Progress in polyolefin clay nanocomposites with polyether based intercalants

In spite of much research and commercial development of clay based nanocomposites, their design, manufacturing, and applications are often empirical, and large scale productions is still in its infancy. The reasons are mainly because of limited theoretical knowledge on such novel nanostructure materials, such as basic guidelines for the selection

of surfactants, and modification of clays for the purposes of targeted polymer matrix, the mechanisms of superior reinforcement observed as compared with their micro- counterparts, and the establishment of a simple processing-structure-property relationship for such nanocomposites. Nanocomposites based on the intercalation of PEO into clay minerals are an attractive substitute of conventional salt compounds [8, 30, 108]. As discussed previously, Poly(ethylene glycol) (PEG) and poly(ethylene oxide) PEO are polymers having an identical structure except for chain length and end groups, and are the most commercially important polyethers [63, 64]. Poly(ethylene glycol) refers to an oligomer or polymer of ethylene oxide with low molecular weight while polyethylene oxide is used for higher molecular weights. While PEO and PEG find use in different applications and have different physical properties (i.e. viscosity) due to chain length effects, their chemical properties are nearly identical.

Lu et al suggested that the challenges lying for in nanocomposites are to improve the compatibility of clay with desirable polymer matrix, and surface modification of clay [130]. Paul et al suggested that a surfactant with multiple alkyl tails leads to more shielding of the silicate surface and increased availability of alkyl material that leads to higher levels of organoclay exfoliation in polyethylene type matrices [131]. PEG is a non-ionic, polar, water soluble polymer. It has been widely used in many fields such as lubricants, pharmaceuticals, cosmetics, surfactants, and as a bio-degradable reagent in metal extraction. Li et al synthesized a complex molecule ((PEG/MDAB - methylacryloyloxy- ethyldodecyldimethylammonium bromide) clinging at the end of PEG molecule and suggested that the complex molecule leads to synergistic effect resulting in intercalation and exfoliation of MMT layers in PP nanocomposites [23]. Li reported that complex intercalation leads to intercalation and exfoliation of clay layers [23]. The addition of surfactants lowered the surface energy of the inorganic host and improved the wetting characteristics of the polymer matrix, resulting in a large interlayer spacing, and making the intercalation of many engineering polymers possible. The exfoliation and intercalation in the matrix is probably attributed to the synergistic effects of the complex intercalant (PEG/MDAB) on the intercalation and exfoliation of MMT. Li also inferred the improvement in rheological and mechanical property of nanocomposite [23]. Choi et al

experimented with poly(ethylene oxide) in preparing organoclay nanocomposite [132].. Choi confirmed that the physical network structure between polymer and organoclay can be confirmed by rheological measurements. Researchers experimented treating poly (ethylene glycol) with kaolinite type clays and inferred that PEG intercalates better compared with conventional surfactants [22].WAXS pattern also shows that the intercalated oxyethylene units were arranged in flattened monolayer arrangements. A PEO smectite nanocomposite with a XRD pattern identical with the PEO-smectite materials prepared via the solution intercalation method was one of the materials prepared [110]. A polytype of smectite clays were intercalated with different ethylene glycols (namely diethylene glycol, triethylene glycol) [111]. Moad et al experimented with different molecules of PEO and suggested saturation ratio of intercalants [8, 9]. Giannelis et al suggested that PEG treated clay improved the crystallization of the polymer in nanocomposites, which could be attributed to polymer chains movement enhanced by PEG

[109]. Maurer et al studied poly (ethylene oxide) / Cloisite Na+ nanocomposites gave rise

to intercalated structures displaying moderate improvement in the mechanical properties [24-26]. Maurer et al suggested that PEG nanocomposites in presence of a polar modifier, gave rise to exfoliated structures, giving a significant increase in mechanical properties [24]. PEO treated clay is thermally stable to 327 °C in an inert atmosphere and 275 °C in air [18, 19, 133]. Maurer et al melt intercalated PEG with laponite, and suggested that the nanocomposites with laponite clay have better thermal stability than the virgin polymer. Evans et al intercalated PEG with long acrylic chain with MMT and suggested that the mechanical property improved with addition of clay [134, 135]. Evans et al also claimed that clay exfoliated well in the polymer matrix, leading to a transparent nanocomposite [135]. The presence of clay substantially increased both Young's modulus and shear modulus of the polymer. Alemdar et al experimented the influence of clay surface modification on morphology and rheology of PEG / MMT nanocomposites. Alemdar suggested that high storage modulus of these intercalated structures reflect the strong clay interactions and a network structure in the polymer-organoclay nanocomposites [27]. Alemdar concluded that the surface modification of MMT resulted in a better intercalation between the MMT and PEG polymer matrix [27]. Giannelis reported that when PEO intercalated clay layers, it replaced the water molecules filling the space between the

hydrated exchangeable cations [30, 109]. Vaia et al suggested that an alkyl chain longer than 12 carbon atoms increases the chance of dispersion of clay layers in polymer matrix [120]. When the alkyl chain length was 12 and higher, exfoliated nanocomposites were formed in the presence of compatibilizer. Typically, an increase in chain length of intercalant corresponds to an increase in interlayer spacing (Figure 2.15).

Figure 2.15 Alkyl chains attached to surface of clay layers increases the chance of exfoliation [120]