Layered-silicate polymer mechanical properties

When electrical and electronic properties are also of interest apart from mechanical and thermal properties, carbon nanotubes can be of better advantages. Nanotubes are inert in nature and, therefore, also require surface modification in order to achieve compatibility with the polymer matrices. Thus, the nanoscale dispersion of the nanotubes is as important and challenging as the layered silicates as the properties are dependant on the generated morphology in the composites. In a representative study, Teng et al. 

Alemdar A, Sain M (2008) Biocomposites from wheat straw nanofibers morphology, thermal and mechanical properties. Compos Sci Technol 68 557-565 Alexandre M, Dubois P (2000) Polymer-layered silicate nanocomposites preparation, properties and uses of a new class of materials. Mater Sci Eng R Rep 28 1-63 Araki J, Wada M, Kuga S, Okano T (2000) Birefringent glassy phase of a cellulose microcrystal suspension. Langmuir 16(6) 2413-2415... 

Clays have long been used as fillers in polymer systems because of low cost and the improved mechanical properties of the resulting polymer composites. If all other parameters are equal, the efficiency of a filler to improve the physical and mechanical properties of a polymer system is sensitive to its degree of dispersion in the polymer matrix (Krishnamoorti et ah, 1996). In the early 1990s, Toyota researchers (Okada et ah, 1990) discovered that treatment of montmorillonite (MMT) with amino acids allowed dispersion of the individual 1 nm thick silicate layers of the clay scale in polyamide on a molecular. Their hybrid material showed major improvements in physical and mechanical properties even at very low clay content (1.6 vol %). Since then, many researchers have performed investigations in the new field of polymer nano-composites. This has lead to further developments in the range of materials and synthesizing methods available. 

However, the field of polymer day silicate has only started to speed up recently, mixing the appropriate modified layered silicate with synthetic layered silicates has long been known (Theng, 1979). The interest in these materials came from two important findings, first has been reported by Toyota research group of a Nylon-6 (N6)/Na-MMT nano-composites (Okada et ah, 1990) where very small amounts of layered silicate loadings resulted in the improvements of thermal and mechanical properties and second the findings of Vaia et ah (1993) about the... 

Of particular interest are the recently developed nanocomposites consisting of a polymer and layered silicate because they often exhibit remarkably improved mechanical and other properties [3] when compared with pure polymer or conventional composites (both micro- and macro-composites). A primary progress in... 

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

On the other hand, Bhattacharya et al. have reported the plasticization effect of organically modified layered silicates on dynamic mechanical properties [13]. In this work, nanocomposites of SBR have been prepared using various nanofillers like modified and unmodified montmorillonite, SP, hectorite etc. It has been observed that the Tg shifts to lower temperature in all the nanocomposites, except for systems from hectorite and NA. This is due to the fact that clay layers form capillaries parallel to each other as they become oriented in a particular direction. Due to wall slippage of the unattached polymer through these capillaries, the Tg is lowered, which could be even more in the absence of organo-modifiers [13]. A similar type of plasticization effect is also noted in the case of the low... 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

For more than a decade, numerous research studies have been carried out on the flame-retardant properties conferred by nanoparticles and mainly by organo-modified layered silicates (OMLS). Earlier work at Cornell University and National Institute of Standards and Technology in the United States showed that nanocomposites containing OMLS reduced polymer flammability and enhanced the formation of carbonaceous residue (char).14 Owing to a strong increase in polymer viscosity, impairing processability, and also due to the breakdown of ultimate mechanical properties, the acceptable rate of incorporation for nanoparticles to improve flame retardancy is generally restricted to less than 10 wt %. 

Intercalation of electroactive polymers such as polyaniline and polypyrrole in mica-type layered silicates leads to metal-insulator nanocomposites. The conductivity of these nanocomposites in the form of films is highly anisotropic, with the in-plane conductivity 10 to 10 times higher than the conductivity in the direction perpendicular to the film. Conductive polymer/oxide bronze nanocomposites have been prepared by intercalating polythiophene in V2O5 layered phase, which is analogous to clays. °° Studies of these composites are expected not only to provide a fundamental understanding of the conduction mechanism in the polymers, but also to lead to diverse electrical and optical properties. 

Choi, M.H. In, J. Mechanical and thermal properties of phenolic resin-layered silicate nanocomposites synthesized by melt intercalation. J. Appl. Polym. Sci. 2003, 90 (9), 2316-2321. 

Ganguli, S. Dean, D. Derrick, J. Kelvin, P. Price, G. Vaia, R. Mechanical properties of intercalated cyanate ester-layered silicate nanocomposites. Polymer 2003, 44 (4), 1315-1319. 

Mechanical Properties Toyota Central Research Laboratories in Japan was the first to obtain significant mechanical improvement of a PA matrix by adding as little as about 2 wt% of montmorillonite (MMT) [Kojima et ah, 1993 Usuki et ah, 1993 Okada and Usuki, 2006], Improvement in the mechanical properties on the vitreous and rubbery plateau by layered silicate nanoparticles depends on several factors, including clay surface modification, polymer chemistry, processing method, level of exfoliation, and clay orientation. In this section we present an overview of the influence of these factors on the dynamic mechanical properties of PLSN. 

Becker, O., Varley, R., and Simon, G., Morphology, thermal relaxations and mechanical properties of layered silicate nanocomposites based upon high-functionality epoxy resins. Polymer, 43, 4365-4373 (2002). 

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