Interaction polymer matrix-sheets

McHugh and Krukonis ( ) have demonstrated with the aid of SEM, that polypropylene preforms can be rendered porous by extraction with supercritical CO2 and propylene. However, these investigators noted that SCF extraction of the preformed polymer sheets at higher pressures and temperatures deformed the polymer matrix leading to a fused appearance when examined by SEM and a concomitant decrease in porosity. Similar interaction between porous polyurethane foam and supercritical CO2 reported by Smith and coworkers (J) lead to a physical alteration of the exposed sorbent. Such pressure-induced structural changes lead to a decrease in available surface area for sorbate adsorption. 

The exfoliated state takes place when the dispersion of the sheets of the clay is fully achieved and, therefore, no interaction between sheets occurs (Figure 8.2, right). The ideal exfoliated state is that in which the clay platelets are homogeneously distributed throughout the polymer matrix. Generally speaking, layered nanocomposites never exhibit a pure structure on the contrary, their morphology is a combination of all three possibilities with one of them dominant. 

This is a newly emo-ging class of polymer-LDH nanocomposites. Ideally, the completely delaminated LDH hydroxide sheets are uniformly dispersed in the polymer matrix, as shown in Figure lOB. The nanometer-scale interaction between polymer and LDH hydroxide sheets and the tortuosity of diffusion through... 

In addition to interfacial interactions between the CNT and the polymer matrix, the dispersion of CNTs in the polymer has signifieant influence on the performance of a CNT-polymer nanocomposite. Many different approaches have been used by researchers in an attempt to disperse CNT in polymer matrix such as physical sonication and chemical modification of CNT surface [124-126]. Functionalization of CNT surface can lead to the construction of chemical bonds between the nanotube and polymer matrix and offers the most efficient solution for the formation of strong interface. A strong interface between the coupled CNT-polymer creates an efficient stress transfer [31]. It should be noted that covalent functionalization of CNT may disrupt the grapheme sheet bonding, and thereby reduce the mechanical properties of the final product. However, noncovalent treatment of CNT can improve the CNT-polymer (Fig. 23.10) composite properties through various specific interactions [127]. 

The preceding study is extended [37] by replacing solvent partides with polymer chains. A set of typical snapshots at the end of simulations is presented in Figures 2.8 and 2.9 with attractive and repulsive interactions between the polymers and the sheets with the polymer matrix density ( ) = 0.2. Let us examine these figures to probe the effect of interaction between the sheets and the polymer chains and their molecular weight. Because of the entropic constraints on the conformations, shorter chains can move and intercalate more easily than the chains with higher molecular weights. 

Snapshots with the lowest molecular weight (Ic = 4-) polymer matrix show that the sheets maintain their layered configuration with e = —1 (attractive) interaction... 

Figure 2.8 Snapshots of the final configurations with the attractive (e= —1) interaction between polymer matrix and sheets) after 10 time steps. Sample size 64 is used with the polymer volume fraction...

Aspect ratios (length/thickness) greater than a thousand are possible [29]. MMT is the most commonly used layered silicate. It is a hydrated alumina-silicate layered clay mineral consisting of an edge-shared octahedral sheet of aluminum hydroxide between two silica tetrahedral layers [6,30]. Highly favorable polymer-surface interactions are essential for thorough clay sheet dispersion [31,32]. Thus, it is conventional to organo-modify these clays with cationic surfactants in order to improve compatibility with the polymer matrix and to faciUtate delamination and exfoliation [3]. 

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