Introductory Chapter: Pillarized Montmorillonite Clay – A Possible Host for New Materials

1. Introduction: pillarized montmorillonite clay - Synthesis and characterization

In the last three decades, many efforts have been made to the synthesis of conducting polymers with improved thermal, mechanical, and electrical properties. A promising way to acquire this purpose is by using clays as hosts of formed polymers or place to be used for the intercalation of monomers followed by polymerization [1, 2, 3]. Mainly smectite clays have been used when the confinement of the polymeric chains is desirable. The smectite clays are structurally formed from MO₄(OH)₂ groups of octahedral symmetrically bound to two MO₄ tetrahedral sheets producing layers designated T:O:T. The octahedral sites have ions such as aluminum, magnesium, and iron, while the tetrahedral centers accommodate silicon and aluminum. As a result, the layers have a negative charge with a parallel orientation, and the electric charge is neutralized by exchangeable hydrated positive ions in the interlayer space. This interlayer space has a lot of water molecules, both interlayer ions and water can be exchangeable with simultaneous change of the interlayer distance. These layered crystalline silicates have a great capacity for surface adsorption and catalytic activity in organic reactions, also including stabilization of radicals formed during polymerization (see Figure 1).

Figure 1.

Schematic representation of T:O:T structure of smectite clay group.

Montmorillonite (MMT) is the most used smectite clay, its negative layer charge arising mainly from the substitution of octahedral aluminum by magnesium (II) ions. Our group has been preparing and characterizing polymer-clay nanocomposites at least the last two decades [4, 5, 6, 7, 8, 9, 10]. The materials have been studied mainly by Raman spectroscopy and X-ray absorption techniques. Raman spectroscopy uses a laser as a source to probe the matter, the radiation inelastic scattered by samples is the Raman signal (stokes and anti-stokes components, see Figure 2). By the scattering process, the data about the vibrational modes can be extracted. Additionally, depending on the electronic structure of the sample, the Raman signal can be amplified by resonance and the vibrational states from excited states can be studied. Hence, by changing the laser energy, it is possible to investigate different structures of the intercalated monomer and polymer. Another powerful technique arises from a synchrotron radiation facility. In these machines, it is possible to tune the adequate photon energy values in relation to a specific element absorption edge. As a result, all possible atoms in the sample can be investigated separately in clay or its polymer-clay material by X-ray absorption techniques. An X-ray absorption spectrum (XAS) is a consequence of the core electron excitations to molecular unoccupied states (or extended states in the case of solid samples). The absorption process at the N K shell (see Figure 2) can be visualized when the photon energy is transferred to a core electron to the atom with sudden changes in the absorption coefficient. The spectra show a lot of information about the electronic states of monomer/polymer and also the clay or other kinds of host.

Figure 2.

(a) Schematic representation of ground and excited electronic states and their respective vibrational levels (the levels are out of scale). The arrows indicated transitions among levels. For Raman scattering, the laser line (ν₀) can be inelastically scattered by molecular vibrations and produce the Raman spectra (the scattered frequency is composed of stokes ν_s and anti-stokes ν_as components). If the laser line has energy like an electronic transition of material, the signal can be intensified by resonance process, this is the resonance Raman effect. (b) N K XANES experiments must be done in ultrahigh vacuum (the pressure inside the chamber is ca. 10⁻⁷ mbar). The measured signal is proportional to the absorption intensity [10, 11]. The arrangement used in our experiments was fully explained elsewhere [10, 11]. Schematic representation of the transitions from 1 s to π* and σ* states of a molecular material (the energy values are related to polyaniline in its emeraldine base form) is also displayed.

Polymer-clay nanocomposites can exhibit better physical properties than their macroscopic composites. The possibility to make a real technological impact is the main reason for preparing new nanocomposites with clays. The polymer confinement into clay layers turns the polymeric chains more organized into nanoscale, as a consequence, this new arrangement at the molecular level creates specific supramolecular arrangements with an improvement of the polymer bulk properties. Several polymers have been synthesized into clay galleries, such as poly(ethylene oxide), poly-(styrene), and conducting polymers such as poly(aniline), poly(pyrrole), poly(o-methoxyaniline), and poly(2-ethynylpyridine) [12, 13, 14, 15, 16]. Although many efforts have been made to develop nanocomposites, nowadays is very difficult to predict and control their properties.

Pillarized clays are materials that have permanent porosity, obtained by the introduction of chemical compounds that function as molecular pillars between the clay lamellae, keeping them apart and giving rise to micropores. The chemical compounds that function as supports, or molecular pillars, between the clay lamellae are called pillaring agents (see Figure 3). The simple introduction of the pillaring agent by ion exchange gives rise to intercalated clays. In our study, the Keggin ion Al₁₃⁷⁺ was used to pillarize the MMT clay (see Figure 3). The study of pillarized clays began in the 1970s with the global oil crisis, due to the need to search for materials potentially applicable in oil cracking that had larger pores than those of the well-known zeolites [17, 18].

Figure 3.

Schematic representation of pillarization process of MMT clay. First step: ion exchange, sodium or potassium ions are replaced by Keggin ions Al₁₃ polycations ([Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ [4]. Typically, MMT-Al₁₃⁷⁺ solution can be prepared by a mixture of a volume of 100.0 mL of an aqueous solution containing the Al₁₃⁷⁺ cation (see Refs. [17, 18, 19] for preparation of solution containing Al₁₃⁷⁺ ions) with 140.0 mL of an MMT clay aqueous suspension (see Refs. [4, 5] for preparation of MMT suspension) under stirring and heated at 50°C. Afterward, the resulting suspension was kept under stirring and heating for another 6 h. Then, the suspension was filtered (0.22 μm Millipore filter), and the solid material was washed with deionized water and dried in a vacuum desiccator with silica gel. Second step: After intercalation, the modified clay is calcinated and there is the formation of stable pillars between the layers.

The objective of the pillarization process is to provide microporosity to the clay, creating materials containing pores with dimensions complementary to those of zeolites, that is, larger than 7 and smaller than 20 Å. If the pillaring agents are distributed homogeneously over the surface of the lamellae, a two-dimensional channel system will be created. It is also necessary that the clay lamella is rigid and does not bend; that the adsorption of pillaring agents on the external surface is negligible, and that all clay lamellae are pillarized [19].

The pillarization process can be characterized by two main techniques: X-ray diffraction (XRD) and Thermogravimetric (Tg) analysis. For instance, Figure 4 shows the X-ray diffraction patterns of the Na⁺-MMT clay before and after the pillarization procedure. It is clearly noted the shift of the d₀₀₁ peak (see Table inside Figure 4) of MMT clay occurs after the pillaring method. This displacement corresponds to an increase in the interlayer space of 4.5 Å, confirming the success of the pillarization process. Thermogravimetry analysis (Tg) can also be used to confirm the pillarization. For Na⁺-MMT clay, there is a loss of water up to a temperature around 100°C, later stabilizing up to a temperature around 650°C, when dihydroxylations occur in its structure (see Figure 5). In pillarized clay, there is a loss of water up to a temperature of around 250°C, when successive dihydroxylations occur whereby the pillar is destroyed, proving pillarization in the clay (see Figure 5).

Figure 4.

XRD patterns of powdered samples of (a) Na + −MMT and (b) PIL-MMT. Inside the figure is placed a table with d₀₀₁ values observed for both samples. XRD patterns of powdered samples were obtained on a Rigaku diffractometer model Miniflex using Cu Ka radiation (1.541 Å, 30 kV, 15 mA, step of 0.05^o).

Figure 5.

TG curves of (a) Na⁺-MMT and (b) PIL-MMT powdered samples. The data were acquired in a Shimadzu TGA-50 by using 10^oC/min heating rate and synthetic air atmosphere.

2. Polyaniline-pillarized montmorillonite clay: an example for new material

Nowadays, the preparation of polymer-clay nanocomposites or materials is an exemplary case of novel bidimensional materials based on clays and clay minerals that will warrant further applications from environmental to industrial concerns. Here, we will show an example that has been studied in our lab and can be replicated for many other cases.

Aniline was added to the PIL-MMT aqueous and then a suspension of An⁺-PIL-MMT was polymerized with potassium persulfate. The solid material (PANI-PIL-MMT) was then characterized by spectroscopic techniques. The FT-Raman technique has been used in our group for systems where the fluorescence is very high. For some clays and materials derived from clays, there is the presence of high intrinsic fluorescence, this is the case for PIL-MMT samples and derived materials. FT-Raman uses exciting radiation close to the infrared region (1064.0 nm) and the problem of fluorescence is bypassed for a major part of cases.

For instance, Figure 6 shows that there is a shift in the bands of the polymer obtained in the pillared clay when compared to the Raman spectrum of polyaniline emeraldine-salt (PANI-ES)—both obtained with exciting radiation in 1064 nm. It is noted that the whole spectrum pattern of PANI-PIL-MMT is like the PANI-ES, the highly doped PANI form, spectrum. Thus, the bands at 1179, 1329, 1372, 1515, and 1623 cm⁻¹ can be associated to the similar bands from PANI-ES at 1172, 1324, 1354, 1505, and 1620 cm⁻¹. For PANI-ES, bands from 1324 to 1354 cm⁻¹ can be assigned to νC–N modes of polarons, and these νC–N bands shift to 1326–1375 cm⁻¹ when the PANI-ES is submitted to mild deprotonation [21], as a consequence of the formation of small conjugated polarons.

Figure 6.

FT-Raman spectra of powdered samples of PANI-ES, PANI-PIL-MMT, and PANI-EB. PANI has different oxidation and protonation states; the emeraldine base state (PANI-EB) is in which the number of reduced or benzenoid units is equal to oxidized or quinoid units. The conducting form of PANI as the emeraldine salt (PANI-ES) has benzenoid, radical cation, and dications segments (see Scheme) [20]. FT-Raman spectra (1064.0 nm laser line) were acquired from powdered samples in a Bruker RFS 100/S FT-Raman spectrometer at room temperature (near 20°C).

However, two bands at 1487 and 1580 cm⁻¹ in the Raman spectrum of PANI-PIL-MMT can be related to the PANI-EB sample (deprotonated form of PANI) at 1460 and 1586 cm⁻¹. These bands are attributed to νC=N and νC=C of quinoid groups [21]. Hence, the PANI was obtained into PIL-MMT probably in a form intermediated to the PANI-ES and PANI-EB forms. It is important to stress that the values of bands of inserted PANI into PIL-MMT can also be affected by the interaction between PANI chains and PIL-MMT walls [22, 23, 24]. In the UV-vis-NIR (not shown), it is not noted PANI bands, however, this behavior is due to two major factors: low stability of PANI-PIL-MMT aqueous suspension and low formation of PANI into PIL-MMT (despite visual color change of sample before and after polymerization).

3. Conclusion and future remarks

In this brief chapter, we give an introductory overview of the use of pillarized MMT clay as a host for the synthesis of polymer-clay nanocomposites. Considering this example and many other cases related to our group, there are two central questions that are possible to address: (i) the molecular structure of the polymer is drastically changed inside the interlayer cavity of clay and (ii) by using the appropriate synthetic or heating route is possible to change the molecular structure of the confined polymer. X-ray diffraction and thermogravimetry can be used in combination to confirm the successful pillarization of the montmorillonite clay. Nowadays, we are changing the experimental conditions to maximize the PANI formation, but many other systems can be prepared following this suggestion. We really think that the polymer-clay area has much more to be investigated. In the last two decades, our group has deeply studied PANI-MMT materials and derivates. These studies have revealed the importance of the use of advanced and complementary spectroscopic techniques to give a more realistic view of the molecular structure formed into the clay layers. The chains formed inside the clay layers can present different segments and molecular arrangements compared to the free polymer. The new Raman instruments can give better Raman imaging of the samples, and open the possibility to study inhomogeneity, chemical modifications and many other aspects of the polymer-clay materials. In addition, new Synchrotron light sources will permit to study changes in these complex materials by in situ measurements, resulting in spectral and imaging data. These new data are crucial for better application design and more vast use of materials derived from polymer-clays.