Ordered exfoliated silicate platelets architecture: hydrogen bonded poly(acrylic acid)–poly(ethylene oxide)/Na–montmorillonite complex nanofibrous membranes prepared by electrospinning technique

Abstract

Hydrogen bonded poly(acrylic acid)–poly(ethylene oxide) interpolymer nanofibrous membranes containing well-arranged/dispersed exfoliated Na–montmorillonite platelets were obtained successfully using the electrospinning technique. The as-spun composite nanostructures were characterized by X-ray diffraction patterns and electron microscopy, and confirm the formation of a favoring ordered exfoliated architecture along the fiber axis. Thermal analysis suggested that an increase in both the thermal stability and the melting point of the as-synthesized composite nanostructures could be a result of ordered exfoliated morphology as well as the well-distributed clay platelets along the fiber axis. Improved morphological and thermal properties assigned to aligned clay platelets induced by the electrospinning technique make the as-spun non-woven membranes more promising candidates in tissue engineering, drug delivery, fuel cell proton exchange membranes and environmental pollution treatment applications.

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1. Introduction

The unique medical and industrial characteristics of clay layered silicate/polymer nanocomposites have attracted much attention in nanoscience studies on both experimental and commercial scales, indicating high stiffness, highly thermal stability, and better barrier properties as well as biocompatibility even with a small amount of silicate.^1–4 However, the alignment of well-dispersed clay platelets in the polymer matrix (intercalation/exfoliation) as well as the interaction between the polymer matrix and clay surface have provided a challenge to synthesis of polymer/layered silicate nanocomposites with improved properties. Unlike the conventional techniques needed for the additional agents, the electrospinning process is an effective technique for preparation of layered silicate/polymer composite nanofibers with high dispersion/orientation of clay layered nanomaterials as well as controllable morphology, especially the exfoliated silicate layer morphology.^5–9 Exfoliation has been the ultimate goal of most researchers in this area because this morphology is expected to lead to dramatic improvement of properties with a reduced loading of fillers compared with traditional composites.^10,11 The overall morphology of electrospun clay nanocomposites is complex, whereas exfoliated structures are classified into ordered, partially ordered and disordered structures to adequately describe nanoscale morphology and to avoid confusion in the structure–properties relationship of nanocomposites.¹² Silvestre et al.¹³ proposed an expanded classification scheme where the intercalated and exfoliated structures can be listed in the ordered or disordered structures (Fig. 1a), depending on the change of spacing and orientation of nanoplatelets. The large shear forces induced by the electric field on the fibers, as well as their rapid solidification resulting from the bending and whipping instability, could result in a high chain orientation along the direction of the fiber axis as well as alignment of anisotropic layered nanofillers in the polymer matrix, mostly leading to formation of a favored exfoliated silicate architecture.^14–18 There are many reports of synthesis of electrospun clay layered silicate/polymer composite nanofibers including one or multi-type polymers.^19–24 Nevertheless, to the best of our knowledge, there are no reports on electrospun poly(acrylic acid)–poly(ethylene oxide) (PAA–PEO) interpolymer complex composites containing clay layered silicate nanoplatelets. In a previous study, we presented a novel approach for preparation of hydrogen bonded PAA–PEO interpolymer complex nanofibrous membranes with unique characterization.²⁵ Herein, for the first time, we synthesized a well-designed electrospun hydrogen bonded poly(acrylic acid)–poly(ethylene oxide) composite nanofibrous membranes containing Na–montmorillonite (Na–MMT) (Fig. 1b). The composite is suggested to be of benefit in exchange-proton membranes in fuel cells, tissue engineering, drug delivery and wastewater treatment, as a result of easy availability of functional groups, high specific surface area, well-arranged pores as well as superior specifications of two-dimensional clay layered nanoplatelets such as enhanced thermal, mechanical and barrier properties along with their biocompatibility.^26–39 The as-synthesized composite fibers were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric (TGA) and differential scanning calorimetric (DSC) analysis.

Fig. 1 (a) Classification of silicate layered morphology in the polymer matrix. (b) Schematic illustration of process.

2. Experimental

2.1. Materials

The polymeric constituents of the samples are poly(acrylic acid) (M_w = 240 000 g mol⁻¹, Aldrich) and poly(ethylene oxide) (M_w = 600 000 g mol⁻¹, Across organic). A standard solution of 1.0 M NaOH (Merck) was used as a pH regulator. Na–MMT is layered clay obtained from Southern Clay Products, USA. Distilled water was used as solvent.

2.2. Preparation of hydrogen bonded PAA–PEO/Na–MMT interpolymer composite solutions

First, appropriate amounts of the clay nanofiller were dispersed into 10.0 mL distilled water for 6 h under magnetic stirring at room temperature. To promote swelling and dispersing, the clay nanomaterials dispersed in distilled water were sonicated for 30 min using an ultrasonic generator equipped with an ice water bath. PEO powder (0.53 g) was added to each of the dispersions and magnetically stirred at 50.0 °C for 12 h to obtain 5.0 wt% PEO homogeneous solutions containing 1.0, 2.0 and 4.0 wt% Na–MMT. Then 5.0 wt% PAA aqueous solution was dropwise added to each of the above-mentioned dispersions and magnetically stirred at 50.0 °C for a further 6 h to obtain PAA–PEO/Na–MMT composites with weight ratio 1 : 1 of PAA–PEO containing 1.0, 2.0 and 4.0 wt% of Na–MMT. The pH of the solutions was regulated for a value of 5.3 using 1.0 M NaOH standard solution.

2.3. Electrospinning of the as-synthesized composite solutions

The electrospinning experiments were performed under ambient conditions at 25.0 °C and 47.0% relative humidity. The polymer solution was put into a 10.0 mL plastic syringe with a steel needle having an inner diameter of 1.0 mm (18 gauges). A clamp connected the high-voltage power supply (MH 100 series, HiTek Power Co., UK) to the needle. A piece of aluminum foil placed at 17.5 cm directly facing the needle, act as a grounded collector. The applied voltage and feed rate of the solution were fixed at 17.0 kV and 0.7 mL h⁻¹, respectively.

2.4. Characterization

The morphology of the as-spun composite fibers was observed under a SEM (ZEISS DSM 960A, 20.0 kV) at an accelerating voltage of 10.0 kV. Prior to scanning under SEM, the samples were sputter-coated for 100 S with gold, using a BALZERS SCD 004 fine coater (Germany). The clay layered silicate architecture was investigated by means of TEM, using a CEM 902A ZEISS, (Germany) with an 80.0 keV accelerating voltage. XRD analysis was performed at room temperature in the reflection mode on a Philips X-ray diffractometer with Cu Kα radiation of a wavelength of 1.54056 Å. A scanning rate of 2° min⁻¹ was used. DSC (DSC Q100, TA) thermograms were analyzed in the temperature range of −50 to 300 °C at a heating rate of 10° min⁻¹. A TA Instruments Q50 TGA was used for TGA analysis. The samples were heated from room temperature up to 600 °C at a heating rate of 10° min⁻¹ in argon atmosphere.

3. Results and discussion

3.1. X-ray diffraction

The spacing between the clay platelets or gallery spacing is an indicator of the extent of the intercalation/exfoliation of clay platelets within a polymer matrix and can be observed by XRD. Generally, intense reflection in the range of 3–9° (2θ) indicates an ordered intercalated architecture. On the other hand, exfoliated morphology, where single silicate layers are homogeneously dispersed into the polymer matrix and XRD patterns with no distinct diffraction peak in the range of 3–9° (2θ) could be observed. The XRD patterns of Na–MMT and its composite nanofibers are shown in Fig. 2. The pristine Na–montmorillonite exhibited a broad diffraction peak at 2θ = 6.61–9.65°. The XRD pattern of hydrogen bonded PAA–PEO/Na–MMT composite nanofibrous membranes, however, did not show the diffraction peak in XRD patterns. These observations confirm the formation of exfoliated Na–MMT layers within PAA–PEO fibers.

Fig. 2 XRD patterns of Na–MMT and PAA–PEO/Na–MMT nanocomposite fibers with 1.0, 2.0 and 4.0 wt% Na–MMT.

3.2. Transmission electron microscopy analysis

TEM analysis is complementary to XRD and can give insights into the spatial distribution of the layers. The exfoliated state is confirmed by TEM analysis as shown in Fig. 3.^40,41 TEM images of the as-spun composite nanofibrous membranes containing 1.0 (a), 2.0 (b) and 4.0 wt% Na–MMT (c) are shown in Fig. 3 (the dark lines represent an individual clay layer, whereas the bright area represents the PAA–PEO fiber matrix). TEM images show that most of the Na–MMT platelets have ordered exfoliated morphology and that they were well-distributed within the fiber matrix, and oriented along the fiber axis (Fig. 4). However, these images indicate that some multilayer stacks of nanoplatelets can be observed within the nanofibers. This clearly indicates the feasibility of the electrospinning technique for controllable preparation of two-dimensional ordered exfoliated platelet architectures with a desired alignment of these platelets along the direction of the fiber axis, which is the critical for the conventional techniques in preparation of such type morphology. In the present study, it is proposed that the strong elongation forces imposed on the PAA–PEO/Na–MMT Taylor cone during the electrospinning process will force the clay platelets to be oriented in the direction of the fiber axis, which causes platelets to mostly show well-ordered exfoliation morphology.⁴²

Fig. 3 TEM images of PAA–PEO/Na–MMT nanocomposite fibers with 1.0 (a), 2.0 (b), and 4.0 wt% Na–MMT (c).

Fig. 4 TEM images of electrospun PAA–PEO interpolymer complex containing 1.0 wt% Na–MMT.

3.3. Scanning electron microscopy analysis

Fig. 5 shows the SEM images of pure PAA–PEO nanofibers (a) and PAA–PEO/Na–MMT composite nanofibers containing 1.0 (b), 2.0 (c) and 4.0 wt% Na–MMT (d). The nanofibers were randomly distributed to form three-dimensional fibrous membranes. The morphology and average diameter of the as-spun nanocomposite fibers were significantly affected by both the loadings of Na–MMT and the working distance, as can be observed in Fig. 5 and 6. By carefully comparing the SEM images, the presence of continued uniform small clay agglomerations can be observed within whole electrospun nanofibers (marked with dashed yellow circles). Such well-dispersed agglomerations are enhanced by increasing the clay amount from 1.0 to 4.0 wt%, confirming the well dispersed clay nanomaterials even in high amounts of clay nanofillers (Fig. 5d). Furthermore, in agreement with TEM images, the presence of some aggregations (marked with dashed red circles) confirms the formation of some multilayer stacks.

Fig. 5 SEM images of PAA–PEO/Na–MMT nanocomposite fibers with 0.0 (a), 1.0 (b), 2.0 (c), and 4.0 wt% Na–MMT (d).

Fig. 6 SEM images of electrospun PAA–PEO interpolymer complex containing 4.0 wt% Na–MMT in the various working distances: (a) 16.0 cm, (b) 17.5 cm, (c) 20 cm and (d) 22 cm.

3.4. Thermogravimetric analysis

Thermogravimetric analysis was conducted to investigate thermal stability of the as-prepared nanofibers. Fig. 7 shows the plots of weight variation versus temperature and the first derivative of the corresponding TGA scan (DTGA) for the clay-containing nanofibers of PAA–PEO with amounts of 0.0, 1.0, 2.0 and 4.0 wt% of Na–MMT. As presented in a previous study,²⁵ investigation of the thermal behavior of PAA–PEO complex (1 : 1 w/w) indicates that two main reaction stages take place during thermal degradation of PAA and PEO complex in an argon atmosphere. The first stage of the thermal degradation in pure PAA–PEO (1 : 1 w/w) nanofibers at around 177.2 °C corresponded to the breaking of chemical bonds along with the formation of cyclic structures (glutaric anhydride).^25,43 The next step of the thermal degradation at about 341.0 °C, is related to destruction of carboxylic groups and CO₂ elevation, as well as main chain scission. The thermal behavior of PAA–PEO/Na–MMT composite nanofibers containing 1.0 and 2.0 wt% clay platelets was similar to those of pure PAA–PEO nanofibers and the thermal degradation started at about 176.3 °C and 183.2 °C, respectively. The as-spun composite nanofibers with 4.0 wt% Na–MMT exhibited different degradation behavior, where degradation in the first stage takes place at higher temperature (about 197.7 °C). Furthermore, in comparison with PAA–PEO/Na–MMT containing 0.0, 1.0 and 2.0 wt% Na–MMT, which revealed residual weights of about 14.7%, 17.1% and 17.9% at 500 °C, respectively, the residual weight in the PAA–PEO/Na–MMT is about 30.2%. Enhanced thermal stability in the composite nanofibers, especially in PAA–PEO/Na–MMT with 4.0 wt% clay is suggested to be the result of dispersion/orientation of exfoliated platelets induced by high tension forces in the electrospinning process.

Fig. 7 TGA (a) and DTGA (b) curves of pure PAA–PEO nanofibers and PAA–PEO/Na–MMT nanocomposite fibers.

3.5. Differential scanning calorimetric analysis

The melting point of PAA–PEO/Na–MMT nanofibers was investigated by DSC, as is observed in Fig. 8. Pure PAA–PEO fibers showed a peak of the melting transition at 53.8 °C. This peak shifted to 63.8 °C, 63.3 °C and 59.9 °C for PAA–PEO/Na–MMT composite nanofibrous membranes with 1.0, 2.0 and 4.0 wt% MMT, respectively. Improvement in the melting temperature of the as-spun composite fibers is suggested to be because of the well-arranged polymer chains and clay platelets by the electric field (SEM and TEM images).

Fig. 8 DSC curves of PAA–PEO nanofibers and PAA–PEO/Na–MMT nanocomposite fibers.

4. Conclusions

The well-dispersed/oriented exfoliated silicate platelets aligned in the direction of the fiber axis were obtained successfully by electrospinning. SEM images show well-distributed clay nanomaterials in electrospun fiber composites even at 4.0 wt% Na–MMT concentration. TEM images as well as XRD patterns confirmed the well-dispersed/oriented exfoliated clay platelets. The as-spun PAA–PEO/Na–MMT composite nanostructures exhibited improved thermal properties assigned to uniformity in morphology, which could be useful for practical applications.

Abbreviations

C [°C]	Celsius temperature
D [nm]	Fiber thickness/diameter
d [cm]	distance
H [h]	Time
M [mol L^−1]	Molarity
Mw [g mol^−1]	Molecular weight average
R [mL h^−1]	Injection rate
S [s]	Time
t [min]	Time
V [mL]	Volume of solution
Wt [g or mg]	Weight percent
ΔE [kV]	Applied voltage
ΔV [keV]	Accelerator voltage
λ [Å]	Diffraction angle
θ [°]	Wavelength of X-ray diffraction

Acknowledgements

This study was supported by Kharazmi Technology Development (SPEC). The authors would like to thank especially Ms Barbora Ehrlichová for proofreading.

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