Polymer/clay-based nanocomposites melt intercalation

Besides melt intercalation, described above, in situ intercalative polymerization of E-caprolactone (e-CL) has also been used [231] to prepare polycaprolactone (PCL)-based nanocomposites. The in situ intercalative polymerization, or monomer exfoliation, method was pioneered by Toyota Motor Company to create nylon-6/clay nanocomposites. The method involves in-reactor processing of e-CL and MMT, which has been ion-exchanged with the hydrochloride salt of aminolauric acid (12-aminodecanoic acid). Nanocomposite materials from polymers such as polystyrene, polyacrylates or methacrylates, styrene-butadiene rubber, polyester, polyurethane, and epoxy are amenable to the monomer approach. 

Three main types of structures, which are shown in Fig. 5.3, can be obtained when a clay is dispersed in a polymer matrix (1) phase-separated structure, where the polymer chains did not intercalate the clay layers, leading to a structure similar to those of a conventional composite, (2) intercalated structure, where the polymer chains are intercalated between clay layers, forming a well ordered multilayer structure, which has superior properties to those of a conventional composite, and (3) structure exfoliated, where the clay is completely and uniformly dispersed in a polymeric matrix, maximizing the interactions polymer-clay and leading to significant improvements in physical and mechanical properties [2, 50-52]. Production of nanocomposites based on polymer/clay can be done basically in three ways (a) in situ polymerization, (b) prepared in solution and (c) preparation of the melt or melt blending [53]. 

This system does not increase the carbon monoxide or soot produced during the combustion, as many commercial FRs do [233]. Other polymer silicate nanocomposites based on a variety of polymers, such as polystyrene, epoxy and polyesters, have been prepared recently by melt intercalation [236]. A direct synthesis of PVA-clay (hectorite) complexes in water solution (hydrothermal crystallization) was reported [237]. It was assumed that the driving force of this phenomenon, at least kinetically, can be described in terms of a simple diffusion reaction of polymers/monomers into clay-layered structures. 

Lee S, Cho W, Hahn P, Lee M, Lee Y, Kim K (2005) Microstructural changes of reference montmorillonites by cationic surfactants. Appl Clay Sci 30(3-4) 174-180 Lee SM, Tiwari D (2012) Organo and inorgano-organo-modified clays in the remediation of aqueous solutions an overview. Appl Clay Sei 59-60 84—102 Lee S Y, Chen H, Hanna M A (2008) Preparation and eharacterization of tapioca stareh-poly(lactic add) nanocomposite foams by melt intercalation based on elay type. Ind Crops Prod 28(1) 95-106 Lee SY, Hanna MA (2009) Tapioca starch-poly(lactic acid)-Cloisite 30B nanocomposite foams. Polym Compos 30(5) 665-672... 

Layered silicate nanoparticles have also been used to prepare PEN-based nanocomposites through the direct intercalation of PEN polymer chains from the melt into the surface-treated clay. An internal mixer was used and exfoliated silicate layers within a PEN matrix were obtained. Mechanical and barrier properties measnred by dynamic mechanical and permeability analysis showed significant improvanents in the storage modulus and water permeabihty when compared to neat PEN (Wu and Liu, 2005). 

To achieve improved dispersibUity of nanoclay fillers within polymer systems, three familiar methods are commonly used, namely, melt intercalation, solution intercalation, and in situ polymerization. The melt-intercalation method is based on the melting point of polymer matrices and is applied by annealing above the melting point of the polymer (Reddy et al., 2013). This method has been chosen by industrial sectors to produce polymer/clay nanocomposites. However, it is not apphcable to the fabrication of biobased polymer/clay nanocomposites based on thermosetting materials such as epoxy and polyester due to their high viscosities (Wypych and Satyanarayana, 2005 Wang et al., 2014). Therefore, the fabrication of biobased thermosetting polymer/clay nanocomposites is mainly based on solution intercalation or in sim polymerization. 

Di Y, lannace S, Di Maio E, Nicolais L (2003) Nanocomposites by melt intercalation based on polycaprolactone and organoclay. J Polym Sci Polym Phys Ed 41 670 Donald AM, Windle AH (1992) Liquid crystalline polymers. University Press, Cambridge Fukushima Y, Okada A, Kawasumi M, Kurauchi T, Kamigaito O (1988) Swelling behaviour of montmorillonite by poly-6-amide. Clay Miner 23 27 Greenland DJ (1963) Adsorption of poly(vinyl alcohols) by montmorillonite. J Colloid Sci 18 647 Han CD (2010) On the mechanism leading to exfoliated nanocomposites prepared by mixing. Adv Polym Sci 231 1-75... 

Unlike melt intercalation, a layered silicate is mixed with monomer before polymerization takes place with in situ polymerization. This method was developed by Toyota researchers [27,28], in which electrostatically held 1-nm thick layers of layered alumina silicates were dispersed in a polyamide matrix on a nanometer level, which led to an exponential growth in the research endeavors, in layered silicate nanocomposites. These nanocomposites were based on the in situ synthesis approach in which a monomer or monomer solution was used to swell the filler interlayers, followed by polymerization. With this process, one can control the nanocomposite morphology through the combination of reaction conditions and clay surface modification. The in situ polymerization method is especially important for insoluble and thermally unstable polymers, which solution blending or melt blending technique cannot process. 

Intercalated nanocomposites are usually formed by mixing in the melt or in situ polymerisation whereas exfoliation may require more complex processing depending on the properties of the clay (Usuki et al, 1993). However, such layered silicate-based polymer nanocomposites have attracted considerable recent interest after the commercialisation of polypropylene-and nylon-6-based materials (Krishnamoorti and Yurekli, 2001, Kiersnowski and Piglowski, 2004). The major barrier to commercialisation has been developing techniques to ensure a reliable and reproducible product which has now been addressed for clay-based composites some thirty or so years after they were first developed. 

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