Francis Reny Costa

Layered Double Hydroxide Based Polymer Nanocomposites

Nanocomposites based on polymers and inorganic filler materials not only create enormous interest among researchers because of their unique way of preparation and properties, but also promise development of new hybrid materials for specific applications in the field of polymer composites. The present article deals with the application of a relatively new class of inorganic materials, namely layered double hydroxides (LDHs), as nanofiller for synthesizing polymer-based nanocomposites. LDHs are mixed metal hydroxides of di- and trivalent metal ions crystallized in the form similar to mineral brucite or magnesium hydroxide (MH) with the incorporation of interlayer anionic species. Several procedures for the synthesis of LDHs, their organic modification, and the synthesis of polymer/LDH nanocomposites are discussed in detail with reference to work done in very recent years. The potential of LDHs, especially magnesium and aluminum-based LDHs (Mg–Al LDH) as nanofillers for the polymer matrix has been investigated. The important aspects in characterizing such hybrid materials (i.e., morphological analysis and melt rheological behavior) have been reported in detail to understand the nature of LDH particle dispersion and its influence on the melt flow behavior of the nanocomposites. The specialty of LDHs as nanofiller is their thermal decomposition behavior, which makes them potential flame retardants for polymers. This aspect has been reported in detail in the case of polyethylene-based systems, where the flame retarding efficiency of organically modified Mg–Al LDH alone and also in combination with conventional flame retardants has been discussed.

Synthesis of Organo Cobalt−Aluminum Layered Double Hydroxide via a Novel Single-Step Self-Assembling Method and Its Use as Flame Retardant Nanofiller in PP

Synthesis of polypropylene/organo-layered double hydroxide (PP/OLDH) has been carried out based on self-assembled organocobalt-aluminum LDH (O-CoAl-LDH). The novel method of synthesizing self-assembled CoAl-LDH and its characterization have also been reported in details. This method is proven to be very efficient way of producing OLDH in a single step with homogeneous composition and structure. As flame-retardant nanofiller, O-CoAl-LDH shows significant decrease in heat release rate (HRR), the total heat release (THR) and the heat release capacity (HRC) of the PP composites, though the thermal stability of the compounds decreases slightly compared to the base polymer. Morphological analyses show that the LDH particles are dispersed in PP matrix in a partially exfoliated form. The activation energy calculation based on the Kissinger method reveals that O-CoAl-LDH has a positive effect on the activation energy of thermal decomposition of PP. However, in the presence of this filler, decomposition of the composites starts at an earlier stage than that of pure PP.

Nanocomposites based on polyethylene and Mg–Al layered double hydroxide: characterisation of modified clay, morphological and rheological analysis of nanocomposites

Abstract Magnesium and aluminium based layered double hydroxide (Mg–Al LDH) after modification with anionic surfactant has been used as nanoclay to prepare polyethylene based nanocomposites using the melt mixing technique. The modified Mg–Al LDH has interlayer separation about four times higher than the unmodified clay and also low surface tension. The modified clay also possesses similar platelet type particle morphology like pristine clay. X-ray diffraction and electron microscopic analyses reveal that the nanocomposites are mainly exfoliated/intercalated type characterised by the presence of hierarchy of clay particle morphology in the matrix. The dispersed particle forms aggregates or localised domains of network structure. As a result, response to steady shear is characterised by the presence of stress overshoot at the start-up flows, especially at higher LDH concentrations. The rheological analysis shows that the dispersed LDH particles caused deviation of linear viscoelastic response of the melt from that of the unfilled polyethylene.

Nonlinear behavior of polyethylene/layered double hydroxide nanocomposites under shear flow

The structural evolution of filler clusters in polyethylene/layered double hydroxide-based nanocomposites is investigated under application of a simple shear flow and is described in the framework of a modified Wagner model. Overall, the structural behavior of these polymer-clay nanocomposites is found to be similar to the behavior of filled elastomers for which breakdown of filler clusters at increasing strain and their reaggregation at decreasing strain were observed under oscillatory shear (Payne effect). Similar to the filled elastomers and other jammed systems, the polymer-clay nanocomposites demonstrate an asymmetric behavior upon approaching the steady state depending on whether the system was initially at higher or lower shear strain. In particular, the reaggregation time of filler structure in the quiescent state is found to be about one order of magnitude larger than the characteristic breakage time in the nonlinear shear regime.