Synthesis and Cyclization-Induced Charge Transfer of Rectangular Bisterthiophenesiloxanes.

Cyclization-modified terthiophene displays the change of emission behavior from locally excited (LE) to the intramolecular charge transfer (ICT) state. The rectangular bisterthiophenesiloxanes (DSiTh) was successfully prepared by π-π-stacking-aided hydrogen-bonding interactions. Cyclization-induced ICT in DSiTh could be observed, which was confirmed by absorption spectra, fluorescence spectra, and quantum chemistry analysis. The cyclization produces a strong intramolecular electron redistribution of a highly packed π-conjugated terthiophene. Thus, a distinctive variation of the dipole moment and a through-space ICT happen.

Transformation of Coiled α-Helices into Cross-ß-Sheets Superstructure.

The fibrous silk produced by bees, wasps, ants, or hornets is known to form a four-strand α-helical coiled coil superstructure. We have succeeded in showing the formation of this coiled coil structure not only in natural fibers, but also in artificial films made of regenerated silk of the hornet Vespa simillima xanthoptera using wide- and small-angle X-ray scatterings and polarized Fourier transform infrared spectroscopy. On the basis of time-resolved simultaneous synchrotron X-ray scattering observations for in situ monitoring of the structural changes in regenerated silk material during tensile deformation, we have shown that the application of tensile force under appropriate conditions induces a transition from the coiled α-helices to a cross-ß-sheet superstructure. The four-stranded tertiary superstructure remains unchanged during this process. It has also been shown that the amorphous protein chains in the regenerated silk material are transformed into conventional ß-sheet arrangements with varying orientation.

Molecular Orientation Enhancement of Silk by the Hot-Stretching-Induced Transition from α-Helix-HFIP Complex to ß-Sheet.

Enhancing the molecular orientation of the regenerated silk fibroin (RF) up to a level comparable to the native silk is highly challenging. Our novel and promising strategy for the poststretching process is (1) creating at first an α-helix-HFIP complex with a hexagonal packing as an intermediate state and then (2) stretching it at a high temperature to induce the helix-to-sheet structural phase transition. Here we show for the first time the significantly high stretching efficiency of the proposed technique compared with the conventional wet-stretching techniques and the successful achievement of higher crystalline orientation and higher Young's modulus compared even with the native silk. The detailed structural analysis based on the time-resolved simultaneous measurement of stress-strain curve, synchrotron X-ray scatterings, and FTIR has revealed the structural transition mechanism from the hexagonally packed α-helix-HFIP complex to the highly oriented ß-sheet crystalline state as well as the critical level of crystal orientation needed for the helix-to-sheet transition.

Molecular assembly of highly symmetric molecules under a hydrogen bond framework controlled by alkyl building blocks: a simple approach to fine-tune nanoscale structures.

To date, molecular assemblies under the contribution of hydrogen bond in combination with weak interactions and their consequent morphologies have been variously reported; however, how the systematic variation of the structure can fine-tune the morphologies has not yet been answered. The present work finds an answer through highly symmetric molecules, i.e. diamine-based benzoxazine dimers. This type of molecule develops unique molecular assemblies with their networks formed by hydrogen bonds at the terminal, while, at the same time, their hydrogen bonded frameworks are further controlled by the hydrophobic segment at the center of the molecule. When this happens, slight differences in hydrophobic alkyl chain lengths (, , and ) bring a significant change to the molecular assemblies, thus resulting in tunable morphologies, i.e. spheres, needles and dendrites. The superimposition between the crystal lattice obtained from X-ray single crystal analysis and the electron diffraction pattern obtained from transmission electron microscopy allows us to identify the molecular alignment from single molecules to self-assembly until the morphologies developed. The present work, for the first time, shows the case of symmetric molecules, where the hydrophobic building block controls the hydrogen bond patterns, leading to the variation of molecular assemblies with tunable morphologies.

Constructing π-Electron-Conjugated Diarylbutadiyne-Based Polydiacetylene under Molecular Framework Controlled by Hydrogen Bond and Side-Chain Substituent Position.

Diarylbutadiyne derivatives are ideal monomers for providing the π-electron-conjugated system of polydiacetylenes (PDAs). The geometrical parameters for diacetylene topochemical polymerization are known. However, control of the molecules under these parameters is yet to be addressed. This work shows that by simply tailoring diarylbutadiyne with amide side-chain substituents, the arrangement of the substituents and the resulting hydrogen bond framework allows formation of π-electron-conjugated PDA.

Shifting from hydrogen bond network to π-π stacking: a key mechanism for reversible thermochromic sulfonated poly(ether ether ketone).

Sulfonated poly(ether ether ketone) (SPEEK) thin film performs reversible thermochromic property by developing the color to be yellowish at the temperature above 190 °C. The detailed analyses based on temperature-dependent techniques suggest the thermal treatment inducing the shifting of the hydrogen bond network between the sulfonated group and the hydrated water molecules to the π-π stacking among aromatic rings in SPEEK chains. Although it is general that the polymer chain packing is unfavorable at high temperature, the present work shows a good example that when the polymer chains can form specific molecular interaction, such as π-π stacking, even in harsh thermal treatment, a rearrangement will effectively occur, which leads to an external stimuli-responsive property.

From Chitin Nanowhisker Colloidal Dispersions to Anisotropic Microfibers: Structural Evolution in Its Transformation Process as Revealed by Synchrotron Wide-Angle X-ray Diffraction Measurements.

Nanowhiskers in a colloidal dispersion are known to form chiral nematic liquid crystals (CNLC), as seen in a cellulose nanowhisker or so-called cellulose nanocrystal and chitin nanowhisker. In our related work, we clarified that once the thus-created chitin nanowhiskers with surface modified by chitosan (CTWK-CS) in CNLC phase were wet-spun, we could directly obtain anisotropic microfibers containing the highly ordered CTWK-CS. This drastic structural transformation from CNLC to anisotropic microfibers might relate to several important stages, i.e., stage (i) is the alignment of CTWK-CS initiated by a specific concentration and flow to create aggregation in the CNLC state, stage (ii) is the coagulation of CTWK-CS in CNLC to transform to microfibers, and stage (iii) is the drying of the thus-extruded microfibers to allow CTWK-CS alignment. The present work sets up the experimental systems simulating stages (i) and (iii) to reveal the orientational behavior of CTWK-CS and the structural evolution, respectively, by synchrotron 2D WAXD measurement. In stage (i), the high degree of the parallel alignments of CTWK-CS with the chain axis oriented along the flow direction of the colloidal dispersions confirms that the flow and concentration synergistically controlled CTWK-CS alignment. In contrast, for stage (iii), the poor c-axial orientation of CTWK-CS in as-spun wet microfibers gradually changed to the high degree of c-axial orientation along the fiber direction during drying process, indicating a reorientation of CTWK-CS along with dehydration. The present study declares an in situ observation of the direct wet spinning of nanowhiskers about their remarkable alignments in the sheared colloidal dispersions (stage (i)) and their random-to-high reorientation during the drying process of the as-spun wet microfiber (stage (iii)).

Polymorphism and Stretch-Induced Transformations of Sustainable Polyethylene-Like Materials.

Herein we demonstrate that polyethylene-like bioderived, biodegradable, and fully recyclable unbranched aliphatic polyesters, such as PE-2,18, develop hexagonal crystal structures upon quenching from the melt to temperatures <∼50 °C and orthorhombic-like packing at higher quenching temperatures or after isothermal crystallization. Both crystal types are layered. While all-trans CH2 packing characterizes the structure of the orthorhombic-like form, there is significant conformational disorder in the staggered long CH2 sequences of the hexagonal crystals. On heating, the hexagonal crystals transform to the orthorhombic type at ∼60 °C via melt recrystallization, but no change is apparent during heating samples with the orthorhombic form up to the melting point (∼95 °C). The hexagonal structure is of interest not only because it develops under very rapid quenching from the melt but also because under uniaxial tensile deformation it undergoes a stretch-induced transformation to the orthorhombic structure. Compared to deformation of orthorhombic specimens that maintain the same crystal type, such transformation results in larger strains and enhanced strain hardening, thus representing a desired toughening mechanism for this type of polyethylene-like materials.

Hybridization of Wide-Angle X-ray and Neutron Diffraction Techniques in the Crystal Structure Analyses of Synthetic Polymers.

The development in the crystal structure analysis of synthetic polymers using the hybridized combination of wide-angle X-ray and neutron diffraction (WAXD and WAND, respectively) techniques has been reviewed with many case studies performed by the authors. At first, the technical development was reviewed, in which the usage of high-energy synchrotron X-ray source was emphasized for increasing the total number of the observable diffraction peaks, and several examples were introduced. Secondly, the usage of the WAND method was introduced, in which the successful extraction of hydrogen atomic positions was described. The third example is to show the importance for the hybrid combination of these two diffraction methods. The quantitative WAXD data analysis gave the crystal structures of at-poly(vinyl alcohol) (at-PVA) and at-PVA-iodine complex. However, the thus-proposed structure models were found not to reproduce the observed WAND data very much. The reason came from the remarkable difference in the atomic scattering powers of the constituting atomic species between WAXD and WAND phenomena. The introduction of statistical disorder solved this serious problem, which reproduced both of the observed WAXD and WAND data consistently. The more systematic combination of WAXD and WAND methods, or the so-called X-N method, was applied also to the quantitative evaluation of the bonded electron density distribution along the skeletal chains, where the results about polydiacetylene single crystals were presented as the first successful study. Finally, the application of WAND technique in the trace of structural changes induced under the application of external stress or temperature was described. The future perspective is described for the development of structural science of synthetic polymers on the basis of the combined WAXD/WAND techniques.

Microscopic Fourier Transform Infrared Characterization on Two Types of Spherulite with Polymorphic Crystals in Poly(heptamethylene terephthalate).

FTIR microspectrometry with in situ temperature variation and IR-peak-mapping capability, and POM characterization were used to study the crystal distribution in dual spherulites in poly(heptamethylene terephthalate). By tracing the crystalline IR bands of the α-crystal and ß-crystal to get the crystal distribution, the techniques resolve that the ringed and ringless spherulites comprise α- and ß-crystals, respectively. In addition, temperature-dependent IR analyses on the spots related to the two crystals also reveal the α- and ß-crystals melt at 98 and 104 °C, respectively. The ringed and ringless spherulites were proven to be correlated with the α- and ß-crystal forms, respectively.

Relationship between morphological change and crystalline phase transitions of polyethylene-poly(ethylene oxide) diblock copolymers. 3. Dependence of morphological transition phenomena on the PE/PEO segmental lengths and its possible origins.

By measurement of the small-angle and wide-angle X-ray scatterings and infrared and Raman spectra and thermal data, microphase separation phenomena have been investigated for a series of polyethylene-poly(ethylene oxide) diblock copolymer (PE-b-PEO) in both the heating and cooling processes and compared with the structural changes occurring inside the PE and PEO domains. The complicated morphological changes between lamella, perforated lamella, gyroid, cylinder, and sphere phases were detected for the copolymer with relatively short PE segments. The orthorhombic crystalline structure of PE was kept unchanged in the lamella-to-gyroid transition. When the PE orthorhombic phase transformed to the pseudohexagonal or rotator phase, the gyroid morphology changed to the cylinder. On the other hand, the diblock copolymer with relatively long PE segment was found to show only the lamellar morphology, in which the order-disorder structural transition between the orthorhombic and pseudohexagonal phases occurred in the PE crystal region. As a possibility, the large difference in morphological change between the copolymers with short and long PE segments has been ascribed to the difference in thermal mobility of PE segments, which is controlled by the conformation of chains and their packing mode, i.e., an extended chain or a folded chain. The extended chains may move thermally and actively along the interfacial boundary in addition to the librational motion around the chain axis, resulting in a variety of morphological changes, whereas the thermal motion of the folded chains may be suppressed because of the geometrical constraint and does not cause such a large-scale morphological change from the lamellar structure. This concept, a thermal activity and geometrical constraint, is considered to be quite important in the interpretation of complicated morphological changes observed for many crystalline-amorphous and crystalline-crystalline diblock copolymers when viewed from the molecular level.

Relationship between morphological change and crystalline phase transitions of polyethylene-poly(ethylene oxide) diblock copolymers, revealed by the temperature-dependent synchrotron WAXD/SAXS and infrared/Raman spectral measurements.

The phase transition behaviors of low-molecular-weight polyethylene-poly(ethylene oxide) (PE-b-PEO) diblock copolymers with the monomeric units of PE/PEO = 17/40 and 39/86 have been successfully investigated through the temperature-dependent measurements of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), infrared and Raman spectra, as well as thermal analysis. These diblock copolymers had been believed to show only order-to-disorder transition of lamellar morphology in a wide temperature region, but it has been found here for the first time that this copolymer clearly exhibits the three stages of transitions among lamella, gyroid, cylinder, and spherical phases in the heating and cooling processes. The WAXD and IR/Raman spectral measurements allowed us to relate these morphological changes to the microscopic changes in the aggregation states of PEO and PE segments. In the low-temperature region the PEO segments form the monoclinic crystal of (7/2) helical chain conformation and the PE segments of planar-zigzag form take the orthorhombic crystalline phase. These crystalline lamellae of PEO and PE segments are alternately stacked with the long period of 165 Angstroms. In a higher temperature region, where the PEO crystalline parts are on the way of melting but the PE parts are still in the orthorhombic phase, the gyroid morphology is detected in the SAXS data. By heating further, the gyroid morphology changes to the hexagonally packed cylindrical morphology, where the orthorhombic phase of PE segments is gradually disordered because of thermally activated molecular motion and finally transforms to the pseudohexagonal or rotator phase. Once the PE segments are perfectly melted, the higher-order structure changes from the cylinder to the spherical morphology. These morphological transitions might relate to the thermally activated motions of two short chain segments of the diblock copolymer, although the details of the transition mechanism are unclear at the present stage.

Crystal structures of n-alkane with three functional groups in the middle and at both ends.

We synthesized a linear alkane, K35DA, with a main-chain carbon number n = 33 and three functional groups, a carbonyl group in the middle and carboxyl groups at both ends, and studied influences of the functional groups as well as chain length on morphologies of samples prepared by solution-grown and bulk crystallization methods (SG-K35DA and BK-K35DA) from differential scanning calorimetry (DSC), X-ray diffraction, and IR absorption measurements. Data analyses reveal that at room temperature an orthorhombic crystal of type P2(1)2(1)2(1), together with a considerable amount of amorphous fraction, is predominantly realized in BK-K35DA due to the van der Waals force between neighboring long methylene sequences, whereas a monoclinic type of crystal belonging to the same space group (P2(1)/c) as reported for linear dicarboxylic acid crystals with odd carbon numbers is coexistent for SG-K35DA. The crystalline structures appear to be distorted with increasing temperature, as the dipole-dipole interaction between the carbonyl groups tends to be weakened, and both orthorhombic and monoclinic crystals undergo the solid-solid phase transition to the hexagonal crystalline structure at a temperature about 10 K below their respective T(m)s, which can be regarded as a new example of the Brill transition.

Structural Evolution Mechanism of Crystalline Polymers in the Isothermal Melt-Crystallization Process: A Proposition Based on Simultaneous WAXD/SAXS/FTIR Measurements.

Time-resolved simultaneous measurements of wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) (and FTIR spectra) were performed for various kinds of crystalline polymers in isothermal melt-crystallization processes, from which the common features of the structural evolution process as well as the different behaviors intrinsic to the individual polymer species were extracted. The polymers targeted here were polyethylene, isotactic polypropylene, polyoxymethylene, aliphatic nylon, vinylidene fluoride copolymer, trans-polyisoprene, and poly(alkylene terephthalate). A universal concept of the microscopically viewed structural evolution process in isothermal crystallization may be described as follows: (i) the small domains composed of locally regular but more or less disordered helical chain segments are created in the melt (this important information was obtained by the IR spectral data analysis); (ii) these domains grow larger as the length and number of more regular helical segments increase with time; (iii) the correlation among the domains becomes stronger and they approach each other; and (iv) they merge into the stacked lamellar structure consisting of the regularly arranged crystalline lattices. The inner structure of the domains is different depending on the polymer species, as known from the IR spectral data.

A study of the extraordinarily strong and tough silk produced by bagworms.

Global ecological damage has heightened the demand for silk as 'a structural material made from sustainable resources'. Scientists have earnestly searched for stronger and tougher silks. Bagworm silk might be a promising candidate considering its superior capacity to dangle a heavy weight, summed up by the weights of the larva and its house. However, detailed mechanical and structural studies on bagworm silks have been lacking. Herein, we show the superior potential of the silk produced by Japan's largest bagworm, Eumeta variegata. This bagworm silk is extraordinarily strong and tough, and its tensile deformation behaviour is quite elastic. The outstanding mechanical property is the result of a highly ordered hierarchical structure, which remains unchanged until fracture. Our findings demonstrate how the hierarchical structure of silk proteins plays an important role in the mechanical property of silk fibres.

pH-induced conformational changes in histamine in the solid state.

Histamine is one of the most basic biogenic amino-compounds, which is composed of imidazole and a flexible ethylamine side chain moiety. Histamine is known to take the form of various types of cations, free base, monocation and dication form, where its conformational change is highly sensitively to the pH conditions. The details of these changes are still controversial due to a lack of detailed information on its crystal structures. Thus, in this study, the molecular packing structures of histidine at various pH were analyzed via X-ray diffraction in combination with vibrational spectroscopy and energy calculations. A variety of molecular conformations including the tautomeric phenomenon was found to be intimately related with intra- and intermolecular hydrogen bonds. The role of the hydrogen bonds was studied also to check the possibility of high proton conductivity of histamine, as predicted by computer simulation. Consequently, the thus-predicted proton conductivity was confirmed for the first time experimentally. During the heating process, the conductivity showed the relatively high maximum value of 10-4 S cm-1 at around 60 °C, which is related to the effective proton transfer between the amino NH group of one histamine unit and the imidazole ring of another.

Electrospinning as a new technique to control the crystal morphology and molecular orientation of polyoxymethylene nanofibers.

Electrospinning is widely accepted as a simple and versatile technique for producing nanofibers. The present work, however, introduces a new concept of the electrospinning method for controlling the crystal morphology and molecular orientation of the nanofibers through an illustration of a case study of polyoxymethylene (POM) nanofibers. Isotropic and anisotropic electrospun POM nanofibers are successfully prepared by using a stationary collector and a rotating disk collector. By controlling the voltage and the take-up velocity of the disk rotator, the morphology changes between an extended chain crystal (ECC) and a folded chain crystal (FCC) as clarified by a detailed analysis of the X-ray diffraction and polarized infrared spectra of the POM nanofibers. Herman's orientation function and dichroic ratio lead us to a schematic conclusion--that (i) molecular orientation is parallel to the fiber axis in both isotropic and anisotropic POM nanofibers, (ii) a single nanofiber consists of a nanofibril assembly with a size of 60-70 A and tilting at a certain degree, and (iii) the higher the take-up velocity, the smaller the nanofibril under the (9/5) helical structure of the POM chains. It should be emphasized here that the electrospinning method is no longer a single nanofiber producer but that it can be applied as a new instrument to control the morphology and chain orientation characteristics of polymer materials, opening a new research field in polymer science where we can understand the relationship between structure at the molecular level and the properties and performance at the macroscopic level.

Investigating the proton transferring route in a heteroaromatic compound part I: a trial to develop di- and trifunctional benzimidazole model compounds inducing the molecular packing structure with a hydrogen bond network.

Di- and trifunctional benzimidazole molecules, 1 and 2, have been synthesized as the model compounds to identify their molecular packing structure and hydrogen bond network, which is possibly involved in the proton transfer system belonging to its heteroaromatic functional groups. By carrying out the simple reaction between acid chloride and diamine, the desired benzimidazole model compounds are obtained with high yield above 60%. The comparison studies between the model compounds and benzimidazole reveal that the model compounds show well-packing structure with intermolecular hydrogen bonds similar to those observed in benzimidazole. The presence of solvent with 2 leads to the unique intermolecular hydrogen bonds between one molecule of 2 and six molecules of solvent (i.e., 2-propanol) resulting in the solvent-assisted intramolecular hydrogen bond network among benzimidazole functional groups. The comparative studies of the effect of temperature on the packing structure and hydrogen bond in the model compounds indicate that the development of the benzimidazole unit from monofunctional to difunctional and finally trifunctional enhances the intermolecular interaction between the molecules and results in the stronger molecular packing structure of the compounds. A study on proton conductivity by preparing the sulfonated poly(ether ether ketone) (SPEEK) membranes with benzimidazole, 1, and 2 for 15 phr equivalent to benzimidazole group clarifies (i) incorporation of benzimidazole compounds improves the proton conductivity of SPEEK in dry condition and (ii) the increase in proton conductivity is relevant to the number of benzimidazole group on molecule.

Effect of Elevated Temperatures on the States of Water and Their Correlation with the Proton Conductivity of Nafion.

For the first time, we report the effects of elevated temperatures, from 80 to 100 °C, on the changes in the states of water and ion-water channels and their correlation with the proton conductivity of Nafion NR212, which was investigated using a Fourier transform infrared spectroscopy study. Experimentally, three types of water aggregates, protonated water (H+(H2O) n ), nonprotonated hydrogen (H)-bonded water (H2O···H2O), and non-H-bonded water, were found in Nafion, and the existence of those three types of water was confirmed through ab initio molecular dynamics simulation. We found that the proton conductivity of Nafion increased for up to 80 °C, but from 80 to 100 °C, the conductivity did not increase; rather, all of those elevated temperatures showed identical conductivity values. The proton conductivities at lower relative humidities (RHs) (up to 50%) remained nearly identical for all elevated temperatures (80, 90, and 100 °C); however, from 60% RH (over λ = 4), the conductivity remarkably jumped for all elevated temperatures. The results indicated that the amount of randomly arranged water gradually increased and created more H-bonded water networks in Nafion at above 60% RH. From the deconvolution of the O-H bending band, it was found that the volume fraction f i (i=each deconvoluted band) of H-bonded water for elevated temperatures (>80-100 °C) increased remarkably higher than for 60 °C.

Infrared Spectroscopy and X-ray Diffraction Characterization of Dimorphic Crystalline Structures of Polyethylenes with Halogens Placed at Equal Distance along the Backbone.

Polyethylenes with halogens placed on each and every 21st, 15th, or ninth backbone carbon display crystallization patterns enabled by the size of the halogen and by changing crystallization kinetics. The different structures have been identified from X-ray patterns combined with a detailed analysis of the infrared spectra of series containing F, Cl, or Br atoms that were either fast or isothermally crystallized from the melt. Under both crystallization modes, all specimens develop layered crystallites that accommodate 5-9 repeating units along the chain's axis. The size of the halogen and intermolecular staggering to maximize packing symmetry are responsible for striking structural differences observed between the series and between the two modes of crystallization. While the small size of the F atom causes a small perturbation to the crystal lattice and the orthorhombic structure is maintained for all members of the series either fast or isothermally crystallized, each Cl or Br-containing system presents dimorphism. Under fast crystallization, Cl and Br containing samples adopt the all-trans conformation (planar Form I), while in slowly crystallized samples gauche conformers set for bonds of the backbone carbons adjacent to the carbon with the halogen due to a close intermolecular staggering of halogens (herringbone Form II). In both forms the methylene sequence between halogens maintains the all-trans conformation. The structural details are extracted from the analysis of the C-halogen stretching region of the IR spectra, and from adherence to the n-alkane behavior of CH2 rocking, CH2 wagging, and C-C stretching progression modes.