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Organically modified layered silicates (OLS) with high thermal stability are critical for synthesis and processing of polymer layered silicate nanocomposites (PLSN). In the current study, the non-oxidative thermal degradation chemistry of alkyl and aryl quaternary phosphonium-modified montmorillonites (P-MMT) was examined using TGA combined with pyrolysis/GC-MS. The morphology evolution at elevated temperature was investigated using in-situ high-temperature X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The onset decomposition temperature via TGA of these P-MMTs ranged from 190 to 230 °C. The initial degradation of the alkyl P-MMTs follows potentially two reaction pathways − β-elimination [Eβ] and nucleophilic displacement at phosphorus [SN(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 radical generation through homologous cleavage of the P−phenyl bond. Overall, 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 that of N-MMT. These observations indicate that, in addition to their conventional purpose as stabilizers, phosphonium salts offer unique opportunities for melting processing polymer layered silicate nanocomposites.

A new series of linear, asymmetrical diphenylaminofluorene-based chromophores (AFX) with various strong π-electron acceptors were synthesized and evaluated for two-photon absorptivity. These chromophores were studied to determine a suitable replacement for 2-(4-vinyl)pyridine, the π acceptor for our previously reported AFX series, which contains a photochemically and thermo-oxidatively unstable olefinic unit. In addition to the benzoyl group (AF-370), these π-electron acceptors include 2-benzothiazolyl (AF-240), 2-benzoxazolyl (AF-390), 2-(N-phenyl)benzimidazolyl (AF-386), and 2-(3,4-diphenyl)imidazolyl (AF-385) moieties (five-membered heterocycles) and the 2-quinoxalinyl (AF-260) group (six-membered heterocycle). From nanosecond nonlinear transmission measurements, these new chromophores have effective two-photon cross sections (σ2‘) at 800 nm, spanning from 3.87 × 10-48 cm4 s/(photon molecule) for AF-385 to 97.46 × 10-48 cm4 s/(photon molecule) for AF-240. Two of them, 2-benzothiazolyl-end-capped AF-240 and benzoyl-containing AF-370 [σ2‘ = 84.32 × 10-48 cm4 s/(photon molecule)] stand out as having relatively good, albeit lower, values of two-photon cross sections, as compared to that of previously reported N,N-diphenyl-7-[2-(4-pyridinyl)ethenyl]-9,9-didecyl-fluorene-2-amine, AF-50 [σ2‘ = 115.6 × 10-48 cm4 s/(photon molecule)]. However, we observed that AF-240 was more photochemically robust than AF-50 when their THF solutions were subjected to repetitive and prolonged exposure to nanosecond laser radiation. Finally, on the basis of our nanosecond TPA cross-section data (σ2‘/MW values), the general trend for π-electron accepting ability, i.e., ability to accept charge transferred from diphenylamine appears to be as follows:  2-(4-vinyl)pyridine > 2-benzothiazolyl > benzoyl > 2-(N-phenylbenzimidazolyl > 2-quinoxalinyl > 2-benzoxazolyl > 2-(4,5-diphenyl)imidazoyl.