PMMA/LDH nanocomposite

FIGURE 11.24 TGA curves of neat PMMA and PMMA/LDH nanocomposite (5% loading) in N2 at 20°C/min. A PMMA/CuA12 B PMMA C PMMA/ZnAl2 D PMMA-CoA12 E PMMA/NiAl2. (From Manzi-Nshuti, C. et al., J. Mater. Chem., 18, 3091, 2008. With permission.)... 

Figure 2.15 Heat release rate curves for PMMA and PMMA-LDH nanocomposite prepared by suspension polymerization with different LDH loadings. Reproduced with permission from reference 142.

Until 2003, Chen s [28], Qu s [29-31], and Hu s [32] groups independently reported nanocomposites with polymeric matrices for the first time the. In Hsueh and Chen s work, exfoUated polyimide/LDH was prepared by in situ polymerization of a mixture of aminobenzoate-modified Mg-Al LDH and polyamic acid (polyimide precursor) in N,N-dimethylactamide [28]. In other work, Chen and Qu successfully synthesized exfoliated polyethylene-g-maleic anhydride (PE-g-MA)/LDH nanocomposites by refluxing in a nonpolar xylene solution of PE-g-MA [29,30]. Then, Li et al. prepared polyfmethyl methacrylate) (PMMA)/MgAl LDH by exfoliation/adsorption with acetone as cosolvent [32]. Since then, polymer/LDH nanocomposites have attracted extensive interest. The wide variety of polymers used for nanocomposite preparation include polyethylene (PE) [29, 30, 33 9], polystyrene (PS) [48, 50-58], poly(propylene carbonate) [59], poly(3-hydroxybutyrate) [60-62], poly(vinyl chloride) [63], syndiotactic polystyrene [64], polyurethane [65], poly[(3-hydroxybutyrate)-co-(3-hydroxyvalerate)] [66], polypropylene (PP) [48, 67-70], nylon 6 [9,71,72], ethylene vinyl acetate copolymer (EVA) [73-77], poly(L-lactide) [78], poly(ethylene terephthalate) [79, 80], poly(caprolactone) [81], poly(p-dioxanone) [82], poly(vinyl alcohol) [83], PMMA [32,47, 48, 57, 84-93], poly(2-hydroxyethyl methacrylate) [94], poly(styrene-co-methyl methacrylate) [95], polyimide [28], and epoxy [96-98]. These nanocomposites often exhibit enhanced mechanical, thermal, optical, and electrical properties and flame retardancy. Among them, the thermal properties and flame retardancy are the most interesting and will be discussed in the following sections. 

Wilkie and co-workers [57] utilized the anions 2-ethylhexyl sulfate, bis(2-ethylhexyl) phosphate, and dodecyl benzenesulfonate as intercalated anions to synthesize organo-LDHs. Nanocomposites of PMMA and PS with organo-LDHs were prepared both by meltblending and by bulk polymerization. XRD and TEM results revealed that the phosphate and sulfonate LDHs in PMMA show fairly good dispersion at the nanometer scale, whereas sulfa LDH is poorly dispersed. Eor PS, the LDHs are poorly dispersed and agglomerated LDH particles are observed. The reductions in PHRR for nanocomposites containing sulfate, phosphate, and sulfonate LDH are 27, 37, and 45% in PMMA and 32, 33, and 49% in PS, respectively. Both PMMA and PS samples obtained from bulk polymerization show poorer dispersion and less reduction in PHRR than samples obtained from melt-blending. 

Figure 2.16 (a) Transmittance of PMMA-MgAl-LDH nanocomposite sheet with a... 

When LDHs are introduced into polymer matrices, the transparency of the polymers is often sacrificed. Only a few transparent polymer-LDH nanocomposite systems can be obtained. Figure 2.16 shows a photograph of a 0.5 mm thick sheet of PMMA-MgAl-LDH nanocomposite with a 33 wt% LDH loading, which indicates the high transparency of the nanocomposite, whereas the PMMA-MgAl-LDH microcomposite is an opaque white sheet even when its thickness is less than 0.1 mm. ... 

In Situ Intercalative Polymerization A variety of polymer nanocomposites have been prepared using this method, that is, PS/graphene, PMMA/expanded graphite, poly(styrene sulfonate) (PSS)/layered double hydroxyl (LDH), PI/LDH, and PET/LDH. 

Not only the divalent metal species but also the metal ratios may influence the fire retardant properties of LDHs. By comparing the cone calorimetry results for PMMA nanocomposites based on ZnAl LDH with Zn Al ratios of 2 1 and 3 1, Wilkie and coworkers found that ZnsAl gives a reduction of 35% in PHRR, whereas ZnaAl gives 26% when both are at 10% LDH loading [86]. 

In another work, series of carboxylates from C-IO to C-22 have been used to examine the effect of anions on fire retardancy of PMMA/MgAl and PS/MgAl nanocomposites [90]. All the carboxylate-modified LDHs are well dispersed in PMMA, whereas none of them is well dispersed in PS. Eigure 13.11 shows the plot of PHRR versus number of carbons in the modifiers for both PMMA and PS nanocomposites. There is a little dependence of PHRR on the length of the carboxylate for PMMA. The reduction in PHRR is between 49 and 58% for all of the carboxylate-modified LDHs at 10% LDH loading. However, the PHRR reduction of the PS system falls off as the carboxylate chain length increases 56%... 

G. A. Wang, C. C. Wang, and C. Y. Chen, The disorderly exfoliated LDHs/PMMA nanocomposite synthesized by in situ bulk polymerization. Polymer, 46 (2005), 5065-lA. 

Figure 2.10 TEM images of nanocomposites of PMMA (Al, Bl) and copolymer PMMA-co-bis[2-(methacryloyloxy)ethyl] phosphate (A2, B2) with HDEHP-LDH prepared from suspension polymerization. Reproduced with permission from reference 142.

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