Polymer nanocomposites surface treatment

Provided in this chapter is an overview on the fundamentals of polymer nanocomposites, including structure, properties, and surface treatment of the nanoadditives, design of the modifiers, modification of the nanoadditives and structure of modified nanoadditives, synthesis and struc-ture/morphology of the polymer nanocomposites, and the effect of nanoadditives on thermal and fire performance of the matrix polymers and mechanism. Trends for the study of polymer nanocomposites are also provided. This covers all kinds of inorganic nanoadditives, but the primary focus is on clays (particularly on the silicate clays and the layered double hydroxides) and carbon nanotubes. The reader who needs to have more detailed information and/or a better picture about nanoadditives and their influence on the matrix polymers, particularly on the thermal and fire performance, may peruse some key reviews, books, and papers in this area, which are listed at the end of the chapter. 

Clays, natural or synthetic, are the most widely investigated and understood nanoadditives used to enhance the flame retardancy of polymers through nanocomposite technology, because of their unique properties, particularly the ease of surface treatment and application in polymer matrices. Clay can be cationic and anionic materials, in accordance with the charge on the clay layers. In this chapter, the focus is on two kinds of clays montmorillonite (MMT), a naturally occurring cationic clay that belongs to the smectite group of silicates, and LDH, an anionic clay that does occur naturally but for which the synthetic form is more common. Other clays will also be mentioned as appropriate. 

However, the simple melt mixing of polyolefins with natural clays, does not guarantee a sufficient level of dispersion of the nanoparticles, which are often present in the form of micron-size agglomerates. In order to overcome this problem, two main strategies have been followed surface functionalization of needle-like clays (usually by alkyl-silanes) or addition of a third polymeric phase (usually maleic anhydrite modified PP PP-g-MA), which acts as a compatibilizer between the matrix and nanofiller. Both methods tend to modify the surface energies of the nanocomposite system, in order to reduce the interparticle interaction and improve the dispersion. In the case of a reactive surface treatment only, the polymer-day interaction is expected to be enhanced, along with better nanoclay dispersion, which is very important for the final mechanical properties. 

From the tensile tests presented, it appeared that the nanocomposites made by alkyl silane-functionalized sepiolite give the best mechanical performances, in particular for what concerns the yield stress. In fact, the sepiolite surface fimctionalization by silane is a reactive treatment, which decreases the interparticle aggregation (improved dispersion) and, at the same time, increases the matrix-filler interactions. The addition of fimc-tionalized polymers is, instead, a nonreactive surface treatment. It leads to a decrease of the particle-particle interaction but can also reduce the matrix-particle interaction, which leads to lower yield stress and ultimate tensile stress. 

M.A. Osman, A. AtaUah, High-density polyethylene microand nanocomposites effect of particle shape, size and surface treatment on polymer crystalltnity and gas permeability, Macromolecular Rapid Communications 25 (2004) 1540-1544. 

EVA/clay/ATH nanocomposites - Effect of particle size and surface treatment of ATH filler. Polymer Degradation and Stability, 93 (2008), 2032-7. 

In addition to interfacial interactions between the CNT and the polymer matrix, the dispersion of CNTs in the polymer has signifieant influence on the performance of a CNT-polymer nanocomposite. Many different approaches have been used by researchers in an attempt to disperse CNT in polymer matrix such as physical sonication and chemical modification of CNT surface [124-126]. Functionalization of CNT surface can lead to the construction of chemical bonds between the nanotube and polymer matrix and offers the most efficient solution for the formation of strong interface. A strong interface between the coupled CNT-polymer creates an efficient stress transfer [31]. It should be noted that covalent functionalization of CNT may disrupt the grapheme sheet bonding, and thereby reduce the mechanical properties of the final product. However, noncovalent treatment of CNT can improve the CNT-polymer (Fig. 23.10) composite properties through various specific interactions [127]. 

Optimizing the hydrophilic-hydrophobic balance of the surface treatment of montmorillonite in the context of the hydrophilic-hydrophobic balance of the polymer continuous phase is also critical to the successful preparation of polymer-montmorillonite nanocomposites. Neither is this a panacea. Useful parameters (for example, solubility parameters) can be employed as guides for the choice of the proper surface modification of the montmorillonite. 

---