ReviewCan nanoparticles really enhance thermal stability of polymers? Part II: An overview on thermal decomposition of polycondensation polymers
Highlights
• The effect of nanoparticles on polymer thermal stability was evaluated. • The introduction of clays into polymeric matrices can accelerate their thermal decomposition. • Thermal stability can be also increased due to lower diffusion of degradation byproducts. • Carbon nanotubes and other nanoparticles were reported that can increase thermal stability of polymers.
Introduction
Polymer nanocomposites are an important class of polymers that have wide applications in a number of different industrial sectors and, thus, organic/inorganic nanocomposite materials have been extensively studied in recent decades. Nanomaterials are classified into three categories; nanoparticles, nanotubes and nanolayers, depending on how many dimensions of the dispersed particles are in the nanometer range [1], [2]. Inorganic nanoscale fillers, which are viewed as being very important, include layered silicates (such as montmorillonite), nanotubes (mainly carbon nanotubes, CNTs), fullerenes, SiO2, metal oxides (e.g., TiO2, Fe2O3 Al2O3), nanoparticles of metals (e.g., Au, Ag), polyhedral oligomeric silsesquioxane (POSS), semiconductors (e.g., PbS, CdS), carbon black, nanodiamonds, etc. All these nanoparticles are used in order to enhance physical and, mainly, mechanical properties [3], [4], [5], [6], [7], [8]. It is well known that nanoparticles can also enhance flame retardancy and thermal degradation of nanocomposites [9], [10], [11], [12], [13]. This is very important since in recent years the demand for non-halogenated flame retardant additives for polymeric end products is increasing. Thus, the nanoscale reinforcement of polymers is becoming an attractive mean of improving the thermal stability of polymers. High thermal stability is one of many important properties required for various industrial applications.
High melt temperature engineering polymers, such as PA-6, PA-6,6, poly(ethylene terephthalate) (PET), and polycarbonate (PC), raise additional concerns when considering preparation of nanocomposites by melt extrusion or when processing (injection molding) the nanocomposites into final molded products [14]. The concern is that most of the used nanomaterials may cause thermal degradation reactions, especially in the case that organic modifiers are used as treatments for these nanoparticles and, mainly, in clays. This is because clays are generally highly hydrophilic species and therefore naturally incompatible with a wide range of polymer types. In clays the space between the closely packed sheets are on the order of ∼1 nm. Thus there is a large entropic barrier associated with the molten polymer diffusing into the gap and hence, intercalation into the layered clay is hindered. For this reason, the clay must be treated before it can be used to make a nanocomposite. One method of lowering this entropic barrier is to anchor short, surfactant-like chains in, or onto, the surface of the clays sheets, thereby forming the so-called organically modified clays (OMMT). The silicate surface is modified by exchanging the cations (ion exchanging) initially present in the interlayer with organic cationic surfactants, mainly including primary, secondary, tertiary and quaternary alkylammonium or alkylphosphonium cations. This can be done because the cations are not strongly bound to the clay surface, so small molecule cations can replace the cations present on the clay. The organic cations lower the surface energy of the inorganic host, improving the wetting with the polymer matrix. Furthermore, their long aliphatic tails, attached with their cationic head via Coulombic interactions to the surface of the negatively charged silicates, result in a larger interlayer spacing. However, the most of organic modifiers have an onset of thermal decomposition at about 200 °C or below [15]. The melt processing temperature of PA-6,6, PET, and PC are above this temperature, and to facilitate rapid manufacturing of filled PA-6, typical industrial melt-processing temperatures for PA-6 are in excess of 300 °C. Thus, new and thermally stable organomodifiers for the preparation of layered silicates nanocomposites should be used.
By studying the reported papers in literature different and sometimes contradictory results have been mentioned concerning the effect of the nanoparticles on polymer thermal stability. There are papers suggesting that nanoparticles have no obviously effect on thermal stability, some of them suggested a small to substantial enhancement and some others suggested acceleration of thermal decomposition. Nanocomposites based on clay or with other nanoparticles with a larger amount of hydroxyl groups can exhibited a much more pronounced degradation because the hydroxyl groups acted as Brønsted acidic sites and accelerated the polymer's degradation [16]. It was suggested that during thermal decomposition in nitrogen the clay can slow down degradation of polymer as a mass transport protective barrier, but the catalytic effect of the metal derivatives in the clays could accelerate the decomposition behavior of a polymer. The combination of these two effects determined the final thermal stability of the nanocomposite. Furthermore, some nanoparticles have reactive groups such as –COOH and –OH, which can also accelerate the decomposition of polymers or to form covalent bonds leading to their thermal degradation enhancement.
Thus, the main question is what is the really effect of nanoparticles on polymer thermal stability?
The aim of this report is to highlight the latest findings concerning the effect of different nanoparticles on thermal stability of polymer/nanocomposites, in order to answer the above question. In this part only condensation polymers are reported and the review is focus to the effects of the kind of used nanoparticles, their used amounts, their dispersion and the interactions with polymer matrices.
Section snippets
Poly(ethylene terephthalate) (PET)
PET is an alipharomatic and semi-crystalline thermoplastic polyester. It has a variety of excellent properties, such as good mechanical properties, resistance to fatigue, and high rigidity, low cost, high transparency, high processability, and moderate recyclability. Thus, PET holds a potential for industrial application, including industrial fibers, packaging, films, bottles and engineering plastics. However, disadvantages such as a low molecular weight and modulus limit its use in engineering
Conclusions
In the present review the thermal degradation studies have been limited to TGA analysis, as a part of a routine characterization for polymer nanocomposites and the characterization of evolved gases using several techniques. It was found that the nanoparticles can behave in a different way on the polymers’ thermal stability.
In the reported studies, it has been shown that the introduction of clays into polymeric matrices can accelerate the thermal decomposition of the polymer matrix, consisted by
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