Biopolymers for composites
The effort to clarify the meanings of the terms ‘clay’, ‘clays’, and ‘clay minerals‘ was the subject of the joint nomenclature committees (JNCs) of the Association Internationale pour l’Etude des Argiles (AIPEA) and the Clay Minerals Society (CMS). The JNCs have proposed the term a class of hydrated phyllosilicates forming the fine-grained fraction of rocks, sediments, and soils and have defined ‘clay’ as ‘‘a naturally occurring material composed primarily of fine-grained minerals, which is generally plastic at appropriate water contents and will harden when dried or fired’’ . According to this definition synthetic clays and clay-like materials are not regarded as clay even though they may be fine grained, and display the attributes of plasticity and hardening on drying and firing.
For phyllosilicates, the terms ‘‘planes’’, ‘‘sheet’’ and ‘‘layer’’ refer to specific parts of the structure, with atomic arrangements that increase in thickness and they cannot be used interchangeably [2,3].
The phyllosilicates are divided by layer type, and within the layer type, and by groups based on charge p.f.u. Further subdivisions by subgroups is based on dioctahedral or trioctahedral character, and finally by species based on chemical composition. Two types of layers, depending on the component sheets are a ‘1:1 layer’ consisting of one tetrahedral sheet and one octahedral sheet, and a ‘2:1 layer’ containing an octahedral sheet between two opposing tetrahedral sheets.
The structure of 2:1 phyllosilicates is composed of tetrahedral (T) and octahedral (O) sheets (Fig. 1). According to the AIPEA Nomenclature Committee , tetrahedral sheet is composed of continuous two-dimensional corner-sharing tetrahedra [TO4]4- involving three basal oxygens and the apical oxygen. The tetrahedral sheet has a composition of [T4O10]4- where T = Si4+, Al3+, Fe3+. The apical oxygens form a corner of the octahedral coordination unit around larger octahedral cations. The octahedral sheet consists of two planes of closely packed O2-, OH- anions of octahedra with the central cations Mg2+ or Al3+. The smallest structural unit contains three octahedral sites. The
Smectites are 2:1 phyllosilicates with a total (negative) layer charge between 0.2 and 0.6 p.f.u.. The octahedral sheet may either be dominantly occupied by trivalent cations (dioctahedral smectites) or divalent cations (trioctahedral smectites).
Vermiculites are generally trioctahedral and are termed (according to the joint nomenclature committees AIPEA and CMS) on the basis of a negative layer charge, which is between 0.6 to 0.9 p.f.u. [4,7,8]. The negative layer charge of vermiculites results from the substitution of Si4+ by trivalent cations in tetrahedral positions [9,10]. Vermiculites were mostly formed by removal of potassium from biotite, phlogopite or muscovite [5,6] and therefore vermiculite chemistry from this perspective is closely linked to that of mica. The thickness of the structural unit (2:1 layer and interlayer space) is about 1.4 nm, depending on the water interlamellar layers and the interlayer cations. Bailey  and Lagaly  described a method to determine the layer charge based on the measurement of basal spacings after exchange with alkylammonium cations of varying chain lengths. Alkylammonium ions in the interlayer spaces of vermiculite acquire distinct arrangements: monolayer and two-layer (bilayer) structures with the alkyl chains parallel to the surface, pseudotrimolecular layers and paraffin type structures. Vermiculites have paraffin-type interlayers if the layer charge is at least 0.75 p.f.u.
The identification of trioctahedral and dioctahedral vermiculites is based on the position of reflection (060) on their XRD patterns. Dioctahedral vermiculites show an interlayer space value
Vermiculite structures contain water interlamellar layers which are subjected to the hydration and dehydration processes [19-26]. The hydration properties are controlled by the interlayer cations Mg2+ and minor amounts of Ca2+, Na+, and K+. The cation radius and charge influence the degree of hydration state in the interlayer and the stacking layer sequences [9,15,19,22,28]. The hydration state of vermiculite was defined by the number of water layers in the interlayer space. The basal space of Mg-vermiculite was declared 0.902 nm for zero-water layer, 1.150 nm for one-water layer and 1.440 nm for two water layer hydration state .
Ordering in vermiculite layered structure occurs when the pseudohexagonal cavities (made of six-membered tetrahedra rings) are facing each other in adjacent tetrahedral layers. In most cases the vermiculites have various possible layer-stacking sequences and therefore the regular arrangement of the layers in vermiculites occurs rarely [30-33].
The structure of vermiculite is called semi-ordered when the transition from a layer to the next layer can be obtained in two or more different ways. For semi-ordered stacks the reciprocal space cannot be described by a set of
The structural formula of vermiculites is often reported on the basis of the structure unit (half unit-cell content). The general formula can be written as:
X4 (Y2-3 ) O10 (OH)2 M.
where M is exchangeable (Mg2+, Ca2+, Ba2+, Na+, K+) cations positioned in the interlayer space, that compensate negative layer charge, Y is octahedral Mg2+,Fe2+ or Fe3+, Al3+, and X is tetrahedral Si, Al.
The half unit cell compositions of vermiculites given in literature are listed below for comparison.
(Si2.86 Al1.14)(Mg2.83Al0.15Fe3+0.02 )O10(OH)2 Mg0.41 3.72 H2O (Llano), 
(Si2.72 Al1.28)(Mg2.36Al0.16Fe 0.58)O10(OH)2 Mg0.32 4.32 H2O (Kenya), 
(Si2.72 Al1.28)(Mg2.59Fe2+0.03 Al0.06Fe3+0.24 Ti0.08)O10(OH)2 Mg0.39 Ca0.02 4.7 H2O (Santa Olalla, Spain), 
(Si2.64Al1.36)(Mg2.48 Fe2+0.04 Al0.14 Fe3+0.32Ti0.01Mn0.01)O10(OH)2 Mg0.44 (Santa Olalla, Spain), 
(Si2.69Al1.31)(Mg2.48Fe3+0.324Fe2+0.036Al0.14 Ti0.01 Mn0.01)O10(OH)2Mg0.39, (Santa Olalla, Spain), 
(Si2.83Al1.17) (Mg2.01Al0.2Fe2+0.16 Fe3+0.40 Ti0.14)O10(OH)2Mg0.235 (Ojen, Spain), 
(Si2.83Al1.17)(Mg2.01Fe3+0.4Fe2+0.16Al0.2Ti0.12) O10(OH)2Mg0.275 (Ojen, Spain), 
(Si2.64 Al1.36)(Mg2.38Fe2+0.02Al0.06Fe3+0.51Ti0.03)O10(OH)2 Mg0.35 Ca0.01Na0.01 4.9 H2O (Letovice, Czech Republic), .
(Si3.02Al0.98) (Mg2.27Al0.12Fe3+ 0.28 Fe2+0.05Ti0.07)O10(OH)2Ca0.09Na0.21K0.50 (West China), .
(Si3.02Al0.79Ti0.05Fe3+0.14)(Mg2.50Fe2+0.38Fe3+0.09)O10(OH,F)2Ba0.29K0.14Ca0.08 (Palabora, South Africa), .
(Si3.43 Al0.57)(Al0.26 Fe0.32 Mg2.34) O10(OH)2 Ca0.064Na0.016K0.047 ( Brazil), .
2. Obtaining vermiculite small particles
Natural vermiculite flakes are characterized by high values of aspect ratio. In clay science,
Exfoliated vermiculite can be produced either by a thermal or a chemical treatment. Heating a particle of vermiculite rapidly above about 200 °C results in the transformation of the interlayer water into steam.
Preparation of submicron-sized vermiculites is usually accomplished by applying
Progressive amorphization and agglomeration of vermiculite particles takes place when grinding time increases .
Balek et al. found that vermiculite from Santa Olalla after grinding for 2 min increased specific surface area from 1 m2g-1 to 39 m2g-1. Prolongation of grinding time to 10 min led to the formation of amorphous phase and the surface area decrease to 20 m2g-1. The thermogravimetry measurement recorded different thermal behavior of original and milled vermiculite samples. Original vermiculite showed two dehydration steps after heating up to 250ºC and one dehydroxylation step at 900ºC. The grounded sample showed no such steps but dehydration was observed as continuous mass loss from 50 to 350ºC and dehydroxylation was completed at 800ºC.
The particle size reduction by sonication may be accompanied by a change of the redox state and the layer charge of the material. Sonication in a 1:1 mixture (volume ratio) of water and hydrogen peroxide (30% H2O2) is a soft method for particle size reduction of phyllosilicate minerals like vermiculites . The oxidation state of the iron in the high-charge vermiculite from Santa Olalla and the low-charge vermiculite from Ojén was found different. In spite that the chemical composition of vermiculites is similar (see unit half cell composition formulas), the layer charge is different 0.78 and 0.52 p.f.u. for the Santa Olalla and Ojén vermiculite, respectively. Both of the original vermiculites had the specific surface area 1 m2g-1. The area in Ojén-vermiculite increased to 51 m2g-1 after eight cycles of sonication in water and after the same numer of sonication cycles in an aqueous 15% H2O2 to 54 m2g-1. The surface area of the Santa Olalla vermiculite after six cycles of sonication was smaller and decreased to the 36 m2g-1 after sonication in hydrogen peroxide and 38 m2g-1 in water. In both vermiculites, one cycle of sonication provided by half greater value increase in the surface area in water in comparison with sonication in hydrogen peroxide. This difference disappeared after three cycles of sonication. Authors stated that delamination by sonication plays a significant role in particle size reduction and in the increase of the surface areas only after a short time. The pH during sonication in water 6.5 increased after three cycles to 7.5–8 and in hydrogen peroxide from 3.7 to 6.5. After sonication no structural change in Santa Olalla vermiculite was observed in comparison with the Ojén-vermiculite. Whereas the ratio Fe3+/Fetotal was found almost constant in the Santa Olalla vermiculite, it increased from 0.79 to 0.85 in case of the Ojén sample together with a decrease of the layer charge.
Kehal et al.  modified vermiculite from Palabora by the combination of the thermal shock (700°C), chemical exfoliation (80°C in the presence of H2O2) and ultrasonic treatments (20 kHz, H2O or H2O2) to improve the adsorption of boron. Authors found that only 1 h treatment of vermiculite in ultrasound and 35 wt.% H2O2 produced small particles of drastically decreased density from 1.026 g cm−3 for raw vermiculite to about 0.23 g cm−3. Furthermore, it was found by acido–basic titrations, that breaking of the particles by 20 kHz sonication induced the generation of OH groups on the edges of the layers which acted as active adsorption sites.
3. Vermiculite particles as carrier of silver nanoparticles
The negative surface charge that results from the ion substitution or from the site vacancies at the tetrahedral and/or octahedral sheets predetermined the use of layered 2:1 phyllosilicates as substrates for the growth of metallic nanoparticles. Uncompensated charges occur at the broken edges of the clay mineral layers, predominantly at the hydroxyl groups. The cations like Al3+ or Fe3+, which typically occupy the octahedral positions, remain at the crystal edges, i.e. at the Lewis acid sites, where they coordinate water molecules. The exchangeable cations between the layers (at the Brønsted acid sites) compensate the negative charge and may be easily exchanged by other metal cations [63,64]. The reduction of the metal cations on the clay minerals matrix initiates the growth of nanoparticles preferably on the surface, because the interlamellar space of clay minerals limits the particle growth [65-70].
The metal nanoparticles are investigated in the function of biosensors, label for cell and cancer therapeutics. The silver nanoparticles have shown to be a promising antiviral material. The strong toxicity of several silver compounds to a wide range of microorganisms is well known. Silver acts as catalyst of the oxidation of the microorganisms, which leads to the disruption of electron transfer in bacteria [71,72]. Silver offers sufficiently small repulsion to oxygen, thus only a small amount of thermal energy is required to move the atomic oxygen readily through its crystal lattice . Monovalent silver ions have a high affinity to sulfhydryl (–S–H) groups in bacteria cells. The resulting stable –S–Ag bonds inhibit the hydrogen transfer and prevent the respiration and the electron transfer. After exposing silver containing molecular oxygen to aqueous media, the molecular oxygen reacts quickly with sulfhydryl (–S–H) groups on the surface of bacteria and replaces hydrogen . Silver nanoparticles exhibit cytoprotectivity towards HIV-1 infected cells . The biological distribution as well as the potential toxicity of silver nanoparticles on the montmorillonite substrate was studied using Swiss mice . The animal study demonstrated that the Ag-montmorillonite was nontoxic, showed no immune response, exhibited increased blood half-life and neurotransmission. Based on the study authors noted that Ag-montmorillonite enables diverse applications in life sciences such as drug development, protein detection and gene delivery for any organs, lungs and brain in particular.
Silver nanoparticles, size about 2 nm, were usually prepared by chemical reduction from silver nitrate under ultraviolet irradiation . Another method employed reduction with sodium borohydride . During this reaction, irregular silver nanoparticles with the size between 1.5 and 2.5 nm nucleate first. This rapid nucleation was followed by the growth of these nuclei into aggregates of circular particles with the size of approximately 12–16 nm. Several approaches have been investigated to prepare silver nanoparticles with well-defined size and morphology. The available free-network spaces between hydrogel networks helped to grow and to stabilize the nanoparticles [81-86].
The antibacterial activity of silver-montmorillonite and copper-montmorillonite studied Magaňa et al. . The authors stated that the overall antibacterial effect was related to the surface characteristics of the sample and to the quantity of silver. Similarly, the antibacterial behaviour of silver grown attached to copper-palygorskite characterized Zhao et al. . Valášková et al.  have prepared and characterized Ag nanoparticles on vermiculite and compared them with Ag nanoparticles on montmorillonite. The antibacterial activity of both nanocomposite types was tested on two bacteria strains. The Gram negative (G-) strain was represented with two bacteria
Nanocrystalline particles of silver reduced from the solution of silver nitrate on the clay mineral substrates were characterized using TEM and HRTEM. Transmission electron microscopy (TEM) revealed silver nanoparticles grown on the surface of both clay minerals of the mean size between 40 and 50 nm. Small Ag particles were substantially smaller than 20 nm. Furthermore, TEM/HRTEM found essential differences in the size distribution of the Ag particles grown on the surface of the montmorillonite and on the surface of the vermiculite. On montmorillonite, all Ag particles grew with a similar size and were well distributed on the surface (Fig. 2a). Huge Ag particles with the size much larger than 50 nm were observed only on the edges of the montmorillonite flakes. The size of the Ag particles grown on the surface of vermiculite was very heterogeneous (Fig. 2b). Some of nanoparticles were agglomerated or grew together (Fig. 2c). A lot of small particles contained smaller domains with high defect density. Microstructure of the Ag nanoparticles showed crystallites disorientation with a lot of defects, especially different planar defects (Fig. 2d).
Based on the results of the microstructure analysis of silver nanoparticles and clay mineral substrates authors [36, 70] assumed the manner of growth of nanoparticles on the surface of vermiculite and montmorillonite.
The particle growth process began after the previous docking of silver (precipitated from the solution of AgNO3) on the clay mineral surface. During their further growth, the Ag nanoparticles separated the clay mineral crystallites into several domains. Single nanocrystalline particles of Ag grow next to each other and then growing to larger nanoparticles with many microstructure defects. Still, the atomic ordering of the Ag nanoparticles at the silver/clay mineral interface is controlled by the local orientation of the clay mineral matrix. During the further growth, larger Ag nanoparticles re-crystallize by reducing the energy of their internal defects through the rearrangement and through the conservation or formation of the low-energy boundaries (as it was observed by HRTEM).
Differences between silver nanoparticles on the vermiculite and montmorillonite substrates authors confronted with the different sources of negative layer charge. When at dioctahedral montmorillonite negative layer charge results from an octahedral substitution of magnesium for iron and the substitution of Si4+ in tetrahedra is negligible  the trioctahedral vermiculite has a negative charge resulting mainly from the substitution of trivalent cations for Si4+ in tetrahedra . Further it was found that montmorillonite and vermiculite after treatment with the silver nitrate solution released the Na+, K+, Ca2+ and Mg2+ cations from the interlayer space of both clay minerals and from the octahedral positions in the montmorillonite. Such structural change allowed the Ag+ to be incorporated in the host structure of the clay minerals through their surface and edges. The total amount of silver on vermiculite was higher than on the montmorillonite. The size distribution of the Ag nanoparticles was much more homogeneous on the montmorillonite substrate than on the vermiculite. The lower negative layer charge of montmorillonite results from the substitution of cations, which are located at the octahedral positions, is responsible for a uniform size of the Ag nanoparticles. The Ag particles with the size much larger than 50 nm were only on the edges. The negative layer charge of vermiculite as a consequence of the charge on tetrahedra can hold higher content of silver than montmorillonite.
Both sample series of silver/vermiculites and silver/montmorillonites, showed good inhibitory action against the Gram negative bacteria strains. The higher content of silver reduced on vermiculite predestines silver/vermiculite to be stronger antibacterial agent than silver/montmorillonite.
4. Vermiculite nanofillers to polymer/clay nanocomposites
Nanocomposites have at least one ultrafine phase dimension, typically in the range of 1–100 nm, and exhibit other properties with comparison to the micro- and macro-composites. The clay–polymer nanocomposite could be considered as “one-nano-dimensional” because the clay filler has one dimension at the nanometer scale . The high aspect ratio of layered silicate nanoparticles is ideal to modify the properties of the polymer, but the hydrophilic nature of silicate surfaces impedes their homogeneous dispersion in the organic polymer phase . On the other hand it was found that polymers can interact with the external surface of clay minerals and also penetrate into the nano- structural spaces, holes and tunnels .
The polymer nanocomposites exhibit new and sometimes improved properties that are not displayed by the individual phases or by their conventional composite counterparts. Significant improvement of mechanical properties, thermal stability, resistance to solvent swelling and suppression of flammability have been achieved with the only up to 5 wt.% exfoliated layered silicates nanoparticles in polymer matrix [94-97]. The development of clay–polymer nanocomposites at low clay loadings (3–7 wt %) as advanced structural materials brings significant improvements in mechanical strength and stiffness, enhanced gas barrier behavior, reduced linear thermal expansion coefficients, and increased solvent resistance in comparison with pristine polymers.
The hydrophilic clay minerals are commonly treated with ammonium cations with long alkyl chains to improve the compatibility between the silicate layers and the polymer matrix. The methods used for the preparation of clay–polymer nanocomposites, include solvent intercalation, in situ polymerization, and melt-compounding [98-102]. Both solvent intercalation and in situ polymerization allow polymer chains to enter into the galleries of silicate clays. Melt blending is more attractive because the nanocomposites can be processed by conventional methods such as extrusion and injection molding. The influences of the addition of various types of clays on the non-isothermal crystallization process of thermoplastics have been studied by several researchers. Fornes and Paul  found that depending on the dispersion of clays in thermoplastic matrices the clay nanofillers can either promote or retard the crystallization of polymers and observed that the degree of crystallinity of nanocomposites showed a strong dependence on the cooling rates.
Polyethylene (PE) is one of the most widely used polyolefins. The molecular structure is generally simply written by the formula (CH2)n, where n is very large. The PE specimen may contain chains of different lengths . Low-density PE (LDPE) contains many statistically placed paraffinic branches. The defects in the arrangement of chains reduce the structure crystallinity . When mixing PE with hydrophilic clay minerals the additives can play a role of a polymeric surfactant, or they may act as a compatibilizer when mixed with organophilized clay minerals. The polar additives or charge of carriers introduce dipole moments in PE [106-109].
Clay mineral vermiculite (VER) was used as the clay mineral nanofiller into polymer matrix and was intercalated with the maleic anhydride into polyamide , polyethylene , and polypropylene [89, 111]. It should be noted that nanocomposites of polypropylene with nanofiller of VER were prepared without any compatibilizer by solid-state shear compounding (S3C) using pan-mill equipment . Vermiculite particles were only partly intercalated with polypropylene and exfoliated in PP matrix (Shao et al., 2006).
Organovermiculite nanofiller octadecylamine/vermiculite (ODA/VER) was exfoliated in polypropylene (PP) . The organovermiculite nanofillers were prepared in three ways. Sample 1 was original VER (Fig. 3a) intercalated using melt intercalation with octadecylamine (ODA). Sample 2 was VER milled in jet mill and intercalated with ODA (Fig. 3b). Sample 3 was VER exfoliated using an oxidizing agent potassium persulfate  and subsequently intercalated with ODA.
The thermal compounding of the organovermiculites (4 wt. %) with maleated PP caused the partial deintercalation of organovermiculites. As the jet milled original VER flakes have corrugated edges  in PP remained intercalated only in their central parts. The frayed edges and small particles showed somewhat higher interlayer distances and the penetration of polymeric segments from the softened polymer matrix was possible. The sample 2 was very well dispersed within PP matrix. The layered vermiculite structure after exfoliation with potassium persulfate was destroyed  and therefore intercalation of ODA molecules into the interlayer was limited (sample 3). On the X-ray diffraction patterns of composite sample 2 in PP authors identified new reflection of the orthorhombic γ-PP in addition to the α-PP. According to the literature it is known that the formation of γ-PP in α-PP takes place when the VER particles reduce the PP chain mobility within the narrow space surrounded by the dispersed clay mineral particles .
Thermal stability of PP and PP/VER nanocomposites was evaluated by thermogravimetric analysis (Fig. 4).
The thermal resistance of the nanocomposites was evaluated by comparing temperatures at the certain weight loss points. Decomposition of pure PP started at 247°C. The degradation point characterized by onset temperature followed the sequence: sample 3 (286°C) → sample 2 (273°C) → sample 1 (267°C).
At 5 % mass loss, the dispersed exfoliated organovermiculite nanofillers sample 2 showed about 51°C, and sample 3 (weekly exfoliated in PP) even about 65°C higher thermal resistance than pure PP.
This agrees with observations of Gilman , who reported that intercalated clay mineral particles in polymer matrix make composite a much more resistant to temperatures than exfoliated particles.
Depending on the structure of dispersed clay-filler in the polymeric matrix, the composites can be classified as intercalated or exfoliated nanocomposites. Polymer nanocomposites can be prepared by two main processes, that is, in situ polymerization and melt compounding.
Use of nano-sized filler particles to form polymer composites has attracted much attention in recent years because of the potential performance advantages that could create new technological opportunities. Potential benefits include increased mechanical strength, decreased gas permeability, superior flame-resistance, and even enhanced transparency when dispersed nanoclay plates suppress polymer crystallization [119-123].
The key issue is to obtain an effective dispersion and exfoliation of the platelets into the polymer matrix to yield well-aligned, high-aspect ratio particles for mechanical reinforcement or a tortuous diffusion pathway for improved barrier properties . In situ polymerization process consists of intercalation of monomer as precursor species, followed by their polymerization inside the interlayer of clay mineral. Numerous nanocomposite materials were prepared by method with clay having inorganic metallic interlayer cation, which can promote formation of monomer radical inducing its polymerization, or modifying agent for nonpolar polymers. Both thermosets and thermoplastics have been incorporated into nanocomposites.
Simulation techniques become an integral part of experimental techniques, since the information about the spatial arrangement of molecules within the interlayer is hard to obtain without the aid of computer simulation. It offers a range of modeling and simulation methods, covering the length range from the subatomic quantum scale, through the molecular level, to the micrometer scale. Methods to model the behavior of systems on each of these scales are combined with analytical instrument simulation and statistical correlation techniques, allowing detailed study of structure, properties, and processes. Molecular modeling using empirical force field represents a way to preliminary estimation of the host-guest complementarity and prediction of structure and properties for nanomaterial design in molecular nanotechnology.
5. Industrial vermiculite nanocomposites
The group of vermiculite–polymeric nanocomposites, where vermiculite or its silicate layers in the final composite stage is altering properties of full system, represents economically and technologically unpretentious materials. Vermiculite here enters the composite system unmodified; it means that it keeps its original characteristics. Usually, polymer and/or layered silicate nanocomposites are synthesized from polymeric macromolecules directly or from monomers followed by in situ polymerization to intercalate into and then enlarge the interlayer of the used clay.
The struggle to prepare nanocomposites is affected by many factors such as the blending or reaction time, temperature and the affinity between the host and the matrix. Moreover, the intercalated nanocomposites instead of the exfoliated nanocomposites are commonly gained in most cases. Generally, dispersion as well as delamination of vermiculite in polymer matrices is observed using electron microscopy and XRD techniques.
Three main approaches have been used in the preparation of clay–polymer composites: (1) melt blending, (2) solution blending, and (3) in situ polymerization. However, homogeneity at nanoscale level may not be fully achieved using these methods. One way of approaching this problem is complete delamination of the clay particles to give colloidal dispersions of single layers in a suitable solvent and restacking the layers in presence of guest species.
Utilization of vermiculite in nanocomposite is not as extensive as for montmorillonite. Most of the applications limits low swell ability of vermiculite compare to montmorillonite. However, several main streams of applications were introduced.
The conventional superabsorbents are based on expensive fully petroleum-based polymers. Their production consumes lots of petroleum and their usage can also cause a nonnegligible environment problem. New types of superabsorbents by introducing naturally available raw materials as additives were desired. The incorporation of clays reduces production cost, and also improves the properties (such as swelling ability, gel strength, mechanical, and thermal stability) of superabsorbents and accelerates the generation of new materials for special applications. The properties of traditional superabsorbent could be enhanced by incorporating vermiculite . It is expected that organomodification of vermiculite can further improve dispersion and performance of the resultant nanocomposite. The superabsorbent nanocomposites were prepared from natural guar gum and organovermiculite by solution polymerization and analyses indicated that organovermiculite was exfoliated during polymerization and uniformly dispersed in the polymeric matrix .
In the superabsorbent field recently, much attention has been paid especially to layered silicate as favorable compound of absorbent composites.
Simple mixing technique of vermiculite into polymer matrix could be applied in case of preparation of
Mixing dilution of
However, it is very difficult to prepare an entirely exfoliated nanocomposite using natural vermiculite as nanofiller by conventional techniques. Therefore, vermiculite has been successfully delaminated with acid treatment and used to mix with engineering polymer directly to synthesize polymer–vermiculite nanocomposites [129, 130]. Acid delaminated vermiculite was successfully utilized for preparation of PVA nanocomposite. The properties of composite significantly depend on the preparation procedure for the reason that chemical reactions and physical interactions involved. Two steps preparation, delamination with hydrochloric acid and then addition to the PVA solution using various mixing times. The positive effect of the vermiculite content on the thermal behavior of the PVA–vermiculite blends was observed .
The utility of the
Nanocomposites poly(4-vinylpyridine(VP)) (PVP) and poly(N-vinyl-2-pyrrolidinone(NVP)) (PNVP) both combined with vermiculite have been synthesized by the intercalative redox polymerization of monomer in the gallery of Cu2+ ion-exchanged vermiculite (Cu2+ serves as polymerization agent). The formation of a single filament of the PVP polymer in the vermiculite gallery is confirmed by the increase in gallery spacing of 0.47 nm as indicated by XRD analysis. XRD analysis following intercalative polymerization of PNVP indicates the presence of two prominent peaks with the corresponding basal spacing
The nanocomposites consisting of the emeraldine salt of polyaniline (PANI) and layered vermiculite were synthesized to improve
An increased understanding, on the micro- and nanolevel, of polymers at surfaces and in confined geometries assists in the development and improvement of new technologies. Of special interest is to understand the nature of the glass transition – not fully solved problem of condensed matter physics. By confining molecules in very small spaces (e.g., clay gallery), the existence or not of a length scale associated with molecular motions responsible for the glass transition can be established. The increment of its relaxation strength in the clay has been related to the 2D geometry of the confinement. These polymers have a dipole moment component parallel to the chain and, therefore, the total dipole vector is proportional to the end-to-end vector and so the overall chain dynamics can be measured by dielectric spectroscopy.
Vermiculite was selected for study the molecular dynamics of oligomeric poly(propylene glycol (PG)) (PPG) liquids (Mw = 1200, 2000, and 4000 g mol–1). The thickness of the liquid layers was 0.55 nm in the case of PG and 0.37 nm for 7-PG and PPG. The PG oligomers form a flat monolayer in vermiculite gallery with their methyl groups pointing in the direction of the clay surfaces. In contrast, the monomers are probable orientated either perpendicular to the clay layers or parallel to the layers, but with the methyl groups pointing toward a clay surface. The dynamics generally become slower with increasing chain length but the dynamics of the 7-mers was established to be faster than for the single monomers. A possible explanation for the fact may be that the OH end groups of the monomers are linked together forming a network, and, as a result, slow down the diffusion at temperatures low enough to keep the network structure intact . The PPG confined in Na-vermiculite has been studied by broadband dielectric spectroscopy. In addition to the temperature dependence of the main (α-) relaxation process and the related high-temperature translational or segmental diffusion, the normal mode relaxation process was studied for all samples in both bulk and confinement. For the normal mode process the relaxation rate and the temperature dependence of the relaxation time in the clay is drastically shifted to lower frequencies compared to that of the bulk material. The α-process relaxation time is only slightly affected by the confinement.
The fact that interactions of PPG with the clay surfaces are very weak was implied based on similar temperature dependence of the relaxation time the α-relaxation in both bulk and confinement . Relaxation process corresponds to the molecular motions of translational character and that it is almost unaffected by the present true 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the confinement. Thus, there is no indication, for none of the confined liquids, that the OH-end groups should form strong hydrogen bonds to the clay surfaces. In fact, the rather small effect of the present confinement on the diffusive dynamics and the main relaxation time suggest that the surface interactions are considerably weaker than in many other model systems .
In situ ring-opening polymerization of the
X-ray diffraction (XRD) analysis confirmed that the main peaks of PPy/VMT nanocomposites are similar to the SAM–VMT particles, which reveal that the crystal structure of SAM–VMT is well maintained after the coating process under polymerization conditions and exhibit semi-crystalline behavior. Thermogravimetric analysis showed that the thermal stability of PPy/VMT nanocomposites was enhanced and these can be attributed to the retardation effect of amine-functionalized VMT as barriers for the degradation of PPy. The morphology of PPy/VMT nanocomposites showed the layered structure and encapsulated morphology. The composites possess high electrical conductivity at room temperature, weakly temperature dependence of the conductivity.
Intercalated nanocomposites comprised of poly(propylene carbonate) (PPC) and organo-vermiculite (OVMT) was
6. Polymeric bio-related nanocomposites
|Poly(lactic acid) (PLA)||aliphatic polyester||Fracture fixation, interference screws, suture anchors, meniscus repair|
|poly(glycolic acid) (PLG)||aliphatic polyester||non-woven fibrous fabrics;suture anchors, meniscus repair, medical devices, drug delivery|
|poly(ε-caprolactone) (PCL)||aliphatic polyester||Suture coating, dental orthopaedic implants, bone tissue implantant|
|poly(hydroxylbutyrate) (PHB)||Polyester||material for waste management strategies; biocompatibility in the medical devices|
Many advantages and disadvantages characterize these two different classes of biomaterials. Synthetic polymers have relatively good mechanical strength and their shape and degradation rate can be easily modified, but their surfaces are hydrophobic and lack of cell-recognition signals. Naturally derived polymers have the potential advantage of biological recognition that may positively support cell adhesion and function, but they have poor mechanical properties. Many of them are also limited in supply and can therefore be costly .
During the last decade, significant attention has been focused on biodegradable polymers. Among all these polymers, poly(lactic acid) (PLA) is one of the most promising because it is thermoplastic, biocompatible and has a high strength, a high modulus, and good processability . PLA also has been revealed an inefficient crystallization process for both the lower crystallization rate and crystallinity as compared with other polymers [144-147].
To maximize therapeutic activity while minimizing negative side effects is the main driven force for the continuous development of new controlled drug delivery systems. Because the release of drugs in drug-intercalated layered materials is potentially controllable (Fig.5), these new materials have a great potential as a delivery host in the pharmaceutical field. Calcium clay has been used extensively in the treatment of pain, open wounds, colitis, diarrhea, hemorrhoids, stomach ulcers, intestinal problems, acne, anemia, and a variety of other health issues.
Chitosan is the deacetylated product of chitin, a natural polymer found in the cell wall of fungi and microorganisms. The active groups in the chitosan structure are the free amine groups, located in the C2 position of the glucose residue in the polysaccharide chain, and the hydroxyl groups . The chitosan/vermiculite nanocomposites have been successfully prepared with different modified vermiculite, which was treated by acid, sodium and CTAB cations. The modification and the nano-scale dispersion of the modified vermiculites were confirmed.
Financial support of the Czech Grant Agency (projects GA ČR 210/11/2215) and the IT4Innovations Centre of Excellence (project reg. no. cz.1.05/1.1.00/02.0070) are gratefully acknowledged.
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