Intercalated nanocomposites diffusivity

The barrier properties of the PCL-based composites were investigated. The transport properties, sorption and diffusion, were measured by a microgravimetric method . The studied model permeants were methylene chloride and water vapour for which the zero concentration diffusion coefficient Dq was determined. The presence of clay (hydrophilic platelets) in the composite gives rise to specific sites on which water molecules can be entrapped and immobilized, thus the water sorption increases on increasing the clay content, particularly for microcomposites containing Cloisite Na It was found out that the microcomposites as well as the intercalated nanocomposites have diffusion parameters for water vapour very near to those of pure PCL. 

As expected, all intercalated nanocomposites show very similar "gas barrier" behaviour, whether they have been prepared by melt blending or by in-situ intercalative polymerization. In contrast, the exfoliated samples obtained by in-situ polymerization of e-CL in presence of Cloisite 30B show enhanced barrier properties, as evidenced by determining the zero concentration diffusion coefficient (Do) by microgravimetry (Figure 6). In... 

Figure 6. Dependence of water vapour diffusion log Do (Do = zero concentration diffusion coefficient) on the clay content of microcomposites, intercalated nanocomposites, and exfoliated nanocomposites.

PPEX compared with the intercalated nanocomposite fibres may be due to enhanced nanodispersion of the silicate platelets. In this respect, despite the fact that the exfoliated silicate platelets may increase point nucleating density, they reduce the volumetric space required for the nuclei to a spherulite growth. In addition, the nanodispersed silicate platelets would lead to a significant volume increase in the silicate layers in the polymer matrix, thus restricting the global self-diffusion ability of the polymer chain. 

In the case of water vapour transport, the micro-composites as well as the intercalated nanocomposites show diffusion parameters very near to PCL, while the exfoliated nanocomposites strongly deviate showing much lower values, even at low montmorillonite content (Fig. 11.4). This is an indication that in the former cases the water molecules on specific sites are not immobilized but can jump fi om one site to another. Only in the case of the exfoliated samples, the inorganic platelets, dispersed in a disordered distribution, can constitute a barrier to the path of the hydrophilic molecules. 

Particularly interesting are the results on the MMT dispersion degree in the polymeric matrix. In the case of dichloromethane, for samples with 3 wt% of MMT it was shown that the diffusion parameter decreases going from microcomposites (values very similar to pure PCL) to exfoliated nanocomposites intermediate values of diffusion were measured for die intercalated nanocomposites. In the case of water, both micro-composites and intercalated nano-... 

Micro-composites as well as intercalated nanocomposites generally have diffusion parameters very near to the pure polymers, while exfoliated nanocomposites show much lower values, even at low silicate content. 

Since the possibility of direct melt intercalation was first demonstrated [11], melt intercalation has become a method of preparation of the intercalated polymer/ layered silicate nanocomposites (PLSNCs). This process involves annealing, statically or under shear, a mixture of the polymer and organically modified layered fillers (OMLFs) above the softening point of the polymer. During annealing, the polymer chains diffused from the bulk polymer melt into the nano-galleries between the layered fillers. 

Nanocomposites of MMT polymer can be obtained by direct polymer melt intercalation where the polymer chains diffuse into the space between the clay galleries. This process can be carried out through a conventional meltcompounding process [6, 4]. 

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