Phase-separated nanocomposites

A layered particle-reinforced bionanocomposite, also known as a layered polymer nanocomposite (LPN), can be classified into three subcategories depending on how the particles are dispersed in the matrix. Intercalated nanocomposites are produced when polymer chains are intercalated between sheets of the layered nanoparticles, whereas exfoliated nanocomposites are obtained when there is separation of individual layers, and flocculated or phase-separated nanocomposites are produced when there is no separation between the layers due to particle-particle interactions. This last class of composites is often named microcomposites as the individual laminas do not separate, thus acting as microparticles dispersed in the polymeric matrix. Their mechanical and physical properties are poorer than exfoliated and intercalated nanocomposites [17, 20, 21, 36, 37]. Figure 11.1 shows a schematic drawing of the structure of layered nanocomposites. 

Tian et al. [56] have studied poly(G-caprolactone)-silica and Sengupta et al. [57] have investigated nylon 66-silica hybrid systems and have observed that the phase separation started when Si/H20 mole ratio is increased above 2 and the resultant hybrid films become opaque. Gao [11] has reported similar observations on sol-gel-derived ionomeric polyethylene-silica system. A wide range of literatures is not available on this topic of mbber-silica hybrid nanocomposites, though Bandyopadhyay et al. [34,35] have reported the hybrid formation with different TEOS/H2O mole ratios from ACM and ENR and also demonstrated detailed structure-property correlation in these systems. The hybrids have been prepared with 1 1, 1 2, 1 4, 1 6, 1 8, and 1 10 TEOS/H2O mole ratios. Figure 3.14 shows the morphology of the ACM-silica hybrid composites prepared from different TEOS/H2O mole ratios. 

It did not give rise to phase separation or precipitation. Similar behavior was observed when other types of polysaccharides were examined [53,54]. By now all the commercially important polysaccharides have been applied to the fabrication of hybrid silica nanocomposites in accordance with Scheme 3.2. What is more, various proteins have been entrapped in silica by the same means. In all instances the THEOS demonstrated good biocompatibility with biopolymers, even though its amount in formulations was sometimes up to 60 wt%. Biopolymer solutions after the precursor admixing remained homogeneous to the point of transition into a gel state. 

The ethylene glycol-containing silica precursor has been combined, as mentioned above, with most commercially important polysaccharides and two proteins listed in Table 3.1. In spite of the wide variety of their nature, structure and properties, the jellification processes on addition of THEOS to solutions of all of these biopolymers (Scheme 3.2) had a common feature, that is the formation of monolithic nanocomposite materials, proceeding without phase separation and precipitation. The syner-esis mentioned in a number of cases in Table 3.1 was not more than 10 vol.%. It is worthwhile to compare it with common sol-gel processes. For example, the volume shrinkage of gels fabricated with the help of TEOS and diglyceryl silane was 70 and 53 %, respectively [138,141]. 

Several main synthesis methods widely applied to produce carbon nanotube-polyurethane nanocomposites were summarized above. In addition, latex technology (27), thermally induced phase separation (28), electrospinning (29,30) and many other methods also show their own advantages and promises, however, these methods will not be discussed here. 

The mechanical properties of the nanocomposites strongly depend on their structure, orientation of the filler, phase separation, and processing conditions. Hence, there is a need for in situ nondestructive characterization technique to probe the internal stress in nanocomposite structures. The shortcomings of many conventional techniques such as low resolution, destructive measurements, complex modeling and applicability to only certain class of materials are overcome by using pRS owing to the sensitivity and nondestructive measurement for monitoring internal stress in various materials [59]. 

Sol-gel-derived materials are popular for developing sensors.16 40"13 The physicochemical properties within these sol-gel-derived nanocomposites is an important factor in designing platforms for sensing applications.18 20 22 44-53 Factors such as polarity, microviscosity, pore size, pore wall chemistry, microscopic phase separation, partition coefficient, and solubility coefficient can dramatically alter the behavior of active dopants within a nanocomposite and may lead to undesirable properties. 

Fig. 2 Different paths to obtain hybrid materials from molecular sources. Path A Sol-gel routes (Al conventional route for hybrid nanocomposites, A2 molecularly homogenous hybrids). Path B Assembly of nanobuilding blocks (ANBB), of prefunctionalized or postfunctionalized clusters or nanoparticles. Route C or D involve the use of templates capable of self-assembly, giving rise to organized phases. Path E involves integrative synthesis combining precedent paths from A to D and other processes, such as the use of lithography, casting, organogels or latex beads as templates, controlled phase separations, or external fields. (From Ref. l) (View this art in color at www.dekker.com.)...

Work on supramolecular shirctures has opened access to useful nanocomposites by influencing phase-separating fluids. Simulations show tliat when low-volume fractions of nanoscale rods are immersed in a binary, phase-separating blend, the rods self-assemble into needle-like, percolating networks. The interconnected network arises tlir ough the dynamic interplay of phase-separation between the fluids, through preferential adsorption of... 

Polymeric nanocomposites are an important class of new emerging nanomaterials that exhibits remarkable improvanents of material properties compared with conventional micro- and macrocomposite materials. The small dimension of the filler particles and, accordingly, large surface of the phase separation give the final product characteristics, which considerably exceed traditional ones at minimal filler concentration (Mikitaev et al. 2008). The formation of the polymeric nanocomposite may be represented as the process of filling of the free space in disperse phase with polymer in the form of melt or solution or with monomer followed by its in situ polymerization by chemical or radiation influence on the formed composite structure. The scheme of the polymeric nanocomposite synthesis under radiation is shown in Figure 18.5. 

Figure 9.2 is a schematic representation of CdSe QDs dispersed in poly(hexyl methacrylate) by in situ polymerization. The polymer with long alkyl branches is expected to prevent or reduce phase separation of the QDs from the polymer matrix during polymerization. This technique resulted in the preparation of a series of QD-based nanocomposite materials for which laser scanned confocal microscopy imaging revealed a nearly uniform dispersion of nanoparticles within the polymethacrylate matrix (Fig. 9.3). Notably, the resulting macroscopic QD-polymer composites appeared to be clear and uniformly colored.

Nanocomposites with carbon nanotubes have been an area of considerable R D ever since the excellent electrical and mechanical properties of carbon nanotubes were demonstrated. However, attempts to prepare carbon nanotube RPs often result in phase separation of the CNT and polymer phases causing premature material failure. Researchers at Nomadic Inc. and Oklahoma State University developed a layer-by-layer (LBL) assembly process that permits preparing polyelectrolyte/CNT RP with a CNT loading greater than 50 wt%. The excellent mechanical properties of these materials can be improved further by additional chemical action crosslinking of the CNT and polymer phases and by parallel alignment of the CNTs. The LBL method has been used to prepare various types of RPs. 

Three main types of structures, which are shown in Fig. 5.3, can be obtained when a clay is dispersed in a polymer matrix (1) phase-separated structure, where the polymer chains did not intercalate the clay layers, leading to a structure similar to those of a conventional composite, (2) intercalated structure, where the polymer chains are intercalated between clay layers, forming a well ordered multilayer structure, which has superior properties to those of a conventional composite, and (3) structure exfoliated, where the clay is completely and uniformly dispersed in a polymeric matrix, maximizing the interactions polymer-clay and leading to significant improvements in physical and mechanical properties [2, 50-52]. Production of nanocomposites based on polymer/clay can be done basically in three ways (a) in situ polymerization, (b) prepared in solution and (c) preparation of the melt or melt blending [53]. 

In recent years, supercritical technology, especially supercritical carbon dioxide (scCCb), has been widely applied in the processing of polymer nanocomposites. A supercritical fluid is defined as "any substance, the temperature and pressure of which are higher than their critical values, and which has a density close to, or higher than, its critical density" (Darr Poliakoff, 1999). Fig. 3 shows a schematic representation of the density and organization of molecules of a pure fluid in solid state, gas state, liquid state and the supercritical domain. No phase separation occurs for any substance at pressures or temperatures above its critical values. In other words, the critical point represents the highest temperature and pressure at which gas and liquid can coexist in equilibrium. 

Disregarding of cause of this effect, the sonication of CNTs in the presence of polymers that are structurally close to the matrix polymer was reported as a desirable approach to ensures compatibility of the flinctionaliznotubes with the polymer matrix to avoid any potential microscopic phase separation in the nanocomposits6,37,38], Interfacial adhesion of polymer matrix to nanotube without any moieties used for functionalizing and/or solublizing also reduce the impurity of final nanocomposite and improve the charge and load transfer efiicency. 

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