Thermal degradation chemistry of organically modified layered silicates and properties of polystyrene-montmorillonite nanocomposites

Organically modified layered silicates (OLS) with high thermal stability are critical for synthesis and processing of polymer-layered silicate nanocomposites (PLSN). The nature of the montmorillonite/surfactant/polymer interface affects dispersion and thus physico-chemical properties of PLSNs. Fundamental studies on the interfacial chemical properties are scarce due to the high temperatures at which the synthesis of PLSNs is conducted. In the present work, thermal gravimetric analysis (TGA) combined with Fourier Transform Infrared Spectroscopy and Mass Spectrometry (TG/FTIR/MS) and Pyrolysis/GC-MS have been applied to characterize the non-oxidative thermal degradation chemistry of montmorillonite and quaternary ammonium/phosphonium modified montmorillonite. In addition, the effect of the presence of nanoclay on the properties of polystyrene nanocomposites has also been addressed.; The initial degradation of the ammonium surfactant generally proceeds by a Hoffmann (β-elimination) process. However, when present in the montmorillonite, additional mechanisms, such as nucleophilic substitution, are observed. The multiple pathways are attributed to the chemically heterogeneous morphology of the layered silicates. Catalytic sites on the aluminosilicate layer reduce the thermal stability of a fraction of the surfactants by an average of 15–25°C. Depending on the analysis technique and procedure employed, the on-set temperature of non-isothermal degradation varies between 165 and 200°C. The release of organic compounds is staged and associated with retardation of product transfer arising from the pseudo-two dimensional morphology of the montmorillonite.; On the other hand, the initial degradation of the alkyl phosphonium modified montmorillonite (P-MMTs) follows potentially two reaction pathways—β-elimination [Eβ] and nucleophilic displacement at phosphorus [S_N(P)]—reflecting the multiple environments of the surfactant in the silicate. Aryl P-MMT decomposition proceeds via either a reductive elimination through a five-coordinate intermediate or a radical generation through homologous cleavage of the P-phenyl bond. In conjunction, the thermal stability of the P-MMT depends to a greater degree on the architecture of the phosphonium surfactant than previously reported for N-MMTs. Additionally, the interlayer environment of the montmorillonite has a more severe effect on stability of the phosphonium surfactant than previously reported for ammonium-modified montmorillonite (N-MMT). Nonetheless, the overall thermal stability of P-MMT is higher than N-MMT.; Polystyrene-organo-montmorillonite (PS-MMT) nanocomposites show improved thermal stability and enhanced mechanical properties compared to virgin polystyrene (PS) as evidenced by TGA and DMA experiments. The alkyl chain length of surfactant used in fabricating organo-MMT affects the mechanical properties of the synthesized PS-MMT nanocomposites. The longer the alkyl chain length of the surfactant, the higher the glass transition temperature of the synthesized PS nanocomposite. The organo-MMT in the nanocomposites seems to have dual roles, as nanofiller to increase the storage modulus and as plasticizer to decrease the storage modulus, resulting in a lower storage modulus of PS-TMOMMT and PS-TMTMMT nanocomposites than that of PS-TMDMMT and PS-TMCMMT nanocomposites. In addition, it was found that P-MMT-based PS nanocomposites exhibit better thermal stability and fire-retardant properties than N-MMT-based PS nanocomposites. This is attributed to the higher decomposition temperature of P-MMT as compared to that of N-MMT.