DMF) And N,N-dimethylacetamide Research Articles

Interactions between Polyacrylonitrile and Solvents: Density Functional Theory Study and Two-Dimensional Infrared Correlation Analysis

Polyacrylonitrile (PAN) is a semicrystalline polymer with high polarity and is usually processed from solutions. Selected solvents for processing influence both the structure and properties of PAN products. We describe the interactions between PAN and various solvents by theoretical calculation based on density functional theories (DFT), and by experimental methods of Fourier transform infrared (FTIR) spectra and two-dimensional infrared (2D-IR) correlation analysis. The selected solvents include dimethyl sulfone (DMSO2), dimethyl sulfoxide (DMSO), ethylene carbonate (EC), propylene carbonate (PC), N,N-dimethyl formamide (DMF), and N,N-dimethyl acetamide (DMAc). Calculation results show that the PAN model monomer (PAN') interacts with each solvent through dipole-dipole interaction and formed PAN'-solvent complexes. Each complex displays an antiparallel alignment of interacting pair between the C≡N group of PAN' and the polar group of solvent molecule (S═O or C═O group). The calculated binding energies (ΔE) reveal that PAN' preferentially interacts with solvent in the order of DMSO2 > DMSO > EC > PC > DMF > DMAc. Red shifts of vibration frequencies are observed for C≡N, S═O, and C═O stretching bands. The C≡N stretching band shifts from 2245 cm(-1) in PAN to 2240, 2242, and 2241 cm(-1) in PAN-DMSO, PAN-EC, and PAN-DMF mixtures, respectively, indicating the existence of PAN-solvent interactions. Moreover, 2D-IR correlation analysis shows that as the PAN content increases, DMSO molecules vary prior to PAN-DMSO complexes, and change earlier than PAN bulk. However, PAN-EC and PAN-DMF mixtures follow the order of PAN bulk > PAN-solvent complexes > solvent molecules. This combination of theoretical simulation and experimental characterization is useful in selection of solvents for PAN or even other polar polymers and can provide an insight into the physical behavior of PAN-solvent complexes.

Solvent Interactions in Methanol/N, N-Dimethylamide Binary Systems Studied by Fourier Transform Infrared–Attenuated Total Reflection (FT-IR/ATR) and Two-Dimensional Correlation Spectroscopy (2D-COS)

The interaction of N,N-dimethyl formamide (DMF) and N,N-dimethyl acetamide (DMA) with methanol in solution mixtures was studied using Fourier transform infrared-attenuated total reflection (FT-IR/ATR) spectroscopy. The concentration-dependent FT-IR/ATR spectra of DMF/methanol and DMA/methanol mixtures were recorded in the wavenumber range 4000-650 cm(-1) to investigate wavenumber shifts as a consequence of hydrogen bonding interactions. In combination with two-dimensional correlation spectroscopy (2D-COS), the positional fluctuations observed in the ν(C=O) and ν(O-H) regions of DMF/DMA and methanol, respectively, have been discussed in terms of changing populations of differently hydrogen-bonded and interacting species of the same and different component molecules.

Study of static permittivity and hydrogen bonded structures in amide–alcohol mixed solvents

Relative static permittivity of the binary mixtures of amides (formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA)) with alcohols (ethyl alcohol (EA) and ethylene glycol (EG)) over the entire composition range have been investigated at 1 MHz and 30 °C. The concentration-dependent relative permittivity increment and the excess permittivity of these binary mixtures reveal that the strength of heterogeneous H-bonded molecular interactions, the dipolar ordering and the molar ratio of stable adduct of amides with EA and EG are influenced by number of hydroxyl groups of the alcohol molecules and the type of substitution in amide molecules.

Temperature dependent static dielectric constant and viscosity behaviour of glycerol–amide binary mixtures: Characterization of dominant complex structures in dielectric polarization and viscous flow processes

The static dielectric constant and viscosity of the binary mixtures of glycerol (Gly) with N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) were measured over the entire composition range at temperatures 288.15, 303.15, 318.15 and 333.15 K. The concentration dependent non-linear behaviour of the measured thermodynamical parameters revealed the formation of hydrogen bond interactions between glycerol and amide molecules with a variety of complexes. The excess dielectric constant and excess viscosity were determined and analyzed for the confirmation of the composition of dominant complex species. Results inferred that the dielectric polarization in both the Gly–DMF and Gly–DMA mixtures is governed by 1:1 complex species with enhanced dipolar ordering at all the investigated temperatures. The complex species of 3Gly:DMF and 2Gly:DMA facilitates the viscous flow process in Gly–DMF and Gly–DMA mixtures, respectively and the density of these species is strongly influenced by the change in temperature. Arrhenius type behaviour of viscosity against the reciprocal of temperature was used to determine the apparent activation energy of the viscous flow. The electric-field-induced increment of the Helmholtz free energy and the entropy were determined from the temperature dependence of the static dielectric constant and its derivative of the binary mixtures. Results were discussed to assess the volume effect of DMF and DMA molecules on hydrogen bonding interactions with glycerol molecules in order to confirm the structural conformations of these mixed solvents.

Modification of Carbon Nanotubes Using Poly(vinylidene fluoride) with Assistance of Supercritical Carbon Dioxide: The Impact of Solvent

We report herein a typical piezoelectric polymer, poly(vinylidene fluoride) (PVDF) to be successfully wrapped on single-walled carbon nanotubes (SWCNTs) using a simple supercritical carbon dioxide (SC CO(2)) antisolvent-induced polymer epitaxy method. Our study focused on the effect of different solvents on the morphology of PVDF wrapping on SWCNTs. Three organic solvents, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc) were chosen for PVDF. When DMSO was used as solvent, the decorating degree of PVDF on the surface of SWCNTs increases significantly with the increase of SC CO(2) pressure, and nanocrystals wrapping on SWCNTs can be observed at high pressure. FTIR and Raman spectra indicated that there exist interactions between SWCNTs and PVDF chains. What's more, FTIR results also show that there exists a transformation from the beta-phase to the alpha-phase of PVDF in DMSO with the assistance of SC CO(2), which is similar to the action of elongation/shear flow field. It indicated that the alpha-phase is the predominant form occurring on the surface of SWCNTs after treatment with SC CO(2). And the helical structure on SWCNTs observed from the TEM image reflected the alternate trans- and gauche-bond conformation of the alpha-form. When DMF or DMAc was used as the solvent, although nanocrystal wrapping and helical structure was not visible, the samples had more excellent dispersion than that in DMSO. Particularly, for DMF, a typical network structure was observed, which is similar to a spider web. Therefore, this work supplies a clue that the various morphologies of nanohybrid structure can be obtained just by changing the solvent during the treatment process of SC CO(2), and accordingly, the tailored nanohybrid structure are promising and important for functional design as a basic component in microfabrication and other fields.

Heterotactic Enthalpy of the Interaction between Protein Model Compounds and Diols in Aqueous Solutions at 310.15 K

The enthalpies of mixing for model protein compounds (glycine, alanine, N,N-dimethyl formamide (DMF) and N,N-dimethyl acetamide (DMA)) with diols (1, 3-butanediol and 2, 3-butanediol) and their respective enthalpies of dilution in aqueous solutions at 310.15 K were determined by flow microcalorimetric measurements. Heterotactic enthalpic interaction coefficients (hxy, hxxy, hxyy) were obtained by analyzing these experimental results according to McMillan-Mayer theory. The results indicated that the hxy values were all positive with a dominant endothermic effect and the coefficients decreased according to the order of hxy(DMA)hxy(Ala)hxy(Gly)hxy(DMF) and hxy(2,3-butanediol) hxy(1,3-butanediol). Based on these results, we discussed solute-solute and solute-solvent interactions with regards to the influence of the relative position of the diol functional groups on the value of hxy. We found that the hydrogen bonds between DMF or DMA and the diols were strong, and because of the good polarizability of DMF′s resonance structure, hydrogen bonds between DMF and the diols were further strengthened.

Dielectric characterization and molecular interaction behaviour in binary mixtures of amides with dimethylsulphoxide and 1,4-dioxane

Hydrogen bond molecular interactions and dipolar ordering in binary mixtures of amides (N-methylformamide (NMF), formamide (FA), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA)) with dimethylsulphoxide (DMSO) and 1,4-dioxane (Dx) have been investigated by dielectric measurements at 30 °C. The static dielectric constant, high frequency limit dielectric constant, excess dielectric constant, effective Kirkwood correlation factor, and corrective Kirkwood correlation factor of the binary mixtures over the entire concentration range were determined to explore the effect of substituted and unsubstituted amides on hetero-molecular interactions. Results confirm that the unlike molecule hydrogen bond interaction strength is comparatively 1.5 times stronger in amides–Dx mixtures as compared to the amides–DMSO mixtures, which vary linearly with the values of dielectric constant and Kirkwood correlation factor of pure amides. Dielectric studies inferred 2:1 complexation of NMF to DMSO/Dx, whereas other amides–DMSO and amides–Dx mixed solvents form most likely 1:1 complexes.

Organic−Inorganic Hybrid Slide-Ring Gels: Polyrotaxanes Consisting of Poly(dimethylsiloxane) and γ-Cyclodextrin and Subsequent Topological Cross-Linking

A novel polyrotaxane consisting of poly(dimethylsiloxane) (PDMS) and γ-cyclodextrin (γ-CD) is synthesized, and the threaded γ-CDs of polyrotaxane are intermolecularly cross-linked in solution to yield a new class of slide-ring gel. Synthesis of the polyrotaxane is achieved by an end-capping reaction of pseudo-polyrotaxane whose both ends are preliminarily activated as p-nitrophenyl ester with p-methoxytritylamine derivatives. The obtained polyrotaxane is soluble in N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc) only in the presence of lithium salts, while PDMS itself is not soluble in these media. Characterization of the polyrotaxane is performed by NMR, FT-IR, and TGA, showing that it is sparsely populated with γ-CDs. Chemical modifications of γ-CDs of the polyrotaxane improve the solubility in various organic solvents and enable further characterizations. An acetylated polyrotaxane is measured by GPC in chloroform to verify the rotaxanation, showing increase of molecular weight and refractive index compared to PDMS. Cross-linking of these polyrotaxanes with and without modifications is carried out with hexamethylene diisocyanate in solution to bridge γ-CDs of the neighboring polyrotaxanes. As a result, an organic−inorganic hybrid slide-ring gel in which PDMS chains are topologically interlocked by figure-of-eight cross-links is materialized after years of the first slide-ring gel consisting of PEG and α-CD reported in 2001.

Convenient synthesis of poly(γ‐benzyl‐L‐glutamate) from activated urethane derivatives of γ‐benzyl‐L‐glutamate

AbstractA series of activated urethane‐type derivatives of γ‐benzyl‐L‐glutamate were synthesized, and their potential as monomers for polypeptide synthesis was investigated. The derivatives of the focus of this work were a series of N‐aryloxycarbonyl‐γ‐benzyl‐L‐glutamate 1, of which aryl groups were phenyl, 4‐chlorophenyl, and 4‐nitrophenyl. These urethanes 1 were reactive in polar solvents such as dimethylsulfoxide, N,N‐dimethylformamide (DMF), and N,N‐dimethylacetamide (DMAc), and were efficiently converted into poly(γ‐benzyl‐L‐glutamate) (poly(BLG)) under mild conditions; at 60 °C without addition of any catalyst. Among the three urethanes, that having 4‐nitrophenoxycarbonyl group 1c was the most reactive to give poly(BLG) efficiently, as was expected from the highly electron deficient nature of the nitrophenoxycarbonyl group. On the other hand, the urethane 1a having phenoxycarbonyl group was also efficiently converted into poly(BLG), in spite of the intrinsically less electrophilicity of the phenoxycarbonyl group. In addition, the successful formation of poly(BLG) by the reaction of 1a favored its diluted concentration (0.1 M) much more than 2.0 M, the optimum initial concentration for 1c. 1H NMR spectroscopic analyses of the reactions in situ revealed that the predominant pathway from 1 to poly(BLG) involved the intramolecular cyclization of 1 into the corresponding N‐carboxyanhydride, with release of phenol and its successive ring‐opening polymerization with release of carbon dioxide. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2649–2657, 2008

Solvent Effect on Optical Limiting Properties of Single-Walled Carbon Nanotube Dispersions

Single-walled carbon nanotubes (SWNTs) were dispersed in N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA), respectively. The nonlinear optical properties of SWNT dispersions were studied using the open aperture Z-scan technique at 532 nm. The nonlinear extinction coefficients strongly increase with increasing SWNT concentration. In the three dispersions, the DMF dispersions show the strongest nonlinear extinction effect. In conjunction with this, the optical limiting performance is also superior for the DMF dispersions. Compared with DMF and DMA, NMP has a much better debundling effect for SWNTs; however, the optical limiting properties of the NMP dispersions is inferior. The SWNT dispersions seem to attenuate the intense light more effectively, as is shown by the stronger optical limiting response compared to the zinc phthalocyanine solution, at the same concentration. The static light scattering experiment revealed that the DMF dispersions have the largest avera...

Thermally stable and organosoluble binaphthylene‐based poly(urea‐ether‐imide)s: one‐pot preparation and characterization

AbstractA new family of wholly aromatic poly(urea‐ether‐imide)s (4a–4f) possessing binaphthylene‐twisted rings was prepared by diphenyl azidophosphate (DPAP)‐activated one‐pot polyaddition reaction of a preformed imide heterocyclic ring‐containing dicarboxylic acid, 2,2′‐bis(4‐trimellitimidophenoxy)‐ 1,1′‐binaphthyl (1) with various kinds of aromatic diamines (3a–3f). At first, with due attention to structural similarity and to compare the characterization data, a model compound 2 was synthesized by the reaction of diimide‐dicarboxylic acid 1 with two mole equivalents of aniline. In this direct method, the polymers were prepared by polyaddition reactions of the in situ‐formed diisocyanate with the aromatic diamines. Molecular weights of the poly(urea‐ether‐imide)s obtained were evaluated viscometrically, and the inherent viscosities (ηinh) measured were in the range 0.10– 0.25 dl/g. All of the polymers were characterized by FT‐IR spectroscopic method and elemental analysis. All of the resulting polymers exhibited an excellent solubility in common polar solvents such as N‐methyl‐2‐pyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N‐dimethylformamide (DMF), and N,N‐dimethylacetamide (DMAc). Crystallinity of the resulted polymers was evaluated by wide‐angle X‐ray diffraction (WXRD) method, and they exhibited nearly a non‐crystalline nature as evidenced by their diffractograms. The glass transition temperatures (Tg) of the polymers determined by differential scanning calorimetry (DSC) thermograms were in the range of 274–302°C. The temperatures at 10% weight loss (Td10%) from their thermogravimetric analysis (TGA/DTG) curves were found to be in the range of 389–414°C in nitrogen atmosphere. Copyright © 2008 John Wiley & Sons, Ltd.

The Effect of Temperature on the Equivalent Conductivities and Ion-Association Constants of Some Tris-(ethylenediamine)chromium(III) Complexes in N,N-dimethylformamide and N,N-dimethylacetamide

The equivalent conductivities of tris-(ethylenediamine)chromium complexes, [Cr(en)3]X3 (where X−= Cl−, Br−, I−; en = ethylenediamine) were measured as functions of temperature (278.15 to 328.15 K) and concentration [(1.948 ×10−4 to 10.728 ×10−4 mol⋅dm−3) and (2.282 ×10−4 to 11.246 ×10−4 mol⋅dm−3)] in N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC), respectively. Equivalent conductivity values for [Cr(en)3]X3 in DMF were found to be higher than those in DMAC. The conductivity data were analyzed with the Robinson-Stokes equations. For [Cr(en)3]X3, the limiting equivalent ionic conductivities of [Cr(en)3]3+ and the ion-association constants (KA) of the ion-pair between [Cr(en)3]3+ and the monovalent halide anions were determined in DMF and DMAC. The values of KA for three complex salts in DMF were higher than those in DMAC. This can be ascribed to an increase of the ion-association constants with a decrease of the relative permittivity of the solvents. The values of KA at 298.15 K decreased in the order Cl−> Br−> I− in DMF and Cl−> I−> Br− in DMAC. The KA values for [Cr(en)3]Cl3 increased with increasing temperature in both DMF and DMAC. For [Cr(en)3]X3(X−= Br−, I−) in both solvents, this indicates increasing disorder occurs with increasing temperature.

Theoretical Studies of the Transition-State Structures and Free Energy Barriers for Base-Catalyzed Hydrolysis of Amides

The transition-state structures and free energy barriers for the rate-determining step (i.e. the formation of a tetrahedral intermediate) of base-catalyzed hydrolysis of a series of amides in aqueous solution have been studied by performing first-principle electronic structure calculations using a hybrid supermolecule-polarizable continuum approach. The calculated results and a revisit of recently reported experimental proton inventory data reveal that the favorable transition-state structure optimized for the tetrahedral intermediate formation of hydroxide ion-catalyzed hydrolysis of formamide may have three solvating water molecules remaining on the attacking hydroxide oxygen and two additional water molecules attached to the carbonyl oxygen of formamide. The calculated results have also demonstrated interesting substituent effects on the optimized transition-state geometries, on the transition-state stabilization, and on the calculated free energy barriers for the base-catalyzed hydrolysis of amides. When some or all of the hydrogen atoms of formamide are replaced by methyl groups, the total number of water molecules hydrogen-bonding with the attacking hydroxide in the transition state decreases from three for formamide to two for N-methylacetamide, N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). The larger substituents of the amide hinder the solvent water molecules approaching the attacking hydroxide oxygen in the transition state and, therefore, destabilize the transition-state structure and increase the free energy barrier. By using the optimized most favorable transition-state structures, the calculated free energy barriers, i.e., 21.6 (or 21.7), 22.7, 23.1, and 26.0 kcal/mol for formamide, N-methylacetamide, DMF, and DMA, respectively, are in good agreement with the available experimental free energy barriers, i.e., 21.2, 21.5, 22.6, and 24.1 kcal/mol for formamide, N-methylacetamide, DMF, and DMA, respectively.

What Can You Learn from a Molecular Probe? New Insights on the Behavior of C343 in Homogeneous Solutions and AOT Reverse Micelles

The behavior of C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media and in aqueous and nonaqueous reverse micelles (RMs). In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission bands showing that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the hydrogen-bonding properties of the solvent. Specifically, in the ground state the molecule displays a bathochromic shift with the polarity polarizability (pi) and the H-bond acceptor (beta) ability of the solvents and a hypsochromic shift with the hydrogen donor ability (alpha) of the media. The carboxylic acid group causes C343 to display greater sensitivity to the beta than to the pi polarity parameter; this sensitivity increases in the excited state, while the dependence on alpha vanishes. This demonstrates that C343 forms a stable H-bond complex with solvents with high H-bond acceptor ability (high beta) and low H-bond donor character (low alpha). Spectroscopy in nonpolar solvents reveals J-aggregate formation. With information from the Kamlet-Taft analysis, C343 was used to explore RMs composed of water or polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/isooctane using absorption, emission, and time-resolved spectroscopies. Sequestered polar solvents included ethylene glycol (EG), formamide (FA), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). Dissolved in the AOT RM systems at low concentration, C343 exists as a monomer, and when introduced to the RM samples in its protonated form, C343 remains protonated driving it to reside in the interface rather than the water pool. The solvathochromic behavior of the dye depends the specific polar solvent encapsulated in the RMs, revealing different types of interactions between the solvents and the surfactant. EG and water H-bond with the AOT sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. In aqueous AOT RMs, C343 interacts with neither the sulfonate group nor the water, perhaps because of intramolecular H-bonding in the dye. DMF and DMA interact primarily through dipole-dipole forces, and the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but retains its H-bond network present in bulk solvent. Surprisingly, FA appears to be the only polar solvent other than water forming a "polar-solvent pool" with macroscopic properties similar to the bulk.

A Raman study of aqueous DMF and DMA solutions at low temperatures

Hydrogen bonding in aqueous N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) solutions was studied by Raman spectroscopy of the OH-stretching spectra of water as functions of solute concentration and temperature (from –50 to 50 °C). The Raman spectra were decomposed successfully into two components (the low- and high-frequency components). The peak area ratio of the low-frequency component to the high-frequency component increases exponentially with decreasing temperature. The frequency of the low-frequency component decreases linearly with lowering temperature down to –50 °C at all solute concentrations (solute mol fraction from 0.3 to 0.7). It is shown that the hydrogen bonds in the clathrate structures around DMA molecules are stronger than those around DMF molecules.Key words: Raman spectrum, DMF, DMA, aqueous solution, low temperature.

Internal rotation about the C–N bond of amides

The internal rotation about the C–N bond of formamide (FA), acetamide (AA), N-methylformamide (NMF), N-methylacetamide (NMA), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) in the gas phase, chloroform and water has been investigated at the HF level of theory with 6-31G(d), 6-31G(d,p) and 6-31+G(d) basis sets in order to figure out the appropriate level of theory in describing the cis–trans isomerization of the X-non-Pro and X-Pro peptide bonds. The conductor-like polarizable continuum model (CPCM) was used to calculate solvation free energies. The conformational electronic energies calculated at three levels predicted satisfactorily the experimental rotational barriers ΔG ‡ of FA, DMF and DMA, and the observed relative enthalpy changes Δ H cis/ trans of cis conformers for NMF and NMA to trans ones in the gas phase. The CPCM method at the HF/6-31+G(d) level predicted well experimental solvation free energies of acetamide derivatives. The ΔG ‡ for FA, NMF, DMF, NMA and DMA, and Δ H cis/ trans for NMF and NMA in chloroform and water were satisfactorily described by the CPCM method at HF/6-31G(d) and HF/6-31+G(d) levels. These results may indicate that the HF/6-31+G(d) level with the CPCM method appears to be the appropriate method in describing the cis–trans isomerization of the X-non-Pro and X-Pro peptide bonds in solutions.

Synthesis of Organic–Inorganic Polymer Hybrids Utilizing Amphiphilic Solvent as a Compatibilizer

Abstract Transparent organic–inorganic polymer hybrids with various organic polymers (polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC) and poly(acrylic acid) (PAA)) that are soluble in amphiphilic solvents such as N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc), could be easily synthesized by the sol–gel reaction of tetramethoxysilane (TMOS) in an amphiphilic solvent. It is because DMF and DMAc form strong hydrogen bonds with the silanol groups of the silica gel and have a high compatibility with organic polymers. DMF and DMAc play a role as compatibilizers between the organic polymer and silica gel. The hydrogen bonding interaction was confirmed by FT-IR spectra. The compatibility of the organic polymer with the amphiphilic solvent was explained by the solubility of the polymer in an amphiphilic solvent. The thermal stability of the polymer hybrids obtained was increased compared to that of bulk polymer by post-treatment. The homogeneity of the obtained polymer hybrids was confirmed by SEM and nitrogen porosimetry studies.

Combining Microsolvation and Polarizable Continuum Studies: New Insights in the Rotation Mechanism of Amides in Water

We present a quantum mechanical investigation of the rotation mechanisms of N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) in water. This rotation can happen through two distinct transition states known as TS1 and TS2, where the nitrogen lone pair is on the opposite side of the oxygen atom or on the same side, respectively. The analysis is focused on complementary descriptions of the solvent, either represented by a limited number of explicit solvent molecules (microsolvation), by an implicit (or continuum) solvation, or by combinations of these two approaches. The combined approach (microsolvation + continuum) can provide quantitative agreement with the experimental results for the gas to solution shift of the rotational barrier. For both amides, continuum effects alone are sufficient to select the correct channels. However, hydrogen-bond effects (via the explicit solvent molecules) are necessary to obtain quantitative agreement with experiment, provided this is combined with a continuum description. In the rotation in DMF, it seems that a single water molecule is directly involved, while the other solvent molecules act as a “mean field” (the bulk), which is well reproduced by a polarizable continuum medium. The mechanism in DMA is less clear. In gas phase the steric repulsive interactions between methyl groups make TS1 clearly favored with respect to TS2. In water, the larger dipole moment of TS2 produces an opposite effect with respect to the repulsion interactions, making the corresponding channel less disfavored than in gas phase. The results are compared with previous Monte Carlo simulations, and this comparison is used to draw a more general picture about how different descriptions of the solvent can take into account long-range and mediated effects on one side and shorter-range and dynamic effects on the other side.

Properties of SPEEK based PEMs for fuel cell application

Comparative studies of membranes prepared using different solvents, have shown that the casting solvent plays a significant role, affecting their proton conductivity and mechanical strength. It has been found that using DMF strongly decreases the membrane conductivity in comparison with other solvents studied. The 1 H NMR results yield an insight into the mechanism of this effect, evidencing the formation of the strong hydrogen bonding of sulfonic acid groups with DMF. This can explain the large discordances of more than an order of magnitude existing between the conductivity results for sulfonated polyetheretherketone (PEEK) in some previous studies and in this work. It is also found that residual sulphuric acid, which is very difficult to eliminate from highly sulfonated polyetheretherketone (SPEEK), also affects its conductivity and under high temperature treatment, enters into reaction with both DMF and N, N-dimethylacetamide (DMAc), causing their degradation. As discussed in the present contribution, the conductivity measurement technique may also be a reason for discrepancy in the reported conductivity characteristics of SPEEK.

Monitoring for N,N-dimethylformamide and N,N-dimethylacetamide with a diffusive sampler using distilled water as an absorbent.

To obtain an appropriate sampling method for evaluating individual workers' exposure concentrations to chemicals highly soluble in water, the suitability of LiPS (liquid passive sampler) as diffusive sampler was examined. Although adsorbed chemical substances with activated carbon felt being used as the adsorbent must be desorbed in such samplers, this process can be omitted by using liquid as the absorbent. This kind of simplification can be utilized for water-soluble solvents such as N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC). In laboratory experiments DMF and DMAC levels determined by this method were sufficiently correlated with exposure concentrations, exposure duration, and cumulative exposure levels. The sampling rates for DMF and DMAC obtained from these results were 0.50 and 0.43 mL/min, respectively, and the theoretical sampling rate for DMF obtained using Fick's diffusion formula was consistent with the experimental value. The effects of relative humidity, wind velocity, and temperature on the measured values were negligible within the range expected in a practical industrial environment. In a field survey the exposure concentrations for 38 workers handling DMF in the process of manufacturing spandex fiber and polyurethane gloves were determined both by this method and by the National Institute of Occupational Safety and Health (NIOSH) method, in which silica gel tubes are used. The DMF concentrations detected by the two methods were comparable (Y = 0.912X, correlation coefficient: 0.932). This LiPS method proved to be useful in determining the concentrations of DMF and DMAC to which workers were exposed.