Hybrid PCNs

Depending on the nature of the components used (layered silicate, organic cation, and polymer matrix) and the preparation method, three types of hybrid PCNs can be obtained [17]. Phase-separated microcomposites (conventional composites) are obtained when the polymer chains are unable to intercalate within the inorganic sheets clay lamellae remain stacked in structures marked as tactoids as in the pristine mineral. Otherwise, when the polymer chains penetrate in between the clay galleries, an intercalative system is obtained. In this case, the nanocomposite shows, at least in principle, a well-ordered multilayer morphology built up with alternating polymeric and clay layers. When clay platelets are randomly dispersed in the polymer matrix and the lamellae are far apart from each other, so that the periodicity of this platelet arrangement is totally lost, an exfoliated structure is achieved. 

Polymer-clay nanocomposites (PCN) are a class of hybrid materials composed of organic polymer matrices and organophilic clay fillers, introduced in late 1980s by the researchers of Toyota (Kawasumi, 2004). They observed an increase in mechanical and thermal properties of nylons with the addition of a small amount of nano-sized clays. This new and emerging class of pol miers has found several applications in the food and non-food sectors, such as in constmction, automobiles, aerospace, military, electronics, food packaging and coatings, because of its superior mechanical strength, heat and flame resistance and improved barrier properties (Ray et al., 2006). 

In one example, the tensile strength of polyamide 6 was increased by 55% and the moduli by 90%, with the addition of only 4wt% of delaminated clay. The enhanced tensile property of PCN suggests that nanocomposite performance is related to the degree of clay delamination, which increases the interaction between the clay layers and the polymers. Several explanations, based on the interfacial properties and the mobility of the polymer chains, have been given for this reinforcement. Kojima et al. reported that the tensile modulus improvement for polyamide 6-clay hybrid originated from a constrained region, where the polymer chains have reduced mobility. The dispersion and delamination of the clay were the key factors for the reinforcement. The delaminated nanocomposite structure produces a substantial increase in modulus. 

Fig. 26 Schemes of (1) rigid triazine ring-aryl junction as the only element of chemical structure in neat PCN networks, tmd (2) the second, soft junction with PTMG crosslink in the PCN-PTMG hybrid networks [148]...

Fig. 29 SAXS patterns obtained with synchrotron radiation for pure PCN and PCN-PTMG hybrid networks differing in composition and molecular weight of PTMG = l.OOOgmor (a), Mn = l.OOOgmor (b), and iW = S.OOOgmol" (c) [151]. It should be noted that the first curve, from below, relates to neat PCN, and the second curve to 70/30 tmd 60/40 compositions in (c)...

Fig. 30 TSDC thermograms obtained for neat PCN and PCN-PTMG hybrid networks with MqPTMG = l,000gmol and different compositions [151]...

Creep rate spectroscopy study of the PCN-PTMG hybrid networks [148,151] demonstrated most distinctly the complicated, heterogeneous nature of glass transition dynamics in these systems providing the detailed information on this problem. Not only the dynamic heterogeneity (Tg plurality) but also the compositional heterogeneity and dynamics/nanostructure interrelationships were found for these amorphous networks. 

Fig. 32 Creep rate spectra obtained at tensile stress of 0,5 MPa for pure PCN and four PCN-PTMG(IOOO) hybrid networks with different compositions [148,151], The inset shows Tg vs cross-linking degree plot found for neat PCN by Georjon et al, [178,179], On this basis (see text), the...

Table 5 Plurality of glass transitions and local compositional heterogeneity in the PCN-PTMG hybrid and neat PCN networks, as estimated from the CR spectra obtained at tensile stress of O.SMPa[148,lSl]...

Fig. 34 Tensile strength and elongation at break of the PCN-PTMG hybrid networks at 20° C as a function of PTMG content [180].

Figure 58 shows the CR spectra of MMT-containing nanocomposites based on the hybrid 90PCN-10PTMG network, in comparison with those of unfilled network and neat PCN. The introduction of 5 wt% of Cloisite 30B results in a decrease of Tg peak with its shift to lower temperatures. A steep rise of creep rates occurs starting already from ca. 230 °C. Unlike that, the introduction of 2 wt% of Cloisite 30B results in a sharp suppression of matrix dynamics at high temperatures, and a steep rise of creep rates only above 270°C, i.e., not far from Tg of the neat PCN. This assumes simultaneously some microplasticity at moderate temperatures and increased high-temperature performance for this perspective nanocomposite. 

In Japan, digital service was based on a national standard, JDC renamed to PDC, and commenced operation in 1994. Despite export promotion it was not adopted outside Japan. Another Japanese mobile standard, PHS, fared even worse. Two years after its introduction, PHS started to lose subscribers to cellular services. PHS is not a cellular but a cordless standard and its failure is described in the next section. Several alternative systems for mobile telephony have been developed and adopted in countries where the benefit of these systems were high, but which never became widely adopted world-wide. In contrast to cellular technology, other technologies such as the cordless and hybrid system, which were suggested and tested on the market as PCNs, could not gain acceptance. Although these alternative mobile systems were initially successful in several countries, they were -even there - swiftly squeezed out by cellular systems. The next section reviews these alternative systems. 

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