fuel cell application

ABSTRACT: The crosslinking of functionalized polystyrene resins is often of critical importance in determining resin properties and performance in the application of these materials as membranes and supports. In this investigation model systems are developed for quantifying the infrared and Raman spectroscopic properties of copolymers based on poly(styrenecodivinylbenzene). Analytical curves appropriate for the quantification of para and metasubstituted species and pendant double bonds are reported, and corrections to previously reported spectroscopic assignments and analytical methods are made. The usefulness of these two analytical methods in characterizing radiationgrafted films and commercial copolymers is compared, and typical characterization results are given. The relative concentrations of the species found in the grafted films are quite different from their concentrations in the grafting solution, and empirical relationships between the two are developed. In addition, the graft composition varies as a function of the base polymer film thickness and type and the penetration depth in the grafted film. Radiationgrafted films are more highly crosslinked in their near surface regions, and thinner films are more extensively crosslinked. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 59–75, 2004 Keywords: crosslinking; polystyrene; Raman spectroscopy; infrared spectroscopy

Infrared and Raman Spectroscopic Investigation of Crosslinked Polystyrenes and Radiation-Grafted Films H.-P BRACK,1 D FISCHER,2 G PETER,2 M SLASKI,1 G G SCHERER1 Labor fuăr Elektrochemie, Paul Scherrer Institut, CH-5232 Villigen, Switzerland Zuărcher Hochschule Winterthur, CH-8401 Winterthur, Switzerland Received 19 May 2003; accepted 27 August 2003 ABSTRACT: The crosslinking of functionalized polystyrene resins is often of critical importance in determining resin properties and performance in the application of these materials as membranes and supports In this investigation model systems are developed for quantifying the infrared and Raman spectroscopic properties of copolymers based on poly(styrene-co-divinylbenzene) Analytical curves appropriate for the quantification of para- and metasubstituted species and pendant double bonds are reported, and corrections to previously reported spectroscopic assignments and analytical methods are made The usefulness of these two analytical methods in characterizing radiation-grafted films and commercial copolymers is compared, and typical characterization results are given The relative concentrations of the species found in the grafted films are quite different from their concentrations in the grafting solution, and empirical relationships between the two are developed In addition, the graft composition varies as a function of the base polymer film thickness and type and the penetration depth in the grafted film Radiation-grafted films are more highly crosslinked in their near surface regions, and thinner films are more extensively crosslinked © 2003 Wiley Periodicals, Inc.* J Polym Sci Part A: Polym Chem 42: 59 –75, 2004 Keywords: crosslinking; polystyrene; Raman spectroscopy; infrared spectroscopy INTRODUCTION Crosslinked Polystyrenes (PSs) Crosslinked copolymers prepared from styrene (St) and divinylbenzene (DVB) are widely used in the preparation of ion-exchange and other functionalized resins, supports, and membranes for use in catalytic, combinatorial chemistry, and ion separation processes.1,2 The DVB content of these materials can be varied to control their extent of crosslinking As a result, many of their properties Correspondence to: H.-P Brack (E-mail: hans-peter brack@psi.ch) Journal of Polymer Science: Part A: Polymer Chemistry, Vol 42, 59 –75 (2004) *© 2003 Government of Switzerland Exclusive worldwide publication rights in the article have been transferred to Wiley Periodicals, Inc such as the swelling,3 selectivity,4 reactivity,5 elasticity,6 chemical stability,7 and thermal stability8 can be readily modified Despite the wide variety of applications of crosslinked PSs and their commercial importance, these copolymer systems are actually quite complex and many aspects are still not well understood or completely investigated For example, technical grade DVB (tech-DVB) is actually a mixture of meta-DVB (m-DVB) and para-DVB (pDVB), as well as m- and p-ethylvinylbenzene (mand p-EVB).9 Not only the polymerization rate constants of m- and p-DVB but also their reactivity ratios with St are known to be quite different.10,11 To add further confusion, the p-DVB monomer is reported10,11 to be either more or less reactive than the m-DVB one In addition, the 59 60 BRACK ET AL kinetic parameters of m- and p-EVB have not been reported yet Because of these differences in the reactivity of the various monomer species, the compositions of poly(St-co-DVB) [P(St-co-DVB)] copolymers are expected to vary significantly as a function of the extent of conversion Another complexity of this system is that the difunctional monomer DVB may be incorporated in the copolymer even if only one vinyl group reacts Therefore, not all bound DVB is active as a crosslinker In a related aspect, Lu and coworkers reported that m-DVB is more effective in crosslinking because of the greater reactivity of its pendant double bond.12 In addition, the extent of reaction of pendant double bonds has been reported to vary as a function of the molar content of DVB.13 Radiation-Grafted Films and Membranes Radiation-grafted copolymers prepared by grafting polymer films with St and DVB and subsequent sulfonation have been used as membranes in electrodialysis,14,15 electrosynthesis,15 protein extraction,14 and purification and recovery in etching processes.14 Such membranes are also used as separators in batteries15 and fuel cells.16 These commercialized applications of radiationgrafted membranes and other applications still under development have been reviewed by Mehnert et al.17 and Clough.18 The use of DVB as a crosslinker in these membranes has been shown to have an important influence on their ex situ properties of swelling,19 ionic resistivity,19 toughness,20 and oxidative and thermal stability.21,22 Crosslinking can also be used to reduce the extent of gas crossover in an actual fuel cell application.23 In addition, the use of DVB has led to improvements in the maximum achievable power densities and lifetime of fuel cells containing these membranes.16,24,25 Despite the importance of crosslinking, typically, only the DVB content in the grafting solutions used to prepare these membranes have been reported.16,19 –27 This is a problem because the actual content of DVB in the resulting membranes may not be the same as that in the grafting solution The extent of incorporation and reaction of the different DVB components is perhaps the most complex in the case of radiation-grafted membranes In this preparation method, only some portion of the grafting monomer is typically converted during grafting Because of thermal polymerization and/or chain-transfer reactions, poly- merization may occur not only in the film but also in the solution Because the rates of monomer reaction and diffusion into the grafting film may also differ for different monomer species, grafted films may have considerable heterogeneity Spectroscopic Characterization of RadiationGrafted Films Indeed, our first qualitative spectroscopic investigations of crosslinked radiation-grafted films indicated that the relative DVB content was higher in the near surface region than in the bulk interior.26 This inhomogeneity was advanced as the root cause for the greater specific resistance and lower swelling of thinner membranes.25,26 It should be noted that no quantitative analysis of the DVB content was actually made in this earlier study Later Mattsson and colleagues reported on a semiquantitative analysis of the relative concentration of crosslinker through the thickness of radiation-grafted poly(vinylidene fluoride) (PVDF) films.28 They used micro-Raman spectroscopy to determined that the relative concentration of crosslinker in the middle of the grafted film was about 50% of that at the surface, and no evidence was found for variations in the crosslinker composition (DVB isomers) through the film thickness However, neither the actual crosslinker composition nor the extent of reaction of the crosslinker in the films was reported In addition, it is very important to note that Mattsson et al.28 made a few incorrect assumptions in their work In particular, they mistakenly assigned two bands to unique species In fact, as we show here, these bands actually result from complex overlapping spectral features of several quite different species As discussed subsequently, Mattsson and coworkers28 therefore actually quantified the ratio of two complicated mixtures of very different species and not one single or representative specie The reasons for the general lack of information or apparent misinterpretation concerning crosslinking in radiation-grafted films and membranes include the absence of available analytical curves and well-developed spectroscopic methods for quantifying the content of the species involved Spectroscopic Characterization of P(St-co-DVB) Copolymers Infrared (IR), Raman, and NMR spectroscopic investigations of P(St-co-DVB) resins have been reported in the literature,13,29,30 but these investi- CROSSLINKED PS AND RADIATION-GRAFTED FILMS gations all have important limitations Some of these problems include very narrow applicable concentration ranges, poor choice of model systems, misidentification of bands, and inappropriate assumptions regarding isomer ratios, which we will discuss In particular, Stokr et al.13 proposed using the Raman spectrum of St as a model system for quantifying the ratio of pendant double bonds to total DVB isomers in P(St-co-DVB) copolymers We show here that this analytical method unfortunately has several problems It is based on a single point model, and it is not known how well this analytical relationship holds for other regions of relative concentration Another problem is overlooked interference because of an overlapping band The most troubling problem is that the band assignment and thus the derived analytical equation are actually incorrect Evidence for these errors will be shown in this work, and methods of correcting them will be presented and discussed It is important to correct this method, because the article of Stokr et al has been widely cited and their analytical method and equation have often been applied,13 as was, for example, the case in the work of Lu and coworkers discussed earlier.12 The use of near IR Fourier transform (FT) Raman spectroscopy for the analysis of P(St-coDVB) and its derivatives has been described more recently by Altava et al.29 They report on the use of two Raman bands corresponding to mono- and parasubstituted benzenes and another band corresponding to residual unreacted double bonds for obtaining a qualitative indication of the degree of crosslinking in these resins The authors caution that this method must be used carefully because commercial DVB materials contain both metaand parasubstituted isomers Unfortunately, their analytical curves are of limited use to others because they are based on DVB from an undisclosed supplier and with an undisclosed ratio of meta- to parasubstituted isomers Thus, they cannot be readily applied to the analysis of copolymers based on DVB from other sources having other potential isomer ratios Another limitation of this analytical method is that it is based on bands associated with the minor parasubstituted isomer, whereas the metasubstituted isomer is usually present in at least a twofold excess.9 Moreover, it is more troubling that the differences in the reactivities of the isomers would be expected to result in P(St-co-DVB) with compositions that vary as a function of the extent of 61 conversion This point is important for radiationgrafted films and membranes because they are quite often prepared with less than 100% conversion of the monomer Therefore, it can be concluded that analytical methods based on only one isomer like that of Altava and coworkers29 are quite limited in their usefulness Solid-state 13C NMR can be used to characterize all the types of carbon atoms and isomeric species present in resins Unfortunately, the required instrumentation is still less widely available today than Raman and IR spectrometers In addition, NMR is less sensitive to species present at low concentrations Perhaps for this reason Law et al have used this method to investigate only highly crosslinked PDVB resins containing no St,30 rather than PS copolymers containing low levels of DVB crosslinker, which is more typically found in most applications This investigation examines the IR and Raman spectroscopic properties of model copolymer, blend, and mixture systems based on analogs of the polymerized forms of St and DVB By careful study of these well-defined model systems, we develop robust quantitative analytical methods for real copolymer systems that are applicable over a wide range of concentrations, as well as a significant correction over prior reported methods We also compare the ease of use of these various methods in evaluating commercial copolymers and then select the best methods for examining the influence of various preparation parameters on the composition of our radiation-grafted copolymers EXPERIMENTAL Preparation of Irradiated Films, Grafted Films, and Membranes The base polymer poly(ethylene-alt-tetrafluoroethylene) (ETFE) was purchased as a 50 ␮m thick film from Nowofol GmbH (Siegsdorf, Germany) Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) was procured as 25–125 ␮m thick films from DuPont The molar weights and copolymer composition of the resins used to make these films were reported previously.31 Grafted films and membranes were prepared with our previously delineated methods.26,27,32 Films were preirradiated in a nitrogen atmosphere in barrier film bags with an electron beam source The dose rate and other information on 62 BRACK ET AL this irradiation source were provided in another study.31 The preirradiated films were subsequently stored under a nitrogen atmosphere at Ϫ80 °C until they were grafted Preirradiated films were grafted for 4.5 h at 60 °C under a nitrogen atmosphere An approximately 10-fold excess of monomer was present in the reactors relative to that consumed in the grafting reactions Therefore, the monomer concentrations in the grafting solutions remained relatively unchanged during grafting The grafting solutions consisted of either pure St (pure grade, Fluka) or its mixtures with DVB (technical grade, Fluka) The DVB monomer is actually a mixture of four species and is typically 70 – 85% active because of the m- and pDVB The remaining 15–30% is 3- and 4-EVB, and the ratio of meta to para isomers in the DVB mixture is about 2.2–2.3.9 This specification was independently confirmed by NMR analysis This technical grade mixture of the four distinct disubstituted benzene monomers is referred to as tech-DVB unless otherwise stated, and the concentration of techDVB in the grafting solutions is referred to as [techDVB]GS The two parasubstituted species (p-DVB and 4-EVB) are jointly referred to here as paradisubstituted benzenes (p-DSBs), and the other two metasubstituted species (m-DVB and 3-EVB) are referred to as metadisubstituted benzenes (mDSBs) It should be noted that vibrational spectroscopic methods readily distinguish between paraand meta-ring substitution patterns, but they cannot directly distinguish between p-DVB and 4-EVB or between m-DVB and 3-EVB Unreacted monomer and ungrafted homopolymer were removed after grafting by Soxhlet extraction with toluene Grafted film samples were then dried under a vacuum before either spectroscopic analysis or subsequent steps in membrane preparation Membranes were prepared from the grafted ETFE-g-PS or FEP-g-PS films with our previously reported methods consisting of sulfonation by chlorosulfonic acid, hydrolysis of sulfonyl chloride groups with NaOH solution, regeneration by HCl solution, and subsequent swelling in deionized water at 80 °C.26,27,32 Characterization of Film and Membrane Properties Graft Level The graft level of each film was determined from the mass of the irradiated film (mi) and its grafted form (mg) as shown in eq 1: graft level ϭ ͑mg Ϫ mi)/mi ϫ 100% (1) Ion-Exchange and Swelling Properties The bulk membrane properties of the ion-exchange capacity (IEC) and swelling were characterized with our standard methods.26,27 The percent swelling is simply the percent difference between the mass of the water-swollen membrane and its dry water-free form, and the IEC is simply the number of moles of acid groups per unit mass of membrane The standard deviation was determined to be about 0.05 mequiv/g for the IEC measurements and approximately mass % for the water swelling measurements Spectroscopic Investigations Preparation of Spectroscopic Standards Homopolymers and copolymers were used as model systems in the development of analytical functions for the compositional analysis of P(St-co-DVB) copolymers The monomers used to prepare the model systems were 3-methylstyrene [3MS, technical grade, Ͼ97% purity as determined by gas chromatogaphy (GC), Fluka], and 4-methylstyrene (4MS, technical grade, Ͼ96% purity by GC, Aldrich) The homopolymers were PS [weight-average molecular weight (Mw) ϭ 45,000 –100,000 Da, Aldrich] and poly(4MS) (P4MS, Mw ϭ 72,000 Da, Aldrich) Copolymers of known composition were prepared by the reaction of St and either 3MS or 4MS at 70 °C with azobisisobutyronitrile (Ͼ98% purity by GC, Fluka) as the initiator The copolymers were directly prepared on the surface of either KBr pellets (IR analysis) or reflective Al plates (Raman analysis) A homopolymer sample of P3MS for blending was polymerized in a similar manner and purified by reprecipitation Gel permeation chromatography analysis of this P3MS homopolymer indicated that it had a Mw of 86,000 Da (PS standards) Benzene and toluene were used as solvents for the preparation of solvent cast films of the homopolymers and their blends on KBr or Al substrates The molar ratio of monosubstituted to para- or metasubstituted benzenes in the blends was varied between about 10:1 and 1:8, and approximately –10 blends were used to develop the analytical curves for each system Solvent cast films were thoroughly dried at elevated temperature under a vacuum before spectroscopic analysis Commercial Copolymer Resins of St and DVB Commercial copolymer resins were used to test the applicability of the spectroscopic methods CROSSLINKED PS AND RADIATION-GRAFTED FILMS that we developed Their characterization also made it possible to compare the compositions of commercial copolymers and our radiation-grafted films The copolymer resins in the form of powder or pellets were purchased from Fluka and Laboratoires Chauny (Rohm & Haas) The copolymers had specified tech-DVB contents of between and 20 mass % IR Spectroscopic Analysis The mid-IR spectra were measured at cmϪ1 resolution with a PerkinElmer FTIR System 2000 spectrometer Samples were either free-standing grafted films or cast films of model systems on KBr plates Spectra for quantitative analysis of the commercial P(St-co-DVB) samples were obtained from KBr pellets containing between about and mass % of the copolymer The spectra of the near surface region of radiation-grafted films were obtained with an attenuated total reflectance (ATR) accessory equipped with a 45° ZnSe crystal (Graseby Specac) Raman Spectroscopic Analysis Raman spectra were measured on a Bruker Equinox 55/S FTIR spectrometer system equipped with a FRA 106/S Raman module The laser intensity was 300 mW for liquid samples and 500 mW for solid samples, and the spectral resolution was cmϪ1 For the development of standard curves for quantifying the content of pendant double bonds to monosubstituted benzenes, the Raman spectra were measured for the monomers DVB, St, and 4MS at different concentrations in toluene (technical grade, Fluka) with a quartz liquid cell with a mirror backing In the case of the standard curves for quantifying the content of monosubstituted and para- or metasubstituted benzenes, the Raman spectra were measured for homopolymers and their blends and copolymer films on reflective aluminum plates The total number of samples and the range of variations of the molar ratios in each system were similar to those described earlier for the IR analysis The Raman spectra of a few solid samples were also measured on a confocal Raman microscope (LabRam, DI-LOR/Instruments S.A.) with the 530.901-nm line of an external Krϩ ion laser (Innova 302, Coherent, Inc.) as a light source The laser power was limited to less than mW at the focal point to avoid potential sample degradation Spectra were recorded with a resolution of 63 cmϪ1, and band positions were calibrated by recording the spectrum of a neon lamp Band-Fitting Analysis of IR and Raman Spectra The curve-fitting feature of GRAMS/386 software (version 3.02) from Galactic Industries was used to fit all vibrational spectra, and the fitted peak shapes were taken to be a mixture of Gaussian and Lorenzian forms All spectra were well defined, and consistent fits were obtained without fixing any fitting parameters in the analysis of the radiation-grafted films The analysis of the commercial copolymers was somewhat more difficult because of their much lower DVB contents and thus their weaker spectral features For these spectra, limits on the band positions and widths in the fitting procedure were set based on the results of the band-fitting analysis of the model homopolymers RESULTS AND DISCUSSION Membrane Graft Level, IEC, and Swelling Shown in Figure are typical results obtained for the graft level, IEC, and water swelling of radiation-grafted films and membranes as the concentration of tech-DVB monomer is increased in the grafting solution The concentration of tech-DVB is given on a mass basis rather than a molar basis because of the complexity of the tech-DVB mixture The graft levels of the films decrease fairly linearly in the figure, and the results fit well in a linear regression analysis to the following equation: graft level ϭ 38.2 Ϫ 0.483 ϫ [tech-DVB] (R2 ϭ 0.895) In parallel to the decreases in the graft level and thus in the P(St-co-DVB) content, the IEC of the membranes also decreases quite linearly as IEC ϭ 2.27 Ϫ 0.0315 ϫ [tech-DVB] (R2 ϭ 0.965) Such dependence of the graft level and IEC on the DVB concentration has been observed earlier.19,33 This behavior is ascribed to crosslinking of the near surface region of films during grafting, which then hinders the transport of monomer further into the interior of the film This hindrance acts to reduce the graft level and thus the IEC of the membrane obtained In contrast, the swelling behavior of the membranes does not show a strong linear decrease with increasing DVB content of the grafting solution (R2 ϭ 0.516), and the uncrosslinked membrane does not fit well to a linear regression The fact that there is no simple linear behavior as in 64 BRACK ET AL Figure The selected region of the mid-IR spectra of PS (spectrum a), P4MS (spectrum b), P3MS (spectrum c), and the radiation-grafted film RGF1 (spectrum d) Figure The (a) graft level and (b) (F) ion-exchange capacity and (E) percent swelling of radiation-grafted films and membranes based on FEP-50 films as a function of the mass percent DVB in the grafting solution the other properties illustrates the importance of this investigation To understand the swelling behavior and other properties of membranes, it is important to be able to measure exactly how much DVB is incorporated and how fully it is reacted Development of Standard Analytical Curves Quantifying Relative Content of DVB Isomers and St IR Spectroscopy Figure presents the mid-IR spectra of PS, P4MS, P3MS, and a typical film grafted with both St and tech-DVB (RGF1) The figure indicates the basis of the choice of IR bands (Table 1) that were subsequently used for the development of analytical curves for determining the concentration of meta- and parasubstituted isomers (m- and p-DSB) relative to that of monosubstituted benzenes (BMS) The mono-, meta-, and parasubstituted benzenes have distinct and unique bands in the mid-IR region between 1470 and 1510 cmϪ1 These bands are attributable to vibrational modes involving semicircular stretch- ing of the carbon ring mixed with COH bending.34 The bands due to the mono- and metasubstituted species overlap somewhat, but their peak centers are nonetheless nearly 10 cmϪ1 apart Thus, these three bands could be easily resolved and their respective areas determined by means of band-fitting analysis The IR spectra of solvent cast films composed of blends of PS with either P4MS or P3MS were used as model systems for the spectra of copolymers of p-DSB and St or m-DSB and St, respectively The results of the linear regression analyses of the band-fitting data are summarized in Table Their related analytical expressions for determining the molar ratio of meta- or parasubstituted benzenes to monosubstituted benzenes in P(St-co-DVB) materials are of the form ͓B MS]/[DSB species] ϭ ((area of band 1/area of band 2) Ϫ b͒/m (2) Table Frequency of Bands Used for Analysis of Grafted Films Functional Group Monosubstituted benzenes Paradisubstituted benzenes Metasubstituted benzenes Double bonds Spectroscopic Method IR Raman IR Raman IR Raman Raman Band Frequency (cmϪ1) 1493 620 1513 642 1486 522 1630 CROSSLINKED PS AND RADIATION-GRAFTED FILMS 65 Table Analysis of Mono- and Disubstituted Benzenes Functional Group Model (x) Paradisubstituted benzenes Metadisubstituted benzenes PS/P4MS PS/P3MS Spectroscopic Method Band (cmϪ1) Band (cmϪ1) Slope (m) Intercept (b) R2 IR Raman IR Raman 1493 620 1493 620 1513 642 1486 522 0.443 0.947 0.122 0.928 0.269 Ϫ0.275 0.373 Ϫ0.404 0.987 0.996 0.942 0.994 The relevant analytical expression is (area of band 1/area of band 2) ϭ m(x) ϩ b Estimates of the error in this analysis of copolymer samples were made based on the statistical values originating from the original regression analysis of the model system data and of the actual copolymer measurements No significant difference was found in the spectra or the analytical curves obtained when either benzene or toluene was used as a solvent or when copolymers were used in place of blends of homopolymers (not shown) A comparison of the slopes summarized in Table demonstrates that the IR bands of the metasubstituted PSs have a stronger molar IR absorbance than the corresponding bands of either the monosubstituted or parasubstituted PSs Thus, the molar IR extinction coefficients vary as meta Ͼ para Ͼ mono Raman Spectroscopy A lower frequency region of the Raman spectra of the homopolymers and the radiation-grafted film sample is shown in Figure The spectral features due to the differently substituted benzenes are only moderately strong, Figure The selected lower frequency region of the Raman spectra of PS (spectrum a), P4MS (spectrum b), P3MS (spectrum c), and the radiation-grafted film RGF2 (spectrum d) and the signal-to-noise ratio is poorer than in the IR Nonetheless, the bands are well resolved and not overlap The bands for the mono- and parasubstituted benzenes between 620 and 645 cmϪ1 result from in-plane quadrant bending modes of the aromatic rings, whereas the band at 522 cmϪ1 for the metasubstituted benzenes results from a benzene ring vibration that involves out of plane ring bending by quadrants.34 The first two bands are particularly stable in frequency and thus useful for analytical purposes because the in-plane quadrant bending modes cannot interact with substituent stretching The Raman bands in this region that are used for the copolymer analysis in this investigation are summarized in Table The Raman spectra of solvent cast films composed of blends of the homopolymers were used as model systems for the spectra of P(St-co-DVB) copolymers The results of a linear regression analysis of these band-fitting data are summarized in Table 2, and their related analytical expressions for quantitative compositional analysis of P(St-co-DVB) copolymers are also in the form of eq As in the IR investigations, we found that the analytical results did not depend on the nature of the solvent or the use of copolymers versus polymer blends It is interesting that the molar Raman scattering intensities of the bands used in this study also vary in a manner similar to that of the molar IR extinction coefficients: meta Ͼ para Ͼ mono (Table 2) The analytical expressions for both the IR and Raman methods were validated by analyzing a three-component blend (PS, P3MS, and P4MS) of known composition Figure shows a higher frequency region of the Raman spectrum of the homopolymers The vibrational modes in the 1550 –1650-cmϪ1 region involve mainly quadrant stretching of the ring COC bonds, and they have only a small interaction with COH in-plane bending modes.34 These Raman bands appear to be fairly strong and have 66 BRACK ET AL Figure The selected midfrequency region of the Raman spectra of PS (spectrum a), P4MS (spectrum b), and P3MS (spectrum c) a good signal-to-noise ratio Also indicated in the figure are the two bands used by Mattsson and coworkers for their analysis of the content of crosslinker in radiation-grafted PVDF films.28 The band at 1583 cmϪ1 was used in their work for the normalization of spectra ([PS] in [crosslinker]/ [PS] ratio), and the intensity of the broad band at 1630 cmϪ1 was taken as a measure of the crosslinker concentration However, the band at 1583 cmϪ1 that Mattsson et al.28 assigned to PS can be seen to actually strongly overlap with two weak bands at 1577 and 1585 cmϪ1 in the spectrum of P4MS (parasubstituted benzenes) and a quite strong band at 1587 cmϪ1 in the spectrum of P3MS (metasubstituted benzenes) This overlap results from the fact that the 1600 cmϪ1 doublet is not very frequency sensitive to changes in substitution In fact, most aromatics exhibit bands in the region from 1565 to 1620 cmϪ1, regardless of whether there is symmetry.34 Therefore, the Raman feature at 1583 cmϪ1 cannot be used to quantify the content of monosubstituted benzenes as was done by Mattson and coworkers, and as a result their spectra are incorrectly normalized to one another.28 At best, the bands in this region could potentially be used for compositional analysis if band-fitting analyses were carried out (not done in that study), but even such a band-fitting analysis is quite difficult in this case because several broad bands strongly overlap with each other In addition, Figure also indicates that the Raman band at 1630 cmϪ1 used by Mattsson et al.28 for their semiquantitative analysis of the content of the DVB crosslinker is actually sensitive to only the less reactive meta isomer and not to the more reactive para isomer An even greater problem however is that the band at 1630 cmϪ1 overlaps completely with a strong Raman band at 1633 cmϪ1 (Fig 5) This 1633-cmϪ1 band is from the pendant double bonds in P(St-co-DVB) copolymers, as illustrated in Figure In fact, Stokr et al already described a Raman spectroscopic method for quantifying the content of pendant double bonds based on the band at 1633 cmϪ1.13 In related work, Witke and Kimmer developed methods to follow the kinetics of the polymerization of styrenic monomers based on monitoring the disappearance of this same band at 1633 cmϪ1.35 Therefore, the analysis of crosslinker content by Mattson and coworkers28 is in error because they failed to consider the overlap of bands because of mono- and disubstituted species and the overlap of metasubstituted benzenes with pendant double bonds This latter point is quite important because it will also have an interfering effect on the quantification of pendant double bonds in P(St-co-DVB) materials with the Raman method of Stokr et al.,13 as is discussed in the next section It can be concluded that Mattson and coworkers have actually quantified the ratio of two complicated mixtures of very different species rather than one representative or even single crosslinker species.28 Quantifying Relative Content of Pendant Double Bonds and St Analytical Raman Spectroscopic Method Figure shows portions of the Raman spectra of mono- Figure Selected regions of the Raman spectra of St (spectrum a), DVB (spectrum b), the radiation-grafted film RGF2 (spectrum c), and toluene (spectrum d) The shorter wave number region of the spectra are multiplied by a constant to improve the visibility of these features CROSSLINKED PS AND RADIATION-GRAFTED FILMS 67 Table Analysis of Pendant Double Bonds by Raman Spectroscopy Model (y) St/(Tol ϩ St) 4MS/Tol (2 ϫ DVB)/Tol Band (cmϪ1) Band (cmϪ1) Slope (m) Intercept (b) R2 620 620 620 1630 1630 1630 0.030 0.0241 0.0253 0.0122 0.0044 Ϫ0.0017 0.997 0.999 0.9981 The relevant analytical expressions are [ϭ]/[BMS] ϭ m(area of band 1/area of band 2) ϩ b Tol, toluene mers St and tech-DVB, as well as the representative radiation-grafted film RGF2 and toluene The lower frequency part of the spectra contains the bands of the in-plane quadrant bending mode for mono- and parasubstituted benzenes at 621 and 642 cmϪ1, respectively.34 The higher frequency part of the spectra contains the CAC stretching band around 1630 cmϪ1 and the bands of the quadrant and semicircle stretching modes of the substituted benzenes at about 1600 and 1500 cmϪ1.34 This same double bond band appears only very weakly in the IR spectrum.34 The St monomer, toluene, and the grafted film RGF2 (grafted PS) all have a medium strong band around 621 cmϪ1 that is attributable to their monosubstituted benzene units; this band is absent from the spectrum of DVB The two monomers (St and tech-DVB) exhibit the double bond band at 1630 cmϪ1, but this band is absent in the spectra of toluene and the film grafted with only St (RGF2) Figures and provide good illustrations of the most serious problem in the analytical method of Stokr et al.,13 namely, their incorrect band assignment and hence their resulting incorrect analytical equation: [ϭ]/[DVB] ϭ 0.037(I1633 /I622 ) (3) where [ϭ] is the molar content of pendant double bonds, [DVB] is the molar content of DVB units, and I1633/I622 is the ratio of the band intensities at 1633 and 622 cmϪ1 As shown here, the band at 622 cmϪ1 is present in the spectra of St and PS but it is not in the spectra of DVB or the model polymers P3MS or P4MS Therefore, this band is due to monosubstituted benzenes (BMS) rather than disubstituted ones (DVB) It is quite logical that this error exists in the equation of Stokr and coworkers13 because they used St, a monosubstituted benzene, as a model system in deriving their analytical relationship Nonetheless, analytical curves for measuring the molar ratio of [ϭ]/[BMS] in P(St-coDVB) based materials could be constructed in this investigation by applying the necessary corrections, as discussed next The parameters of the analytical curves for the determination of the relative content of pendant double bonds are given in Table for the three model mixture systems: St and toluene, tech-DVB and toluene, and 4MS and toluene The appropriate analytical equation is [ϭ]/[BMS] ϭ m͑A1633 /A622 ͒ ϩ b (4) where m is the slope, A1633/A622 is the ratio of the integrated band areas at 1633 cmϪ1 to that at 622 cmϪ1, and b is the intercept The slope is the largest for the purely monosubstituted system (St and toluene) and lowest for the disubstituted monomers in toluene, and the magnitudes of the slopes differ by up to about 20% in the most extreme case This difference in slopes is most likely due to electronic interactions of the double bond with the benzene ring Through their influence on the aromatic ␲ system, additional substituents of the aromatic ring may affect the polarizibility of the double bond and thus the intensity of its Raman band For example, alkyl groups act as weak electron-donating substituents and, of course, a second vinyl substituent could interact through overlap of its ␲ orbital system with that of the intervening benzene ring Thus, there is still another important difference between the earlier analytical relationship in eq and the results of this investigation in eq 4, namely, the slope and intercept Stokr et al developed their analytical expression based on the measurement of a single point (St).13 In this investigation we used several model mixture systems and a subsequent linear regression analysis of their data The slope here was determined to be significantly lower (ca 19 –35%) than that determined by Stokr and coworkers13 and to have a 68 BRACK ET AL value of between about 0.024 and 0.030 (Table 3) The analytical relationship in our eq was also found to have a small but nonzero intercept The two disubstituted systems (4MS/toluene and techDVB/toluene) gave approximately the same results for the parameters in this expression We selected the slope and intercept from the model mixture system (4MS/toluene), for use in our further analytical work because the composition of this model system is somewhat better defined As mentioned before, the analytical method of Stokr et al.13 has yet another problem that is due to the overlap of the strong double bond band with a weaker band for m-DSBs To estimate the magnitude of this interference, the ratios of the band areas (1630/522 cmϪ1) were determined from the Raman spectra of both the monomer (3MS) and the polymer (P3MS) This peak area ratio is 0.422 for P3MS and 9.96 for 3MS Therefore, the molar Raman scattering coefficient of the metasubstituted benzene band is only about 1/20th that of the double bond band The magnitude of the resulting error on the quantitative analysis of the double bond content in many P(St-co-DVB) materials is likely to be even less because tech-DVB consists of both meta and para isomers Nevertheless, if the meta content of a P(St-co-DVB) copolymer is known or if the area of the Raman band at 522 cmϪ1 is measured, this interference can now be corrected This correction has been made to all analytical results reported here Another modest difference between this and earlier work is that we have used integrated band areas whereas the earlier work simply used peak heights In summary, the differences in the band assignment, analytical expression, and details of the analytical method all have a significant influence on the magnitude of the calculated content of pendant double bonds, as we show later Spectroscopic Analysis of Commercial Copolymers Content of Para- and Metasubstituted Isomers It was difficult to obtain Raman spectra of the copolymer pellets having strong and well-resolved bands in the 500 – 650-cmϪ1 region Thus, alternative Raman bands for the quantitative analysis of these species were evaluated These bands are at 1030 cmϪ1 for monosubstituted species (semicircle ring stretch), at 740 and 1242 cmϪ1 for metasubstituted benzenes (quadrant in-plane bending mode involving substituent bond inphase stretching and ring stretching vibration, respec- Figure The molar ratio of (E) para-/monosubstituted benzenes and (ᮀ) meta-/monosubstituted benzenes of commercial P(St-co-DVB) copolymers as determined by IR spectroscopy tively), and at 830 cmϪ1 for parasubstituted species (quadrant in-plane bending mode).34 Unfortunately, these bands were also insufficiently strong and well resolved in the spectra to allow for a quantitative compositional analysis of the copolymers For this reason we decided instead to carry out the analysis based on the IR bands given in Table Figure summarizes the results of the IR analysis of the commercial copolymer samples The meta species band at 1486 cmϪ1 was only poorly resolved as a shoulder in the spectra of the samples having less than 5% DVB content The para species band at 1513 cmϪ1 was a distinct band in all cases, but it was barely visible in the spectra of the samples having less than 5% DVB content As a result, there is considerable scatter in the results of the copolymer samples containing less than 5% DVB In addition, the content of the meta isomers is clearly overestimated and the para isomers are apparently underestimated for these samples with low DVB content (assuming that the meta/para ratio in these copolymers is similar to that of tech-DVB) The meta content of the 7% DVB sample is in agreement with the predicted value, but the para content is not For the case of the 20% DVB sample, both the experimentally measured meta and para contents and their ratio agree well with the expected values On the basis of these various results, we conclude that this IR method is only applicable for crosslinked PSs having at least about 10% DVB content CROSSLINKED PS AND RADIATION-GRAFTED FILMS 69 value of the DSB concentration in this ratio It can be seen from this figure that the ratio of [ϭ]/[DSB] decreases quite strongly as the mass percent DVB in the copolymers increases to 20 mass %, and the data fits fairly well to a decaying exponential function (R2 ϭ 0.837) The very different magnitudes and x dependencies of the data plotted in Figures 7(a) and 8(a) serve to highlight the importance of the correction of the assignment of the band at 620 cmϪ1 (our eq vs eq from Stokr et al.13) In particular, their eq would predict that the ratio of [ϭ]/[DVB] would look rather like the curve in our Figure instead of Figure The large-scale decrease in the content of unreacted pendant double bonds as the DVB content of these commercial copolymers increases may be a result of the gel effect In these materials, the gel effect could reduce the rates of termination reactions, thereby ensuring that the pendant double bonds are more fully reacted Figure The molar ratio of double bonds to monosubstituted benzenes in (a) commercial P(St-co-DVB) copolymers from (E) Chauny and (F) Fluka, and (b) (E) radiation-grafted FEP-g-P(St-co-DVB) copolymers and (F) their graft levels Content of Pendant Double Bonds The two Raman bands at 620 and 1630 cmϪ1 required for the analysis of pendant double bonds are reasonably strong and well resolved in the spectra of all of the commercial copolymers Hence, the molar ratio of double bonds to monosubstituted benzenes ([ϭ]/[BMS]) could be readily determined for all samples Therefore, we conclude that this Raman method is very sensitive and readily applicable for the analysis of crosslinked PSs having DVB contents as low as 1% The content of pendant double bonds in these copolymers is shown in Figure 7(a) These data indicate that the ratio ([ϭ]/[BMS]) in the commercial copolymers increases steadily as the DVB content increases to approximately mass %, and then it does not appear to increase further There is a fair amount of scatter in the data, and they fit only moderately well to an inverse first-order polynomial regression (R2 ϭ 0.736) The same data are shown in Figure 8(a) in the form of the molar ratio of the pendant double bonds to DSB ([ϭ]/[DSB]) The manufacturers’ product specifications were used to provide the Figure The molar ratio of double bonds to DSB in (a) commercial P(St-co-DVB) copolymers from (E) Chauny and (F) Fluka, and (b) radiation-grafted FEPg-P(St-co-DVB) 70 BRACK ET AL Spectroscopic Analysis of Radiation-Grafted Films Content of Para- and Metasubstituted Isomers As was the case with the commercial copolymers, the bands of the para-, meta-, and monosubstituted benzenes were much stronger and better resolved in the IR spectra of the radiation-grafted films than in their Raman spectra For this reason, the analytical results reported here are based on an IR spectroscopic analysis However, it should be noted that Cardonna et al showed that the micro-Raman technique is useful for investigations of the penetration depth of grafted PS (no DVB crosslinker) onto fluoropolymer substrates.36 FEP-Based Grafted Films The absorption spectra of the radiation-grafted films based on a 50 ␮m thick FEP film were measured in both transmission and ATR modes The swelling data of their membranes corresponding to these films were given in Figure The compositions of these films were analyzed with the same method as just described for the commercial copolymers The results of this analysis are shown in Figure 9(a) for the ATR mode measurements and in Figure 9(b) for the transmission mode measurements The transmission mode measurements probe the bulk of the film whereas the ATR measurements probe a near surface region having a penetration depth on the order of micrometers The transmission mode results indicate that the ratio of para- to metasubstituted isomers in the bulk of the grafted films increases quite strongly and linearly over the entire concentration range under study In contrast, the ratio of both para-/monosubstituted species and meta-/ monosubstituted species increases only very slowly as the concentration of tech-DVB in the grafting solution ([tech-DVB]GS) increases to about 40 mass %, and thereafter they increase quite rapidly Regression analyses were carried out on the transmission mode data, and the resulting relationships were found to give the best fit: p-DSB/m-DSB ϭ 6.40 ϫ 10Ϫ3[tech-DVB]GS ϩ 0.089 (R2 ϭ 0.866), p-DSB/BMS ϭ Ϫ0.0074[tech-DVB]GS ϩ 0.0001([tech-DVB]GS)2 ϩ 0.125 (R2 ϭ 0.986), and m-DSB/BMS ϭ Ϫ0.0163[techDVB]GS ϩ 0.0003([tech-DVB]GS)2 ϩ 0.383 (R2 ϭ 0.939) The data from the ATR mode measurements in Figure 9(a) have a qualitatively similar appearance to that of the transmission mode data, but the magnitudes of the ratios are quite different Figure (a) Attenuated total reflection and (b) transmission mode IR spectroscopic determinations of the molar ratio of (E) para-/monosubstituted benzenes, (F) para-/metasubstituted benzenes, and (ᮀ) meta-/monosubstituted benzenes of radiation-grafted FEP films In particular, the para-/monosubstituted and meta-/monosubstituted ratios both reach a final value at high tech-DVB concentrations (in the grafting solution) that is more than twice that of the transmission mode data The resulting relationships were found to give the best fit to the ATR mode data: p-DSB/m-DSB ϭ 4.37 ϫ 10Ϫ3[tech-DVB] ϩ 0.1489 (R2 ϭ 0.831), p-DSB/ BMS ϭ Ϫ0.0206[tech-DVB] ϩ 0.0004[tech-DVB]2 ϩ 0.310 (R2 ϭ 0.976), and m-DSB/BMS ϭ Ϫ0.0383[tech-DVB] ϩ 0.0008[tech-DVB]2 ϩ 0.704 (R2 ϭ 0.966) From the results of the various regression analyses it is clear that there is a strong tendency for all three ratios, (p-DSB/m-DSB, m-DSB/BMS, and p-DSB/BMS) to increase as the tech-DVB concentration in the grafting solution increases This conclusion holds for both transmission mode (bulk) and ATR mode (near surface) spectroscopic analyses In the bulk the increases in the ratios have about the same magnitude In contrast, in CROSSLINKED PS AND RADIATION-GRAFTED FILMS Figure 10 The molar ratio of (E) para-/monosubstituted benzenes and (F) meta-/monosubstituted benzenes and (ᮀ) the (sum of para- ϩ meta-)/monosubstituted benzenes in the grafting solutions the near surface region the increase in the two ratios of the di- to monosubstituted species (mDSB/BMS and p-DSB/BMS) dominate over the increase in the isomer ratio (p-DSB/m-DSB) A linear model provided the best fit for the increase in p-DSB/m-DSB isomer ratio as a function of [tech-DVB]GS The initial value of this ratio at low DVB concentrations is significantly higher in the near surface region, but it increases more rapidly in the bulk as the concentration of DVB in the grafting solution is increased A quadratic model had the best fit to the data for the other two ratios (p-DSB/BMS and m-DSB/BMS) The initial value of these ratios is again quite a bit higher in the near surface region, but in this case they also increase more rapidly in that same region as the tech-DVB concentration in the grafting solution increases It is of interest to compare the relative concentrations of the grafted species in the films with their concentrations in the grafting solutions To facilitate this comparison, Figure 10 shows the relative content of the monomeric species in the grafting solutions As mentioned earlier, the meta/para ratio is constant at a value of about 2.2–2.3 Most striking in the comparison of Figures and 10 is that the relative contents of tech-DVB and its para and meta isomers are all greater in the grafting solution than in either the bulk or near surface regions of the grafted films For all three ratios, the following relationship appears to hold: ([tech-DVB species]/[S])GS Ͼ ([tech-DVB species]/[BMS])NS Ͼ ([tech-DVB 71 species]/[BMS])bulk, where NS indicated the near surfacen region To better quantify the relationship between the grafted film regions and the grafting solution, the two ratios (p-DSB/BMS and m-DSB/BMS) were divided by their values in the grafting solution and then plotted in Figure 11 The data for the p-DSB/ BMS ratio from the transmission measurements show some scatter, but the values are centered around 0.21 Ϯ 0.03 and show no significant tendency to increase or decrease (R2 Ӷ 0.1) In contrast, the data for the m-DSB/BMS ratio decrease as [tech-DVB]GS increases, and the data fit well (R2 ϭ 0.94) to a second-order polynomial equation of the form m-DVB/BMS ϭ 0.959 Ϫ 2.62 ϫ 10Ϫ2([tech-DVB]GS) ϩ 2.16 ϫ 10Ϫ4 ([techDVB]GS)2 In the near surface region the values of the p-DSB/BMS ratio are centered around 0.46 Ϯ 0.15 with at most only a very weak tendency to increase (R2 ϭ 0.34) The m-DSB/BMS ratio has a Figure 11 (a) Attenuated total reflection and (b) transmission mode IR spectroscopic determinations of the ratios of (E) para-/monosubstituted benzenes, (F) para-/metasubstituted benzenes, and (ᮀ) meta-/monosubstituted benzenes and their division by their values in the grafting solution 2.2 0.86 0.83 0.96 1.2 0.13 0.21 0.19 0.16 0.10 0.29 0.18 0.15 0.16 0.12 1.0 0.62 0.63 0.78 0.95 0.045 0.047 0.043 0.037 0.021 0.047 0.029 0.027 0.029 0.020 0.60 0.66 0.74 0.78 0.72 0.27 0.31 0.23 0.23 0.25 The grafting solution consisted of 16.9 mass % DVB with the remainder St 0.18 0.20 0.17 0.18 0.18 0.66 0.64 0.72 0.78 0.74 0.32 0.36 0.27 0.27 0.27 0.23 0.23 0.20 0.21 0.20 17.7 19.4 21.4 30.2 30.3 25 50 75 125 50 FEP FEP FEP FEP ETFE 80 80 30 30 10 ATR Trans ATR Trans/ ATR ATR Trans Trans/ ATR ATR Trans Thickness (␮m) Type Irradiation Dose (kGy) Graft Level (mass %) m- ϩ p-DSB/BMS m-DSB/BMS Molar Ratios Table Representative Analytical Results for Radiation-Grafted Film Samples Different FEP Base Polymer Film Thicknesses It was next of interest to determine if these empirical relationships between the grafting solution and the grafted film compositions would hold for other base polymers Therefore, IR spectroscopic analyses were carried out in a similar manner on radiation-grafted films prepared from FEP films of other thicknesses The variation in the ratios of the different species as a function of the tech-DVB concentration in the grafting solution qualitatively resembled those of the grafted films based on the 50 ␮m thick FEP film, but on a quantitative level there were considerable differences A few representative values are given in Table for comparison purposes Several trends can be seen in these data In the case of the p-DSB/BMS ratio, this ratio decreases by up to about 40% in the bulk as the base polymers become thicker The decrease in this ratio is smaller, up to about 20%, for the near surface region The difference between this ratio in the bulk and in the near surface region becomes even greater as the films become thicker The two ratios that give a measure of the amount of techDVB incorporated into the grafted film (p-DSB/ BMS and m-DVB/BMS) are both significantly lower in the near surface and bulk regions of all of the grafted films than their values in the grafting solution It is interesting that the difference between the bulk and near surface region is not quite as great Trans p-DSB/BMS Trans/ ATR p-DSB/m-DSB dependency in this region that is similar to that in the bulk, but it is shifted to somewhat higher values and shows much more scatter: m-DSB/BMS ϭ 1.27 Ϫ 2.88 ϫ 10Ϫ2([tech-DVB]GS) ϩ 2.69 ϫ 10Ϫ4([tech-DVB]GS)2 (R2 ϭ 0.618) It is perhaps not too surprising that the concentrations of the tech-DVB species are higher in the grafting solution than in the grafted films The reported reactivity ratios for m-DVB and pDVB with St indicate that St will be preferentially incorporated, especially compared to the mDVB.37 Such behavior has been observed for copolymers prepared in solution.11 Nonetheless, the amount of tech-DVB species incorporated into the grafted films is much lower than would be expected based on the solution work We propose that there may be other important factors unique to the radiation-grafting system that act to distinguish it from polymerization in solution.26 These special factors could include differences in solubility and/or diffusion rates of different monomeric species in the films during grafting Trans/ ATR BRACK ET AL Base Polymer 72 CROSSLINKED PS AND RADIATION-GRAFTED FILMS as the 50% difference reported by Mattsson and coworkers.28 However, as noted earlier, it is not quite clear what was actually measured in that investigation because of problems with their band assignments and other factors To have a better understanding of this difference, we calculated some of the relative crosslinker contents of our samples with their method In general, we found that the method of Mattsson et al.28 gave quite scattered results and relative crosslinker contents that were much too high, particularly for films prepared with grafting solutions with a higher crosslinker concentration The fact that each thickness of the same fluoropolymer film from the same manufacturer yields a somewhat different grafted film composition is perhaps not that surprising because the different thickness films are necessarily made under somewhat different processing conditions We reported earlier that many properties of these films like the crystallinity and orientation vary with thickness and these differences have a significant effect on the grafting rate Presumably, differences in the base polymer free volume and consequently the diffusion rates of St into the base polymer are the root cause of this behavior Our results here suggest that the relative diffusion rates of the individual monomeric species are also affected to varying extents by these differences in the properties of the base polymer film The behavior of the p-DSB/m-DSB ratio is quite different from the others This ratio decreases by about 50% in the bulk as the base polymer films increase in thickness; but it appears to increase by about the same amount, but with some scatter, in the near surface region The net result is that this ratio is significantly higher in the near surface region in the 25 ␮m thick base polymer, but it is approximately the same in both regions in the case of the other FEP base polymers The total DVB content (p-DSB ϩ m-DSB/BMS) of the different thickness, grafted FEP films shows only a weak tendency to increase (ca 10 – 20%) as the thickness of the base polymer films decreases These differences here tend to be smaller in magnitude than those of our earlier reported differences in properties between crosslinked membranes based on different thickness FEP films.25,26 For example, radiationgrafted membranes based on 25 ␮m FEP films were found in the latter work to have up to 60% higher specific resistances and 30% lower water swelling than ones based on 75 ␮m FEP films 73 However, it should be noted that only a qualitative comparison between the results of the two investigations is possible because some preparative parameters and membrane properties like the graft levels were different in the earlier cited investigations In addition, we not yet have a quantitative relationship between the DVB content and material properties such as the resistance and swelling Nonetheless, there appears to be qualitative agreement and this investigation confirms that the membranes based on thinner base polymers tend to be more highly crosslinked ETFE-Based Grafted Films Our next interest was to investigate the influence of different base fluoropolymer types on the composition of the incorporated graft component IR spectroscopic measurements and their subsequent analyses were thus carried out on a few radiation-grafted films prepared from ETFE films Some of the same qualitative trends were found as with FEP, but the results are quantitatively quite different from those obtained with the FEP film of the same thickness Some representative values are given in Table The ETFEbased grafted films appear to be significantly different from the FEP-based ones in several important ways For example, there appears to be much less of a difference between the grafted film composition in the bulk and near surface regions of the grafted ETFE films It would thus appear that there is no simple universal empirical relationship to describe the grafting behavior of different base polymer film types, and it is therefore necessary to develop models for each individual system Content of Pendant Double Bonds of Grafted Films Raman spectroscopic analysis was used to determine the content of pendant double bonds in the radiation-grafted films The results of this analysis are shown in Figures 7(b) and 8(b) The data in Figure 7(b) indicate that over the entire range of investigation the [ϭ]/[BMS] ratio increases in the films as the DVB concentration of the grafting solutions increases The data fit quite well to a quadratic polynomial equation of the form [ϭ]/[BMS] ϭ 0.0120 ϩ 0.0019[tech-DVB]GS ϩ 0.0001([DVB]GS)2 (R2 ϭ 0.974) These data are shown again in Figure 8(b) as the molar ratio of the pendant double bonds to total DSBs ([ϭ]/[DSB]) It is assumed for the calculation of this ratio that the Raman measure- 74 BRACK ET AL ments are representative of the bulk properties of the grafted films This assumption is reasonable because a Nd:YAG laser (1064-nm wavelength) was used, and the transparent grafted film was placed on top of a mirror for the Raman measurements Based on this assumption, the IR spectra measured in the transmission mode could be used for the quantification of the DSB concentration in this ratio According to the data in Figure 8(b), the molar ratio of pendant double bonds to DSB is centered around a value of 0.50 Ϯ 0.16 with some scatter, and it is difficult to discern a large-scale or systematic variation in this ratio as the DVB content of the grafting solution is increased (linear regression analysis, R2 ϭ 0.036) It is noteworthy that this behavior is quite different than what was found in the case of the commercial copolymers in Figure 8(a) It was relatively simple to develop empirical relationships between the membrane IEC (Fig 1) and the approximate crosslinking level {e.g., ([m-DSB] ϩ [p-DSB] Ϫ [ϭ])/[BMS]} of the grafted films from which these membranes were made (not shown) because the crosslinking level and IEC were both found to be strong functions of the composition of the grafting solutions However, it was quite difficult to correlate the swelling data for the membranes from Figure with the crosslinking level of the corresponding grafted films It was qualitatively clear that the swelling decreased as the crosslinking level increased, but it was not possible to find a strong regression relationship between the two This lack of correlation is perhaps not surprising in that the graft level of the membranes also decreased as higher tech-DVB concentrations were used in the grafting solution The swelling of uncrosslinked membranes is well known to decrease as their graft level and thus sulfonated PS content decreases.23,27 To better separate these two effects (crosslinking and graft level), future studies will examine membranes prepared to have the same graft level but different levels of crosslinking CONCLUSIONS We developed vibrational spectroscopic methods appropriate for the characterization of PSs crosslinked by DVB IR spectroscopy was more suitable for the quantification of meta- and parasubstituted isomers, but Raman spectroscopy was better suited for the quantification of pendant double bonds The IR method and band-fitting analysis for determining the relative content of substituted benzenes worked quite well for the samples having relative DVB contents of at least about 10% In contrast, the Raman method for quantifying the double bond content worked quite well for all samples, even those with only 1% DVB Significant differences between the relative content of the various species in commercial copolymers and the radiation-grafted films were found In the case of one type of radiation-grafted film, empirical relationships between the grafting solution composition and the grafted film composition were developed Nevertheless, we found that several parameters like the base polymer film composition and thickness also have significant influences on the composition of the resulting grafted film It therefore appears to be necessary to develop empirical relationships for each type of radiation-grafted film system The authors thank M Steinemann (Hot Laboratory, Paul Scherrer Institut) for ␥ irradiation of the samples, C Guănthard (Studer AG, Daăniken) for electron irradiation of the samples, and O Schweizer and D Jahn (Fachhochschule beider Basel Nordwestschweiz) for the preparation of some of the radiation-grafted samples and the measurement of certain spectra REFERENCES AND NOTES Sherrington, D C J Polym Sci Part A: Polym Chem 2001, 39, 2364 –2377 Alexandratos, S D.; Crick, D W Ind Eng Chem Res 1996, 35, 635– 644 Wallner, A S.; Ritchey, W M J Appl Polym Sci 1995, 57, 1– Jones, I L.; Carta, G Ind Eng Chem Res 1993, 32, 107–117 Geethakumari, K.; Sreekumar, K Proc Ind Acad Sci Chem Sci 1998, 110, 499 –506 Tiihonen, J.; Markkanen, I.; Laatikainen, M.; Patero, E J Appl Polym Sci 2001, 82, 1256 –1264 Bibler N E.; Orebaugh, E G Ind Eng Chem Prod Res Dev 1976, 15, 136 –138 Li, Y.; Fan, Y.; Ma, J Polym Degrad Stab 2001, 73, 163–167 van Look, G Fluka Chemie AG, 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