| Abstract | In this review we discuss clay-reinforced polymeric nano-composites (PNC). The advantages of using clays are availability, cost, and aspect ratio; the main disadvantage is their hygroscopic character. It is a relatively simple task to disperse clay platelets in water-soluble solvents, monomers, or oligomers. However, preparation of PNC in a hydrophobic, high molecular weight, polymer (e.g., polypropylene) is difficult. The way to approach the problem is to consider the process from the perspective of compatibilization of antagonistically immiscible components and diffusion-cont rolled mixing. The preferred cl ay is montmorillonite (MMT) with micron-sized particles formed by stacks of hundreds of layered crystals, each of 0.96 nm thickness and an average diameter of 100–2000 nm. The MMT unit cell offers two types of reactive sites: anions on the silicate fl at surfaces and hydroxyl (–OH) groups on the edges. Historically, “compatibilization” involved forming an ionic bond between the clay surface and organophilic onium cation, e.g., ammonium. More recently, “compatibilizers” containing epoxy or acid anhydride groups were reacted with the side –OH groups. Since the solid–solid interactions between MMT layers are about 100 times stronger than liquid–liquid ones, good bonding between clay particle and the matrix is imperative. To prepare PNC with well-exfoliated clay, the best strategy is to do it in multiple steps. Initial swelling of MMT in water may expand the interlayer spacing from the initially dry state of 0.96 to about 1.3 nm. Intercalation with suitable organophilic molecules, onium or Lewis-base type increases it to about 4 nm. Reactive compatibilization of the organo–clay/matrix polymer system under flow results in exfoliation of the clay platelets (interlayer spacing larger than 8.8 nm). |
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