Composites of Engineering Plastics with Layered Silicate Nanofillers: Preparation and Study of Microstructure and Thermomechanical Properties

Abstract

Polymer nanocompositesPolymer/layered silicate nanocomposites have recently attracted a great deal of interest because of their unique properties, such as enhanced mechanical property, increased thermal stability, improved gas barrier properties and reduced flammability. According to the arrangement of silicate layers within the polymer matrix, two types of morphology can be achieved in nanocomposites: namely intercalated or exfoliated structures. Exfoliated structures have been well recognized as superior morphology for high performance at lower clay loadings, but are difficult to achieve. Attempts to improve processability and ensure efficient dispersion of the above fillers have led to modifications, e.g. with quaternary ammonium surfactants, which is expected to increase the inter-gallery spacing and provide enough hydrophobicity to clay particles in order to make them miscible with the polymer matrix. A typical example is sodium montmorillonite (MMT), one of the most commonly used clays, which is hydrophilic and therefore shows restricted compatibility with many polymers. To obtain good interfacial adhesion for improved mechanical properties, the clay needs to be modified prior to incorporating into the usual organophilic polymer matrices. Clay modification can be achieved by an ion exchange reaction with organophilic cations (Utracki, 2004). There are two reasons for this modification: (i) the addition of an intercalating agent to increase the space between the layered silicates and make it more uniform and (ii) the addition of small organic molecules bonded to silicates to make MMT more miscible with the polymer matrix. Therefore, polymer molecules are allowed to enter the enlarged intergallery of silicates for further intercalation or exfoliation. In general the intercalated agents are small molecules of cationic surfactant, such dodecyl ammonium chloride and 1-hexadecyl ammonium blomide. Clay possesses net negative charge on its lamellar surface and, therefore, it can absorb cations, such as Na + or Ca 2+ . Alkyl ammonium ions can replace metal cations through a cation exchange process and occupy the gallery space between nanoscaled layers of the clay to alter the original silicate surface from hydrophilic to organophilic (Burnside and www.intechopen.com Nanocomposites and Polymers with Analytical Methods 150Giannelis, 1995). Because of the negative charge of the silicate layer, the cations head group of an alkyl ammonium molecule preferentially resides at the layer surface with the aliphatic tails being removed from the surface (Chen and Yoon, 2005). Nanocomposites can be prepared by solvent casting, by in situ polymerization or by melt compounding (Pinnavaia & Beall, 2002). So far, melt intercalation method has been the most commonly used procedure because of advantages especially in terms of commercial versatility and mass production ability. Three types of polymer/clay composites are usually recognized: a)"conventional composites", in which the clay is added as a common filler, b)"intercalated nanoc...