the phaseseparated blend solutlons.
spection of the 7525 PS/PVME data, as well as data from several other blend ratios, there is still some separation between the data points representative of the two polymers. This remaining separation implies that the sample is somewhat segregated on the surface even though it is "miscible" in the bulk. This segregation might be due in part to the thickness of the polymer blend films we were able to produce (-400-500 nm). Perhaps when the sample is too thin,the interdiffusion of different polymer chains is reduced. Such an effect would reduce the Gibbs free energy of the system; therefore, reducing the amount of intimate mixing between the two polymers on the surface. Coincidence counting can also provide information on the yield of secondary ions. From eq 1, if Q = 1.0, then P(A+)P(B+)= P(A+,B+) (4)
(5)
100P(A+)=
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(7) The yield of a secondary ion of interest is thus determined by calculating the percent coincidence of other secondary ions with it. However, some care must be taken to ensure that the secondary ions analyzed are unrelated to the ion of interest, Le., Q = 1.0. If correlations exist between the secondary ions, then the yield observed via % (A+,B+)will be distorted. For the case of the PS/PVME blends, we determined the percent yield of selected secondary ions by their percent coincidence and plotted these versus the percentage of PVME in the bulk, as shown in Figure 6. The points plotted represent the average percent coincidence between the ion of interest and unrelated (background) secondary ions. To ensure that these values were not distorted by correlations between the secondary ions, we also calculated the yield ratios of mass 91 and 101 by comparing the peak areas of mass 101 and 91 in the conventional spectrum. No significant difference was observed between the data in Figure 6 and the yield ratios determined by comparing secondary ion peaks. Using coincidence counting, we were able to directly determine the yield of individual secondary ions without prior knowledge of the
number of fission fragments actually striking the sample. The data shown in Figure 6 are for the phase-separated polymer blend samples. The square and triangle data points are representative of the two most intense PVME peaks: mass 75 and 101. The circular data points represent the yield of the mass 91 peak characteristic of PS. Due to differences in the ionization probabilities of the characteristic peaks, the yield of mass 91 is 5 times less than mass 75 for the case of the two homopolymers. Such discrepancies are normalized in the calculation of percent coincidence by dividing the peak areas in the coincidence spectra by the area of the corresponding peaks in the conventional spectrum. In order to understand the nature of the polymer blend surface, it is more important to look at the trends in the data (Figure 6) than the actual values of the percent coincidence (percent yield). The yield of mass 91 from PS is highest at 0% PVME (bulk), where the yield corresponds to the amount of mass 91 detected from the pure PS homopolymer. The yield of mass 91 rapidly decreases at high PVME/PS ratios, leveling off at a value above 50% PVME in the bulk. A reciprocal trend is evident in the case of the characteristic PVME peaks. The yield of PVME seems to level off at higher bulk concentrations,indicating a saturation coverage of PVME at the surface. This phenomenon has been observed elsewhere7and has been attributed to the enrichment of PVME at the surface due to its lower surface energy. When discussing the film surface, it is more appropriate to assume a constant volume and use the Hemholtz free energy rather than Gibbs free energy." Polymer molecules on the surface are subjected to a different environment than those in the bulk. Because polymer surface molecules are subjected to intermolecular attraction from one side only, the packing at least will differ from surface to bulk. At the surfaceair interface the migration of the lower surface energy component, in this case PVME, to the surface minimizes the interfacial free energy, thus lowering the total free energy of the system. Several systems have been reported in which this migration has the largest effect in physical and chemical differences between the composition of the surface and the bulk of a binary blend.7J2J3 The temperature at which two polymers can mix intimately enough to be considered a single-phase system is known a8 the critical temperature T,.For most polymer pairs, the T, exceeds their thermal decomposition temperature Td;thus most polymer pairs are only partially miscible at accessible temperatures. For the uncommon case of PS/PVME blend systems the interaction parameter (which is a measure of the free energy of the system) is negative resulting in T,< Td. The two polymers can therefore interdiffuse until equilibrium between two coexisting phases is reached. For the case of the toluene-dissolved PS/PVME blend, the polymers have been shown to diffusively mix until a single homogeneous phase existsa8Figure 7 shows, however, that even under conditions of intimate mixing in the bulk between the two polymer pairs, there still exists some degree of 'micro" segregation. The data from Figure 7 show the weight fraction of PVME detected from the film surface versus the percent PVME in the bulk solution. The straight line represents an idealized homogeneous mixture of the two polymers at the surface. Both the miscible (0) and phase-separated (0)solvent-cast blends show an enrichment at the surface of PVME. The magnitude of the PVME enrichment is largest for the phase-separated blend and, for both cases, tends to decrease at higher PVME ratios in the bulk. These results agree with those reported by Pan et al.7
CONCLUSIONS Coincidence counting combined with TOF-MS proved to be an efficient tool for the analysis of molecular heterogeneity.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 8, APRIL 15, 1992
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cidences. Poly(viny1 methyl ether), due to ita lower surface energy, showed a nonlinear increase in surface concentration. This nonlinear increase was apparent in both the miscible and phase-separated samples. The relative enrichment is higher for the phase-separated samples, particularly in the region between the extremes (homopolymers). We believe that more intimate mixing in the bulk solutions resulta in a decrease in the PVME surface enrichment. Interesting future polymer blend experiments might include varying the molecular weight of the two polymers while looking for changes in the surface homogeneity and the amount of polymer migration. Registry No. Polystyrene,9003-53-6;poly(viny1methyl ether), 9003-09-2.
REFERENCES
%-PVME (bulk) Figurr 7. Comparison of percent PVME at the surface of various weight fractbn miscible (0)and phaseseparated(0)polymer blends vs the composttion of the bulk solutions.
Spatial differences between phase-separated and miscible PS/PVME blends were revealed in the form of the Q value and percent coincidence. For each concentration ratio of PS/PVME, phase-separated samples showed the largest degree of segregation (Le., Q # l). Miscible samples had Q values much closer to unity. Differences between Q values calculated for the miscible samples and those predicted for a homogeneous blend (Q = 1) tend to confirm the presence of Ymicro"segregations on the polymer surface.!' Differences in the polymer surface energy between PS and PVME were revealed by comparison of their percent coin-
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ter, 1965; pp 6-25. . 58, 2881-2870. (7) Pan, D. H.;Prest, W. M., Jr. J . Appl. f h ~1885, (6) Bank, M.; Lefflngwell, J.; Thies. C. M e c r o " l e s 1871, 4 (l), 43-47. (9) Summers, W. S.; Schwelkert, E. A. Rev. Sci. Instrum. 1888, 57, 692-697. (IO) Festa, E.; Sellem, R.; Tasson-Qot, L. Nucl. Instrum. Melhods mys. Res., Sect. A 1886. 234, 305-311. (11) cherry, 6. W. Polymer Surfeces; Cambridge Unlverslty: London, 1981; Chapters 1 and 2. (12) Galnes, 0. L., Jr. J . fhys. Chem. 1888, 73, 3143-3149. (13) Gardella, J. A., Jr.; Pireaux,J. J. Anal. Chem. 1880, 62, 645A-661A.
RECEIVED for review October 21,1991. Accepted January 21, 1992.
