Self-Segregation Behavior of N-Ethyl-pentadecafluorooctanamide

Oct 20, 2009 - Polybutylene isophthalates (PBI) end-capped with N-ethyl-pentadecafluorooctanamide were synthesized from dimethyl isophthalate, 1 ...
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J. Phys. Chem. B 2009, 113, 15204–15211

Self-Segregation Behavior of N-Ethyl-pentadecafluorooctanamide-Terminated Polybutylene Isophthalate and Its Effects on Film Morphology and Wettability Zhonggang Wang,*,† Wenjuan Li,†,‡ Xiaolin Zhao,† Dongjun Zhu,† and Jia You† Department of Polymer Science and Materials, School of Chemical Engineering, Dalian UniVersity of Technology, Zhongshan Road 158, Dalian, 116012, China, and College of Chemistry and Chemical Engineering, Shihezi UniVersity, Shihezi 832000, Xinjiang, China ReceiVed: July 23, 2009; ReVised Manuscript ReceiVed: September 25, 2009

Polybutylene isophthalates (PBI) end-capped with N-ethyl-pentadecafluorooctanamide were synthesized from dimethyl isophthalate, 1,4-butanediol, and N-(2-hydroxyethyl)-perfluorooctanamide with fluorine monomer content from 0 to 5 mol %. The results demonstrated that there was an obvious gradient drop of fluorine concentration from the film surface down to the inner bulk, and the enriched perfluoroalkyl segments covered the rough disordered crystalline topography of PBI, leading to greatly decreased roughness values of fluorinated films. Moreover, the film with only 3 mol % fluorine monomer content exhibited the surface tension of 16.7 mN/m, even lower than that of polytetrafluoroethylene (18.5 mN/m), indicating that the fluorinated groups not only enriched on the film surface but also tended to orient the CF3 group upward. This study is of significance for further understanding the effects of polymeric structural factors on the migration ability of fluorine in the polymer film. Introduction Self-segregation of fluorine, principally driven by the difference in the surface tension between fluorinated groups and nonfluorinated moieties in the polymer backbone, has been proven to be a useful strategy to achieve a fluorine-enriched layer at the surface of polymer materials. In this way, a very hydrophobic surface can be obtained by only a small amount of fluorine content in the polymer.1-5 Especially, in the polymer coating application field, the water/oil repellency property, to a large degree, depends on the chemical composition of the outmost surface within several nanometers of depth. From the viewpoint of both cost-saving and good cleanability, it is desirable that the polymer coating layer has a pronounced enrichment of fluorinated groups on the surface since the expensive fluorine throughout the bulk of the film can not contribute to the low surface energy property. Over the past decade, a variety of polymers with a perfluorinated group in the main chain or on the side chain have been reported in the literature, including fluorinated polyacrylates/methacrylates,6,7 polyethers,8,9 polyurethanes,10,11 polyesters,12,13 polysiloxanes,14 as well as various random, block, and graft copolymers.15,16 Among them, perfluoroalkyl end-capped polymers have received much attention due to their ease of synthesis, low surface energy, low friction coefficient, and nonsticking behavior while maintaining the preferred bulk properties such as good solubility, melting processability, and film-forming characteristic, exhibiting predominating advantages over the traditional wholly fluorinated polymers, e.g., polytetrafluoroethylene, in which the high cost and harsh processing conditions still remain a big problem.17 The most extensively investigated polymers with an end capped by perfluoroalkyl group are polyethylene glycol, polystyrene, and polyesters.18-24 The research results showed that * Corresponding author. E-mail: [email protected]. † Dalian University of Technology. ‡ Shihezi University.

after attaching the fluorinated group to the end of the chain polymers exhibited enhanced biocompatibility, surface activity, and low surface energy. The research by Hirao indicated that for the same polystyrene backbone the fluoroalkyl group at the end of the polymer chain led to the film surface with significantly higher fluorine enrichment and hydrophobicity than that inserted within the chain.25 Recent studies revealed that besides the difference of surface tension between the fluorinated group and other components the segregation ability of fluorinated groups could be affected by a variety of aspects, such as interchain interaction, molecular weight, glass transition temperature, crystallinity degree, melting temperature, phase separation, and film forming process, e.g., melting or solution method as well as the post annealing temperature and time. The length of the perfluoroalkyl and the nature of the linking group by which perfluoroalkyl is attached with the polymer chain can also be directly related to the surface concentration level of fluorine in polymer films.26 Moreover, even though within the same fluorine segment, the surface tension of CF3 (15 mN/m) is much lower than that of CF2 (23 mN/m).27-29 The work by Pospiech showed that the polyester containing the -C10F21 side group attached with a main chain via -(CH2)10- formed a liquid crystalline structure that resulted in low values of surface energy.30 Our previous studies investigated the segregation behavior of the -C7F15 end group in polyester films which was linked with a main chain through the -(CH2)5- segment.31,32 However, the current knowledge of structural influencing factors on wettability and morphology of fluorinated polymer film is still insufficient. A more accurate understanding of structure-property relationships is essential to develop new materials with a low energy surface as expected. The present work was undertaken to synthesize a series of polybutylene isophthalates end-capped with a perfluorooctanamide group attached via an -ethyl- spacer. The bulk physical properties of fluorinated samples were compared to that of the nonfluorinated one by means of solubility test, differential scanning calorimetry, and wide-angle X-ray diffraction methods.

10.1021/jp907000d CCC: $40.75  2009 American Chemical Society Published on Web 10/20/2009

N-Ethyl-pentadecafluorooctanamide-Terminated PBI The surface morphology and segregation behavior of fluorinated groups for the samples with different bulk fluorine content were investigated through AFM, ATR-FTIR, and angle-resolved XPS spectra in both survey and high-resolution modes. The film wettabilities were characterized by contact angles of water and hexadecane, from which the surface tensions were calculated through the two-liquid method. Experimental Section Materials. Fluoroalkyl monomer N-(2-hydroxyethyl)-perfluorooctanamide (Rf-OH) was prepared according to the synthesis procedure described in the literature with some modifications.33 The melting point was 74-76 °C. HRMS calcd. for [C10H6F15NO2]+ was 457.0159, and the found value was 457.0019. FTIR (cm-1): 3323, 1693, 1547, 1201, 1229, 1101, 970. Dimethyl isophthalate (DMIP) was obtained from Qingdao Sanli Chemical Corp. and purified by recrystallization from ethanol, with a melting point of 67-69 °C. 1,4-Butanediol (BD) was purchased from Shuangyi Chemical Corp. and distilled under reduced pressure prior to use. Unless stated otherwise, all the other regents including catalyst tetrabutyl titanate (TBT) were purchased from Shanghai Reagents Corp. and used as received. Synthesis of Nonfluorinated PBI. A 250 mL four-necked flask fitted with a nitrogen inlet, a distillation apparatus, mechanical stirrer, and thermometer was charged with 9.71 g of DMIP (50 mmol), 4.51 g of BD (50 mmol), and TBT (0.0168 g, 0.11 wt %). The system was heated to 180 °C and allowed to react at this temperature for 2 h under the protection of a nitrogen atmosphere. The methanol which formed during the course of the reaction was distilled out. Then the temperature was raised to 220 °C, and the pressure was reduced to 1-2 mmHg. The reaction was maintained at this temperature and pressure for 8 h to further remove the byproduct methanol. Upon cooling, the product was dissolved in 60 mL of chloroform, and the solution was poured into 500 mL of ethanol to remove the unreacted monomers. Repeating this purification procedure three times, the white solid product was dried at 100 °C under vacuum to constant weight, with 92.8% yield. Synthesis of Perfluoroalkyl End-Capped PBIs. Perfluoroalkyl end-capped PBIs with Rf-OH amount charged in the polymerization system over the range from 0 to 5 mol % were prepared by the similar procedure, so only a typical synthesis of FPBI-1.5 is described here as an example. A 250 mL fournecked round-bottomed flask fitted with a nitrogen inlet, a distillation apparatus, mechanical stirrer, and thermometer was charged with 9.71 g (50 mmol) of DMIP, 4.31 g (48.5 mmol) of BD, TBT (0.0168 g, 0.11 wt %), and 0.66 g (1.48 mmol) of Rf-OH. The system was heated to 180 °C, and the reaction was allowed to proceed at this temperature for 2 h. A slow stream of nitrogen was maintained through the reaction. The methanol which formed during the course of the reaction was distilled out. Then the temperature was raised to 220 °C and reduced the pressure to 1-2 mmHg and maintained at this temperature and pressure with agitation for another 10 h to further remove the byproduct methanol. After cooling to room temperature, the product was dissolved in 60 mL of chloroform, and the solution was poured into 500 mL of ethanol to remove the unreacted monomers and catalyst. Repeating this purification procedure three times, the white solid product was dried at 100 °C under vacuum to constant weight, with 91.5% yield. For the sake of brevity, the polymer synthesized in this study is named as FPBI-n, where n refers to the percentage of molar

J. Phys. Chem. B, Vol. 113, No. 46, 2009 15205 ratio of Rf-OH to the sum of DMIP and BD charged in the polymerization system. Film Preparation. The silicon wafer (25 × 25 mm) was used as substrate. Before using, the wafer was put into a ultrasonic bath with piranha solution (4:1 volume ratio of H2SO4 to H2O2) at 80 °C for 1 h. Then it was rinsed with deionized water three times, blown dry with a nitrogen stream, and finally vacuumdried at 40 °C for 2 h. Polymer solution of 4 wt % concentration in chloroform was filtrated through a 0.25 µm polytetrafluoroethylene filter to remove particles just prior to spin coating. The spin time is 30 s, and the spin speed is 2000 r/min. At first the solvent was allowed to evaporate at room temperature for 12 h, and then the films were transferred to a vacuum oven and further dried at 40 °C and 2 mmHg for an additional 24 h. Measurements. A mass spectrum was obtained on a Bruker Microtof-Q spectrometer (Germany) for electrospray ionization in positive and negative modes. The mass-to-charge (m/z) ratios of the ions were determined with a quadrupole mass spectrometer, which was scanned from 65 to 2000 amu. The polarity of the ions detected was rapidly switched between + and -, and the data were recorded. Nuclear magnetic resonance proton spectra (1H NMR) were recorded in deuterated chlroform (DCCl6) on a Varian INOVA400 spectrometer (USA) with tetramethylsilane (TMS) as an internal reference. Fourier transform infrared spectra (FTIR) were measured on a Nicolet 20XB FT-IR spectrophotometer (USA) in the 400-4000 cm-1 region at room temperature with a resolution of 2 cm-1. Samples were prepared by dispersing the sample in KBr and compressing the mixtures to tablets. All spectra were baseline corrected. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) was performed on a NEXUS FT-IR apparatus equipped with an ATR accessory (USA), using an IR beam with an incident angle of 45°. The unmodified ATR element and wafer were measured, respectively, as background spectra. Molecular weight and molecular distribution were determined by GPC with a Water Associates model PL-GPC-220 (England) apparatus at room temperature with chloroform as eluent, at a flow rate of 1 mL/min, calibrated with polystyrene standards. Differential scanning calorimetries (DSC) were run on NETZSCH DSC204 (Germany) in the temperature range of 100-400 °C at a heating rate of 10 °C · min-1 in nitrogen atmosphere. Wide-angle X-ray diffraction (WAXD) measurements were performed at room temperature (about 25 °C) on a D/Max-B X-ray diffractometer (Japan) using Cu KR radiation at a wavelength of 1.54 Å (40 kv, 15 mA). The scanning rate was 2 °C · min-1 over a range of 2θ ) 5° - 60°. Angle-resolved XPS measurements were performed in both survey and high-resolution mode on a Thermo ESCALAB 250 (England), with a Monochromatic Al KR (hν ) 1486.6 eV) X-ray light source of 150 W at 15 kV. Spectra were acquired at three different takeoff angles: 0°, 30°, and 60°, which are defined as the angle between the sample surface normal and the optical axis of the photoelectron spectrometer. The survey spectra from 0 to 1000 eV were collected with a pass energy of 50 eV, while C1s, F1s, and O1s high-resolution spectra were acquired with a pass energy of 20 eV. The atomic force microscopy (AFM) images were recorded with a PicoPlus II microscope (Agilent, USA) in the tapping model at room temperature. The images were recorded in the so-called “soft” tapping mode to avoid the possible deformation

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SCHEME 1: Synthesis of Polybutylene Isophthalates with a Perfluoroalkyl End Group

Figure 1. WAXD patterns of FPBI-0 and FPBI-5.

and indentation of the polymer surface by the tip. The values of root-mean-square roughness (rms) were calculated over the whole captured film area using the accessory software of instrument. Contact angles of deionized water and hexadecane were measured on an OCAH200 contact angle goniometer (Dataphysics, Germany) at room temperature. Each syringe used to place small drops of the liquids on the film surface was dedicated to handling only one of the liquids and was washed with its respective liquid before being filled. For each sample, two spincoated films were used for contact angle measurements. Three to four drops on different positions for one sheet were tested, and the obtained values were averaged. The standard deviation of the contact angle was found to be within (2°. The surface tension γS of the polymer thin film was calculated by the twoliquid method according to eqs 1-334,35 d p γL1(1 + cos θ1) ) 2√(γSdγL1 ) + 2√(γSpγL1 )

(1)

d p γL2(1 + cos θ2) ) 2√(γSdγL2 ) + 2√(γSpγL2 )

(2)

γS ) γSd + γSp

(3)

where γ is the surface tension and the superscripts d and p correspond to dispersion and polar components of the surface tension, respectively. γL and γS denote liquid surface tension and solid surface tension of the thin films, respectively. γ is the sum of γp and γd. γL, γLp, and γLd are constants for a specific liquid.36 Results and Discussion Synthesis of Polymers and Their Bulk Physical Properties. Polybutylene isophthalates end-capped with perfluoroalkyl (FPBIs) were synthesized by the melting polycondensation method from dimethyl isophthalate with 1,4-butanediol and N-(2hydroxyethyl)-perfluorooctanamide (Rf-OH), using tetrabutyl titanate as the catalyst (Scheme 1). The perfluorinated monomer was attached to the main chain via an ester bond, and the fluorine content in the polymer was controlled by the amount of Rf-OH charged in the reaction system. The GPC data in Table 1 showed that the number-average molecular weights (Mn) were in the range from 6454 to 13886 g/mol with the polydispersity indexes (PDI) from 1.6 to 3.0. As a monofunctional monomer, the

incorporation of Rf-OH indeed caused the decrease of molecular weight to some extent, especially for FPBI-4 and FPBI-5 with relatively higher fluorine content. Despite this influence, all the samples synthesized still had weight-average molecular weights (Mw) above 14 000 g/mol. In the 1H NMR spectrum of FPBI-5 (Figure S1, Supporting Information), the small peaks at 3.94 and 4.52 ppm are assigned to the CH2 protons belonging to the N-ethyl-pentadecafluorooctanamide end group, linked to the amide group and ester groups, respectively, demonstrating that the fluorinated group was indeed covalently attached to the terminal of the polybutylene isophthalate chain. DSC measurements were performed to determine the melting points of the polymers. For both nonfluorinated and fluorinated PBIs, the melting points (Table 1) were similar, at around 148 °C. In comparison with other commercial fluorinated polymers, such as polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (CTFE), polyethylene-chlorotrifluoroethylene (ECTFE), and polytetrafluoroethylene (PTFE), the relatively low melting temperature endowed these materials the possibilities for potential powder coating or melt spinning applications at greatly decreased energy cost. After the polymer chain was attached with the fluorinated end group, the FPBIs did not display obvious variation in solubility. All the polymers prepared could not dissolve in N,Ndimethyformamide, tetrahydrofuran, N-methylpyrrolidone, ethyl acetate, and acetone but were easily soluble in polar chloroalkane solvents such as chloroform, dichloromethane, and 1,1,2trichloroethane at room temperature. All the FPBIs could be prepared into films by solution casting or spin coating techniques. The WAXD pattern of FPBI-0 in Figure 1 displays the typical semicrystalline polymer characteristic, with crystalline diffraction peaks at 13.9°, 17.6°, 20.3°, and 24.8°, which are essentially in the same position with respect to the previously reported WAXD spectrum of polybutylene isophthalate.37,38 Moreover, after being end-capped with the perfluoroalkyl group, the fluorinated FPBI-5 has almost the same diffraction pattern as nonfluorinated FPBI-0, indicating that the introduction of the perfluorinated end group did not apparently affect the aggregation structure of the polymer chain. Self-Segregation Behavior of the Fluorinated End-Group in the Five Polymers. As a surface sensitive method, in this study, ATR-FTIR was employed to reveal the fluorine enrichment information on the near surface of polymer films. For FPBI-5 film, relative to the routine FTIR spectrum, the -CH2absorption (2954-2968 cm-1) in the ATR-FTIR became very weak (Figure 2). Moreover, the peak intensities for ester carbonyl and benzene ring absorption also decreased. On the

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TABLE 1: Synthesis and Physical Properties of the Five Polymers

a

sample

DMIP (mmol)

BD (mmol)

Rf-OH (mmol)

Rf-OHa (mol %)

Mn (g/mol)

Mw (g/mol)

PDI

Tm (°C)

FPBI-0 FPBI-1.5 FPBI-3 FPBI-4 FPBI-5

50 50 50 50 50

50 48.5 47.1 46.2 45.2

0 1.48 2.93 3.85 4.76

0 1.5 3 4 5

13886 10052 9313 8258 6454

26844 33562 17423 14923 16041

1.93 3.34 1.87 1.81 1.64

151 148 152 146 149

The percentage of molar ratio of Rf-OH to the sum of DMIP and BD added in the polymerization reaction.

TABLE 2: Binding Energies of C1s, F1s, and O1s of FPBI-5 Film atom C1s

F1s O1s

functional group

FPBI-5 (eV)

CF3 CF2 CONH COO CH2O CH2 aromatic C C-F C-O CdO

293.4 291.3 289.2 288.3 286.5 285.1 284.3 688.3 533.8 532.3

contrary, the band absorptions in the range from 850 to 1250 cm-1 became broadened and enhanced obviously. All of these broad bands could be related to the C-F complex vibration absorption peaks. Figure 3 illustrates the ATR-FTIR spectra for the FPBI polymers with different fluorine content. The peak area from 850 to 1250 cm-1 was integrated (named as A1100) and used as quantitative information of the fluorinated group. The variation of peak area ratio of fluorinated group to ester group (A1100/ A1735) with fluorine content of polymer is given in Figure 4. It could be seen that, in the initial stage, the ratio of A1100/A1735 increased rapidly, and then the variation trend became gentle after fluorine monomer content was over 3 mol %. The results showed that there was an obvious enrichment and saturation of the fluorine in the near surface of thin film at a very low concentration of fluorinated group in the polymer bulk. Angle-dependent X-ray photoelectron spectroscopy (XPS) is another useful technique to examine the segregation behavior of the fluorinated group due to its ability to provide information about elemental composition and chemical bonding of the outer few nanometers of solid polymer films in a quantitative way.39,40 The sample information depth (d) is calculated according to the equation: d ) 3λ cos θ, where λ is the mean free path of photoelectron and θ is the takeoff angle.41 In our case, XPS spectra were performed at three takeoff angles: 0°, 30°, and 60°, corresponding to the information depths of ca. 8, 6.9, and 4 nm, respectively. The XPS survey spectra for the polymer films recorded at the takeoff angle of 0° are shown in Figure 5. The assignments and binding energy data in Table 2 showed that the F1s had a singlet symmetrical peak at 688.3 eV, while O1s displayed doublet peaks at 532.3 and 533.8 eV of similar areas, corresponding to the two different types of C-O and CdO oxygens, respectively. The fluorinated PBI in this study contained seven chemical different types of carbon, exhibiting a complex pattern of peaks extending over the 9 eV range. The peaks at binding energies of 284.3 (A) and 285.1 eV (B) were attributed to the carbon atom of the benzene ring and aliphatic -CH2- segments, respectively. The signal at 286.5 eV (C) corresponded to the carbon bonded to the oxygen or nitrogen atom. The peak at 288.3 eV (D) showed the presence of a carbonyl carbon of the

Figure 2. FT-IR and ATR-FTIR spectra of FPBI-5.

Figure 3. ATR-FTIR spectra of the fluorinated polymers.

ester group, whereas the peak at 289.2 eV (E) was assigned to the carbonyl carbon of the amide group. The signals of CF2 and CF3 groups were found to be at 291.3 (F) and 293.4 eV (G), respectively. The assignments above were consistent with previously published values of related fluorinated polymers.31,42 The elemental compositions were calculated from the peak area of carbon (C1s), fluorine (F1s), and oxygen (O1s) as well as the instrumental sensitivity factors, which are 1.00, 1.80, and 4.43 for carbon, oxygen, and fluorine, respectively. Figure 6 displays the variations of the F/C atomic ratio found at the takeoff angle of 30° and 60° as a function of Rf-OH charged in the polymerization reaction. The results showed that within the surface depths of 4 and 6.9 nm, with the initial introduction of the perfluorinated group, the F/C atomic ratios increased rapidly, exhibiting an obvious fluorine enrichment effect. After the fluorine content was over 3 mol %, the increasing tendency of

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Figure 4. Plot of area ratio of 1100/1735 cm-1 versus fluorine content of polymers.

Figure 5. XPS wide-scan spectra of the polymer films recorded at the takeoff angle of 0°: (a) FPBI-1.5; (b) FPBI-3; (c) FPBI-4; and (d) FPBI-5.

the F/C ratio became flat, which agreed well with the experimental results of ATR-FTIR measurements. Figure 7 shows the C1s spectra and curves of resolved peaks for the four polymer films. It could be seen that the peak intensities corresponding to CF2 (F) and CF3 (G) greatly enhanced from FPBI-1.5 to FPBI-5. With the end-capping of fluorinated groups to the polymer chains, the relative area ratio of (CF3 + CF2) to the total carbon displayed a significant increase from 0.111 (FPBI-1.5) to 0.184 (FPBI-3), and then the continuous increasing only resulted in a slow growth of the fluorinated group on the film surface (Figure S4, Supporting Information). The trend of fluorine enrichment as a function of Rf-OH amount was consistent with the observation using the ATR-FTIR method. Figure 8 presents the relationship plots of the F/C atomic ratio versus different surface depth of polymer films. There was an obvious gradient drop of fluorine concentration from the surface down to the bulk. The more approaching the film surface, the more fluorine concentration was found. For sample

Wang et al.

Figure 6. Plots of F/C atomic ratio obtained from the XPS of the film surface versus fluorine content of polymer films at the takeoff angle of 30° and 60°: (2) 30° and (1) 60°.

FPBI-5, which has an Rf-OH of 5 mol %, the F/C atomic ratio even reached 0.73 at the surface of 4 nm depth. Moreover, Figure S5 (Supporting Information) also displayed a decrease of F/O atomic ratio with surface depth. The variations of O/C atomic ratio at surface depths of 4 and 6.9 nm were plotted against Rf-OH content. As opposed to the F/C and F/O atomic ratios, Figure S6 (Supporting Information) showed that the oxygen content on the outermost surface significantly decreased with the increased perfluoroalkyl groups. As a direct evidence of surface segregation of fluorinated groups, the above results demonstrated that the big difference in surface tension between perfluorinated end moieties and components of polymer backbone made the fluorine atoms preferentially enrich on the surface, and on the contrary, the polar ester groups tended to immerse in the bulk of polymer film. The ratio of surface fluorine concentration to the bulk one, i.e., the fluorine enrichment factor (Q), was used as a measure of the enrichment extent of fluorine on the film surface. The fluorine over carbon (F/C) atomic ratios measured from XPS at different incidence angles, the calculated F/C values in the bulk calculated according to the charged fluorine monomer content, as well as the Q values were given in Table 3. For FPBI-1.5 with Rf-OH of 1.5 mol %, the calculated F/C ratio in the bulk was only 0.0365, whereas the F/C value in 4 nm depth of the film surface was 0.393, increased by about 10.8-fold. Additionally, in this study, a term named as degree of segregation (DS) was introduced to indicate the migration ability of the fluorinated group within the bulk of polymer, which is defined as the difference of Q values between different surface depths. The plots of DS against Rf-OH amount charged in the polymerization reaction are illustrated in Figure 9. It was observed that the fluorine enrichment factor dramatically decreased from the topmost surface to the inner bulk, and this trend was found to be more significant for the samples with the lower bulk fluorine content. For FPBI-1.5 as an example, the DS value between the surface depths of 4 and 8 nm was as high as 7.45, exhibiting a great gradient drop of fluorine concentration from the surface to the inner bulk. On the contrary, the DS value between surface depths of 4 and 8 nm was only 2.73. The self-segregation effect of fluorinated terminal groups in FPBI-1.5 was significantly more pronounced than that in FPBI-5. The reason could be attributed to that the fluorinated

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TABLE 3: F/C Enrichment Data on the Film Surface from XPS Spectra sample

surface depth (nm)

bulk F/C atomic ratio

surface F/C atomic ratio

fluorine enrichment factor (Q)

FPBI-1.5

4 6.9 8 4 6.9 8 4 6.9 8 4 6.9 8

0.0365

0.393 0.13 0.121 0.614 0.375 0.341 0.676 0.473 0.385 0.72 0.541 0.406

10.77 3.56 3.32 8.60 5.25 4.78 7.21 5.05 4.11 6.26 4.70 3.53

FPBI-3 FPBI-4 FPBI-5

0.0714 0.0937 0.115

groups occupied on the topmost surface layer hampered the migration of additional fluorinated moieties to the surface when the bulk fluorine content reaches a certain level. Morphologies of the Five Polymer Films. Similar to other commercial polyesters like polyethylene terephthalate and polybutylene terephthalate, PBI is an easily crystalline polymer, possessing an apparent and sharp melting peak on the DSC curve. To investigate the influence of surface-enriched fluorine element on the morphology of fluorinated PBI films, the AFM

Figure 7. XPS high-resolution C1s spectra of the polymer films recorded at the takeoff angle of 0°: (a) FPBI-1.5; (b) FPBI-3; (c) FPBI4; and (d) FPBI-5.

height and phase images (10 × 10 µm2) were simultaneously recorded at the moderate-force tapping mold. Usually, in the phage image, the crystalline region appears brighter and has a higher phase angle than the amorphous part due to its greater stiffness.43 The evolution of phase images of the five polymer films with different fluorine content is illustrated in Figure 10. In this study, all the polymer films, for both fluorinated PBIs and nonfluorinated PBI, were obtained by the spin-coating method from their chloroform solutions. Chloroform is a low boiling point solvent, so in the course of film-forming, there is not enough time for polymer chains to adjust their conformation and well align their chains to obtain an ordered crystalline pattern. As shown in Figure 10a, the phase image of nonfluorinated PBI exhibited many bright peaks due to the disordered polycrystalline aggregates and dark valleys corresponding to the amorphous part. After some of the PBI chain ends were capped with fluoroalkyl groups, for FPBI-1, the sharp peak edges of crystalline clusters appeared to be blurred. This trend became more and more obvious with the increased fluorine content and finally displayed very uniform morphology for FPBI-5 film. As revealed by the XPS results, because of the difference in surface tension between the fluorinated group and the polyester backbone, the perfluoroalkyl end groups tended to migrate onto the film-air interface,whereas the nonfluorinatd PBI chains remained in the bulk. For example, FPBI-5 film had fluorine enrichment at the surface 11 times higher than that in bulk. Therefore, the film surface topography reflected the packing

Figure 8. Variations of F/C atomic ratio with surface depth for FPBI films (9) FPBI-1.5; (b) FPBI-3; (2) FPBI-4; and (1) FPBI-5.

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Figure 9. Variations of DS values with the fluorine content for FPBI films: (9) DS between surface depths of 4 and 6.9 nm and (b) DS between surface depths of 4 and 8 nm.

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Figure 12. Plots of contact angles of water and hexadecane and surface tensions versus fluorine content of polymer films: (9) water contact angle; (b) hexadecane contact angle; and (1) surface tension.

TABLE 4: Roughness, Contact Angle (θ), and Surface Tension Data of the Five Polymer Films sample FPBI-0 FPBI-1.5 FPBI-3 FPBI-4 FPBI-5

Figure 10. AFM phase images (10 × 10 µm2) of the five polymer films: (a) FPBI-0; (b) FPBI-1.5; (c) FPBI-3; (d) FPBI-4; and (e) FPBI5.

Figure 11. AFM three-dimensional height images of the five polymer films: (a) FPBI-0; (b) FPBI-1.5; (c) FPBI-3; (d) FPBI-4; and (e) FPBI5.

characteristics of fluorinated moieties to a large extent. From the observation of the AFM phase image of FPBI-5, it showed that its surface was covered by continuous fluorinated moieties scattered with some bright spots attributed to the trace of the crystalline feature of PBI backbones. The three-dimensional height images in Figure 11 present more direct observation on the topographies of films with

roughness θwater θhexadecane γSp γSd γS (nm) (°) (°) (mN/m) (mN/m) (mN/m) 28.5 19.8 17.8 6.14 5.04

82.2 98.7 109.5 109.8 110.4

16.2 35.5 57.3 60.4 61.6

6.3 2.2 1.0 1.1 1.1

26.9 22.4 15.7 15.2 14.9

33.2 24.6 16.7 16.3 16.0

different fluorine content. The film surface of FPBI-0 displayed an image of fluctuating mountain peak shape and was very rough with roughness values (rms) of 28.5. Then the rms amplitude decreased rapidly to 6.14 for FPBI-4 and 5.04 for FPBI-5, respectively. It was believed that the rough surface of PBI arose from the crystalline behavior of PBI chains. With the increase of fluorine content, the film surface was gradually covered by a layer of fluorinated components, for which the thickness depended on the fluorine content. Investigation on Wettabilities of the Five Polymer Films. Figure 12 depicts the plots of contact angles of water and hexadecane as a function of Rf-OH charged in the polymerization reaction. For FPBI-0 film, the measured contact angle of water was only 82°. With the introduction of a perfluoroalkyl group, the water contact angle increased rapidly and reached 109.5° for FPBI-3 which has the fluorine monomer of 3 mol %. After that, the increase of the contact angle with fluorine content leveled off to a plateau. The variations of hexadecane contact angles with fluorine content had a similar behavior. The surface tensions of the polymer films were calculated according to the two-liquid method. The resultant γS as well as its dispersion (γdS) and polar components (γpS) are given in Table 4, and the variation of surface tension with fluorine content is also presented in Figure 12. The dependence of surface energy on fluorine content displayed the trend opposite to the liquid contact angle. For FPBI-0, which does not contain a fluorine group, the calculated surface energy was 33.2 mN/m. When the fluorine content was over 3 mol % (FPBI-3), the surface tension was greatly decreased to 16.7 mN/m, reflecting the high fluorine enrichment on the topmost surface of FPBI film, as convinced by the angle-resolved XPS and ATR-FTIR results. In addition, as a comparison, PTFE has a long perfluoroalkyl

N-Ethyl-pentadecafluorooctanamide-Terminated PBI -CF2-CF2- chain with a surface tension of 18.5 mN/m.24 Considering that the surface tension of CF2 (23 mN/m) is much higher than that of CF3 (15 mN/m), the reason that FPBI-3 even had the lower surface tension than PTFE might be attributed to that not only the FPBI film had the enriched fluorine atoms on the surface but also the perfluoroalkyl end group had the tendency to orient the CF3 group to the air-film interface. Conclusions In this paper, a series of semicrystalline polybutylene isophthalates (PBI) end-capped with N-ethyl-perfluorooctanamide were synthesized successfully with fluorine monomer content varied from 0 to 5 mol %. In comparison with the nonfluorinated PBI, the fluorinated PBIs maintained similar melting temperatures and solubilities in the common organic solvents and almost the same chain aggregation structures. The ATR-FTIR spectra revealed that the peak area ratio of C-F stretching absorption to the carbonyl absorption of ester group (A1100/A1735) increased with the fluorine content in the film bulk, indicating that there was an enrichment of the fluorine in the near surface of the thin film. This observation was further confirmed by the angleresolved XPS results, which showed that there was an obvious gradient drop of fluorine concentration from the surface down to the inner bulk, and the O/C atomic ratio displayed an opposite result. In addition, the enriched fluorine components covered the rough disordered crystalline topography of PBI and displayed greatly decreased roughness values with the increase of fluorine content. As evidence of the enrichment of fluorinated groups on the surface, the contact angles of both water and hexadecane for the five films were measured, through which the surface tensions were calculated by the two-liquid method. It was found that the surface tensions decreased from 33.2 mN/m of nonfluorinated FPBI-0 to 16.7 mN/m of FPBI-3, which was even lower than that of PTFE (18.5 mN/m), and this trend became flat with the continuous increase of fluorine content, demonstrating that, after fluorine monomer content was over 3 mol %, the film surface had reached saturation with the fluorine groups and the fluorinated groups tend to orient with exposed CF3 on the film surface. Acknowledgment. The support from the Program for New Century Excellent Talents in University of China (Grant No. NCET-06-0280) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Grant No. 2005-546), is gratefully acknowledged. Supporting Information Available: This section contains seven figures (S1-S7). Among them, Figure S1 and Figure S2 present 1H NMR spectra of fluorinated and nonfluorinated FPBI, respectively. Figure S3 gives the GPC traces of five polymers synthesized. Figure S4 shows the relative area ratio of (CF3 + CF2) to the total carbon for FPBI films. Figure S5 and Figure S6 describe the variation of F/O atomic ratio with surface depth and O/C atomic ratio versus fluorine content of polymer films at different takeoff angles. Figure S7 exhibits the AFM height images and phase images of the five polymer films. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Anton, D. AdV. Mater. 1998, 10, 1197–1205. (2) Brantley, E. L.; Jennings, G. K. Macromolecules 2006, 37, 1476– 1484. (3) Bantz, M. R.; Brantley, E. L.; Weinstein, R. D.; Moriarty, J.; Jennings, G. K. J. Phys. Chem. B 2004, 108, 9787–9794.

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