nanocomposite materials, in which nano-sized silicate plates of clay are uniformly dispersed in the polymer matrix, exhibit superior physical properties such as high dimensional stability, gas barrier performance, flame retardancy, and mechanical strength that cannot be achieved by pure polymer or conventional composites (micro- and macro composites) [1-3]. Furthermore, polymer layered silicate nanocomposites (PLS) avoid processing techniques (e.g. extrusion) which are used for materials with a higher content of reinforcement. This polymer/clay nanocomposites can be prepared in several ways, namely, solution exfoliation, melt intercalation, in situ polymerization and template synthesis [4]. Solution exfoliation can be only used with water-soluble polymers to produce mostly intercalated nanocomposites, because of the need of large amounts of solvent to ensure a good clay dispersion [5]. Melt intercalation is a solvent-free method which enables mixing of the layered silicate with the polymer matrix in the molten state. However, very careful attention has to be paid to finely tune the processing conditions to increase the compatibility of clay layer surfaces with the polymer matrix. In the in situ polymerization technique, the monomer, together with the initiator and/or catalyst, is intercalated within the silicate layers and the polymerization is initiated by external stimulation such as thermal, photochemical or chemical activation [6-10]. The chain growth in the clay galleries triggers the clay exfoliation and hence the nanocomposite formation. Unlike melt intercalation, the low viscosity of the monomer (if compared with the polymer) in the in situ polymerization makes it more easy to break up particle agglomerates by using high shear devices, resulting in a more uniform mixing of particles in the monomer. In template synthesis clay layers are formed by crystallization in an aqueous polymer gel. However, the layers show a limited length and the size are not comparable to pristine clays. Furthermore, it is possible to control nanocomposite morphology through the combination of reaction conditions and clay surface modification.
Since the discovery of polymer/clay nanocomposites by the Toyota research group [11] in the early 1990s, over 5.000 papers have been published up to now with the concept of clay as filler for polymer matrices. In the work of the Toyota group, ε-caprolactam monomers were polymerized between silica layers resulting in polyamide/clay nanocomposites showing highly improved thermal rheological and mechanical properties of the polymer.
Fig 1. Schematic representation of polymer/clay nanocom posites by various in situ polymerization techniques (A. monomer immersion, B. intercalation, C. exfoliation).
Various different living and controlled/living polymerization methods were used in the production of well-dispersed silicate layers, including atom transfer radical polymerization (ATRP) [12-20], nitroxide mediated polymerization (NMP) [21,22], and reversible additionfragmentation chain transfer (RAFT) polymerization [23-26] , ring-opening polymerization (ROP) [27-32], ring-opening metathesis polymerization (ROMP) [33-35] , living cationic polymerization [10,36] and living anionic polymerization (Figure 1) [37,38]. The common approach throughout the literature is to immobilize polymerization initiators in between the clay layers. This can be done by replacing the cations of the clay surface with silane coupling agents or with organic salts, mainly quaternary ammonium salts which comprise functional groups. During the polymerization step the layers exfoliate and a highly dispersed nanocomposite can be gained [22].
In this presentation, we will focus on the recent progress of the in situ synthesis of polymer/clay nanocomposites with well-defined structures and highly exfoliated morphologies. The methods used for the preparation were classified according to the individual polymerization mechanisms. Other possibilities such as multi-mode polymerization combining different polymerization methods and click chemistry are also described. A special emphasize is devoted to the structures and morphologies of the obtained nanocomposites rather than their practical properties.
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