Tubular nanoclays

Haroosh, H.J., Chaudhary, D., and Dong, Y. (2012) Electrospun PLA/PCL fibers with tubular nanoclay morphological and structural analysis. J. Appl. Polym. Sci., 124, 3930-3939. 

Nanoclay fillers are categorized as platelet-like nanoclays or layered silicates and tubular nanoclays in terms of filler shape. With the configuration of two tetrahedral sheets of silicate and a sheet layer of octahedral alumina, platelet-like nanoclays or phyllosilicates are formed, which include smectite, mica, vermiculite, and chlorite. In particular, smectite clays are widely employed with further subcategories of MMT, saponite, hectorite, and nontronite. The typical MMT clays are regarded as one of the most effective nanofillers used in polymer/clay nanocomposites due to their low material cost and easy intercalation and modification (Triantafillidis et al., 2002). On the other hand, the fundamental structure of tubular nanoclays contains an aluminum hydroxide layer and a silicate hydroxide layer. They are also known as dio-ctahedral minerals with two different types of halloysite nanotubes (HNTs) and imo-golite nanotubes (INTs). Notwithstanding their material role as clay minerals, these two types of tubular nanoclays resemble the hollow tubular structure of carbon nanotubes (CNTs). In this section, three different types of clay nanofillers, namely MMTs, HNTs, and INTs are reviewed in detail along with the development of clay modification. 

HNTs are different from MMT nanoclays with respect to their fundamental structure. Consisting of an octahedral aluminum layer and a tetrahedral silicate layer with a hydrated characteristic, HNTs are also known as 1 1 phyllosUicate.. Similar to a coil, the structure of HNTs is in the form of multilayered tubes rolled up by layers of aluminosilicates as observed in Figure 6.3. HNTs belong to the kaolin famUy with a chemical formula of Al2Si205(0H)4 H20 (Liu et al., 2007). HNTs are naturally abundant and available as hydrated clays with n = 2 for the water content. In general, the evaporation process of natural kaolin leads to the HNT structure (Lvov and Abdullayev, 2013). Lvov and Abdullayev (2013) also found the HNT length is approximately 0.5—2 pm whereas the inner and outer diameters are about 10—30 nm and 50—70 nm, respectively. Moreover, ecofriendly HNTs have become more cost competitive when compared with CNTs. Their unique cylindrical tubular... 

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