Liquid crystalline main chain polymers with a poly(p-phenyleneterephthalate) backbone. 2. Fiber spinning and mechanical properties of polyesters with alkoxy side chains

Fiber spinning and mechanical properties of four rigid polyesters with alkoxy substituents of different length and placement were evaluated. Properties of oriented fibers from the polymer with dodecyloxy substituents on the terephthalate moiety, PTA12HQ, were significantly affected by the crystal modification. At room temperature the following properties (tensile modulus E, tensile strength σ_b, and strain at break ε_b) could be obtained: E = 9.5 GPa, σ_b = 85 MPa and ε_b = 1.1% for phase L_f (the “frozen in layered mesophase”); E = 10.4 GPa, σ_b = 59 MPa and ε_b = 0.6% for modification A; E = 17.3 GPa, σ_b = 158 MPa and ε_b = 1.2% for modification B. Because of the higher amount of main chains per cross sectional area the polymer with hexyloxy side chains, PTA6HQ, showed better properties at a comparable degree of molecular orientation: E = 24 GPa, σ_b = 270 MPa, ε_b = 1.4%. Fibers obtained from the polyester with dodecyloxy substituents on the hydroquinone moiety, PTAHQ12, were too brittle to handle. The polyester with dodecyloxy substituents on both moieties, PTA12HQ12, was spun from the isotropic melt. Because of the obtained low degree of orientation, properties (E = 1 GPa, σ_b = 40 MPa, and ε_b = 6.3%) were governed by interactions between the chains (the main chains are not load-bearing). © 1993 John Wiley & Sons, Inc.     

Intermolecular hydrogen bond energies in crystals evaluated using electron density properties: DFT computations with periodic boundary conditions

The hydrogen bond (H‐bond) energies are evaluated for 18 molecular crystals with 28 moderate and strong O? H···O bonds using the approaches based on the electron density properties, which are derived from the B3LYP/6‐311G** calculations with periodic boundary conditions. The approaches considered explore linear relationships between the local electronic kinetic G_b and potential V_b densities at the H···O bond critical point and the H‐bond energy E_HB. Comparison of the computed E_HB values with the experimental data and enthalpies evaluated using the empirical correlation of spectral and thermodynamic parameters (Iogansen, Spectrochim. Acta Part A 1999 , 55, 1585) enables to estimate the accuracy and applicability limits of the approaches used. The V_b?E_HB approach overestimates the energy of moderate H‐bonds (E_HB < 60 kJ/mol) by ～20% and gives unreliably high energies for crystals with strong H‐bonds. On the other hand, the G_b?E_HB approach affords reliable results for the crystals under consideration. The linear relationship between G_b and E_HB is basis set superposition error (BSSE) free and allows to estimate the H‐bond energy without computing it by means of the supramolecular approach. Therefore, for the evaluation of H‐bond energies in molecular crystals, the G_b value can be recommended to be obtained from both density functional theory (DFT) computations with periodic boundary conditions and precise X‐ray diffraction experiments. © 2012 Wiley Periodicals, Inc.     

Dehydration of solids in different modes as a proof of their primary congruent dissociative vaporization

The main purpose of this paper is to prove the applicability of the mechanism of congruent dissociative vaporization (CDV) to the solid-state decomposition kinetics through the comparison of the fundamental theoretical relationship E_i/E_e=(a+b)/a resulted from this mechanism with experiment. It has been shown that the ratios of E_i and E_e parameters of the Arrhenius equation measured in the isobaric and equimolar modes (in the presence and absence of H₂O vapour) for 22 reactants with the general formula aSalt⋅bH₂O or aOxide⋅bH₂O are in agreement with the values of (a+b)/a. The relative standard deviation is only 17% and the correlation coefficient is close to 0.99. A probability of accidental correlation for all set of the E parameters taken from the literature is lower than 4⋅10^–16 . This strongly supports the validity of the CDV mechanism. The problem of stability of polyatomic molecules of inorganic salts in the gaseous state, which are the primary decomposition products of crystalline hydrates, was also discussed on the basis of recent mass spectroscopy studies. It was concluded that any doubts in the applicability of the CDV mechanism as a general mechanism of solid-state decomposition reactions are unsound.     

Effects of high pressure on the conductivity and dielectric constant of conductive organic polymers

Previous theory for pressure effects on conduction in amorphous macromolecular solids is extended to include pressure effects on hyperelectronic polarization. A theoretical estimate of hopping activation energies is given. Experimental studies of the frequency effects of the pressure coefficients of conduction and permittivity permits resolution of the salient factors. Carrier mobility in ekaconjugated polymers is best represented by activated hopping, for conductivity increase with frequency, and permittivity drops. Conductivity and hyperelectronic polarization increase markedly with pressure. Their activation energies, E_a0, and the pressure coefficients of their activation energies (b_E + b_H) are nearly identical. E_a0 is frequency-dependent, but the (b_E + b_H) terms are not, showing the frequency dependence of E_a0 to lie in the hopping activation enthalpy. Similar arguments on the “total” pressure coefficients b and b for conduction and polarization show the latter coefficients to have a frequency dependence lying in the hopping activation entropy.     

A combinatorial chemistry approach to new materials for non‐linear optics. I. Five Schiff bases

A combinatorial chemistry approach has been used to synthesize an array of Schiff bases, five of which, namely N‐[(E,2E)‐3‐(4‐methoxy­phenyl)‐2‐propenyl­idene]‐3‐nitro­aniline, C₁₆H₁₄N₂O₃, (1a), N‐[(E,2E)‐3‐(4‐methoxy­phenyl)‐2‐propenyl­idene]‐4‐nitro­aniline, C₁₆H₁₄N₂O₃, (2a), N‐{(E,2E)‐3‐[4‐(di­methyl­amino)­phenyl]‐2‐propenyl­idene}‐3‐nitro­aniline, C₁₇H₁₇N₃O₂, (1b), N‐{(E,2E)‐3‐[4‐(di­methyl­amino)­phenyl]‐2‐propenyl­idene}‐4‐nitro­aniline, C₁₇H₁₇N₃O₂, (2b), and N‐{(E,2E)‐3‐[4‐(di­methyl­amino)­phenyl]‐2‐propenyl­idene}‐2‐methyl‐4‐nitro­aniline, C₁₈H₁₉N₃O₂, (3b), have been structurally characterized. A stack structure is observed for (1a) and (1b) in the crystal phase. Experimental and calculated molecular structures are discussed for these compounds which belong to a chemical class having potential applications as non‐linear optical materials.     

Competing intermolecular interactions in some `bridge‐flipped' isomeric phenylhydrazones

To examine the roles of competing intermolecular interactions in differentiating the molecular packing arrangements of some isomeric phenylhydrazones from each other, the crystal structures of five nitrile–halogen substituted phenylhydrazones and two nitro–halogen substituted phenylhydrazones have been determined and are described here: (E)‐4‐cyanobenzaldehyde 4‐chlorophenylhydrazone, C₁₄H₁₀ClN₃, (Ia); (E)‐4‐cyanobenzaldehyde 4‐bromophenylhydrazone, C₁₄H₁₀BrN₃, (Ib); (E)‐4‐cyanobenzaldehyde 4‐iodophenylhydrazone, C₁₄H₁₀IN₃, (Ic); (E)‐4‐bromobenzaldehyde 4‐cyanophenylhydrazone, C₁₄H₁₀BrN₃, (IIb); (E)‐4‐iodobenzaldehyde 4‐cyanophenylhydrazone, C₁₄H₁₀IN₃, (IIc); (E)‐4‐chlorobenzaldehyde 4‐nitrophenylhydrazone, C₁₃H₁₀ClN₃O₂, (III); and (E)‐4‐nitrobenzaldehyde 4‐chlorophenylhydrazone, C₁₃H₁₀ClN₃O₂, (IV). Both (Ia) and (Ib) are disordered (less than 7% of the molecules have the minor orientation in each structure). Pairs (Ia)/(Ib) and (IIb)/(IIc), related by a halogen exchange, are isomorphous, but none of the `bridge‐flipped' isomeric pairs, viz. (Ib)/(IIb), (Ic)/(IIc) or (III)/(IV), is isomorphous. In the nitrile–halogen structures (Ia)–(Ic) and (IIb)–(IIc), only the bridge N—H group and not the bridge C—H group acts as a hydrogen‐bond donor to the nitrile group, but in the nitro–halogen structures (III) (with Z′ = 2) and (IV), both the bridge N—H group and the bridge C—H group interact with the nitro group as hydrogen‐bond donors, albeit via different motifs. The occurrence here of the bridge C—H contact with a hydrogen‐bond acceptor suggests the possibility that other pairs of `bridge‐flipped' isomeric phenylhydrazones may prove to be isomorphous, regardless of the change from isomer to isomer in the position of the N—H group within the bridge.     

From the Principle of Areno-Analogy to Heterocyclopolyaromatic Compounds

Organometallic linkage of heteroaromatic compounds provided a means of synthesizing complicated combinations of heteroaromatic compounds; not only nucleophilic aromatic substitution, but also “Ar? Cu/Ar? Hal linkage”, “organometallic oxidative linkage”, and “metal amide linkage” have been employed. The heterocyclopolyaromatic compounds are made of one, two, or three kinds of heteroaromatic species as ring members. These syntheses illustrate the construction of heterocycles from large, performed structural units. Competition experiments showed that the reactivity typical of the individual species is enhanced in open-chain combinations (Ar_Nu)_n and (Ar_E)_n (Ar_Nu, Ar_E: nucleo- and electrophilic heteroaromatic systems, respectively); the opposite situation is mostly encountered in the case of Ar_Nu? A_E combinations.—Cycloocta[1,2-b:4,3-b′5,6-b″:8,7-b?]tetrathiophene, the only heterocyclopolyaromatic system yet to have been studied in detail, proved surprisingly inclined to undergo monosubstitution reactions.     

Linearity relationship between solvatochromic shifts of stilbazolium-like dyes and modified reaction field function

We analyzed statistically the linear correlation of the solva-tochromic shifts of the stilbazolium-like dyes in the nonselected solvents with the reaction field function, L(εr) - bL( n2), and the solvent polarity parameter, ETN, respectively, and observed that there were not perfectly linearity relationships between them, so we introduced ETN into L(εr) - bL(n2) to form a new reaction field function, L(εr) - bL(n2) g ETN, called as the modified reaction field function, which can be perfectly linearly correlated with the solvatochromic shifts of the stilbazolium-like dyes in the nonselected solvents.     

Asymptotic Behavior of the t Expansion

The asymptotic behavior of a system's ground-state energy from the t expansion of Horn and Weinstein has been suggested to have the form E ₁(t)=E ₁+exp(–a _n t+b _n ). In the limit of very large t, this becomes E ₁(t)=E ₁+exp(–a ₁ t+b ₁). A simple analysis shows that the parameters are a ₁=E ₂–E ₁ and b ₁=ln[(E ₂–E ₁)|c ₂|²/|c ₁|²]. Functions are introduced which allow determination of a ₁, b ₁ and lower bounds to E ₁.     

Rayleigh–Schrödinger perturbation expansions for a hydrogen atom in a polynomial perturbation

Rayleigh–Schrouml;dinger (RS ) perturbation expansions for the eigenvalues E(λ) of a hydrogen atom in the general polynomial perturbation V(r) = aλr + b>²r², a, b > 0, are studied. When a² = 2b, the ground state energy is exactly E(λ) = -(1/2) + (3/2)a>, i.e., the RS series is truncated. In the case a² > 2b, the RS series is negative Stieltjes. In general, when λ < 0, a well of depth ω ≈ -a²/(4b²) (note the λ independence) is situated at r_ω = a/(2b|λ|). When a² > 2b/N², and interaction between this well and the hydrogenic state ψ_NLM(λ) is possible, thus creating a pair of asymptotically degenerate eigenstates separated by a “gap” δE(λ). The large order behavior of the RS coefficients E may be computed from the asymptotics of δE(λ), which is, in turn, related to the tunnelling integral. For excited states, stricter inequalities must be obeyed for Stieltjes behavior. The E⁽ⁿ⁾_NLM may be calculated either numerically or in closed form via the “so(4, 2) Lie algebra technology” for such hydrogenic problems.     

Hooke numbers of polymers

Hooke numbers He ≡ σ_b/(E) are calculated from published ultimate tensile strengths σ_b, tensile moduli E, and ultimate elongations ?_b. Data for common thermoplastics and natural fibers each follow a function He = [1 + (?_b/?_crit)^ab]^?1/b with a Hookean region I (He = 1) at ?_b ? ?_crit, a non-Hookean region III at ?_b ? _crit, and a transition region II for ?_b ≈ ?_crit. Only non-Hookean regions III were found for semisimultaneous interpenetrating networks from polyisobutylene-polymethyl methacrylate, thermoplastic elastomers from segmented polyamide-polyethers, molecular composites from poly(p-phenylene benzobisthiazole) and poly[2,5(6)-benzimidazole], and three groups of various synthetic fibers. The Hooke numbers of lyotropic and thermotropic liquid-crystalline polymers vary with the heat treatment and depend on orientation angles for orientation angles greater than ca. 10°. Hooke numbers much greater than 1 are observed for highly stressed polymers. ©1995 John Wiley & Sons, Inc.     

X-ray photoelectron spectroscopic characterization of the acetylene cyclotrimerization catalyst NbCl2(CnHn) (n = 10–12)

The elemental and ionic compositions of the surface of NbCl₂(C _n H _n ) (n = 10–12), an active catalyst for acetylene cyclotrimerization into benzene, have been determined by X-ray photoelectron spectroscopy. Binding energy data for the sample sputtered with argon ions E _b (Nb3d _5/2) = 203.8–204.2 eV) suggest that the oxidation state of niobium in the active catalyst is +2 or +3. The narrow C1s line indicates the equivalence of all carbon atoms, and the corresponding binding energy, E b(C1s) = 284.0 eV, is close to the BE value for cyclic unsaturated hydrocarbons with conjugate double bonds. Interacting with atmospheric oxygen and moisture during sample preparation, niobium ions on the catalyst surface oxidize to their highest oxidation state, +5, characterized by E _b (Nb3d _5/2) = 207.3–207.7 eV. These data suggest that niobium oxychlorides or oxides form ion the sample surface. The catalyst is stable in a high vacuum and undergoes slight charging under the action of an X-ray beam, showing poor dielectric properties.     

TL glow-curve deconvolution functions for various kinetic orders and continuous trap distribution: Acceptance criteria for E and s values

The glow curve deconvolution (GCD) analysis of a composite thermoluminescence (TL) glow curve into its individual glow-peaks needs appropriate equations describing a single glow peak. In the present work, new single glow peak equations are presented, which are produced by transformation of the I(n ₀,E,s,T) and I(n ₀,E,s,b,T) single glow-peak equations into I(I _m,T _m,E,T) and I(I _m,T _m,E,b,T), respectively. Moreover, equations of the forms I(I _m,T _m,w,b,T) are also introduced. The proposed equations have two basic advantages: (1) they use parameters, which are directly obtained from the experimental glow peaks and (2) their accuracy is equal to that of the original thermoluminescence single glow-peak equations.     

In Situ Study of the Selective Oxidation of Methanol to Formaldehyde on Copper

Combined use of X-ray photoelectron spectroscopy (XPS) and in situ mass spectrometry made it possible to simultaneously obtain the O1s spectra and the mass spectrometric signal of formaldehyde (m/z = 30) in the course of heating (420–670 K) of polycrystalline foil in a flow of the reaction mixture of methanol and oxygen (with a total pressure of 0.1 mbar and a ratio of 3/1). It is shown that the O1s spectra contain two lines with E _b = 530.1 and 531.2 eV, whose relative intensities depend on the sample temperature. At a low temperature (420 K) the line with a lower binding energy dominates, whereas sample heating leads to a drastic decrease in its intensity and its replacement by a line with a higher value of E _b. A decrease in the intensity of the latter line occurs at T > 550 K, in the same temperature range as a drastic increase in the intensity of the formaldehyde signal. These lines were assigned on the basis of literature data and data obtained by the authors for the known forms of oxygen on copper and for the intermediate species of the reaction, such as methoxy and formate. The O1s line with E _b = 530.1 eV was assigned to methoxy groups, and the line with E _b = 531.2 eV was assigned to suboxide oxygen. The correlation of the intensity of the XPS signal of suboxide oxygen with the yield of formaldehyde was supported by stationary experiments using in situ XPS that prove its participation in the key step of the selective oxidation of methanol to formaldehyde.     

Comprehensive understanding of size‐, shape‐, and composition‐dependent polarizabilities of SimCn (m,n = 1–4) clusters

We performed a comprehensive study of the size‐, shape‐, and composition‐dependent polarizabilities of Si_mC_n (m, n = 1–4) clusters on the basis of the density‐functional‐based coupled perturbed Hartree–Fock calculations. We found better correlations between the polarizabilities and both the binding energies (E_b) and change in charge distribution (Δq) than the energy gaps. The α values exhibit overall decreasing and increasing trends with increases in the E_b and Δq values, respectively. For isomers with the same E_b values and different polarizabilities, Δq can well explain the difference in polarizabilities. The π‐electron delocalization effect is the best factor for understanding the shape‐dependence. For a given m/n value, the linear clusters have an obviously larger polarizability than both the prolate and compact clusters, irrespective of the cluster size. We fit a quantitative expression [α = A ? (A ? B) × exp(?k(m/n))] to describe the composition‐dependent polarizabilities. © 2012 Wiley Periodicals, Inc.     

Asymptotic behavior of the RHF energy of the PPP model of alternant cyclic polyenes

The large N expansion of the restricted Hartree–Fock (RHF) exchange energy per atom E(N) of the Pariser–Parr–Pople (PPP) model of cyclic polyenes (annulenes) C_NH_N is derived in detail. We explicitly derive the coefficients E₀ and E₁ of the asymptotic expansion: E(N)=E₀+E₁ ln N/N²+O(N⁻²), N→∞, in the very simple case of half-filling and no bond alternation. The exchange energy per atom in the infinite chain can be written as E_ex=(2/π²)∑^∞_j=1{[γ_∞(2j−1)]/[(2j−1)²]}, where γ_∞ is the two-electron repulsion integral in the infinite chain. On the other hand, the second coefficient E₁ giving a finite-size correction is found to be 1/2b, where b is the bond length. This value of E₁ differs slightly from that of a linear chain with periodic boundary conditions because the distance between sites depends upon the radius of the ring, i.e., upon N. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 397–407, 1998     

Theoretical Investigation on the Topochemical Polymerization of Diacetylenes

Abstract

The solid-state polymerization of diacetylenes (MDA-PBT-PDA) is studied with a concerted reaction model and the calculation method of EHMO-ASED and EHCO-ASED, where MDA = crystalline molecular diacetylenes, PBT = polybutatrienes, and PDA = polydiacetylenes. As the reaction goes on, the symmetry of frontier orbitals inverts at state PBT, HOCO from C ₂-antisymmetry to C ₂-symmetry and LUCO from C ₂-symmetry to C ₂-antisymmetry, which means completion of the 1,4-addition. Two necessary conditions must be satisfied for the reaction to take place: 1) the geometric parameters must undergo a series of concerted changes to make the conformation suitable for the intermolecular 1,4-addition, which should overcome an energy barrier E_b ; 2) the symmetry match between the frontier crystal orbitals of the reactant and the product must be satisfied-electrons of the reactant should be excited from HOCO (C ₂-antisymmetry) into LUCO (C ₂-symmetry), which faces an energy gap E _g. At state MDA, there is E _g(MDA) ≈ 5.6 eV. If MDA and PDA are analyzed according to Woodward-Hoffmann's rules, this reaction would be considered photochemically allowed but thermochemically forbidden. It has been shown that the E _g gradually decreases along the reaction coordinate from state MDA to PBT. At state PBT there is E _g(PBT) ≤ 0.1 eV, and the electrons of the reactant can be easily excited there. Since E_b ≤ 1.0 eV is not very large and E_g (PBT) ≤ 0.1 eV is very small, the two necessary conditions mentioned above can be satisfied thermally. Therefore, thermal polymerization can take place smoothly. By this pathway the apparent activation energy of the reaction will be E_a ≤ 1.0 eV, which is consistent with the activation energies of the polymerizations of diacetylenes in the literature.     

Film formation stages for poly(vinyl acetate) latex particles: a photon transmission study

Photon transmission technique was used to monitor the evolution of transparency during film formation from poly(vinyl acetate) (PVAc) latex particles. The latex films were prepared below the glass transition temperature (T _g) of PVAc. These films were annealed at elevated temperatures in various time intervals above the T _g of PVAc. It is observed that transmitted photon intensity (I _tr) from these films increased as the annealing temperature is increased. It is seen from I _tr curves that there are two film formation stages. These successive stages are named void closure (viscous flow) and interdiffusion. The activation energies for viscous flow (ΔH) and backbone motion (ΔE _b) were obtained by using well-defined models. The averaged values of the backbone (ΔE _b) and the viscous flow activation energies (ΔH) were found to be 188.6 and 5.6 kcal/mol, respectively. The minimum film formation (τ _M,T _M) and healing points (τ _H,T _H) were determined. Minimum film formation (ΔE _M) and healing activation energies (ΔE _H) were measured using these time–temperature pairs. ΔE _M and ΔE _H were found to be 32.5 and 28.3 kcal/mol, respectively.     

Thermodynamic characteristics of poly(cyclohexylethylene‐b‐ethylene‐co‐ethylethylene) block copolymers

A series of poly(cyclohexylethylene‐b‐ethylene‐co‐ethylethylene) (C‐E/E_E) diblock copolymers containing approximately 50% by volume glassy C blocks and varying fraction (x) of E_E repeat units, 0.07 ≤ x ≤ 0.90, was synthesized by anionic polymerization and catalytic hydrogenation. The effects of ethyl branch content on the melt state segment–segment (χ) interaction parameter and soft (E/E_E) block crystallinity were studied. The percent crystallinity ranged from approximately 30% at x = 0.07 to 0% at about x ≥ 0.30, while the melting temperature changed from 101 °C at x = 0.07 to 44 °C at x = 0.28. Dynamic mechanical spectroscopy was employed to determine the order–disorder transition (ODT) temperatures, from which χ was calculated assuming the mean‐field prediction (χN_n)_ODT = 10.5. Previously published results for the temperature dependent binary interaction parameters for C‐E (x = 0.07), C‐E_E (x = 0.90), and E‐E_E (x = 0.07 and x = 0.90) fail to account for the quantitative x dependence of χ, based on a simple binary interaction model. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 566–574, 2010     

Simple Access to 2-Arylbenzo[b]tellurophenes

Arguably, the simplest way for the preparation of benzo[b]tellurophenes has been elaborated. Cyclization of o-haloarylalkynes in the presence of Te-NaOH-DMSO triad occurs by simple mixing of starting materials and heating overnight without the need for dry and inert atmosphere and provides the corresponding benzo[b]tellurophenes in up to 90 % GC-yield. Catalytic telluration of aryl iodide substrates extends the applicability of the developed cyclization cascade to substrates that are not suitable for the direct S_NAr/5-endo-dig cyclization approach. Successful S_EAr in the third position of benzo[b]tellurophene provides a synthetic intermediate for the preparation of Te analogues of selective estrogen receptor modulator (SERM) raloxifene.