Mesomorphous Structure and Properties of Non-equimolar

Langmuir 2004, 20, 10737-10743

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Mesomorphous Structure and Properties of Non-equimolar Complexes of Poly(Ethylenimine) and Perfluorooctanoic Acid Biye Ren, Zhen Tong,* Xinxing Liu, Chaoyang Wang, and Fang Zeng Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China Received July 23, 2004. In Final Form: September 3, 2004 A series of solid complexes, PEI-PFAO, made of poly(ethylenimine) (PEI) and perfluorooctanoic acid (PFOA) with different compositions were prepared through a “starving addition” method, where PFOA was fed into PEI solution at the molar ratio, φfeed, of acid group to the amino group of PEI, never beyond unity. Wide-angle X-ray diffraction diagrams confirmed amorphous structure of these complexes. Smallangle X-ray scattering indicated two ordered mesomorphous structures of R and β lamellar phases, with respective long periods of 2.29 and 1.15 nm in the complexes. By increasing the actual molar ratio, φ, of PFOA to the amino group of PEI, the complex structure was altered from R-phase dominant to β-phase dominant. All complexes exhibited two thermal degradation processes induced by decomposition of the bound PFOA below 230 °C and PEI backbone at about 350 °C. The initiating degradation temperature, Tid, decreases with increasing φ due to the preferential degradation of the PFOA chain bound to the tertiary amino groups. The glass transition temperature, Tg, of the complex increases with φ up to the degradation of the complex of φ ) 1. This increase in Tg with φ also supports an ordered alignment of the bound PFOA chains, which greatly restricts the PEI mobility. The solid surface tension, γS, and critical surface tension, γc, of the complex are between 15.4 and 16.8 mN/m and between 13.5 and 15.4 mN/m, respectively. The latter is very close to or even smaller than γc of PTFE (15 mN/m), suggesting the enrichment of CF2 and CF3 groups at the complex surfaces. The fact that the PEI-POFA complex combines high hydrophobicity with selective thermal degradation of bound fluorinated chains promises a potential of selective change and local functionalization of the surface in a well-controlled manner.

1. Introduction The solid polyelectrolyte-surfactant complex (PSC) is a novel class of functional materials with ordered mesomorphous structures and has received considerable attention in the past decade due to its simple preparation and interesting properties.1-4 In recent years, focus has been turned to the preparation, structure, and properties of functional PSC for special application purposes.5-7 Generally, there are three approaches to realize the functional PSC. The first is based on the polyelectrolyte with optical and/or photoelectric functions, such as fluorescence-labeled,8,9 azo- and diazo-polyelectrolytes,9,10 conjugated polyelectrolytes such as cationic poly(p-phenylene) (PPP)11 and anionic poly(1,4-phenylene-ethynylene carboxylate) (PPE)6 and poly(2,5-methoxy-propyloxy- sulfonatephenylene vinylene) (MPS-PPV),5 etc. The second * Author to whom correspondence should be addressed. Tel/ Fax: +86-20-8711-2886. E-mail: [email protected]. (1) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolarcules 1994, 27, 6007. (2) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1869. (3) Antonietti, M.; Burger, C.; Effing, J. Adv. Mater. 1995, 7, 751. (4) MacKnight, W. J.; Ponomarenko, E. A.; Tirrell, D. A. Acc. Chem. Res. 1998, 31, 781. (5) Chen, L.; Xu, S.; McBranch, D.; Whitten, D. J. Am. Chem. Soc. 2000, 122, 9302. (6) Thu¨nemann, A. F.; Ruppelt, D. Langmuir 2000, 16, 3221. (7) Ikkala, O.; Brinke, G. T. Science 2002, 295, 2407. (8) Ren, B.; Gao, F.; Tong, Z.; Liu, X.; Zeng, F. Polymer 2001, 42, 7291. (9) Thu¨nemann, A. F.; Schno¨ller, U.; Nuyken, O.; Voit, B. Macromolecules 1999, 32, 7414. (10) Thu¨nemann, A. F.; Schno¨ller, U.; Nuyken, O.; Voit, B. Macromolecules 2000, 33, 5665. (11) Thu¨nemann, A. F.; Ruppelt, D.; Schnablegger, H.; Blaul, J. Macromolecules 2000, 33, 2124.

is utilizing functional surfactants, such as fluorinatedsurfactants.12 The third is mixing functional small molecules into the complex, as the dye-doped complex.13 The functional PSC reveals a possibility for the development of new materials with specific properties and applications. Numerous studies demonstrated that the solid PSC usually exhibits an ordered supramolecular structure at nanometer scale and the structure can be tuned by the charge density, flexibility, and hydrophobicity of polyelectrolyte, as well as the nature of surfactant, such as polar group, alkyl chains, and polar-to-nonpolar volume ratio.3,4,14-16 Hence, the complexation of polyelectrolyte with oppositely charged surfactant also promises a simple but versatile method to construct various ordered nanostructures. Antonietti et al.1 observed from complexes of poly(styrenesulfonate) and alkyltrimethylammonium chlorides that the undulating lamellar structure and interfacial curvature varied with increasing the length of surfactant alkyl tails and interpreted the structure with an “egg-carton” stacking model. Chu et al.17-19 found multiple ordered supramolecular structures, such as Pm3n space group cubic, face-centered cubic close packing of (12) Antonietti, M.; Henke, S.; Thu¨nemann, A. Adv. Mater. 1996, 8, 41. (13) Wang, L.; Yoshida, J.; Ogata, N. Chem. Mater. 2001, 13, 1273. (14) Kogej, K.; Theunissen, E.; Reynaers, H. Langmuir 2002, 18, 8799. (15) Pe´rez-Camero, G.; Garcıá-Alvarez, M.; Martıńez de Ilarduya, A.; Fernańdez, C.; Campos, L.; Munˇoz-Guerra, S. Biomacromolecules 2004, 5, 144. (16) Kim, B.; Ishizawa, M.; Gong, J.; Osada, Y. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 635. (17) Sokolov, E. L.; Yeh, F.; Khokhlov, A.; Chu, B. Langmuir 1996, 12, 6229. (18) Sokolov, E.; Yeh, F.; Khokhlov, A.; Grinberg, V. Y.; Chu, B. J. Phys. Chem. B 1998, 102, 7091. (19) Zhou, S.; Chu, B. Adv. Mater. 2000, 12, 545.

10.1021/la048148+ CCC: $27.50 © 2004 American Chemical Society Published on Web 10/14/2004

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spheres (fcc), Ia3d space group cubic, 2D hexagonal close packing of cylinders (hcpc), hexagonal close packing of spheres (hcps), and bilayer lamellar structures in the poly(sodium methacrylate-co-N-isopropyl acrylamide) geltetradecyl- or dodecyltrimethylammonium bromide complex, dependent on the charge density of the polyelectrolyte gel. Fluorine polymers exhibit an extremely low surface tension and high oil and water repellency due to the preferential arrangement of the fluorinated groups at the air/polymer interface, taking poly(tetrafluoroethylene) (PTFE) as an example.18,19 These fluorine-rich surfaces have been widely studied for environmentally friendly coatings against biological and other types of fouling.20 The complexes of polyelectrolytes with fluorinated surfactants are particularly attractive due to the facility of preparation compared with other fluorine polymers, which makes them particularly promising low-surface-energy materials useful in the protection of buildings and machines, high performance coatings, self-lubricating machine parts, etc.9,10,21-24 Antonietti et al.25 investigated the complexes of either poly(acrylic acid) (PAA) or poly(diallyldimethylammonium chloride) (PDADMAC) with commercially available fluorinated surfactants and found that complexes exhibited the lamellar structures with low solid surface tension, γS, of 12.7-17.8 mN/m. Recently, Thu¨nemann26 examined the surface energy and supramolecular structures of complexes of poly(ethyleneimine) (PEI) and perfluorinated carboxylic acids with a tail chain length from 4 to 16 carbon atoms. The solid surface tension, γS, was found to decrease from 19 to 9 mN/m as the surfactant chain length increased. Smallangle X-ray scattering (SAXS) indicated interesting mesomorphous stacking structures when the fluorinated surfactant had seven or more carbon atoms in the chain. The relative reflection positions and intensities of the SAXS diagrams from these complexes have to be explained with a model of two coexistent lamellar mesophases of R phase and β phase, with the long period of R being about twice that of β. The dominant phase in the complex is determined by the surfactant tail length, and there is only the β phase when the carbon number, n, is smaller than 10. All of above-mentioned investigations, however, were based on the complexes of 1:1 stoichiometry for polyelectrolytes and surfactants with respect to the charges. To reveal the charge density effect of polyelectrolytes on the supramolecular structure and surface property of the polyelectrolyte-perfluorinated surfactant complex, Thu¨nemann and Lochhaas21 prepared a series of copolymers with different contents of charged monomers to adjust the charge density and build the complexes with surfactants at 1:1 stoichiometry. By increasing the charge density, the surface tension of the complex with perfluorooctadecanoic acid decreased to 9 mN/m and the stacking density increased. These results imply that the relative amount of bound fluorinated surfactants plays an important role in the formation of the mesophase of complexes. At the same time, it raises a question that the binding mechanism of surfactant on polyelectrolyte and the supramolecular structure of the complex may be altered by the monomer sequence distribution of the (20) Tsibouklis, J.; Nevell, T. G. Adv. Mater. 2003, 15, 647. (21) Thu¨nemann, A. F.; Lochhaas, K. H. Langmuir 1998, 14, 4898. (22) Thu¨nemann, A. F.; Lochhaas, K. H. Langmuir 1999, 15, 4867. (23) Thu¨nemann, A. F.; Kubowicz, S.; Pietsch, U. Langmuir 2000, 16, 8562. (24) Thu¨nemann, A. F. Prog. Polym. Sci. 2002, 27, 1473. (25) Antonietti, M.; Henke, S.; Thu¨nemann, A. Adv. Mater. 1996, 8, 41. (26) Thu¨nemann, A. F. Langmuir 2000, 16, 824.

Ren et al. Table 1. Composition of PEI-PFOA Complexes sample

yield (wt%)

φfeed

N (wt%)

φ

PEI-PFOA-A PEI-PFOA-B PEI-PFOA-C PEI-PFOA-D PEI-PFOA-E PEI-PFOA-F

94 93 93 92 93 86

1.0 0.90 0.80 0.70 0.60 0.40

3.04 3.48 3.76 4.28 4.95 6.18

1.0 0.87 0.78 0.69 0.58 0.44

copolymer because the monomer reactivity ratios are usually unequal to unity, which leads to a nonstatistic sequence distribution in the copolymer. In the present study, we used PEI-perfluorooctanoic acid (PFOA) complexes of non-1:1 stoichiometry to investigate the effect of the relative amount of the bound fluorinated surfactants on the ordered mesophase structure and thermal properties. PEI is a homopolycation after protonation with acid and is expected to produce the non1:1 stoichiometric complex, to which, to our knowledge, much less attention has been paid. 2. Experimental Section Materials. PEI (Aldrich, MW 25 000) is highly branched, with a molar ratio of primary to secondary to tertiary amino groups of 34:40:26,26 and used as received. Perfluorooctanoic acid (PFOA, 99%) was purchased from Shanghai 3F New Materials Company and the solvent, 1,1,1,3,3,3-hexafluoropropanol, purchased from Aldrich; both were used as received. Complex Preparation. PEI-PFOA complexes with different compositions were prepared with a “starving addition” method in which the moles of added PFOA were never greater than the moles of amino groups in PEI. PFOA (4.14 g) was dissolved in 300 mL of water (purified with a Millipore apparatus) at 80-90 °C and slowly added dropwise into a PEI aqueous solution of equal volume under stirring. The PEI-PFOA complex was formed as a white precipitate in the solution and refined by filtration, washed with warm water, and dried under vacuum for 48 h. The yield ranged from 86% to 94%, depending on the feed molar ratio, φfeed, of PFOA to the amino group of PEI. The nitrogen, N, content of the complex was determined with a Heraeus CHN-O elemental analyzer, and the actual molar ratio, φ, of PFOA to the amino group of PEI was estimated from the N wt% value. The composition of these PEI-PFOA complexes is listed in Table 1. Measurements. Fourier transform infrared (FTIR) spectra of all samples were recorded on a Bruker Vector 33 FTIR spectrometer using a KBr pellet at room temperature. Wideangle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurements of the complex power were performed in transmission geometry with a Rigaku D/max-IIIA powder diffractometer (40 kV and 40 mA) using Cu KR radiation (wavelength λ ) 0.1542 nm) at room temperature. The scan step was 0.01° in 2θ with a counting time of 0.3 S/step. The measured 2θ angles ranged from 10° to 25° for WAXD measurements. For SAXS measurements, the scattering vector, s, ranged from 0.113 to 1.693 nm-1, where s ) 2/λsinθ. Differential scanning calorimetry (DSC) experiments were performed with 3-5 mg of samples in a 6-mm aluminum pan on a Netzsch DSC 204 under nitrogen atmosphere at a heating/ cooling rate of 10 °C/min following the temperature sequence as room temperature f 100 °C f -60 °C f 100 °C. Thermogravimetry (TG) was measured using a Netzsch TG 209 under nitrogen atmosphere at a heating rate of 10 °C/min from room temperature to 500 °C. Contact angles were measured on a Kyowa CA-A contact angle meter at 20 ( 1 °C with eight test liquids for evaluating the surface tension of the PEI-PFOA complex films. The complex was dissolved in 1,1,1,3,3,3-hexafluoropropanol to 0.5 wt% concentration at room temperature, and the complex thin film was prepared on a glass substrate by spin coating at room temperature and dried under vacuum at 60 °C (Caution: the experiment must be carefully carried out in a ventilated cabinet because this solvent is highly toxic.). The data of contact angles were averaged at least over three measurements.

Structure and Properties of Non-equimolar Complexes

Langmuir, Vol. 20, No. 24, 2004 10739

Complex Formation. The structure of the PEI-PFOA complex is sketched in Figure 1. The complex formation can be recognized from the FTIR stretch band shift of the carbonyl group of PFOA, as shown in Figure 2. The carbonyl stretch bands of pure PFOA appear at 1698 and 1765 cm-1, the former corresponds to the carbonyl group with hydrogen bonding and the latter corresponds to the carbonyl group free of hydrogen bonding.26-28 In contrast, the PEI-PFOA complex exhibits only one band of carbonyl stretch at a lower frequency of 1680 cm-1. This red-shift of about 20 cm-1 compared with pure PFOA for hydrogenbonded carbonyl groups suggests the formation of ionic bonds between PFOA anions and PEI amine cations, owing to the proton transfer, as shown in Figure 1. In addition, the absorption bands of the CF2 and CF3 groups appear at the same positions for both pure PFOA and the PEI-PFOA-A complex, i.e., at 1150, 1207, and1242 cm-1, which further supports the binding of PFOA on PEI to form the complex. Similar results were observed from the other complexes with different compositions. The actual molar ratio, φ, of PFOA to the amino group of PEI for the PEI-PFOA complexes estimated from their nitrogen content is almost the same as the feed molar ratio, φfeed, of PFOA to amino groups of PEI, as seen in Table 1. This indicates that the non-equimolar polyelectrolyte-ionic surfactant complex can be fabricated with this “starving addition” method, in which the amount of surfactant in the solution is never beyond that required for forming an equimolar (1:1) stoichiometric complex. Usually, the binding process for complex formation of ionic

surfactant molecules with oppositely charged polyelectrolytes in aqueous solution is highly cooperative, where the binding neighboring a bound molecule is much easier than the primary binding, preferentially in the binding free energy. Such a binding process is sometimes called as the “zipper mechanism”, which may give rise to a strict 1:1 stoichiometry for the complex3 if the surfactant is adequate. The present data illustrate that the complexation of PEI and PFOA does not follow the 1:1 stoichiometry, similar to that reported by Chen and Hsiao for the complexation of PEI and dodecylbenzenesulfonic acid (DBSA).29 This may be attributed to the mixing sequence of PFOA added into PEI solution and the φfeed values never higher than 1. PEI contains three kinds of amino groups, i.e., primary, secondary, and tertiary. The relative basicity and the tendency of the amino group to be protonated, judged from pKb values of 3.35, 3.27, and 4.22 at 25 °C for CH3NH2, (CH3)2NH, and (CH3)3N, respectively, are in the order of 2° amine > 1° amine > 3° amine.30 If the three kinds of amino groups are distributed statistically along the PEI backbone, the zipper binding may be disturbed because the surfactant molecules encounter the amino groups having uneven reactivity. Once the acidic PFOA is dropped into an aqueous solution with an excess of PEI, the secondary amino group will be preferentially protonated and bound, then primary, and then tertiary to form the non-equimolar precipitate. Further added PFOA may diffuse into swollen complex precipitates and lead to an increase in the φ value due to the favorable free energy for the binding process. When the φfeed is as low as 0.4 (PEI-PFOA-F), there remain 60 mol% amino groups in the PEI chain free of binding, which causes considerable water-solubility of the complex. Therefore, φ of the precipitated PEI-PFOA-F sample is higher than its φfeed and the yield is the lowest (86 wt%). Inversely, if PEI is dropped into PFOA solution and enough PFOA molecules are bound to all kinds of the amino groups in PEI, the PEI-PFOA complex of 1:1 stoichiometry will precipitate from the solution soon. Mesomorphous Structures. The ordering of fluorinated chains in solid polyelectrolyte-fluorinated surfactant complexes with the 1:1 stoichiometry is reported to be semicrystalline for long-tail surfactants23,26 and for highly charged polyelectrolytes.21 To clarify the stacking structure of the PEI-PFOA complexes with non-equimolar stoichiometry, WAXD and SAXS measurement were performed and the diagrams are presented in parts a and b of Figure 3, respectively. Only one broad diffraction peak appears at 2θ ≈ 17° for every complex in Figure 3a, corresponding to a Bragg spacing of 0.52 nm. This result confirms the noncrystalline structure for all of these solid PEI-PFOA complexes due to the short carbon-fluorine chain in the PFOA. Neither birefringence nor bright spot was observed from all samples with a polarization optical microscope at room temperature. In addition, no firstorder transition was observed in the DSC thermogram for these complexes either (in next section). Thu¨nemann also illustrated that the narrow reflection began to appear only when the bound fluorinated alkyl tail was as long as 12 carbon atoms.26 Some investigators observed ordered mesomorphous structure with a length scale of 1-10 nm from polyelectrolyte-fluorinated surfactant complexes.1-4 Figure 3b shows SAXS curves of our complex powders. One can see

(27) Sukhishvili, S. A. Macromolecules 2002, 35, 301. (28) Zhang, Y.; Guang, Y.; Yang, S.; Xu, J.; Han, C. C. Adv. Mater. 2003, 15, 832.

(29) Chen, H.-L.; Hsiao, M.-S. Macromolecules 1999, 32, 2967. (30) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 4th ed.; Allyn and Bacon: Boston, 1983; pp 887-889.

Figure 1. Schematic representation of the poly(ethylenimine) (PEI)-perfluorooctanoic acid (PFOA) complex.

Figure 2. FTIR spectra of PFOA (dashed line) and the complex PEI-PFOA-A (solid line).

3. Results and Discussion

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Figure 3. WAXD patterns (a) and SAXS patterns (b) of indicated PEI-PFOA complexes.

obvious reflection peaks in each scattering curve, indicating the presence of a supramolecular order in these complexes. These curves imply that fluorinated surfactant chains are strong mesogen for forming ordered stacking of structures if only 44 mol% of charged sites of a polyelectrolyte are bound. There are two peaks at s ) 0.85 and 1.30 nm-1 for PEI-PFOA-A (φ ) 1.0), three peaks at s ) 0.44, 0.88, and 1.33 nm-1 for both PEI-PFOA-D and -E (φ ) 0.69 and 0.58), and two peaks at s ) 0.43 and 0.87 nm-1 for PEI-PFOA-F (φ ) 0.44). The corresponding relative position is 2.0:3.0 for PEI-PFOA-A, 1.0:2.0:3.0 for PEI-PFOA-D and -E, and 1.0:2.0 for PEI-PFOA-F, and the Bragg spacing corresponding to the first, second, and third peak is about 2.29, 1.15, and 0.76 nm, respectively. Such diffraction data is indicative of lamellar mesomorphous structure of the complexes,1,23-25 similar to that proposed by Thu¨nemann26 for the complexes of PEI-perfluorinated carboxylic acids with the fluorinated alkyl chain longer than seven carbon atoms (heptanoic acid). The fluorinated alkyl chain in the complex acts as the mesogen in a way similar to that in the fluorinated side-chain liquid crystalline polymer.31 Chen and Hsiao’s observation on the PEI-DBSA complexes has indicated that the irregular branching structure of the PEI backbone does not hamper the formation of the lamellar supramolecular structure.29 The main question is how to interpret the SAXS relative intensities with a reasonable packing model. Here, we adopt the Thu¨nemann assumption of two coexistent lamellar phases, R and β.26 The long period of the R phase is 2.29 nm, about twice as large as the 1.15-nm long period of the β phase, so that the reflections from indices 1, 2, and 3 of the β phase can overlap those from indices 2, 4, (31) Corpart, J.-M.; Girault, S.; Juhue´, D. Langmuir 2001, 17, 7237.

Ren et al.

and 6 of the R phase. According to the relative intensity, the complex PEI-PFOA-A consists of almost the β phase that is the same as that in Thu¨nemann’s results,26 but the complex PEI-PFOA-F consists of dominantly the R phase and the complexes PEI-PFOA-D, -E consist of both the R and β phases. One of the most important findings of this work is that changing the molar ratio, φ, of surfactant to charged group of polyelectrolyte can alter the mesomorphous structures of the solid complex, rather than changing the length of the fluorinated tail chains in the bound surfactant reported by Thu¨nemann.26 In other words, the mesomorphous structure of polyelectrolytesurfactant complexes can be adjusted not only by the length of bound surfactant chain but also by the relative amount of bound surfactant. It is straightforward expected that an increase in the bound surfactant tail length will increase the complex packing density and long period; however, it is surprising that decreasing the molar ratio, φ, of the bound surfactant to the polyelectrolyte charge can also increase the lamellar long period from the β phase to the R phase. This seems to suggest that the mesomorphous structure would be more complicated than the proposed monolayer and bilayer. The fact that the long period of the lamellar R phase reduces to half of that for the β phase with increasing the bound fluorinated surfactant content in the complex implies an increase in the stacking density of the mesomorphous complex. Thu¨nemann and Lochhaas21 have also observed a density increase when perfluorodecanoic acid binds on a cationic copolymer with increasing charge density. Therefore, we can conclude that increasing the relative amount of the bound surfactant can bring about denser packing in the solid polyelectrolyte-surfactant complex and the densest structure can be achieved in the equimolar stoichiometric complex. Thermal Properties. The thermal properties of the complexes were characterized using TG and DSC. TG thermograms are shown in Figure 4a compared with those of PFOA and PEI. The temperature at which the weighloss starts is defined as the initiating degradation temperature, Tid, and the temperature where the weight-loss rate becomes maximal is defined as the degradation temperature, Td. A two-step weight-loss process is observed for all complexes. The first weight-loss step occurs below 230 °C mainly due to the loss of the PFOA side chain bound to PEI because the lost weight for a given complex during the first degradation step accords well with the overall weight of PFOA bound in this complex, as shown in the second and third columns of Table 2. The second weight-loss step at about 350 °C is due to the degradation of PEI backbones in the complex because the rapid weight-loss of pure PEI was observed at 356 °C. In addition, the Td of the second weight-loss step does not show an obvious dependence on the complex composition, which further supports our conclusion that the bound PFOA side chains have already been lost during the first degradation step. In contrast, Chen and Hsiao29 found that the thermal stability of PEI was enhanced by the complexation with DBSA and the degradation temperature was raised by as much as 50 °C for their most thermalstable complex. There is only one weight-loss step in their samples and the bound side hydrocarbon chains are thermally stable and protect the PEI backbone from degradation. However, for our complex samples, the bound fluorocarbon side chains degrade at temperatures lower than that for PEI degradation during the first weightloss step (see the following discussion). Therefore, the thermal stability of PEI in the present samples cannot be enhanced.


Figure 4. TG thermograms of PFOA, PEI, and PEI-PFOA complexes (a). Initial decomposition temperature Tid (b) and decomposition temperature Td (9) plotted against the complex composition φ (b). Table 2. Weight Loss during the First Degradation Step and Overall Weight of Bound PFOA

complex

PFOA (wt%)a

weight loss in the first step (%)

PFOA on 3° amino groups (wt%)b


90.8 89.6 88.5 87.2 85.1 81.3

89.6 88.1 86.8 85.5 83.1 81.2

23.6 13.3 4.5 0 0 0

weight loss during initiating degradation (%)c 21.2 10.2 6.7 4.2 0 0

a From the N content determined by elemental analysis. Estimated from PFOA wt% and PEI composition. c Determined from Tid to the temperature 40 °C below Td.

b

Td and Tid for the first degradation step are plotted against the complex composition, φ, in Figure 4b. It can be seen that Td slightly increases from 214 to 231 °C when φ increases from 0.44 to 0.69 and then levels off with further increasing φ. Obviously, the thermal stability of PFOA in the complex is significantly raised by 70-90 °C compared with Td ) 142 °C for pure PFOA. This enhancement should be attributed to the strong electrostatic interaction between PFOA and PEI accompanying the hydrophobic interaction among the PFOA tails in the complex, which greatly restricts the mobility of PFOA molecules and deduces an ordered arrangement of PFOA. The thermal degradation of perfluorinated alkyl side chains may be used as a new approach to selectively change the surface property and to locally pattern the surface function for the complex films.32

Langmuir, Vol. 20, No. 24, 2004 10741

Figure 5. DSC thermograms of PEI-PFOA complexes recorded during the second heating (a) and Tg plotted against the complex composition φ (b).

Tid in Figure 4b is lower than Td for the same complex and decreases with increasing φ from 172 to 104 °C. This initiating degradation is considered to be a decomposition of the weakest part in the complex. The PFOA bound to the 3° amino group may be easily lost and then degrades during heating, owing to the weakest basicity of this amino group.30 The present opinion is supported by the agreement of the weight percent of PFOA bound to the 3° amino group, with the weight loss during the initiating degradation of the complex, as shown in the fourth and fifth columns of Table 2. The weight loss from Tid to a temperature 40 °C below the corresponding Td is taken for the initiating degradation because the Tid of complexes PEI-PFOA-E and -F is about 40 °C lower than their Td and there would be no PFOA bound to the 3° amino group in these two complexes. Consequently, the weight loss between Tid and Td - 40 °C is induced by the breaking away and degradation of the PFOA bound on the 3° amino group. With an increase in φ of the complex, more and more PFOA molecules will bind to the 3° amino groups to form the complex because the 1° and 2° amino groups have been occupied. This will bring about the decrease in Tid with φ. Figure 5a depicts the second heating DSC thermograms of the PEI-PFOA complexes. These DSC curves confirm that there is no crystalline structure in all the complexes due to lack of melting transition. This is consistent with the WAXD results described above. The glass transition is observed for the complexes with φ ranging from 0.44 up to 0.87, and there is no glass transition observed for the complex PEI-PFOA-A with φ ) 1.0. Variation of observed Tg vs φ is illustrated in Figure 5b. Tg reflecting

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Table 3. Surface Tension, γL, of Test Liquids and Static Contact Angle, θ, on Complex Films at 20 °C θ (deg) test liquid

γL (mN/m)

PEI-PFOA-A

PEI-PFOA-B

PEI-PFOA-C

PEI-PFOA-D

PEI-PFOA-E

PEI-PFOA-F

n-hexane n-octane n-decane n-dodecane n-tetradecane n-hexadecane water

18.4 21.8 23.9 25.4 26.7 27.6 72.8

40 52 59 65 68 70 102

38 51 58 64 67 69 101

36 51 57 61 65 67 101

35 50 56 60 64 66 101

33 48 55 60 63 65 102

31 46 53 58 63 65 101

Table 4. Surface Tension, γS, and Critical Surface Tension, γc, of Complex Films complex

φ

γS (mN/m)

γSd (mN/m)

γSp (mN/m)

γC (mN/m)


1.0 0.87 0.78 0.69 0.58 0.44

15.4 15.9 16.3 16.5 16.5 16.8

12.4 12.7 13.3 13.6 14.0 14.0

3.0 3.2 3.0 2.9 2.5 2.8

13.5 13.9 13.9 14.1 14.7 15.4

the PEI segment mobility significantly increases with φ. The increase in Tg is attributed to the stiffening of polymer chains resulted from the binding of PFOA to the PEI backbone,29 which forms an ordering packing structure similar to the nematic liquid crystal. Consequently, only the complexes PEI-PFOA-F and -E are soft and deformable at ambient temperature compared with the other complexes. It is possible the Tg of PEI-PFOA-A, estimated from extrapolation of the curve in Figure 5b to φ ) 1, is higher than its Tid shown in Figure 4b. This complex is of 1:1 stoichiometry for PFOA bound to the amino groups of PEI, so that the bulky nematic packing of the bound PFOA into the densest β phase results in a strong restriction to the PEI mobility and a high glass transition temperature. Thu¨nemann and Lochhaas21 reported a contrary result that Tg for the cationic copolymerperfluorodecanoic acid complexes decreased with an increase in the fluorine content induced by increasing ionic moieties of the copolymer. They attributed this to the increase in the fluorinated chain mobility, similar to that observed from the fluorinated side-chain copolymers.33 Surface Energy. The surface tension, γ, can be considered to be a sum of both the dispersion (γd) and polar (γp) components, and the interfacial tension γ12 between two contacting liquids can be expressed as34,35

γ12 ) γ1 + γ2 - 2xγ1dγ2d - 2xγ1pγ2p

cos θ ) 1 + k (γL - γC)

(3)

where k is the slope of cos θ vs γL plots. Thus, γC can be evaluated by extrapolating γL to cos θ ) 1. γC means a limit value for a test liquid to completely wet the surface (i.e., a liquid with surface tension, γL, lower than γC will spread on the surface and wet it completely). The smaller the value of γC, the more hydrophobic and oleophobic the surface is. Zisman plots of cos θ vs γL of hydrocarbon liquids (Table 3) are presented in Figure 6 for the six complex

(1)

Substituting the Yong’s equation relating the contact angle (θ), surface tension of solid (γS), liquid (γL), and the solid/ liquid interface (γSL) into eq 1, the following semiempirical equation can be obtained31,34,35

γL(1 + cos θ) ) 2(γSdγLd)1/2 + 2(γSpγLp)1/2

To verify the surface enrichment of the bound PFOA chain in the PEI-PFOA complexes, surface tension of spinning coated complex films was investigated using static contact-angle measurement with several test liquids. The experimental results are summarized in Table 3 and the solid surface tension γS () γSd + γSp) of these complexes determined from eq 2 using water and n-hexadecane as test liquids is 15.4-16.8 mN/m, as listed in Table 4 with γSd and γSp values. The γS slightly decreases with increasing φ of the complexes, as shown in Table 4. The critical surface tension, γc, defined by Zisman’s experienced equation is read as37,38

(2)

Therefore, the solid surface tension, γS, γSd, and γSp, can be determined from a measured contact angle, θ, of two test liquids with known γLd and γLp, e.g., water (γL ) 72.8 mN/m, γLd ) 21.8 mN/m, and γLp ) 51.0 mN/m at 20 °C) and n-hexadecane (γL ) γLp ) 26.3 mN/m at 20 °C).36 (32) Bo¨ker, A.; Reihs, K.; Wang, J.; Stadler, R.; Ober, C. K. Macromolecules 2000, 33, 1310. (33) Ho¨pken, J.; Mo¨ller, M. Macromolecules 1992, 25, 1461. (34) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (35) Oshibe, K. Y.; Uozumi, H.; Kawai, S.; Yamada, Y.; Ohmura, H.; Yamamoto, T. J. Appl. Polym. Sci. 1993, 47, 2207. (36) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40.

Figure 6. Zisman plots for the spin-coated PEI-PFOA complex surface-tested with six hydrocarbon liquids.

films. The values of the critical surface tension, γC, of these complexes are listed in Table 4. We have found that n-hexadecane exhibits a high contact angle (g65°) on the complex surface and the γC value of 13.5-15.4 mN/m slightly decreases with increasing φ of the complex, as listed in Table 4. γC is always smaller than γS for the same complex, as expected from its definition of the lowest limit for the test surface. The changes in surface tension with φ summarized in Table 4 are probably due to the packing density of the CF2 (37) Hare, E. F.; Shafrin, E. G.; Zisman, W. A. J. Colloid Sci. 1954, 58, 236. (38) Ellison, A. H.; Fox, H. W.; Zisman, W. A. J. Phys. Chem. 1953, 57, 622.


and CF3 groups aligned at the complex surface, which is influenced by the lamellar structure within the bulk complex. It is worth noting that water (the most polar test liquid with γL ) 72.8 mN/m36 ) exhibits static contact angles larger than 100° on all the complex surfaces, as given in Table 3. The above results indicate that the PEI-PFOA complexes can result in a highly hydrophobic surface whether the composition is of 1:1 or non-1:1 stoichiometry. The present procedure may suggest a new method to build low-surface-energy materials easily and economically. According to Zisman,39 when the surface contact angle of n-hexadecane reaches a value around 54°, this surface can be classified as highly oleophobic. Thus, the present contact-angle data, which are close to the highest contact angle of 75-78° for hexadecane on the CF3-group-covered surface, reveal the oleophobicity for all the PEI-PFOA complexes. Zisman et al.37 observed that a uniform monolayer of hexagonally close packed array of perfluorocarboxylic acids with a surface consisting purely of CF3 groups yielded γC ) 6 mN/m. In the case of PTFE, where the surface nearly completely consists of CF2 groups, the critical surface tension rises to 15 mN/m.38 The γC value of 13.5-15.4 mN/m, for the PEI-PFOA complexes slightly varying with φ, is very close to and even smaller than that of PTFE. The surface tension obtained here is in good agreement with that of poly(sytrene-b-butadiene) copolymer, having perfluorooctanated ester side chains previously reported by Antonietti et al.,40 and other polyelectrolyte-perfluorinated surfactant complexes.12 Therefore, we can conclude that the surface of the PEI-PFOA complex is nearly exclusively occupied with CF3 and CF2 groups. On the other hand, we have found that the thermal cleavage of the bound fluorinated chains can effectively reduce the contact angle. For example, n-hexadecane exhibits a contact angle of 70° (Table 3) on the PEI-PFOA-A film before heating, which is reduced to 60° after heat treatment at 190 °C for 0.5 h. It is particularly interesting that water can rapidly spread on the complex surface after heat treatment, indicating change in the complex surface composition from dually hydrophobic and oleophobic to hydrophilic but still oleophobic due to partial decomposition of the bound fluorinated chains in the complex. Similar results were observed for (39) Shafrin, E. G.; Zisman, W. A. Contact Angle and Wettability; Advances in Chemistry Series 43; American Chemical Society: Washington, DC, 1964; p 151. (40) Antonietti, M.; Fo¨rster, S.; Micha, M. A.; Oestreich, S. Acta Polym. 1997, 48, 262.

Langmuir, Vol. 20, No. 24, 2004 10743

all the other complexes. Clearly, the change in surface composition of the complex due to the selective thermal cleavage of the bound fluorinated chains may be used to selectively change the surface property and to locally functionalize the polymer surface in a controllable manner. 4. Conclusion In summary, PEI-PFOA complexes with the molar ratio, φ, of PFOA to the amino group in PEI ranging from 0.44 to 1.0 can be prepared by controlling the feed ratio of PFOA to PEI via the “starving addition” method. By increasing φ to unity, one can alter the mesomorphous structure of lamellar packing from R-phase dominant to β-phase dominant, the latter has a long period half that of the former. One of the most important findings of this work is that changing the molar ratio of surfactants to charged groups of the polyelectrolyte can alter the mesomorphous structure of the solid complexes. Thermal degradation occurs for the decomposition of the bound PFOA below 230 °C and the PEI backbone at about 350 °C. Due to preferential degradation of the PFOA chains bound to the 3° amino group, the initiating degradation temperature, Tid, decreases with increasing φ, for the binding strength of the 3° amino group is lower. The increase in Tg of the complexes with φ also indicates an ordering alignment of the bound PFOA chains, which greatly restricts the PEI mobility. Furthermore, all the complexes exhibit the solid surface tension, γS, of 15.4-16.8 mN/m and the critical surface tension, γC, of 13.5-15.4 mN/m, which are very close to or even smaller than that of PTFE (15 mN/m), suggesting the enrichment of CF2 and CF3 groups at the complex surface. The fact that the PEI-POFA complexes combine high hydrophobicity with thermal degradation of the bound fluorinated chains will promise a selective change and local functionalization of the surface in a controllable manner, as well as a material for low-surface-energy coatings. In addition, the potential application of this result is to improve substrate adhesion without spoiling the ultrahydrophibicity too much. Acknowledgment. The financial support to this work by the Specialized Research Fund for the Doctoral Program of the Education Ministry (20020561014) and the NSF of China (20374021 and 90206010) and the NSF of Guangdong Province (015036) and is gratefully acknowledged. LA048148+

Mesomorphous Structure and Properties of Non-equimolar

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