Polymer-organoclay nanocomposites thermal stability

A recent publication [13] illustrates the value of designing surface treatments for montmorillonite that are expressly developed for the preparation of organoclays with thermal stabilities commensurate or superior to the thermal stabilities of polymers utilized to prepare polymer-clay nanocomposites. The merging of oligomeric silsesquioxanes (POSS) with imidazolium charged head groups provides a very impressive thermally stable organomontmorillonite. 

Polyimide-clay nanocomposites constitute another example of the synthesis of nanocomposite from polymer solution [70-76]. Polyimide-clay nanocomposite films were produced via polymerization of 4,4 -diaminodiphenyl ether and pyromellitic dianhydride in dimethylacetamide (DMAC) solvent, followed by mixing of the poly(amic acid) solution with organoclay dispersed in DMAC. Synthetic mica and MMT produced primarily exfoliated nanocomposites, while saponite and hectorite led to only monolayer intercalation in the clay galleries [71]. Dramatic improvements in barrier properties, thermal stability, and modulus were observed for these nanocomposites. Polyimide-clay nanocomposites containing only a small fraction of clay exhibited a several-fold reduction in the... 

The improvement in thermal stability of the nanocomposites compared to the neat EVA/natural rubber is due to the barrier effect and insulating properties of organoclay. The well dispersed plate-like silicate layers form a tortuous path in the polymer matrix which gives a barrier effect and inhibits the diffusion of volatile degradation product from the inside of the polymer matrix. Moreover the well-dispersed silicate layers restrict the movement of polymeric chains, hence reducing the free volume for diffusion of volatile degradation products. Other researchers also confirm that organoclay tends to form a compact char-like residue on the surface of the nanocomposites when it is burnt. This char-like structure is incombustible and acts as an insulator which inhibits heat transfer to the inside of the nanocomposites. At 8 phr... 

Cellulose has been identified as a source of biopolymer that can be used as a substitute for petroleum polymers. EPN have been successfully synthesized from cellulose acetate, triethyl citrate plasticizer, and organically modified clay (Misra 2004). The polymer matrix for nanocomposite contains 80 wt% pure cellulose acetate and 20 wt% triethyl citrate plasticizer. Results show that better exfoliated and intercalated structure were obtained from nanocomposites containing 5 and 10 wt% organoclay compared with that of 15 wt% organoclay. Tensile strength of ceUulosic plastic reinforced with 10 wt% organoclay improved by 180 % and thermal stability of the cellulosic plastic also increased. 

The decomposition of ammonium compounds, as considered for neat organoclays, may not necessarily be the exact degradation process occurring in a polymer matrix. It may be altered by complex interactions between nanocomposite components and products of their thermal decomposition. The composition of commercial polymeric materials containing dispersed nanoparticles is quite complex, as it incorporates a set of additional additives, such as UV and thermal stabilizers, plasticizers, and dyes and other fillers. These additives may considerably influence the degradation mechanism [38, 39]. 

Other thermally stable surfactants have been used in the preparation of organoclays to produce PS nanocomposites [57]. Triphenylhexadecylstibonium trifluoromethylsulfonate (Table 3.4) was prepared with sodium montmorillonite to prepare polystyrene nanocomposites by bulk polymerization. The organoclay was not uniformly distributed throughout the polymer matrix, but there was evidence of polymer intercalation and a small amount of clay exfoliation (Figure 3.13). The nanocomposite showed enhanced thermal stability. 

Charring polymers such as PA-6 and PA-6 nanocomposites were used by Bourbigot et al. [27] to improve the flame retardancy of EVA. The organoclay increased the efficiency of the char as a protective barrier by thermal stabilization of a phosphorcarbonaceous structure in the inmmescent char and additionally the formation of a ceramic layer. Hu et al. [28] used a blend of PA-6 and EVA nanocomposite to improve the flame retardancy of PR... 

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