Solution-grown Single Crystals Research Articles

Anisotropic Charge Transport in Organic Single Crystals Based on Dipolar Molecules

AbstractWe studied the anisotropic charge transport properties of solution-grown organic single crystals based on a dipolar molecule 4HCB (4-hydroxy-cyanobenzene) by electrical transport measurements, current-voltage and space charge limited current (SCLC), and by X-ray diffraction analyses.Optical excitation differently affects the flow of charge carriers along the two main planar crystal axis, altering the charge transport anisotropy induced by the molecular π-orbitals stacking. We attribute this behaviour to the presence of an intrinsic molecular dipole and to its different orientation within the crystal lattice. The anisotropy of transport along the three crystallographic directions has been assessed by electrical characterization and correlated to the crystal molecular packing as determined by X-ray analyses.

大型放射光による生分解性ポリエステル結晶の動的構造解析

Poly[(R)-3-hydroxybutyrate] (P(3HB)) and its copolymers, produced by various microorganisms as an intracellular carbon and energy reserve, are biodegradable thermoplastic materials. These polymers are thus expected as a new material for a solution to problems concerning global environment. For the biodegradable polymer materials, the crystalline regions in the material play an important role in the regulation of both the degradability and the physical properties. Toward developing the high-performance biodegradable materials with desirable physical properties, we aim to reveal the morphology of crystalline region and its transformation by external field such as stress or heat at a molecular level. Here, we discuss the real-time annealing experiments of P(3HB) and its copolymer solution-grown single crystals, followed by synchrotron radiation small-angle X-ray scattering and wide-angle X-ray diffraction. The solid state reorganization of crystal thickening is attributed to partial melting-recrystallization process via the mutual rearrangement of molecular chains, which are released from the crystal lattice, between the adjacent crystals.

Charge Order Superstructure with Integer Iron Valence inFe2OBO3

Solution-grown single crystals of Fe(2)OBO(3) were characterized by specific heat, Mössbauer spectroscopy, and x-ray diffraction. A peak in the specific heat at 340 K indicates the onset of charge order. Evidence for a doubling of the unit cell at low temperature is presented. Combining structural refinement of diffraction data and Mössbauer spectra, domains with diagonal charge order are established. Bond-valence-sum analysis indicates integer valence states of the Fe ions in the charge ordered phase, suggesting Fe(2)OBO(3) is the clearest example of ionic charge order so far.

The Effect of Substrate on the Annealing of Poly(butylene succinate) Single Crystals

The morphological evolution of poly(butylene succinate) (PBS) solution-grown single crystals during annealing was studied using hot-stage atomic force microscopy. Their morphology changed with increasing temperature and annealing time. The annealing behavior and melting temperature were found to be affected by the substrate. Morphological changes occurred at a much lower temperature on an amorphous carbon film than that on a mica surface. Moreover, the pattern of morphological evolution of the single crystals on a carbon film was different from that on a mica surface. Since the PBS melt had a larger contact angle on the mica surface, these differences in the melting behavior were ascribed to the different interfacial interactions between the chain-folded surface of the single crystal and the substrate.

Effect of crystallization conditions on the shape of polymer single crystals: Experimental and theoretical approaches

A modified system of Mansfield equations with different step propagation rates to the right and left was applied to describe the shape of solution-grown single crystals of long-chain alkanes with asymmetrically curved faces ⨑ub;110⫂ub;. Solution of this system of equations and simulation of the shape of single crystals of polyethylene, α-polyvinylidene fluoride, and polyethylene oxide shows good agreement with the experimental data. Comparison of the simulated shapes and experimental micrographs makes it possible to determine the ratio of the intensity of secondary nucleation to the average propagation rate of the layer as well as the ratio of the rates of lateral steps to the right and left and to calculate the absolute values of the main crystallization parameters when the growth rates of single crystals in certain directions are known. It is shown that the asymmetry of the step propagation rate to the right and left is significant only for crystallization at very small supercoolings.

모델 시스템을 이용한 Poly(l-lactide)의 분해거동

The hydrolytic kinetics of biodegradable poly(l-lactide) (PLLA) have been studied by using two model systems, solution-grown single crystal (SC) and Langmuir monolayer techniques, for elucidating the mechanism for both alkaline and enzymatic degradations. The present study investigated the parameters such as degradation medium and time. The Langmuir mono layers of PLLA showed faster rates of hydrolysis when they were exposed to a basic subphase rather than they did when exposed to neutral subphase. Both degradation mediums had moderate concentrations to show a maximized activity, depending on their sizes. An alkaline degradation of SCs of PLLA showed the decrease of molecular weight of the remained crystals due to the erosion of chain-folding surface. However, the enzymatic degradation of SCs of PLLA occurred in the crystal edges thus the molecular weight of remained crystals was not changed. This behavior might be attributed to the size of enzymes which is much larger than that of alkaline ions; that is, the enzymes need larger contact area with monolayers to be activated.

Enzymatic Degradation and Aminolysis of Microbial Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) Single Crystals

Solution-grown single crystals of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] were hydrolyzed by polyhydroxybutyrate (PHB) depolymerase from Ralstonia pickettii T1. Enzymatic degradation proceeded from the edges of lamellar crystals, yielding serrated contour and small crystal fragments. Gel permeation chromatography analysis revealed that the molecular weights of the crystals decreased during enzymatic degradation, suggesting that the enzymatic hydrolysis of chain-folding regions at the crystal surfaces occurred in addition to the enzymatic degradation at crystal laterals or edges. After P(3HB-co-4HB) single crystals were aminolysed in 20% aqueous methylamine solution to remove the folded-chain regions and enzymatic degradation by lipase from Rhizopus oryzae to remove 4HB components at crystal surfaces of single crystal aminolyzed, it was found that a small amount (up to ca. 2 mol%) of 4HB component can be incorporated into the P(3HB) mother crystal lattice irrespective of the 4HB content.

Role of Columnar Mesophase in the Morphological Evolution of Polymer Single Crystals upon Heating: A Combined Atomic Force Microscopy and Electron Diffraction Study

The crystal-to-hexagonal columnar mesophase transition in solution-grown single crystals of poly(di-n-propyl siloxane), PDPS, was studied in situ with selected-area electron diffraction and variable-temperature atomic force microscopy. It was observed that, upon the transition, the hexagonal columnar mesophase displays two mutually orthogonal orientations of the lattice. The 2a* and 2b* reciprocal lattice vectors of the α-phase crystal coincide with the a* vectors of each of the two differently oriented mesophase lattices. The in situ atomic force microscopy study of the evolution of a single PDPS crystal upon heating shows that crystal thickening can occur via a short-term dwelling in the mesophase. The high chain mobility in the mesophase can account for the exceptional range of accessible crystal thicknesses of PDPS, which is much larger than that observed for polyethylene single crystals.

Vickers microhardness studies on solution-grown single crystals of magnesium sulphate hepta-hydrate

A Vickers microhardness study has been carried out on (100) and (010) faces of solution-grown single crystals of magnesium sulphate hepta-hydrate (MgSO4·7H2O) over a load range of 10–80g. The Vickers hardness numbers (Hv) are found to decrease initially with increase in load and then appear to level-off. The (100) face is the softest one. The Meyer index ‘n’ of the two faces is less than 2 as expected theoretically if the particular crystal system belongs to the soft material category. Neither Kick's law nor Hays and Kendall's law can fully explain the nonlinear variation of microhardness with load. Instead, preference is given to Li and Bradt's proportional specimen resistance (PSR) model. The elastic stiffness coefficient, c11, has also been calculated using Wooster's empirical relation from the hardness data and it shows reasonable agreement with a reported value. A hardness anisotropy for both planes has been observed in accordance with the orientation of the crystallographic planes.

Structural study on polymorphism of long-chain dicarboxylic acids using oblique transmission method for micro FT-IR spectrometers

Polymorphism and higher-order structures of even-number dicarboxylic acids from hexadecanedioic acid to eicosanedioic acid have been studied by micro-FTIR spectroscopy and microscopic observation. In order to obtain the three-dimensional structural information, the oblique transmission method was incorporated into a micro-FTIR spectrometer equipped with a couple of Cassegrain lenses. The IR spectra showed that the solid states of dicarboxylic acid bore a marked structural similarity to those of n-saturated fatty acids. It was found that a solution-grown single crystal of dicarboxylic acids was an aggregate of lamellae whose thickness exceeded by far their molecular length. The surfaces of lamellae were covered with hydrocarbon segments taking a hairpin structure, and the linkage of hydrogen-bonded dicarboxylic acids formed a folded chain structure. The result of oblique transmission measurements showed that the stacking mode of lamellae varied depending on crystallization conditions.

Chain Unfolding in Single Crystals of Ultralong Alkane C390H782 and Polyethylene: An Atomic Force Microscopy Study

Ultralong alkanes (CnH2n+2, n > 150) are generally considered as the most appropriate model for polyethylene, PE, and studies of these compounds can shed light on fine structural features of polymer crystals and the mechanisms of polymer crystallization. High-temperature atomic force microscopy was applied for comparative studies of solution-grown single crystals of C390H782 (the longest available alkane) and PE. While the structures of single crystals of C390H782 and PE are similar, the details of their thermal behavior are quite different. The structural transitions in C390H782 crystals follow the typical oligomer-type integral chain-folding scheme. Upon annealing, the alkane crystal undergoes a complete series of transformations corresponding to stepwise unfolding from the folded-in-five conformation toward the fully extended-chain crystal. The chain unfolding in PE crystals is a continuous and slower process. The morphological evolution of single crystals on heating is found to be dependent on the presence of solvent traces. Melting of the alkane crystals on graphite is accompanied by spreading of the alkane onto the substrate leading to the formation of a thin epitaxial layer. The complete melting of this film occurs at 185 °C, i.e., approximately 55 °C above the melting point of the bulk material.

In situ observation of heterogeneous melting of poly[( R)-3-hydroxybutyrate] single crystals by temperature-controlled atomic force microscopy

Morphological changes of lath-shaped single crystals of poly[( R)-3-hydroxybutyrate] (P(3HB)) during annealing were followed in situ by a temperature-controlled atomic force microscopy (TC-AFM) in order to elucidate thermal stability or chain-mobility within the individual single crystal. Two kinds of the solution-grown single crystals prepared under different crystallization conditions were used to also investigate the influence of crystallization temperature on the annealing behavior. The significant morphological changes during annealing were observed at temperatures above the original crystallization temperature. The TC-AFM observations have revealed that ridges on the lamellar surface running across the crystal width (parallel to the b-axis), and cracks along the long axis of the crystal (parallel to the a-axis) are occasionally formed by thermal treatment. At a higher annealing temperature, numerous notches along the b-axis were frequently formed at the lateral sides of crystal. The notch formation may be due to preferential melting of the defective or less perfect regions within the lamellar crystal. The whole crystal became narrow while the notches progressed along b-axis, and some cavities developed at the central portion of the crystal as annealing temperature was elevated. The isothermal annealing experiment has demonstrated that the melting of P(3HB) crystals gradually progresses from the lateral outer faces. On the basis of the results, it has been concluded that the less perfect regions along the a-axis and b-axis exist within the P(3HB) single crystal, and that melting of the regions at the lateral sides of crystal is more easily initiated.

Morphology and Enzymatic Degradation of Solution-Grown Single Crystals of Poly(ethylene succinate)

Solution-grown lamellar single crystals of poly(ethylene succinate) (PES) were prepared from a 0.025% solution of monochlorobenzene under an isothermal crystallization condition. Lozenge-shaped lamellar crystals with and without spiral growth were obtained at different crystallization temperature. The crystals produced well-resolved electron diffractograms. Polyethylene decoration of the crystals resulted in a “cross-sector” surface morphology and showed that the average direction of chain-folding is parallel to the crystal growth planes. In some cases, the lamellar crystals showed spiral growths originating from screw dislocations with both left-handed and right-handed forms. Two extracellular poly(hydroxybutyrate) (PHB) depolymerases purified from Alcaligenes faecalis T1 and Pseudomonas stutzeri YM1006 were used for the enzymatic degradation of the PES lamellar crystals. Adsorption of both the extracellular PHB depolymerases onto the crystal surface was observed using immuno-gold labeling technique. The enzyme molecules were homogeneously distributed on the surface of PES lamellar crystals. When the extracellular PHB depolymerase from A. faecalis T1 was applied to a suspension of PES crystals, the turbidity of crystal suspension did not decrease. This indicated that the A. faecalis T1 depolymerase could not degrade the PES crystals even though it can adsorb onto the crystal surface. On the other hand, enzymatic hydrolysis by the extracellular PHB depolymerase from P. stutzeri YM1006 was confirmed by the decrease in turbidity. The degradation progressed from the edges of the lamellar crystals without decreasing the molecular weights and the lamellar thicknesses. The difference in enzymatic activity seems to be due to the recognition of the distance between ester bonds in molecular chain by the active site of the enzyme molecule.

Direct Observation of Enzymatic Degradation Behavior of Poly[(R)-3-hydroxybutyrate] Lamellar Single Crystals by Atomic Force Microscopy

The enzymatic degradation behavior of solution-grown single crystals of poly[(R)-3-hydroxybutyrate] (P(3HB)), which were adsorbed on the surface of highly ordered pyrolytic graphite, with an extracellular PHB depolymerase from Alcaligenes faecalis T1 was directly observed by atomic force microscopy. At the initial stage of degradation, enzymatic action of both adsorption and hydrolysis produced smaller crystal blocks by predominant degradation of distorted regions of a single crystal. Subsequently, the hydrolysis progressed from a generated end of the crystal block, resulting in the formation of cracks. The degradation also took place at a middle part of the crystal, yielding slits. Several cracks and slits stood in a line along the long axis of crystal, and further enzymatic hydrolysis combined the cracks and the slits into longer cracks. With these processes, the enzymatic degradation converted a lath-shaped lamellar single crystal into lathlike fingers and crystal fragments. It has been proposed from t...

Alkaline Hydrolysis of Solution-Grown Poly(L-lactic acid) Single Crystals.

The changes in morphology and crystalline state of solution-grown single crystals of poly(L-lactic acid)(PLLA) in aqueous NaOH solution of various strengths were investigated by means of transmission electron microscopy, atomic force microscopy and gel permeation chromatography. The degradations of hexagonal and lozenge shaped PLLA single crystals started from the lateral side of the crystal to generate the notched morphology at initial stage of hydrolysis. As hydrolysis proceeded, the molecular weight of degraded PLLA crystals corresponded to equal the value calculated from lamellar thickness measured by atomic force microscopy, suggesting that the tight chain-folding region of PLLA molecules on crystal surface was degraded by alkaline hydrolysis. Based on the results, alkaline hydrolysis with NaOH solution firstly occurs at the loosely chain-packing region on crystal edges, and then gradually progresses from both crystal edges and tight chain-packing region with chain-folding on the crystal surface.

Alkaline Hydrolysis of Solution-Grown Poly[(R)-3-hydroxybutyrate] Single Crystals

The changes in morphology and crystalline state of solution-grown single crystals of bacterial poly[(R)-3-hydroxybutyrate] (P(3HB)) in NaOH solution of various strengths were investigated by means of transmission electron microscopy, atomic force microscopy, and gel permeation chromatography. Lath-shaped P(3HB) single crystals, which had widths of 2−3 μm, were split to narrow ribbonlike crystals of around 0.1 μm at the initial stage of hydrolysis without a decrease in lamellar thickness. As hydrolysis time increased, ribbonlike crystals were degraded from the lateral side of the crystal to generate the notched morphology. Furthermore, the molecular weight of degraded P(3HB) crystals corresponded to equal the value calculated from lamellar thickness measured by atomic force microscopy, suggesting that the tight chain-folding region of P(3HB) molecules on the crystal surface was degraded by alkaline hydrolysis. This surface degradation was further supported by the disappearance in the electron micrograph of straight lines on the surface of the degraded crystal decorated with polyethylene. Despite the unselective degradation of alkaline hydrolysis from both crystal edges and crystal surfaces, narrow ribbonlike crystals were generated at the initial stage of hydrolysis. This observation suggests that single crystals with chain-folding have two regions of tight chain-packing region with adjacent reentry folds and a loosely chain-packing region with random reentry folds with interval of 100 nm or less. The model developed in this paper suggests that alkaline hydrolysis with NaOH solution first occurs at the loosely chain-packing region on crystal surface and then gradually progresses from both crystal edges and tight chain-packing region with chain-folding on the crystal surface.

Optically Active Polyethers. 3. On the Relationship between Main-Chain Chirality and the Lamellar Morphology of Solution-Grown Single Crystals

The morphology of chiral (R)- and (S)-poly(epichlorohydrin) (PECH) single crystals grown from solution was studied to obtain insight into the mechanism by which the chiral character of the main chain can be transmitted to higher organizational levels, such as the lamellar twisting orientation in banded spherulites. Poor solvents were selected so that the crystallization from solution occurred at high temperatures, where the resulting lamellar habits simulate most closely those obtained in crystallization from the melt. Interestingly, the TEM examination of folded-chain single-crystal lamellae showed no morphological evidence that could be related to the chiral character, R or S, of the polyenantiomer. In addition, it was established that the handedness (or sense) of screw dislocations in these lamellae is not controlled by the main-chain chirality. This result supports previous evidence which shows that, although the screw dislocations may contribute to the banding through lamellar branching, they appear not to be primary determinants of either the handedness or magnitude of the twist of lamellae in banded spherulites. Similar morphology patterns were obtained for lamellar single crystals of chiral (R)- and (S)-poly(propylene oxide) and of the optically active biopolyester, (R)-poly(3-hydroxyvalerate), supporting the generality of these features. In searching further for chiral effects, the examination of lamellar morphology in large PECH aggregates, e.g., highly dendritic crystals and hedrites, grown from solution indicated that some of the lamellae are nonplanar while in solution. Some considerations pertaining to the factors that may contribute to the nonplanar habits observed in the morphology of these large aggregates are discussed as well.

Structure Relaxation of n-alkanes Observed by the Simultaneous DSC-FTIR Method

Phase transitions of hexatriacontane (C36) and octacosane (C28), both as the solution grown single crystal (SGC) and polycrystalline aggregates (MCC) prepared by cooling at 1 K min−1 from the isotropic liquid state, were measured by the simultaneous DSC-FTIR method. MCC of C36 showed the freezing of the high temperature stable phase, which had a slight lower order of the lattice vibration mode comparing with the room temperature stable phase. MCC of C28 demonstrated thermo-reversible phase transition, however, had a binomial distribution of crystal stability of the monoclinic phase.

Electron paramagnetic resonance and electron nuclear double resonance of NaCl:Rh2+: A comparison between similarly doped Bridgman- and solution-grown single crystals

In a recent electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) study several new rhodium related centres in a Bridgman-grown crystal were reported. For two centres a detailed microscopic model was established. The primary Rh2+ centre was identified as a [RhCl6]4− complex with a single charge compensating Na+ vacancy in a next nearest neighbour position. A second centre was identified as a rhodium dimer. Two other Rh2+ centres, with apparent axial symmetry, remained unidentified. In the present study a detailed comparison between Rh2+ defects in Bridgman- and solution-grown NaCl crystals is presented. One of the previously unidentified Rh2+ centres is shown not to be axial but to have orthorhombic-I symmetry and is identified as a cis-[RhCl6]4− complex with two next nearest neighbour Na+ vacancies. For the primary centre additional ENDOR interactions confirm the model mentioned above. Finally a comparison is made between the temperature stability of the primary and orthorhombic-I centre in Bridgman-grown and similarly doped solution-grown NaCl crystals.

Frustrated Crystal Structure of Poly(l-hydroxyproline)

The crystal structure of poly(l-hydroxyproline) (PHypro) with trans planar conformation of the peptide group is reexamined. The electron diffraction pattern obtained from solution-grown triangular single crystals confirms the unit-cell geometry determined by Sasisekharan (Acta Crystallogr. 1959, 12, 903) (trigonal unit cell with parameters a = b = 12.3 and c = 9.15 , three left-handed 3-fold helices per unit cell). Analysis of the diffraction data and the single-crystal morphology indicates that PHypro crystallizes in a frustrated structure characterized by the fact that the azimuthal orientation of one of the helices differs significantly from that of the other two. Such an arrangement is, however, compatible with hydrogen bonding between the hydroxyl groups and the carbonyl groups of the neighbor helices. It further accounts for the highly unusual triangular morphology of solution-grown single crystals.