Nylon-6/Nylon-12/clay hybrid composites were the first exfoliated smectic clay composites made. [40] Montmorillonite, with a CEC of 119 mEq/100 g, was intercalated with 12-aminolauric acid, which increased the intergallery spacing from 1.0 to 1.7 nm. This ‘12-montmorillonite’ was then mixed with ε-caprolactam, which increased the intergallery spacing even further, to 4.0 nm, indicating that the ε-caprolactam had intercalated into the galleries. Heating to 250 oC led to polymerization, forming a clay/Nylon-6 nanocomposite. Further research [41]
determined that ε-caprolactam could intercalate directly into the galleries of montmorillonite in a hydrochloric acid solution and, upon intercalation, becomes oriented vertically in the galleries. The modified montmorillonite then mixed easily with additional molten ε-caprolactam and 6-aminocaproic acid, yielding a Nylon-6 homopolymer/clay nanocomposite. The montmorillonite was completely exfoliated.
Recently [42], montmorillonite/Nylon 6 nanocomposites were processed by melt intercalation. Although the degree of exfoliation was not as high as in nanocomposites produced by the above methods, at weight fractions less than 0.1 the composites were primarily exfoliated.
1.5.2 Polyimide Matrices
The preparation of polyimide matrix clay nanocomposites involves several steps [43] (Figure 1-16). By intercalating montmorillonite with the ammonium salt of dodecylamine, it becomes soluble in dimethylacetamide (DMAC). DMAC is also a solvent for 4,4’-diaminodiphenylether and pyrometllitic dianhydride, the precursors for polyamic acid and, as such, polyimides. After intercalation of the ammonium salt of dodecylamine, x-ray studies [44] showed that hectrite (CEC = 55 mEq/100 g) has
one monolayer of organic material between the layers, whereas saponite, montmorillonite, and synthetic mica (all with CEC > 100 mEq/100 g) have two. After composite formation, however, only the montmorillonite and the synthetic mica have exfoliated completely, but the hectrite and saponite remain in a somewhat aggregated state. Lan et al. [45] found aggregates of montmorillonite after using a similar procedure. More recently, P-phylenediamine in an HCl solution was also found to form organic-modified montmorillonite that dissolves in DMAC. [46] This same study showed that the presence of a small amount of nanoscale organoclay can decrease the imidization temperature by 50 oC (from 300 oC to 250 oC), and at 250 oC the imidization time decreased by 15 min. The activation energy decreased by 20 %.
Clearly, the organoclay surface is acting as a catalyst.
Figure 1-16 Schematic of the synthesis of polyimide-clay hybrid film. [40]
1.5.3 Polypropylene and Polyethylene Matrices
Nonpolar polymers are very difficult to intercalate into smectic clays, because the clays are strongly polar. This challenge has been met [47, 48] by first intercalating stearylamine into montmorillonite and synthetic mica. Melt-mixing the organoclays with maleic anhydride-modified polypropylene oligomers results in PP-MA intercalation. The modified organoclay is then melt-mixed with a polypropylene matrix. There is a balance between creating a polar oligomer with enough maleic anhydride to intercalate well, but nonpolar enough to mix with the polypropylene.
Unfortunately, the oligomer limits the extent of property improvement achieved to date. Polyethylene has also been successfully melt-mixed with modified montmorillonite and saponite after ion exchange with dioctadecyldimethylammonium bromide. The degree of dispersion is not excellent, and the layers are certainly not exfoliated; yet, significant modification of both the crystal structure and properties has been observed. [49]
1.5.4 Polymethylmethacrylate/Polystyrene Matrices
The processing of clay/PMMA or clay/PS composites was first done by directly intercalating the monomer into the clay, followed by polymerization. [50] This method was not successful in exfoliating the clays. At issue again is the compatibility between the clay and the monomer. One solution for PMMA has been to use appropriate ammonium salts, [51, 52] which may be reactive. Another solution is to use a comonomer as a compatibilizer. [53] A similar solution was found for polystyrene by using the reactive cationic surfactant vinylbenzyldimethyldodecylammonium as the intercalant. Exfoliated graphite/polystyrene composites have been made by similar processing methods.
Recently, a commercially viable process was developed [54] for polystyrene in which
montmorillonite intercalated with octadecyl trimethyl ammonium chloride was melt-mixed with a styrene methylvinyloxazoline copolymer. This process resulted in complete exfoliation, which could not be achieved with pure polystyrene. The hypothesis is that the hybridization is due to strong hydrogen bonding between the oxazoline groups and oxygen groups in the silicate clays.
1.5.5 Epoxy and Polyurethane Matrices
Epoxy is a widely used thermoset, with applications ranging from household glues to high-performance composites. To improve performance, increasing the Tg of epoxy and improving its properties above the Tg are desirable. Adding clays and layered silicic acids to epoxy [55-57] can greatly improve its mechanical performance, particularly at temperatures above Tg. The processing has been studied in detail. In the smectic clay/epoxy composites, the length of the intercalated organic amine determines the ease of exfoliation, and only clays with primary and secondary onium ions form exfoliated nanocomposites. After intercalation of the organic amines, the epoxy resin or a combination of resin and curing agent can be intercalated into the smectic clays or layered silicic acids. If enough resin and curing agent are intercalated and the curing process is controlled, exfoliated nanocomposites result. The acidic onium ions catalyze the intragallery polymerization or curing of the resin. If this reaction occurs more rapidly than extragallery curing, then the clay exfoliates.
Otherwise, an intercalated nanocomposite results. Therefore, careful control of temperature and time is required, or the ratio of resin to curing agent must be significantly less than the stoichiometric ratio in order to achieve exfoliation. An approach similar to that used for epoxy composites was used to make intercalated montmorillonite/polyurethane composites.
1.5.6 Polyelectrolyte Matrices
Polyelectrolytes can be used in electrochemical devices such as solid-state batteries, electrochromic devices, and sensors. [69] The addition of layered silicates to polyelectrolytes increases the conductivity, improves the mechanical stability, and improves the interfacial stability with electrode materials. Polyelectrolytes are characterized by a large number of ionizable groups and thus are highly polar. This makes them excellent candidates for intercalation into smectic clays.
Polyvinylpyridines are of particular interest because of the variety of processing methods available. Intercalated nanocomposites can be formed easily from the water-soluble hydrobromide salt of the 1,2 or 1,6 polyelectrolyte (1,2 or 2,6 polyvinylpyridinium cations). However, only a single layer of polymer intercalates, and exfoliation does not occur. A slower, but ultimately more effective process, uses neutral poly-4-vinylpyridine and results in an exfoliated composite. A second method involves intercalation of 4-vinylpyridinium salts, followed by polymerization.
Poly(ethylene oxide) (PEO) matrix composites have also been processed both by intercalating PEO in solution into organically modified smectic clays [69] and by melt- mixing clay with PEO and PEO/PMMA mixtures. [70, 71] In neither case does an exfoliated composite result. Aranda and Ruiz-Hitzky [69] dissolved PEO in acrylonitrile and found that the structure of the PEO changed when the interlayer cation was changed. Use of Na+ montmorillonite or NH4+ montmorillonite resulted in either a helical PEO or a bilayer zigzag PEO structure in the galleries. The PEO arrangement was reversible with exchange of the interlayer cations.
1.5.7 Rubber Matrices
Several applications of rubbers might benefit from inclusion of exfoliated clays.
Their greatly reduced permeability [72] would be useful for the inner liners of tires and inner tubes. [73] In addition, modification of the glass transition temperature and/or the loss modulus might be useful in a variety of damping applications.
Montmorillonite has been ion-exchanged with a protonated form of butadiene and acrylonitrile copolymer. This was subsequently mixed with nitrile butadiene rubber in the presence of crosslinking agents and resulted in highly dispersed nanocomposites.
Nanocomposites have also been prepared from dioctadecyldimethyl ammonium- exchanged montmorillonite in poly(styrene-b-butadiene) matrices. [74]
1.5.8 Others
Clay/polymer nanocomposites that include poly(e-caprolactone) have been made via in-situ polymerization. Composites that include poly(p-pheylenevinylene) have been made via intercalation of poly(xylylenedimethylsulfonium bromide) and subsequent elimination of the dimethylsulfide and HBR. [75] Those including cyclic polycarbonate [76] or polyethyleneterephthalate have been made via monomer intercalation and subsequent polymerization; and those including polyaniline via in-situ polymerization of aniline monomer. [77]
1.6 Properties of Nanocomposites