Behavior of pressure dependence of melting temperatures in aromatic polyesters: Influence of polymer structure

The thermal behavior of three aromatic polyesters in a homologous series, poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), and poly(butylene terephthalate) (PBT) was studied under hydrostatic pressure up to 200 MPa by using a high pressure differential thermal analysis apparatus. Confining fluid high pressure dilatometer was used to establish the volume–temperature curves (in both solid and liquid regions) from which volume change on melting of these polyesters at atmospheric pressure was determined. Single endothermic peak was seen for PET and PTT, whereas PBT showed double peaks above 50 MPa. Pressure coefficient of melting temperature at atmospheric pressure (dT_m/dp₍₀₎), was obtained from the quadratic fit. The dT_m/dp₍₀₎ for PTT was newly determined to be 0.445 KMPa^?1, whereas for PET and PBT were 0.503 and 0.455 KMPa^?1, respectively, comparable to reported values. The dT_m/dp₍₀₎ exhibited the odd‐even behavior corresponding to odd and even number of methylene groups in the repeat unit. Enthalpy and entropy of fusion had the most influence on this coefficient. Entropy related to conformational and volume change were evaluated and the former was found to have a significant impact on the value of dT_m/dp₍₀₎. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1799–1808, 2009     

Crosslinked liquid crystal polymers from liquid crystal monomers: Synthesis and mechanical properties

Difunctional acrylates and methacrylate monomers have been made which are high order smectic liquid crystal (or crystalline) at room temperature. This report discusses materials with the following structure: F–S–M–S–F, where F is a functional group, acrylate or methacrylate (A or M); S is a spacer (CH₂)_n(n), and M is a mesogen—in this case 4,4′-dioxybiphenyl (B). They are codified as BnA or BnM where n is the number of methylenes in the spacer. High conversion with high T_g can be obtained when polymerizing in the smectic state because the reactive end groups are concentrated in a small volume and can react well with little or no diffusion. B2A, B3A, B6A, B11A, and B3M were polymerized in the smectic state and compared to polymers made at temperatures where the monomers were isotropic. High conversion was obtained below final T_g—even then, probably because the polymers were ordered. All the polymers were studied by WAXD and dynamic mechanical spectroscopy. Solid-state NMR on B3A showed that there was very high conversion of the double bonds at all temperatures. B3A photopolymerized in the smectic state (60–76°C) produced a crystalline polymer with T_g = 185°C (1 Hz). When photopolymerized at 85°C, above the isotropization temperature (T_i), a poorly organized polymer was obtained with a T_g of 155°C (1 Hz). Monomers with an odd number of methylene groups as spacers were crystalline after polymerization. With an even number of methylene groups, they lost most of their crystallinity on polymerization below T_i, but retained a low order smectic structure. Similar structures were obtained with all the monomers when they were polymerized above T_i. There was little effect of polymerization temperature on T_g when the spacers had an even number of methylene groups. © 1993 John Wiley & Sons, Inc.     

Effects of combining of different flexible spacers in liquid crystal trimers and tetramers on transition properties

Liquid crystal trimers and tetramers containing two kinds of flexible spacers, namely O(CH₂) _m O and COO(CH₂) _n O, were divided into four classes according to the odd/even nature of the number of atoms in the flexible spacers: specifically, even–even, odd–odd, even–odd, and odd–even trimers, and even–even–even, odd–odd–odd, odd–even–odd, and even–odd–even tetramers. The transition properties of the four types of trimers and of tetramers were compared. Although the nematic–isotropic transition temperature and the associated entropy changes were primarily related to the number of the even-membered flexible spacers in these molecules, the different combinations of the flexible spacers significantly affected their transition properties.     

Nylon 6 9 can crystallize with hydrogen bonding in two and in three interchain directions

Nylon 6 9 has been shown to have structures with interchain hydrogen bonds in both two and in three directions. Chain-folded lamellar crystals were studied using transmission electron microscopy and sedimented crystal mats and uniaxially oriented fibers studied by X-ray diffraction. The principal room-temperature structure shows the two characteristic (interchain) diffraction signals at spacings of 0.43 and 0.38 nm, typical of α-phase nylons; however, nylon 6 9 is unable to form the α-phase hydrogen-bonded sheets without serious distortion of the all-trans polymeric backbone. Our structure has c and c^* noncoincident and two directions of hydrogen bonding. Optimum hydrogen bonding can only occur if consecutive pairs of amide units alternate between two crystallographic planes. The salient features of our model offer a possible universal solution for the crystalline state of all odd–even nylons. The nylon 6 9 room-temperature structure has a C-centered monoclinic unit cell (β = 108°) with the hydrogen bonds along the C-face diagonals; this structure bears a similarity to that recently proposed for nylons 6 5 and X3. On heating nylon 6 9 lamellar crystals and fibers, the two characteristic diffraction signals converge and meet at 0.42 nm at the Brill temperature, T_B · T_B for nylon 6 9 lamellar crystals is slightly below the melting point (T_m), whereas T_B for nylon 6 9 fibers is ≅ 100°C below T_m. Above T_B, nylon 6 9 has a hexagonal unit cell; the alkane segments exist in a mobile phase and equivalent hydrogen bonds populate the three principal (hexagonal) directions. A structure with perturbed hexagonal symmetry, which bears a resemblance to the reported γ-phase for nylons, can be obtained by quenching from the crystalline growth phase (above T_B) to room temperature. We propose that this structure is a “quenched-in” perturbed form of the nylon 6 9 high-temperature hexagonal phase and has interchain hydrogen bonds in all three principal crystallographic directions. In this respect it differs importantly from the γ-phase models. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1153–1165, 1998     

The Missing Link in the Magnetism of Hybrid Cobalt Layered Hydroxides: The Odd–Even Effect of the Organic Spacer

A dramatic change in the magnetic behaviour, which solely depends on the parity of the organic linker molecules, has been found in a family of layered Co^II hydroxides covalently functionalized with dicarboxylic molecules. These layered hybrid materials have been synthesized at room temperature using a one-pot procedure through the epoxide route. While hybrids connected by odd alkyl chains exhibit coercive fields (H_c) below ca. 3500 Oe and show spontaneous magnetization at temperatures (T_M) below 20 K, hybrids functionalized with even alkyl chains behave as hard magnets with H_c>5500 Oe and display a T_M higher than 55 K. This intriguing behaviour was studied by density functional theory with the incorporation of a Hubbard term (DFT+U) calculations, unveiling the structural subtleties underlying this observation. Indeed, the different molecular orientation exhibited by the even/odd alkyl chains, and the orientation of the covalently linked carboxylic groups modify the intensity of the magnetic coupling of both octahedral and tetrahedral in-plane sublattices, thus strongly affecting the magnetic properties of the hybrid. These findings offer an outstanding level of tuning in the molecular design of hybrid magnetic materials based on layered hydroxides.     

A quantum-chemical study on the structures of linear conjugated systems that absorb in the near IR range

Calculations have been performed in the AM1 approximation on the equilibrium geometry, charge distribution, and energy levels of [R−(CH)_m−R]² symmetrical conjugated systems in which z=0, ±1, ±2; R=CN₂, NH₂, O; m=1÷46; the symmetrical structure is disrupted in a long molecule having an odd number of CH groups. In a long even-numbered system, there are two parts in which there are bond equalization and substantial charge alternation, which features are also characteristic, of odd systems, and in which they differ from neutral even ones, which have a uniform electron-density distribution and CC bond length alternation. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 3, pp. 150–154, May–June, 1999.     

Liquid Nickel Salts: Synthesis,Crystal Structure Determination,and Electrochemical Synthesis of Nickel Nanoparticles

New nickel‐containing ionic liquids were synthesized, characterized and their electrochemistry was investigated. In addition, a mechanism for the electrochemical synthesis of nanoparticles from these compounds is proposed. In these so‐called liquid metal salts, the nickel(II) cation is octahedrally coordinated by six N‐alkylimidazole ligands. The different counter anions that were used are bis(trifluoromethanesulfonyl)imide (Tf₂N^?), trifluoromethanesulfonate (OTf^?) and methanesulfonate (OMs^?). Several different N‐alkylimidazoles were considered, with the alkyl sidechain ranging in length from methyl to dodecyl. The newly synthesized liquid metal salts were characterized by CHN analysis, FTIR, DSC, TGA and viscosity measurements. An odd‐even effect was observed for the melting temperatures and viscosities of the ionic liquids, with the complexes with an even number of carbon atoms in the alkyl chain of the imidazole having a higher melting temperature and a lower viscosity than the complexes with an odd number of carbons. The crystal structures of several of the nickel(II) complexes that are not liquid at room temperature were determined. The electrochemistry of the compounds with the lowest viscosities was investigated. The nickel(II) cation could be reduced but surprisingly no nickel deposits were obtained on the electrode. Instead, nickel nanoparticles were formed at 100 % selectivity, as confirmed by TEM. The magnetic properties of these nanoparticles were investigated by SQUID measurements.     

Odd-even effect in melting properties of 12 alkane-α,ω-diamides

Fusion and solid-to-solid transitions of a homologous series of 12 linear alkane-α,ω-diamides H₂NCO-(CH₂)_(n−2)-CONH₂, where n = (2 to 12 and 14), were investigated by differential scanning calorimetry (d.s.c.). The temperatures of fusion of even terms decreased from T_fus ≈ 572 K to about 460 K, whereas those of odd terms remained substantially constant at about 450 K. Solid-to-solid transitions were also detected for oxamide, malonamide, succinamide, adipamide, suberamide, and dodecanediamide. Regular odd-even alternation was displayed by the temperature, enthalpy, and entropy of fusion values, terms with even number of carbon atoms showing higher values than odd terms.This behaviour was attributed to different crystal packing allowing consonance between hydrogen bonding and dispersive interaction in even terms, which are characterised by multilayer structure, whereas in odd terms a strained three-dimensional network results in looser packing. Parallel alternation of densities in solid alkane-α,ω-diamides supports this interpretation. Comparison was made with literature values for temperatures, enthalpies, and entropies of fusion of isoelectronic linear alkanes, dicarboxylic acids, and alkyldiamines.     

Absorption spectra of molecules consisting of groups of the same types

The UV spectra of chains with links of the same type (including SiR₂, GeR₂, SnR₂) show a direct relation of chain length to bathochromic shift of the first strong band, whose wavelength ₁ tends to a limiting value. This tendency can be described approximately via MO and via the model of an electron in a potential box. The first trend is due to nonuniformity in the potential, while the second is due to the values of the resonance integrals for interaction between adjacent bonds. It would seem that interaction between more distant bonds is of secondary importance. The results indicate that the additive scheme is rather crude even for saturated compounds. There is no very pronounced difference between polyenes and compounds whose chains consist of -bonds as regards the effects of interaction between groups. The degree of coupling between parts of the chain runs in the sequence CR ₂< SiR ₂, GeR ₂3). It seems that heavy atoms affect a -bond system in much the same way via a SiH₂ group as via a CH₂ group.     

Covalently Palladated Oligonucleotides Through Oxidative Addition of Pd0

An 11-mer oligonucleotide incorporating a central (2-iodobenzoylamino)methyl residue has been synthesized and palladated by oxidative addition of Pd₂(dba)₃. UV melting profiles of the duplexes formed by the palladated oligonucleotide with its natural complements were biphasic and the higher melting temperatures (T_m) exhibited considerable hysteresis. CD spectra, in turn, resembled those of canonical B-type double helices. Two-step denaturation, with the “low-T_m” melting involving only canonical base pairs and the “high-T_m” melting involving also dissociation of a Pd^II-mediated base pair, appears the most likely explanation for the observed UV melting profiles. As the latter step in all cases takes place at a higher temperature than denaturation of natural duplexes of the same length, the putative Pd^II-mediated base pairs are stabilizing.     

1H-NMR-Spektroskopische Untersuchungen an isomeren Alkendiinen

¹H NMR spectra of several aliphatic and phenyl substituted alkenediynes have been obtained. Chemical shifts and coupling constants of these compounds are discussed in conjunction with some compounds described in the literature. Chemical shifts of the protons from isomeric alkenediynes R? C?C? C?C? CH?CH₂, R? CH?CH? C?C? C?CH and R? CH?CH? C?C? C?C? CH₃ (R = H, alkyl, C₆H₅, C₆H₄OCH₃-p) are well correlated with cis/trans-isomerism and electronic effects of substituents at the C?C bond. The coupling constants were found to be only slightly dependent on the substitution at the double bond. We could resolve couplings over a maximum of eight bonds in the alkenediyne system.     

Mean field analysis of four liquid crystalline odd–even ester dimers

A mean field analysis is presented for four liquid crystalline ester dimers, Dn, containing the dimethylbenzalazine mesogen, alkanedioyloxy flexible spacers from 7 to 10 carbon atoms and acetate terminal groups. The conformations of the dimers, in the RIS approximation, were generated from the known crystallographic coordinates of D8 and D9. The energy of each conformer is split into an internal (conformation dependent) part and an external (orientation dependent) part. After proper averaging over all orientations and conformations, the orientation–conformation partition function is evaluated and, from that, the Helmholtz free energy. A qualitative agreement between calculated and observed thermodynamic properties is obtained. In fact, the theoretical analysis correctly predicts strong odd–even fluctuations for the mesogenic group order parameter, S, as well as for transition entropy, ΔS _NI, and transition temperature, T _NI. The distribution of conformers is similar for dimers having the same parity of the spacer. For even dimers, the calculated fraction of linear extended conformers in the nematic phase at the N–I transition is around 47%, whereas it is less than 3.6% for odd dimers.     

Methylene-linked liquid crystal dimers

A range of symmetric liquid crystal dimers which differ in the nature of the link, either ether or methylene, between the spacer and mesogenic units has been prepared and their transitional properties characterized. The nematic-isotropic transition temperature, T _NI, and the associated entropy change, ΔS _NI/R, are sensitive to the chemical nature of this link. Specifically, T _NI falls on replacing ether links with methylene links for both odd and even members although this reduction is more pronounced for odd members. In comparison, ΔS _NI/R increases on changing ether links for methylene links for even dimers, but decreases for odd-membered dimers. These observations are completely in accord with the predictions of a model developed by Luckhurst and co-workers in which the difference between the ether-linked and methylenelinked dimers rests exclusively in their shapes. Furthermore, the highly non-linear pentamethylenelinked dimers show a greater tendency to exhibit smectic behaviour; this is interpreted in terms of molecular packing giving rise to an alternating smectic phase.     

Carbenoid Reactions in Rhodium(II)-Catalyzed Decomposition of Iodonium Ylides

The intermediacy of metallocarbenes in decomposition reactions of iodonium ylides with [Rh₂(OAc)₄] was established by comparison with reactions of the corresponding diazo compounds. The sensitivity of the Rh^II-catalyzed intermolecular cyclopropane formation from substituted styrenes and bis(methoxycarbonyl)(phenyliodono)methanide ( 1a ) or dimethyl diazomalonate ( 1b ) is identical. The Hammett plot (with σ⁺) has a slope of ?0.47. Iodonium ylides and diazo compounds afford the same products in [Rh₂(OAc)₄]-catalyzed cyclopropane formations, cycloadditions, and intramolecular CH insertions, and exhibit the identical selectivity in intramolecular competitions for cyclopropane formation and insertion. The intramolecular CH insertion of the ylide 20c , when carried out in the presence of a chiral catalyst ([Rh₂{(?)-(S)-ptpa}₄]), results in formation of 21a having an ee of 67%, identical to the ee obtained with the diazo compound 20b .     

Kinetics of epoxidation of poly(trans-1,4-butadiene) crystals in suspension

The kinetics of the epoxidation reaction between metachloroperbenzoic acid (MCPBA) and poly-(trans-1,4-butadiene), (PTBD), crystals in toluene suspension was investigated in the 6–21°C range using infrared spectroscopy. Crystals of PTBD with M?_n = 36,000 grown from heptane and from toluene solutions and crystals of PTBD with M?_n = 5500 grown from heptane were studied. For toluene-grown crystals the total number of double bonds available for reaction increases with reaction temperature. For all preparations studied the epoxidation reaction is initially second-order—first-order with respect to [MCPBA] and first-order with respect to the concentration of the available double bonds. The second-order rate constant is found to be dependent on temperature, on molecular weight, and also on the crystal preparation conditions. The bromination of PTBD crystals was studied in CCl₄ suspension at 0°C; this reaction was found to be complete within 1 hr with the fraction of double bonds brominated consistent with the epoxidation results. The IR spectra for dried mats of brominated and of epoxidized PTBD crystals were obtained: Changes in the amorphous band at 1335 cm^?1 due to reaction of double bonds at crystal surfaces were observed. The results of this investigation are discussed in terms of the amorphous content of the PTBD lamellas.     

Order–Disorder Transitions in Model Liquids of Mesogenic Trimers

Monte Carlo simulations have been performed for realistically dense liquids of model trimers consisting of rigid cores connected by virtual bonds representing semiflexible spacers. The conformational characteristics of the trimers are approximately regulated in two cases to mimic (CH₂)_n spacers with n odd or even. All simulated systems undergo reversible isotropic–nematic phase transitions at well defined temperatures, showing odd–even oscillations in good agreement with experiments. The transitions are coupled with a conformational selection favoring extended conformations in the nematic liquids. The odd–even oscillations and the conformational distribution in the nematic liquids are fully explained on the basis of the intrinsic conformational characteristics of the model trimers.     

Enthalpy-entropy compensation in ionic micelle formation

The enthalpy-entropy compensation in ionic surfactant micellization process over a large temperature range is examined. The surfactants SDS and C₁₆TAB are investigated experimentally, and the enthalpy and entropy changes are evaluated based on phase separation or mass action models together with the other three surfactant systems. The relationship between compensation temperature and the reference temperatures is discussed.Notations C _p heat capacity change, J/mol-K - CMC critical micelle concentration,M - CMC₀ critical micelle concentration atT=T ₀,M - G Gibbs free energy change, kJ/mol - H enthalpy chang, kJ/mol - h _c enthalpy change for transfer of a methylene group to water, kJ/mol - R gas constant, 8.314 J/mol-K - S entropy change, J/mol-K - S _c entropy change for transfer of a methylene group to water, J/mol-K - S ^* entropy change atT=T ^*, J/mol-K - T temperature,K - T _c compensation temperature, K - T _H temperature at which H=0, K - T ₀ temperature at the minimum point, K - T ^* 112°C Greek Letters degree of dissociation     

An Alkyne‐Appended,Click‐Ready PtII Complex with an Unusual Arrangement in the Solid State

To better understand the range of cellular interactions of Pt^II‐based chemotherapeutics, robust and efficient methods to track and analyze Pt targets are needed. A powerful approach is to functionalize Pt^II compounds with alkyne or azide moieties for post‐treatment conjugation through the azide–alkyne cycloaddition (click) reaction. Herein, we report an alkyne‐appended cis‐diamine Pt^II compound, cis‐[Pt(2‐(5‐hexynyl)amido‐1,3‐propanediamine)Cl₂] ( 1 ), the X‐ray crystal structure of which exhibits a combination of unusual radially distributed CH/π(C?C) interactions, Pt? Pt bonding, and NH:O/NH:Cl hydrogen bonds. In solution, 1 exhibits no Pt? alkyne interactions and binds readily to DNA. Subsequent click reactivity with nonfluorescent dansyl azide results in a 70‐fold fluorescence increase. This result demonstrates the potential for this new class of alkyne‐modified Pt compound for the comprehensive detection and isolation of Pt‐bound biomolecules.     

An Alkyne‐Appended,Click‐Ready PtII Complex with an Unusual Arrangement in the Solid State

To better understand the range of cellular interactions of Pt^II‐based chemotherapeutics, robust and efficient methods to track and analyze Pt targets are needed. A powerful approach is to functionalize Pt^II compounds with alkyne or azide moieties for post‐treatment conjugation through the azide–alkyne cycloaddition (click) reaction. Herein, we report an alkyne‐appended cis‐diamine Pt^II compound, cis‐[Pt(2‐(5‐hexynyl)amido‐1,3‐propanediamine)Cl₂] ( 1 ), the X‐ray crystal structure of which exhibits a combination of unusual radially distributed CH/π(CC) interactions, Pt Pt bonding, and NH:O/NH:Cl hydrogen bonds. In solution, 1 exhibits no Pt alkyne interactions and binds readily to DNA. Subsequent click reactivity with nonfluorescent dansyl azide results in a 70‐fold fluorescence increase. This result demonstrates the potential for this new class of alkyne‐modified Pt compound for the comprehensive detection and isolation of Pt‐bound biomolecules.     

Thermally stimulated current studies of thermotropic polyesters with polymethylene spacers (I): The glass transition and relaxation behavior of anisotropic and isotropic glasses

Thermally stimulated current (TSC) and relaxation map analysis (RMA) were used to study the glass transitions and relaxation phenomena of the anisotropic and isotropic glasses for the semiflexible polyesters with various polymethylene spacers (n = 7 ˜ 10). TSC analysis indicated that the glass transition temperatures (T_g) of polymers at around 40 ˜ 50°C exhibited an apparent even-odd behavior not only for the anisotropic glasses but also for the isotropic glasses. The T_g of the isotropic glass for the polymer with even n value was observed to be higher than those for the two neighboring polymers with odd n values. The anisotropic glass for the polymer with even n value had a lower T_g than those for the two neighboring polymers with odd n values. The lower value was attributed to the configuration and orientation effects on the behavior of polarization. RMA revealed that the relaxation modes of the investigated polymers were also influenced by the configuration and orientation effects. The dipolar relaxation of the anisotropic glass for the polymer with odd n value occurred at a higher temperature and had a lower entropy (enthalpy) of activation than that of the isotropic glass for the same polymer due to the orientation effect. However, an inverse relation was found to occur for the polymer with even n value, which came from the trans configuration of the even polymethylene spacers. Finally, the thermokinetic properties evaluated by RMA (e.g., the final state after depolarization) correlated quite well with the results obtained by TSC. © 1995 John Wiley & Sons, Inc.