Nanocomposite intercalated layered

Nanohybrids can be prepared in the form of intercalated layered nanocomposites produced by co-assembly of guest biomolecules in the presence of exfoliated organoclay sheets (Section 8.4), or by wrapping single biomolecules in ultrathin layers of condensed organoclay oligomers (Section 8.5). Such approaches should provide new general routes towards the development of functional biomaterials with numerous applications. 

Challier, T. and Slade, R. C. T. (1994). Nanocomposite materials - polyaniline-intercalated layered double hydroxides. J. Mater. Chem. 4, 367. 

Rheology of various polymer layered-silicate nanocomposites - intercalated, exfoliated and end-tethered exfoliated (prepared by in-situ polymerization from reactive groups tethered to the silicate surface), have been performed in a conventional melt-state rheometer in both oscillatory and steady shear modes. These experimental studies have provided insight into the relaxation of polymer chains when confined by the layers of inorganic silicates, as well as the role of shear in orienting the layered nanocomposites. 

In this method, clay is first ion-exchanged using an organic compound in order for the monomer to be intercalated into the layers of the clay. The monomers that form the intercalated layer will become a polymerized interlayer. The authors succeeded in producing a polyamide nanocomposite for the first time using this method. The details will be described in Sect. 3. The basic concept of the technique is as follows. 

Figure 15.2 Schematic representation of intercalated and exfoliated nanocomposite from layered silicate clay filler and polymer.

TEM images revealed an intercalated nanocomposite with layer spacings of 5-8 nm. No physical properties of the materials were reported. 

Ai-Vinylcarbazole is polymerized in MMN at 64°C or in benzene at 50°C due to the presence of cations in the MMN. The thickness of the intercalated layer of guest-host particles in this nanocomposite is 33 lOnm, and the conductivity (10 S cm ) is 10 orders of magnitude higher than that of polyvinylcarbazole. 

The bad quality of the 001 reflections above 400°C only allows assumption on the size of the perovskite clusters formed by thermal degradation. Taking into account the average value of dool for the nanocomposite, ca. 14 A, and the layers thickness, ca. 9.6 A, the size of the intercalated layer can be estimated to about 4 to 5 A, if the composite can be described as a regular sequence of silicate layers and oxide layers. 

O. C. Wilson, T. Olorunyolemi, A. Jaworski, L. Borum, D. Young, A. Siriwat, E. Dickens, C. Oriakhi, and M. Lemer, Surface and interfacial properties of polymer-intercalated layered double hydroxide nanocomposites. Applied Clay Science, 15 (1999), 265-79. 

These materials, unlike the other nanophase materials described in this chapter, are nano-sized in only one dimension and thereby act as nanoplatelets that sandwich polymer chains in composites. Mont-morillonite (MMT) is a well-characterized layered silicate that can be made hydrophobic through either ionic exchange or modification with organic surfactant molecules to aid in dispersion [5,23]. Polymer-layered silicates may be synthesized by exfoliation adsorption, in situ intercalative polymerization, and melt intercalation to yield three general types of polymer/clay nanocomposites. Intercalated structures are characterized as alternating polymer and siHcate layers in an ordered pattern with a periodic space between layers of a few nanometers [13], ExfoHated or delaminated structure occurs when silicate layers are uniformly distributed throughout the polymer matrix. In some cases, the polymer does not intercalate... 

Bio-nanocomposites can be prepared by several methods which include in situ polymerization, solution exfoliation, and melt inteicalatioa In the in situ polymerization method, monomers are migrated into the galleries of layered silicates and subsequently polymerized via heat, radiation, or catalyst. In solution exfoliation, layered clays are exfoliated into single platelets. Exfoliation is achieved by dispersing the layered clays in a solvent. The polymer is adsorbed onto the platelets by mixing in the clay suspension. The solvent is removed either by evaporation or by precipitation. In melt intercalation, layered clays are mixed with the polymer matrix in molten state (Zeng et al., 2005). 

Keywords layered fillers, layered silicates, montmorillonite, elastomers, nanocomposites, intercalation, exfoliation, ionic liquids. 

WAXD difiractograms of intercalated or otherwise stacked tactoid structures may show second-order and even third-order reflections, which are assodated to very ordered stacking of the intercalated layers [24,26,28,45-48]. On the other hand, the absence of high order (001) peaks in the WAXD patterns of nanocomposites, especially with low amounts of MMT fillers, can be due to different reasons, such as a decrease in the regularity of the stacking layers or a diminution of the size and number of tactoids with highly parallel stacking, and a consequent increase in the proportion of exfoliated silicate layers dispersed in the polymer matrix [27,49]. 

Nanometer-scale composites prepared from layered inorganic materials, especially clay, and polymers have also attracted much attention because of their unique optical, thermal, mechanical, gas barrier, and electrical properties. There are many reports describing polymer-clay nanocomposites. " The clay can be, for example, a sihca or silicate. In such a hybrid composite, weak dipolar and van der Waals forces provide the driving force for interactions between the layers, and they result in galleries being formed. There are three types of clay-polymer composites conventional, intercalated, and exfohated. Three mediods are widely used for the preparation of polymer-clay hybrid nanocomposites intercalation by in situ polymerization, direct intercalation, and polymer melt intercalation. Each of these methods has its advantages and disadvantages. For example, the in situ polymerization works only in tiie... 

Dietsche, F., Thomann, Y, Thomann, R., Mulhaupt, R. Translucent acrylic nanocomposites containing anisotropic laminated nanoparticles derived from intercalated layered silicates , Jb wa/ of Applied Polymer Science, 2000,75,396-405. 

Albrecht M., Ehrler S. andMuhlebachA. (2003), Nanocomposites from Layered Silicates Graft Polymerization with Intercalated Ammonium Peroxides. Macromol Rapid Commun, 24, 3S2-7. 

In general, according to the different dispersion states of the LDH layers in the polymer matrix, two main types of polymer-LDH nanocomposites, intercalated and/or exfoliated nanocomposites, can be obtained. In an intercalated nanocomposite, the dispersion of the LDH in the polymer matrices is similar to those in traditional microcomposites however, the interlayer of LDH particles is intercalated by a few molecular layers of polymer. In an exfoKated nanocomposite, the LDH particles are completely exfoliated and the individual LDH layers are uniformly dispersed in a continuous polymer matrix. Usually, the exfoliated nanocomposites attract more attention because they have nanoscale dispersion of high aspect ratio LDH layers in polymer nanocomposites and thus can give some improved properties compared with microdispersed and conventional composites. 

X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses are generally applied for the characterization of polymer-LDH nanocomposites. XRD analysis can provide useful information on the types of the layered structure, i.e. intercalated and/or exfoliated structures, because the peaks change with the gallery distance of the LDHs. However, the XRD results cannot provide a complete picture of the state of the nanocomposites. Moreover, for the exfoliated nanocomposites, LDH layers are often randomly dispersed in the polymer matrices and the basal spaces are generally larger than 10 nm, which is beyond the limit that the XRD can analyze. Therefore, to obtain complete information on the materials, actual images, such as TEM images, are required. 

It should be noted that an obvious weight loss occurs at about 120-300 °C in all TGA curves for PS-LDH nanocomposites. This weight loss has been ascribed to the evaporation of physically absorbed water in the intercalated layers, the loss of hydroxide on LDH layers and the thermal decomposition of LG alkyl chains. In contrast, no weight loss was observed in the temperature range when the surfactant-free PS-LDH nanocomposite prepared by the soap-free emulsion method was heated. This result indicates that the thermal decomposition of LG alkyl chains is the main reason for the weight loss. 

Figure 11.1. The first type is conventional composites, where non-swollen clay tactoids with layers are dispersed simply as a segregated phase in a polymer matrix, which results in poor mechanical properties of the composite material. The second type is intercalated polymer-clay nanocomposites, which are well-ordered multilayered structures formed by the insertion of polymer chains into the gallery space between parallel individual clay layers. Although the layer spacing of clay increases, there are still attractive forces between the silicate layers to stack the layers with uniform spacing. The last type is exfoliated polymer-clay nanocomposites, in which the clays are well dispersed in the polymer matrix. Compared with the intercalated polymer-clay nanocomposites, the layer spacing in the exfoliated polymer-clay nanocomposites increases to the point that there are no longer sufficient attractions between the silicate layers to maintain a uniform layer spacing. The last two hybrid structures are nanocomposites and the exfoliated polymer-clay nanocomposites are especially desirable for improved properties because of the homogeneous dispersion of clay and huge interfacial area between the polymer and clay.

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