Clay delamination

In one example, the tensile strength of polyamide 6 was increased by 55% and the moduli by 90%, with the addition of only 4wt% of delaminated clay. The enhanced tensile property of PCN suggests that nanocomposite performance is related to the degree of clay delamination, which increases the interaction between the clay layers and the polymers. Several explanations, based on the interfacial properties and the mobility of the polymer chains, have been given for this reinforcement. Kojima et al. reported that the tensile modulus improvement for polyamide 6-clay hybrid originated from a constrained region, where the polymer chains have reduced mobility. The dispersion and delamination of the clay were the key factors for the reinforcement. The delaminated nanocomposite structure produces a substantial increase in modulus. 

Organophilic clays have also been blended with PLA. Ogata et al. [133] modified MMT with distearyldimethylam-monium chloride and that was solvent-cast blended with PLLA (Af = 2 x 10 ) using chloroform. Results showed that the tactoids (several stacked sificate monolayers) lay almost parallel to the film surface and were stacked with insertion of PLLA lamellae in the thickness direction of the film. Such preferred orientation of the tactoids is assumed to give excellent water barrier properties to the blends, as reported for nylon 6-clay hybrids (NCH) by Kojima et al. [134] In this nylon blend, the absence of clay delamination gave relatively poor tensile improvements compared to true nanocomposites. 

Yoonessi M, Toghiani H, Daulton TL, Lin J-S, Pittman CU (2005) Clay delamination in clay/ poly(dicyclopentadiene) nanocomposites quantified by small angle neutron scattering and high-resolution transmission electron microscopy. Macromolecules 38(3) 818-831... 

Initiation of e-CL polymerization from the clay surface accounts for the tethering of the in-situ grown chains onto the clay surface and for clay delamination. As reported elsewheregrafted and/or tethered polymer chains have a key role in preventing the exfoliated silicate layers from aggregating during post-thermal and processing treatments of the composites. 

As an alternative, in-situ intercalative polymerization of e-c rolactone directly in between the silicate layers was also approached. In this way, intercalated nanocomposites could be obtained even with Cloisite Na. Furthermore, exfoliated nanocomposites have been recovered starting from Cloisite 30B, thus a montmorillonite surface-modified by ammonium cations bearing two primary hydroxyl functions. Such a morphology results from the direct grafting of the growing polyester chains onto the hydroxyl functionalized silicate layers. As expected, clay delamination gives rise to an additional improvement of materials properties, in comparison with intercalated nanocomposites obtained by melt blending. 

The clay delamination as well as the development of the silica matrix give rise to an enhancement of the surface area and porosity compared with the pristine layered silicates (Table 14.2). An extraordinary increase (>20 times) in the BET surfece area and in the total pore volume ( 100 times) is observed in the nanoarchitectures prepared from vermiculite, making these materials of potential interest as adsorbents, support of catalysts, and nanofillers of polymers. 

A process for upgrading kaohu by grinding in a stirred bead miU has been reported (Stanczyk and Feld, U.S. Bur Mines Rep. Invest. 6327 and 6694, 1965). By this means the clay particles are delaminated, and the resulting platelets give a much improved surface on coated paper. 

Many different polymers have already been used to synthesize polymer-clay nanocomposites. In this section, an overview of the advances that have been made during the last 10 years in the intercalation and the delamination of organoclay in different polymeric media is given. The discussion mainly covers the work involving thermoset nanocomposites along with a brief discussion about thermoplastic-based nanocomposites. 

Films of polyolefins, polyamides and poly(vinylidene dichloride) are made using this technique. As most of the films are used for flexible packaging, further down-stream surface treatments are usually applied to improve performance. For example, aqueous polymer emulsions, e.g., poly(vinylidene dichloride), or delaminated clay particles improve the barrier properties as will metallising with aluminium vapour. Corona discharge, causing slight surface oxidation, improves printability. 

The assumption that the water is adsorbed in uniform layers on all the clay surfaces for a wide range of mixtures has been criticized (2, 20). The argument is that the individual clay particles in the clay-water mixture do not expand beyond a certain distance regardless of the quantity of water which is added. The clay layers group themselves into tactoids resulting in two populations of water those molecules which are found between the tactoids and those directly perturbed by the clay layers. If true, this would invalidate the procedure used to calculate the thermodynamic properties of the adsorbed water. However, other workers have reported complete delamination of certain smectites (21., 22). It is not clear under what conditions tactoids will form, or not, and this uncertainty is underlined in (21) (see remarks by Nadeau and Fripiat, pages 146-147). 

LDH materials are not readily exfoliated in contrast to other layered materials such as smectite clays or MS2 type-chalcogenides (M0S2, NbSe2, etc...) [15-17] the delamination of LDH sheets requires elaborate syntheses as discussed above in Sect. 2.1. 

A recent investigation has demonstrated the usefulness of ultrasonic irradiation in the preparation of delaminated zeolites, which are a particular type of modified oxides - microporous crystalline aluminosilicates with three-dimensional structures - having a greater catalytic activity than the layered structures (clays) and mesoporous catalysts. In an attempt to increase the pore size of zeolites, a layered zeolite precursor was... 

The exfoliation or delamination configuration is of particular interest because it maximizes the polymer-clay interactions, making the entire surface of layers available for a polymer, and thereby yields better properties than intercalated ones. 

Figure 7 shows the representative bright field HRTEM images of nanocomposites of NR and unmodified montmorillonite (NR/NA) prepared by different processing and curing techniques. It is apparent that the methodology followed to prepare the nanocomposites by latex blending facilitates the formation of exfoliated clay structure, even with unmodified nanoclays. It has been reported in the literature that hydration of montmorillonite clay leads to extensive delamination and breakdown of silicate layers [94, 95]. It has also been shown that NA disperses fully into the individual layers in its dilute aqueous dispersion (clay concentration <10%)... 

Dispersion The degree of dispersion of the nanoplatelets is determined by the degree of delamination of the clay. The fully delaminated (exfoliated) nanocomposite presents much higher values for the tortuosity factor and the aspect ratio in comparison with the partially delaminated (intercalated) nanocomposite. This means that the clay particles that grow as aggregates or books of sheets must be broken up or exfoliated into individual sheets that have a thickness of the order of 1 nm, with lengths and widths of the order of 500 nm. 

In order to prove the importance of the delaminated layer structure in the direct (melt) intercalation, the intercalation of stearic acid molecules into the clay galleries has been studied by premixing them with organoclay and then successfully incorporating this modified clay into the rubber matrix [59]. A schematic presentation of such modification of clay is given in Fig. 37. 

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