2: Recent Progress on Nafion-based Nanocomposite Membranes for
2.4 Preparation and Characterization of Nafion-based Nanocomposite
2.4.1 Clay-containing Nafion-based Composite Membranes
In recent years, one of the most extensively used nanoparticles for the preparation of polymer nanocomposites is the layered silicate or clay. The layered silicate particles generally used for the preparation of nanocomposites belong to the same general family of 2:1 layered or phyllosilicates.[31, 32] The layer thickness is around 1 nm, and the lateral dimensions of these layers may vary from 30 nm to several microns or larger depending on the particular layered silicate. Montmorillonite (MMT) is the most commonly used clay mineral for the preparation of nanocomposites.[31, 32]
Scheme 2.2 Schematic illustration of three different types of thermodynamically achievable polymer/layered silicate nanocomposites [48b]. Reproduced from Sinha Ray and Okamoto with permission from American Chemical Society, USA.
The presence of high aspect ratio (length to width) filler is expected to decrease significantly the methanol cross-over of Nafion membrane as a result of a much longer path way. The success of this strategy relies, however, on the degree of dispersion of silicate layers in the polymer matrix, which may lie between three typical situations such as (i) microcomposites, where the silicate layers are not intercalated by the polymer chains, (ii) intercalated nanocomposites, where insertion of polymer chains into the silicate structure occurs in a crystallographically regular fashion, regardless of polymer to layered silicate ratio, and a repeat distance of a few nanometers, and (iii) exfoliated nanocomposites, in which the individual silicate layers are separated in polymer matrix by average distances that totally depend on the layered silicate loading (see Scheme 2.2). [32–47]
For the barrier properties of the polymer matrix to be improved, particularly towards methanol permeability, the fine dispersion of the silicate layers in the Nafionmatrix is a prerequisite for maximizing the tortuosity factor () that retards the progress of molecules through the matrix resin. According to the Nelsen model, when plates of length (L) and width (D) of the layered silicate are dispersed in the polymer matrix, then the tortuosity factor () can be expressed as: [31]
= 1+ (L/2D) (2.8)
where is the volume fraction of dispersed layered silicate particles into the polymer matrix.
Two particular characteristics of layered silicates that are generally considered for the preparation of PLS-nanocomposites: the first is the ability of the silicate particles to disperse into the polymer matrix as individual layers. The second characteristic is the ability to modify their surface chemistry through ion exchange reactions with organic and inorganic cations. These two characteristics are, of course, interrelated since the degree of dispersion of layered silicate in a particular polymer matrix depends on the interlayer cation.
One successful method to prepare layered silicates containing polymer nanocomposites is to intercalate polymer chains into the silicate galleries. Generally, the intercalation of polymer chains into the silicate galleries is done by using one of the following two approaches: the insertion of suitable monomers in the silicate galleries and subsequent polymerization or direct
insertion of polymer chains into the silicate galleries from either solution or melt. [32] Recently, the melt intercalation technique has become the main stream for the preparation of polymer nanocomposites because it is quite compatible with the recent industrial polymer processing techniques. This method involves annealing, statically or under shear, a mixture of the polymer and layered silicate above the softening point of the polymer. During the annealing, the polymer chains diffuse from the bulk polymer melt into the galleries between the silicate layers. [48] On the other hand, during polymer intercalation from solution, a relatively large number of solvent molecules have to be desorbed from the host to accommodate the incoming polymer chains. The desorbed solvent molecules gain one translational degree of freedom, and the resulting entropic gain compensates for the decrease in conformational entropy of the confined polymer chains.[48]
Therefore, there are many advantages to direct melt intercalation over the solution intercalation.
For example, direct melt intercalation is highly specific for the polymer, leading to new hybrids that were previously inaccessible. In addition, the absence of a solvent makes direct melt intercalation an environmentally sound and an economically favorable method for industries from a waste perspective.[31–33] Up to this date various research groups used different methods for the fabrication of layered silicates containing Nafion membranes. H+–MMT is a proton conductor with reported ionic conductivities of ~10–4 S/cm at room temperature. [49]
In 2003 Jung et al. first reported the preparation of Nafion/MMT composite membranes for fuel cell applications.[20] They used both pristine and organically modified MMT (oMMT) for the preparation of nanocomposite membranes. In a typical preparative method, a perfluorosulfonyfluoride copolymer and MMT were first mechanically mixed by internal mixer.
The prepared composite was then pulverized by universal grinder and was taken in a stainless steel grinder and preformed as a sheet shape of 130–140 m thick by hot pressing at temperatures of about 200–230C at 6000 psi. The composite material based membranes were fabricated by annealing the membrane at a temperature greater than the glass transition temperature (Tg) of copolymer resin. The protonation of composite membranes were made by immersing the membrane in a solution of 20% sodium hydroxide (NaOH) and MeOH.
The XRD patterns revealed that the polymer chains were not intercalated into the two–
dimensional gallery of silicate when pristine MMT was used for the preparation of
nanocomposite. However, XRD patterns of oMMT based nanocomposites showed the formation of intercalated nanocomposite.
Figure 2.3 TEM micrographs for Nafion membranes filled with 2 wt% of: (a) CNa+, (b) CH+, (c) C25A, and (d) C30B. Reprinted from Ref. [21] with permission from Elsevier Science Ltd.
Thomassin et al. [21, 50, 51] prepared clay-containing Nafion nanocomposite membranes by melt- mixing a Nafion precursor (R1100 pellets) with clays. They have used four different types of oMMT such as (a) pristine MMT (CNa+), (b) H-substituted MMT (CH+), (c) MMT modified with nonfunctional alkyl-ammonium (C25A), and (d) MMT modified with a hydroxyl- containing alkylammonium (C30B) for the preparation of nanocomposites with Nafion. TEM micrographs for four different types of composite membranes are shown in Figure 2.3. C30B is homogeneously dispersed within the Nafion matrix, although the nanoclay particles are not completely exfoliated. Individual platelets can be observed within stacks, which results in a highly anisotropic filling of the polymer. This improvement in dispersion is due to the favorable
interactions between the –OH groups of C30B and the sulfonic acid functions of Nafion.
Dispersion of C25A is not as good as that one of C30B, this might reflect the negative impact on the shorter spacing between the nonfunctional alkyl-ammonium in C25A on the interaction with the sulfonic acid function of Nafion matrix. The barrier properties were also improved as assessed by lower MeOH permeability. This improvement was, however, observed at the expense of the ionic conductivity, more likely as a result of the exchange of protons of Nafion with much less mobile ammonium cations of the nanoclay. The best performances were observed with C30B content as low as 0.5 wt%.
However, the addition of MMT particles, whether organically modified or not, results in decreased ionic conductivity in parallel to lower methanol permeability.[21, 50, 51] Rhee et al. have made claims on overcoming this drawback by grafting an organic sulfonic acid containing compound onto the surface of the alumina-silicates layers by silane condensation. [52] But this did not yield significant improvement on the ionic conductivity of composite membrane.
In another method, a Nafion solution in DMA was ultrasonically mixed with organically modified MMT clay (here Cloisite® C10A, C10A) at an elevated temperature.[53] Nafion/MMT composite membranes were casted on a glass substrate above 100C in a vacuum oven. The casted composite membranes were boiled in 1 M hydrogen peroxide and rinsed with deionised water. Finally, the composite film was boiled in 1 M sulphuric acid, followed by rinsing with deionised water several times in order to remove the excess acid.
Figure 2.4 represents the XRD patterns of Nafion/MMT composite membranes at different clay loadings. Compared with unmodified MMT, the relatively large initial interlayer spacing of the modified MMT was observed, and it generally makes polymer chains intercalation much easier.
Both neat Nafion and Nafion/MMT composite membranes showed featureless diffraction patterns in the region from 2 = 2 to 10 until the loading of MMT reached 20 wt%. This strongly implies that a disordered and exfoliated nanocomposite was formed.
Figure 2.4 X-ray diffraction patterns of recast Nafion membrane and Nafion/MMT nanocomposite membranes. Reprinted from Ref. [53] with permission from Elsevier Science Ltd.
Like Jung et al.,[20] Silva et al.[54] also used the solvent casting method to prepare MMT containing Nafion membranes and results showed that the addition of a very small amount of MMT (~1 wt%) did not alter the ionic conductivity, however, the technique or their method produced a marked decrease in MeOH permeability.
Kim et al. attempted to overcome the problem of MeOH permeability while at the same time increasing the ion conductivity of the composites membrane by preparing Nafion/sulfonated clay (S–MMT) nanocomposite membranes via the film coating process.[55] Although, their attempt was not successful, their results, however, showed that many factors strongly affect MeOH permeability as well as the conductivity of the membranes. Results also showed that the humidity should be considered and controlled during synthesis and characterization membranes, especially for mechanical properties improvements.
Recently, Zhang et al. used sol-gel method for the fabrication of clay containing Nafion composite materials.[56] In a reported method, a desire amount of oMMT was added to a Nafion solution, then stirred mechanically for 8 h followed by ultrasonication degassing so as to make the contents of oMMT (based on the Nafion resin) in the mixture 3, 5, 8, and 10 wt%, respectively. The prepared mixture was then slowly poured into a Petri dish to form a composite membrane with a thickness of about 0.1 mm. The filled glass dish was placed in a vacuum oven and dried slowly by increasing the temperature from 80 to 130C to prevent the formation of crevices in the composite membrane. The residual solvent in the composite membrane was fully removed by evacuation at 130C for 12 h. The composite materials were finally obtained by annealing the membranes at temperatures higher than the Tg of Nafion resin.
XRD studies showed that the d001-spacing of oMMT in nanocomposite increased moderately compared to that of neat oMMT and d001-spacing of oMMT in nanocomposite systematically increases with oMMT loading. This observation indicates that the side chain groups of Nafion resin are probably not intercalated into the two-dimensional gallery of oMMT and most of the Nafion chains are probably located outside the galleries of oMMT.
In a recent report, Alonso et al.[57] report the preparation of Nafion/MMT nanocomposite membranes by the solvent casting method. They used H+ exchanged MMT clay for the preparation of the nanocomposite membranes. To find out the effect of the solvent on the properties of composite membranes, two different types of casting techniques were used. The first involves heating the solution over a glass substrate in an oven at 75°C overnight, and in the second procedure, the dispersion was placed under high pressure at 180C and 180 psi for 8h. To maintain a constant pressure of 180 psi and to keep the environment inert, dry nitrogen gas was continuously fed to the chamber.
The morphology of the nanocomposite membranes as a function of clay loading was investigated with small-angle X-ray scattering (SAXS), and results showed that the composite membranes are composed of polymer aggregates oriented parallel to the membrane surface. Results also showed that clay particles are also tending to orient parallel to the surface. With increase in clay loading in the composite membranes, the ionic cluster peak tends to shift monotonically to a higher
scattering vector value. Because of this preferred orientation of the clay particles, membranes are much stiffer and can withstand much higher temperatures compared to the neat polymer based membrane.
The same group also reported the preparation of Nafion/MMT hybrid membranes using the depletion aggregation of suspended particles method.[58] This phenomenon is very popular in the colloids area. In a typical preparative method, a known amount of Na+–MMT was added to the Nafion solution, the mixture was stirred at 70C for 24 h, and then left untouched for seven more days. Membranes were then fabricated by casting the Nafion/Na+–MMT mixture on to a Teflon mold and placing it into a high-pressure chamber at 160C and 160 psi (under dry nitrogen) for 6 h. Morphological analyses using XRD patterns and cryo-TEM images showed that the clay particles in the hybrid gels form a network structure with an average cell size of 500 nm.