Nanocomposites exfoliated

Exfoliated nanocomposites Exfoliated nanocomposites are those where the degree of intercalation is so high that distance between the layers increases to a great extent and they no longer remain as stacked strucmre (Figure 2.6b). 

When a solvent diffuses across a neat polymer, it must travel the thickness of the sample (do). When the same solvent diffuses through a nanocomposite film with nanoclays, its path length is increased by the distance it must travel around each clay layer it strikes. According to Lan et al. [99] the path length of a gas molecule diffusing through an exfoliated nanocomposite is... 

Novel room-temperature-vulcanized silicone mbber-organo-MMT nanocomposites were prepared by a solution intercalation process by Wang et al. [104]. A new strategy was developed by Ma et al. [105] to prepare disorderly exfoliated nanocomposites, in which a soft siloxane surfactant with a weight-average molecular weight of 1900 was adopted to modify the clay. 

Polyimide-clay nanocomposites constitute another example of the synthesis of nanocomposite from polymer solution [70-76]. Polyimide-clay nanocomposite films were produced via polymerization of 4,4 -diaminodiphenyl ether and pyromellitic dianhydride in dimethylacetamide (DMAC) solvent, followed by mixing of the poly(amic acid) solution with organoclay dispersed in DMAC. Synthetic mica and MMT produced primarily exfoliated nanocomposites, while saponite and hectorite led to only monolayer intercalation in the clay galleries [71]. Dramatic improvements in barrier properties, thermal stability, and modulus were observed for these nanocomposites. Polyimide-clay nanocomposites containing only a small fraction of clay exhibited a several-fold reduction in the... 

Most nanocomposite researchers obdurately believe that the preparation of a completely exfoliated structure is the ultimate target for better overall properties. However, these significant improvements are not observed in every nanocomposite system, including systems where the silicate layers are near to exfoliated [29]. While, from the barrier property standpoint, the development of exfoliated nanocomposites is always preferred, Nylon 6-based nanocomposite systems are completely different from other nanocomposite systems, as discussed [3,8]. 

The kinetics of PAA, synthesized from 4,4 -oxydianiline and pyromellitic dianhydride, solid-state imidization both in filler absence and with addition of 2 phr Na+-montmorillonite was studied [1], The nanofiller was treated by solution of P-phenylenediamine in HC1 and then washed by de-ionized water to ensure a complete removal of chloride ions. The conversion (imidization) degree Q was determined as a function of reaction duration t with the aid of Fourier transformation of IR-spectra bands 726 and 1014 cm 1. The samples for FTIR study were obtained by spin-coating of PAA/Na+-montmorillonite mixture solution in N,N-dimethylacetamide on KBr disks, which then were dried in vacuum for 48 h at 303 K. It was shown, that the used in paper [1] method gives exfoliated nanocomposites. The other details of nanocomposites polyimid/Na+-montmorillonite synthesis and study in paper [1] were adduced. The solid-state imidization process was made at four temperatures 7) 423, 473, 503 and 523 K. 

Du L, Qu B, Meng Y, Zhu Q (2006) Structural characterization and thermal and mechanical properties of poly (propylene carbonate)/MgAl-LDH exfoliation nanocomposite via solution intercalation. Compos Sci Technol 66 913-918... 

In exfoliated nanocomposites, delaminated structures are obtained if a large number of polymer chains are present between the layers and the layers stand >10 nm apart. Thus, the interlayer expansion is comparable to the radius of gyration of the polymer rather than that of an extended chain, as in the case of intercalated hybrids [35]. 

Maiti et al. [64] took typical fluorocarbon-based exfoliated nanocomposites and observed that they reagglomerate while they are processed in a hydraulic press during the curing operation. Their XRD studies revealed that the level of exfoliation was, however, not disturbed if oven curing was performed, instead of press curing. 

The lowering of die swell values has a direct consequence on the improvement of processability. It is apparent that the processability improves with the incorporation of the unmodified and the modified nanofillers. Figure lOa-c show the SEM micrographs of the surface of the extrudates at a particular shear rate of 61.2 s 1 for the unfilled and the nanoclay-filled 23SBR systems. The surface smoothness increases on addition of the unmodified filler, and further improves with the incorporation of the modified filler. This has been again attributed to the improved rubber-clay interaction in the exfoliated nanocomposites. 

Dispersion The degree of dispersion of the nanoplatelets is determined by the degree of delamination of the clay. The fully delaminated (exfoliated) nanocomposite presents much higher values for the tortuosity factor and the aspect ratio in comparison with the partially delaminated (intercalated) nanocomposite. This means that the clay particles that grow as aggregates or books of sheets must be broken up or exfoliated into individual sheets that have a thickness of the order of 1 nm, with lengths and widths of the order of 500 nm. 

Su, S., Jiang, D. D., and Wilkie, C. A. Novel polymerically-modified clays permit the preparation of intercalated and exfoliated nanocomposites of styrene and its copolymers by melt blending, Polym. Degrad. Stab. (2004), 83, 333-346. 

Polyvinyl chloride-montmorillonite nanocomposites were prepared either by in-situ suspension polymerisation using initiator- and comonomer-modified montmorillonite in the presence of free-radical initiators, such as AIBN, and compared. It was found that monomer conversion was low when the nanocomposites were prepared using initiator-modified montmorillonite. Exfoliated nanocomposites... 

Exfoliated composites in an exfoliated nanocomposite, the individual clay layers are separated in a continuous polymer matrix by average distances that depend on loading. Usually, the clay content of an exfoliated composite is much lower than that of an intercalated nanocomposite. 

Fig. 30 A,B. Proposed model for the fracture of A a glassy B a rubbery polymer-clay exfoliated nanocomposite with increasing strain...

It is generally accepted that thermal stability of polymer nanocomposites is higher than that of pristine polymers, and that this gain is explained by the presence of anisotropic clay layers hindering diffusion of volatile products through the nanocomposite material. It is important to note that the exfoliated nanocomposites, prepared and investigated in this work, had much lower gas permeability in comparison with that of pristine unfilled PE [12], Thus, the study of purely thermal degradation process of PE nanocomposite seemed to be of interest in terms of estimation of the nanoclay barrier effects on thermal stability of polyolefin/clay nanocomposites. 

Both intercalated and exfoliated nanocomposites, containing 3-5% of nanoparticles (w/w), reportedly show better or comparable flame resistance compared with plastics filled up to 30-50% with traditional flame retardants. Another way to increase flame retardancy is to combine ATH or magnesium hydroxide with organo-clays. It was reported that organoclays and some classical flame retardants, such as brominated compounds, showed a synergism between them [13]. 

The rheological behavior of these materials is still far from being fully understood but relationships between their rheology and the degree of exfoliation of the nanoparticles have been reported [73]. An increase in the steady shear flow viscosity with the clay content has been reported for most systems [62, 74], while in some cases, viscosity decreases with low clay loading [46, 75]. Another important characteristic of exfoliated nanocomposites is the loss of the complex viscosity Newtonian plateau in oscillatory shear flow [76-80]. Transient experiments have also been used to study the rheological response of polymer nanocomposites. The degree of exfoliation is associated with the amplitude of stress overshoots in start-up experiment [81]. Two main modes of relaxation have been observed in the stress relaxation (step shear) test, namely, a fast mode associated with the polymer matrix and a slow mode associated with the polymer-clay network [60]. The presence of a clay-polymer network has also been evidenced by Cole-Cole plots [82]. 

Improvements in the reduction of flammability of polymers with low clay contents and better processability have been reported, in addition to reductions in the concentration of toxic vapors produced in the combustion stage [116-120]. In connection to their flame-retardant properties, exfoliated nanocomposites based on PP [121, 122, 115, 123], PS [115, 123, 124], poly(ethylene-vinyl acetate) [125, 126], styrene-butadiene rubber [127], PMMA [128], polyesters... 

MEH-PPV clay hybrid nanocomposite materials prepared by in situ polymerization in organically modified montmorillonite show a comparatively higher current and a lower tum-on voltage. By controlling the ratio of monomeric precursors to montmorillonite, exfoliated nanocomposites can be obtained. In intercalated structures, the polymer chains are merely inserted into the interlayer spaces of the montmorillonite. However, in the exfoliated structure, two-dimensional nanospaces will no longer be present. 

Intercalation refers to the inclusion of molecules between the layers. Exfoliation refers to the separation of the layers. In other words, intercalated polymer chains are sandwiched in between clay layers. In exfoliated nanocomposites the individual layers are separated, and somewhat uniformly dispersed in the polymeric matrix. This class of materials has been widely investigated because of their unique properties. The efficiency of PPV-related electroluminescent devices is summarized in Table 3.5. 

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