Morphology, polymer clay nanocomposites

Solvent-polymer and solvent-clay interactions are very important in determining the morphology of polymer/clay nanocomposites. There are many reports describing the preparation of PNCs by solution mixing [65, 231-235]. Ho and Glinka [38]... 

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. 

The previous session has shown that the preparation method has a fundamental role in the dispersion of the nanofiller and the morphology of the polymer/clay nanocomposite. Different degrees of dispersion are obtained for different systems and, to a lesser extent, even within the same sample (Figure 12.12). 

The main issue in processing of polymer/clay nanocomposites is to achieve sufficient interaction between the nanofiller and the polymer so as to achieve a favourable morphology. Due to the laminar structure of the most common nanoclays, the morphology of nanocomposites can be classified in three different types depending on the structure of the nanoclays and the interaction with the polymer chains aggregated, intercalated and exfoliated. Figure 8.2 shows the three possible morphologies of a polymer/clay nanocomposite. 

Figure 8.2 Possible morphologies of a polymer/clay nanocomposite.

F.C. Bragan9a, L.F. Valadares, C.A.P. Leite, F. Galembeck, Counterion effect on the morphological and mechanical properties of polymer-clay nanocomposites prepared in an aqneous medium. Chemistry of Materials 19 (2007) 3334-3342. 

Processing operations such as injection and compression moulding, extrusion, fibre spuming, etc. are likely involved for the preparation and commercialization of polymer/clay nanocomposites. The flow applied during processing play a decisive role in crystallization kinetics and in the morphology obtained. For neat polymers, it has been demonstrated that the flow accelerates crystallization kinetics when compared with the quiescent melts (Keller and Kolnaar 1997 Kumaraswamy et al. 1999) and some crystallization mechanism has been proposed (Somani et al. 2000 Seki et al. 2002), whereas for polymer/clay nanocomposites, studies are still lacking and most of results have been reported for iPP and nylon 6/clay nanocomposites. 

Van der Hart et al. [51] first used solid-state NMR ( H and as a tool for gaining greater insight into the morphology, surface chemistry and, to a very limited extent, the dynamics of exfoliated polymer/clay nanocomposites. The major objective in solid-state NMR measurement is to connect the measured longitudinal relaxations (T s) of H or nuclei with the quality of clay dispersion [52]. 

David, L. S. and Gupta, R. K. A finite element analysis of the infiuence of morphology on barrier properties of polymer-clay nanocomposites. Excerpt from the Proceedings of the COMSOL Conference, Boston (2007). 

Therefore, polymer/clay nanocomposites can be deflned as a new class of composites with polymer matrices in which the dispersed phase is the silicate constituted by particles that have at least one of the dimensions at nanometer level. One of the components is the matrix, in which the particles of the second material are dispersed. The most used mineral particles in these nanocomposites are smectitic clays (montmorillonite, saponite, and hectorite), having their particles lamellae morphology with sides at micrometer level and thickness around one nanometer (Alexandre and Dubois, 2000 Esteves, Barros-Timmons, and Trindade, 2004). 

The theory, processes, and characterization of short fiber reinforced thermoplastics have been reviewed by De and White [31], Friedrich et al. [32], Summerscales [33], in an introductory text by Hull and Clyne [34], and in a handbook by Harper [35]. Natural fibers and composites have been reviewed by Wallenberger and Weston [36]. The introduction of new composite materials, called nanocomposites, has resulted in new materials that are being applied to various industrial applications. These materials have in common the use of very fine, submicrometer sized fillers, generally at a very low concentration, which form novel materials with interesting morphology and properties. Nanocomposites have been discussed in a range of texts including two focused on polymer-clay nanocomposites by Pinnavaia and Beall [37] and Utracki [38]. 

Polymer-clay nanocomposites are characterized by improved thermal, mechanical, barrier, fire retardant, and optical properties compared to the matrix of conventional composites, commonly called particulate microcomposites, because of their unique phase morphology deriving from layer intercalation or exfoliation that maximizes interfacial contact between the organic and inorganic phases and enhances bulk properties [8]. 

This book will assess the structure-property benefits of these nanoparticles in polymers as a function of their degree of exfoliation, alignment, and dispersion. Methods for the preparation of these polymer nanocomposites will be assessed in relation to the attainment of the optimal morphology for nanoparticle structure within the polymer. This book will not attempt to give an exhaustive review of the literature since many recent works have served that purpose quite adequately. Rather, this book will focus on assessing what is truly understood about polymer-clay nanocomposites and which areas require further study. 

The fourth significant independent variable that increases the thermal stability of polymer-clay nanocomposites is the barrier properties provided by the anisotropic morphology of the clay particles [3]. The mechanism of barrier performance of montmorillonite is fully developed... 

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