Polymer layered silicate nanocomposites: Structure, morphology, and properties

Layered silicates are important fillers for improving various mechanical, flame retardant, and barrier properties of polymers, which can be attributed to their sheet-like morphology. Layered silicates can be modified with organic surfactants to render them compatible with polymer matrices. Organically modified silicates (organoclays) having large surface areas are very cost-efficient non-toxic nanofillers effective at very low loads and are readily available. Upon amalgamation of organoclays with polymer matrix nanocomposites, polymer chains can penetrate in between the silicate layers and result in an intercalated structure where the clay stack remains intact but the interlayer spacing is increased. When penetration becomes more severe, disintegration of clay stacks can occur, resulting in an exfoliated structure. It has often been observed that exfoliation is not complete down to the level of isolated silicate layers; rather, the large clay stacks are broken up into shorter stacks termed 'tactoids' together with a few individual silicate layers, resulting in a kind of mixed intercalated-exfoliated structure. Organoclay particles are mostly intercalated, having a preferred orientation with the clay gallery planes being preferentially parallel to the plane of the pressed film. Preferential orientation of organoclays affects the barrier properties of polymer membranes. Additional fillers like carbon black can induce a change in the orientation of organoclays. The effect of carbon black on the orientation of organoclays was elucidated and a relationship between orientation and permeability of air through such membranes was established.;We have also investigated the flammability properties of a series of polymer nanocomposites, containing various Transition Metal Ion (TMI) modified organoclays. The improved fire retardation in nanocomposites with TMI-modified organoclays can be attributed to enhanced carbonaceous char formation during combustion, i.e., charring promoted by the presence of catalytically active TMI. Polymer nanocomposite materials depend not only on the properties of individual components but also on their morphology and interfacial interactions. In polymer nanocomposites, the interfacial interactions are maximized due to the large surface area of the filler particles exposed to the polymer matrix, resulting in unique anisotropic properties. Thus, it will be of great importance to achieve exfoliation of the lamellar stacks prior to mixing with the polymer matrix, in the dry powder state or in a solution state. In layered silicates the lamellar stacks are held by electrostatic interactions between the basal charges and ions present within the basal spacing. Lamellar stacks of layered silicates can be exfoliated if the amount of energy gained by them is higher than the electrostatic energy required to hold the lamellar stacks together. Using 'Microwave radiation', exfoliation of organoclays was achieved. Various characterization techniques were used to evaluate structure, morphology and properties of fillers and polymer nanocomposites.