From Trans to Cis Conformation: Further Understanding the Surface Properties of Poly(m-phenylene isophthalamide).

Controlling the membrane surface properties such as hydrophobicity and hydrophilicity is critical to achieving desirable performance. For a long time, the difference between the surface wettabilities of superhydrophilic poly(m-phenylene isophthalamide) (PMIA) electrospun nanofibrous membranes (ESMs) and the superhydrophobic parent material is believed to arise from the poling effect induced by the electrospinning process and wicking caused by the porous membrane structure. In this study, the short-term poling effect was eliminated using a reference, centrifugally spun nanofibrous membrane that has a similar fiber morphology and specific surface area to those of the ESM; consequently, we investigated the changes in the surface properties of the ESM in detail. Chemical analysis by angle-resolved X-ray photoelectron spectroscopy revealed that the N and O concentrations are greater at the surface of the ESM. In addition, analysis of the membranes by attenuated total reflectance Fourier-transform infrared spectroscopy and Raman spectra revealed that large amounts of cis conformations of the amide bonds appeared on the surface of ESM after the electrospinning process. The results indicate that the remarkable surface properties of PMIA ESMs mainly arise from the change in the conformation of the amide groups from the stable trans form to the cis form. This change was confirmed by the subsequent in situ growth of Pt nanoparticles and the excellent dye adsorption capacity of the membrane.

Introduction

For polymer materials, control of the surface properties is crucial to achieving desirable performance. For instance, poly(m-phenylene isophthalamide) (PMIA) is a highly crystalline fibrous polyaramid that is widely used because of its high thermal resistivity and excellent mechanical properties.[1−3] According to the X-ray analysis of PMIA, the polymer chains are contracted from a fully extended state by the bonds between the phenyl rings and the amide groups and the chains interact via intermolecular hydrogen bonds.[4,5] Furthermore, mutually perpendicular hydrogen bonds are repeated alternately along the chain axis, forming a three-dimensional “jungle-gym”-type network, which contributes to the hydrophobicity, thermal and mechanical stability, and flame resistance of the fibers.

However, owing to the intermolecular hydrogen bonds, high crystallinity, and lack of polar functional groups in the polymer chains, the surfaces of the PMIA fibers are chemically inert, resulting in their poor interactions with many dyes and solvents and low adhesion to other materials, which poses difficulties in processing PMIA.[6−8] Therefore, the surface properties such as the wetting behavior of PMIA must be improved.

Various alternative processing routes have been developed to modify the relatively inert polymer surface, including chemical treatment, plasma discharge, and even treatment with supercritical carbon dioxide.[9−11] For example, N-substitution of the amide hydrogen by allyl or alkyl groups reduces the stiffness of the polymeric chain, increasing its solubility in organic solvents.[12,13] In addition, the insertion of carboxylic acid groups into the PMIA main chain improves both the polymer wetting behavior and its interaction with adhesive resins. Furthermore, exposure to ultraviolet radiation improves the affinity of the polymer for dyes.[14] Finally, the deposition of a poly(dopamine) layer on PMIA enables the binding of metal ions, allowing metal reduction and formation of nanoparticles (NPs), producing a polymer with electrical resistivity.[15]

Recently, a mixture of LiCl in N,N-(dimethylacetamide) (DMAC) has been shown to be a solvent that, to some extent, improves the processability of PMIA, making the polymer flexible.[16,17] The lithium ions can form strong interactions with the carbonyl group of DMAC, whereas the chloride ions are left unbound and can act as highly active nucleophilic bases, which play a major role in the disruption of the inter- and intramolecular hydrogen bonds.[18] Upon elimination of hydrogen bonds, the three-dimensional jungle-gym-type network is broken, resulting in an increase in polymer flexibility; furthermore, the asymmetric molecular structure shows some weak self-charged characteristics, opening the possibility of its use as a nanofiltration membrane for reaction with heavy metal ions.[19,20] Such a processed membrane still shows low degree of hydrophilicity.

More recently, electrospun nanofibers of PMIA have been shown to have improved specific surface areas, significantly enhanced surface properties (hydrophilicity), and broad applications.[21,22] For example, hydrophilic PMIA nanofibers have been applied as a nanofibrous membrane to remove SiO2 NPs from water.[23] In addition, by combining PMIA nanofibers and an in situ polymerized fluorinated polybenzoxazine functional layer incorporating SiO2 NPs, Tang et al. prepared a superhydrophobic and superoleophilic nanofibrous membrane for water/oil separation.[24]

Interestingly, the surface properties of PMIA nanofibers are dramatically different from those of commercial inert fibers, becoming superhydrophilic when the nanofibers are electrospun. This advantageous surface property is commonly recognized as the result of the electrical poling effect caused by the high-voltage electrospinning process and the wicking effect resulting from the porous structure of the PMIA membranes. However, the effect of poling can be easily lost. In addition, for a nanofibrous material, the hydrophilicity of the surface may arise from the surface charge characteristics or the wicking effect, which always arises from the porous structure of the fibrous material. However, to date, the cause of the dramatic surface changes observed for PMIA, whether the charge resulting from the residual poling, wicking, or another effect, is unknown.

In this study, to minimize the influence of wicking from the porous structure of nanofibers, we prepared a PMIA electrospun nanofibrous membrane (ESM), and by applying a centrifugally spun nanofibrous membrane (CSM) as a reference sample, the changes in the surface properties of the membranes at the molecular level were investigated.

Experimental Section

Materials

Commercial PMIA pulp (fiber length 0.5–3 mm) was provided by Chengdu Boxing Co. Ltd. (China). Analytical grade DMAC and LiCl, used as the solvent, were purchased from Aladdin Reagents. Chloroplatinic acid hexahydrate (H2PtCl6·6H2O) and l-ascorbic acid were analytical grade reagents and were purchased from Aladdin Industrial Corporation. All of the other chemicals were of analytical reagent grade.

Preparation of the PMIA Solution

LiCl was dried in a vacuum dryer at 120 °C for 2 h, and the primed PMIA pulps were treated with methanol, acetone, and ethanol successively, followed by drying in a vacuum dryer at 80 °C for 2 h.

LiCl (0.85 g) was added to DMAC (10.7 mL) and stirred until completely dissolved. Then PMIA pulp (1.58 g) was added to the solution and stirred at 90 °C until completely dissolved, yielding a 15 wt % solution.

Preparation of the PMIA Membranes

The 15 wt % PMIA solution (1 mL) was fed through the plastic microtip of a 10 mL syringe. Electrospinning was carried out at an applied voltage of 30 kV, the distance between the collector and the tip of the syringe was 10 cm, and the flow rate of the solution was 0.35 mL h–1. The relevant temperature and humidity were 25 ± 2 °C and 40 ± 2%, respectively. The obtained ESM was immersed in water and ethanol for 12 h alternately and dried at 80 °C for 10 h to remove the solvent completely.

The CSM was prepared by a homemade centrifugal spinning machine,[25] which includes a needle, a spinneret, an annular fiber collector, and a motor. The needle (l = 1 cm, d = 0.2 mm) was mounted on the spinneret, and the spinneret was fixed on a shaft and controlled by the motor. In this study, the rotational speed of the spinneret was set to 6000 rpm. The distance between the needle tip and the rod collector was 12 cm. All fiber spinning was performed at 25 ± 2 °C and 40 ± 2%, respectively.

A cast membrane (CM) was also prepared from the 15 wt % PMIA solution. The post-treatment processes for the CSM and CM were the same as those for the ESM. To ensure complete removal of the solvent, we controlled the thickness of the CM to be less than 50 μm.

In Situ Growth of Pt NPs on Membrane Fibers

The membrane was first immersed in an aqueous solution of H2PtCl6 (15 mL, 0.01 M, 0.1 M) with agitation for 12 h to ensure Pt adsorption on the self-charged PMIA fibers. These sites become active sites following the nucleation and growth of Pt NPs. Then, a certain amount of l-ascorbic acid (0.5 M), which was used as a reducing agent, was added to the above membrane/Pt mixture, and the reaction mixture was stirred and heated to reflux at 80 °C for 15 min. After reflux, the membrane was removed, thoroughly rinsed in deionized water, and dried in an oven at 60 °C overnight. The obtained samples are denoted ESM/Pt, CSM/Pt, and CM/Pt in the following discussion.

Dye Adsorption Experiments

The adsorption of a dye on the membrane in solution was studied at room temperature in batch mode. The dye adsorption isotherms of the ESM/CSM were measured by varying the initial dye concentrations. The membrane adsorbent (0.025 g) was placed in the aqueous solution (10 mL) containing different concentrations of the dyes (ranging from 2 to 100 mg L–1) at room temperature. The solutions were then vigorously stirred until equilibrium was reached. After adsorption, the sample membrane was removed and the absorption spectrum of the solution was measured with a UV–visible (UV–vis) spectrophotometer.

The amount of dyes adsorbed per unit mass was calculated bywhere C0 and Ce are the initial and equilibrium concentrations of dyes (mg L–1), m is the mass of the PMIA membrane sample (g), and V is the volume of the solution (L).

In the mixed dye adsorption experiment, the adsorbent (0.025 g) and the aqueous solution (10 mL) with a total concentration of 30 mg L–1 of the dye were used. All of the adsorption experiments were performed at approximately pH 7 and at room temperature.

Characterization

The scanning electron microscopy (SEM) images were taken using a field-emission scanning electron microscope (FESEM, ZEISS VLTRA-55, 10 kV). Energy-dispersive X-ray analysis (EDS, IncaEnergy-200) was used to investigate the sample compositions. The static contact angle (CA) of water on the surface of a sample pellet was measured by a contact angle goniometer (Rame-Hart 100-10 Model). UV–vis absorbance spectra were measured using a Perkin-Elmer Lambda 35 scanning spectrophotometer.

Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectra were recorded using a Nicolet 5700 FTIR spectrometer equipped with an ATR apparatus. The resolution was 4 cm–1, and the average result of 60 automatic scans from 4000 to 650 cm–1 was output as the test result.

The Raman spectra were acquired with a Trivista CRS Raman spectrometer (Princeton Instruments) with a laser wavelength (λ) of 514.5 nm. All Raman spectra were parallel-polarized, which means that the polarization of the incident photons from the laser and the polarization of the scattered photons were parallel.

X-ray photoelectron spectroscopy (XPS) was performed using an ESCALab220i-XL spectrometer with 300 W Al Kα radiation. The base pressure was about 3 × 10–9 mbar, and the spectra were collected mainly at the takeoff angle of 90°. The binding energies were referenced to the C 1s line at 284.6 eV from adventitious carbon. Wide energy survey scans were acquired from 0 to 1100 eV, and detailed high-resolution spectra were acquired from 280 to 292 eV for C (1s), 394 to 406 eV for N (1s), and 524 to 536 eV for O (1s). The spectra were fitted using XPSpeak41 software and Shirley background subtraction. Atomic compositions were calculated from high-resolution spectral peak areas after normalizing with individual atomic sensitivity factors. For angle-resolved measurements, the electron detector angle was varied from 5 to 30, 60, and 90°, corresponding to sampling depths of approximately 2.5, 4.5, 7.0, and 9.5 nm.

The apparent CAs of the samples of water were measured by the sessile drop method using a DSA-100 optical contact-angle meter (Kruss Company Ltd., Germany) using liquid droplets of water (3 μL in volume).

Results and Discussion

In this study, commercial PMIA was first dissolved in a solution of LiCl/DMAC. Subsequently, using either electrospinning or centrifugal spinning, nanofibers were prepared. Finally, the solvent was completely removed, yielding either the PMIA ESM or CSM, both of which had very similar morphologies. Note that, first, to eliminate the influence of solvent and any possible charge storage of the electrospun nanofibers (that is, the poling effect from the electrospinning process) completely, the following procedures were carried out. Both the ESM and CSM samples were treated with water and ethanol for 12 h alternately and dried at 80 °C for 10 h to remove the solvent completely. The results of energy-dispersive X-ray spectroscopy (EDX) and the wide energy survey XPS scans demonstrate that the surfaces of both ESM and CSM contain carbon, oxygen, and nitrogen only, and the elemental compositions of the ESM and CSM are the same (see Figure S1). This result suggests that no new functional groups or substances have been introduced. At the same time, alternately washing the membranes with water and ethanol and heating at a high temperature was found to be an efficient way to eliminate the temporary charge storage of the electrospun nanofibers.[26,27] In addition, to minimize the influence of wicking of the nanofibrous material, the thicknesses of the ESM and CSM samples were controlled to be less than 50 μm.

Characterization of the PMIA Nanofibers

Figure shows the representative SEM images of the PMIA ESM (Figure b,c) and CSM (Figure e,f). The ESM nanofibers have a relatively uniform size with an average diameter of 250 ± 50 nm, whereas those of CSM have a broader fiber diameter distribution, which ranged from 100 to 600 nm. In addition, the fibers of CSM were loosely packed compared to the densely packed nanofibers of the ESM formed from the same quantity of spinning solution. During electrospinning, the fibers were densely deposited on the collector by the force of the electric field, resulting in a compact structure. However, during centrifugal spinning, spiral break-up jets freely extend outwards until they meet at the annular collector, where fibers intertwine into membranes, resulting in a less dense membrane.

Figure 1

Static contact angles of water droplets on the PMIA (a) ESM and (d) CSM. Representative SEM images of nanofibers of the (b, c) PMIA ESM. (e, f) Representative SEM images of nanofibers of the PMIA CSM.

Figure a,d shows the water contact angle values for the PMIA ESM and CSM, respectively. Interestingly, the samples show very different wetting behaviors: the static contact angle of the water droplet is 121.6° (hydrophobic) for the CSM and 0° (superhydrophilic) for the ESM (N.B., the values of static contact angles for both samples were taken at different locations for each measurement. In particular, for the CSM, the water droplet is in the Wenzel state and some water is left in the porous structure of PMIA, which will influence the measured values if measurements are made at the same point). According to wetting theories, the ESM and CSM have nanofibrous morphologies with rough surfaces, large surface areas (the Brunauer–Emmett–Teller surface area (SBET) of the PMIA ESM is 21 m2 g–1 and that of the CSM is 18 m2 g–1), and high porosities, allowing the trapping of air or liquid. However, the two nanofibers showed significantly different wettability by water. The results of the wide energy survey XPS scans demonstrate that the surfaces of both CSM and ESM consist of carbon, oxygen, and nitrogen, and the elemental compositions of the samples are identical (Figure S1), indicating that no new functional groups or substance was introduced during processing.

To further determine the differences in the membrane surfaces, angle-resolved X-ray photoelectron spectroscopy (ARXPS) was used to construct a depth profile of the surface at the nanometer scale by varying the signal detection angle. We determined the surface composition (quantitative evaluation of the C, N, and O concentrations) by tilting the sample relative to the analyzer in four angular steps (5, 30, 60, and 90°), which correspond to test depths of 2.5, 4.5, 7.0, and 9.5 nm.[28] The results are shown in Figure . For the CSM, there is no obvious concentration gradient for the elements at different detecting depths. However, for the ESM, the C concentration was found to gradually decrease at lower sample angles and lower depths, whereas the N and O concentrations increase. These changes in the nanofiber surface chemical compositions may be caused by the migration of polar amide groups to the fiber surface during the electrospinning process.

Figure 2

ARXPS spectra of the ESM and CSM at different test depths.

Further surface investigations were carried out to investigate the differences between the two samples. ATR-FTIR spectroscopy is an effective surface technique, the IR rays penetrating little into the sample (about 0.4 μm in this work); thus, ATR-FTIR can be used to detect changes in the surface molecular structure. ATR-FTIR spectra are shown in Figure , and both samples show similar absorption bands over the whole detection range, although some band intensities are different. Figure a shows the N–H stretching region (3100–3500 cm–1). The band at 3394 cm–1 is assigned to the “free” N–H stretching modes,[29,30] where the band intensity for the ESM is larger than that for the CSM. In the range of 650–1700 cm–1, the band at 1608 cm–1 is assigned to the C–C stretch of the phenyl rings,[31] and the band intensity in the spectrum of the ESM is slightly lower than that in the spectrum of the CSM (Figure b).

Figure 3

(a) ATR-FTIR spectra of the ESM and CSM. Enlarged spectra in the ranges of (b) 1200–1700 cm–1 and (c) 650–850 cm–1.

There are five adsorption bands assigned to the amide groups. Aside from the bands of amide II (1541 cm–1, corresponding to N–H bending vibration) and amide III (1249 cm–1, corresponding to C–N stretching vibration), which are complicated by coupling with the backbone and aromatic rings and thus show a slight enhancement in intensity, the amide I (1650 cm–1, corresponding to the C=O stretching vibration), amide IV (720 cm–1, corresponding to the N–H out-of-plane bending vibration), and amide V (685 cm–1, corresponding to the O–C–N bending vibration) bands exhibit significant enhancement in intensity in the spectrum of the ESM (Figure b,c). The appearance of an amide V band indicates O–C–N torsions and N–H out-of-plane deformations.[32,33] Compared to the broad and flat band in the spectrum of the CSM, a sharp and strong band was detected in the spectrum of the ESM, suggesting that the application of the electric field results in significant torsion and out-of-plane deformations of the amide groups.

Furthermore, slight blue shifts in the characteristic amide I and amide V bands indicate an increase in the bond strength of the amide groups.

Figure shows the Raman spectra of the ESM and CSM. The trans amide bond is stable and is the predominant conformation for polypeptides based on the dipole moment.[34,35] In addition, the trans conformation is also the dominant conformation for LiCl/DMAC-dissolved PMIA (CM of PMIA, see Figure S2).[19,36] Notably, both the FTIR and Raman spectra of the CSM are similar to those of the CM (see Figure S2), and even the water contact angles for both membranes indicate hydrophobicity (see Figure S4). This suggests that the trans amide bond is also the predominant conformation in the CSM. The bands at ca. 1268, 1571, and 1622 cm–1 are derived from amide III, II, and I, respectively, of the trans amides of the CSM.[37,38] The trans amide III band is derived from a combination of C–N stretching, N–H bending, and some C=O stretching, whereas the trans amide II vibration is a combination of C–N stretching and some N–H bending motions. The amide I vibration mainly results from a combination of C=O stretching and N–H bending.

Figure 4

Raman spectra of the CSM (black line) and ESM (red line).

The band at ca. 1480–1490 cm–1 is derived from cis amide II, arising mainly from C–N stretching.[37,38] In the Raman spectrum of the ESM at ca. 1476 cm–1, a cis amide II band appeared that has a significantly increased intensity compared to that in the CSM; in contrast, the spectrum of the CM contains almost no cis amide II band, and the intensity of the band in the CSM spectrum was weak. The fact that the spectrum of the ESM also contains a strong trans amide band indicates that only the conformation of the surface amide groups of the PMIA ESM changed to cis after the high-voltage treatment, although, especially inside the nanofibers, the stable trans conformation is still predominant.

All of the data, including ARXPS, ATR-FTIR, and Raman spectra, reveal that after electrospinning a large number of amide groups on the surface of the PMIA nanofibers are in the cis conformation, which results in the exposure of a large number of C=O groups at the surface, leading to a charged surface and interactions between water molecules and the ESM surface. When the fibers are formed in the absence of a high-voltage electric field, the stable trans conformation of the surface amide groups is predominant, although a few cis amide groups may be present occasionally because of the flexibility of the PMIA chain.

Scheme shows a possible formation mechanism for the cis conformation of the PMIA ESM. At first, because of the three-dimensional jungle-gym-type hydrogen-bonded network within the PMIA chains, the bulk PMIA is hydrophobic. Using LiCl/DMAC as the solvent, the inter- and intramolecular hydrogen bonds are broken, increasing the flexibility of the PMIA chains and enabling the occasional emergence of cis amide groups.

Scheme 1

Schematic Illustration of the Formation of the cis-PMIA ESM

The flexible PMIA can be processed to yield CSM and ESM nanofibrous materials. The nanofibrous morphology of the CSM results in the exposure of more cis amide groups (surface self-charged characteristics) on the surface of the fibers. However, this self-charged characteristic in the CSM is not sufficient to yield a hydrophilic material.

In contrast, during the electrospinning process, a Taylor cone of the PMIA solution, which is enriched with positive charge on the cone surface, is formed first. Then, the PMIA solution is elongated and stretched into nanofibers by the electric field.[39,40] Due to electrostatic attraction, the electronegative C=O groups of PMIA are pulled to the surface. Therefore, in an electric field, the conformation of the amide groups on the surface of nanofibers changes to the higher-energy cis conformation.

Importantly, the change from trans to cis conformation is stable, unlike that in the electrical poling process phenomena. Although the poling effect can be removed easily, the PMIA ESM maintains its superhydrophilicity as an intrinsic feature; for example, even after storage for 1 year, superhydrophilicity can be retained (see Figure S3).

The water contact angle on the surface is an important parameter characterizing the wetting behavior of the polymer. Although useful for hydrophobic surfaces, contact angle measurements are limited to hydrophilic polymers, especially for nanofibrous polymer materials where wicking must be considered. To minimize the influence of wicking, the thickness of membranes must be less than 50 μm, but a liquid residue may lead to subsequent slow wicking of the whole CSM. Therefore, to prove the charge properties derived from the enrichment of the cis amide groups on the surface of the PMIA ESM, the in situ growth of Pt NPs was studied and dye adsorption experiments were carried out.

Membrane Surface Decoration with Pt NPs

The two sample membranes were placed into a mixed aqueous solution of the Pt precursor and l-ascorbic acid to study the surface adsorption of Pt. As shown in Figure b,c, after 15 min exposure to a low concentration of the Pt precursor, Pt NPs densely covered the surface of the ESM nanofibers. At higher concentrations of the Pt precursor, the size of the Pt NPs increased significantly and the Pt NPs uniformly covered the PMIA nanofibers, forming a several hundreds of nanometers thick Pt layer. In contrast, the CSM showed an entirely different Pt NP loading pattern. At a low concentration of the Pt precursor, Pt NPs were deposited irregularly on the CSM nanofibers (Figure e). At high concentrations of the Pt precursor, the reduced Pt NPs were still irregularly deposited on the fibers but were larger than those formed at low Pt concentrations (Figure f). Furthermore, we noticed that at the beginning of the reaction the CSM floated on the surface of the solution and was gradually wetted by the solution. This slow wetting can be attributed to the wicking effect of the nanofibrous material. For use a reference sample, we also prepared a CM and found that the membrane had a similar Pt surface decoration behavior to that of the CSM (see Figure S4). Notably, the CM remained floating on the surface of the solution over the whole reaction process, indicating that there was little wicking in the CM membrane.

Figure 5

Representative SEM images of the (a) ESM and (d) CSM, SEM images of PMIA ESM/Pt at Pt precursor concentrations of (b) 0.01 M and (c) 0.1 M, and SEM images of PMIA CSM/Pt at Pt precursor concentrations of (e) 0.01 M and (f) 0.1 M.

Clearly, the cis-conformation PMIA ESM, which results in the exposure of more C=O groups on the surface of the nanofibers, is advantageous for the absorption of Pt ions via electrostatic attraction, resulting in increased in situ growth of Pt NPs.

Inspired by the effects of the cis-conformation PMIA ESM, we tested the membrane for its effectiveness in removing organic pollutants from water. Furthermore, because the ESM is a macroscopic material, solid–liquid separation is easy, occurring by settlement in a short time period. Here, we selected Rhodamine B (RhB) as a model organic pollutant to study the dye-capturing ability of the PMIA ESM.

Figure a,b shows the time-dependent UV–vis absorption spectra of the RhB solutions treated with the PMIA membrane samples. The initial dye concentration was fixed at 30 mg L–1. Strikingly, a significant difference in adsorption between the ESM and CSM was observed. In comparison with the ESM, the CSM has a poor dye adsorption capability. Notably, the PMIA ESM achieved an ultrahigh adsorption capacity of 24.87 mg g–1 for RhB, whereas for the CSM and CM (see Figure S5), the adsorption capacities are 10.25 and 0.93 mg g–1, respectively.

Figure 6

UV–vis spectra of adsorption of RhB on the (a) ESM and (b) CSM. The membrane sample (0.025 g) was placed in an RhB solution (10 mL, 30 mg L–1) at room temperature. The inset image shows the ESM with absorbed RhB. (c) The adsorption capacity of the membrane samples for RhB as a function of contact time. (d) Adsorption isotherm curves for the adsorption of methylene blue (MB), malachite green (MG), direct bordeaux (DB), and Congo red (CR) on the ESM.

Furthermore, the PMIA ESM exhibits excellent adsorption capacity for a variety of water-soluble cationic dyes (Figure ), a property not observed for the CSM. Because of the electronegative amide groups on the surface, the hydrophilic PMIA ESM exhibits excellent adsorption capability for cationic dyes. MG, DB, and MB were used to evaluate the adsorption capacity of the ESM, and the adsorption capacities were found to be 32, 24, and 23.15 mg g–1, respectively. The adsorption of an anionic dye, CR, was also studied, but the adsorption capacity for this dye was only 3.02 mg g–1. By treating the PMIA ESM with an acidic solution to change the surface charge, the adsorption of CR was improved by 1 order of magnitude.

Figure 7

UV–vis spectra of the hydrophilic PMIA ESM on absorption of mixed dye solutions. (a) From left to right, the dye mixture is composed of basic red 18, astrazon pink FG, RhB, MB, and MG. (b) The time-dependent UV–vis absorption spectra of the mixed dye solution. After 30 min, dye removal is completed. The insets are the digital pictures of (a) the individual dye solution and (b) the mixed dye solution before and after the adsorption treatment with the PMIA ESM.

The ability of the PMIA ESM to absorb dyes may be attributed to its advantageous membrane structure such as high specific surface area, large pore volume, and unique nanofibrous morphology. However, by comparing the two types of nanofibers, the main contributor to the absorption appears to be the electrostatic interactions arising from the cis amide groups on the surface of the nanofibers.

Furthermore, the adsorbed dye molecules could be removed by dipping the adsorbed ESM in an ethanol solution (Figure b). As shown, the adsorbed dye molecules are released from the ESM quickly, allowing the ESM to be reused. Figure a shows that after repeated adsorption–release cycles, there is little change in the adsorption capacity of the membrane for RhB. Consequently, the PMIA ESM has potential applications for the rapid and deep treatment of wastewater containing a high concentration of dye.

Figure 8

(a) Adsorption recyclability of the ESM for RhB. (b) Images of the removal processes of RhB from the ESM using ethanol.

Conclusions

In summary, on the basis of the experimental results for both in situ Pt NP growth and dye adsorption, the following conclusions can be made. When the flexible PMIA is dissolved by LiCl/DMAC, it can be processed to yield three different membranous materials.

In comparison with the CM, the CSM is less hydrophobic because of two reasons: first, the CSM has a larger specific surface area owing to its nanofibrous morphology, which results in the exposure of more cis amide groups (a surface self-charged characteristic) on the surface of the fiber, and, second, wicking, arising from the porosity of the nanofibrous material, occurs more easily in the CSM than in the CM.

For the nanofibrous samples, the wettability of the ESM is very different from that of the CSM, and the superhydrophilicity of the PMIA ESM can be attributed to the large number of cis amide groups on the surface resulting from the conformational changes induced by the electrospinning process. Clearly, the cis amide conformation significantly enhances the electrostatic attraction between the ESM and cations, increasing the adsorption capacity of the membrane for the dye molecules and enhancing the in situ growth of metal NPs.