Anal. Chem. 2003, 75, 3744-3750
Quantitative Characterization of a Polystyrene/ Poly(r-methylstyrene) Blend by MALDI Mass Spectrometry and Size-Exclusion Chromatography Renata Murgasova and David M. Hercules*
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
The use of coupled size-exclusion chromatography and MALDI mass spectrometry for quantitative measurement of the composition of technical polymer blends at the molecular level is described. The method is illustrated with a model binary blend consisting of polystyrene and poly(r-methylstyrene), both polymers having similar molecular weights and polydispersities. The proposed MALDI data treatment allows determination of the chemical composition of the blend, the molecular weights of the constituents, and the distributions of the homopolymers. Polymer blends are typically mixtures of two or more polymeric components with somewhat different chemical structures, molecular weights, or both. Blending polymers can produce materials with improved properties, but little is understood about the mechanisms involved. Therefore, to expand the knowledge base for polymer blends, development of new measurement methods, which provide precise definition of material components, is required. In particular, the properties of polymer blends are largely dependent on chemical composition and molecular weight distributions of individual polymers used in their preparation. Thus, detailed quantitative as well as qualitative characterization of polymer blends is crucial. Liquid chromatographic techniques can be used for compositional characterization of polymer blends. In particular, sizeexclusion chromatography (SEC) with selective double (UVrefractive index (RI)) detection enables one to determine the chemical composition distribution in multicomponent AES blends formed by ethylene-propylenediene elastomers and styreneacrylonitrile copolymers.1 However, the results obtained by SEC coupled with multiple detectors suffer from severe limitations: (1) the operating separation mechanism is based on the hydrodynamic volume of the solute rather than molecular weight and (2) different detector sensitivities. In recent years, there has been increased interest in using interactive liquid chromatography for quantitative characterization of polymer blends to determine their chemical composition, molecular weight, or both.2 Polymer blends can be separated via * To whom correspondence should be addressed. E-mail: hercules@ ctrvax.vanderbilt.edu. (1) Chiantore, O. Ind. Eng. Chem. Res. 1997, 36, 1276-1282. (2) Pasch, H. Surfactant Sci. Ser. 1999, 80, 387-434 (Interfacial Phenomena in Chromatography).
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precipitation, redissolution, or both using gradient polymer elution chromatography.3,4 The use of an evaporative light scattering detector (ELSD) allows quantitative analysis of polymer blends without baseline drift caused by the solvent gradient.5 Unfortunately, the ELSD response is nonlinear with sample concentration and depends strongly on experimental conditions. Moreover, the above-mentioned separation mechanisms almost exclusively depend on molecular weight distribution, which further complicates quantitative data processing. Binary polymer blends can be separated by liquid chromatography at the critical adsorption point, by coupling exclusion and adsorption separation mechanisms. At the critical point, the effects of both separation mechanisms mutually compensate so that molar mass-independent separation occurs.6,7 Operating at the critical point of one blend component, binary polymer blends of polymethacrylates were separated and molar mass distribution of the second component, which was eluted in a size-exclusion mode, was determined using either conventional SEC calibration8 or an absolute molar mass detector.9 The major limiting factor of this technique, which uses multicomponent eluents to control retention, is preferential solvation that often complicates quantitative detection of macromolecules by nonspecific detectors. IR and nuclear magnetic resonance spectroscopy have been used to determine the average chemical composition of polymer blends. With the increasing popularity of evaporative interfaces, detection using Fourier transform (FT)-IR spectrometry is becoming more important in SEC. Using the semi on-line fraction deposition approach, FT-IR spectra can be obtained without solvent interference.10 Recently, coupled SEC-FT-IR has been used for quantitative determination of the variation in chemical composition across an SEC chromatogram of a polystyrene/poly(3) Staal, W. J. Neth. Chem. Magn. (The Hague) 1997, 2, 61-63 (4) Bungellar, J. H. J.; Leenen, A. J. H. Competence Cent. Plast., Philips PMF Eindhoven, The Netherlands, Lemstra, P. J., Kleintjens, L. A., Eds.; Integr. Fundam. Polym. Sci. Technol. 5 [Proc. Int. Meet. Polym. Sci. Technol., Rolduc Polym. Meet. 5], Meeting Date 1990, 1991; pp 323-9. (5) http://www.waters.com/WATERS_WEBSITE/Applications/Polymer/ palden_gpc.htm. (6) Pasch, H. Polymer 1993, 34, 4095-4099. (7) Pasch, H.; Much, H.; Schultz, G. J. Appl. Polym. Sci., Appl. Polym. Symp. 1993, 52, 79-90. (8) Pasch, H.; Rode, K.; Chaumien, N. Polymer 1996, 37, 4079-4083. (9) Pasch, H.; Rode, K. Polymer 1998, 39, 6377-6383 (10) Cheung, P. C.; Balke, S. T.; Schunk, T. C. Adv. Chem. Ser. 1995, No. 247, 265-279 (Chromatographic Characterization of Polymers). 10.1021/ac020593r CCC: $25.00
© 2003 American Chemical Society Published on Web 07/01/2003
(methyl methacrylate) blend.11 Partial least-squares prediction with internal calibration, using the second derivative of solvent annealed spectra, was found to provide the best compromise among processing time, accuracy, and precision. Since publication of the first MALDI spectrum of poly(ethylene glycol),12 mass spectrometry has become an increasingly important technique for characterization of average molecular weights, oligomer repeat units, and end groups of polymers. MALDI-TOF, operating in the reflector mode with time lag focusing, can yield isotopically resolved mass spectra, which are extremely rich in information, allowing detailed qualitative analysis of a variety of homopolymers and copolymers.13 Mass discrimination effects that hinder extraction of quantitative information have been reduced by coupling MALDI MS with separation techniques, in particular liquid chromatography.14 The SEC/MALDI MS technique provides accurate determination of average molecular weights and distributions for polydisperse synthetic homopolymers without the use of external calibrants. In contrast to SEC/viscometry and SEC/light scattering, structural information about repeat units and end groups, as well as information about the presence of minor species, is also provided by SEC/MALDI MS. To our knowledge, no methods have been proposed for quantitative analysis of polymer blends using MALDI MS. In fact, the prevailing opinion seems to be that quantitative analysis by MALDI MS, related to blends of different types of polymers, is not possible.13 The major concern has been about different ionization efficiencies of the individual polymer blend components, the most important parameter in determining the relative intensities of peaks in the MALDI mass spectra of polymer blends.15 The present article describes a novel approach for both quantitative and qualitative characterizations of polymer blend classes consisting of components with similar chemical structures and molecular weights, using combined SEC/MALDI MS. A MALDI MS data processing method suitable for the determination of both molecular weight and chemical composition distribution of a polystyrene/poly(R-methylstyrene) blend across the SEC chromatogram is proposed. The advantages and disadvantages of this technique over the above-mentioned techniques are discussed. EXPERIMENTAL SECTION Materials. Narrow dispersity polystyrene (PS; M h w ) 2100, M h w/M h n ) 1.05) was obtained from Polymer Laboratories (Amherst, MA), and poly(R-methylstyrene) (PAMS) (M h w ) 1700, M h w/M h n ) 1.1) was purchased from Polymer Source, Inc., (Quebec, Canada). MALDI MS. All MALDI spectra were acquired using a Voyager-DE STR, MALDI-TOF mass spectrometer from Applied Biosystems (Framingham, MA). The instrument was equipped with a N2 laser emitting at 337 nm. Spectra were acquired in the positive-ion mode using a reflectron. The acceleration voltage was (11) Torabi, K.; Karami, A.; Balke, ST.; Schunk, T. C. J. Chromatogr., A 2001, 910, 19-30. (12) Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T. Rapid Commun. Mass Spectrom. 1988, 2 (8), 151-153. (13) Nielen, M. W. F. Mass Spectrom. Rev. 1999, 18, 309-344. (14) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Rapid Commun. Mass Spectrom. 1995, 9, 1158-1163. (15) Puglisi, C.; Samperi, F.; Alicata, R.; Montaudo, G. Macromolecules 2002, 35, 3000-3007.
25 kV. Single-shot mass spectra (225) were summed to give a composite spectrum. Polymer samples were dissolved in tetrahydrofuran (THF; HPLC grade, Fisher Scientific, Pittsburgh, PA) at 10 mg/mL total concentration. A PS/PAMS blend was prepared having PS weight fractions of 0.5 each. A series of PS/PAMS blends with the PS weight fraction ranging from 0.09 to 0.7 was used for MALDI calibration. The dithranol (Fluka, Buchs, Switzerland) matrix solution was prepared by dissolving 30 mg in 1 mL of THF; matrix and polymer solutions were mixed in a 4:1 ratio. To aid sample ionization, the MALDI target was prespotted with 1 µL of 2 mg/mL silver trifluoroacetate (AgTFA) in THF and allowed to air-dry. One microliter of the polymer/matrix mixture was deposited on top of the AgTFA and air-dried. All samples were analyzed at least in quadruplicate. SEC. SEC was performed at ambient temperature on a Jordi Gel DVB mixed-bed column (250 × 10 mm) (Jordi Associates, Inc., Bellingham, MA) with a refractive index detector from Knauer (Berlin, Germany). THF (HPLC grade, Fisher Scientific) plus 2% CH3CN was used as the eluent at a flow rate of 0.5 mL/ min. Polystyrene standards from Polymer Laboratories with molecular weights ranging from 580 to 195 900 and at concentrations of 10 mg/mL were used for calibration of the SEC system. Polymer samples at 10 mg/mL total concentration in THF were injected from a 100-µL loop using a Rheodyne (Cotati, CA) injection valve. For SEC fractionation of the 0.5 weight faction PS/PAMS blend, 15-20 fractions (0.2 mL) were collected manually in test tubes and evaporated to dryness. A total of 25-50 mL of dithranol solution (30 mg/mL in THF) was added prior to MALDI analysis. Samples were analyzed in triplicate. Multiangle light scattering (MALS)/SEC measurements were carried out using a Wyatt OPTILAB 903 RI and Wyatt miniDAWN MALS detector (Wyatt Technology Corp., Santa Barbara, CA). The same experimental conditions as described above for conventional SEC with an RI detector were followed with the PAMS polymer solution, except that the mobile-phase flow rate was kept at 1.0 mL/min. To determine the molecular weight of the polymer sample, a specific refractive index increment (dn/dc) was measured off-line using a Wyatt Optilab 903 RI detector. Data were processed by a Wyatt software package (ASTRA, version 4.72.03; DNDC, version 5.20). RESULTS AND DISCUSSION SEC and MALDI MS Analysis. The PS/PAMS mixture was chosen as the model polymer blend for this study for several reasons. PS and PAMS are miscible polymers, which have similar chemical structures, so one of them can be used as a reference. The molecular weights and distributions of both components are sufficiently close that the polymers cannot be separated readily using liquid chromatography. This is shown by the SEC chromatogram in Figure 1, which has a single peak for the PS/PAMS binary blend with a 0.5 weight fraction of PS. Because of nearly identical hydrodynamic volumes, PS and PAMS homopolymer peaks overlap and move along the column in one peak zone. Therefore, SEC alone cannot discriminate between the individual blend components. In contrast, the MALDI mass spectrum of the same PS/PAMS blend, shown in Figure 2, clearly distinguishes between the PS and PAMS oligomer distributions. Only singly charged, silverattached oligomer ions are observed in the MALDI MS spectrum Analytical Chemistry, Vol. 75, No. 15, August 1, 2003
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Figure 1. SEC chromatogram of PS/PAMS binary blend having 0.5 weight fraction PS. SEC cannot discriminate between the individual blend components.
Figure 2. MALDI mass spectrum for PS/PAMS binary blend with 0.5 weight faction PS. The spacing between PS oligomer ion peaks is 104.15, the mass of the styrene monomer unit (MSt0); the spacing between PAMS oligomer ion peaks is 118.18, the mass of R-methylstyrene (MAMS0).
of the PS/PAMS blend. The spacing between PS oligomer ion peaks is 104.15 Da, corresponding to the mass of the styrene monomer unit (MSt0), while the spacing between PAMS oligomer ion peaks is 118.18 Da, corresponding to the mass of R-methylstyrene (MAMS0). Since the ion peaks of individual PS and PAMS oligomers do not overlap in the spectrum (Figure 2), MALDI MS data can be used to determine molecular weights, distributions, and chemical composition of the individual blend components as will be discussed below. Typically, commercial polymer blends have broad polydispersities. To be able to extract correct quantitative information from the mass spectrum of a highly polydisperse sample, SEC needs to be employed, providing fractions with very narrow molecular weight distributions. These can subsequently be analyzed by MALDI MS without mass discrimination. Mass discrimination by MALDI MS is a common phenomenon that has been widely documented for polydisperse synthetic polymers.15 The reason for bias toward lower mass oligomers in the MALDI spectra of polydisperse polymers lies in both the MALDI ionization process and the instrument. The ionization efficiency is not the same for all molecules, and the detector response is also molecular weight 3746 Analytical Chemistry, Vol. 75, No. 15, August 1, 2003
Figure 3. Stacked MALDI mass spectra for selected SEC fractions from PS/PAMS blend containing 0.5 weight fraction PS.
dependent. Overall, discrimination in ionization, transmission, or detection often causes the high-mass signal to be lost in the baseline noise. On the other hand, the molecular weights of the SEC fractions measured by MALDI MS can effectively calibrate the SEC system to give absolute values for the average molecular weights of highly disperse (unfractionated) polymer samples, something that is rarely obtained by SEC alone. In the present study, fractions of the model PS/PAMS blend (0.5 weight fraction PS) were collected from an SEC to illustrate this approach, which would be used to improve the ability of MALDI MS to obtain quantitative information about highly disperse commercial blends. Figure 3 shows stacked MALDI mass spectra for selected PS/PAMS blend fractions. The MALDI mass spectra clearly show an elution profile of coeluting homopolymers; the PS homopolymer elutes first (see fraction 2) followed by the PAMS homopolymer (see fraction 6 and higher). The MALDI deconvolution feature will be used to further determine the molecular weight and chemical composition distribution along the SEC chromatogram. It should be noted that, in our study, we have not focused on determination of the average molecular weight and the polydispersity for the whole blend since the principle of this methodology has been already demonstrated for various synthetic polymers.17-21 Quantitative Interpretation. Molecular Weight Determination. Quantitative determination of molecular weights for PS/PAMS blend components using MALDI is based on SEC data interpreted by MALDI. The SEC chromatogram, which represents the molecular weight distribution of the polymer sample, can be divided into a number of equal-volume increments, so that each increment is associated with a specific height (Hi), representative of detector response, and a mass (Mi), typically obtained from a calibration curve (see Figure 4). When a concentration detector is used, the height of each increment for any volume interval is (16) Byrd, M. C. H.; McEwen, C. N. Anal. Chem. 2000, 72, 4568-4576. (17) Danis, P. O.; Saucy, A. D.; Huby, F. J. Polym Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1996, 37 (1), 311-312. (18) Liu, J.; Loewe, R. S.; McCullough, R. D. Macromolecules 1999, 32, 57775785. (19) Hanton, S. D.; Liu, M. X. Anal. Chem. 2000, 72, 4550-4554 (20) Esser, E.; Keil, C.; Braun, D.; Montag, P.; Pasch, H. Polymer 2000, 41, 4039-4046. (21) Nielen, M. W. F.; Malucha, S. Rapid Commun. Mass Spectrom. 1997, 11, 1194-1203.
Table 1. MALDI Data Analysis of Selected SEC Fractions from the PS/PAMS Blend Containing 0.5 Weight Fraction PSa fraction
Figure 4. Schematic of molecular weight calculation by SEC.
Wi
Mn,i
Mw,i
2 6 8 10 16
(a) Polystyrene 0.292 2659 0.333 2251 0.1607 2162 0.1449 2083 0.0689 1961
2750 2324 2239 2166 2020
5 6 8 10 16
(b) Poly(R-methylstyrene) 0.175 2166 0.225 1999 0.208 1841 0.242 1700 0.151 1616
2215 2056 1923 1763 1661
a Data obtained for the polystyrene blend constituent are shown in (a); data for poly(R-methylstyrene) are shown in (b). Wi, Mn,i, and Mw,i are as defined in eq 1.
individual homopolymers in the blend using the combined SEC/ MALDI technique can be expressed as
M h n)
Figure 5. Enlarged detail of MALDI mass spectra for two selected PS/PAMS fractions illustrating selectivity of the technique toward individual blend constituents.
proportional to the polymer concentration ci in that volume, which is equal to the product of NiMi, where Ni is the number of the molecules of one relative molecular mass Mi. Molecular weight averages can be calculated by, M h x ) ∑ciMin/∑ciMin-1, where n ) 0 specifies the number-average molecular weight M h n, n ) 1 specifies the weight average molecular weight M h w, and the molecular weight distribution (polydispersity) is given by PD ) M h w/M h n. The chromatogram is then normalized to give the percent weight of polymer (or percent fraction) at unit volume increment, Wi, which involves dividing each value of Hi by the sum of Hi: Wi ) % dW/dV ) Hi/∑Hi. In our experiment, the SEC chromatogram of the PS/PAMS polymer blend is fractionated into a series of finite increments. Thus, each fraction represents one slice of the SEC chromatogram (see Figure 4). MALDI MS is then used as an absolute molecular weight detector selective to each homopolymer constituent. MALDI molecular weight averages for increments can be calculated by, Mn ) ∑IiMi/∑Ii, Mw ) ∑IiMi2/∑IiMi, where Mi is the mass and Ii is the signal intensity of the ith oligomer in the spectrum. Oligomerically resolved MALDI spectra of the PS/ PAMS blend fractions, such as those shown in Figure 5, provide Mi (molecular weight) for each homopolymer in a particular fraction. By analogy to SEC normalization, the percent weight, Wi, can be determined as the integral intensity of one homopolymer (e.g., PS) in the MALDI spectrum of the ith fraction, Ii, divided by the sum of Ii, expressed as, Wi ) Ii/∑Ii. Thus, the equations for determination of the molecular weight averages of
∑W M /∑W i
n,i
i
and M h w)
∑W M i
2
w,i
/
∑W M i
w,i
(1)
where Mn,i and Mw,i are molecular weights of the homopolymer in the ith fraction. To obtain Ii values, spectral peak integration was performed using GRAMS/32 AI 6.0 software (Galactic Industries, Corp., Salem NH). The software calculates the area of a peak in the trace using the trapezoidal rule employing the absolute value of the regions between the baseline and the curve. Values are determined both before and after subtracting the baseline. The results of MALDI data analysis of SEC fractions from the PS/PAMS blend containing 0.5 weight fraction PS are illustrated in Table 1. Mn,i and Mw,i, as well as Wi values obtained for PS and PAMS homopolymers in the individual blend fractions, were than used to determine molecular weight averages (M h n and M h w) for the PS/PAMS blend constituents using eq 1. Resulting data along with the method of comparison are listed in Table 2. From Table 2, it is evident that the combined SEC/MALDI method for determining homopolymer molecular weights and polydispersities in the PS/PAMS blend yields M h n and M h w values that agree with the corresponding MALDI and SEC values obtained for individual PS and PAMS standards. The average molecular weights for the whole blend measured by SEC tend to be smaller than those obtained using MALDI and SEC/MALDI. This can be caused by the mutual influence of coeluting polymers on their hydrodynamic volumes and consequently on corresponding retention times. Attractive forces between polymers with similar chemistry can lead to smaller hydrodynamic volumes. Consequently, smaller molecular weights are obtained. Chemical Composition Determination. Quantitative determination of the variation of chemical composition across the SEC chromatogram of the PS/PAMS blend is based on the external standard calibration method. MALDI MS is used as an SEC detector selective for both PS and PAMS blend components. Quantitative interpretation of MALDI data is based on the following simple assumptions: Analytical Chemistry, Vol. 75, No. 15, August 1, 2003
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Table 2. Method Comparison for PS/PAMS Blend Analysis MALDI
c
SEC
SEC/MALDI
sample
M hn
M hw
PD
M hn
M hw
PD
PSa PAMSa PSb PAMSb PAMS/PSc
2308 1637
2466 1877
1.07 1.15
1917 1756d
2067 1932d
1.08 1.11
2026
2234
1.1
1406
1620
1.16
M hn
M hw
PD
2311 1866 1801
2414 2006 2075
1.05 1.08 1.15
a Molecular weight averages for PS and PAMS standard samples. b Molecular weight averages for PS and PAMS measured for the blend. Molecular weight averages measured for the whole blend. d Molecular weight averages determined by MALS/SEC.
(i) For a two-component blend, the detector response is proportional to the concentrations of the different chemical species eluting from the separation column according to the relationships
signal intensityPS ) DPSCPS signal intensityPAMS ) DPAMSCPAMS
(2)
where DPS and DPAMS are MALDI response factors for the PS and PAMS homopolymers and CPS and CPAMS are their concentrations, respectively. Furthermore, the signal intensities in eqs 2 can be expressed by areas APS and APAMS, which are equal to integral ∫DPSCPS and ∫DPAMSCPAMS over a mass increment, respectively. (ii) The concentrations of species can be expressed in relation to the total sample concentration, C, and their individual weight fractions, wPS and wPAMS: Figure 6. Standard curve for polystyrene (PS)/poly(R-methylstyrene) (PAMS) blends. Data points are from four replicate MALDI experiments.
CPS ) wPSC CPAMS ) wPAMSC ) (1 - wS)C
(3)
Substituting signal intensities into eq 2 by areas, concentrations by eqs 3, and taking the ratio, one obtains following equation:
APS DPS wPS ) APAMS DPAMS (1 - wPS)
(4)
where APS/APAMS is the ratio between the areas of the PS and PAMS homopolymer constituents in the MALDI spectrum of the blend. Based on eq 4, calibration was performed using a series of PS/PAMS standard blends with the PS weight fraction ranging from 0.09 to 0.7; the concentration of PAMS was kept constant. A standard curve was obtained by plotting the ratios between the areas of the analyte (PS) and the reference (PAMS) versus the ratio, wPS/(1 - wPS). Four replicate MALDI measurements were carried out for each standard. The areas APS and APAMS were calculated by summing integral intensities of PS and PAMS oligomer peaks in the PS/PAMS MALDI spectra, which were determined by direct peak integration using GRAMS/32 AI 6.0 software. The standard calibration curve for PS/PAMS blend with PAMS as the reference is shown in Figure 6. The data points were fitted to a straight line using a standard linear least-squares algorithm. An excellent linear curve fit was obtained having a coefficient of variation (R2) greater than 0.997. The relative standard deviation of the slope is less than 5%, and in the residual 3748
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plot obtained from regression analysis (not shown here for brevity), the residuals for the intensity ratio/concentration data indicate only random variation. This suggests that the standard linear least-squares algorithm fits the data well. In addition to graphical residual analysis, the F ratio for six concentrations analyzed in four replicates was used to test the goodness of fit to the data. The observed F ratio was 0.0433 for 3 and 20 degrees of freedom; the critical F value is F(3, 20) ) 3.09, indicating linear functionality. At the 95% confidence level, the t value for the y-intercept term (ta ) 0.129 464) is less than the critical value tc ) 2, indicating that the intercept term is not significantly different from zero. Therefore, the calibration method is considered to be free of systematic errors. The linear regression calibration equation (y ) 0.0118 + 2.776x, where a y value represents intensity ratio and an x value represents concentration ratio, respectively) was then applied to quantify the polymer blend composition (x values) across the SEC chromatogram for a series of APS/APAMS ratios (y values) determined from the MALDI spectra of the polymer blend fractions. Variation of chemical composition across the SEC chromatogram for the PS/ PAMS blend with 0.5 weight faction PS is shown in Figure 7. As expected, the PS content in the blend decreases in the fractions with lower molecular weights (higher retention times). Therefore, it is reasonable to consider that the latter fractions consist mainly of PAMS. This corresponds to the MALDI mass spectra for SEC fractions from the PS/PAMS blend; see for
nent, for which the ionization efficiency is much larger. To overcome this difficulty, a sample preparation recipe optimized for both polymer components needs to be developed. Likely the sample preparation procedure would involve the combination of several matrixes and ionization agents. Except for the extreme case of a polymer blend formed from polymers with significantly different ionization efficiencies, signal suppression does not affect the results as the relative signal intensities are accounted for calculations. It is also worthwhile to mention the possibility of applying the above-proposed MALDI data treatment for determining the chemical composition of binary blends to three-component mixtures. The relationship between the detector signal intensity and concentrations of individual blend components Ca, Cb, and Cc can be expressed as Figure 7. Variation of the chemical composition across the SEC chromatogram for PS/PAMS blends with 0.5 weight faction PS. Data points are from three replicate SEC experiments. Calculation is based on external calibration.
signal intensitya ) Aa ) DaCa signal intensityb ) Ab ) DbCb signal intensityc ) Ac ) DcCc
(5)
where Aa, Ab, and Ac are the areas of the blend constituents in the MALDI spectrum. The concentrations of species are expressed as products of their individual weight fractions wa, wb, and wc and the total sample concentration, C:
Ca ) waC Cb ) wbC Cc ) wcC
(6)
Figure 8. Three-dimensional plot for the PS/PAMS blend with 0.5 weight faction PS. See text for details.
example Figure 3, which shows the elution profile, where PS molecules elute from the column first (smaller retention times) closely followed by PAMS molecules. Finally, the data obtained using SEC/MALDI were combined and the bimodal distribution for the PS/PAMS blend was illustrated as shown by the three-dimensional plot of Figure 8. The first dimension (x axis) in the plot represents the chemical composition distribution (weight fraction PS); the second dimension (y axis) represents the PS molecular weight distribution in the PS/PAMS blend; and the signal intensity of the SEC concentration detector for the blend is plotted as the third dimension (z axis). From the plot of Figure 8, values for the molar mass of one constituent and composition of the PS/PAMS blend along the SEC chromatogram can be estimated. For instance, at the signal intensity of 7.0 au, corresponding to a retention time of 25.4 min in Figure 7, the PS/PAMS blend contains ∼50% PS with Mw ) 2400 g/mol (see Figure 8). It should be noted, however, that, for more complex systems prepared from chemically unlike polymers, the MALDI sample preparation step might be complicated by differences in ionization efficiencies. In the case of a blend formed from chemically different types of polymers, significant differences in ionization efficiencies could lead to total signal suppression of one component while the mass spectrum is dominated by a second compo-
Substituting concentrations into eqs 5 by eqs 6, assuming Cb ) wbC ) 0, and taking the ratios of the remaining areas Aa and Ac, one obtains following equation:
Aa Da wPS ) Ac Dc (1 - wc)
(7)
Thus, the weight fraction, wa, can be determined from eq 7 using the approach described for the two-component blend. The same step can be repeated when considering Ca ) waC ) 0, so that wb can be determined based on following equation:
wb Aa Db ) Ac Dc (1 - wb)
(8)
Knowing wa and wb values, the weight fraction of the third component in the blend, wc, can be easily calculated using wa + wb + wc ) 1. Nevertheless, the present results represent the first quantitative study of synthetic polymer blends by SEC/MALDI, and the abovementioned issues indicate the need for future work in this area. CONCLUSIONS The combined SEC/MALDI MS technique was utilized to quantitatively examine a PS/PAMS model blend. The most Analytical Chemistry, Vol. 75, No. 15, August 1, 2003
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important finding of this study is that the combined SEC/MALDI technique can provide valuable quantitative data for a certain class of polymer blends. In particular, this technique has been successfully used to analyze polymer blends of constituents with similar chemical structures and molecular weights, for which the differences are often too small to allow separation by liquid chromatography. In this case, MALDI MS can be used as a highly selective SEC detector for discrimination of individual blend homopolymer constituents. The proposed MALDI data treatment can provide, in one experimental setup, precise, quantitative information on the blend chemical composition distribution as well
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as on molecular weights and distributions of individual blend components. ACKNOWLEDGMENT This work was supported by the National Science Foundation, Grant CHE-9985864.
Received for review September 25, 2002. Accepted May 8, 2003. AC020593R
