Nafion/ORMOSIL nanocomposites via polymer-in situ sol-gel reactions. 1. Probe of ORMOSIL phase nanostructures by Si solid-state NMR spectroscopy

A series of Nafion^®/ORMOSIL hybrids, generated by in situ sol-gel co-polymerizations of tetraethylorthosilicate (TEOS) and semi-organic R′_nSi(OR)_4−n co-monomers (SOC), were developed to generate a spectrum of nanoscale chemical environments within the Nafion^® morphological template. The molecular structures of the ORMOSIL phases were analyzed by means of ²⁹Si solid-state NMR (SSNMR) spectroscopy. A high average degree of Si atom coordination about SiO₄ molecular sub-units can be achieved, but a significant number of unreacted SiOH groups on reacted Q=Si(O_1/2)₄ units is always present. The shifting, with relative ORMOSIL composition, of D=RR′Si(O_1/2)₂ or T=R″Si(O_1/2)₃ (R, R′ and R″ are organic moeities) peak envelopes for difunctional or trifunctional SOCs is suggested to reflect random co-condensation rather than distinct Q and D (or T) block formation. The numbers of membrane-incorporated Q and D (or T) units per fixed sulfonate group were calculated from the ²⁹Si SSNMR spectra for those particular hybrids that had a reasonably low noise/signal aspect. Spectra for hybrids based on in situ sol-gel reactions for TEOS (no SOC present) suggest that inserted hydroquinone molecules interfere with condensation reactions between (RO)_4−xSi(OH)_x molecules and silanol oligomers to yield silicate structures with lower average coordination. While earlier small angle X-ray scattering studies indicated that ORMOSIL structures can be grown within the polar regions of Nafion^®, the results reported here address the specific compositions of these nanoscale structures.     

Synthesis and characterization of low-refractive-index fluorinated silsesquioxane-based hybrids

Novel low-refractive-index silsesquioxane-based hybrids were synthesized via hydrolytic condensation of fluorinated triethoxysilane precursors, which were prepared by reacting 3-aminopropyl triethoxysilane with acrylates containing fluoroalkyl groups, 1H,1H,5H-octafluoropentyl acrylate (OFPA) and 2,2,2-trifluoroethyl acrylate (TFEA). The hydrolytic condensations proceeded as a homogeneous system in acetone in the presence of aqueous HF solution (3.2%) at 30 °C. The products were soluble in a variety of organic solvents, including CHCl₃, THF, and acetone, but were insoluble in hexane and water. The structures of the products were confirmed by ¹H NMR, ¹³C NMR, ¹⁹F NMR, and FT-IR spectroscopy. The low polydispersities and reasonable molecular weights of the resulting fluorinated silsesquioxanes (M_n = 5800, M_w/M_n = 1.01; and M_n = 4700, M_w/M_n = 1.04 for the OFPA- and TFEA-based products, respectively) were confirmed by size-exclusion chromatography. Scanning force microscope (SFM) measurements indicated the formation of spherical hybrids having relatively narrow size distributions (average particle diameter < 3.0 nm) without aggregation. The sizes of the hybrids were also confirmed by X-ray diffraction (XRD). The refractive indexes of the TFEA- and OFPA-based silsesquioxane hybrids were 1.43 and 1.40, respectively. These results indicate the formation of novel fluorinated silsesquioxane-based hybrids having good solubility, narrow size distribution, and a low refractive index. Co-condensation of the TFEA- and OFPA-based triethoxysilane precursors affords a series of fluorinated hybrids whose refractive index and various other properties can be manipulated by varying the composition of the feed.     

Silicon-29 solid-state nuclear magnetic resonance spectroscopy of composite interfaces

The types of structures and bonds that are formed with silicons in the composite interface were studied using ²⁹Si cross-polarization/magic angle sample spinning (CP/MAS) nuclear magnetic resonance spectroscopy (NMR). The change in mobility of silane coupling agent bonded to silica, as compared with bulk hydrolyzed silane coupling agent, can be monitored by the change in line width and the shift of resonances to higher fields, as well as by the change in the silicon-proton cross-polarization time T_SiH. In the silane coupling agent-matrix resin interface, the T_SiH values reflect the change in mobility as a function of the concentration and degree of hydrolysis of the silane coupling agent. It has been demonstrated that quantitative measurements of T_SiH can be used to investigate relative mobilities.     

Solid-state F MAS and H→F CP/MAS NMR study of the phase transition behavior of vinylidene fluoride-trifluoroethylene copolymers: 1. Uniaxially drawn films of VDF 75% copolymer

The changes in the phase structures and molecular mobility caused by the ferroelectric-paraelectric phase transition of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer, P(VDF₇₅/TrFE₂₅), were analyzed using variable temperature (VT) solid-state ¹⁹F MAS and ¹H→¹⁹F CP/MAS NMR spectroscopy. The CF₂ signal of the VDF chain sequence and the CHF signal at the head-to-head linkage of VDF-TrFE sequence showed higher frequency shift in the temperature range 43-92 °C, whereas no change was found for the CHF signal at the head-to-tail linkage of VDF-TrFE up to 92 °C. Hence, VT ¹⁹F MAS spectra revealed that the VDF-TrFE head-to-tail sequence is the most stable part in polymer chains against trans-gauche conformational exchange motions below the phase transition temperature (Curie temperature, T_c) on heating. However, all chain sequences including TrFE units undergo conformational exchange at around T_c. The phase transition behavior is clearly recognized in the ¹⁹F spectral shapes, in which the broad signals of the ferroelectric immobile phase disappeared between 115 and 119 °C. In addition, T_1ρ^F for all peaks decreased to a unique value (ca. 20 ms) at 119 °C, indicating that uniform molecular motion accompanied by a full chain rotation occurred at the temperature. The significantly longer T_1ρ^F for all peaks (ca. 20 ms) in the paraelectric phase (119 °C) than that in the amorphous domain (<4 ms) at ambient temperature supports the conclusion that there is restricted rotational motion of polymer chains around the chain axis in the paraelectric phase. On cooling from 119 to 85 °C, a gradual decrease in gauche conformers in the paraelectric phase was confirmed by the low-frequency displacement of CF₂ signals in VDF sequences accompanied by slight decreases in T₁^F and T_1ρ^F. The phase transition was observed between 85 and 77 °C on cooling, in which the characteristic signals of the paraelectric phase disappeared, the T_1ρ^F values of all peaks quickly increased, and the broad crystalline signals abruptly appeared at 77 °C.     

Organic–inorganic hybrid materials. II. Chain mobility and stability of polysiloxaneimide–silica hybrids

Polysiloxaneimide–silica hybrid materials (PSI‐SiO₂) were obtained using the sol–gel technique by polycondensation of tetramethoxysilane (TMOS) in a polyamic acid solution. IR, ²⁹Si‐ and ¹³C‐NMR spectroscopy, and thermogravimetric analysis were used to study hybrids containing various proportions of TMOS and hydrolysis ratios. The morphology, dynamics, and thermal stability of the hybrids were investigated. The chain mobility of the hybrids was investigated by spin–spin relaxation time (T₂) measurements. The apparent activation energy (E_a) for degradation of the hybrids in air was studied by the van Krevelen method. The T₂ value was independent of the silica content whereas that of the E_a decreased as silica content increased. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 965–973, 2001     

Pyrolytically prepared carbon from fluorine-GIC

Formation mechanism, crystallinity, porosity and chemical reactivity were studied on the carbon prepared by pyrolysis of single phase, stage-1 fluorine-graphite intercalation compound (fluorine-GIC; C_xF). The stage-1 C_2.5F directly decomposes to fluorocarbon gases and carbon above 650 K, without forming higher stage compounds. The pyrocarbon prepared from C_2.5F gives hkl diffraction peaks indicating graphite-like stacking order of graphene layers. This carbon possesses average crystallite sizes along the c- and a-axes (L_c and L_a) of about 5 and 50 nm, respectively. The specific surface area of the pyrocarbon (about 40 m² g⁻¹) is only twice as large as that of the original crystalline graphite. Chemical behavior of the pyrocarbon as an intercalation host for sodium and potassium is similar to that of crystalline graphite, but it is easily fluorinated by elemental fluorine even at 573 K to give poly(carbon monofluoride) [(CF)_n] probably due to the small crystallite size and the mesopores formed by formation/decomposition processes of C_2.5F.     

Corrosion inhibitors : Part II: Quantum chemical studies on the corrosion inhibitions of steel in acidic medium by some triazole, oxadiazole and thiadiazole derivatives

The corrosion inhibition efficiencies of some triazole, oxadiazole and thiadiazole derivatives for steel in presence of acidic medium have been studied by using AM1, PM3, MINDO/3 and MNDO semi-empirical SCF molecular orbital methods. Geometric structures, total negative charge on the molecule (TNC), highest occupied molecular energy level (E_HOMO), lowest unoccupied molecular energy level (E_LUMO), core-core repulsion (CCR), dipole moment (μ) and linear solvation energy terms, molecular volume (V_i) and dipolar-polarization (π^*), were correlated to corrosion inhibition efficiency. Four equations were proposed to calculate corrosion inhibition efficiency. The agreement with the experimental data was found to be satisfactory; the standard deviations between the calculated and experimental results ranged between ±0.03 and ±4.18. The inhibition efficiency was closely related to orbital energies (E_HOMO and E_LUMO) and μ. The correlation between quantum parameters and experimental inhibition efficiency has been validated by single point calculations for the semi-empirical AM1 structures using B3LYP/6-31G^** as a higher level of theory. The proposed equations were applied to predict the corrosion inhibition efficiency of some related structures to select molecules of possible activity from a presumable library of compounds.     

Effect of molecular structures and mobility on the thermal and dynamical mechanical properties of thermally cured epoxy-bridged polyorganosiloxanes

The effects of molecular structures and mobility on the thermal properties of epoxy-bridged polyorganosiloxanes have been investigated by solid-state ²⁹Si and ¹³C solid state NMR in this study. Epoxy-bridged alkoxysilanes precursors with mono-(APDES), di-(APMDS), tri-(APTES) functional ethoxysilane terminal groups have been synthesized and thermally cured with or without the addition of catalysts to obtain epoxy-bridged polyorganosiloxanes. Three kinds of catalysts including acidic, basic, and organometalllic compounds have been used as the curing catalyst for the direct thermal curing of epoxy-bridged polyorganosiloxane precursors. The structures of epoxy-bridged polyorganosiloxanes with respect to the catalysts are quantitatively investigated. Acidic BF₃·MEA shows the best catalytic effects on the formation of T³ and D² structures in the epoxy-bridged polyorganosiloxanes from tri-functional epoxy-APTES and di-functional epoxy-APMDS precursors, but basic NBu₄·OH has better enhancement on the formation of M¹ structure in the epoxy-bridged polyorganosiloxanes from mono-functional epoxy-APDES precursor. TEM spectra show that the epoxy-bridged polysilsesquioxanes of epoxy-APTES precursors exhibit polysilsesquioxanes nano domain around 45-55 nm under the catalysis of dibutyltindilaurate (DBTDL), but show bigger polysilsesquioxanes nano domain around 50-150 nm under the catalysis of basic tetrabutylammonium hydroxide (NBu₄·OH) in epoxy matrix after direct thermal curing process.The coefficient of thermal expansion of the epoxy-bridged polyorganosiloxanes are affected by the functionality of terminated alkoxysilanes and the species of catalyst used during curing process. The epoxy-bridged polysilsesquioxanes of epoxy-APTES precursor possesses the lowest coefficient of thermal expansion compared with the other two epoxy-bridged polyorganosiloxanes from mono-, and di-functional epoxy-bridged polyorganosiloxanes precursors. There is no obvious T_g observed in the epoxy-bridged polysilsesquioxanes of epoxy-APTES precursor from the analysis of TMA and DMA. ¹³C solid state NMR has been used to investigate the molecular motion behaviors of epoxy-bridged polyorganosiloxanes structures with respect to the changes in T_g and CTE. The (the relaxation time of ¹³C after the spin lock process) of the epoxy-bridged polysilsesquioxanes of epoxy-APTES precursor is longer than that of epoxy-APMDS precursor, which indicates that the molecular mobility of epoxy-bridged polysilsesquioxanes of epoxy-APTES is highly restricted due to the strong intermolecular interaction of nano hybrid network.     

Morphological and conductivity studies of di-ureasil xerogels containing lithium triflate

Sol-gel derived poly(oxyethylene)/siloxane hybrids doped with lithium triflate, LiCF₃SO₃, have been investigated. The host hybrid matrix of these materials, named di-ureasil and represented by U(600), is composed by a siliceous framework to which polyether chains containing 8.5 oxyethylene repeat units are covalently bonded through urea linkages. Xerogel samples U(600)_nLiCF₃SO₃ with n (where n is the molar ratio of oxyethylene moieties per Li⁺ ion) between ∞ and 0.1 have been examined. X-ray diffraction and differential scanning calorimetry have provided conclusive evidence that the xerogels analyzed are entirely amorphous. The salt-rich material with n=1 exhibits the highest conductivity over the whole range of temperature analyzed (e.g. 4.3×10⁻⁶ and 2.0×10⁻⁴ Ω⁻¹ cm⁻¹, respectively, at 25 and 94 °C).     

Formaldehyde Oligomerization on Silicalite: An FTIR and NMR Study

Upon adsorption of formaldehyde onto silicalite, hydrogen-bonded adducts are formed between the (slightly acidic) SiOH groups of the purely siliceous zeolite and formaldehyde, as seen by using both FTIR and ¹H-NMR spectroscopies. These species then evolve to form polymerization products of the type –(CH₂O) _n –. On the contrary, when silicalite is contacted with trioxane hydrogen-bonded adducts are formed, but no polymerization ensues.     

Fragmentation of Fluorinated Model Compounds Exposed to Oxygen Radicals: Spin Trapping ESR Experiments and Implications for the Behaviour of Proton Exchange Membranes Used in Fuel Cells

Low‐molecular weight model compounds (MCs) for Nafion membranes used in fuel cells were exposed at 300 K to ^·OH radicals produced by UV irradiation of aqueous H₂O₂ solutions. The MCs contained fluorinated and partially fluorinated groups terminated by sulphonic or carboxylic acid groups. The fragmentation process in the MCs was studied by spin trapping electron spin resonance (ESR) methods, using 5,5‐dimethylpyrroline‐N‐oxide (DMPO), N‐tert‐butyl‐α‐phenylnitrone (PBN) and 2‐methyl‐2‐nitrosopropane (MNP) as the spin traps. The objective of these experiments was to assess the effect of the type of ionic groups (sulphonic or carboxylic) and of fluorine substitution on the spin adducts detected. DMPO experiments led to the detection of spin adducts of ^·OH and of carbon‐centred radicals (CCRs), and allowed the determination of the ^·OH attack site on the ionic and/or on the protiated or fluorinated groups. CCR adducts were also detected when using PBN as a spin trap; a key point in the interpretation of the PBN results was, however, the realisation that MNP is formed during PBN exposure to UV irradiation and oxygen or other oxidants such as H₂O₂. Experiments with MNP as the spin trap were the most informative in terms of structural details for adducts obtained from each MC. The results allowed the identification of CCRs present as adducts, based on large hyperfine splittings (hfs) from, and the number of, interacting ¹⁹F nuclei; in addition, oxygen‐centred radicals (OCRs) as MNP adducts were also identified, with much lower hfs from ¹⁹F nuclei. Taken together, the results deduced by spin trapping suggest that both sulphonic acid and acetic acid groups can be attacked by ^·OH radicals and confirm two possible degradation mechanisms in Nafion membranes: initiated at the backbone and at the side chain.     

Divalent silver oxide-diatomite hybrids: Synthesis,characterization and antibacterial activity

To produce better antibacterial and low water-soluble submicron powders of divalent silver oxide (AgO), divalent silver oxide-diatomite (AgO-d) hybrids were studied. AgO-d hybrids were prepared by chemical oxidation, using silver nitrate and diatomite as raw materials and potassium persulfate as oxidant. The results show that AgO-d hybrids with AgO weight percentage up to 20.8% are obtained by oxidation of Ag⁺ adsorbing on diatomite in alkaline solution (n(KOH)/n(AgNO₃)=7.5) for 1.5 h at 333.15 K. Products were characterized by laser particle sizer, SEM, XRD, XPS, FT-IR and atomic absorption spectrophotometer (AAS). AgO-d hybrids are composed of tetragonal cristobalite, amorphous silica, monoclinic divalent silver oxide and a few of cubic silver oxide. Element Ag can be released from AgO-d hybrids but the dissolution speed is slow, which is about 3.20×10^?2 mg (L h)^?1. Antibacterial effectiveness of AgO-d hybrids was tested against Staphylococcus aureus (S. aureus ATCC6538) and Escherichia coli (E. coli ATCC8099) by the shake-flask method. Results show that AgO-d hybrids possess excellent antibacterial properties. When the concentration of AgO-d hybrids is 10 mg L^?1 and the contact time with S. aureus and E. coli is 30 min, the bactericidal rates reach up to 99.974% and 99.944%, respectively.     

Synthesis and emission analysis of RE (Eu or Dy):Li2TiO3 ceramics

The development and photoluminescence analysis of Eu³⁺or Dy³⁺ ions in the matrix of lithium titanate (Li₂TiO₃) ceramics by using a solid state reaction method are reported. Emission spectra of Eu³⁺:Li₂TiO₃ ceramics have shown strong red emission at 611 nm (⁵D₀ → ⁷F₂) with λ_exci = 392 nm (⁷F₀ → ⁵L₆) and from the Dy³⁺:Li₂TiO₃, a blue emission at 493 nm (⁴F_9/2 → ⁶H_15/2) and also an yellow emission at 582 nm (⁴F_9/2 → ⁶H_13/2) have been observed with λ_exci = 366 nm (⁶H_15/2 → ⁶P_5/2). Both the rare-earth ions containing ceramics have displayed their brighter emission performance from their measured spectral results. In addition, X-ray diffraction (XRD), Fourier transform infra red (FTIR) spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) have been used to characterize the structural properties of (Eu³⁺ or Dy³⁺):Li₂TiO₃ ceramics.     

Brønsted acidity of amorphous silica–aluminas studied by 1H NMR

¹H broad-line (4 K) and MAS (room temperature) NMR have been used to study the acid strength of two amorphous silica–aluminas interacting or not with adsorbed water. The study is more difficult than for zeolites, because the acidic SiO(H)Al bridges are reversibly destroyed by dehydration. However, an acidity coefficient value (H₃O⁺ concentration per Brønsted acid site when one water molecule interacts with each Brønsted site) of 0.34±10%; has been determined. This value is equal to that obtained for H-faujasite and H-mordenite samples with Si/Al ratios high enough for maximum acid strength.     

Quantitative determinations in the molecular structures of polyaluminocarbosilane

The molecular structure of polyaluminocarbosilane (PACS) derived from polycarbosilane (PCS) was investigated by ¹H, ²⁹Si, ²⁷Al NMR and elemental analyses. Quantitative methods based on experimental data were established to determine the exact amounts of O-containing groups, namely Si−O−Al and Si−O−Si, and the relative numbers of linear and cyclic Si−C segments in PACS. The HSiC₂O groups and the SiH/SiH₂ in linear Si−C segments, single and condensed six-member Si−C rings were quantitatively distinguished. The results showed that PACS molecules consisted predominantly of single six-member Si−C rings. The introduction of Al into PCS led to H consumption through which Al was closely bonded to O by breaking Si−H or Si−CH₃ to form Si−O−Al cross-linking bonds and AlO_x (x = 4-6) groups in PACS. The AlO_x (x = 4-6) groups acted as linkage centers to bridge smaller PCS molecules through which the molecular weight and its distribution coefficient of PACS were both apparently increased.     

Three hydrogen-bonded nanotubular zinc(II) complexes of N-(9-anthracenyl)-N′-(4-pyridyl)-urea

Three hydrogen-bonded nanotubular zinc(II) complexes of a monodentate ligand N-(9-anthracenyl)-N′-(4-pyridyl)urea (L), [Zn(OAc)₂L₂]?H₂O (1), [ZnBr(OAc)L₂]?H₂O (2) and [ZnCl(OAc)L₂]?4H₂O (3), were synthesized and structurally characterized. In the complexes, the central metal ion is tetrahedrally coordinated by the pyridyl nitrogen atoms of two ligands and different anions, while the urea groups of the ligands self-associate into the typical urea tape through R₂¹(6) hydrogen bonds, which are essential for the formation of the nanotubes. The fluorescence properties of ligand L and the complexes were studied in the solid state at room temperature.     

NMR study of the gamma radiolysis of poly(dimethyl siloxane) under vacuum at 303 K

The structural changes which occur on the γ-radiolysis of poly(dimethyl siloxane) (PDMS) under vacuum at 303 K have been investigated using ²⁹Si and ¹³C NMR. New structural units consistent with main chain scission and crosslinking through both H-linking and Y-linking reactions have been identified. The results obtained at various absorbed doses have been used to calculate the G-values for scission and crosslinking. G-values for scission of G(S)=1.3±0.2, for H-linking of G(D^CH₂-R)=0.34±0.02 and for Y-linking of G(Y)=1.70±0.09 were obtained for radiolysis under vacuum at 303 K. Thus crosslinking predominates over scission for radiolysis of PDMS under these conditions, and, by contrast with previous studies, Y-links have been shown to be the predominant form of crosslinks.     

Oxa-perfluoroalkyl end-capped PEG-based amphiphilic fluorocarbon polymer: synthesis and self-assembly behavior in water

Oxy-perfluoroalkylated end-capped polyethylene glycol (PEG) was synthesized by the reaction of PEG and polyethylene glycol monomethylether (PEG(MeO)) with trimer of hexafluoropropylene oxide (HFPO). The structures of the fluorinated polymers were characterized by ¹H NMR, ¹⁹F NMR and FTIR. Their surface activities study shows that the fluorinated polymers have high surface activity. Core-shell micellization of the fluorinated polymers was inferred from the ¹⁹F NMR spectra derived in two different solvent environments (CDCl₃ and D₂O). Their self-assembly behaviors in water were studied in detail by pyrene fluorescence techniques, dynamic light scattering and scan electron microscopy. The results demonstrated that due to different fluorine content PEGF with two end-capped fluorocarbon groups can form large aggregates with the diameter size of about 132 nm, whereas PEG(MeO)F with only one-end-capped fluorocarbon groups forms small micelles with the size of round 12 nm.     

Zingerone Modulates Neuronal Voltage-Gated Na+ and L-Type Ca2+ Currents

Zingerone (ZO), a nontoxic methoxyphenol, has been demonstrated to exert various important biological effects. However, its action on varying types of ionic currents and how they concert in neuronal cells remain incompletely understood. With the aid of patch clamp technology, we investigated the effects of ZO on the amplitude, gating, and hysteresis of plasmalemmal ionic currents from both pituitary tumor (GH₃) cells and hippocampal (mHippoE-14) neurons. The exposure of the GH₃ cells to ZO differentially diminished the peak and late components of the I_Na. Using a double ramp pulse, the amplitude of the I_Na(P) was measured, and the appearance of a hysteresis loop was observed. Moreover, ZO reversed the tefluthrin-mediated augmentation of the hysteretic strength of the I_Na(P) and led to a reduction in the I_Ca,L. As a double ramp pulse was applied, two types of voltage-dependent hysteresis loops were identified in the I_Ca,L, and the replacement with BaCl₂-attenuated hysteresis of the I_Ca,L enhanced the I_Ca,L amplitude along with the current amplitude (i.e., the I_Ba). The hysteretic magnitude of the I_Ca,L activated by the double pulse was attenuated by ZO. The peak and late I_Na in the hippocampal mHippoE-14 neurons was also differentially inhibited by ZO. In addition to acting on the production of reactive oxygen species, ZO produced effects on multiple ionic currents demonstrated herein that, considered together, may significantly impact the functional activities of neuronal cells.