Crystal structure nanocomposites

Metal oxide nanocomposites were synthesized by electrical discharge method using a combination of aluminum and copper electrodes submerged into water. The crystal structure, lattice parameters and grain size of the nanopowders were determined by XRD using Cu K radiation (Fig. 3b). The XRD pattern exhibited the presence of cubic copper with a lattice constant of 0.3615 nm, as well as aluminum and copper oxide and hydroxide phases. The positions of all peaks were in agreement with the JCPDS standards. 

DSC measurements were performed on a TA Instruments 2190 DSC with temperature and enthalpy calibrations performed using an indium reference. Experiments were performed under a nitrogen atmosphere with a flow rate of 50ml/min. Extruded granules of nanocomposites were heated at 2°C/min to 250°C, held isothermally for 5mins, then cooled to room temperature at 2°C/min. All samples were heated twice, first to examine the properties post extrusion and secondly to examine the preferred crystal structures with slow cooling. 

The crystal structure of the nanocomposites was studied with XRD and DSC. The XRD spectra in Figure 5 shows the effect of alkyl chain length on the different crystal structure formation in the skin (Figure 5a) and at the core (Figure 5b) of the nanocomposites. By observing the diffraction pattern of the injection moulded test bars core (Figure 5b) between 10 and 40°, the relative content of amorphous material and a and y crystals in the polymer matrix can be determined. The peak at 24.6° corresponds to the y crystal structure and the peaks at 23.7° and 27.3° to the ai and a2 crystal in nylon 6 respectively. As reported in previous literature (16-18) the peak at 21.4° corresponds to the amorphous content in the matrix, but it is not prominent in Figure 5b Contrary to this literature however, the a and y peaks are not located at the same 20 values... 

This structural information can also help explain changes observed in the mechanical properties of the nanocomposites. As the amorphous content of the samples decreases from UM to dPC and the material becomes more crystalline, the nanocomposites become stronger. Also in the core of the injection moulded test bars where slow cooling is prevalent, the more stable a structure appears to form readily. As the y crystal structure is said to be more ductile than the a, it would be expected that the tensile strength of materials containing mostly a crystals, like DdPC-OdPC, to be much stronger than those with high levels of y crystal in the core. So not only is the increase in modulus due to the reinforcement provided by the clay layers and increase in crystallinity, but also the reduction in y crystal content. 

For the comprehension of mechanisms involved in the appearance of novel properties in polymer-emhedded metal nanostructures, their characterization represents the fundamental starting point. The microstructural characterization of nanohllers and nanocomposite materials is performed mainly by transmission electron microscopy (TEM), large-angle X-ray diffraction (XRD), and optical spectroscopy (UV-Vis). These three techniques are very effective in determining particle morphology, crystal structure, composition, and particle size. 

Variations in the preparation of nanocomposites have now been investigated extensively. Liu et al. [202] proposed the preparation of nylon-6/clay nanocomposites by a melt-intercalation process. They reported that the crystal structure and crystallization behaviors of the nanocomposites were different from those of nylon-6. The properties of the nanocomposites were superior to nylon-6 in terms of the heat-distortion temperature, strength, and modulus without sacrificing their impact strength. This is attributed to the nanoscale effects and the strong interaction between the nylon-6 matrix and the clay interface. More recently, nanocomposites of nylon-10,10 and clay were prepared by melt intercalation using a twin-screw extruder [203]. The mechanical properties of the nanocomposites were better than those of the pure nylon-10,10. 

The reinforcement of polypropylene and other thermoplastics with inorganic particles such as talc and glass is a common method of material property enhancement. Polymer clay nanocomposites extend this strategy to the nanoscale. The anisometric shape and approximately 1 nm width of the clay platelets dramatically increase the amount of interfacial contact between the clay and the polymer matrix. Thus the clay surface can mediate changes in matrix polymer conformation, crystal structure, and crystal morphology through interfacial mechanisms that are absent in classical polymer composite materials. For these reasons, it is believed that nanocomposite materials with the clay platelets dispersed as isolated, exfoliated platelets are optimal for end-use properties. 

Clays are classified on the basis of their crystal structure and the amount and locations of elelectric charge (defidt or excess) per unit cell. Crystalline days range from kaolins, which are relatively uniform in chemical composition, to smectites, which vary in their composition, cation exchange p>rop>erties, and ability to expand. The most commonly employed smectite clay for the preparation of polymeric nanocomposites is bentonite, whose main mineral component is montmorillonite (Utracki, 2004). 

In the same NMR spectrum the pure polymer resonates at a separate chemical shift value. Host-guest cross peaks are diagnostic of the nanoscale topological relationship, giving an insight into the incommensurate crystal structure. This kind of spectroscopy was extended from polymethylene chains to a number of polymer nanocomposites, including rubbery polymers. The most interesting examples, those are formed with elastomers, where the crystalline adducts act as reinforcement for the elastomeric material [55]. 

Unlike polymer-clay nanocomposites, in rubber-clay nanocomposites complete exfoliation of clay layers results in disappearance of the diffraction maxima in their XRD patterns. However, this can also occur due to other reasons, like extremely low concentration of clay materials in the composites, crystal defects, etc. The majority of the reports on rubber-clay nanocomposites display the intercalated or swollen nature of the clay structures. The presence of the basal reflections in the XRD patterns of such type of nanocomposites indicates that the clay crystal structure is not destroyed completely. But, shifting of their positions to lower 26 values is interpreted as an expansion of the interlayer region by the macromolecular rubber chains. Besides, broadening of the characteristic reflections in nanocomposites is often related to the defects in the crystal layer stacking caused by the interlayer polymeric species. 

Chen J, Chen J, Zhu S, Cao Y, Li H (2011) Mechanical properties, morphology, and crystal structure of polypropylene/chemically modified attapulgite nanocomposites. J Appl Polym Sci 121 899-908... 

In-situ synthesis of caprolactone in the interlayers of Cr -modified fluorohec-torite was reported by Messersmith et al. [35). The microstracture development was studied by using XRD as shown in Figure 1.17. The unintecalated filler had a basal plane spacing of 12.8 A, which was increased to 14.6 A for the intercalated filler swollen with caprolactone. After the polymerization of caprolactone in and around of filler interlayers, a final basal plane spacing of 13.7 A was observed for the nanocomposite. The observed basal plane spacing correlated well with 4-A interchain distance in the crystal structure of poly(caprolactone). 

Shah D, Maiti P, Gunn E, Schmidt DF, Jiang DD, Batt CA, et al. Dramatic enhancements in toughness of polyvinylidene fluoride nanocomposites via nanoclay-directed crystal structure and morphology. Adv Mater August 2004 16 1173-7. 

Thermotropic LC polyester nanocomposites based on a small quantity of multi-walled carbon nanotubes (M WCNTs) can be prepared by in situ polymerization of l,4-bis(4-hydroxybenzoyloxy) butane and terephthaloyl dichloride. Significant change in the crystal structure of LC polyester cannot be observed even after forming the nanocomposite. The evidence from various instrumentation results indicates interaction of MWCNT with the surrounding liquid crystal molecules, most likely through aromatic interactions (H-stacking), The thermal stability and transition temperature of the hybrid is always better than pure LC polyester [71]. 

The new technique involves using chemical precursors, that facilitate the synthesis of nanomaterials containing phases of desired composition and crystal structure. Metalorganic precursors are particularly attractive since they can be used to (i) yield a high fraction of end product and (ii) obtain phases with a selected stoichiometry. Consequently, the possibility now exists for producing nanocomposite materials in significant amounts. These may contain a diverse range of phases such as intermetallic, silidde, boride, nitride, carbide and oxide phases. 

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