Polymer-silicate interactions

In any case, dispersion obtained via in-situ polymerization may improve the barrier and thermal properties based on a tortuosity argument alone but may or may not improve the mechanical performance, depending on the level of polymer/silicate interactions. Without thermodynamic compatibility, the silicate layers may even collapse to form multilayer stacks if the nanocomposite is heated. Finally, all of these techniques become somewhat complicated by any changes the silicate layers cause in terms of the polymer s molecular weight distribution as compared to silicate-free controls. 

Polymer-silicate interactions can stabilize a polymer metastable phase and induce polymorphism. The examples include PAs and PVDF. Tlie PA6 monocUnic a phase, with shorter hydrogen bonds, is thermodynamically most stable and crystallizes during slow cooling from the melt. The /crystals can be obtained by highspeed melt spinning or iodine treatment of the a phase... 

Enhanced crystallization of PA6 in the form of / phase (indexed as monoclinic or pseudohexagonal) in the presence of the clay platelets was attributed to conformational changes of the chains induced by the proximity of the surface limiting the formation of hydrogen-bonded sheets [9,97]. The stabilization of the /phase by silicate platelets and promotion of the molecular orientation due to polymer-silicate interaction in a commercial PA6-based copolymer was also reported... 

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. 

Based on the similarity of high polymer silicate pK values to that of silica gel one might expect these species to interact with metal ion in solution in a manner analogous to... 

Compared to conventional composites, polymer layered-silicate (PLS) nanocomposites have maximized polymer-clay interactions since the clay is dispersed on a nanometer scale. 

Chrissopoulou, K., Altintzi, L, Anastasiadis, S. H., Giannelis, E. P., Pitsikalis, M., Hadjichristidis, N., and Theophilou, N. 2005. Controlling the miscibility of polyethylene/layered silicate nanocomposites by altering the polymer/surface interactions. Polymer 46 12440-12451. 

From a thermodynamic point of view, the intercalation of a polymer into the galleries of OLS can be approached by considering entropic and enthalpic effects the entropy loss due to the confinement (the macromolecules pass from 3D coils to 2D structures) has to be compensated by the entropy gain of the surfactant chain and by the balance of different possible interactions polymer-surfactant, polymer-silicate surface, and surfactant-silicate surface interactions (Ciardelli et al. 2008) (Figure 11.2). 

The thermodynamic spontaneity of the interacalation process is determined by an interplay of entropic and enthalpic factors [38, 39]. The confinement of the polymer inside the interlayer results a decrease in the overall entropy of the polymer chains. However, the entropic penalty may be compensated by the increased conformational freedom of the tethered surfactant chains in a less confined environment, as the layers separate out. Even then, there is an overall decrease in entropic factor as the increase in gallery height is very small. Thus the degree of layer separation depends on the establishment of very favorable polymer surface interaction to over come the penalty of polymer confinement. There are two different types of interactions, unfavorable apolar interaction and the favorable polar interaction, which originates from the Lewis acid/Lewis base character of the layered silicates. Thus depending on the interactions and entropic factors three possible equilibrium states are generated namely immiscible (conventional), intercalated and exfoliated. 

Among the existing nanofillers, MMT clays are the most commonly employed for the production of nanocomposites. They are 2 1 layer silicates. The MMT particles are made up of layers having a length within 100 to 1000 nm, a thickness of 1 nm, and a high aspect ratio within 10 to 1000, thus possessing very high surface area for polymer/filler interaction. 

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