Crystallisation nanocomposite

In nanocomposites, exchange coupling occurs between hard and soft magnetic nanograins. The hard phase provides coercivity and the soft phase contributes to magnetisation enhancement (see section 4.3). The grain size of both phases must be of the order of the hard phase domain wall thickness. The crystallisation behaviour plays an important role in the microstructural control of the magnetic properties. 

Nd-Fe-B and Pr-Fe-B nanocomposites are rare-earth deficient with respect to the R2Fei4B stoichiometry. The soft phase which is formed is Fe3B or a-Fe. The control of the soft grain size is crucial to preserve a significant coercivity. This can be obtained by crystallisation of amorphous ribbons. The annealing temperature must be high enough to allow crystallization of the... 

PP/silver nanocomposite fibres were prepared with the aim of achieving permanent antibacterial activity in a common synthetic textile. The fibres were melt-spun by coextmsion of PP and PP/silver masteibatches using general conjugate spinning. Masteibatches were made up of a mixture of PP chips and nano-sized silver powder. The antibacterial efficacy of spun fibres was high when the masteibatch was used as the sheath rather than the core. The antibacterial activity of nano-silver in fibres was evaluated after a certain contact time and calculated by percent reduction of two types of bacteria. Staphylococcus aureus and Klebsiela pneumoniae. DSC and wide-angle X-ray diffraction were used for analysis of stractuie, thermal properties and crystallisation behaviour of the spun fibres. SEM was carried out in order to observe particle distribution on the nanocomposite fibres. 17 refs. (2nd International Conference on Polymer Fibres, Manchester, UK, July 2002)... 

Polymer clay nanocomposites have, for some time now, been the subject of extensive research into improving the properties of various matrices and clay types. It has been shown repeatedly that with the addition of organically modified clay to a polymer matrix, either in-situ (1) or by melt compounding (2), exfoliation of the clay platelets leads to vast improvements in fire retardation (2), gas barrier (4) and mechanical properties (5, 6) of nanocomposite materials, without significant increases in density or brittleness (7). There have been some studies on the effect of clay modification and melt processing conditions on the exfoliation in these nanocomposites as well as various studies focusing on their crystallisation behaviour (7-10). Polyamide-6 (PA-6)/montmorillonite (MMT) nanocomposites are the most widely studied polymer/clay system, however a systematic study relating the structure of the clay modification cation to the properties of the composite has yet to be reported. 

The crystallisation behaviour of PA-6/MMT nanocomposites is complicated to analyse because its polymorphic nature produces a monoclinic a and psuedohexagonal y structure, both of which are affected by the presence of clay particles in the matrix (9). The clay particles themselves have been shown to act as nucleating agents, increasing the crystallisation process in some instances while retarding crystal growth in others. Clearly, the cause of these phenomena must be understood so some measure of control can be employed to produce useful nanocomposite materials with desired properties. 

In this section, the production of PP nanofibres containing silver nanoparticles using the above technique, together with their characterisations using X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis are presented. Additionally, the antibacterial properties of nanofibres are evaluated using the quantitative American Association of Textile Chemists and Colorists (AATCC) 100 test. The inclusion of nanosilver into polymers to form a nanocomposite has been demonstrated to have a profound effect on the crystallisation of the polymer, which in turn affects the properties of nanofibres, including their antibacterial properties. 

The introduction of conductive fillers in a polymer matrix except for the obvious effect on electrical conductivity which was discussed previously, has the possibility to affect the overall morphology of the nanocomposite. Here though we must note that the changes or their absence in the overall morphology are heavily dependent on the polymer, the type, regularity and size of the filler, the preparation methods as they affect the level of dispersion and the crystallisation conditions (Gkourmpis 2014). 

As a final note we can say that the addition of CNTs in a semicrystalline polymer has the potential to impede, promote or have no effect on the nanocomposite crystallisation behaviour. This of course will be heavily dependent on the preparation method, the level of dispersion and the chemical configuration of both polymer and filler. It has also been suggested that the amount of CNTs in the system might... 

Electrically conductive polymer nanocomposites are widely used especially due to their superior properties and competitive prices. It is expected that as the level of control of the overall morphology and associated properties increases we will see an even wider commercialisation on traditional and totally novel applications. In this section we have discussed the basic principles of the percolation theory and the different types of conduction mechanisms, outlined some of the critical parameters of controlling primarily the electrical performance and we have provided some indications on the effect such conductive fillers have on the overall morphology and crystallisation of the nanocomposite. The latter becomes even more critical if we take into consideration that modem nanosized fillers offer unique potential for superior properties at low loadings (low percolation thresholds) but have a more direct impact on the morphology of the system. Furthermore we have indicated that similar systems can have totally different behaviour as the preparation methods, the chain conformation and the surface chemistry of the fillers will have a massive... 

Nanocomposites formed by polypropylene/clay without change to either polymer or clay were prepared using two methods and their molecular structures investigated solid-state NMR. From the C, Si and Al solid-state NMR spectra, it was found that the crude clay interferes in the polypropylene crystallisation process due to a specific interaction between both components. " ... 

It is significant that the reinforcement degree corresponds to a class of polymer forming a nanocomposites matrix. The largest values of / are obtained for polymers whose chains are able to stretch on the silicate platelet surface (rigid-chain polyimide, crystallising polypropylene and thermotropic liquid crystalline polyester), intermediate values for polymers whose chains are able to stretch only partly (polycarbonate, poly (butylenes terephthalate) and amorphous polyamide-6) and the smallest values for nanocomposites on the basis of epoxy polymer, the capability of chains stretching of which decreases sharply because of the availability of transverse covalent bonds network [30]. 

Eet us consider crystallisation mechanism changes for HDPE/EP nanocomposites in comparison with the initial HDPE, which define the degree of reduction in crystallinity with an increase in c p. As it is known [8], the crystallisation kinetics of polymers is often described with the aid of the Kolmogorov-Avrami equation, obtained for low-molecular substances (Equation 4.19), in which the exponent n value can be changed within the range of 1-4 [9]. 

The dependence K c p), calculated according to Equation 8.2, is shown in Figure 8.3 by a dashed line. As one can see, this dependence demonstrates much slower reduction in K with increase in observed experimentally. This means that for the considered nanocomposites the value of K is defined not so much by the polymer chain flexibility, characterised by parameter C, as by the variation of the characteristics of nucleation and crystallisation mechanisms, from which the exponent n is the most important. 

Hence, the results stated above have shown that reduction in the degree of crystallinity of HDPE/EP nanocomposites is due to variation of the characteristics of nucleation and crystallisation mechanisms. The change of polymer molecular characteristics has less influence on the crystallinity degree. These effects can be accurately described within the frameworks of the fractal model. 

Eigure 8.4 The dependence of crystallisation rate constant z on melt flow index MFI for HDPE/EP nanocomposites [2]... 

This book covers both fundamental and applied research associated with polymer-based nanocomposites, and presents possible directions for further development of high performanee nanocomposites. It has two main parts. Part I has 12 chapters which are entirely dedicated to those polymer nanocomposites containing layered silicates (clay) as an additive. Many thermoplastics, thermosets, and elastomers are included, such as polyamide (Chapter 1), polypropylene (Chapter 4), polystyrene (Chapter 5), poly(butylene terephthalate) (Chapter 9), poly(ethyl acrylate) (Chapter 6), epoxy resin (Chapter 2), biodegradable polymers (Chapter 3), water soluble polymers (Chapter 8), acrylate photopolymers (Chapter 7) and rubbers (Chapter 12). In addition to synthesis and structural characterisation of polymer/clay nanocomposites, their unique physical properties like flame retardancy (Chapter 10) and gas/liquid barrier (Chapter 11) properties are also discussed. Furthermore, the crystallisation behaviour of polymer/clay nanocomposites and the significance of chemical compatibility between a polymer and clay in affecting clay dispersion are also considered. 

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