1. Introduction
1.1 Clays and clay materials
The synthesis and study of materials based on clays and/or clay minerals with polymers are one of the widest fields of research in materials science and technology. Characterization followed by application of many derived clay materials is a big deal in clay science and technology. Figure 1A shows that in 2021, more than 16,000 papers having “clay” and “polymer” as keywords were published since 1998. In addition, Figure 1B shows that a group of conducting polymers represents more or less 20% of total polymers studied in composites and/or nanocomposites with clays. In addition, Figure 2 displays that at least 15 different research fields mainly concentrated on chemistry and polymer sciences, environmental sciences, materials and nanotechnology. These two graphs clearly show that polymer clay-derived materials are one of the focuses in the science of advanced materials. Our group has been dedicated to the preparation and characterization of materials consisting of conducting polymers from polyaniline family with clays [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Among the different techniques used for structural investigation, resonance Raman spectroscopy and Synchrotron X-ray techniques are the most important for these systems.
The term “clay” was defined as “…a naturally occurring material composed primarily of fine-grained minerals (< 4μm), which is generally plastic at appropriate water contents and will harden with dried or fired” [12, 13, 14, 15]. Likewise, the term “clay mineral” can be defined as “…phyllosilicate minerals and minerals which impart plasticity to clay and which harden upon drying or firing.” Since the origin of the mineral is not part of the definition, clay mineral (unlike clay) may be synthetic. Hence, analyzing the fine-grained clay particles in detail is only possible by a combination of advanced spectroscopic and microscopic techniques [16]. Clays are aluminosilicates where the aluminum can be replaced by magnesium and iron atoms, resulting in an excess of negative charge that is balanced by alkaline or alkaline earth elements. Each clay has different composition, dehydration properties, structural failure limits, decomposition products, cation exchange capacity (CEC), and other useful properties and economic interests.
Clays layers are formed by tetrahedral (T) and octahedral (O) sheets bonded by oxygen atoms. Unshared oxygen atoms are present in hydroxyl form. Two main arrangements of T and O layers are observed: (1: 1) and (2: 1) (T:O) clays. The (1: 1) group is known as kaolin group, with the general composition of Al2Si2O5(OH)5 and the layer thickness of ~0.7 nm [12, 13, 14, 17]. In addition, the second main arrangement (2: 1) is well known as phyllosilicates, where one octahedral sheet is sandwiched between two tetrahedral sheets (2:1) with a total thickness of 0.94 nm. When silicon in tetrahedral sheets is substituted by aluminum, the 2:1 structure is called mica. The negative charge generated by this change is equilibrated by the presence of potassium cations between the layers. Because of the equal size of K+ cation and the hole created by Si/Al tetrahedral sheets, there is no inter-layer spacing. Hence, no swelling or exfoliation of 2:1 layers is possible. When the aluminum cations in the octahedral sheets are partially substituted by Mg2+ or Fe2+ cations, the smectite clay group is formed, whose structure consists of a central sheet containing octahedral groups (MO4(OH)2) bonding to two tetrahedral layers (MO4) producing layers designated T:O:T [17] (see Figure 3). Ions of aluminum, iron and magnesium occupy the octahedral sites, while the centers have tetrahedrons of silicon and aluminum ions.
The T:O:T layers assume a parallel orientation, and the negative electric charge is neutralized by the presence of hydrated positive ions that are in the interlayer region [17]. These clays have high-surface adsorption capacity and catalytic activity in organic reactions. The montmorillonite (MMT) is the most common smectite clay, and the negative charges of the layer arise mainly from substitution of aluminum by magnesium ions in octahedral sites. In addition, the swelling properties, and thermal, chemical and colloidal dispersion stabilities probably are responsible for the extensive use of the MMT clay (ca. 50%) in the polymer clay-derived materials. In this introductory chapter, we will give en passant about the main points about polymers and clays —a fruitful combination.
1.2 Polymer clay materials
A polymer clay material is made by a physical and/or chemical combination of a polymer and synthetic or natural clay. The clay platelets can improve the mechanical, thermal, gas and fire barrier retardancy properties of the polymer. The improvements depend strongly on mechanical and physical dimensions of the clay, the interfacial adhesion between polymer and clay and especially on the aspect ratio of the clay. The aspect ratio of the clay is very important and crucial for many properties in composites, such as electrical, mechanical and thermal properties [18, 19].
Polymer nanocomposite is defined as a composite material in which at least one dimension of at least one component is in the nanometer size scale (< 100 nm) [20]. In recent years, the nanocomposite research area has created efficient and powerful strategies to upgrade the structural and functional properties of natural and synthetic polymers. Polymer nanocomposites have generated great attention because of superior properties such as strength, toughness and fire resistance far from those of regular micro-composites and comparable with those of metals. The existence of one nanoscale phase tremendously increases the interfacial contact between the polymer and clay. As a consequence, the improvement of the polymer properties, such as mechanical, thermal barrier and flame retardancy, durability and chemical stability, scratch/wear resistance and biodegradability, as well as optical, magnetical and electrical properties, has been observed [21, 22, 23, 24]. Recent investigations using continuum mechanics modeling show that the enhancement of nanocomposites properties is highly dependent on the peculiar features of nanofiller material, in particular, its content, aspect ratio and the ratio of filler mechanical properties to those of the matrix [25].
Polymer clay nanocomposites can be made by direct mixture of two aqueous solutions containing the monomer and the clay suspension, respectively (see Figure 4), followed by polymerization induced by thermal or light sources or by adding chemical oxidants [26, 27]. Afterward, if the major part of the polymer is produced outside the interlayer space, the resulting compound is named as
1.3 Conducting polymer-clay materials
Among the polymers used for production of polymer clay materials are the intrinsically conducting polymers (ICPs). These classes of polymers are conjugated polymers that can be doped by chemical, electrochemical or photochemical processes with an increase of their conductivities (see Figure 5). The doping is reversible, and the polymer can return to its original state without major changes in its structure [29, 30, 31, 32, 33, 34]. In the doped state, the presence of counter ions stabilizes the doped state. All conductive polymers (and their derivatives), for example, poly(aniline), poly(pyrrole), poly(thiophene)s, poly(
The adsorption of aromatic compounds such as aniline on MMT clay was investigated a long time ago, and the clay property to generate colored species by the adsorption of aromatic amines was on the first experimental observation. The well-known behavior is the blue color generated by the adsorption of benzidine (4,4′-diaminobiphenyl) into clay layers. First, investigations reported that films of MMT containing metal ions become black after immersion in aniline, suggesting the polymerization of the monomer [35, 36, 37, 38, 39]. By using resonance Raman spectroscopy (RR), Soma and Soma [40, 41, 42, 43, 44] discovered that the adsorption of liquid aniline on Cu2+ or Fe3+-MMT causes the formation of the polymer. Soma and Soma suggested that the polymeric structure is equal to that generated electrochemically but with the presence of azo bonds (-N=N-). Another important observation is that intercalated PANI showed FTIR bands at 1568, 1505, 1311 and 1246 cm−1 characteristic of the conducting emeraldine state, but they were shifted to higher frequencies to the spectrum of free polymer. According to the authors, this displacement is a consequence of geometric restrictions imposed over the aromatic rings. The intercalation was confirmed by changes in the interlayer distance from 1.47 to 0.36 nm after the polymerization of aniline. Absorption bands were observed at 420 and 800 nm in the UV–vis–NIR spectrum of the material, which are the characteristic of conducting PANI form [45].
In fact, many authors prepared PANI-MMT using ammonium persulphate as an oxidizing agent, and for these authors the electronic spectra and FTIR bands were enough to characterize the PANI in its conductive state (emeraldine salt) [45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55]. However, when our group started to study this material by using resonance Raman spectroscopy (like Soma and Soma one decade earlier [40, 41, 42, 43, 44]) together with X-ray absorption techniques, it was revealed that the structure of intercalated PANI into MMT layers is much more complex and different from the free PANI [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. The early stages of the polymerization of intercalated anilinium ions were monitored by
2. Conclusion and future remarks
We really think that the polymer clay area has much more to be investigated, but with the employment of new synthetic strategies and the use of advanced spectroscopic techniques. The accumulated experience with PANI-MMT materials revealed the importance of the use of advanced and complementary spectroscopic techniques in order to give a more realistic view of the molecular structure formed in the clay layers. The screening of the electronic and vibrational structure of PANI-MMT through resonance Raman and X-ray absorption spectroscopy has been decisive in the determination of their “real” structure and in the study of the interactions between the clay layers and polymer chains. In fact, by selecting the appropriate laser line, it is possible to study in particular each polymeric segment into polymer backbone. 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 enable us to study changes in these complex materials by
References
- 1.
Do Nascimento GM, Constantino VRL, Temperini MLA. Spectroscopic characterization of a new type of conducting polymer–clay nanocomposite. Macromolecules. 2002; 35 :7535 - 2.
Do Nascimento GM, Landers R, Constantino VRL, Temperini MLA. Aniline polymerization into montmorillonite clay: A spectroscopic investigation of the intercalated conducting polymer. Macromolecules. 2004; 37 :9373 - 3.
Do Nascimento GM, Constantino VRL, Temperini MLA. Spectroscopic characterization of doped poly(benzidine) and its nanocomposite with cationic clay. The Journal of Physical Chemistry B. 2004; 108 :5564 - 4.
Do Nascimento GM, Landers R, Constantino VRL, Temperini MLA. Spectroscopic characterization of polyaniline formed in the presence of montmorillonite clay. Polymer. 2006; 47 :6131 - 5.
Do Nascimento GM, Barbosa PSM, Constantino VRL, Temperini MLA. Benzidine oxidation on cationic clay surfaces in aqueous suspension monitored by in situ resonance Raman spectroscopy. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2006; 289 :39 - 6.
Do Nascimento GM, Padilha ACM, Constantino VRL, Temperini MLA. Oxidation of anilinium ions intercalated in montmorillonite clay by electrochemical route. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2008; 318 :245 - 7.
Do Nascimento GM, Temperini MLA. Structure of polyaniline formed in different inorganic porous materials: A spectroscopic study. European Polymer Journal. 2008; 44 :3501 - 8.
Do Nascimento GM, Temperini MLA. Spectroscopic study of the polymerization of intercalated anilinium ions in different montmorillonite clays. Journal of Molecular Structure. 2011; 1002 :63 - 9.
Do Nascimento GM. X-ray absorption spectroscopy of nanostructured polyanilines. Chemical Papers. 2013; 67 :933 - 10.
Do Nascimento GM, Pradie NA. Deprotonation, Raman dispersion and thermal behavior of polyaniline–montmorillonite nanocomposites. Synthetic Metals. 2016; 217 :109 - 11.
Do Nascimento GM. Structure of clays and polymer–clay composites studied by X-ray absorption spectroscopies. In: Do Nascimento GM, editor. Clays, Clay Minerals and Ceramic Materials Based on Clay Minerals 1st ed. London: InTech; 2016 - 12.
Bergaya F, Lagaly G. General introduction: Clays, clay minerals, and clay science. In: Bergaya F, Theng BKG, Lagaly G, editors. Handbook of Clay Science. Amsterdam: Elsevier; 2006. pp. 1-18 - 13.
Hall PL. Clays: Their significance, properties, origins and uses. In: Wilson MJ, editor. A Handbook of Determinative Methods in Clay Mineralogy. Glasgow: Blackie; 1987. pp. 1-25 - 14.
Guggenheim S, Martin RT. Definition of clay and clay mineral: Joint report of the AIPEA nomenclature and CMS nomenclature committees. Clays and Clay Minerals. 1995; 43 :255 - 15.
Moore DM, Reynolds RC Jr. X-ray Diffraction and the Identification and Analysis of Clay Minerals. 2nd ed. Oxford: Oxford University Press; 1997 - 16.
Brown G. Associated minerals. In: Brindley GW, Brown G, editors. Crystal Structures of Clay Minerals and their X-ray Identification. London: Mineralogy Society; 1980. pp. 361-410 - 17.
Yariv S. Introduction to organo-clay complexes and interactions. In: Yariv S, Cross H, editors. Organo-Clay Complexes and Interactions. New York: Marcel Dekker, Inc.; 2002 - 18.
Zhang R, Ni QQ, Natsuki T, Iwamoto M. Mechanical properties of composites filled with SMA particles and short fibers. Composite Structures. 2007; 79 :90 - 19.
Meneghetti P, Qutubuddin S. Synthesis, thermal properties and applications of polymer-clay nanocomposites. Thermochimica Acta. 2006; 442 :74 - 20.
Hussain F, Hojjati M, Okamoto M, Gorga RE. Review article: Polymer-matrix nanocomposites, processing, manufacturing, and Application: An Overview. Journal of Composite Materials. 2006; 40 (17):1511 - 21.
Armentano I, Dottori M, Fortunati E, Mattioli S, Kenny JM. Biodegradable polymer matrix nanocomposites for tissue engineering: A review. Polymer Degradation and Stability. 2010; 95 (11):2126 - 22.
Cosoli P, Scocchi G, Pricl S, Fermaglia M. Many-scale molecular simulation for ABS-MMT nanocomposites: Upgrading of industrial scraps. Microporous and Mesoporous Materials. 2008; 107 :169 - 23.
Ma H, Xu Z, Tong L, Gu A, Fang Z. Studies of ABS-graft-maleic anhydride/clay nanocomposites: Morphologies, thermal stability and flammability properties. Polymer Degradation and Stability. 2006; 91 :2951 - 24.
Pandey JK, Reddy KR, Kumar AP, Singh RP. An overview on the degradability of polymer nanocomposites. Polymer Degradation and Stability. 2005; 88 :234 - 25.
Sheng N, Boyce MC, Parks DM, Rutledge GC, Abes JI, Cohen RE. Multiscale micromechanical modeling of polymer/clay nanocomposites and the effective clay particle. Polymer. 2004; 45 :487 - 26.
LeBaron PC, Wang Z, Pinnavaia TJ. Polymer-layered silicate nanocomposites: An overview. Applied Clay Science. 1999; 15 :11 - 27.
Thostenson ET, Li C, Chou TW. Nanocomposites in context. Composites Science and Technology. 2005; 65 :491 - 28.
Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Progress in Polymer Science. 2008; 32 :1119 - 29.
Shirakawa H, Ikeda S. Infrared spectra of poly(acetylene). Polymer Journal. 1971; 2 (2):231 - 30.
Shirakawa H. The discovery of polyacetylene film: The dawning of an era of conducting polymer (Nobel Lecture). Angewandte Chemie, International Edition. 2001; 40 :2575 - 31.
MacDiarmid AG. ‘Synthetic Metals’: A novel role for organic polymers (Nobel Lecture). Angewandte Chemie, International Edition. 2001; 40 :2581 - 32.
Heeger AJ. Semiconducting and metallic polymers: The fourth generation of polymeric materials (Nobel Lecture). Angewandte Chemie, International Edition. 2001; 40 :2591 - 33.
MacDiarmid AG, Epstein AJ. The polyanilines: A novel class of conducting polymers. In: Conducting Polymers, Emerging Technologies. New Jersey: Technical Insights Inc.; 1989. p. 27 - 34.
Do Nascimento GM, Souza MA. Spectroscopy of nanostructured conducting polymers. In: Eftekhari A, editor. Nanostructured Conducting Polymers. Londres: Wiley and Sons; 2010. pp. 341-375 - 35.
Yariv S, Michaelian KH. Structure and surface acidity of clay minerals Organo-Clay Complexes and Interactíons; Yariv, S.; Cross, H., Marcel Dekker: New York; 2002. p. 1 - 36.
Hauser EA, Leggett MB. Color reactions between clays and amines. Journal of the American Chemical Society. 1940; 62 :1811 - 37.
Yariv S, Heller L, Safer Z. Sorption of aniline by montmorillonite. Israel Journal of Chemistry. 1968; 6 :741 - 38.
Furukawa T, Brindley GW. Adsorption and oxidation of benzidine and aniline by montmorillonite and hectorite. Clays and Clay Minerals. 1973; 21 :279 - 39.
Cloos P, Moreale A, Broers C, Badat C. Adsorption and oxidation of aniline and p-chloroaniline by montmorillonite. Clay Minerals. 1979; 14 :307 - 40.
Soma Y, Soma M. Adsorption of benzidines and anilines on Cu-montmorillonites and Fe-montmorillonites studied by resonance Raman-spectroscopy. Clay Minerals. 1988; 23 :1 - 41.
Soma Y, Soma M. Chemical-reactions of organic-compounds on clay surfaces. Environmental Health Perspectives. 1989; 83 :205 - 42.
Soma Y, Soma M, Harada I. Raman-spectroscopic evidence of formation of para-dimethoxybenzene cation on Cu-montmorillonite and Ru-montmorillonite. Chemical Physics Letters. 1983; 94 :475 - 43.
Soma Y, Soma M, Harada I. The reaction of aromatic-molecules in the interlayer of transition-metal ion-exchanged montmorillonite studied by resonance Raman-spectroscopy. 1. Benzene and para-phenylenes. The Journal of Physical Chemistry A. 1984; 88 :3034 - 44.
Soma Y, Soma M, Harada I. Reactions of aromatic-molecules in the interlayer of transition-metal ion-exchanged montmorillonite studied by resonance Raman-spectroscopy. 2. 4, 4′-Disubstituted biphenyls. The Journal of Physical Chemistry. 1985; 89 :738 - 45.
TeC C, Ho SY, Chao KJ. Intercalation of polyaniline in monmorillonite and zeolite. Journal of the Chinese Chemical Society. 1992; 39 :209 - 46.
Wu Q, Xue Z, Qi Z, Wang F. Synthesis and characterization of PAN/CLAY hybrid with extended chain conformation of polyaniline. Acta Polymerica Sinica. 1999; 10 :551 - 47.
Wu Q, Xue Z, Qi Z, Hung F. Synthesis and characterization of PAn/clay nanocomposite with extended chain conformation of polyaniline. Polymer. 2000; 41 :2029 - 48.
Biswas M, Ray SS. Water-dispersible nanocomposites of polyaniline and montmorillonite. Journal of Applied Polymer Science. 2000; 77 :2948 - 49.
Lee DK, Lee SH, Char K, Kim J. Expansion distribution of basal spacing of the silicate layers in polyaniline/Na+−montmorillonite nanocomposites monitored with X-ray diffraction. Macromolecular Rapid Communications. 2000; 21 :1136 - 50.
Lee D, Char K, Lee SW, Park YW. Structural changes of polyaniline/montmorillonite nanocomposites and their effects on physical properties. Journal of Materials Chemistry. 2003; 13 :2942 - 51.
Yeh JM, Liou SJ, Lai CY, Wu PC, Tsai TY. Enhancement of corrosion protection effect in polyaniline via the formation of polyaniline-clay nanocomposite materials. Chemistry of Materials. 2001; 13 :1131 - 52.
Zeng QH, Wang DZ, Yu AB, Lu GQ. Synthesis of polymer–montmorillonite nanocomposites by in situ intercalative polymerization. Nanotechnology. 2002; 13 :549 - 53.
Kim BH, Jung JH, Joo J, Kim JW, Choi HJ. Charge transport and structure of nanocomposites of polyaniline and inorganic clay. Journal of the Korean Physical Society. 2000; 36 :366 - 54.
Kim BH, Jung JH, Kim JW, Choi HJ, Joo J. Effect of dopant and clay on nanocomposites of polyaniline (PAN) intercalated into Na+−montmorillonite (N+-MMT). Synthetic Metals. 2001; 121 :1311 - 55.
Kim BH, Jung JH, Kim JW, Choi HJ, Joo J. Physical characterization of polyaniline-Na+−montmorillonite nanocomposite intercalated by emulsion polymerization. Synthetic Metals. 2001; 117 :115