Silicate nanolayers

Stabilizing effect of A50°C (Fig.4 b) of PP-MAPP-Cloisite 20A over neat PP calculated with the maximum rate of mass loss can be explain by means of the barrier effect of the silicate nanolayers which operate in the nanocomposite level against oxygen diffusion, shielding the polymer from its action. 

The addition of layered silicate to a polymer can result in three different morphologies formation of micro- (or even macro-) composites, or nanocomposites with intercalated polymer species (where the polymer is sandwiched between 2D silicate nanolayers) or with exfoliated/delaminated structures. In the last case, the 2D silicate nanolayers are completely separated, at least at distances of a few nanometers from one another. 

Recently, a few papers [221-228] on PCN networks, modified by montmoril-lonite (MMT) silicate nanolayers, were published. Data on structure and mechanical and thermal properties of these nanocomposites were obtained. The 2D shape of MMT nanolayers, the enormous interfacial area, and strong PCN-MMT interactions, due to both covalent bonding between cyanate groups and the functional groups of MMT surface as well as between triazine rings and A1 and Si atoms (charge transfer interactions), provided the enhancement of some PCN properties in the nanocomposites. Well-dispersed MMT additive resulted in the substantial increase in fracture toughness and crack resistance of PCN [223,225], better thermal stability, and reduced coefficient of thermal expansion [221]. 

The inclusion of nanoclay in polymeric materials has been proven to elucidate remarkable tensile, fracture, and thermal properties enhancements (Lan and Pinnavaia, 1994 LeBaron, Wang, and Pinnavaia, 1999), attributed to the high aspect ratio of the silicate nanolayers. The largest improvement in properties are observed for those nanocomposites where the polymer is able to penetrate and expand the gallery... 

In the case of layered silicates, the filler is present in the form of sheets one to a few nanometer thick and hundreds to thousands nanometer long. In general, the organically modified silicate nanolayers are referred to as nanodays or organo-silicates [12]. It is important to know that the physical mixttue of a polymer and layered silicate may not form nanocomposites [13]. Pristine-layered sUicates usually contain hydrated Na" " or K ions [13]. To render layered silicates misdble with other polymer matrices, it is required to convert the normally hydrophUic sUicate surface into an organophilic one, which can be carried out by ion-exchange reactions with... 

Silicate nanolayers, which are widely used in polymer clay nanocomposites (PCNs), are montmorillonites from the smectite family, that is, 2 1 phyllosilicates composed of octahedral and tetrahedral crystalline sheets. The octahedral sheet consists of hydroxyl groups as well as oxygen, aluminum, iron, and magnesium atoms. On the other hand, the tetrahedral sheet comprises a central silicon atom and four oxygen atoms or hydroxyl groups [5]. The structure of 2 1 phyllosilicates is shown in Figure 21.1. 

Solution blending, in situ polymerization, and melt blending are three processing techniques used for preparing polymer-layered silicate nanocomposites. In solution blending, a polymeric solution is prepared and layered silicates are swollen in a cosolvent (water, toluene, chloroform, etc.). By mixing the polymeric and silicate nanolayers solutions. 

Figure 21.2 Schematic representation of organic modification of silicate nanolayers. Pavlidou and Papaspyrides [2]. Reproduced with permission of Elsevier.

The enhanced corrosion protection effect of PCN compared with bulk polymers is believed to have resulted from dispersing silicate nanolayers of clay in the polymer matrix to increase the tortuosity of diffusion pathways for H2O and O2 molecules, and is further evidenced by the O2 and H2O permeability stupes on the free-standing film of as-prepared PCN. 

In recent years, MMT has attracted great academic and commercial interest because of its high aspect ratio of silicate nanolayers, high surface area and wide applications in polymer materials.Much attention has been paid to polymer nanocomposites, essentially polymer MMT or polymer-SS nanocomposites, which exhibit physical and chemical properties that are dramatically... 

Incorporation of silicate nanolayers in semi-crystalline polymers like polypropylene ean have a two-fold effect on the barrier properties, (1) well oriented large aspeet ratio platelets will increase the tortuosity of the diffusion path and (2) the nanolayers will affect the crystalline order (size and interlamellar spacing) and possibly affect the barrier properties. The extent of orientation is greater in blown film than in extmsion cast film and this leads to similar trends in barrier properties of polypropylene nanocomposites with 7wt.% 1.3 IPS (silated) clay as reported by Qian et al. With cast films, the nanoeomposite had a lower permeability to oxygen by a faetor of 1.5 compared to neat polypropylene. With blown films, the nanocomposite permeability to oxygen was lower by a factor of 2.5 compared to neat polypropylene. However, Ellis and D Angelo were able to prepare only intercalated polypropylene nanocomposites with the same 1.31 PS and obtained no improvement in permeability to a solvent over that for neat polypropylene. This underlines the greater sensitivity of barrier performanee to the level of dispersion and orientation. 

The high aspect ratio of silicates nanolayers in exfoliated nanocomposites has been found to greatly reduce the gas permeability in films prepared fi-om sueh nanomaterials. The dependence on factors such as the relative orientation and... 

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