Stress-optical behavior of cycloaliphatic polyesters

Stress-strain-birefringence measurements were carried out on elastomeric networks of poly(oxymethylene-1,4-cis-cyclohexylenemethyleneoxysebacoyl) at several temperatures between 5 and 80°C. The dependence of both the birefringence Δn and the true stress f/A on temperature was found to be linear for T > 30°C; for T < 30°C an anomalous increase in the birefringence and a sharp decrease in the stress was observed. This behavior suggests that crystallinity is developed in the strained networks at low temperatures, and the crystallites are oriented in the direction of the elongation. Values of the optical configuration parameter Δa ranged from 9.15 to 8.28 in units of 10²⁴ cm³ in the temperature range studied. The value at 40°C of 10²⁴Δa, obtained from experiments performed on swollen networks, amounted to 7.47 cm³. These results suggest that intermolecular interactions enhance the birefringence of the strained networks. The quantities Δa and d In Δa/dT were calculated by using the valence optical scheme. Although the calculations reproduce the temperature coefficient fairly well, the theoretical values of Δa are smaller than the experimental ones. The agreement between theory and experiment is better assuming that the CH₂CH₂? COOCH₂ segment is freely rotating.     

Random-coil configurations of aromatic polyesters: Birefringence of poly(triethylene glycol terephthalate) networks in elongation

Hydroxyl-terminatd poly(triethylene glycol terephthalate) was crosslinked with an aromatic triisocyanate. Birefringence–stress–strain experiments, performed on the networks at 70°C, showed an anomalous increase in the modulus and a downturn in the birefringence–strain isotherms at high elongations. These results suggest that crystallinity is not responsible for the non-Gaussian behavior of the chains at high extension. The same kind of experiments were performed over the range 20–70°C. Values of the optical configuration parameter Δa of the order of 13.3 × 10^?24 cm³ with negligible temperature coefficient were found for these networks. The quantities Δa and d In Δa/dT were calculated by means of the rotational isomeric state model. Better agreement between the theoretical and experimental values of these parameters was found for poly(triethylene glycol terephthalate) than for poly(diethylene glycol terephthalate). Since the polarities of the two chains are similar, intermolecular interactions involving terephthaloyl residues may be responsible for the discrepancies observed between theory and experiment for Δa in aromatic polyesters.     

Birefringence of amorphous polyoxides: Stress-optical behavior of poly(3-methyltetrahydrofuran)

The stress-optical behavior of an unswollen elastomeric network of poly(3-methyltetrahydrofuran) was measured for elongation ratios a in the range 1.182–1.549, at several temperatures between 20 and 60°C. No evidence of strain-induced crystallization was found; moreover, the dependence of birefringence Δn on true stress f/A was linear in the interval of α investigated. Values of Δa ranged from 2.4 to 2.8 in units of 10^?24 cm³, in the temperature range studied, with a temperature coefficient 3.1 × 10^?3 K^?1. Theoretical calculations carried out with the rotational-isomeric-state model gave values of Δa noticeably smaller than the experimental results; however, a small increase in the backbone valence angles θ improved the theoretical result of Δa without worsening that of the dipole ratio. Analysis of the Δa results seems to corroborate the conclusion obtained through the study of dipole moments concerning the preference for nucleophilic attack on the less hindered α carbon in the monomer. Theoretical and experimental values of the temperature coefficient of Δa were in clear disagreement; a qualitative explanation for this discrepancy is discussed.     

Optical anisotropy of polyisobutylene: Strain birefringence

Strain birefringence measurements on crosslinked polyisobutylene (butyl rubber) confirm earlier work of Stein and Tobolsky on the linear polymer indicating the optical anisotropy to be much greater than should have been expected from the structural symmetry of the polyisobutylene (PIB) chain. The configuration–optical anisotropy parameter Δa for PIB at 25°C is 4.1(±0.1) × 10^?24 cm³, or about half the value for crosslinked polymethylene, both polymers being undiluted and amorphous. Swelling with cyclohexane, CCl₄, and CBrCl₃ lowers Δa to values of 3.8, 3.4, and 2.8 × 10^?24 cm³, respectively. Contributions from intermolecular correlations in the bulk polymer and from form anisotropy in the diluted systems are small, if not negligible. Temperature coefficients measured isometrically yield d In Δa/dT ≈ 0.2 × 10^?3 deg^?1. Both Δa and its temperature coefficient are much greater than calculated from rotational isomeric state theory assuming additivity of bond polarizabilities. The disparity (more than tenfold for Δa) cannot be relieved by any rational adjustment of the structural parameters. It is suggested that the severe crowding of groups in the PIB chain may affect the anisotropies of group polarizabilities.     

Stress-optical properties of poly(oxypropylene) networks

Strain birefringence of poly(oxypropylene) was studied using several poly(oxypropylene) model networks of different crosslink densities. Most of the measurements were carried out in elongation, in both the unswollen and the swollen states, over the temperature range 10–70°C. The optical configuration parameter was found to be Δa = (4.33 ± 0.09) × 10^?24 cm³ at 25°C with a very small temperature coefficient. Theoretical calculations based on rotational isomeric state theory were employed to interpret the experimental data. The results indicate the intermolecular correlations to be low for this polymer, and, contrary to other systems, the stress-optical coefficient C decreased with increasing average molecular weight between crosslinks. This fact was attributed to the end-group effect introduced by the crosslinking agent.     

Stress-optical behavior of polydimethylsilmethylene [?Si(CH3)2CH2?], a hydrocarbon analog of polydimethylsiloxane,and a silicon analog of polyisobutylene

Elastomeric networks were prepared from polydimethylsilmethylene (PDMSM) [? Si(CH₃)₂CH₂? ], a polymer closely related to polydimethylsiloxane (PDMSO) [? Si(CH₃)₂O? ] and polyisobutylene (PIB) [? C(CH₃)₂CH₂? ]. The birefringence of PDMSM in elongation was found to be qualitatively similar to that of PDMSO, in that there was no evidence for strain-induced crystallization. However, the values of the optical-configuration parameter Δa were considerably larger, and both Δa and its temperature coefficient are essentially the same as those of PIB. Swelling the PDMSM networks generally decreased Δa; the largest decreases, obtained as expected with the most nearly symmetrical diluent, were also approximately the same as those observed for PIB. Use of cyclic PDMSO pentamer as diluent in a PDMSM network was found to increase the birefringence, presumably because of orientational effects. Various network-diluent combinations involving the significantly anisotropic PDMSM and the more nearly isotropic PDMSO should be extremely useful for elucidating the nature of these intermolecular correlations. Results obtained from rotational isomeric state theory considerably underestimate both Δa and its temperature coefficient for PDMSM, as they do for PIB. Although the origin of the discrepancy is not necessarily the same for both polymers, the results on PDMSM suggest that the discrepancy for PIB is not due to the severe steric congestion known to be present in this polymer.     

Response of the mean-square optical anistropy of chain molecules to the imposition of excluded volume

The influence of the chain expansion produced by excluded volume on the mean-square optical anisotropy has been studied in six types of polymers. The mean-square optical anistropy for a specified configuration is calculated using the valence optical scheme. Realistic rotational isomeric state models are used for the configurational statistics of the unperturbed chains. Excluded volume is introduced by hard sphere interactions. Results obtained with chains of 100, 200, 300, and 400 bonds permit extrapolation to the behavior expected for much longer chains. The mean-square optical anisotropy of polyethylene is insensitive to excluded volume. A similar conclusion was obtained several years ago in a study of chains confined to a tetrahedral lattice and weighted in a manner appropriate for the short-range interactions in polyethylene.² Different behavior is seen in poly(vinyl chloride), poly(vinyl bromide), polystyrene, poly(p-chlorostyrene), and poly(p-bromostyrene). The mean-square optical anisotropy of these five vinyl polymers is sensitive to the imposition of excluded volume if the stereochemical composition is exclusively racemic. Much smaller effects are seen in meso chains and in chains with Bernoullian statistics and an equal probability for meso and racemic dyads.     

catena‐Poly­[[[cis‐bis(1‐phenyl­tetra­zole‐κN4)copper(II)]‐di‐μ‐chloro] hemi(1‐phenyl­tetrazole)]

The title compound, {[CuCl₂(PhTz)₂]·0.5PhTz}_n (PhTz is 1‐­phenyl­tetrazole, C₇H₆N₄), has a polymeric structure, with uncoordinated disordered PhTz mol­ecules in the cavities. The coordination polyhedron of the Cu atom is a highly elongated octahedron. The equatorial positions are occupied by two Cl atoms [Cu—Cl = 2.2687 (9) and 2.2803 (7) Å] and two N atoms of the PhTz ligands [Cu—N = 2.0131 (19) and 2.0317 (18) Å]. The more distant axial positions are occupied by two Cl atoms [Cu—Cl = 3.0307 (12) and 2.8768 (11) Å] that lie in the equatorial planes of two neighbouring Cu octahedra. The [CuCl₂(PhTz)₂] units are linked by Cu—Cl bridges into infinite chains extending parallel to the a axis. The chains are linked into two‐dimensional networks by intermolecular C—H⋯N interactions between the phenyl and tetrazole fragments, and by face‐to‐face π–π interactions between symmetry‐related phenyl rings. These two‐dimensional networks, which lie parallel to the ac plane, are connected by intermolecular π–π stacking interactions between phenyl rings, thus forming a three‐dimensional network.     

Effects of the alkyl side-chain length on the structures of poly[oxy(N-alkylsulfonylmethyl)ethylene]s

The structures of poly[oxy(N-alkylsulfonylmethyl)ethylene]s (ASE-Ns) were examined with X-ray, IR, DSC, and polarized optical microscopy. The structures of ASE-Ns were strongly dependent on the alkyl side-chain length. ASE-2 and ASE-3, the shortest ones, were amorphous materials. ASE-4 and ASE-5 showed nematic characteristics. ASE-6–ASE-12 had smectic A structures. ASE-14 and ASE-16 could be labeled as more ordered structures higher than smectic A (probably smectic B or smectic E). The d-spacings of the first small-angle reflections were double the most extended side-chain length and linearly increased with a slope of 2.50 Å per methylene unit, regardless of the structural phases. The maintenance of the double-layered structure in all ASE-Ns may be due to the strong dipole–dipole interactions at both sides of the layers against the main chain for all amorphous, nematic, and smectic phases of ASE-Ns. The double-layered structure was maintained above the isotropic temperature, indicating that dipole–dipole interactions were not destroyed although the alkyl side chains melted during the isotropic transition. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1868–1874, 2004     

Structure and side-chain interactions in poly(δ-N-carbobenzoxy L-ornithine)

Two crystal modifications are found in α-helical poly(δ-N-carbobenzoxy L -ornithine). In films as cast, the two-dimensional unit cell is pseudohexagonal and contains two chains. This form transforms irreversibly into a pseudotetragonal form at about 140°C. A second-order transition associated with the onset of the side-chain motion is observed at about 30°C for the bulk sample (by dilatometry) and for the crystalline phase (by x-ray diffraction). The dielectric behavior of the side-chain dispersion suggests that the side chains interact with one another. The temperature dependence of the infrared absorbance due to the NH stretching vibration reveals that about half the side chains are associated via hydrogen bonds at room temperature and become dissociated at higher temperature. The enthalpy and the entropy of the hydrogen bond formation is estimated to be ΔH = ?5.0 ± 0.5 kcal mol^?1 and ΔS = ?15 ± 1 e.u. mol^?1.     

Acoustic and Thermodynamic Properties of Binary Liquid Mixtures of Benzaldehyde in Hexane and Cyclohexane

Densities and ultrasonic speeds of binary mixtures of benzaldehyde with n-hexane and cyclohexane at 30 °C were measured over the entire composition range. From these experimental data, the adiabatic compressibility (K _S ), intermolecular free length (L _f), acoustic impedance (Z), relative association (R _a) and relaxation strength (r) were calculated. Also, the excess adiabatic compressibility (K _S ^E), intermolecular free length (L _f^E), acoustic impedance (Z ^E), and ultrasonic velocity (U ^E) were calculated. The observed variation of these parameters helps in understanding the nature of interactions in these mixtures. Further, theoretical values of the ultrasonic speed were evaluated using theories and empirical relations. The relative merits of these theories and relations were discussed.     

Molecular valence calculations on cyclohexasulfur,cycloheptasulfur, and cyclooctasulfur

A theoretical study of homocyclic sulfur species S₆, S₇, and S₈ was carried out using a molecular valence method involving stepwise approximations for orthogonality and core-valence interactions. The valence shell orbitals are described at the minimal basis level. The geometries of the molecules are predicted well as compared with other theoretical studies and the experimental values. The slight overestimation of the SS bond length is typical to the nonpolarized basis sets. The energies of the valence orbitals are well in accord with the conventional all-electron ab initio results. The trend in the stabilities of the three molecules is discussed. The present method provides an attractive possibility to study homocyclic and heterocyclic systems involving heavier chalcogens with no increase of the computing time.     

The salt–cocrystal spectrum in salicylic acid–adenine: the influence of crystal structure on proton‐transfer balance

At one extreme of the proton‐transfer spectrum in cocrystals, proton transfer is absent, whilst at the opposite extreme, in salts, the proton‐transfer process is complete. However, for acid–base pairs with a small ΔpK_a (pK_a of base ? pK_a of acid), prediction of the extent of proton transfer is not possible as there is a continuum between the salt and cocrystal ends. In this context, we attempt to illustrate that in these systems, in addition to ΔpK_a, the crystalline environment could change the extent of proton transfer. To this end, two compounds of salicylic acid (SaH) and adenine (Ad) have been prepared. Despite the same small ΔpK_a value (≈1.2), different ionization states are found. Both crystals, namely adeninium salicylate monohydrate, C₅H₆N₅⁺·C₇H₅O₃^?·H₂O, I , and adeninium salicylate–adenine–salicylic acid–water (1/2/1/2), C₅H₆N₅⁺·C₇H₅O₃^?·2C₅H₅N₅·C₇H₆O₃·2H₂O, II , have been characterized by single‐crystal X‐ray diffraction, IR spectroscopy and elemental analysis (C, H and N) techniques. In addition, the intermolecular hydrogen‐bonding interactions of compounds I and II have been investigated and quantified in detail on the basis of Hirshfeld surface analysis and fingerprint plots. Throughout the study, we use crystal engineering, which is based on modifications of the intermolecular interactions, thus offering a more comprehensive screening of the salt–cocrystal continuum in comparison with pure pK_a analysis.     

Quantum chemical calculation of exchange interactions in supramolecularly arranged N,N′‐dioxy‐2,6‐diazaadamantane organic biradical

Quantum mechanical exchange effects in purely organic N,N′‐dioxy‐2,6‐diazaadamantane biradical derivatives with promesogenic substituents have been studied. To determine intermolecular exchange energies, packing conditions of the radical core units in layered liquid crystalline phases are simulated using the Gaussian 09 program. The broken symmetry approach gives J ≈ 7 cm^?1 for intramolecular ferromagnetic exchange interactions between nitroxyl radical centers in one molecule. Both ferromagnetic and antiferromagnetic intermolecular interactions are possible in this kind of systems according to the obtained calculation results. Depending on the mutual positioning and orientation of molecules, the intermolecular antiferromagnetic exchange constant can reach a value of ?50 cm^?1, and the intermolecular ferromagnetic constant a value of 10 cm^?1. The simultaneous presence of intramolecular and intermolecular exchange between spin‐carrying centers in this kind of supramolecularly ordered multispin systems is favorable for the formation of magnetically interacting chains and two‐dimensional networks. © 2016 Wiley Periodicals, Inc.     

Extended conformations of isolated molecular bottle-brushes: Influence of side-chain topology

A Monte Carlo study is presented to discuss the influence of the side-chain topology on the enhancement of the persistence length of a molecular bottle-brush in a dilute athermal solution due to the excluded volume interactions between the side chains. The structures investigated consisted of freely jointed backbones of 100 hard spheres (beads) of diameter 1 to which 50 equally flexible side chains were grafted. The diameter of the side-chain beads was varied from 1 to 3 in the same units. For every given size of the side-chain bead, the length of the side chains was varied from 4 to 20 beads. The ratio between the persistence length and the bottle-brush diameter, which is the determining factor for lyotropic behavior of conventional semi-flexible chains, was found to be almost independent of the side-chain length. At the same time, it was found to increase considerably with increasing size of the side-chain beads, suggesting that by a proper choice of the chemistry lyotropic behavior of molecular bottle-brushes due to excluded-volume interactions between the side chains might be achieved. Moreover, relatively short side chains can be used since the side chain length has only a minor influence on the ratio between the persistence length and the diameter. These findings are in a good agreement with recent experimental observations.     

Study on the rapid deswelling mechanism of comb-type N-isopropylacrylamide gels

The shrinking mechanism of comb-type grafted poly(N-isopropylacrylamide) gel was investigated by fluorescence spectroscopy and small-angle X-ray Scattering (SAXS). The SAXS reveals that the microdomain structure with characteristic dimension of 460 Å is developed in the comb-type grafted poly(N-isopropylacrylamide) gel during the shrinking process. Fluorescence spectroscopy together with SAXS observation suggests that the freely mobile characteristics of the grafted chains are expected to show the rapid dehydration to make tightly packed globules with temperature, followed by the subsequent hydrophobic intermolecular aggregation of the dehydrated graft chains. The dehydrated grafted chains created the hydrophobic cores, which enhance the hydrophobic aggregation of the networks. These aggregations of the NIPA chains contribute to an increase in void volume, which allow the gel having a pathway of water molecules by the phase separation.     

4-(1H-Benzimidazol-3-ium-2-yl)­benzoate dihydrate

In the title compound, C₁₄H₁₀N₂O₂·2H₂O, the water mol­ecules are involved in hydrogen bonds and interactions. Intermolecular and intramolecular O—H⃛O hydrogen bonds connect the complex into chains along the a axis, whereas N—H⃛O intermolecular hydrogen bonds and C—H⃛O interactions interconnect these layers forming a three-dimensional network.     

Crystal structures,packing features,Hirshfeld surface analyses and DFT calculations of hydrogen‐bond energy of two homologous 8a‐aryl‐2,3,4,7,8,8a‐hexahydropyrrolo[1,2‐a]pyrimidin‐6(1H)‐ones

The crystal structures and packing features of two homologous Meyer's bicyclic lactams with fused pyrrolidone and medium‐sized perhydropyrimidine rings, namely, 8a‐phenyl‐2,3,4,7,8,8a‐hexahydropyrrolo[1,2‐a]pyrimidin‐6(1H)‐one, C₁₃H₁₆N₂O ( 1 ), and 8a‐(4‐methylphenyl)‐2,3,4,7,8,8a‐hexahydropyrrolo[1,2‐a]pyrimidin‐6(1H)‐one, C₁₄H₁₈N₂O ( 2 ), were elucidated, and Hirshfeld surface plots were calculated and drawn for visualization and a deeper analysis of the intermolecular noncovalent interactions. Molecules of 1 and 2 are weakly linked by intermolecular C=O…H—N hydrogen bonds into chains, which are in turn weakly linked by other C=O…H—C_ar interactions. The steric volume of the substituent significantly affects the crystal packing pattern.     

The sesquiterpenoid nootkatone and the absolute configuration of a di­bromo derivative

Nootkatone, or (4R,4aS,6R)‐4,4a,5,6,7,8‐hexa­hydro‐4,4a‐di­methyl‐6‐(1‐methyl­ethenyl)­naphthalen‐2(3H)‐one, C₁₅H₂₂O, a sesquiterpene with strong repellent properties against Formosan subterranean termites and other insects, has the valencene skeleton. The di­bromo derivative (1S,3R,4S,4aS,6R,8aR)‐1,3‐di­bromo‐6‐iso­propyl‐4,4a‐di­methyl‐1,2,3,4,5,6,7,8‐octa­hydro­naphthalen‐2‐one, C₁₅H₂₄Br₂O, has two independent mol­ecules in the asymmetric unit, which differ in the rotation of the iso­propyl group with respect to the main skeleton. The C—Br distances are in the range 1.950 (4)–1.960 (4) Å. Both independent molecules form zigzag chains, with very short intermolecular carbonyl–carbonyl interactions, having the perpendicular motif and O⋯C distances of 2.886 (6) and 2.898 (6) Å. These chains are flanked by intermolecular Br⋯Br interactions of distances in the range 4.067 (1)–4.218 (1) Å. The absolute configuration of the di­bromo derivative was determined, from which that of nootkatone was inferred.     

Helical Folding of Conjugated Oligo(phenyleneethynylene): Chain‐Length Dependence,Solvent Effects,and Intermolecular Assembly

As a representative folding system that features a conjugated backbone, a series of monodispersed (o‐phenyleneethynylene)‐alt‐(p‐phenyleneethynylene) (PE) oligomers of varied chain length and different side chains were studied. Molecules with the same backbone but different side‐chain structures were shown to exhibit similar helical conformations in respectively suitable solvents. Specifically, oligomers with dodecyloxy side chains folded into the helical structure in apolar aliphatic solvents, whereas an analogous oligomer with tri(ethylene glycol) (Tg) side chains adopted the same conformation in polar solvents. The fact that the oligomers with the same backbone manifested a similar folded conformation independent of side chains and the nature of the solvent confirmed the concept that the driving force for folding was the intramolecular aromatic stacking and solvophobic interactions. Although all were capable of inducing folding, different solvents were shown to bestow slightly varied folding stability. The chain‐length dependence study revealed a nonlinear correlation between the folding stability with backbone chain length. A critical size of approximately 10 PE units was identified for the system, beyond which folding occurred. This observation corroborated the helical nature of the folded structure. Remarkably, based on the absorption and emission spectra, the effective conjugation length of the system extended more effectively under the folded state than under random conformations. Moreover, as evidenced by the optical spectra and dynamic light‐scattering studies, intermolecular association took place among the helical oligomers with Tg side chains in aqueous solution. The demonstrated ability of such a conjugated foldamer in self‐assembling into hierarchical supramolecular structures promises application potential for the system.