Fabrication and Wettability of Colloidal Layered Double Hydroxide

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Ind. Eng. Chem. Res. 2010, 49, 5610–5615

Fabrication and Wettability of Colloidal Layered Double Hydroxide-Containing PVA Electrospun Nanofibrous Mats Lina Zhao,† Dayong Yang,‡ Mingdong Dong,§ Ting Xu,† Yu Jin,‡ Sailong Xu,†,* Fazhi Zhang,† David G. Evans,† and Xingyu Jiang‡ State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China, National Center for Nano Science and Technology, Beijing 100190, China, and Rowland Institute, HarVard UniVersity, Cambridge, Massachusetts 02142, U.S.A.

We report a facile protocol of preparing colloidal layered double hydroxide/polyvinyl alcohol (cLDH/PVA) electrospun nanofibrous mats via direct incorporation of low-content cLDH nanoplatelets with PVA without aid of any surfactant. The as-prepared cLDH/PVA nanofibrous mats exhibit a uniform, smooth surface and enhanced temperatures of the onset decomposition and inflection compared with those of the electrospun mats of pristine PVA and solid powdery LDH/PVA (pLDH/PVA) with the same LDH content. Subsequent surface chemical modification enables the resulting cLDH/PVA fibrous mats to exhibit superhydrophobicity and adhesion to water droplets, which are distinctly different from the hydrophilicity of the unmodified cLDH/ PVA fibrous mats. Our results involving the cLDH/PVA electrospun nanofibrous mats may allow designing a variety of electrospun composite nanofibers with homogeneous morphology and multiple properties. Introduction Inorganic/polymer nanostructured mats have generated diverse scientific and technological interests because of the possibilities of endowing materials with combined attractive properties of polymer matrix and thermally or mechanically stable inorganics.1-5 Inorganic/polymer fibrous mats have shown various promising applications, such as ultrafiltration, protective clothing, composite-fiber-reinforced materials, biomedical applications, core/shell fibers for optical applications, and nanocables for microelectronics applications. Electrospinning has been widely recognized as a simple, flexible technique to enable large-scale production of polymer ultrathin fibers with diameters ranging from tens of nanometers to tens of micrometers.6-9 In addition to a wide variety of polymer nanofibers with uniform external morphologies, inorganic-containing nanofibers, such as ceramic,10 oxide,11 and carbon,12 can also be fabricated using this simple technique, generally via calcination of polymer nanofibers containing inorganic precursors. As one kind of inorganic additives, layered materials such as anionic clays (e.g., layered double hydroxides) and cationic clays (e.g., montmorillonite) have been investigated in polymer solutions for fabrication of composite fibrous mats. Layered double hydroxides (LDHs), also known as hydrotalcitelike materials, are a large class of anionic layered functional materials.13-16 LDHs can be expressed by the general formula [MII1-xIII M x(OH)2]x+(An-)x/n · yH2O, where MII and MIII cations occupy octahedral holes in a brucite-like layer and An- anion is located in the hydrated interlayer galleries. LDHs have a wide variety of applications in additives to polymers,17,18 in biology and medicine,19 in catalysis and environmental remediation.20-22 Previous studies of LDHs/polymer nanofibers have shown that LDH additives were utilized usually in the form of powders.17,18 Addition of LDHs powders, * To whom correspondence should be addressed. E-mail: xusl@ buct.cn. † Beijing University of Chemical Technology. ‡ National Center for Nano Science and Technology. § Rowland Institute, Harvard University.

however, usually resulted in the formation of necklace-like electrospun fibers, due to the strong interparticle interactions involving electrostatic forces and hydrogen bonding.23 Improvement on the necklace-like morphology was achieved typically via treatment of LDHs powder, viz. via intercalation of surfactants into LDH galleries to prevent conglomeration of powdery LDHs. For example, intercalation of 12-hydroxydodecanoic acid into MgAl-LDH powder was found to highly improve the electrospinnability of polycaprolactone (PCL) electrospun nanofibers.18 Recently, the sodium laurateintercalated MgAl-LDH powder was demonstrated in our laboratory to allow the enhancement of the compatibility between LDH powder and polylactide (PLA) matrix.17 However, a decrease in melting temperature of the inorganic/ polymer nanofibers was observed concomitant with the obtained morphological improvement, with the increasing relative content of the organically modified LDH powders. Polyvinyl alcohol (PVA) is generally used as an effective water-soluble polymer for dispersing inorganic nanoparticles or powders.24,25 The hydroxyl groups of PVA molecules might adjust the interfacial bonding in composites, promote electrospinnability of the PVA electrospun nanofibers, thereby resulting in an intrinsic hydropholicity of the obtained mats.26 In the point of view of applications, the molecular layers of water adsorbed on surfaces could affect remarkably the performance of film transistors.27 In case of PVA-based mats, the hydropholicity allows water or moisture to reach the underlying surface and thus may lead to a decrease in transistor performance and thermal stability. These problems may limit the utilization of the PVA-based electrospun mats in humid environments. In the present study, we report the preparation of colloidal LDH/PVA (cLDH/PVA) electrospun nanofibrous mats with a uniform, smooth surface and enhanced temperatures of the onset decomposition and inflection. The cLDH/PVA nanofiber was prepared by electrospinning aqueous solution of PVA matrix with low-content colloidal LDHs preincorporated without aid of any surfactant. The morphological structure and thermal property of the cLDH/PVA electrospun nanofibers were investigated with the control of pure PVA nanofibers and powdery LDH/PVA (pLDH/PVA). The improved performances were

10.1021/ie902037z  2010 American Chemical Society Published on Web 05/11/2010

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illustrated in terms of hydrogen-bonding interaction on the basis of the results of Fourier transform infrared (FT-IR) spectra and X-ray diffraction (XRD). The wetting properties were investigated for the cLDH/PVA nanofibrous mat modified chemically with fluorosilane. Experimental Section Preparation of cLDH/PVA Composite Mats. Fully hydrolyzed (98-99%) granulated polyvinyl alcohol (from Alfa Chem. Co., Mw ) 88000 - 97000) was used without further purification. Organic reagents, such as toluene (from Beijing Chemical Co., purity g99.5%, A. R., water content: 0.03%) and perfluorodecyltrichlorosilane (denoted as PFDTCS, from Alfa Chemical Co., purity g96%) were used without further purification. Colloidal CoFe-LDH was prepared based on a reported procedure with a slight modification.28,29 Briefly, dissolution of 0.15 M NaOH and 0.013 M Na2CO3 was performed with 40 mL of deionized water to give rise to a mixed basic solution, followed by rapid addition of 10 mL of mixed salt solution containing Co(NO3)2 · 6H2O (3.0 mmol) and Fe(NO3)3 · 9H2O (1.0 mmol) with vigorous stirring for 30 min. Pure LDH slurry was obtained via centrifuge separation and washed several times and then manually dispersed in 40 mL of deionized water. This aqueous suspension was then transferred into a stainless steel autoclave with a Teflon lining, followed by hydrothermal treatment at a temperature of 100 °C for 16 h. The resulting suspensions generally have a solid CoFe-LDH concentration of 0.4% as the LDHs yield of 60 ( 5%. CoFe-LDH crystallites were collected with high speed centrifugation (14000 r/min) for physical characterizations of XRD and microscopic observations. PVA aqueous solution with a concentration of 8 wt % was used for electrospinning. The mixture of PVA solution and colloidal LDH was prepared for electrospinning by dissolving PVA into CoFe-LDH colloid solution, which eventually resulted in an 8 wt % aqueous solution of cLDH/PVA and 0.4% of colloidal LDH with respect to the total mixture, respectively. The mixture of PVA solution and powdery CoFe-LDH was prepared by mixing the slurry of powdery LDH with PVA aqueous solution. The powdery LDH was prepared via a conventional coprecipitation,30 and the contents of powdery LDH used for electrospinning were 0.4, 0.8, and 1.6% with respect to the total mixture, respectively. The preparation of composite nonwoven mats of cLDH/PVA and pLDH/PVA was performed with an electrospinning setup previously described in the literature.8 Electrospinning of composite fibers was done at room temperature at a high voltage of 20 kV (DC Power Supply, Spellman High Voltage Electronics Corporation). The spinneret used in the experiments had an inner diameter of 0.8 mm. A copper wire was mounted in the spinneret and used as positive electrode. Grounded aluminum foil was used as counter electrode and mounted at a distance of 10 cm from the spinneret. Continuous electrospun fibers were collected on the aluminum foil in the form of fibrous mat. Electrospinning conditions were optimized to produce electrospun nanofibrous mats composed of individual fibrils less than 1000 nm in diameter and without bead formation. The collector could be changed into silica pieces or glass plates. Superhydrophobic Surface Modification. Surface fluorination was performed by treatment with fluoroalkylsilane to yield a chemically modified surface. The cLDH/PVA electrospun mats were immersed into 100 mL toluene of PFDTCS (1%, V/V) at room temperature for 60 min. The as-modified nanofibrous mat was rinsed with toluene and subsequently ethanol to remove

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physically adsorbed molecules, followed by heat treatment at 100 °C in air for 15 min. Characterizations. Powder XRD measurements were performed on a Rigaku XRD-6000 diffractometer, using Cu KR radiation (λ ) 0.15406 nm, Ni-filter) at 40 kV and 30 mA. The samples without preferred orientations were scanned in steps of 0.02° in the 2θ range 3° to 70° using a count time of 4 s per step. Thermo-gravimetric (TG) investigation was carried out in air on a Perkin-Elmer Diamond thermal analysis system with a heating rate of 10 °C/min, between 25 and 1000 °C. Roomtemperature Fourier transform infrared (FT-IR) spectra were recorded in the range of 4000 to 400 cm-1 with 2 cm-1 resolution on a Bruker Vector-22 Fourier transform spectrometer using the KBr pellet technique with a standard of 1 mg of sample in 100 mg of KBr. Morphological observations were performed with a scanning electron microscopy (SEM; JEOL JSM-T300). A small section of fiber mat was placed on the SEM sample holder and sputter-coated with gold prior to the analysis. Imaging and measurement of atom force microscopy (AFM) was performed using a Nanoscope IIID multimode scanning probe microscope (Digital Instruments Inc., Santa Barbara, CA). The images were recorded in tapping mode in air using silicon cantilevers (NSC11/AIBS, MicroMasch, Tallinn, Estonia). The AFM images were flattened using the NanoScope software (version 5.31r1, Veeco), followed by the section analysis to yield the profile across the imaged fibers. The drive frequency of the cantilever was in the range of 300-360 kHz. Static contact angles were measured with a sessile drop at three locations of each sample using a commercial drop shape analysis system (DSA100, Kru¨ss GmbH, Germany) at ambient temperature. For each sample, contact angles were obtained from three separate locations. The water droplets used for measurement had a volume of 5 µL. Results and Discussion Morphology of Electrospun Fibers. Typical SEM images are shown in Figure 1 for the fibrous mats of pure PVA and cLDH/PVA. The pristine PVA nanofibers exhibit a uniform, smooth surface and random orientation (Figure 1A). The obtained cLDH/PVA nanofibers exhibit a similar morphology, without the appearance of “beads on a string” (Figure 1B),31 indicating that the addition of the colloidal LDH component caused no noticeable change in morphology compared with pure PVA nanofibers. In contrast, the pLDH/PVA nanofibers, containing powdery LDH with the same content of 0.4%, clearly show the bead-like protrusion (Figure S1A, Supporting Information). The bead-like morphology obtained is similar to that of the polylactide electrospun fibers containing surfactant-modified powder LDH reported previously.17 By comparison of morphology between cLDH/PVA and pLDH/PVA nanofibers, one can hypothesize that the colloidal LDH nanosheets enhance the possible compatibility between LDH platelets and PVA matrix thereby effectively preventing the conglomeration of LDH. This compatibility may be evidenced by TEM observation of the cast film of the mixture of cLDH/PVA used for electrospinning (Figure S1B, Supporting Information). No aggregates of LDH colloid was observed in the cast film, but well-dispersed LDH nanoplatelets instead (as indicated by arrow in Figure S1B, Supporting Information). The typical dimensional sizes of the LDH nanoplatelets range from 50 to 100 nm, as determined by SEM study (Inset of Figure S1B, Supporting Information). The diameter distribution of the electrospun nanofibers was quantitatively determined using AFM analysis. AFM observations show high-resolution network structures with numerous

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Figure 1. SEM images of the electrospun mats of (A) pristine PVA fibers and (B) cLDH/PVA fibers with 0.4% colloidal LDH.

Figure 2. XRD patterns of the electrospun mats of (A) pristine PVA fibers and (B) cLDH/PVA fibers, and (C) colloidal LDH nanoplatelets.

fibers of pristine PVA and cLDH/PVA oriented randomly on the substrate (Figure S2A and S2B, Supporting Information). This morphology is actually similar to the integrally interweaved network observed by SEM visualization (Figure 1B). The mean diameters are 464 ( 73 nm and 305 ( 43 nm for pure PVA and cLDH/PVA fibers, respectively, as determined by measuring the width measured at half-height (Figure S2C and S2D, Supporting Information). The decrease in fibrous diameter suggests that the addition of colloidal LDH nanoplatelets improves the electrospinnability of the mixture. Similar decrease in fibrous diameter was also observed for polycaprolactone nanofibers without and with LDH powder premodified with 12hydroxydodecanoic acid.18 XRD Diffraction Patterns. XRD patterns are shown in Figure 2 for pristine PVA nanofibers, cLDH/PVA nanofibers, and colloidal LDH nanoplatelets. The colloidal CoFe-LDH nanoplatelets exhibit the characteristic hydrotalcite-like XRD pattern with diffraction peaks of (003), (006), and (009) (Figure 2C).28,29,32 The pure PVA fibers show a broad peak around 2θ ) 23°, corresponding to the (101) plane of semicrystalline PVA (Figure 2A).33,34 The cLDH/PVA composite fibers exhibit two strong reflection peaks centered at 2θ ) 11.38°, 23° (Figure 2B), corresponding to CoFe-LDH and pure PVA, respectively. XRD analysis of cLDH/PVA composite fibers show a better signal-to-noise ratio for PVA component, indicative of an improved crystallinity of PVA upon hybridization with colloidal LDH. If there is no interaction between colloidal LDH and PVA molecules in the blend fibers, each component would show its own crystal region in the blend fibers and XRD patterns could thus exhibit a simple mixed pattern of cLDH and PVA. However, compared to pristine PVA fibers and colloidal LDH, cLDH/PVA composite nanofibers show a relatively obtuse and broad peak around at 23° with incorporation of colloidal LDH.

This evidence indicates that interactions occurred between the intrinsically hydrophilic colloidal LDH nanoplatelets and PVA molecules in the composite fibers. The interactions could involve both the highly efficient hydrogen bonding, and the coordination to metal atoms accessible on the surface of LDH sheets and to metal atoms located along the edges of the LDH platelets, in which the transition metal cations of Co2+ and Fe3+ were chosen. Similar interactions were reported for the layer-by-layer multifilms of PVA and montmorillonite clay nanoplatelets.2,35 Moreover, the changes in broadness and the relative intensity in cLDH/PVA composite fibers also come possibly form that the reflection peak (006) of colloidal LDH was overlapped with the characteristic peak (101) of PVA molecules. Thermal Properties. TG investigation was performed to evaluate the thermal stability of cLDH/PVA nanofibers, with reference materials of the pure PVA nanofibers and pLDH/PVA nanofibers containing the same content of powdery LDH. In TG curve, the temperature where the weight loss exhibits 5 wt % is described as the onset decomposition temperature, denoted as Tonset.31 Figure 3 shows that the TG curves of three samples exhibit distinctly different thermal decomposition, as summarized in Table S1 (Supporting Information) by comparison of Tonset and inflection temperature. The degradation of PVA fibers was observed very rapidly after 182.6 °C (Figure 3A, also see Table S1, Supporting Information). This can be assigned to both the elimination of water, and the loss of 90% weight before the inflection point of 446.6 °C where a quantity of volatile was produced and the random scission of molecular main chain thus occurred.36 In contrast, the values of Tonset and inflection point of cLDH/PVA fibers were improved to 259.3 and 617.6 °C, respectively (Figure 3B, also see Table S1, Supporting Information). Obviously, the presence of colloidal LDH in the PVA matrix was found to delay both the onset

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Figure 3. TG curves of the electrospun mats of (A) pristine PVA fibers, (B) cLDH/PVA fibers with 0.4% LDH colloid, and (C) pLDH/PVA fibers with 0.4% powdery LDH.

Figure 4. FT-IR spectra of the electrospun mats of (A) pristine PVA fibers, and cLDH/PVA fibers (B) before and (C) after the surface modification.

degradation and the inflection point. This improvement could be due to the high efficiency of hydrogen bonding between the colloidal LDH and PVA molecules. We therefore hypothesize that a strong composite assembly was formed between PVA and colloidal LDH on the microscale, instead of an amorphous structure. For comparison, in the case of addition of same content of powdery LDH in PVA matrix, we indeed observed a decrease in thermal stability, Viz., both the Tonset at 158.7 °C and the inflection point at 356.9 °C (Figure 3C, also see Table S1, Supporting Information). Upon increasing the content of powdery LDH, no further significant decrease was observed. The decreased thermal stability was due to the decreased crystallinity of PVA resulting from the aggregated powdery

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LDH, thereby leading to starting decomposing at relatively lower temperatures.17 The above comparisons thus reveal that the degradation of PVA could occur at an elevated temperature upon the hybridization with colloidal LDH, which could deserve further research in the field of inorganic/polymer hybrid materials. To gain further insight into the hypothesis, FT-IR investigation was performed for the above mats with the same relative quality (1 mg of fibrous mats in 10 mg of KBr).37 In the case of pure PVA mat (Figure 4A), the band ranging between 3400 and 3550 cm-1 was observed with a low intensity, corresponding to the O-H stretching vibration of free hydroxyl groups.38 The bands at 877 and 779 cm-1 exhibit the characteristics of skeletal vibrations of syndiotactic and isotactic PVA stereosequences, respectively.39 In the case of cLDH/PVA composite mat (Figure 4B), the absorption peaks centered at 1096 and 852 cm-1, corresponding to the stretch vibrations of C-OH and C-O,40 respectively, are similar to those of pure PVA films. Comparing cLDH/PVA mat with pure PVA mat, however, one can observe that the peak centered at 3441 cm-1 for pure PVA mat was shifted to lower wavenumbers at 3416 cm-1 for cLDH/ PVA mat. The change associated with -OH stretching vibrations strongly suggests that hydrogen bonding occurs between PVA matrix and inorganic LDH colloid,2,41,42 which eventually favors the improvement on thermal stability of cLDH/PVA mats. This can be verified by comparisons of pLDH/PVA mats with pure PVA mats. With increasing content of powdery LDH, no shift in wavenumber of the peak of -OH stretching vibration was observed (Figure S3, Supporting Information). This clearly evidences that hydrogen bonding was formed only between colloidal LDH and PVA molecules, resulted from O atoms of the colloidal LDH surface and H atoms of PVA rather than the network with hydroxide layers and water molecules.2 Superhydrophobicity of cLDH/PVA Composite Mat. Superhydrophobicity is generally achieved by fabrication of microand nanoscale hierarchical structures with a low interfacial free energy.22,43,44 We first performed AFM observation of the premodified cLDH/PVA nanofibrous mat. The AFM result shows that the root-mean-square (rms) roughness over the size (40 µm × 40 µm) mats is 241.2 nm, indicative of that the morphological roughness could generally favor the hydrophobicity of LDH/PVA nanofibrous mats. We then performed chemical surface modification of cLDH/PVA mats with perfluorodecyl-trimethoxysilane (PFD-TMS).45 The FT-IR spectroscopy of the postmodified cLDH/PVA mat is shown in Figure 4C. The absorption peaks centered at 1070 and 712 cm-1, respectively corresponding to Si-O-C and F-C stretching vibrations of PFD-TMS molecules,46 clearly indicate the anchoring of PFD-TMS on the surface of cLDH/PVA nanofibers. The surface modification was actually based on the reaction of -OCH3 groups in F(CH2)10Si(OCH3)3 molecules. The modi-

Figure 5. Contact angles with the values of 69.2° and 152.4° for the cLDH/PVA composite mat (A) before and (B) after the surface chemical modification. The markers are superimposed to the profiles of a 5 µL water droplet just for guiding the eyes.

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fication was actually achieved by the hydrolization between PFD-TCS and a thimbleful of water in toluene, with C-OH groups in [CH2CHOH]n macromolecule giving rise to the formation of Si-O-C covalent linkages.2,46 Contact angle measurement was carried out to evaluate wetting properties of the pre- and postmodified composite fibrous mats. Figure 5A shows that the premodified cLDH/PVA fibrous mat exhibits a contact angle of 69.2°, displaying the hydrophilic property of the intrinsically hydrophilic PVA matrix. This value of the premodified cLDH/PVA is much higher than that of 19° measured for the smooth composite film prepared by casting of the mixture of PVA solution containing colloidal LDH, strongly revealing the contribution to obtained increased contact angle from the rough surface of cLDH/PVA nanofibrous mat. Upon surface chemical modification, the resulting cLDH/ PVA fibrous mat exhibits an average contact angle of 152.4°. This value, higher than that of 139.2° observed for the surfacemodified PVA nanofibrous mat (Figure S4, Supporting Information), strongly suggests that the surface wettability of cLDH/ PVA nanofibrous mats was converted from hydrophilicity to superhydrophobicity by chemical surface modification treatment. Furthermore, we observed that the water droplet adhered to the surface of postmodified cLDH/PVA nanofibrous mat even when the electrospun nanofibrous mats were titled vertically or inverted in a continuous procedure (Figure S5, Supporting Information). The video shows the changes in contact angle observed in a continuous procedure, where the cLDH/PVA nanofibrous mat was set horizontally, vertically, and inverted, respectively (Figure S5, Supporting Information). This adhesion of water droplet strongly reveals the “sticky” hydrophobic behavior of the postmodified cLDH/PVA nanofibrous mats, possibly following Wenzel wetting behavior characteristic of nonpenetration of the liquid into the network of nanofibrous mats.22,47 Conclusions We have demonstrated the preparation of cLDH/PVA electrospun composite mats via direct incorporation of colloidal LDH with PVA matrix without surfactant. Microscopic observations show the nonbead-like morphology of the obtained cLDH/ PVA electrospun composite mats. The addition of colloidal LDH is found to dramatically increase the temperatures of both the onset degradation and the inflection of the cLDH/PVA electrospun mats, which are distinctly different from the decreased thermal performances of the nanofibers of pure PVA and pLDH/ PVA. The superhydrophobicity of the surface chemically modified cLDH/PVA fibrous mats may initiate designing antimoisture mats in the fields of electrics and microdevices. Acknowledgment Prof. Min Wei is acknowledged for stimulating discussion. This work was financially supported by the National Natural Science Foundation of China, the 111 Project (B07004), the Program for New Century Excellent Talents in Universities, the Beijing Nova Program (2007B021), and the Youth Innovative Program of Beijing University of Chemical Technology. Supporting Information Available: SEM images, AFM image, FT-IR spectra of PVA and cLDH/PVA fibrous mats, contact angle of the surface-modified PVA cast film, and video for contact angle in a continuous procedure. This material is available free of charge via the Internet at http://pubs.acs.org.

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ReceiVed for reView July 26, 2009 ReVised manuscript receiVed March 20, 2010 Accepted April 14, 2010 IE902037Z