Metamagnetism and Single‐Chain Magnetic Behavior in a Homospin One‐Dimensional Iron(II) Coordination Polymer

Reaction of FeCl₂?4 H₂O with KNCSe and pyridine in ethanol leads to the formation of the discrete complex [Fe(NCSe)₂(pyridine)₄] ( 1 ) in which the Fe^II cations are coordinated by two N‐terminal‐bonded selenocyanato anions and four pyridine co‐ligands. Thermal treatment of compound 1 enforces the removal of half of the co‐ligands leading to the formation of a ligand‐deficient (lacking on neutral co‐ligands) intermediate of composition [Fe(NCSe)₂(pyridine)₂]_n ( 2 ) to which we have found no access in the liquid phase. Compound 2 is obtained only as a microcrystalline powder, but it is isotypic to [Cd(NCSe)₂(pyridine)₂]_n and therefore, its structure was determined by Rietveld refinement. In its crystal structure the metal cations are coordinated by two pyridine ligands and four selenocyanato anions and are linked into chains by μ‐1,3 bridging anionic ligands. Magnetic measurements on compound 1 show only paramagnetic behavior, whereas for compound 2 an unexpected magnetic behavior is found, which to the best of our knowledge was never observed before for a iron(II) homospin compound. In this compound metamagnetism and single‐chain magnetic behavior coexist. The metamagnetic transition between the antiferromagnetically ordered phase and a field‐induced ferromagnetic phase of the high‐spin iron(II) spin carriers is observed at a transition field H_C of 1300 Oe and the single‐chain magnetic behavior is characterized by a blocking temperature T_B, estimated to be about 5 K.     

Magnetic Bistability of Isolated Giant‐Spin Centers in a Diamagnetic Crystalline Matrix

Polynuclear single‐molecule magnets (SMMs) were diluted in a diamagnetic crystal lattice to afford arrays of independent and iso‐oriented magnetic units. Crystalline solid solutions of an Fe₄ SMM and its Ga₄ analogue were prepared with no metal scrambling for Fe₄ molar fractions x down to 0.01. According to high‐frequency EPR and magnetic measurements, the guest SMM species have the same total spin (S=5), anisotropy, and high‐temperature spin dynamics found in the pure Fe₄ phase. However, suppression of intermolecular magnetic interactions affects magnetic relaxation at low temperature (40 mK), where quantum tunneling (QT) of the magnetization dominates. When a magnetic field is applied along the easy magnetic axis, both pure and diluted (x=0.01) phases display pronounced steps at evenly spaced field values in their hysteresis loops due to resonant QT. The pure Fe₄ phase exhibits additional steps which are firmly ascribed to two‐molecule QT transitions. Studies on the field‐dependent relaxation rate showed that the zero‐field resonance sharpens by a factor of five and shifts from about 8 mT to exactly zero field on dilution, in agreement with the calculated variation of dipolar interactions. The tunneling efficiency also changes significantly as a function of Fe₄ concentration: the zero‐field resonance is significantly enhanced on dilution, while tunneling at ±0.45 T becomes less efficient. These changes were rationalized on the basis of a dipolar shuffling mechanism and transverse dipolar fields, whose effect was analyzed by using a multispin model. Our findings directly prove the impact of intermolecular magnetic couplings on SMM behavior and disclose the magnetic response of truly isolated giant spins in a diamagnetic crystalline environment.     

Structure–Property Relationship of Supramolecular Ferroelectric [H‐66dmbp][Hca] Accompanied by High Polarization,Competing Structural Phases,and Polymorphs

Three polymorphic forms of 6,6′‐dimethyl‐2,2′‐bipyridinium chloranilate crystals were characterized to understand the origin of polarization properties and the thermal stability of ferroelectricity. According to the temperature‐dependent permittivity, differential scanning calorimetry, and X‐ray diffraction, structural phase transitions were found in all polymorphs. Notably, the ferroelectric α‐form crystal, which has the longest hydrogen bond (2.95 Å) among the organic acid/base‐type supramolecular ferroelectrics, transformed from a polar structure (space group, P2₁) into an anti‐polar structure (space group, P2₁/c) at 378 K. The non‐ferroelectric β‐ and γ‐form crystals also exhibited structural rearrangements around hydrogen bonds. The hydrogen‐bonded geometry and ferroelectric properties were compared with other supramolecular ferroelectrics. A positive relationship between the phase‐transition temperature (T_C) and hydrogen‐bond length (<d>) was observed, and was attributed to the potential barrier height for proton off‐centering or order/disorder phenomena. The optimized spontaneous polarization (P_s) agreed well with the results of the first‐principles calculations, and could be amplified by separating the two equilibrium positions of protons with increasing <d>. These data consistently demonstrated that stretching <d> is a promising way to enhance the polarization performance and thermal stability of hydrogen‐bonded organic ferroelectrics.     

Preparation and phase behavior of side‐chain cholesteric liquid‐crystalline elastomers

A series of new side‐chain cholesteric elastomers derived from cholesteryl 4‐(10‐undecylen‐1‐yloxy)‐4′‐ethoxybenzoate and phenyl 4,4′‐bis(10‐undecylen‐1‐yloxybenzoyloxy‐p‐ethoxybenzoate) was synthesized. The chemical structures of the monomers were confirmed by elemental analyses, Fourier transform infrared, and ¹H NMR and ¹³C NMR spectra. The mesomorphic properties of elastomers were investigated with differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy, and X‐ray diffraction measurements. The influence of the content of the crosslinking unit on the phase behavior of the elastomers was examined. Monomer M₁ showed a cholesteric phase, and M₂ displayed smectic and nematic phases. The elastomers containing <15 mol % of the crosslinking units revealed reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3315–3323, 2005     

A Nuclear Singlet Lifetime of More than One Hour in Room‐Temperature Solution

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are supremely important techniques with numerous applications in almost all branches of science. However, until recently, NMR methodology was limited by the time constant T₁ for the decay of nuclear spin magnetization through contact with the thermal molecular environment. Long‐lived states, which are correlated quantum states of multiple nuclei, have decay time constants that may exceed T₁ by large factors. Here we demonstrate a nuclear long‐lived state comprising two ¹³C nuclei with a lifetime exceeding one hour in room‐temperature solution, which is around 50 times longer than T₁. This behavior is well‐predicted by a combination of quantum theory, molecular dynamics, and quantum chemistry. Such ultra‐long‐lived states are expected to be useful for the transport and application of nuclear hyperpolarization, which leads to NMR and MRI signals enhanced by up to five orders of magnitude.     

Graphene‐coated magnetic‐sheet solid‐phase extraction followed by high‐performance liquid chromatography with fluorescence detection for the determination of aflatoxins B1, B2, G1, and G2 in soy‐based samples

A new method named graphene‐coated magnetic‐sheet solid‐phase extraction based on a magnetic three‐dimensional graphene sorbent was developed for the extraction of aflatoxins prior to high‐performance liquid chromatography with fluorescence detection. The use of a perforated magnetic‐sheet for fixing the magnetic nanoparticles is a new feature of the method. Hence, the adsorbent particles can be separated from sample solution without using an external magnetic field. This made the procedure very simple and easy to operate so that all steps of the extraction process (sample loading, washing, and desorption) were carried out continuously using two lab‐made syringe pumps. The factors affecting the performance of extraction procedure such as the extraction solvent, adsorbent dose, sample loading flow rate, ionic strength, pH, and desorption parameters were investigated and optimized. Under the optimal conditions, the obtained enrichment factors and limits of detection were in the range of 205–236 and 0.09–0.15 μg/kg, respectively. The relative standard deviations were <3.4 and 7.5% for the intraday (n = 6) and interday (n = 4) precisions, respectively. The developed method was successfully applied to determine aflatoxins B₁, B₂, G₁, and G₂ in different soy‐based food samples.     

Unusual Electro‐Optic Kerr Response in a Self‐Stabilized Amorphous Blue Phase with Nanoscale Smectic Clusters

We investigated the electro‐optic response in the “foggy” amorphous blue phase (BPIII) as well as in the isotropic phase. To the best of our knowledge, such a study has not yet been performed due to the very limited thermal range of BPIII. In this study, we used a single‐component chiral bent‐core liquid crystal with a self‐stabilized BPIII, which is stable over a wide temperature range. The results show that the response time is on the order of hundreds of microseconds in the isotropic phase and increases to 1–2 ms in the BPIII (at T_I?BP–T <1), then drastically increases up to a few tens of milliseconds upon further cooling in BPIII. Such an unusual behavior was explained on the basis of the high rotational viscosity and/or the existence of nanoscale smectic (Sm) clusters. The Kerr constant was also measured and found to be ～500 pm V^?2, which is the largest among bent‐core BP systems reported so far and comparable with that of polymer‐stabilized BPs.     

Investigation on LCST behavior of a new amorphous/crystalline polymer blend: Poly(n‐methyl methacrylimide)/poly(vinylidene fluoride)

The liquid–liquid phase‐separation (LLPS) behavior of poly(n‐methyl methacrylimide)/poly(vinylidene fluoride) (PMMI/PVDF) blend was studied by using small‐angle laser light scattering (SALLS) and phase contrast microscopy (PCM). The cloud point (T_c) of PMMI/PVDF blend was obtained using SALLS at the heating rate of 1 °C min^?1 and it was found that PMMI/PVDF exhibited a low critical solution temperature (LCST) behavior similar to that of PMMA/PVDF. Moreover, T_c of PMMI/PVDF is higher than its melting temperature (T_m) and a large temperature gap between T_c and T_m exists. At the early phase‐separation stage, the apparent diffusion coefficient (D_app) and the product (2Mk) of the molecules mobility coefficient (M) and the energy gradient coefficient (k) arising from contributions of composition gradient to the energy for PMMI/PVDF (50/50 wt) blend were calculated on the basis of linearized Cahn‐Hilliard‐Cook theory. The kinetic results showed that LLPS of PMMI/PVDF blends followed the spinodal decomposition (SD) mechanism. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1923–1931, 2008     

Matrix solid‐phase dispersion with chitosan‐zinc oxide nanoparticles combined with flotation‐assisted dispersive liquid–liquid microextraction for the determination of 13 n‐alkanes in soil samples

In this study, chitosan‐zinc oxide nanoparticles were used as a sorbent of miniaturized matrix solid‐phase dispersion combined with flotation‐assisted dispersive liquid–liquid microextraction for the simultaneous determination of 13 n‐alkanes such as C₈H₁₈ and C₂₀H₄₂ in soil samples. The solid samples were directly blended with the chitosan nanoparticles in the solid‐phase dispersion method. The eluent of solid‐phase dispersion was applied as the dispersive solvent for the following flotation‐assisted dispersive liquid–liquid microextraction for further purification and enrichment of the target compounds prior to gas chromatography with flame ionization detection. Under the optimum conditions, good linearity with correlation coefficients in the range 0.9991 < r² < 0.9995 and low detection limits between 0.08 to 2.5 ng/g were achieved. The presented procedure combined the advantages of chitosan‐zinc oxide nanoparticles, solid‐phase dispersion and flotation‐assisted dispersive liquid–liquid microextraction, and could be applied for the determination of n‐alkanes in complicated soil samples with acceptable recoveries.     

Influence of Isotope Substitution on Lattice and Spin‐Peierls‐Type Transition Features in One‐Dimensional Nickel Bis‐dithiolene Spin Systems

Four new 1D spin‐Peierls‐type compounds, [D₅]1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate ([D₅]R‐Py; R=F, I, CH₃, and NO₂), were synthesized and characterized structurally and magnetically. These 1D compounds are isostructural with the corresponding non‐deuterated compounds, 1‐(4′‐R‐benzyl)pyridinium bis(maleonitriledithiolato)nickelate (R‐Py; R=F, I, CH₃, and NO₂). Compounds [D₅]R‐Py and R‐Py (R=F, I, CH₃, and NO₂) crystallize in the monoclinic space group P2₁/c with uniform stacks of anions and cations in the high‐temperature phase and triclinic space group P$\bar 1$ with dimerized stacks of anions and cations in the low‐temperature phase. Similar to the non‐deuterated R‐Py compounds, a spin‐Peierls‐type transition occurs at a critical temperature for each [D₅]R‐Py compound; the magnetic character of the 1D S=1/2 ferromagnetic chain for [D₅]F‐Py and the 1D S=1/2 Heisenberg antiferromagnetic chain for others appear above the transition temperature. Spin‐gap magnetic behavior was observed for all of these compounds below the transition temperature. In comparison to the corresponding R‐Py compound, the cell volume is almost unchanged for [D₅]F‐Py and shows slight expansion for [D₅]R‐Py (R=I, CH₃, and NO₂) as well as an increase in the spin‐Peierls‐type transition temperature for all of these 1D compounds in the order of F>I≈CH₃≈NO₂. The large isotopic effect of nonmagnetic countercations on the spin‐Peierls‐type transition critical temperature, T_C, can be attributed to the change in ω₀ with isotope substitution.     

Optically Biaxial,Re‐entrant and Frustrated Mesophases in Chiral,Non‐symmetric Liquid Crystal Dimers and Binary Mixtures

Sixteen optically active, non‐symmetric dimers, in which cyanobiphenyl and salicylaldimine mesogens are interlinked by a flexible spacer, were synthesized and characterized. While the terminal chiral tail, in the form of either (R)‐2‐octyloxy or (S)‐2‐octyloxy chain attached to salicylaldimine core, was held constant, the number of methylene units in the spacer was varied from 3 to 10 affording eight pairs of (R & S) enantiomers. They were probed for their thermal properties with the aid of orthoscopy, conoscopy, differential scanning calorimetry and X‐ray powder diffraction. In addition, the binary mixture study was carried out using chiral and achiral dimers with the intensions of stabilizing optically biaxial phase/s, re‐entrant phases and important phase sequences. Notably, one of the chiral dimers as well as some mixtures exhibited a biaxial smectic A (SmA_b) phase appearing between a uniaxial SmA and a re‐entrant uniaxial SmA phases. The mesophases such as chiral nematic (N*) and frustrated phases viz., blue phases (BPs) and twist grain boundary (TGB) phases, were also found to occur in most of the dimers and mixtures. X‐ray diffraction studies revealed that the dimers possessing oxybutoxy and oxypentoxy spacers show interdigitated (SmA_d) phase where smectic periodicity is over 1.4 times the molecular length; whereas in the intercalated SmA (SmA_c) phase formed by a dimer having oxydecoxy spacer the periodicity was found to be approximately half the molecular length. The handedness of the helical structure of the N* phases formed by two enantiomers was examined with the aid of CD measurements; as expected, these enantiomers showed optical activities of equal magnitudes but with opposite signs. Overall, it appears that the chiral dimers and mixtures presented herein may serve as model systems in design and developing novel materials exhibiting the apolar SmA_b phase possessing D_2h symmetry and nematic‐type biaxiality.     

Magnetic Field‐Induced Shape Transitions in Multiphase Polymer‐Liquid Crystal Blends

Summary: This paper presents a computational study of phase separation‐phase ordering‐texturing in blends of polymer coils and rod‐like nematic liquid crystals under the presence of magnetic fields, using an extended version of the Matsuyama‐Evans‐Cates model (Phys. Rev. E 2000 , 61, 2977). This work demonstrates that demixing in these blends leads to droplet morphologies with tunable droplet shapes and director textures. In contrast to filled nematics, where solids are suspended in a nematic liquid crystal matrix, demixing in coil‐mesogenic rods blends leads to nematic emulsions, in which the deformable viscoelastic polymer drops are suspended in a nematic matrix. Under strong anchoring conditions, the imposition of a magnetic field leads to a director re‐orientation that due to strong anchoring produces a droplet shape change. Magnetic field‐induced shape transitions in these blends are shown to be second order with a finite critical field threshold that diverges as anchoring strength vanishes. A morphological‐texture diagram summarizes the magnetic field‐anchoring conditions that promote anisotropic shapes. This work presents additional material processing routes to design and control bi‐phasic morphologies in polymer‐liquid crystal blend.

Computed morphology phase diagram in terms of magnetic field strength Λ_M and anchoring strength. Λ_ϕQ.     

Phase Biaxility in Smectic‐A Side‐Chain Liquid Crystalline Elastomers

²H NMR investigations on the biaxial phase behavior of smectic‐A liquid crystalline side‐chain elastomers are presented. Biaxiality parameters were determined by measuring the quadrupolar splitting of two spin probes, namely benzene‐d₆ and hexamethylbenzene‐d₁₈, at various angles between the principal director and the external magnetic field: while for a uniaxial sample the angular dependence can be described by the second Legendre polynomial, an additional asymmetric term needs to be included to fit the data of the two investigated biaxial systems. Two elastomers synthesized from mesogens that differ in the molecular geometry in order to study the molecular origin of biaxiality were compared. Biaxiality is observed for both elastomers when approaching the glass transition, suggesting that the network dynamics dominate the formation of the biaxial phase.

     

Broad Hexagonal Columnar Mesophases Formation in Bioinspired Transition‐Metal Complexes of Simple Fatty Acid meta‐Octaester Derivatives of meso‐Tetraphenyl Porphyrins

A series of meta‐substituted fatty acid octaester derivatives and their transition‐metal complexes of meso‐ tetraphenyl porphyrins (TPP‐8OOCR, with R=C_n?1H_2n?1, n=8, 12, or 16) have been prepared through very simple synthesis protocols. The thermotropic phase behavior and the liquid crystalline (LC) organization structures of the synthesized porphyrin derivatives were systematically investigated by a combination of differential scanning calorimetry (DSC), polarized optical microscopy (POM), and variable‐temperature small‐angle X‐ray scattering/wide‐angle X‐ray scattering (SAXS/WAXS) techniques. The shorter octanoic acid ester substituted porphyrin (C8‐TPP) did not show liquid crystallinity and its metal porphyrins exhibited an uncommon columnar mesophase. The lauric acid octaester (C12‐TPP) and the palmitic acid octaester (C16‐TPP) series porphyrins generated hexagonal columnar mesophase Col_h. Moreover, the metal porphyrins C12‐TPPM and C16‐TPPM with M=Zn, Cu, or Ni, exhibited well‐organized Col_h mesophases of broad LC temperature ranges increasing in the order of TPPNi<TPPCu≤TPPZn with their increased effective ionic radii in the square‐planar coordination. The simplicity in synthesis, the well intercolumnar organization of Col_h mesophase, the broadness of the discotic LC range, and the specific UV/Vis absorption and fluorescence emission behaviors make the symmetrically substituted fatty acid octaester porphyrins and their metal complexes very attractive for variant applications.     

High‐resolution solid‐state NMR study of the domain structure and molecular motion in drawn films of a vinylidene fluoride and trifluoroethylene copolymer with 73 mol % vinylidene fluoride

The heterogeneous higher order structure and molecular motion in a single crystalline film of a vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer with 73 mol % VDF was investigated with the ¹H–¹³C cross‐polarization/magic‐angle spinning NMR technique. A transient oscillation was observed in plots of the ¹³C peak intensity versus the contact time for the CH₂, CHF, and CF₂ groups. On the basis of the extended cross‐relaxation theory of spin diffusion, we determined that the oscillation behavior was caused by the TrFE‐rich segments in the chain and that the crystal consisted of VDF‐rich and TrFE‐rich domains. The former had TrFE‐rich segments in VDF and TrFE fractions of 0.24 and 0.27, respectively, and the latter had VDF‐rich segments in a VDF fraction of 0.49. The spin–lattice relaxation time T_1ρH in the rotating frame for each group was minimal in the three temperature regions of β, α_b, and α_c (↑) on heating and in the two temperature regions of α_1D and α_c (↓) on cooling. The α_c (↑) and α_c (↓) processes depended on the first‐order ferroelectric phase‐transition regions on heating and cooling, respectively. The motional modes for the other processes were confirmed by the T_1ρH minimum behavior of the VDF and TrFE groups in the TrFE‐rich domain and the VDF‐rich segments in the VDF‐rich domain. The β and α_b processes were attributed to the flip–flop motion of the TrFE‐rich segments and the competitive motion of the TrFE‐ and VDF‐rich segments in the ferroelectric phase, respectively. The α_1D process was due to the one‐dimensional diffusion motion of the conformational defects along the chain in the paraelectric phase, accompanied by the trans and gauche transformation of the VDF conformers of ttg⁺tg^? and g⁺tg^?tt. The effect of the competitive motion of the TrFE‐rich segment on the thermal stability of the VDF‐rich segment in the chain near the Curie temperature was examined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1026–1037, 2002     

Charge‐Transfer Phase Transition of a Cyanide‐Bridged FeII/FeIII Coordination Polymer

Heterometallic Prussian blue analogues are known to exhibit thermally induced charge transfer, resulting in switching of optical and magnetic properties. However, charge‐transfer phase transitions have not been reported for the simplest FeFe cyanide‐bridged systems. A mixed‐valence Fe^II/Fe^III cyanide‐bridged coordination polymer, {[Fe(Tp)(CN)₃]₂Fe(bpe)?5 H₂O}_n, which demonstrates a thermally induced charge‐transfer phase transition, is described. As a result of the charge transfer during this phase transition, the high‐spin state of the whole system does not change to a low‐spin state. This result is in contrast to FeCo cyanide‐bridged systems that exhibit charge‐transfer‐induced spin transitions.     

Chemical Inhomogeneity,Short‐Range Order,and Magnetism in the LiNiO2‐NiO Solid Solution

NiO:Li is an early exemplar for which hole‐doping of a correlated insulator gives rise to rich and varied magnetic behavior. It is also an important system from the viewpoint of p‐type transparent conducting oxides, and is representative of a large class of materials that have been used in lithium ion batteries, since the end‐member compound, LiNiO₂, belongs to the class of layered cathode materials. Despite the deceptive structural and compositional simplicity of this system, a complete understanding of its complex magnetic properties has remained elusive. Here a comprehensive investigation of the solid solution Li_xNi_2?xO₂, examining samples of precise stoichiometry using a combination of high‐resolution synchrotron X‐ray powder diffraction and SQUID magnetometry, is provided. The focus is on the interesting region between 0.40<x<1.00 in which the magnetic ordering temperature changes drastically with composition. The magnetism evolves from strong G‐type antiferromagnetism of x=0.40 with T_N=327 K to robust uncompensated magnetic order at T_N=240 K when x is close to 0.7, and to glassy A‐type antiferromagnetism of x=1.00 at T_N=9 K. This study demonstrates this magnetic behavior is linked to the Li–Ni chemical order that develops from short‐ to long‐range. The interfaces between ordered domains give rise to magnetic exchange bias, which manifests as a shift in the magnetization‐field loop for samples with nanoscale coherence lengths (0.54<x<0.66).     

Thermotropic Liquid Crystals Based on Extended 2,5‐Disubstituted‐1,3,4‐Oxadiazoles: Structure–Property Relationships,Variable‐Temperature Powder X‐ray Diffraction,and Small‐Angle X‐ray Scattering Studies

A class of extended 2,5‐disubstituted‐1,3,4‐oxadiazoles R¹‐C₆H₄‐{OC₂N₂}‐C₆H₄‐R² (R¹=R²=C₁₀H₂₁O 1 a , p‐C₁₀H₂₁O‐C₆H₄‐C?C 3 a , p‐CH₃O‐C₆H₄‐C?C 3 b ; R¹=C₁₀H₂₁O, R²=CH₃O 1 b , (CH₃)₂N 1 c ; F 1 d ; R¹=C₁₀H₂₁O‐C₆H₄‐C?C, R²=C₁₀H₂₁O 2 a , CH₃O 2 b , (CH₃)₂N 2 c , F 2 d ) were prepared, and their liquid‐crystalline properties were examined. In CH₂Cl₂ solution, these compounds displayed a room‐temperature emission with λ_max at 340–471 nm and quantum yields of 0.73–0.97. Compounds 1 d , 2 a – 2 d , and 3 a exhibited various thermotropic mesophases (monotropic, enantiotropic nematic/smectic), which were examined by polarized‐light optical microscopy and differential scanning calorimetry. Structure determination by a direct‐space approach using simulated annealing or parallel tempering of the powder X‐ray diffraction data revealed distinctive crystal‐packing arrangements for mesogenic molecules 2 b and 3 a , leading to different nematic mesophase behavior, with 2 b being monotropic and 3 a enantiotropic in the narrow temperature range of 200–210 °C. The structural transitions associated with these crystalline solids and their mesophases were studied by variable‐temperature X‐ray diffractometry. Nondestructive phase transitions (crystal‐to‐crystal, crystal‐to‐mesophase, mesophase‐to‐liquid) were observed in the diffractograms of 1 b, 1 d , 2 b, 2 d , and 3 a measured at 25–200 °C. Powder X‐ray diffraction and small‐angle X‐ray scattering data revealed that the structure of the annealed solid residue 2 b reverted to its original crystal/molecular packing when the isotropic liquid was cooled to room temperature. Structure–property relationships within these mesomorphic solids are discussed in the context of their molecular structures and intermolecular interactions.     

Crystal modifications and multiple melting behavior of poly(L‐lactic acid‐co‐D‐lactic acid)

The crystal modifications and multiple melting behavior of poly(L ‐lactic acid‐co‐D ‐lactic acid) (98/2) as a function of crystallization temperature were studied by wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). It was found that the disorder (α′) and order (α) phases of poly(L ‐lactic acid) (PLLA) were formed in cold‐crystallized poly(L ‐lactic acid‐co‐D ‐lactic acid) samples at low (<110 °C) and high (≥110 °C) temperatures, respectively. A disorder‐to‐order (α′‐to‐α) phase transition occurred during the annealing process of the α′‐crystal at elevated temperatures, which proceeded quite slowly even at the peak temperature of the exotherm P_exo but much more rapidly at higher temperature close to the melting region. The presence or absence of an additional endothermic peak before the exotherm in the DSC thermograph of the α′‐crystal was strongly dependent on the heating rate, indicating that a melting process involved during the α′‐to‐α phase transition. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Crystal Structure,Magnetic Exchange Interaction,and DFT Study of 2,2′‐(1‐Oxidopyridine‐2,6‐diyl)bis[4,5‐dihydro‐4,4,5,5‐tetramethyl‐3‐oxido‐1H‐imidazol‐1‐oxyl] Hydrate

The structure of a novel oxido‐aminoxyl (=`nitronyl nitroxide') biradical, 2,2′‐(1‐oxidopyridine‐2,6‐diyl)bis[4,5‐dihydro‐4,4,5,5‐tetramethyl‐3‐oxido‐1H‐imidazol‐1‐oxyl] hydrate ( 1 ⋅H₂O) was established by X‐ray analysis in the solid state: monoclinic, space group P2₁/c, Z=4 with a=12.621(4), b=15.704(5), and c=13.001(4) Å, and β=115.202(6)°. Variable‐temperature magnetic susceptibilities show a weak antiferromagnetic interaction between the two oxido‐substituted aminoxyl moieties of 1 , indicative of a singlet ground state. AM1 Calculations located minima for the possible structure based on the X‐ray crystal structure. A hybride density‐functional‐theory calculation with the UB3LYP method from the X‐ray crystal structure establishes the same spin sign in the two aminoxyl moieties and shows that a small spin density is localized at the C‐atoms of the pyridine moiety. These theoretic results are in good agreement with the determined weak antiferromagnetic interaction of 1 .