Surface modification of halloysite nanotubes: role of external hydroxyl groups

Halloysite nanotubes (HNTs) are naturally occurring aluminosilicate clay minerals consisting of two adjacent sheets of gibbsite octahedra (Al (OH)3) and tetrahedrally coordinated silicon dioxide (SiO2), which are rolled up as a result of mismatch between these two sheets (Figure 18.1, Chapter 15). The rolled-up sheets occur within a few layers (5-6) in the wall of nanotubes resembling multi-walled carbon nanotubes (MWCNTs). These layers are separated by a monolayer of water molecules, which can be easily removed by heating (60-110 °C). Halloysite nanotubes are low cost and abundant. Compared with other nanofillers, used in polymer nanocomposites like CNTs and montmorillonite (MMT), the modification of HNTs and their dispersion is much easier to achieve because of their unique hollow tubular shape and crystalline structure. HNTs are non-toxic, environment-friendly, and biocompatible minerals. These unique properties have drawn attention to finding new potential applications for HNTs such as nanocontainers (Shchukin and Möhwald, 2007; Shchukin et al., 2008), sustained release (Levis and Deasy, 2003; Qi et al., 2010), immobilization (Machado et al., 2008; Zhai et al., 2010), cosmetics (Suh et al., 2011), nanoreactors (Shchukin et al., 2005; Tierrablanca et al., 2010), and reinforcement of polymeric matrixes (Du et al., 2006; Ning et al., 2007; Ye et al., 2007; Guo et al., 2008; Ismail et al., 2008; Deng et al., 2009; Guo et al., 2009; Jia et al., 2009; Handge et al., 2010; Tang et al., 2011; Soheilmoghaddam et al., 2013; Yin and Hakkarainen, 2013). Recent studies showed that HNTs can effectively improve mechanical and thermal properties of polymers such as polypropylene (PP) (Du et al., 2006; Ning et al., 2007), polyamide (PA) (Guo et al., 2009; Handge et al., 2010), ethylene propylene diene monomer (EPDM) (Ismail et al., 2008), styrene-butadiene rubber (SBR) (Guo et al., 2008; Jia et al., 2009), and epoxy (Ye et al., 2007; Deng et al., 2009; and Tang et al., 2011).