Multipurpose Polymeric Coating for Functionalizing Inert Polymer

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Multipurpose Polymeric Coating for Functionalizing Inert Polymer Surfaces A. Evren Ozcam, Kirill Efimenko, Richard J. Spontak, Daniel A Fischer, and Jan Genzer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12216 • Publication Date (Web): 27 Jan 2016 Downloaded from http://pubs.acs.org on January 30, 2016

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ACS Applied Materials & Interfaces

Multipurpose Polymeric Coating for Functionalizing Inert Polymer Surfaces A. Evren Özçam,1,† Kirill Efimenko,1 Richard J. Spontak,1,2 Daniel A. Fischer3 and Jan Genzer1,* 1

Department of Chemical & Biomolecular Engineering North Carolina State University Raleigh, North Carolina 27695-7905 2

Department of Materials Science & Engineering North Carolina State University Raleigh, North Carolina 27695-7907 3

Ceramics Division National Institute of Standards and Technology Gaithersburg, Maryland 20899

Abstract In this work, we report on the development of a highly functionalizable polymer coating prepared by the chemical coupling of trichlorosilane (TCS) to the vinyl groups of poly(vinylmethyl siloxane) (PVMS). The resultant PVMS-TCS copolymer can be coated as a functional organic primer layer on a variety of polymeric substrates, ranging from hydrophilic to hydrophobic. Several case studies demonstrating the remarkable and versatile properties of PVMS-TCS coatings are presented here. In particular, PVMS-TCS is found to serve as a convenient precursor for the deposition of organosilanes and the subsequent growth of polymer brushes, even on hydrophobic surfaces, such as poly(ethylene terephthalate) and polypropylene. In this study, the physical and chemical characteristics of these versatile PVMS-TCS coatings are interrogated by an arsenal of experimental probes, including scanning electron microscopy, water contact-angle measurements, ellipsometry, Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, and near-edge x-ray absorption fine structure spectroscopy.

Keywords: silicones, ultra-violet/ozone treatment, surface modification, trichlorosilane, PVMS, NEXAFS, semifluorinated, PET

† *

Present address: 3M Purification Inc., 3M Center, St. Paul, MN 55144. To whom correspondence should be addressed ([email protected], +1-919-515-2069).

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Introduction Controlled modification of polymer surfaces is crucial in endowing soft materials with properties needed for a diverse range of contemporary applications ranging from, for example, biomedical devices to filtration membranes.1-14 Most homopolymers are, however, inherently hydrophobic,15-18 which makes them cumbersome in technologies requiring a hydrophilic surface. Over the past few decades, various methods have been developed to facilitate the modification of polymer surfaces and make them more amenable for such intentions. These methods are, in most instances, based on physical surface treatments and include plasma, corona or flame discharge, all of which endow polymer surfaces with a large concentration of hydrophilic chemical groups.19-22 McCarthy and co-workers have recently provided a comprehensive summary of plasma oxidation of various PDMS elastomers (commercial versus model) and discussed processing conditions capable of promoting longer-lasting hydrophilicity.23 While widely practiced as viable routes to hydrophilic polymer surfaces, these methods possess drawbacks. Because of the uncontrolled nature of the generated hydrophilic functionalities (in terms of their identity, concentration and stability), physical treatments intrinsically limit the general applicability of surface-modified polymers. Moreover, physical changes alone are often accompanied by nontrivial degradation of organic polymers, frequently resulting in compromised material integrity in terms of reduced mechanical strength, toughness and other physical properties.24,25 This is particularly detrimental in some situations, such as those involving polymer fibers, wherein the surface-to-volume ratio can be large (especially with regard to nano/microfibers).26 In these cases, surface coatings alternatively generated by relatively mild chemical reactions27 can be more beneficial than those produced exclusively by physical means. The development of a hybrid, general approach by which to tailor the physicochemical attributes of polymer surfaces through the use of a functional coating strategy is therefore highly desirable as it could greatly benefit polymer surface modification efforts while ensuring bulk property retention.28 In this spirit, we employ a functional polymer based on polysiloxane chemistry to generate versatile elastomeric coatings that can be applied to most (if not all) organic and inorganic substrates because of its highly reactive hydroxyl groups. With hydrophobic surfaces (i.e., polyolefins), limited hydrophilization of the substrate may likewise be required to generate a small population of hydrophilic groups that can serve as attachment points for the polysiloxane –2– ACS Paragon Plus Environment

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coating. Unobtrusive, surface-sensitive physical methods, such as ultraviolet/ozone (UVO) treatment, are perfectly suited for this purpose as they are known to convert only the topmost region of a polymer surface into functional groups.29,30 Because the polysiloxane coating is thin compared to typical substrates, the bulk properties of the underlying polymer remain unchanged. The hydroxyl groups in the polysiloxane coating can be utilized to not only anchor the coating to the substrate but also attach additional functional groups that would ultimately generate an entirely new functional surface.31 Here, we demonstrate the benefit of this capability by introducing initiator molecules that are subsequently used to grow functional polymers via surface-initiated polymerization. Alternatively, the polysiloxane coating can itself be modified by UVO treatment without compromising dramatically its integrity and topography as it is converted into a silica-like layer32 that can serve as a basis for tethering small-molecule functional modifiers, such as organosilanes. We perform this modification for illustrative purposes to establish that fluorinated organosilanes, often used to promote scratch resistance and hydrophobicity, can be chemically grafted to a UVO-treated polysiloxane coating. These organosilanes exhibit densities and molecular packing that are reminiscent of those observed on silica-based surfaces. Lastly, on the basis of results reported elsewhere,28 we point out that the mechanical properties of the elastomeric polysiloxane coating, such as the modulus, can be tailored by independently varying the chemical composition and/or coating thickness.

Experimental33 Materials Poly(vinylmethylsiloxane) was synthesized using the step-growth polymerization of short hydroxyl-terminated oligomeric vinyl methyl siloxane chains, as reported previously.34 Briefly, the precursor monomer was obtained by slow hydrolysis of methylvinyldichlorosilane in the presence of dilute aqueous HCl. The reaction products consisted of vinylmethyl siloxane cycles (VD3, VD4 and VD5) and linear hydroxyl-terminated chains.

The cyclic products were

separated by vacuum distillation to obtain linear chains (yield of ≈25-35%, depending on the quantity of HCl and water present in the reaction mixture). All polymers used here were prepared from hydroxy-terminated linear chains.

Solvent-free siloxane polymerization was

initiated by a minute amount of lithium hydroxide (10 - 20 ppm) at 100°C for various reaction



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times under constant nitrogen flow; this facilitated the removal of water molecules formed during the reaction. The reaction was terminated by the addition of carbon dioxide, which resulted in the formation of α,ω-hydroxy terminated PVMS chains. The final polymer was filtered under vacuum using the Celite 545 filtering aid system. Unreacted short oligomeric chains were removed by precipitation in methanol. The resulting PVMS was dissolved in diethyl ether and then added drop-wise to chilled methanol, after which the polymer was collected and dried in vacuum for 72 h. The same procedure was repeated two times. Complete removal of low-molecular-weight compounds was verified with size-exclusion chromatography equipped with light scattering and refractive index detectors; the monomer conversion was ~92%. Infrared spectroscopy confirmed that the amount of vinyl functional groups remained unchanged, which suggested that no backbone branching occurred during the polymerization. The experiments described in this study were conducted using only hydroxy-terminated PVMS with a molecular weight of 35 kDa Trichlorosilane (TCS), reagent-grade anhydrous toluene, anhydrous tetrahydrofuran (THF), chloroform, acetone, methanol, and ethanol were all purchased from Sigma-Aldrich (St. Louis, MO) and used without further purification. α-ω-vinyl-terminated PDMS (Mn=62 kDa), tetrakis(dimethylsiloxy)silane

(TDSS)

and

Pt(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane

complex were obtained from Gelest Inc. (Morrisville, PA). 1H,1H,2H,2H-Perfluorodecyl trichlorosilane (tF8H2) was supplied by Alfa-Aesar (Ward Hill, MA) and used as-received, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], a free-radical polymerization (FRP) initiator, was purchased from Wako Specialty Chemicals (Richmond, VA). Methyl methacrylate (MMA) monomer was acquired from Sigma Aldrich (Allentown, PA) and used without further purification. Several commercial polymers used in this study were generously donated: polypropylene (PP, Sunoco, CP360H), polyethylene (PE, Dow, 6850), poly(ethylene terephthalate) (PET, DuPont-Teijin, Mylar-DL), and polyimide (PIm, DuPont, Kapton®). Since these polymers were only used as substrates, their molecular weight details do not influence our PVMS-based surface coatings and are deemed unimportant in the present study.


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Reactive modification The hydrosilylation reaction used to couple TCS to PVMS, depicted in Scheme 1, was performed in the presence of the Pt(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in anhydrous toluene (or THF). The PVMS liquid was dissolved in anhydrous toluene (or THF) at a predetermined concentration in a glass vial, after which a corresponding amount of TCS and the Pt(0) complex were added and the vial was capped under nitrogen. [Caution: TCS reacts violently with water and liberates HCl. Refer to the MSDS sheet of TCS for safety guidelines.] Reaction mixtures were stirred with a magnetic stir bar for 1-2 h at ambient temperature and subsequently spin-coated onto various substrates, including PET, PE, PP, PIm, glass, and silicon wafer, by using a Headway Research PWM-32 spin-coater operated at a speed of 2000 rpm and an acceleration of 1000 rpm/s for 60 s. In the presence of moisture, the silane groups on TCS convert to silanol groups (cf. Scheme 1). The UVO treatment of all substrates was performed in a UVO chamber, Model 42 manufactured by Jelight Company Inc. The standard fused quartz lamp used here emits ≈65% of its total radiation at 184.9 nm and has an output of 6.2 mW/cm2 at a distance 6 mm away from the source, as measured by a UV light detector. Substrates were placed onto glass slides, inserted into the UVO chamber at a distance of ≈5 mm from the lamp, and exposed to radiation on one side for predetermined periods of time. In the remainder of this report, we denote samples exposed to the UVO treatment with the appendix [UVO]. The UVO treatment employed in this study did not increase the roughness of the coatings, since the rootmean-square roughness of all sample surfaces, as measured by atomic force microscopy, was consistently 1-2 nm.

Scheme 1. Schematic illustration depicting the formation of PVMS-TCS by coupling TCS to PVMS macromolecules.



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Semifluorinated self-assembled monolayers (SAMs) were prepared on UVO-treated substrates from tF8H2, which was first mixed with a fluorinated oil (perfluoromethyl decalin) in a 1:5 w/w ratio. A small drop of this mixture was placed on the bottom of a Petri dish. The substrates, including UVO-treated substrates coated with PVMS-TCS, untreated PET, UVOtreated PET, and silica, were taped to the lid of the Petri dish, and the lids were placed back on the Petri dishes so that the samples hung face-down at a distance of ≈1 cm from the tF8H2/oil mixture. After exposure for 15 min at ambient conditions, the lid was removed and the samples were washed thoroughly with ethanol to remove physisorbed tF8H2 molecules and subsequently blown-dry with pressurized nitrogen gas. The azo-based FRP initiator was first dissolved in dry THF and then mixed with a THF solution containing dissolved PVMS-TCS to enable coupling with the chlorosilane groups of PVMS-TCS. This reaction mixture was stirred at ambient temperature for 1-2 h and applied directly onto PET, PP and silica substrates. The resultant functionalized substrates were immersed in MMA monomer, de-oxygenated and heated to 90ºC for 2 h to promote generation of poly(methyl methacrylate) (PMMA) chains. At the end of the polymerization, the substrates were vigorously rinsed with THF, sonicated, rinsed again with THF to remove physisorbed PMMA chains, and then dried with nitrogen gas.

Surface characterization Water contact-angle (WCA) tests were performed via the sessile drop technique with deionized water (resistivity >15 MΩ-cm) using a Ramé-Hart Model 100-00 contact-angle goniometer equipped with a CCD camera. Data were analyzed with the Ramé-Hart Imaging 2001 software. The WCAs were determined after placing an 8 µL droplet of deionized water on each surface of interest. At least 4 different measurements were performed across each sample surface, and average WCA values are reported along with their corresponding standard errors. The thicknesses of films deposited on non-transparent substrates were measured by variableangle spectroscopic ellipsometry (VASE) on a J.A. Woollam Co. instrument. All ellipsometric data were collected at an incidence angle of 70° with respect to the surface normal and at wavelengths ranging from 400 to 1100 nm in 10 nm increments. Scanning electron microscopy (SEM) images showing the surface morphology of UVO-modified samples coated with 8 nm of gold were collected with a Hitachi S-3200 microscope operated at an accelerating voltage of


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5 kV. Fourier-transform infrared spectroscopy (FTIR) spectra were recorded on a Nicolet 6700 spectrometer, and the data were analyzed by means of the OMNIC software. Transmission FTIR was used to monitor the extent of coupling reaction, whereas FTIR performed in attenuated total reflection (ATR) mode with a Ge crystal was employed to follow chemical changes that occurred on coatings after surface modification. In the first case, a drop of PVMS-TCS solution extracted at the end of the coupling reaction was spread on a KBr crystal, and spectra were collected after complete solvent evaporation. For each sample, 256 scans were collected and averaged (after correcting for the background) at a resolution of 4 cm-1. The surface chemical composition of freshly prepared samples was determined by X-ray photoelectron spectroscopy (XPS) performed on a Kratos Axis Ultra DLD spectrometer with monochromated Al Kα radiation and charge neutralization. Survey and high-resolution spectra were collected with pass energies of 80 and 20 eV, respectively, using both electrostatic and magnetic lenses. Elemental chemical compositions were calculated from spectral regression with the Vision software. Near-edge x-ray absorption fine structure (NEXAFS) spectroscopy conducted at the carbon, oxygen and fluorine K-edges was utilized to examine the composition and molecular orientation of the tF8H2 SAMs and the surface chemistry of the UVO-modified PET substrates coated with PVMS-TCS. The NEXAFS experiments were conducted at the NIST/Dow Soft X-ray Materials Characterization Facility of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). This technique involves the resonant soft x-ray excitation of a K-shell electron to an unoccupied low-lying antibonding molecular orbital of σ symmetry (σ*) or π symmetry (π*). The initial-state K-shell excitation endows NEXAFS with elemental specificity, whereas the final-state unoccupied molecular orbitals provide NEXAFS with bonding or chemical selectivity. Measurement of the partial electron yield (PEY) intensity of the NEXAFS spectral features thus allows identification of chemical bonds and determination of their relative population densities on the sample surface (the probing depth is ≈1-2 nm into the film). Because the incident x-ray is polarized, collecting NEXAFS spectra at various sample/x-ray beam geometries yields information about the molecular orientation of tF8H2 molecules adsorbed on the surface. For this purpose, NEXAFS spectra were collected at θ = 20º, 50º and 90º, where θ denotes the angle between the surface normal and the X-ray polarization vector.



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Figure 1. FTIR-ATR spectra of PET (black), PVMS (green), a 200 nm PVMS-TCS layer deposited on PET (blue), and a ≈1 µm PVMS-TCS layer deposited on PET (red).


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Results and Discussion The role of coating thickness is first determined by spin-coating the functional PVMS-TCS coating onto PET and silicon wafer (SiOx) substrates (hereafter designated as PET/PVMS-TCS and SiOx/PVMS-TCS, respectively), and then investigating the surface properties before and after UVO treatment, as well as after subsequent deposition of semifluorinated tF8H2 SAMs (see a note about semifluorinated molecules in the Supporting Information). According to ellipsometry measurements performed on SiOx, the coating thicknesses are 102.1 ± 1.7 nm, 204.8 ± 2.7 nm and ≈1 µm for coatings applied from 1.5, 3.3 and 10% w/w PVMS-TCS solutions, respectively, in toluene. Here, we assume that the coating thicknesses on PET are identical to those measured on SiOx. Figure 1 displays FTIR-ATR spectra of untreated PET, PVMS elastomer cross-linked through end-terminal silanol groups and PET/PVMS-TCS (at coating thicknesses of ≈1 µm and 200 nm). After TCS coupling to PVMS, the signal intensities of the peaks due to C=O stretch (≈1714 cm-1), C-H stretch (722 cm-1) and C-C-O stretch (1250 cm-1) are all observed to decrease with increasing PVMS-TCS thickness.35 These spectra thus confirm that the coupling of TCS to the vinyl group of PVMS is efficient and, as discerned from related results reported elsewhere,28 comes close to being quantitative. Further examination of these spectra reveals that the populations of Si-O-Si bonds and –OH groups increase relative to bare PVMS. Spontaneous formation of –OH groups from the silane groups on TCS in the presence of moisture is confirmed (in the spectra for TCS-containing specimens at ≈3300 cm-1), and these highly reactive groups can be used for further surface modification with organosilanes or acid chlorides. Since both PET/PVMS-TCS and SiOx/PVMS-TCS specimens generated by spin-coating PVMS-TCS solutions exhibit similar surface properties and to avoid the production of unnecessarily thick (≈1 µm) coatings, only results from the PVMS-TCS coating measuring 200 nm thick are discussed further below. As mentioned earlier, one effective route by which to hydrophilize the surface of an inherently hydrophobic material is through UVO treatment. Figure 2 displays the wettabilities of UVO-treated PET (hereafter designated PET[UVO]), PET/PVMS-TCS[UVO] and SiOx/PVMSTCS[UVO] expressed in terms of WCA as a function of UVO dosage. As discussed elsewhere,36 UVO exposure promotes chain scission and introduces hydrophilic moieties along the surface of PET. As a result, the WCA initially decreases (and the wettability of PET increases) with



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increasing UVO time until it reaches a plateau at just under 40º after ≈5 min of exposure. In marked contrast, the WCA of PET/PVMS-TCS[UVO] and SiOx/PVMS-TCS[UVO] become completely wettable (with WCA ≈ 0°) in only 6 min, indicating that the PET/PVMS-TCS[UVO] surface becomes more hydrophilic than the uncoated PET[UVO] surface. One explanation for this observed difference contends that UVO treatment of PET produces water-soluble lowmolecular-weight organic compounds, which are removed during specimen cleaning (water rinsing) to yield less hydrophilic PET surfaces.37 Conversely, the silicon atoms residing on the surface of the PVMS-TCS layer should exist predominantly in the form of silanols. During UVO treatment, these and other hydrophilic groups react with each other to form a thin silica-like layer.29,38 The wettabilities of PET/PVMS-TCS[UVO] and SiOx/PVMS-TCS[UVO] in Figure 2 are nearly identical for the same UV exposure time, strongly suggesting that the properties of both surfaces are dictated primarily by the PVMS-TCS coating before and after UVO treatment.

Figure 2. Advancing WCA values of (a) PET[UVO] and (b) PET/PVMS-TCS[UVO] (triangles) and SiOx/PVMS-TCS[UVO] (squares) surfaces as a function of UVO exposure time.

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Figure 3. Atomic concentrations of (a) oxygen, (b) carbon and (c) silicon measured from the surfaces of PET[UVO] (circles), PET/PVMS-TCS[UVO] (triangles) and PVMS[UVO] elastomer (squares) as a function of UVO exposure time.

To elucidate the chemical reasons for the WCA results provided in Figure 2, we have performed XPS measurements to ascertain the surface atomic concentrations of carbon, oxygen and silicon for PET[UVO], PET/PVMS-TCS[UVO] and PVMS[UVO] specimens as functions of UVO exposure time. The results displayed in Figure 3a, for example, confirm that the oxygen concentration on the surfaces of these three material systems initially increases and then levels



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off (to between 38 and 54%, depending on the specimen considered) with increasing UVO treatment. The opposite trend is evident for carbon in Figure 3b, and the concentration of surface silicon appears to be nearly independent of UVO exposure in Figure 3c. Close examination of all the data in Figure 3 further verifies that the PVMS[UVO] and PET/PVMSTCS[UVO] measurements are nearly identical with respect to all three elements, supporting our earlier contention that the PVMS elastomer regulates surface property development after surprisingly little UVO treatment, which effectively reduces the population of carbon-containing groups in favor of hydrophilic anchoring moieties (e.g., hydroxyl, carboxyl and ketone groups) on specimen surfaces. It immediately follows from these results that the UVO-induced decrease in WCA observed for all specimens in Figure 2 can most likely be attributed to a concurrent increase in surface oxygen content. The oxygen concentration on PET after UVO exposure, however, is consistently lower, while the carbon concentration remains significantly higher, than that of the PVMS[UVO] and PET/PVMS-TCS[UVO] specimens, which explains why the PET[UVO] specimen possesses the least hydrophilic surface. In the case of PET, UVO treatment induces chain scission and promotes the removal of small oxidized PET fragments from the PET surface, which prevents a systematic increase in PET wettability. Before UVO treatment, the PET/PVMS-TCS surface exhibits evidence of a higher surface oxygen concentration than the PVMS surface in Figure 3a due to the presence of Si-O linkages formed during the hydrolysis of the TCS moieties upon exposure to atmospheric moisture. After extended UVO exposure, the oxygen concentration in both specimens reaches nearly the same oxygen level. The high-resolution XPS spectra presented in Figure 4 provide detailed information regarding the chemical environments of oxygen, carbon and silicon for these specimens after different UVO exposure times. Whereas the silicon atom of unmodified poly(vinylmethylsiloxane) has 2 neighboring oxygen atoms with a binding energy (Si 2p) centered at 99.5 eV, each silicon atom in SiOx has, on average, 4 neighboring oxygen atoms and a binding energy of 103.6 eV (corresponding to tetrahedral coordination). In contrast, the Si 2p peak of the PVMS elastomer without UVO treatment is centered at 102.3 eV (calibrated with respect to the invariant C 1s peak at 285 eV). In Figure 4, this value is observed to increase slightly to 102.8 eV for the untreated PET/PVMS-TCS specimen due to the initiation of a threedimensional Si-O-Si network during the hydrolysis of TCS (which shifts the binding energy towards that of SiOx). As the PET/PVMS-TCS specimen is exposed to UVO, its Si 2p peak


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position undergoes a further shift as the organosilicon gradually converts to SiOx and ultimately reaches a limiting value at 103.6 eV, which corresponds to a net change of 1.3 eV in peak position. This observation is consistent with the previous studies of Ouyang et al.,38 who have reported Si 2p peak shifts of 1.4 and 1.2 eV for PDMS and PDMS-co-PVMS copolymers, respectively, after 120 min of UVO treatment. Included in Figure 4 are high-resolution XPS spectra acquired for the O 1s peak, which shifts to higher binding energy from PVMS elastomer to PET/PVMS-TCS (due to the presence of Si-O linkages that form upon exposure of TCS to moisture) and shifts further (and grows in intensity) upon UVO treatment as additional oxygencontaining species are formed.

Figure 4. High-resolution XPS spectra for surface oxygen (529~539 eV), carbon (279~289 eV), and silicon (99~109 eV) of bare PVMS elastomer (bottom) and UVO-treated PET/PVMS-TCS (i.e., PET/PVMS-TCS[UVO]) at several UVO treatment times (labeled). The dashed lines are centered at the peaks corresponding to the O 1s, C 1s, and Si 2p, transitions of untreated PET/PVMS-TCS[UVO] (at 0 min) as described in the text.



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Figure 5. SEM images displaying the surface topographies of (a) PET[UVO] and (b) PET/PVMS-TCS[UVO] at different UVO exposure times (labeled). The inset in (a) is a 2x enlargement.

As with all physical modification methods, exposure to a high-energy source promotes chemical changes in and potential degradation of organic polymers. Shown for comparison in Figure 5 are planar SEM images acquired from unmodified PET and PET/PVMS-TCS surfaces, as well as these surfaces after 2, 8 and 30 min of UVO treatment. Comparison of the images corresponding to the virgin specimens (at 0 min) indicates that they are, for the most part, featureless except for some discrete protrusions that are attributed to surface contamination. Close examination of the images in Figures 5a and 5b establishes that the surface topographies do not change noticeably in the first 2 min of UVO exposure. After an exposure time of 8 min, however, discrete spheroidal grains measuring 82 ± 4 nm in diameter become faintly visible (see the enlargement for a contrast-enhanced view) on the PET[UVO] film in Figure 5a, but not on the PET/PVMS-TCS[UVO] surface in Figure 5b. Further exposure reveals that the size of the grains, which appear only on the PET[UVO] surface, increases to 113 ± 7 nm in diameter. These features, which are clearly seen in the image collected after 30 min of UVO exposure in Figure – 14 – ACS Paragon Plus Environment

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5a, are attributed to the semi-crystalline nature of PET, wherein the crystalline and amorphous regions possess different etching/degradation rates. The surface topography of the PET/PVMSTCS[UVO] specimen, on the other hand, does not change discernibly during the course of the investigation, as shown in Figure 5b, because the etching/degradation rates for PVMS-TCS are relatively uniform throughout the coating. To explore the functionality of the PVMS-TCS coatings, we have performed two independent analyses: (1) chemical attachment of small-molecule organosilanes, and (2) surface polymerization of long macromolecules. In the first case, hydrophilic moieties introduced into PET and PET/PVMS-TCS during UVO treatment are employed to attach reactive hydrophobic organosilane precursors that react with surface-bound hydroxyl groups. For this purpose, we have used a semifluorinated organosilane with a TCS head group (tF8H2) because it is known to react rapidly, even at ambient temperature. As detailed in the Experimental section, tF8H2 SAMs are vapor-deposited on the surface of UVO-treated specimens, and the wettabilities of the resultant PET[UVO]/tF8H2 and PET/PVMS-TCS[UVO]/tF8H2 materials are examined by WCA in Figure 6. In this figure, WCA data collected from tF8H2 SAMs formed on the surface of silicon wafer (SiOx/tF8H2) are included for comparison. These WCA data reveal that tF8H2 attaches to SiOx and PET/PVMS-TCS substrates even without any UVO treatment. In marked contrast, tF8H2 does not attach to the PET substrate, as indicated by the low WCA. Whereas the WCA values measured from SiOx/tF8H2 specimens lie between 113 and 118º for all UVO times examined, which agrees with previously reported39 results and demonstrates that the normally hydrophilic surface has been transformed into a hydrophobic surface due to the presence of the SAM, the WCA values determined for the PET[UVO]/tF8H2 system are consistently and significantly lower, decreasing with increasing UVO exposure time after ≈2 min. This reduction is due to the removal (during ethanol rinsing) of low-molecular-weight organic compounds that reacted with tF8H2. Comparison of the WCA values of PET[UVO] (40° in Figure 2a) and PET[UVO]/tF8H2 (72° in Figure 6) at long UVO treatment times (≈14-15 min) confirms that a portion of the tF8H2 SAM remains even after rinsing. In sharp contrast, WCA values measured from the PET/PVMS-TCS[UVO]/tF8H2 and SiOx/tF8H2 specimens appear nearly identical. In addition, the PET/PVMS-TCS sample that was not UVO-treated but exposed to the tF8H2 SAM exhibits a WCA of 110.9 ± 1.2º, which is substantially higher than that without the SAM (cf. Figure 2b) and thus reveals direct attachment of tF8H2 to the PVMS-TCS coating. This



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attachment is a consequence of the –OH groups present on the PVMS-TCS layer spin-coated on PET, as evidenced by the FTIR-ATR spectra displayed in Figure 1.

Figure 6. WCA measurements of PET[UVO] (circles), PET/PVMS-TCS[UVO] (triangles) and SiOx on silicon wafers (squares) modified with a semifluorinated SAM as a function of UVO exposure time.

We have utilized NEXAFS spectroscopy to further examine the molecular-level details of our functionalized surfaces.40 While a comprehensive study reporting on the orientation of tF8H2 on surface was published earlier,41,42 our intention here is to compare the results of molecular orientation of tF8H2 on our current surfaces to those reported in previous studies. As a prelude to the following discussion, representative PEY NEXAFS spectra collected at the carbon K-edge from PET films and F8H2 monolayers deposited onto a flat silica substrate are provided for reference in Figure 7. Each spectrum displays a series of characteristic peaks, the locations and descriptions of which are listed in Table 1.


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Table 1. Assignment of transitions in the carbon K-edge NEXAFS spectra of PET and F8H2. Peak energy (eV)

Compound

284.5

PET

285.1

S

Description

Reference

0.486

1s→π*ring

43,44

PET

0.485

1s→π*ring

43,44

287.9

PET

0.300

1s→π*C=O

43,44

290.1

PET

0.282

1s→π*ring

43,44

291.1

PET

0.255

1s→π*C=O

43,44

292.7

PET

≈0

1s→σ*C-C

43,44

295.8

PET

-0.403

1s→σ*C-C

43,44

303.0

PET

-0.286

1s→σ*C-C

43,44

287.7

F8H2

-0.522

1s→σ*C-H

41,42

292.0

F8H2

-0.436

1s→σ*C-F

41,42

295.5

F8H2

0.282

1s→σ*C-C

41,42

298.5

F8H2

-0.235

1s→σ*C-F

41,42

300.3

F8H2

0.236

1s→σ*C-C

41,42

According to these assignments, the spectrum acquired from PET is dominated by strong peaks located at 284.5 and 287.9 eV, which correspond to the presence of phenyl rings and carbonyl groups, respectively. The major peaks observed in the spectrum of F8H2 confirm the occurrence of C-F and C-C bonds, located at 292.0 (as well as 298.0) and 295.5 eV, respectively. A series of PEY NEXAFS spectra has been collected at three different geometries defined by the angle between the incident beam and the sample (θ), as defined in the Experimental section. Variation in the intensity of each peak in the NEXAFS spectra with θ indicates that the molecular groups within the first ≈2 nm of the sample surface are oriented. Qualitative assessment of these spectral features implies that the phenyl rings and carbonyl groups of PET lie approximately along the surface plane. In the case of F8H2, however, the C-F bonds are oriented nearly parallel to the surface, whereas the C-C bonds are oriented relatively close to the surface normal. Quantitative analysis of the orientation of the individual bonds can be achieved by evaluating the uniaxial orientation parameter, S, which ranges from +1 (σ* or π* aligned perfectly along the surface normal) to −1/2 (σ* or π* lying along the surface plane).45 The values of S included in Table 1



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have been evaluated by following the model proposed by Stöhr and Samant,45 which is described in detail in the Supporting Information.

Figure 7. Normalized PEY NEXAFS spectra collected at the carbon K-edge at 3 incident angles (θ): 20° (red), 50° (blue) and 90° (green) from (a) PET films and (b) tF8H2 SAMs deposited on a flat SiOx-covered silicon wafer.

In addition, NEXAFS spectroscopy is employed here to determine the population and molecular orientation of tF8H2 molecules composing the SAMs46,47 deposited on the surfaces of PET[UVO] and PET/PVMS-TCS[UVO] substrates. The PEY NEXAFS intensities measured at θ = 50º (the so-called “magic angle”) are independent of molecular orientation and thus provide


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a measure of the concentration of chemical species on the specimen surface. Time-resolved PEY NEXAFS spectra acquired at the carbon K-edge are shown at different UVO exposure times in Figures 8a (PET[UVO]/tF8H2) and 8b (PET/PVMS-TCS[UVO]/tF8H2). In Figure 8b, only the peaks identifying the 1s→σ*C-H, 1s→σ*C-F and 1s→σ*C-C transitions are evident, and they are present at all exposure times, thereby indicating that all the spectra collected from PET/PVMSTCS[UVO]/tF8H2 specimens appear similar. These are also comparable to those obtained from tF8H2 SAMs deposited on SiOx (cf. Figure 7b), as well as from PET[UVO]/tF8H2 specimens at short UVO exposure times (< 5 min) in Figure 8a. At longer exposure times in Figure 8a, two new spectral features positioned at 284.5 and 285.1 eV appear, which correspond to the 1s→π* transition of the C=C signal in the phenyl ring in PET. This observation verifies that, under these conditions, PET lies within the first ≈2 nm of the sample surface (the typical probing depth of PEY NEXAFS).48,49 Appearance of these signals is accompanied by a decrease in the intensity of the 1s→σ*C-F peak, which also indicates that the fluorine concentration on the PET[UVO]/tF8H2 surface decreases as the UVO treatment time is increased. This result is consistent with the WCA data provided in Figure 6 and is attributed to the removal of low-molecular-weight compounds from the PET[UVO]/tF8H2 surface, as depicted schematically in Figure 9. As the SEM images in Figure 5 demonstrate, the PET surfaces roughen with increasing UVO treatment time, in which case the quality of the SAM deposited on the surface is expected to deteriorate, especially after long exposure times.37 In marked contrast, the tF8H2 SAMs deposited on PET/PVMSTCS[UVO] surfaces remain stable regardless of UVO exposure time.



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Figure 8. Carbon K-edge PEY NEXAFS spectra collected at θ=50º from (a) PET[UVO] and (b) PET/PVMS-TCS[UVO] exposed to UVO for various times (labeled and color-coded) and modified subsequently with a semifluorinated tF8H2 SAM. The short dashed line at the post-edge coincides with the pre-edge PEY intensity.


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Figure 9. Proposed mechanism by which tF8H2 organosilanes are removed from PET[UVO] (top) and oriented in PET/PVMS-TCS[UVO] layers (bottom).

We propose 3 reasons for such remarkable coating stability: (1) the PET/PVMS-TCS[UVO] surface possesses silanol groups, which form stable Si-O-Si bonds with tF8H2 molecules; (2) the PET/PVMS-TCS[UVO] surface inhibits formation of low-molecular-weight PET fragments that can be rinsed away (and compromise surface contiguity in doing so); and (3) the PET/PVMSTCS[UVO] surface is intrinsically smoother than that of PET[UVO]. Another consideration involves the orientation and packing of the tF8H2 molecules that comprise the SAMs. As discussed in relation to Figure 7, such molecular-level information can be gleaned from NEXAFS spectroscopy. For this purpose, PEY NEXAFS spectra from PET[UVO]/tF8H2 and PET/PVMS-TCS[UVO]/tF8H2 specimens have also been acquired at three different angles, since the orientation of anti-bonding orbitals relative to the electric vector of the polarized x-ray beam alters the intensity of the 1s→σ* transition, as mentioned earlier. To elucidate the effect of the underlying PVMS-TCS layer on the molecular orientation of tF8H2, we have prepared specimens at two different PVMS-TCS concentrations in solution (x): 1.5 and 3.0%. Normalized



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PEY NEXAFS spectra acquired from PET[UVO]/tF8H2 and PET/PVMS-TCS(x)[UVO]/tF8H2 specimens at θ=20º and 90º are presented for different UVO treatment times in Figure 10. [The Supporting Information provides more comprehensive sets of PEY NEXAFS spectra collected from all three specimens.] Of particular interest here are the peaks corresponding to the presence and orientation of tF8H2 and positioned near 292.0 eV (1s→σ*C-F) and 295.5 eV (1s→σ*C-C).


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Figure 10. Normalized PEY NEXAFS spectra collected at the carbon K-edge at 2 incident angles (θ): 20° (red) and 90° (green), from (a) PET[UVO], (b) PET/PVMS-TCS(1.5%)[UVO] and (c) PET/PVMS-TCS(3.0%)[UVO] surfaces exposed to UVO for 2, 4, 8, and 16 min and modified with a semifluorinated F8H2 SAM (labeled, from top to bottom). Parenthetical values in the specimen designations indicate the concentration of the deposited PVMS-TCS solution. – 23 – ACS Paragon Plus Environment


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Figure 11. Orientation parameter (S) presented as a function of UVO treatment time for PET[UVO]/tF8H2 (red), PET/PVMS-TCS(1.5%)[UVO]/tF8H2 (blue), and PET/PVMSTCS(3.0%)[UVO]/tF8H2 (green) corresponding to C-F (closed symbols) and C-C (open symbols) bonds. The inset provides a more detailed view of the data in the vicinity of S=0.

A complete analysis of these spectra, conducted by fitting each spectrum to a series of Gaussian peaks, an ionization edge and a background for each sample/x-ray beam orientation, yields the orientation parameter (S) for the C-F and C-C bonds. Figure 11 shows S as a function of UVO treatment time for the C-F and C-C bonds for the three series discussed in relation to Figure 10. Close examination of the data reveals that tF8H2 does not exhibit strong molecular orientation when deposited onto UVO-treated PET. Some degree of orientation develops when tF8H2 molecules are anchored to PET/PVMS-TCS(1.5%)[UVO] specimens exposed to UVO at times shorter than 10 min. At longer times, the orientation appears lost since most of the PVMSTCS primer has been removed from the substrate during washing and the underlying PET


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substrate becomes exposed (cf. Figures 10 and S2 in the Supporting Information). Conversely, the PET/PVMS-TCS(3.0%)[UVO]/tF8H2 specimens exhibit highly oriented C-F and C-C bonds. In fact, an increase in UVO treatment time causes SC-F to approach −0.3 and SC-C to increase above zero. Taken together, these results indicate that the tF8H2 molecules tend to orient progressively in the direction along the substrate normal. Determination of the molecular orientation of the fluorinated “finger” on the basis of SC-C data alone is challenging because the 1s→σ*C-C signal originates from both the fluorinated and hydrogenated parts of the tF8H2 molecule. We therefore focus on SC-F. At short UVO times in Figure 11, SC-F starts at a relatively large negative number and then increases (i.e., becomes less negative) with increasing UVO dosage up to ≈4 min, after which time it decreases. To put these values in perspective, recall that SC-F = −0.5 corresponds to the physical situation wherein the F8 section of the tF8H2 molecule orients perfectly along the substrate normal. With this limit in mind, we reconcile the SC-F data presented in Figure 11 as follows. At very brief UVO times, the tF8H2 molecules attach to the substrate as clusters of tF8H2 networks that are stabilized by in-plane crosslinks among neighboring molecules. In this scenario, only a few attachment points, generated by UVO treatment, are needed on the substrate to anchor the tF8H2 SAM. As the UVO treatment time is increased, more reactive groups (attachment points) are generated on the PVMS-TCS surface. According to the data displayed in Figure 11, the organization of the tF8H2 molecules becomes modestly compromised as the crosslinked in-plane assemblies incorporate additional tF8H2 chains at UVO times between 3 and 5 min. At long UVO treatment times, the tF8H2 molecules ultimately adopt and retain a preferred orientation approaching the surface normal (cf. Table 1). Overall, short UVO treatment promotes the formation of imperfect assemblies of tF8H2 on the PET/PVMS[UVO] surfaces, whereas extended UVO treatment creates a large density of surfaceanchored groups that are capable of anchoring tF8H2 in well-organized fashion. Our results are in accord with previously published findings.41,42



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Figure 12. In (a), FTIR-ATR spectra acquired from PP/PVMS-TCS/azo, PMMA, PET/PVMSTCS/azo/PMMA and PP/PVMS-TCS/azo/PMMA (labeled and color-coded). The thin black lines correspond to the spectra of the unmodified substrates (PP and PET), and the dashed lines identify the characteristics peaks of PMMA. In (b), optical images showing the change in color due to the surface-initiated growth of PMMA chains on silicon wafers due to the functional PVMS-TCS/azo primer.


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Scheme 2. Chemical coupling of azo-based free radical polymerization initiator to PVMS-TCS.

The second test addressing the functionality of the PVMS-TCS coating involves synthesizing long macromolecules from polymer substrates coated with a PVMS-TCS layer. The general strategy to achieve this objective couples an azo-based FRP initiator to PVMS-TCS, which yields FRP-functionalized PVMS-TCS copolymers (PVMS-TCS-azo), as illustrated in Scheme 2 and described in the Experimental section. Subsequent spin-coating of the PVMS-TCS-azo copolymer on PET, PP and SiOx substrates permits surface attachment of the FRP initiators. The FTIR-ATR spectrum of a PP film coated with the PVMS-TCS-azo copolymer is displayed for example in Figure 12a and confirms the presence of the characteristic peaks associated with the azo (1510 cm-1), amide (1650 and 1530 cm-1) and hydroxyl (3300 cm-1) groups of the initiator. Subsequent surface-initiated FRP of MMA monomer in the presence of PET/PVMS-TCS-azo, PP/PVMS-TCS-azo and SiOx/PVMS-TCS-azo substrates has been successfully used to generate surface-tethered poly(methyl methacrylate) (PMMA) chains on the substrates. Corresponding FTIR-ATR spectra are included for comparison in Figure 12a, and the existence of PMMA is confirmed from its signature peaks located at 1730, 1270, 1241, 1150, and 1074 cm-1. Chain growth is further corroborated by the observation that the initial thickness of the PVMS-TCS-azo layer on SiOx wafer (≈100 nm) increases after polymerization of MMA (to ≈185 nm), as measured by ellipsometry. This result establishes that a PMMA layer measuring ≈85 nm thick has been grown on the SiOx wafer and is solely responsible for the color difference in the optical photographs of SiOx/PVMS-TCS-azo and SiOx/PVMS-TCS-azo/PMMA provided in Figure 12b.



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Figure 13. Optical images qualitatively showing the wettability of various polymer and glass substrates with (left) and without (right) the PVMS-TCS coating before (top) and after (bottom) UVO exposure for 5 min.

Summary and outlook We report herein a novel and straightforward method of expeditiously modifying the surface properties of various polymeric substrates. The basic strategy relies on brief preactivation of a polymer surface by UVO treatment (which only affects the uppermost surface layer and has no measurable effect on bulk properties), followed by deposition of an active PVMS-TCS layer. Upon exposure to atmospheric moisture and subsequent hydrolysis of the silane groups, the PVMS-TCS coating contains reactive hydroxyl groups that can be used for further chemical alteration. In this study, we have explored the benefits of introducing such surface-bound hydroxyl groups on polymer substrates by (1) investigating surface wettability, (2) depositing organosilanes and (3) growing tethered polymer chains. In the first case, examination of UVO treatment on substrates coated with PVMS-TCS reveals that, after an initial transient period, the


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surfaces become completely wettable due to an increase in the surface concentration of oxygen and the formation of a nanoscale silica-like layer, as evidenced by WCA and XPS measurements, respectively. Incorporation of a PVMS-TCS coating on PET, followed by UVO treatment, results in a stable surface, whereas UVO treatment of bare PET promotes fragmentation and roughening of the PET surface as low-molecular-weight fragments are formed during UVO exposure and removed by subsequent rinsing. Deposition of semifluorinated SAMs onto UVOtreated PET with and without a PVMS-TCS coating generates a tailorable hydrophobic surface from WCA measurements. Analysis of the SAMs by NEXAFS spectroscopy provides insight into both composition and molecular orientation. As noted above, SAMs deposited on UVOtreated PVMS-TCS coatings exhibit remarkable stability even for long UVO exposure times. Functionalization of PVMS-TCS coatings with a FRP initiator permits surface-initiated polymerization of PMMA on polymers such as PET and PP. The remarkable ability of these coatings to generate stable surfaces with designer physico-chemical properties makes them beneficial in a wide range of applications, thereby allowing them to be considered highly versatile. Use of the PVMS-TCS copolymer as an active primer for modifying polymer (or inorganic) surfaces opens up new opportunities for generating functional surfaces. Although not discussed here, we have demonstrated elsewhere28 that the modulus of a silicone surface modified with a PVMS-TCS coating can likewise be tailored by adjusting the concentration of TCS in the layer, as well as the layer thickness. Additional functionality can be introduced into PVMS-TCS coatings through the physical incorporation of nanoscale objects, such as fumed or colloidal silica nanoparticles, as well as layered silicates. Embedding silica nanoparticles into PVMS-TCS surface coatings is expected to increase both hardness and abrasion resistance, and the ability of such nanoparticles to react with the chlorosilane groups of the PVMS-TCS copolymer would likely promote the formation of three-dimensional networks. Physical addition of layered silicates (e.g., nanoclays) to PVMS-TCS coatings would not only increase hardness but also enhance barrier properties. With all these elegant modifications aside, application of functional PVMS-TCS coatings on polymer surfaces, followed by brief UVO activation, serves a broadly valuable purpose: they can be used to create hydrophilic surfaces in facile and straightforward fashion, thereby making commonly encountered hydrophobic polymers suitable for biomedical and related applications requiring surface hydrophilicity. The photograph displayed in Figure 13



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shows a PVMS-TCS coating applied to PE, PP, PIm, and glass substrates. The wettabilities of the bare, as well as coated, substrates appear qualitatively similar, despite the underlying substrate, from the side-by-side comparison provided. After exposing the bare and coated substrates to UVO for 5 min, their wettability generally improves, but it is significantly higher (resulting in nearly complete wettability) for all the coated specimens. This observation is consistent with the results reported here indicating that the surface characteristics of various substrates coated with a thin PVMS-TCS layer are largely (if not altogether) governed by the coating irrespective of the underlying substrate.

Acknowledgments This study was supported by the United Resource Recovery Corporation and the Office of Naval Research. The NEXAFS spectroscopy experiments were conducted at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences.


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TOC figure



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Scheme 1. Schematic illustration depicting the formation of PVMS-TCS by coupling TCS to PVMS macromolecules.

Scheme 2. Chemical coupling of azo-based free radical polymerization initiator to PVMS-TCS.


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Figure 1. FTIR-ATR spectra of PET (black), PVMS (green), a 200 nm PVMS-TCS layer deposited on PET (blue), and a ≈1 µm PVMS-TCS layer deposited on PET (red).



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Figure 2. Advancing WCA values of (a) PET[UVO] and (b) PET/PVMS-TCS[UVO] (triangles) and SiOx/PVMS-TCS[UVO] (squares) surfaces as a function of UVO exposure time.


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Figure 3. Atomic concentrations of (a) oxygen, (b) carbon and (c) silicon measured from the surfaces of PET[UVO] (circles), PET/PVMS-TCS[UVO] (triangles) and PVMS[UVO] elastomer (squares) as a function of UVO exposure time.



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Figure 4. High-resolution XPS spectra for surface oxygen (529~539 eV), carbon (279~289 eV), and silicon (99~109 eV) of bare PVMS elastomer (bottom) and UVO-treated PET/PVMS-TCS (i.e., PET/PVMS-TCS[UVO]) at several UVO treatment times (labeled). The dashed lines are centered at the peaks corresponding to the O 1s, C 1s, and Si 2p, transitions of untreated PET/PVMS-TCS[UVO] (at 0 min) as described in the text.


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Figure 5. SEM images displaying the surface topographies of (a) PET[UVO] and (b) PET/PVMS-TCS[UVO] at different UVO exposure times (labeled). The inset in (a) is a 2x enlargement.



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Figure 6. WCA measurements of PET[UVO] (circles), PET/PVMS-TCS[UVO] (triangles) and SiOx on silicon wafers (squares) modified with a semifluorinated SAM as a function of UVO exposure time.


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Figure 7. Normalized PEY NEXAFS spectra collected at the carbon K-edge at 3 incident angles (θ): 20° (red), 50° (blue) and 90° (green) from (a) PET films and (b) tF8H2 SAMs deposited on a flat SiOx-covered silicon wafer.



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Figure 8. Carbon K-edge PEY NEXAFS spectra collected at θ=50º from (a) PET[UVO] and (b) PET/PVMS-TCS[UVO] exposed to UVO for various times (labeled and color-coded) and modified subsequently with a semifluorinated tF8H2 SAM. The short dashed line at the post-edge coincides with the pre-edge PEY intensity.


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Figure 9. Proposed mechanism by which tF8H2 organosilanes are removed from PET[UVO] (top) and oriented in PET/PVMS-TCS[UVO] layers (bottom).



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Figure 10. Normalized PEY NEXAFS spectra collected at the carbon K-edge at 2 incident angles (θ): 20° (red) and 90° (green), from (a) PET[UVO], (b) PET/PVMS-TCS(1.5%)[UVO] and (c) PET/PVMS-TCS(3.0%)[UVO] surfaces exposed to UVO for 2, 4, 8, and 16 min and modified with a semifluorinated F8H2 SAM (labeled, from top to bottom). Parenthetical values in the specimen designations indicate the concentration of the deposited PVMS-TCS solution. – 42 – ACS Paragon Plus Environment

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Figure 11. Orientation parameter (S) presented as a function of UVO treatment time for PET[UVO]/tF8H2 (red), PET/PVMS-TCS(1.5%)[UVO]/tF8H2 (blue), and PET/PVMSTCS(3.0%)[UVO]/tF8H2 (green) corresponding to C-F (closed symbols) and C-C (open symbols) bonds. The inset provides a more detailed view of the data in the vicinity of S=0.



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Figure 12. In (a), FTIR-ATR spectra acquired from PP/PVMS-TCS/azo, PMMA, PET/PVMSTCS/azo/PMMA and PP/PVMS-TCS/azo/PMMA (labeled and color-coded). The thin black lines correspond to the spectra of the unmodified substrates (PP and PET), and the dashed lines identify the characteristics peaks of PMMA. In (b), optical images showing the change in color due to the surface-initiated growth of PMMA chains on silicon wafers due to the functional PVMS-TCS/azo primer.


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Figure 13. Optical images qualitatively showing the wettability of various polymer and glass substrates with (left) and without (right) the PVMS-TCS coating before (top) and after (bottom) UVO exposure for 5 min.



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TOC figure


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Multipurpose Polymeric Coating for Functionalizing Inert Polymer

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