Enhancing nucleation and controlling crystal orientation by rubbing/scratching the surface of a thin polymer film

We used the tip of an atomic force microscope (AFM) in the contact mode to scratch/rub the surface of a glassy polymer thin film, i.e., isotactic polystyrene (i-PS) at room temperature. After subsequent isothermal crystallization, an extremely high nucleation density of edge-on crystals within the rubbed region or at the edge of the scratched area was observed. Furthermore, a transition from edge-on to flat-on lamellae occurred beyond a certain distance from the edge of the scratched region. Our results demonstrate that both, soft rubbing or hard scratching, allow to lower the nucleation barrier for polymer crystallization and to control the orientation of the resulting crystalline lamellae. The role of scratching/rubbing on chain deformation and its relation to nucleation and crystal orientation in polymer thin films is discussed.     

Lamellar orientation on the surface of a polymer determined by ToF-SIMS and AFM

The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), containing both aliphatic CH₂ and aromatic segments, were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At low and high crystallization temperatures, the surfaces of the polymer films were shown to consist mainly of edge-on and flat-on lamellae, respectively. Surfaces with a mixture of edge-on and flat-on lamellae were produced at intermediate temperatures. The edge-on and flat-on lamellae were identified by using ions that recognize the flexible and rigid segments of the polymer. Ion images produced using selected ions that are related to the edge-on or flat-on orientation can be used to identify the location of these lamellae on the polymer surface. Our results indicate that ToF-SIMS can be used to detect different lamellar orientations at the surfaces of semi-crystalline polymers.     

Melt crystalfization and crystal morphology of two low molecular weight linear polyethylene fractions

Melt-crystallization behavior and single-crystal morphology of two low molecular weight (LMW) linear polyethylene (PE) fractions of 3900 and 5800 have been investigated. Linear growth rates along the b axis (G _b) of these fractions were measured via polarized light microscopy (PLM). The two fractions show a growth rate change at an undercooling of 17°C (at 117°C and 120°C, respectively, for these two fractions), which may be identified as the regime I/II transition. This transition does not correspond to a single-crystal morphological change from a truncated lozenge with curved (200) and (110) planes to a lenticular crystal as proposed previously. However, this morphological change can be observed at a temperature higher than the regime transition (at 122°C and 124°C), at which the cusps of the G _b data can be observed for these two fractions. Based on our morphological study via PLM, transmission electron microscopy, electron diffraction, small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) experiments, it is found that within a 2°C temperature region, the G _b change is accompanied by a sharp long period increase and a drastic change in single-crystal morphology from a truncated lozenge with curved (200) and (110) planes to a lenticular-shape crystal. The morphological change may result from a sudden increase in the G _b coupled with a smaller change in the growth rate along the a axis with undercooling. This implies that, within this temperature region (2°C), the crystals may undergo substantial changes in the geometry of the (110) and (200) crystal growth fronts and chain folding behavior.     

Anisotropic thermoelectric power of TTF-TCNQ

We have measured the thermoelectric power along the a-axis as well as the highly conducting b-axis in TTF-TCNQ crystals. Measurements were taken both parallel and perpendicular to the long axis on two sets of crystals. One set grew along the a-axis and the other along the b-axis. The thermopowers are of opposite sign except near 60°K where both cross zero at slightly different temperatures. The a-axis thermopower is consistent with non-metallic diffusive transport in the a direction.     

Investigation of dielectric, piezoelectric and ferroelectric properties of b-axis grown triglycine sulphate single crystal

Large single crystal of triglycine sulphate (dimension 100 mm along monoclinic b-axis and 15 mm in diameter) was grown using the unidirectional solution growth technique. The X-ray diffraction studies confirmed the growth/long axis to be b-axis (polar axis). The dielectric studies were carried out at various temperatures to establish the phase transition temperature. The frequency response of the dielectric constant, dielectric loss and impedance of the crystal along the growth axis, was monitored. These are typically characterized by strong resonance peaks in the kHz region. The piezoelectric coefficients like stiffness constant (C), elastic coefficient (S), electromechanical coupling coefficient (k) and d ₃₁ were calculated using the resonance-antiresonance method. Polarization (P)-Electric field (E) hysteresis loops were recorded at various temperatures to find the temperature-dependent spontaneous polarization of the grown crystal. The pyroelectric coefficients were determined from the pyroelectric current measurement by the Byer and Roundy method. The ferroelectric domain patterns were recorded on (010) plane using scanning electron microscopy and optical microscopy.     

Magneto-optical study of ferromagnetically coupled cobalt (II) pairs in cadmium bromide

Low temperature polarized absorption and magnetic circular dichroism spectra are reported for crystals of CdBr₂ doped with varying concentrations of Co²⁺ in the region near 17 700 cm^-1. Sharp zero phonon lines are identified as the trigonal field components of U_g ′(² T _1g , ² H) in dilute crystals, and as states arising from ferromagnetically coupled in-plane pairs in the concentrated ones. Measurements of the temperature dependence of the lines, and Zeeman spectra recorded with applied fields up to 50 kG parallel and perpendicular to the crystal c-axis, lead to estimates of the anisotropy of the exchange interaction in the ground state, and to both isotropic and anisotropic exchange in the excited state. The preferred direction of spin alignment in both states is parallel to the c-axis.     

СИММЕТРИЯ И ПЬЕЗОЭЛЕ КТРИЧЕСКИЕ СВОЙСТВА КРИСТАЛЛОВ

The necessary and sufficient conditions for the existence of the direct and inverse piezoelectric effect in crystals and textures are deduced. It is found that under certain conditions the piezoelectric properties in crystals of certain classes are richer than those obtained from the piezoelectric tensor. The results are given of the piezoelectric polarization of quartz in the direction of the optical axis and the excitation of oscillations of the plates perpendicular to the optical axis when the field is applied in the direction of the optical axis.     

Crystallization Kinetics of Lamellar Crystals Confined in Polymer Thin Films

We used dynamic Monte Carlo simulations to investigate the crystallization kinetics of flat-on lamellar polymer crystals in variable thickness films. We found that the growth rates linearly reduced with decreasing film thickness for the films thinner than a transition thickness d_t , while they were constant for the films thicker than d_t . Moreover, the mean stem lengths (crystal thickness) we calculated decreased with film thickness in a similar way to the growth rates, and the intramolecular crystallinities we calculated confirmed the film thickness dependence of the crytsal thickness. Besides, the crystal melting rates in thin films were calculated and increased with decreasing film thickness. We proposed a new interpretation on the film thickness dependence of the crystal growth rate in thin films, suggesting that it is dominated by the crystal thickness in terms of the driving force term (l–l _min) expressed by Sadler, rather than the chain mobility based on experiments. The crystal thickness can determine whether a crystal grows or melts in a thin film at a fixed temperature, indicating the reversibility between the crystal growth and melting.     

A tem study on polyoxymethylene rodlike crystals grown epitaxially on NaCl

Rodlike crystals of polyoxymethylene (POM) were isothermally grown onto the newly cleaved (001) face of NaCI from a 0.1 wt% solution in nitrobenzene and then were studied by transmission electron microscopy (TEM). Morphological observation in bright- and dark-field imaging modes and selected-area electron diffraction analysis directly evidenced that POM chain stems in the rodlike crystals are set parallel to the substrate surface and also perpendicular to the rod axis. That is, each of the rodlike crystals is an “edge-on” lamella of POM, the lateral face of which is in contact with the substrate surface. The contact plane of the edge-on lamella in question, namely, the crystallographic plane parallel to the substrate surface, is determined as the (100) plane of the hexagonal crystal lattice of POM.     

Investigation of the Melt‐Crystallization of Polypropylene by a Temperature Slope Method

Abstract

To investigate the in‐situ ordering process of isotactic polypropylene (iPP) from a melt state, a stationary growth front was prepared by the temperature slope crystallization (TSC) method. During the melt‐crystallization, iPP was crystallized into the α‐phase or β‐phase depending on the crystallizing conditions. The mechanism of the melt‐crystallization at the growth front was precisely observed by wide‐angle and small‐angle x‐ray scattering (WAXS and SAXS) using a strong synchrotron beam. In the TSC apparatus, the sample was crystallized in between a heater, controlled to 220°C, and a cooler, cooled by water to 25°C. We define the z‐axis parallel to the temperature gradient. A‐lamellae and B‐lamellae are also defined as those whose lamellar normal are perpendicular and parallel to the z‐axis, respectively. In a sample‐stop (SS) stage before the TSC, the original α‐phase lamellae became thicker, approaching to the melt‐solid boundary by annealing. The annealing process showed that the α‐phase B‐lamellae remained and the SAXS reflection was stronger on the meridian near the melt‐solid boundary in the SS stage. In the beginning of the TSC, the α‐phase B‐lamellae developed as a primary crystallization. During secondary crystallization under high supercooling, the SAXS cross pattern appeared showing that the α‐phase developed both A‐ and B‐lamellae. As the growth direction of A‐lamellae is parallel to the z‐axis, A‐lamellae grow faster than B‐lamellae. By the self‐epitaxial mechanism on the side surface of the A‐lamellae, the B‐lamellae grow on the base of the A‐lamellae. Following appearance of a spontaneous β‐nucleus, the β‐phase lamellae grew preferentially, excluding the α‐phase, and occupied the whole area of the sample. In this case also, A‐lamellae are advantageous to grow because of the growth direction parallel to the z‐axis. As a result, the SAXS β‐phase reflection appeared on the equator.     

K_(0.8)Fe_2Se_2晶体c轴向载流子输运特性的研究

成功制备了超导临界温度为27 K的K0.8Fe2Se2晶体,并详细研究了晶体c轴向的载流子输运特性.结合X射线衍射、光学显微镜下的形貌、变温电阻率的测试结果表明,样品存在有"相分离",但是这类层状铁基超导体材料的两个相不是简单沿c轴向层状交替排布的,而应该是沿着c轴向存在弱联系的金属相链接通路,金属相部分形成近3维的空间网状链接模式.热导率测试和复阻抗谱z(ω,T0)的研究表明超导晶体沿着c轴方向存在有大量的相界面,所束缚的极化电荷致使相对介电常数达到106数量级,相应地在10 MHz附近出现负的相位特征.     

Texture transitions in the liquid crystalline alkyloxybenzoic acid 6OBAC

The 4,n-alkyloxybenzoic acid 6OBAC has a very rich variety of crystalline structures and two nematic sub-phases, characterised by different textures. It is a material belonging to a family of liquid crystals formed by hydrogen bonded molecules, the 4,n-alkyloxybenzoic acids (n indicates the homologue number). The homologues with 7?≤?n?≤?13 display both smectic C and N phases. In spite of the absence of a smectic phase, 6OBAC exhibits two sub-phases with different textures, as it happens in other materials of the homologue series which possess the smectic phase. This is the first material that exhibits a texture transition in a nematic phase directly originating from a crystal phase. Here we present the results of an image processing assisted optical investigation to characterise the textures and the transitions between textures. This processing is necessary to discriminate between crystal modifications and nematic sub-phases.     

The effect ofpH on the growth of the cytosine monohydrate single crystals

The habit of cytosine monohydrate crystals is influenced bypH of the growth solution. Changes in crystal faces growth rates were interpreted in terms of the adsorption of hydrated hydroxonium ions. This effect was used for preparation of single crystals excessively elongated inc axis direction.     

Novel approach to study liquid crystal phase transitions using Legendre moments

A novel approach is proposed to investigate the phase transitions of cholesteric liquid crystals using the Legendre moments. The textures of cholesteric liquid crystals (cholesteryl butyrate, cholesteryl n-valerate, cholesteryl decanoate, and cholesteryl myristate) are captured as a function of temperature using high-resolution camera attached to the arthroscopic mode of polarizing optical microscope with hot stage. A recurrence formula is used to compute the Legendre moments of the liquid crystal textures based on the Legendre polynomial using MATLAB software. The abrupt change in the values of Legendre moments as a function of temperature gives the phase transitions of liquid crystals. The investigated transition temperatures of cholesteric liquid crystals are compared with other techniques.     

Growth Rates of Edge-on Lamellar Crystals Confined in Polymer Thin Films

The growth rates of edge-on lamellar polymer crystals in variable thickness films were investigated in terms of dynamic Monte Carlo (MC) simulations. The growth rates linearly decreased with decreasing film thickness for the thinner films and were nearly constant for the thicker films. The mean stem lengths (crystal thickness) were also constant in different thickness films. The crystal widths parallel to the film thickness increased more slowly with increasing film thickness in the thinner films than that in the thicker films, indicating they were restrained by the film thickness. We propose that the growth rate of edge-on lamellar crystals in thin films is dominanted by the crystal width in the thinner films and by the crystal thickness in the thicker films; the variation of the film thickness can change the three-dimensional shape of the crystal growth front, also affecting the growth rate of the edge-on lamellar crystal.     

Determination of proton diffusion anisotropy by thermal decay of fixed holograms with K-vector perpendicular to the c-axis in LiNbO3:Fe

In this paper the thermal fixing of holographic gratings with K-vectors perpendicular to the crystallographic c-axis of LiNbO₃ is considered in order to obtain information about anisotropy of the proton thermal diffusion in this crystal. Specifically, thermal decays of fixed holograms in particular crystallographic directions are measured and related with proton diffusion. The values obtained are compared with previous data of decays of fixed holograms with K-vector parallel to the c-axis. The results show a high anisotropy of the thermal diffusion of protons in lithium niobate crystals.     

Effects of V/III ratio on the growth of a-plane GaN films

The non-polar a-plane GaN is grown on an r-plane sapphire substrate directly without a buffer layer by metal-organic chemical vapour deposition and the effects of V/III ratio growth conditions are investigated. Atomic force microscopy results show that triangular pits are formed at a relatively high V/III ratio, while a relatively low V/III ratio can enhance the lateral growth rate along the c-axis direction. The higher V/III ratio leads to a high density of pits in comparison with the lower V/III ratio. The surface morphology is improved greatly by using a low V/III ratio of 500 and the roughness mean square of the surface is only 3.9 nm. The high resolution X-ray diffraction characterized crystal structural results show that the rocking curve full width at half maximum along the m axis decreases from 0.757° to 0.720°, while along the c axis increases from 0.220° to 0.251° with the V/III increasing from 500 μmol/min to 2000 μmol/min, which indicates that a relatively low V/III ratio is conducible to the c-axis growth of a-plane GaN.     

Trigonal warping of the Fermi surface in n-Bi2Se3

Shubnikov-de Haas investigations on n-type Bi₂Se₃ single crystals were performed in magnetic fields up to 10.5 Tesla (105 kG). The shape of the single-valley Fermi surfaces is almost an ellipsoid of revolution around the trigonal k_c-axis, the longest main axis of the Fermi surface. There is a small amount of flattening in the direction of the k_c-axis and of trigonal warping around the k_c-axis, caused by higher order terms than [k²] in the ?(k) relation.     

3 T强磁场对真空蒸发Zn薄膜晶体结构的影响

在3 T强磁场下采用真空蒸发沉积在玻璃基片上制备了三种厚度分别为1,2,3 μm的Zn薄膜,并和无磁场下制备的薄膜进行了对比研究.对施加磁场和无磁场环境下制备的试样分别进行了X射线衍射研究.研究表明,3 T磁场下制备的Zn薄膜都是沿（002）面取向,而0 T磁场下制备的薄膜随着厚度的增加c轴取向逐渐减弱. 3 T磁场的取向作用可以维持Zn晶粒沿着c轴取向.利用扫描电子显微镜对薄膜表面形貌的研究发现,施加磁场制备的Zn薄膜表面晶粒要比无磁场条件下制备的薄膜有明显的细化.对磁场下Zn原子团形成进行了热力学分析,推导了磁场作用下的临界形核半径r^*_M和临界形核自由能ΔG^*_M.初步分析表明,r^*_M和ΔG^*_M减小从而增加临界形核浓度是Zn晶粒细化的原因. 关键词： 强磁场 晶体结构 真空蒸发沉积 薄膜     

On asymmetric propagation on transverse acoustic waves near the structural phase transition in RbH3(SeO3)2

Recently ultrasonic measurements of the RbH₃(SeO₃)₂ crystal have shown that a transverse acoustic wave propagating along the modulation axis of the incommensurate structure (c-axis) and polarized along the polar axis (b-axis) exhibits greater temperature anomalies of its velocity and damping than a wave propagating along the b-axis and polarized along the c-axis. The possible explanation of this experimental result is given on the basis of the Landau theory.