剪切对玻璃珠填充聚丙烯结晶行为的影响

应用Linkam CSS450剪切仪、广角X射线衍射仪(WAXD)和小角X射线散射仪(SAXS)等研究了剪切对玻璃珠填充聚丙烯结晶行为的影响, 结果表明, 与纯聚丙烯相比, 填加玻璃珠的聚丙烯体系中, 玻璃珠起到成核剂的作用, 不利于β晶的生成. 玻璃珠直径较小(4 μm)时, 剪切对聚丙烯β晶的生成影响较小; 当玻璃珠直径增加到35 μm时, 剪切速率为20 s-1左右最有利于β晶生成; 剪切速率和玻璃珠直径的增加, 有利于聚丙烯片晶的取向, 而且玻璃珠含量越高, 片晶的取向程度越大.     

Investigation of the Crystallization Kinetics of Zn(Leu)SO4·0.5H2O in Mixing Solvent by Microcalorimetry

The crystal growth process of Zn(Len)SO₄·0.5H₂O from water and acetone was investigated using a Calvet microcalorimeter. The heat and the rate of heat production during the crystal growth process at 293.15 K, 295.15 K, 298.15 K and 300.15 K were measured. On the basis of experimental and calculated results, the rate constant and the kinetic parameters (the activation energies, the pre‐exponential) during the crystal growth process were obtained. The results show that the crystal growth process accorded with the Burton‐Cabrera‐Frank dislocation theory.     

Effects of Branch Content and Branch Length on Polyethylene Crystallization: Molecular Dynamics Simulation

Influences of branch content (BC) and branch length (BL) on isothermal crystallization of precisely branched polyethylene are studied by molecular dynamics simulation. Branch acts as a defect both in nucleation and crystal growth process. BC affects not only crystallization kinetics but also final morphologies. Crystallization rate and crystallinity decrease as BC increases. Morphology Regimes change from lamellae crystal to bundle crystal at critical BC (20/1000 C) because of different folding pattern. 50 CH₂ is the critical methyl sequence length to form lamellae crystal. Lamellae thicknesses of final morphologies decrease in gradient corresponding to Morphologies Regimes. BL has no influence on the crystallization kinetics, and only affects the final morphologies when more branches inclusion happens with BL increasing. Trans‐rich phenomenon in pre‐crystalline state is observed. Crystallization process begins at the end of induction stage when trans state population reaches a critical value, and this value is independent of BC and BL.     

Surface anchoring of nematic liquid crystal 8OCB on a DMOAP‐silanated glass surface

Dynamic light scattering spectroscopy has been used to determine the temperature dependence of the anchoring strength of the nematic liquid crystal 8OCB on DMOAP‐silanated glass surfaces inducing homeotropic alignment. Wedge‐type glass cells with known thickness profile starting from 150 nm to several microns have been used in the experiments. The relaxation rates of the nematic fluctuations with the wave vector perpendicular to the confining surfaces have been measured as a function of the cell thickness. Fitting of the thickness dependence of the relaxation rate allows for straightforward determination of the surface extrapolation length and therefore also the strength of the surface anchoring, which is 1×10^?4 J m^?2. The overall experimental accuracy of the experiments is discussed.     

Structure of the B4 liquid crystal phase near a glass surface

The B4 liquid crystal phase of bent-core molecules, a smectic phase of helical nanofilaments, is one of the most complex hierarchical self-assemblies in soft materials. We describe the layer topology of the B4 phase of mesogens in the P-n-OPIMB homologous series near the liquid crystal/glass interface. Freeze-fracture transmission electron microscopy reveals that the twisted layer structure of the bulk is suppressed, the layers instead forming a structure with periodic layer undulations, with the topography depending on the distance from the glass. The surface layer structure is modeled as parabolic focal conic arrays generated by equidistant parabolas whose foci are defect lines along the glass surface. Nucleation and growth of toric focal conics near the glass substrate is also observed. Although the growth of twisted nanofilaments, the usual manifestation of structural chirality in the B4 phase, is suppressed near the surface, the smectic layers are intrinsically chiral, and the helical filaments that form on top of them grow with specific handedness.     

Oriented Two‐Dimensional Porous Organic Cage Crystals

The formation of two‐dimensional (2D) oriented porous organic cage crystals (consisting of imine‐based tetrahedral molecules) on various substrates (such as silicon wafers and glass) by solution‐processing is reported. Insight into the crystallinity, preferred orientation, and cage crystal growth was obtained by experimental and computational techniques. For the first time, structural defects in porous molecular materials were observed directly and the defect concentration could be correlated with crystal growth rate. These oriented crystals suggest potential for future applications, such as solution‐processable molecular crystalline 2D membranes for molecular separations.     

Mechanism of dissolution of sparingly soluble electrolytes

Recent constant composition dissolution studies of sparingly soluble calcium phosphates have revealed an interesting and unusual behavior in that the rates decreased, eventually resulting in effective suppression, even though the solutions remained undersaturated. Contrary to traditional theories of dissolution, these experimental results indicated the importance of not only the particle size on the dissolution rate but also the participation of critical phenomena. In these theories, it is assumed that when the dissolution reactions are initiated, they continue spontaneously until all solid phase has disappeared. In terms of these mechanisms, there are no critical phenomena in the dissolution mechanism. Although the crystal size decreases during dissolution, when the reaction is controlled by polypitting (formation and growth of pits), the edge free energy increases at the very first stage due to the creation of pits and dissolution steps. The constant composition experimental results demonstrate the development of surface roughness as the dissolution steps are formed, implying an increase of the total edge length during the reactions. In an exactly analogous mechanism to crystal growth, the participation of critical conditions involving dissolution steps is a possibility. In contrast to crystal growth, dissolution is a process of size reduction and, when the particle size is sufficiently reduced, critical phenomena become important so that the influence of size must be taken into consideration. This paper proposes such a model for dissolution reactions, and although these unusual phenomena probably apply to all mineral phases, they are more evident for sparingly soluble electrolytes in which the critical conditions are attained much more readily.     

Kinetics and mechanisms of crystal growth and diffusion in a glass-forming liquid

Extensive data on the viscosity, covering 15 orders of magnitude, and crystal growth rate, covering seven orders of magnitude, of liquid diopside (CaO.MgO.2SiO(2)) were collected in a wide range of undercoolings from 1.10T(g) to 0.99T(m) (T(g) is the glass transition temperature and T(m) the melting point). The raw growth rate data were corrected for the increased interfacial temperature produced by the heat released during crystallization. A detailed analysis confirms that growth mediated by screw dislocations reasonably explain the experimental data in these wide ranges of temperatures and growth rates. Effective diffusion coefficients were calculated from crystal growth rates and from viscosity, and were then compared with measured self-diffusion coefficients of silicon and oxygen in diopside melt. The results show that oxygen and silicon control the diffusion dynamics involved in crystal growth and viscous flow. This study not only unveils the transport mechanism in this complex liquid, but also validates the use of viscosity (through the Stokes-Einstein or the Eyring equations) to account for the kinetic term of the crystal growth expression in a wide range of temperatures.     

Glass-forming ability of butanediol isomers

In this study, the thermal behavior of butanediol isomers is investigated for temperatures ranging from 103 to 303 K using differential scanning calorimetry, complemented, when necessary, by polarized light thermal microscopy. The butanediol isomers display quite different thermal behaviors: for 1,2- and 1,3-isomers, glass transition is the only thermal event observed; for 1,4-butanediol, crystallization occurs on cooling even at a high scanning rate and no glass formation was detected; and for the 2,3-isomer, glass or crystal formation is dependent on the experimental conditions employed. The glass-forming ability of the isomers is correlated with data available on their molecular symmetry.     

Molecular mechanism leading to memory effect of mesomorphic isotactic polypropylene

In this study, memory effect of mesomorphic isotactic polypropylene (iPP) was investigated using polarized optical microscope and small‐angle X‐ray scattering. Differing from classical memory effect, mesomorphic iPP melt had a higher growth rate and a higher memory temperature. The relative growth rate increased with increasing crystallization temperature. Lauritzen–Hoffman plots indicated that the increased growth rate arose from reduced surface nucleation barrier. The highest memory temperature was estimated to be 185 °C, which was close to the equilibrium melting point of iPP crystal. Additionally, Small‐angle X‐ray scattering measurements showed that a liquid crystal layer might exist between lamellar and amorphous layers. Based on above results, a crystallization model was proposed. In the mesomorphic iPP melt, there exist aggregates structurally similar to β phase except α‐phase crystal residuals, which cannot act as nucleation sites or transform to β crystal through surface nucleation. The only way for the aggregate is to transform to α crystal during crystal growth. The aggregate decreases the surface nucleation barrier and promotes the helical growth, leading to higher growth rate. Only when the aggregate relaxes to polymer coils through thickening at a higher temperature, can the memory effect be erased. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1573–1580     

沸石分子筛的生成机理与晶体生长(Ⅻ)——非自发成核体系中沸石分子筛的晶体生长模型

依据沸石分子筛液相机理,建立了非自发成核体系的晶体生长模型,计算所得27条L型、Ω型和ZSM-5型沸石分子筛晶体生长理论曲线与实验值相符合,确证了该模型的合理性。     

Molecular relaxation of isotactic polystyrene: Real‐time dielectric spectroscopy and X‐ray scattering studies*

The molecular relaxation processes and structure of isotactic polystyrene (iPS) films were investigated with real‐time dielectric spectroscopy and simultaneous wide‐ and small‐angle X‐ray scattering. The purpose of this work was to explore the restrictions imposed on molecular mobility in the vicinity of the α relaxation (glass transition) for crystallized iPS. Isothermal cold crystallization at temperatures of T_c = 140 or 170 °C resulted in a sigmoidal increase of crystallinity with crystallization time. The glass‐transition temperature (T_g), determined calorimetrically, exhibited almost no increase during the first stage of crystal growth before impingement of spherulites. After impingement, the calorimetric T_g increased, suggesting that confinement effects occur in the latter stages of crystallization. For well‐crystallized samples, the radius of the cooperativity region decreased substantially as compared with the purely amorphous sample but was always smaller than the layer thickness of the mobile amorphous fraction. Dielectric experiments directly probed changes in the amorphous dipole mobility. The real‐time dielectric data were fitted to a Havriliak–Negami model, and the time dependence of the parameters describing the distribution of relaxation times and dielectric strength was obtained. The central dipolar relaxation time showed little variation before spherulite impingement but increased sharply during the second stage of crystal growth as confinement occurred. Vogel–Fulcher–Tammann analysis demonstrated that the dielectric reference temperature, corresponding to the onset of calorimetric T_g, did not vary for well‐crystallized samples. This observation agreed with a model in which constraints affect primarily the modes having longer relaxation times and thus broaden the glass‐transition relaxation process on the higher temperature side. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 777–789, 2004     

聚乳酸的非等温结晶行为

用差示扫描量热法(DSC)研究聚乳酸(PLA)从熔体及玻璃态为初始条件下的非等温结晶行为,采用Ozawa方程、Mo法、Khanna法和Kissinger法对结晶动力学参数进行计算处理。 实验结果表明,这几种方法均适合处理PLA的非等温结晶过程,而Khanna法提出的结晶速率系数(CRC)能够方便地评价PLA相对结晶速率的大小。 PLA从玻璃态升温结晶比从熔体降温结晶容易得多,升温过程有利于晶核生成,而降温有利于晶体生长。 升温结晶时,升温速率2.0 ℃/min时,结晶焓(ΔH_c)达到最大为27.1 J/g。 从熔体等速降温过程中,随着冷却速率的降低ΔH_c单调增加,冷却速率为0.25 ℃/min时ΔH_c增加到28.3 J/g。 在较低温度下从玻璃态结晶,主要表现为异相成核的二维生长方式。 在较高的温度下从玻璃态升温结晶及从熔体冷却结晶时,以均相成核的三维生长方式结晶为主。 与升温过程相比,冷却不利于晶核的生成,所以导致冷却过程总体ΔH_c偏低,扩散活化能偏大。     

Molecular dynamics simulation on devitrification: isothermal devitrification and thermodynamics of PbF2 glasses

The vitrification and devitrification features of lead fluoride are investigated by means of molecular dynamic simulations. The influence of heating rate on the devitrification temperature as well as the dependence of the glass properties on its thermal history, i.e., the cooling rate employed, is identified. As expected, different glasses are obtained when the cooling rates differ. Diffusion coefficient analysis during heating of glass and crystal, indicates that the presence of defects on the glassy matrix favors the transition processes from the ionic to a superionic state, with high mobility of fluorine atoms, responsible for the high anionic conduction of lead fluoride. Nonisothermal and isothermal devitrification processes are simulated in glasses obtained at different cooling rates and structural organizations occurring during the heat treatments are clearly observed. When a fast cooling rate is employed during the glass formation, the devitrification of a single crystal (limited by the cell dimensions) is observed, while the glass obtained with slower cooling rate, allowing relaxations and organization of various regions on the glass bulk during the cooling process, devitrifies in more than one crystalline plane.     

Crystallization kinetics of colloidal spheres under stationary shear flow

A systematic experimental study of dispersions of charged colloidal spheres is presented on the effect of steady shear flow on nucleation and crystal growth rates. In addition, the nonequilibrium phase diagram as it relates to the melting line is measured. Shear flow is found to strongly affect induction times, crystal growth rates, and the location of the melting line. The main findings are that (1) the crystal growth rate for a given concentration exhibits a maximum as a function of the shear rate; (2) contrary to the monotonic increase in the growth rate with increasing concentration in the absence of flow, a maximum of the crystal growth rate as a function of concentration is observed for sheared systems; and (3) the induction time for a given concentration exhibits a maximum as a function of the shear rate. These findings are partly explained on a qualitative level.     

A kinetic model to simulate protein crystal growth in an evaporation-based crystallization platform

The quality, size, and number of protein crystals grown under conditions of continuous solvent extraction are dependent on the rate of solvent extraction and the initial protein and salt concentration. An increase in the rate of solvent extraction leads to a larger number of crystals. The number of crystals decreases, however, when the experiment is started with an initial protein concentration that is closer to the solubility boundary. Here we develop a kinetic model capable of predicting changes in the number and size of protein crystals as a function of time under continuous evaporation. Moreover, this model successfully predicts the initial condition of drops that will result in gel formation. We test this model with experimental crystal growth data of hen egg white lysozyme for which crystal nucleation and growth rate parameters are known from other studies. The predicted and observed rates of crystal growth are in excellent agreement, which suggests that kinetic constants for nucleation and crystal growth for different proteins can be extracted by applying a kinetic model in combination with observations from a few evaporation-based crystallization experiments.     

Determining crystal growth rate-type of curves in glasses by differential thermal analysis

A crystal growth rate vs. temperature (U vs. T)-type of curve for a Li₂O.2SiO₂ glass was determined up to a temperature of 690 °C using a differential thermal analysis (DTA) method. As determined from this (U vs. T)-type of curve, the temperature corresponding to the onset of crystal growth was 570±3 °C which is in excellent agreement with the same value determined by the conventional method. The complete range of temperature where crystal growth can occur and the temperature (T_max) where the crystal growth rate is a maximum for this glass could not be determined. It is concluded that for temperatures <T_max, the DTA method can delineate only a small, initial portion of the (U vs. T)-type of curve, where the crystal growth rate is sufficiently low so that the glass does not totally crystallize after the required (for at least 5 min) isothermal heat treatment at these temperatures. For temperatures >T_max, the curve cannot be determined by the DTA method if the temperature for isothermal heat treatment is approached by heating the glass from room temperature. For determining the curve for temperatures >T_max, the desired temperature (>T_max) for isothermal heat treatment of the glass is suggested to be approached from the melting temperature of the glass.     

Crystallization Kinetics of Lithium Diborate Glass by DTA

The crystallization process of Li₂B₄O₇ in the glass of stoichiometric composition, characterized by the crystal growth of pre-existing nuclei, was analyzed kinetically by means of DTA. Because the number of pre-existing nuclei for the subsequent growth varies depending on the cooling rate of the glass-forming melt and heating rate of the as-prepared glass, a modified Kissinger plot was applied for evaluating the apparent activation energy to the crystal growth in the glass samples with three different thermal histories, i.e., the pre-annealed, slowly quenched and quickly quenched glasses. The process was characterized by the three dimensional growth of pre-existing nuclei with the apparent activation energy of ca 340 kJ mol⁻¹. This revised version was published online in August 2006 with corrections to the Cover Date.     

In situ optical microspectroscopy approach for the study of metal transport in dielectrics via temperature- and time-dependent plasmonics: Ag nanoparticles in SiO2 films

This study proposes in situ optical microspectroscopy as a means for the investigation of particle growth and metal transport in nanocomposite systems based on the temperature- and time-dependent optical response of the material. The technique has been successfully employed for the real-time monitoring of the growth of Ag nanoparticles (NPs) in SiO(2) films deposited on soda-lime glass during thermal processing in nitrogen atmosphere. By fitting the surface plasmon resonance (SPR) profiles with spectra calculated by Mie theory in the quasi-static regime, the time variation in effective Ag particle size was determined and subsequently analyzed in the context of crystal growth theory. The Ag NPs were indicated to grow first through a diffusion-based process and subsequently via Ostwald ripening. The experimental determination of the activation energies associated with each one of the particle growth mechanisms was carried out based on the time evolution of the SPR of Ag NPs. Arrhenius-type analyses of a set of time-dependent isotherms allowed for estimating the activation energies at 2.3 eV for the diffusion-based growth and 2.8 eV for the ripening stage.     

PbF_2·SiO_2基玻璃陶瓷的溶胶-凝胶法制备及结构转变研究

采用溶胶-凝胶法制备了 PbF2?SiO2纳米晶玻璃陶瓷块体;利用 TG-DSC 和 IR 技术分析了干凝胶在热处理过程中有机基团的分解及内部原子键合方式的演变;结合 XRD 和 TEM 研究了凝胶玻璃中 PbF2纳米晶粒的长大过程,并分析了 Er3+掺杂的影响。结果表明,采用此方法制备的 PbF2?SiO2纳米晶玻璃陶瓷具有较好的成形性,晶化温度在 320℃左右;经 480℃热处理,镶嵌在玻璃基体中的 PbF2晶粒尺度约为 10～25nm,材料透明性良好。在 PbF2?SiO2系统中掺入少量的 Er3+,将提高 PbF2的晶化温度,降低玻璃陶瓷的显微硬度;掺杂可能对 PbF2晶粒表面原子的活性起抑制作用,阻碍晶粒的表面迁移,使晶粒的生长速度明显降低。