Selection of suitable intercalant

The objective of this research is to determine an intercalant, and to characterize the properties of PP nanocomposite made using the intercalant and the clay. As discussed in earlier sections, alcohols of higher n-series (n-dodecanol, n-octanol), acetone, di-isobutyl ketone, formaldehyde, formamide did not intercalate well with clay. Theng et al suggested that hydroxylic compounds (alcohols) are hydrogen bonded to the clay surface, which will not increase the d-spacing appreciably [7]. Sorbitan monostearate (SMS) is a monoalkyl, lilophilic, non-ionic surfactant. SMS intercalates by pushing the clay layers apart to 1.45 nm. SMS has got a long chain that should be compatible with PP and PP-g-MA. Thermal stability of SMS was comparable with other intercalants, but d-spacing was not good compared with 18-crown-6-ether treated clay. Polyethers compounds were experimented because of their interaction with metal ions. With polyethers, complexes are much stronger compared with sorbitan monostearate. 18-crown – 6 ether treated clay possesses better thermal stability than other treated clays (in this research). Yao et al suggested that the crown ether treated clay polystyrene nanocomposites exhibits better thermal stability and flame retardancy than pure polystyrene [12]. Crown ether does not have a long chain to react with PP and it has less chance to exfoliate the clay layers. Sodium ions are complexed within the crown ether molecules, and hence have less chance to overcome the forces between the clay layers and exfoliate the clay layers. PEG with different molecular weights 300, 600, and 1000 g/mol were treated with clay. D-spacing increased better than with other intercalants mentioned before.

PEG based compounds can react with sodium ions of MMT and surface oxygen atoms of silicate sheets. Thermal stability was comparable with other literature where authors treated clay with poly(ethylene glycol) based surfactants [18, 19]. TGA analyses revealed that complete decomposition of the organic component of oxyethylene-based clay did not occur until greater than 320 °C. Weight loss above 320 °C could be associated with release of carbonaceous material intercalated with in the clay matrix. PEG based compounds can operate as ligands of the interlayer cations inherent to the clay matrix, giving stable complexes. Hitzky suggested that in inert atmosphere, PEG treated clay were thermally stable between 227 and 327 °C [19]. Above 320 °C, the intercalated material is

progressively eliminated giving a collapsed phase of the starting MMT. Ray et al and Utracki et al suggested that for an intercalant to be strongly adsorbed, it would be necessary either that the molecule be ionically bound to the surface, or that it be large and sufficiently flexible to make a large number of point contacts with the surface [3, 4]. This condition is satisfied by PEG monolaurate, which has six to eight ethylene glycol units and a long alkyl chain (12 carbon atoms) (Figure 5.7, Section 5.2.3.3). The hypothesis was supported by the WAXS pattern (Figure 5.6) PEG 400 ML and PEG 600 ML intercalated comparably well, but the thermal stability was appreciable compared with other intercalants except 18-crown-6 ether, at high temperatures. It would be reasonable to suppose that an increase in the number of long chains would increase the basal spacing. PEG 400 ML and PEG 600 ML intercalated better and had a long alkyl chain that could interact with PP and PP-g-MA. This lead to the selection of clay treated with poly(ethylene glycol) monolaurate for the preparation of nanocomposites. The effect of the surfactants on the dry clay spacing is shown in Table 5.1. Clearly, the basal spacing between the clay plates increased with chain length and number of long chains. The basal layer spacings in the dry clays scale as expected, with longer chains increasing the basal spacing. We can also observe that d-spacing is relatively closer to commercially available clays (Cloisite 30B).

Clay d-spacing (in nm) (calculated using Bragg’s equation)

Sodium MMT (untreated) 1.18

Clay treated with

Sorbitan monostearate 1.45

18-Crown-6 ether 1.58

Poly(ethylene glycol) 1000 1.60

Poly(ethylene glycol) 600 1.64

Poly(ethylene glycol) 300 1.69

Polyethylene glycol monolaurate (Mn=600 g/mol) 1.79

Polyethylene glycol monolaurate (Mn=400 g/mol) 1.82

Commercial clay (Southern clay products)

Cloisite 25A 1.85

Cloisite 30B 1.86

Table 5.1 Basal spacing (d001) expansion with different intercalants observed in this

research

5.4 Conclusion

Layered clay has been intercalated by polyethers that can complex with the clay surfaces instead of ion-exchange of sodium ions (Figure 5.7, Section 5.2.3.3). Interlamellar adsorption of organic compounds by MMT can be measured by WAXS methods, and it has been shown that a wide range of organic compounds can be adsorbed. Thermogravimetric analysis was applicable to study the thermal stability and clay content of the composites. WAXS provides support that adsorbed polar organic compounds, interact with exchangeable cations rather than with the silicate surface. Increase in d-spacing attested our hypothesis of choice of intercalants from a pool of fifty different compounds, initially proposed. The d-spacing increment in this research was consistent with the d-spacing achieved in alkyl ammonium treated clays. TGA analyses supported the selection of intercalants, based on its mass loss reduction with temperature. Non-ionic organic compounds (PEG and SMS) are adsorbed and strongly retained by the clay. Sodium ions between clay layers influenced the amount of adsorption, because of its dependence on the

dispersion of the clay, and the accessibility of intralamellar surfaces. It should be noted that not all clay platelets can be fully treated due to natural defects and charge heterogeneities that pre-exist. Therefore, some of the organic surfactants may not be complexed with sodium ions and were labile to washing in treatment of clay. The clays were intercalated well using a conventional boiling technique that does not include any special requirements for treatment of clay. In the process of surface modification of MMT, attempt was made to obtain a surface featuring a high hydrophobicity and increased affinity to non-polar polymer. Chapters 6, 7, 8 and 9 will discuss the characterization and properties of PP nanocomposites based on PEG based surfactants with longer chains, which not only intercalates the clay but has a long molecule to compatbilize with PP. Clay treated with PEG ML was chosen, as a starting point in making nanocomposites. The following chapter will discuss structural property of PP – PEG ML treated clay nanocomposites.