Layer silicates, model system

Why can layer silicates serve as models for replication processes The answer is simple they have properties which are observed in replicating systems. Montmorillonite crystals contain similar parallel layers, the distance between which... 

Bartholmai, M. and Schartel, B. 2004. Layered silicate polymer nanocomposites New approach or illusion for fire retardancy Investigations of the potentials and the tasks using a model system. Polymers for Advanced Technologies 15 355-64. 

The static and dynamic properties of polymer-layered silicate nanocomposites are discussed, in the context of polymers in confined spaces and polymer brushes. A wide range of experimental techniques as applied to these systems are reviewed, and the salient results from these are compared with a mean field thermodynamic model and non-equilibrium molecular dynamics simulations. 

On a global scale, the linear viscoelastic behavior of the polymer chains in the nanocomposites, as detected by conventional rheometry, is dramatically altered when the chains are tethered to the surface of the silicate or are in close proximity to the silicate layers as in intercalated nanocomposites. Some of these systems show close analogies to other intrinsically anisotropic materials such as block copolymers and smectic liquid crystalline polymers and provide model systems to understand the dynamics of polymer brushes. Finally, the polymer melt-brushes exhibit intriguing non-linear viscoelastic behavior, which shows strainhardening with a characteric critical strain amplitude that is only a function of the interlayer distance. These results provide complementary information to that obtained for solution brushes using the SFA, and are attributed to chain stretching associated with the space-filling requirements of a melt brush. 

In this work, the starch/PVOH/silicate clay blend films were prepared by a solution casting method. Four different types of layered silicate clays were used to investigate the compatibility of multiphased system. Special attention is driven to study the effect of the clay content on the film properties based on barrier and mechanical modeling predictions of nanocomposite films. 

Gianuelis E P (1996) Polymer layered silicate nanocomposites, Adv Mater 8 29-35. Giaiinelis E P, Krishiiamoorti R and Manias E (1999) Polymer-silicate nanocomposites - model systems for confined polymers and polymer brushes, in Advances in Polymer Science Polymers in Confined Environments (Ed. Granick S) Springer, Berlin, Vol. 138, pp. 107-147. 

Free volume present in nanocomposite systems plays a major role in determining the overall performance of the membranes. Positron annihilation lifetime spectroscopy (PALS) is an efficient technique used for the analysis of free volume. The diffusion of permeant through polymeric membranes can be described by two theories, namely, molecular and free-volume theories. According to the free-volume theory, the diffusion is not a thermally activated process as in the molecular model, but it is assumed to be the result of random redistributions of free-volume voids within a polymer matrix. Cohen and Turnbull developed the free-volume models that describe the diffusion process when a molecule moves into a void larger than a critical size, Vc- Voids are formed during the statistical redistribution of free volume within the polymer. It is found that the relative fractional free volume of unfilled polymer decreases on the addition of layered silicates. The decrease is attributed to the interaction between layered silicate and polymer because of the platelet structure and high aspect ratio of layered silicates. The decrease is explained to the restricted mobility of the chain segments in the presence of layered silicates. This results in reduced free-volume concentration or relative fractional free volume [49]. 

These simple models have been widely used to describe the barrier properties of polymer layered silicate nanocomposites (Yano et al, 1997 Gusev and Lusti, 2001 Lu and Mai, 2007), however, some issues like the effect of clay platelets exfoliation / intercalation / aggregation ratio and multiphase polymer matrix system (polymer blend) remain a challenging problem for barrier property modeling. 

---