Surface Organization of a Perfluorocarbon-Functionalized Polystyrene

May 1, 2012 - Surface Organization of a Perfluorocarbon-Functionalized Polystyrene ... and Technology, Gaithersburg, Maryland 20899, United States ...
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Surface Organization of a Perfluorocarbon-Functionalized Polystyrene Homopolymer Michael D. Dimitriou,†,‡ Elisa Martinelli,§ Daniel A. Fischer,∥ Giancarlo Galli,*,§ and Edward J. Kramer*,†,⊥,‡ †

Materials Department, University of California, Santa Barbara, Santa Barbara, California 93106, United States Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, California 93106, United States § Dipartimento di Chimica e Chimica Industriale, Università di Pisa, 56126 Pisa, Italy ∥ Materials Science and Engineering Laboratory, National Institute for Standards and Technology, Gaithersburg, Maryland 20899, United States ⊥ Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States ‡

S Supporting Information *

ABSTRACT: We use the perfluorocarbon-functionalized p o l y m e r , p o l y ( 4 - ( ( 1H ,1H ,2H ,2 H-per flu o r o d e c y l ) oxycarbonyl)styrene) [PPFOCS], as a model system with both surface molecular segregation and molecular orientation to test the capabilities of a near-edge X-ray absorption fine structure (NEXAFS) spectroscopy analysis scheme for polymer surfaces. Both NEXAFS spectroscopy and angleresolved X-ray photoelectron spectroscopy (XPS) show segregation of the −(CF2)7CF3 chain to the air/polymer interface with the styrenic portion underneath. Postedge analysis of the NEXAFS spectra indicates a low carbon atom density surface layer, of thickness 1.0−1.4 nm, due to the overlayer of perfluorocarbon chains. An analysis of the NEXAFS C 1s → π*CC and C 1s → σ*C−F transitions accounting for the different depth distributions of the phenyl rings and fluorocarbon helices reveals strong orientational ordering with the orientational order parameter SCC for the phenyl ring equal to −0.27 and for the C−F bonds in the fluorocarbon helix SC−F equal to −0.13. The SCC and SC−F determined for the polymer with the esterlinked side chain are considerably higher than those reported previously (−0.039 and 0, respectively) for a polymer [poly(4(1H,1H,2H,2H-perfluorodecyl)oxymethylstyrene)] with an identical side chain that was ether linked to the styrene phenyl ring.1 We tentatively attribute the high orientation in the PPFOCS to the partial conjugation between the ester group and the phenyl ring providing a relatively stiff linkage between the perfluorocarbon helix and the phenyl ring.

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INTRODUCTION A useful surface characterization technique that complements the information obtained from in-house methods such as photoelectron spectroscopy (XPS, UPS, AES), sum frequency generation, ellipsometry, reflectivity, and scanning force microscopy is near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.2 Unlike the techniques listed above, NEXAFS can determine both surface structure and chemical composition through the use of tunable, polarized, soft X-rays and has demonstrated its applicability in characterizing polymeric surfaces for antifouling coatings,3−5 organic electronics,6−8 and lithography.9−11 NEXAFS has proven particularly useful in characterizing fluorocarbons at a polymer surface.4,5,12 Fluorocarbons offer a range of useful properties such as high transparency13 as well as thermal resistance and chemical resistance14 and have low surface energy,12 refractive index, and dielectric constant.15 By harnessing these properties, the incorporation of fluoropolymers has extended the industrial applicability of polymers. © 2012 American Chemical Society

This paper serves to examine the near-surface of one such fluorocarbon-functional polymer, poly(4-((1H,1H,2H,2Hperfluorodecyl)oxycarbonyl)styrene) [PPFOCS]. Because of the low surface energy of perfluorocarbons, these materials display “surface activity” in polymer systems and will segregate to the surface upon thermal processing.12,16−19 Li et al. demonstrated the applicability of NEXAFS by showing that poly(4-(1H,1H,2H,2H-perfluorodecyl)oxymethylstyrene), along with other derivatives, form a smectic mesophase at the surface of the polymer film.20 Additionally, they established that the phase stability and surface coverage are dependent on perfluorocarbon chain length and hydrocarbon spacer length between the phenyl ring and perfluorocarbon chain. The susceptibility to rearrangement when exposed to water is also dependent on these parameters, findings that have provided Received: January 10, 2012 Revised: April 18, 2012 Published: May 1, 2012 4295

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flux was ∼1 × 1011 photons/s at a typical storage ring current of 750 mA. A spherical grating monochromator was used to obtain monochromatic soft X-rays at an energy resolution of 0.2 eV. The C 1s NEXAFS spectra were acquired for incident photon energy in the range of 270−320 eV. The incidence angle of the X-ray beam, measured from the sample surface, was varied from 20° to 125° to obtain information on molecular orientation and depth profile. To avoid prolonged X-ray exposure of the sample during multiple angular scans, the X-ray beam was moved to a different location for each scan. The PEY signal was collected using a channeltron electron multiplier with an adjustable entrance grid bias (EGB) that was set at −150 V for these experiments. The channeltron partial electron yield (PEY) detector was positioned at an angle of 36° relative to the incoming Xray beam in the plane defined by sample normal and the X-ray beam and at an angle of 35° out of that plane. The experimental geometry is depicted in the Supporting Information. All PEY spectra were normalized by the incident beam intensity obtained from the photoyield of a clean gold grid followed by the subtraction of a linear pre-edge baseline. The photon energy was calibrated by adjusting the peak position of the lowest π* phenyl resonance from polystyrene to 285.5 eV.

fundamental knowledge for analogues of these systems developed for antifouling coatings.3−5,21,22 Sohn et al. used a similar system as a model to investigate new analysis methods for NEXAFS spectroscopy. By analyzing the postedge NEXAFS spectra of poly(4-(1H,1H,2H,2Hperfluorodecyl)oxymethylstyrene), it was possible to deduce a thickness of the perfluorocarbon layer populating the surface.1 This was done by accounting for differences in the carbon atom density as a function of depth from the surface, a parameter that was not accounted for in earlier works. With further analysis of the C 1s → π*CC and C 1s → σ*C−F transitions, a distance from the polymer/air interface was quantified for phenyl rings and C−F bonds. Accounting for these depths proves useful in evaluating a more accurate orientational order parameter for the phenyl rings and C−F bonds. However, an orientation of the perfluorocarbon chain was not detected in the system studied by Sohn et al., and as a result the analysis was not tested on a highly ordered surface.1 Hence, the following study will serve as further validation of postedge NEXAFS analysis, using PPFOCS as a model system. By offering more quantitative insight into the order parameter and depth distribution of bonds in a system, this analysis methodology contributes significantly to more accurate NEXAFS analysis of the surface structure of polymer films, which in turn can be correlated with surface properties.

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RESULTS AND DISCUSSION Polymer Structure and XPS Surface Characterization. Poly(4-((1H,1H,2H,2H-perfluorodecyl)oxycarbonyl)styrene), a perfluorocarbon-functional polystyrene [PPFOCS], contains a −(CF2)7CF3 group linked to a phenyl ring by an ester bond (Scheme 1). The perfluorocarbon chain of PPFOCS is

EXPERIMENTAL SECTION

Scheme 1. Chemical Structure of Perfluoro-Functional Polystyrene-Based Homopolymer Poly(4-((1H,1H,2H,2Hperfluorodecyl)oxycarbonyl)styrene)

Materials. Benzene and THF were refluxed and distilled over Na/ K alloy under nitrogen. Triethylamine was refluxed and distilled over KOH under nitrogen. α,α′-Azobis(isobutyronitrile) (AIBN) (from Fluka) was recrystallized from methanol. 4-Vinylbenzoic acid and 1H,1H,2H,2H-perfluorodecanol (from Sigma-Aldrich) were used without further purification. Experimental conditions for the synthesis of the monomer 4-((1H,1H,2H,2H-perfluorodecyl)oxycarbonyl)styrene and polymer poly(4-((1H,1H,2H,2H-perfluorodecyl)oxycarbonyl)styrene) [PPFOCS] are reported in the Supporting Information. Surface Preparation. Synthesis of surfaces of PPFOCS were prepared by spin-coating from a 1 wt % solution of polymer in trifluorotoluene onto a silicon wafer to yield a film of ∼30 nm in thickness. The films were then annealed at 150 °C for 12 h in a reduced pressure of 1.0 × 10−8 Torr and allowed to cool slowly to room temperature before being removed from vacuum. Instrumentation. 1H (vs TMS) and 19F (vs CF3COOH) spectra were recorded on a Varian Gemini VRX 300 spectrometer. Size exclusion chromatography (SEC) was carried out with a Jasco PU1580 liquid chromatograph equipped with two PL gel 5 μm Mixed-D columns, a Jasco 830-RI refractive index detector, and a Perkin-Elmer LC75 UV detector. Polystyrene standards (0.4−400 kg/mol) were used for calibration. XPS measurements were performed using a Kratos Axis Ultra spectrometer (Kratos Analytical, Manchester, UK) with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 225 W under a vacuum of 1.0 × 10−8 Torr. Charge compensation was carried out by injection of low-energy electrons into the magnetic lens of the electron spectrometer. The pass energy of the analyzer was set at 40 eV for high-resolution spectra with an energy resolution of 0.05 eV. The spectra were analyzed using CasaXPS v.2.3.14 software. The C−C peak at 285 eV was used as the reference for binding energy calibration. NEXAFS experiments were carried out on the U7A NIST/Dow materials characterization end-station at the National Synchrotron Light Source at Brookhaven National Laboratory (BNL). The general underlying principles of NEXAFS and a description of the beamline at BNL have been previously reported.9,19 The X-ray beam was elliptically polarized (polarization factor = 0.85), with the electric field vector dominantly in the plane of the storage ring. The photon

sufficient in length to complete a helix, making the perfluorocarbon side group rigid.17 Coupled with hydrophobic interactions of the low surface energy perfluorocarbons, this rigidity induces a smectic mesophase at the surface of polymer films,12,23,24 easily detectable using near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). XPS is an accurate and facile method to monitor the surface activity of such −(CF2)7CF3 groups as carbons bonded to fluorine are distinct from carbons bonded to less electronegative elements.3,21 The surface activity of −(CF2)7CF3 in a PPFOCS film was investigated by angle-resolved XPS (Figure 1) at two electron emission angles relative to the surface normal (ϕ) of 0° and 75° that correspond to effective probing depths of 3.1 and 0.8 nm, respectively.25 The spectra show four distinct peaks due to carbon 1s electrons bonded in the following environments: C−C/CC (285 eV), O−CO (289 eV), 4296

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Figure 1. Carbon 1s high-resolution XPS of PPFOCS recorded at ϕ = 0° (solid) and 75° (dashed). The observed peaks in order of increasing binding energy are C−C/CC (285 eV), O−CO (289 eV), CF2 (292 eV), and CF3 (294 eV). At ϕ = 75°, where the measurement is more surface sensitive, the area of CF2 and CF3 peaks is higher due to the perfluorocarbon chain wetting the polymer surface.

CF2 (292 eV), and CF3 (294 eV). By comparing scans from both emission angles, it is apparent that the relative areas of the CF2 and CF3 peaks are slightly higher for ϕ = 75°. This indicates that the −(CF2)7CF3 groups are surface active with an increased concentration of CF2 and CF3 carbons at the surface, an observation consistent with XPS analyses of polymeric systems containing similar perfluorocarbon chains. NEXAFS Spectroscopy of PPFOCS. Although XPS gives definite information on chemical surface composition, it is unable to provide information about bond orientation as the Xrays can neither be tuned in energy or polarization. NEXAFS spectroscopy, however, capitalizes on polarized, tunable, soft Xrays from a synchrotron X-ray source to monitor not only bond composition at the surface of a film but also bond orientation. NEXAFS spectroscopy is an ideal technique for answering questions that XPS is unable to, and in conjunction with XPS, can offer valuable insight into surface structure and composition over depths from 0 to 3 nm.1,26 The NIST/Dow U7A end-station at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratories (BNL) is organized so that concurrent variation of the angle θ between the incoming X-ray beam and sample surface and the angle ϕ between the Auger photoelectrons ejected and sample normal can be performed. By varying θ, valuable information can be obtained about bond orientation, while by varying ϕ, the effective electron escape depth (EED) is varied, changing the depth resolution of the technique. The Supporting Information illustrates the geometric setup of the U7A end-station, how θ and ϕ are related, and how the effective EED is determined. All partial electron yield (PEY) data in this study are first divided by I0, the intensity of the incoming X-ray beam, and then undergo a pre-edge subtraction to zero by averaging the intensity between 270 and 280 eV. Figure 2A depicts the spectra for PPFOCS after setting the postedge to unity at 320 eV; this assumes the same number of carbon atoms are probed for all θ, allowing for angular comparison of spectra. The spectra show peaks from the aromatic portion of PPFOCS at 285.5 eV due to the C 1s → π*CC transition. Between 288 and 290 eV the carbons from both the ester group and (CH2)2 group can be observed due to C 1s → π*CO, σ*C−O, and σ*C−H transitions, while the perfluorocarbon chain

Figure 2. NEXAFS spectra of PPFOCS at the carbon K-edge for various values of θ (A). Spectra underwent a pre-edge subtraction and postedge normalization at 320 eV to unity. Visible peaks corresponding to C 1s → antibonding orbital transitions, in order of increasing energy, are C 1s → π*CC (285.5 eV); σ*C−H, σ*C−O, and π*CO (288−290 eV); σ*C−F (293 eV); and σ*C−C (296−300 eV). The spectra were fit using Gaussian-type peaks for the near-edge resonances and an arctan function for the carbon ionization edge (B).

can be monitored with peaks from the C 1s → σ*C−F and σ*C−C transitions at 293 and 296−300 eV, respectively. Intensities of the listed peaks change when varying θ indicating orientational order of the bonds at the surface (Figure 2A). For each θ the spectra were fit using Gaussian-type peaks for the near-edge resonances and an arctan function for the carbon ionization edge.2 Figure 2B depicts the fitting for θ = 90° highlighting the peak areas determined for the C 1s → π*CC and σ*C−F transitions. Peak areas for the remaining transitions were not calculated due to smaller intensity variation with θ and overlap of peaks. Changes in peak area as a function of θ, when 20° ≤ θ ≤ 90°, are indicative of orientational ordering of the transition dipole moment (TDM) of each π* or σ* orbital at the film surface. Assuming there is rotational isotropy of bond orientation in the film plane, the peak area should vary as I(θ ) = A + B cos2(θ )

(1)

where A and B are the isotropic and anisotropic amplitudes of the peak, respectively, and can be determined by fitting the data.1,2,27 These values can be further used to determine the orientational order parameter of the TDM, S, defined by B S= (2) 3AP + B where P is the polarization of the X-rays (ca. 0.85), and when S = 1 the TDM of the bonds are completely oriented perpendicular to the film while for S = −1/2 the TDM of 4297

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the bonds are oriented in the plane of the film.2,20,26 Figure 3 depicts plots of areas under the two peaks highlighted in Figure

Figure 3B is a perfect example of this issue. Since the perfluorocarbon chain is populating the surface, its intensity will increase when ϕ is increased because a higher concentration of C−F bonds are probed at lower effective EED. This is what gives rise to discrepancies in calculated areas where cos2(θ) is the same but ϕ is different. NEXAFS Spectroscopy Postedge Analysis. Recent advances in NEXAFS spectroscopy analysis have enabled a more quantitative determination of orientation by accounting for changes in carbon atom number density as a function of depth. Sohn et al. developed a methodology to account for these variations in carbon atom density.1 In this method the PEY intensity, I, at an energy in the postedge ionization region, where the cross section σcont is the same for all carbon atoms, can be determined from the equation

Figure 3. Area under C 1s → π*CC (A) and C 1s → σ*C−F (B) peaks as a function of cos2(θ). The slope and intercept of a fitted line for data between 20° ≤ θ ≤ 90°(circles) are used to calculate an order parameter of the two bonds. SCC = −0.28 and SC−F = −0.18. Areas calculated for θ > 90° are indicated by crosses.

I=

2 as a function of cos2(θ). A linear fit for data between 20° ≤ θ ≤ 90° (circles) was used to determine A and B that were subsequently used in eq 2 to determine S for each bond. This calculation resulted in orientational order parameters of SCC = −0.28 and SC−F = −0.18, indicating an orientation of the TDMs of both orbitals, with that of the phenyl ring being the most oriented. The negative values denote that on average the angle between the TDM orientation of these orbitals and the surface normal, α, is greater than 54°, where α is defined by ⟨cos2(α)⟩ =

2S + 1 3

∞ Ω I0A 0 dz nv(z) σcont 0 4π sin(θ ) ⎡ ⎤ z exp⎢ − ⎥ λ cos(ϕ) cos(ω) ⎦ ⎣

∫

(4)

where I0 is the intensity of the incoming X-ray beam, Ω is the solid angle subtended by the electron detector, A0 is the area irradiated by the incident X-rays, λ is the escape depth of Auger electrons, z is the distance below the sample surface, and nv is the number density of carbon atoms.1 For a sample that has a constant carbon atom number density as a function of depth, a plot of (I/I0)[ sin(θ)/cos(ω)] versus cos(ϕ) will yield a straight line through the origin with a slope of (Ω/4π) A0σcontnvλ. Deviations from the straight line can be fit approximately using the following two-layer model

(3)

This means that both phenyl ring and −(CF2)7CF3 chain tend to orient normal to the surface since the π*CC TDM is perpendicular to the phenyl ring and the σ*C−F TDM is parallel to the C−F bond (normal to the −(CF2)7CF3 helix). Data for θ > 90° (crosses) were not considered when fitting A and B because the geometric organization of the end-station is such that ϕ is significantly higher at these angles, decreasing the effective EED. As a result, if certain bond concentrations vary as a function of depth at the surface, their PEY intensities will change at larger ϕ. This is particularly apparent in the C 1s → σ*C−F transition, where the intensities at θ > 90° do not overlay with those at θ ≤ 90° because the −(CF2)7CF3 chains are surface active. At decreased probing depths (θ > 90°) the C−F bonds are more concentrated, yielding a more intense C 1s → σ*C−F peak after postedge normalization. The opposite effect is visible for the CC bonds as they are buried beneath the perfluorocarbon chains. The above methodology used to calculate S has been reported on similar systems where an ether group linked the phenyl ring and perfluorocarbon chain.20 The Supporting Information compares the values of S determined for PPFOCS in this work to those determined for the ether analogue of PPFOCS, poly(4-(1H,1H,2H,2H-perfluorodecyl)oxymethylstyrene). Li et al. investigated the ordering of these ether polymers and the effect of varying perfluorocarbon chain length; however, there is a limitation using this methodology. Namely, this analysis assumes that the densities of carbon atoms are consistent with depth and do not vary when changing the effective EED. If differences in the density of a certain bond exist as a function of depth, it will adversely affect the values calculated for S. The plot of the C 1s → σ*C−F in

I=

⎡⎛ nv ,1 ⎞ Ω I0A 0 ⎟⎟ λ cos(ϕ) cos(ω)σcontnv ,2⎢⎜⎜1 − ⎢⎣⎝ 4π sin(θ ) nv ,2 ⎠ exp( −t /λ cos(ϕ) cos(ω)) +

nv ,1 ⎤ ⎥ nv ,2 ⎥⎦

(5)

where the ratio of carbon atom number densities, nv,1/nv,2 = y, between the top layer of carbon atom number density nv,1 and thickness t, and the bottom (thick) layer of carbon atom number density nv,2 is accounted for. The value for λ used for all calculations was 1.95 nm, as determined previously.1 As a result, by using eq 5, it is possible to determine the thickness of any low carbon density layer that is present at the surface of a film. This analysis is particularly useful for PPFOCS as a low carbon atom density layer would arise from a surface populated with perfluorocarbons. The ability to acquire this additional information highlights an advantage of using the postedge analysis over the earlier method used to determine SCC and SC−F. In order to apply this methodology to the measured data, the processing of spectra does not need any further normalization after dividing by I0 and subtracting a pre-edge between 270 and 280 eV (Figure 4). Spectral integrated intensities were recored at three energies, labeled in Figure 4, for all θ. The spectral integrated intensity at 319 eV was recorded to monitor the postedge intensity, and eq 5 was used to calculate the thickness of a low carbon atom density layer on the surface, corresponding to perfluorocarbons on top of a polystyrenelike underlayer. 4298

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(Figure 5B). The first extreme can be regarded as a layer of amorphous polytetrafluoroethylene (PTFE) on top of polystyrene, and hence dividing the carbon densities of those yields y = 0.51.28 The second scenario yields a value lower than 0.51 and along with the thickness t of the surface layer was varied to obtain a best fit result with a value of y = 0.30 and t = 1.0 nm. The second scenario could be due to either a more dispersed surface composition of −(CF2)7CF3 chains on the surface, or a concentration of low density carbonaceous contamination, or a combination of both. Fitting the plots in Figure 5 determines the thickness of the surface overlayer to be between 1.0 and 1.4 nm depending on y. This value is consistent with the length of −(CF2)7CF3 which is calculated to be ∼1.0 nm, whereas by accounting for the −COO(CH2)2(CF2)7CF3 this distance is increased to ∼1.5 nm. Besides determining the thickness of the perfluorocarbon layer, this methodology can be used to assess intensities that correspond to particular electron transitions. In this way the order parameter S can directly be determined by recording the intensity at a specific energy, as a function of cos(ϕ). Figure 4 highlights two additional points along the energy axis at 285.5 and 293 eV where the spectral intensities for these energies were recorded at all angles to determine the order parameters of CC and C−F bonds. By plotting the normalized intensity as a function of cos(ϕ) (Figure 6), it is possible to determine overlayer thickness relative to a specific bond. The plots in Figure 6 show data with the calculated thicknesses required to make a fit; however, the data show inconsistent values at cos(ϕ) > 0.75. This is because θ is changing within this range, and hence, ordering will have an effect on the overall intensity,

Figure 4. NEXAFS of PPFOCS at carbon K-edge for varying θ. Spectra were divided by I0, and a pre-edge subtraction was performed with no further data normalization. Three energies are labeled in the figure: 285, 293, and 319 eV where the intensities are recorded as a function of θ. I285/I0 and I293/I0 are used to determine order parameters of C 1s → π*CC and σ*C−F transitions, respectively. I319/ I0 is used to calculate the thickness of a low carbon atom density layer on the surface corresponding to the −(CF2)7CF3 chain.

The PEY intensity at 319 eV is plotted as a function of cos(ϕ) and fit with two values of y by envisioning two scenarios where either a perfect perfluorocarbon layer is obtained (Figure 5A) or a more dispersed layer of perfluorocarbon groups

Figure 5. Postedge analysis data for PPFOCS showing normalized I319 as a function of cos(ϕ). Fitting was performed using two values for y, the ratio of the density of carbon atoms in the surface layer to that of a thick layer below. In (A) the system was modeled as an amorphous polytetrafluoroethylene layer atop polystyrene, and a thickness of 1.4 nm was determined. When y was varied to obtain a best fit, the carbon atom density ratio y was 0.3 and the layer thickness obtained decreased to 1.0 nm (B).

Figure 6. C 1s → π*CC (A) and C 1s → σ*C−F (B) peak analysis as a function of cos(ϕ). A value of t cannot be fit to the data because the bonds display orientational order causing differences in data at 0.75 < cos(ϕ) < 1.0. 4299

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−(CF2)7CF3 overlayer, the calculated thickness will be less. The analysis for the C−F bonds shows they are buried beneath an overlayer of thickness t = 0.2 nm. If they were at the utmost top of the polymer surface, they would overlay with the “clean” perfluorocarbon trace with an S = −0.13. The 0.2 nm overlayer is most likely due to small amounts of low carbon atom number density contamination that adsorbed to the surface upon atmospheric exposure, before NEXAFS measurements were conducted. With the t values determined for each bond the orientational order parameter can alternatively be calculated by plotting the normalized PEY signal (left side of eq 6) as a function of cos2(θ) (Figure 8). In contrast to Figure 3 where PEY

something that the fit does not account for. As a result, the fitted values for thickness are not accurate and should be recalculated by accounting for S values. To determine S, Sohn et al. derived the following relation for the intensity of a spectral peak, sin(θ ) I exp[t /λ cos(ω) cos(ϕ)] I0 cos(ω) cos(ϕ) ⎛1 ⎞ ΩA 0 S = λnσ ⎜ − + SP cos2(θ )⎟ ⎝ ⎠ 4π 3 3

(6)

where n and σ are the carbon atom density and cross section now for the transition to the orbital in question, S is the orientational order parameter for the orbital in question, and P is the polarization of the X-rays (ca. 0.85). The rightmost term in parentheses is a factor derived to determine the effect of S and P on total intensity and is substituted into eq 4 to give the intensity at a depth t for a given transition. By plotting the left side of eq 6 as determined for the C 1s → π*CC and σ*C−F transitions versus cos2(θ), the data are only accounting for carbon atoms that have either CC or C−F bonds. But to calculate an accurate value for t, the plots in Figure 6 must be reevaluated using eq 6 to account for S. The revised plots are depicted in Figure 7 showing the best fit t and S values.

Figure 8. Normalized C 1s → π*CC and C 1s → σ*C−F intensities as a function of cos2(θ). The orientational order parameters determined are SCC = −0.27 and SC−F = −0.13.

intensities differ for θ ≤ 90° and θ > 90°, by using the overlayer thickness determined for each transition the data now give straight line plots where PEY intensities acquired at the same θ, but different ϕ, are now equal. Linear fits to these data yield values for SCC and SC−F that are equal to those determined from the best fits in Figure 7. Both S values in Figure 8 are negative, indicating that α is greater than 54° for the TDM orientation of CC and C−F bonds, a result similar to what was determined using eq 2 where nv is assumed constant with z. There are however differences when comparing S values obtained using eq 6, where changes in nv with z are accounted for, to S values acquired using eq 2, where nv is assumed constant with z. These differences highlight the importance of considering carbon atom number density depth profiles when calculating the orientational order parameter of bonds. The value calculated for SCC remains relatively consistent, however, the value for SC−F decreases by ∼25% when accounting for changes in nv with z. There are a number of reasons why this could be the case. For the C−F bonds, peak area calculations for the C 1s σ*C−F illustrated in Figure 2B are complicated as they exist in the region of the decaying carbon ionization edge. This could lead to increased error in the initial SC−F values calculated while the C 1s → π*CC occurs before the carbon edge, or any other transitions, and can be easily quantified. Also, the model assumes uniform layers of coverage, when in fact, these layers are not discrete and there is intermixing of the bonds as a function of depth. For example, −(CF2)7CF3 does not only exist at the near surface, but there are underlying repeat units beneath the wetting layer that are

Figure 7. C 1s → π*CC (A) and C 1s → σ*C−F (B) peak analysis as a function of cos(ϕ) after accounting for calculated order parameters for SCC and SC−F. Considering the order parameters allows for improved fitting of the intensity plots to give a depth of the CC bonds at 0.6 nm and the C−F bonds at 0.2 nm.

For the C 1s → π*CC transition, the fitting yields an overlayer thickness of 0.6 nm. This value is lower than the thickness calculated from the postedge data, probably because the overlayer may contain a concentration of phenyl rings. This method is measuring what is above the aromatic carbon rings, and if there is a concentration of them mixing into the 4300

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additionally contributing to some of the C 1s → σ*C−F signal, and these helices may not be as ordered as those on the surface. The calculations performed in Figure 7 better accounts for such concentration changes as a function of depth and is the reason for a more accurate determination of S. The preceding NEXAFS analysis of PPFOCS validates the postedge methodology for a polymer system that displays both a carbon bond number density depth profile and orientational order of carbon bonds. With the values determined for SCC and SC−F above it is worth noting how they compare to those determined for poly(4-(1H,1H,2H,2H-perfluorodecyl)oxymethylstyrene), a polymer with an identical −(CF2)7CF3 side chain but with an ether linkage to the styrene phenyl ring. This system displayed negligible ordering of either the phenyl ring or the perfluorocarbon chain with SCC = −0.039 and SC−F = 0, even though the samples were prepared under similar conditions.1 The increased orientational order observed in PPFOCS is likely due to the ester linkage and its partial conjugation with the phenyl ring. This results in an increased stiffness between the phenyl ring and −(CF2)7CF3 chain as compared to the ether bonded linkage. Such comparisons exemplify the importance of decoupling depth profile and orientational order from NEXAFS data and highlight how NEXAFS postedge analysis can be used effectively to compare the surfaces of different polymer systems.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.G.); [email protected] (E.J.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was primarily supported by the Office of Naval Research (ONR) through Award No. N00014*02-1-0170. M.D.D. and E.J.K. acknowledge partial support from the NSF Polymers Program (DMR-0704539) as well as the use of facilities funded by the NSF-MRSEC program (UCSB MRL, DMR-1121053). E.M. and G.G. thank the Italian MiUR (fondi PRIN) for partial support of the work.

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CONCLUSION The near surface molecular composition and orientation of a perfluoro-functionalized polystyrene homopolymer was investigated using NEXAFS spectroscopy. A methodology developed by Sohn et al., previously tested on an unordered system, was applied to the PPFOCS system which displayed increased orientational order at the surface. The NEXAFS analysis conducted on this system shows that the previously reported methodology can quantify both carbon bond orientation and a carbon bond number density depth profile at the polymer film surface. Postedge analysis data are used to calculate the thickness of a low carbon density overlayer between 1.0 and 1.4 nm, consistent with the length of a −(CF2)7CF3 chain. Analysis of peaks from C 1s → π*CC and σ*C−F transitions allows for the determination of bond depth and orientational order parameter. SC−F and SCC were calculated to be −0.13 and −0.27, respectively, indicating a tendency for both the phenyl rings and −(CF2)7CF3 chains to orient perpendicular to the polymer surface. The values for SC−F and SCC are considerably higher than previously reported values for poly(4(1H,1H,2H,2H-perfluorodecyl)oxymethylstyrene), an ether analogue to PPFOCS. The higher degree of orientational order is attributed to the partial conjugation between the ester group and the phenyl ring providing a relatively stiff linkage between the perfluorocarbon helix and the phenyl ring. The surface analysis of PPFOCS documented in this work offers valuable insight into the near surface character of perfluorofunctional polymers and supports a robust method for obtaining surface bond orientation and depth profile using NEXAFS spectroscopy.

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Article

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S Supporting Information *

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