Plywood-like structures of injection-molded polypropylene

N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide, a commercial nucleating agent, can dissolve into molten polypropylene (PP) and self-organize into needle-like crystals when the melt temperature reaches 260°C. As temperature decreases, PP molecules crystallize on the surface of the needle-like crystals with their c-axis perpendicular to the long axis of the crystal. Under a flow field in injection molding, PP chains are parallel or perpendicular to the flow direction at different depths owing to the difference in crystallization kinetics and applied shear rate in the melt, forming a plywood-like material. This unique structure can simultaneously improve the toughness and strength of plastic parts. Herein, we present a novel method of producing high-performance plastic articles based on general plastics by manipulating their morphology and structure.     

Cholesteric order in aqueous solutions of the polysaccharide Xanthan

Summary Measurements of birefringence, optical rotation and laser light diffraction show that aqueous solutions of the polyelectrolytic polysaccharide Xanthan at a concentration of 7.5 % (vol/vol) have long range order very similar to cholesteric liquid crystals. The cholesteric screw is left handed implying a rod-like conformation and right handed helicity of individual molecules. The cholesteric phase separates from a less concentrated isotropic solution and spherolites are formed.     

The role of liquid crystals in the formation of graphitizable carbons (cokes)

The formation of cokes and graphites proceeds via the creation from the isotropic fluid phase of carbonization of pitch and coal, of lamellar nematic liquid crystals or mesophase. This anisotropic fluid, deformable mesophase, develops as spheres within which constituent molecules are stacked parallel to an equatiorial plane. This type of structure facilitates coalescence to a coherent mass which eventually forms a graphitisable carbon. The ‘onion-skin’ structure of mesophase spheres cannot so coalesce. Different optical textures of cokes and graphites owe their origin to different chemical reactivities and fluidities of mesophase, the lower the fluidity the smaller the size of the optical texture. Mesophase from lameller molecules is compared with conventional rod-like nematic liquid crystals. Structures in needle-cokes, metallurgical coke, coke from solvent refiend coal and carbon fibre from pitch are discussed in terms of formation and properties of lamellar nematic liquid crystals.     

Formation of solid lipid nanoparticles in a microchannel system with a cross-shaped junction

This work presents a novel method for generating solid lipid nanoparticles (SLNs) in a microchannel system with a cross-shaped junction formed by a main microchannel and two branches. A lipid solution by dissolving the lipid in a water-miscible organic solvent was passed through the main channel, while an aqueous surfactant solution was introduced into the branches simultaneously. These two liquids met together at the cross-shaped junction and passed along the main channel. The solvent diffused from the lipid solution stream into the aqueous phase, which resulted in the local supersaturation of lipid and thus led to the formation of SLNs. The flow behaviors of lipid and aqueous phase zones were measured by a digital inversion microscope system. The mean particle diameter and the particle size distribution of the obtained SLNs were measured by dynamic light scattering (DLS) method and the particle morphology was examined by transmission electron microscopy (TEM). The effects of liquid flow velocity and lipid concentration on the properties of SLNs were investigated experimentally. The formation mechanism of SLNs in the present microchannel system was discussed and analyzed.     

Melt viscosity reduction of polycarbonate with low molar mass liquid crystals

The phase behavior and rheological behavior of low molar mass liquid crystal (LLC) and polycarbonate blends is firstly reported. The results of small angle light scattering (SALS) indicate that the LLC is miscible in the mixture for weight fractions of LLC less than 6%. Mixtures of two different liquid crystals with two different molecular weight of polycarbonate were prepared inside the miscible regime of the blends. Both the complex and steady shear viscosities of the blends were found to be significantly decreased upon addition of small amounts of liquid crystal (1% by weight). At low shear rate, the steady state shear viscosity was similar to the pure polycarbonate, whilst, at higher shear rates, three further regimes of behavior, as has been described for liquid crystals and liquid crystal polymers, were found despite the low concentration of LLC; hence, the rheological properties of the blends can be significantly modified by small concentrations of LLC (as low as 1%). The decrease in melt viscosity of polycarbonate that we observe upon addition of LLC is not due to lubrication effects at the interfaces, as shown by reproducible oscillatory shear flow sweeps. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Measurement of the size distribution of fat crystals using a laser particle counter

A new method that uses a laser particle counter has been developed to measure the size of fat crystals. The fat crystals were suspended in isobutanol in the ratio of 1∶25,000. The solid and liquid fat recovery after being in contact with isobutanol for 24 hr, fatty acid composition of the solid and liquid fractions, DSC melting behavior and polarized light microscopy proved that isobutanol did not affect the structural integrity of the fat crystals. The mean crystal size of commercial shortenings ranged from 5 to 9 μm. The isobutanol suspension method makes it possible to quantitatively separate the solid and liquid components of a fat. It also provides a convenient way of preparing fat crystals for scanning electron microscopy.     

溶剂对维生素C单晶生长动力学的影响

引　言当过饱和溶液中产生晶核或加入晶种时 ,溶质分子在过饱和度的推动下使晶核或晶种长大的过程就是晶体生长 .晶体生长动力学决定了晶体产品的形态及其物理性质 ,是结晶过程设计、优化和操作的基础 ,在结晶过程研究中占有重要地位[1,2 ] .晶体生长的外部条件例如过饱和度、温度、溶剂等都直接影响到晶面吸附情况的变化、晶体比表面自由焓 ,从而影响晶面的生长速度[3,4 ] .单晶研究是借助于显微镜、照相机、衍射仪等观察手段测定单晶各方向的尺寸随时间的变化 ,确定晶体生长动力学的方法[5] .作者曾采用间歇动态法实验测试维生素C在水 乙…     

The technological importance of the crystallographic and surface properties of copper phthalocyanine pigments

New and previously reported results are combined to emphasise the importance of fundamental crystallographic and surface chemical properties on the technology of copper phthalocyanine pigments. Particular attention is paid to undesirable aggregation of pigment crystals and to the changes in aggregate structure and crystal phase which occur during ageing. The two most commercially significant forms of copper phthalocyanine are the α- and β-polymorphs. The crystals of both consist of parallel molecular stacks. In β-phase crystals, the coordinate bonding between adjacent molecules within the stacks promotes crystal growth in the direction of the stacks, producing commonly observed rod-shaped crystals. Moreover, the molecular stacks of the β-phase, being interlocked, are not readily distorted. In contrast, the distortion of lattice planes in the α-phase can accommodate mismatch between contiguous crystals, and promote their fusion into strongly coherent aggregates. Although the strength of aggregation is thus influenced by the crystal lattice, the geometric structure of aggregates is largely determined by crystal shape, brick-shaped crystals being more prone to aggregation than rod-shaped crystals. Aggregates of brick-shaped crystals are porous, and solvent molecules can penetrate some way into their internal structure. Ageing may then occur. Changes in crystal phase and aggregate structure are discussed at some length. New evidence is presented for the existence of an intermediate metastable state in the α-→β-phase transformation induced by solvents. The β-→α-phase change induced by mechanical grinding is shown to start at the surface of β-phase crystals, and then to extend gradually throughout their bulk. Examples are given of different kinds of ageing of aggregate structures. Ageing of aggregates of α-phase crystals in liquid toluene involves not only Ostwald ripening and a transformation to the β-phase, but also crystal fusion (as occurs during ageing in toluene vapour). In liquid n-propanol, on the other hand, the aggregates are converted to highly consolidated structures.     

Morphology evolution in polytetrafluoroethylene as a function of melt time and temperature. Part III. Effect of prior deformation

In parts I and II the effect of melt time and temperature on dispersed dispersion particles of various molecular weights was described: substantial molecular motion on the substrate occurs with the development of both planar folded chain single crystals, and single molecule single crystals and banded structures with parallel double-striations oriented along the long axis of the crystal or band. The molecules in the single molecule single crystals and bands are parallel to the substrate, an individual double-striation appearing to consist of a ‘double edge’, folded chain, lamella more or less normal to the substrate. In this paper, the effect of deformation and subsequent heat treatment of the nascent particles is described. Similar to the known effect of deformation of compacted nascent PTFE dispersion particles, large scale shear of dispersed dispersion particles can lead to nano-fibrils of indefinite length for both standard size resins and nano-emulsions, of both high and low molecular weight. Sintering of the nano-fibrils in contact with the substrate leads to the development of shish-kebab structures with the nano-fibril as a core and epitaxially nucleated, ‘double edge’, folded chain single crystal kebabs oriented perpendicular to the fibril axis and the substrate. No change is seen in similar fibrils remaining suspended between the dispersion particles. The molecules forming the ‘kebabs’ have ‘wandered’ individually on the substrate (glass) before attaching to the growing kebabs.     

Molecular dynamics simulation of a flexible polymer network in a liquid crystal solvent; structure and equilibrium properties

A flexible regular tetrafunctional polymer network containing a low molecular liquid crystal (LC) solvent was simulated with molecular dynamics. The LC solvent comprises of anisotropic rod-like semi-flexible linear molecules composed of beads bonded by a FENE potential. Flexibility was induced by a bending potential proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard-Jones potential. The average length of the network chain was chosen to be close to the length of a mesogen. The number of network cells was constant and the simulated systems differ from each other by the number of LC layers. The simulations of a system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. Increasing the density of the composite system the LC solvent experiences the same phase transition as the pure LC: isotropic, nematic and smectic. The presence of the network shifts the isotropic-nematic transition to higher densities but does not significantly change the position of nematic-smectic transition. Transition of the LC solvent into the smectic state changes the morphology of the network. The periodicity of LC phase determines the number of network layers. The presence of linear chains in the LC solvent decreases the number of LC layers in the smectic phase.The LC order induces some stretching of the network chains along the direction of orientation and at the same time causes shrinkage in the perpendicular direction especially in the smectic phase.     

Rod-like crystal growth of dioctyl substituted polyfluorene from nematic and isotropic states

The growth behavior of the rod-like α-form crystals from both the liquid crystal and the isotropic melt state was studied in situ and in real time using a hot-stage atomic force microscopy. The growth rate and direction of the poly(9,9-dioctylfluorene) (PFO) α-form crystals are greatly affected by the orientation of the polymer chains in the liquid crystal matrix. The pre-existence of chain alignment in the liquid crystal matrix serves as a precursor for crystallization, which greatly reduces the growth energy barrier and promotes the overall growth rate of PFO α-form crystals. Our results provide some valuable information for understanding the relationship between morphology and photoexcited emission behaviors.     

Molecular dynamics simulation of a flexible polymer network in a liquid crystalline solvent; dynamical properties

Molecular dynamics simulation was performed for the systems consisting of a flexible regular tetrafunctional polymer network and a low molecular liquid crystal (LC) solvent. The LC solvent comprises of anisotropic rod-like semiflexible linear molecules composed of beads bonded by a FENE potential. Rigidity was induced by a bending potential, proportional to the cosine of the angle between neighbouring valence bonds. All interactions between non-bonded beads are described by the repulsive part of the Lennard–Jones potential. For comparison the simulations of the system of flexible polymer chains in a low molecular LC solvent and a system of pure low molecular LC solvent were also carried out. The influence of the network and linear chain polymer on the translational and rotational mobility of low molecular LC solvent was studied. The influence of the LC solvent ordering on the local translational mobility of the polymer chains was also observed.     

Capillary and slit flow behavior of concentrated solutions of a rod-like polymer

This paper is concerned with the flow behavior of isotropic solutions of the rod-like polymer, poly(p-phenyleneterephthalamide) (PPT), in 100 percent sulfuric acid. Studies include entry flow visualization in a slit die and solution fracture, and die swell in capillaries and a slit die. It was observed that solutions of PPT exhibit nearly negligible die swell, a slip-stick type of fracture that disappears at high shear rates, and radial entry flow patterns similar to Newtonian fluids. Fracture was associated with the plateau in the shear stress vs shear rate curve. Because values of the wall shear stress (τ_w.) obtained from capillary measurements were in good agreement with those obtained from a cone-and-plate rheometer and values of the loss modulus (G″) obtained from small-strain dynamic oscillatory measurements, it is believed that the rheological properties lead to the flow instability. These results are in agreement with the predictions of a recent theory by Doi and Edwards for concentrated solutions of rod-like molecules. Data are also presented for a flexible chain polyamide, nylon 6,6, in 100 percent H₂SO₄ for the purpose of comparing the flow characteristics of rigid and flexible chain polymers.     

Quantitative Characterization of Internal Defects in RDX Crystals

Size, shape, internal defects are very important properties of explosives crystals. These parameters play a role on both the explosive formulation processing and the detonic behavior of the explosive formulations. The use of explosive crystals free of solvent inclusions leads to decrease the shock sensitivity of cast explosive formulations. Many efforts for processing such high quality explosive crystals have been done and are still in progress. Qualitative observations of internal crystal defects can be performed by optical microscopy with matching refractive index. The purpose of this paper is to provide two accurate quantitative tools for internal crystal defects measurements. The first method is based on accurate measurements of the crystal apparent density. The second method records the mass of the species entrapped in the crystal internal cavities. Experiments are performed on two RDX batches. The strong correlation recorded between the results of the two complementary methods validates the measurements. Apparent density measurements provide an accurate global characterization of the internal defects population of a crystal batch sorting the crystals in function of their apparent density. The second method is a tool to identify the species entrapped in the crystals.     

Continuous production of solid lipid nanoparticles by liquid flow-focusing and gas displacing method in microchannels

The liquid flow-focusing and gas displacing method is developed to produce solid lipid nanoparticles (SLNs) continuously in a microchannel, which has a cross-junction for the focus of lipid and aqueous solutions and a T-junction for the injection of gas bubbles. The liquid flow-focusing was achieved by introducing a lipid solution with a water-miscible organic solvent and an aqueous surfactant solution simultaneously through the two branches of the cross-junction into the main channel, while the gas displacing was accomplished by injecting an inert gas (N₂) through the T-junction at the downstream of the cross-junction into the main flow streams upward to form gas-liquid slug flow. Solid lipid nanoparticles were formed due to the local supersaturation of lipid induced by the diffusion of the solvent from the lipid solution stream into the aqueous phase. The liquid suspension containing solid lipid nanoparticles then passed freely through the microchannel without any blockage by the contribution of gas slug flow. The flow behaviors were observed by a digital inversion microscope system and the hydrodynamics of the liquid flow-focusing streams and the gas slug flow were investigated. Particle size distributions of the solid lipid nanoparticles obtained under various conditions were measured by dynamic light scattering and the particle morphology was examined by transmission electron microscopy. The influences of liquid velocity and lipid concentration under the gas displacing condition on the properties of solid lipid nanoparticles were studied experimentally. The solid lipid nanoparticles with small size (the mean size in the range of 120-200 nm) and narrow particle size distribution (with values of polydispersity index in the range of 0.14-0.19) had been produced by this method. The crucial roles of Taylor bubbles and liquid slugs in the formation of solid lipid nanoparticles were considered and the transfer mechanism of slug flow on the formation and passage of solid lipid nanoparticles in the microchannel were also discussed. Compared with other production methods for SLNs (e.g., hot homogenization, warm microemulsions and supercritical fluid technique), the proposed method in this work is simple and no overcritical operations are needed during the preparing process. Therefore, it can be employed to prepare SLNs with small sizes and a narrow diameter distribution.     

Modeling of a Continuous Crystallization Process for Solvent Dewaxing

Abstract

A steady 1-D parallel flow model was established for a continuous solvent dewaxing process; the model equations were numerically solved to obtain the wax supersaturations, amount of crystals, crystal sizes and crystal size distributions in the wax-oil-solvent solution along the flow direction. The effects of the operating conditions, including temperature distributions, multiple dilutions, and the composition of dewaxing solvent, the system's characteristics, including nucleation constant, nucleation order, and wax compound, and the crystallizer dimensions on the formed wax crystals were examined and discussed in detail. The system's nucleation constant was determined by an experimentally obtained recovery of wax; further predictions are in acceptable agreement with the results from a pilot plant.     

Computational rheology of carbonaceous mesophases

Mesophase pitches are multicomponent discotic nematic liquid crystals (DNLCs), whose characteristic molecular weight is intermediate between low molar mass and polymeric nematic liquid crystals. Flow modelling of these fluids is performed using a previously formulated mesoscopic viscoelastic rheological theory [J. Non-Newtonian Fluid Mech. 94 (2000) 87] that takes into account flow-induced texture transformations. A complete extra stress tensor equation is developed from first principles for liquid crystal materials under non-homogeneous arbitrary flow. This mesoscopic viscoelastic model has been adapted to describe the rheology of flow-aligning thermotropic DNLCs as models of mesophase pitches. We develop a fundamental understanding of the relations between rheology and flow of carbonaceous mesophases using theory and simulation by characterizing the steady and transient shear rheological material functions of flow-aligning DNLCs. Predictions for simple shear flow (under non-homogeneous conditions) for the apparent shear viscosity and first normal stress differences are presented. The predicted relations among rheological properties, shear-induced microstructure, processing conditions and material parameters of discotic mesophases are characterized and discussed.     

Crystal and molecular structure of an enantiomeric gossypol-acetic acid clathrate

Single crystals of gossypol with three molecules of acetic acid (gossypol triacetic acid) were grown from solutions of gossypol acetic acid and acetone. The crystals were unstable in air but could be stabilized for X-ray diffraction analysis by coating the crystal surfaces with a thin layer of mineral oil. The gossypol triacetic acid complex (C₃₀H₃₀O₈·3C₂H₄O₂) forms an orthorhombic crystal system with P2₁2₁2₁ (Z=4) symmetry. Unit cell dimensions were a=9.0208(7) Å, b=17.4884(10) Å, and c=24.358(2), Å yielding a volume of 3842.7(5) ų and a density of 1.2077(2) g/cm³. As with all previously reported crystals of gossypol, the gossypol molecules were of the aldehyde tautomer, and the two planar naphthalene rings were approximately perpendicular. Acetic acid molecules were found to lie in channels within the gossypol matrix. Individual crystals contained only one gossypol enantiomer, but both enantiomers crystallized from solution. Although the crystal habit could not be used to distinguish between the gossypol enantiomers, a fragment of the crystal could be derivatized and analyzed by high-performance liquid chromatography for this purpose. The ability to grow large, nonracemic crystals leads to a simple procedure for separating small quantities of the individual gossypol enantiomers.     

Formation of cylindrite structures in shear-induced crystallization of isotactic polypropylene at low shear rate

The shear-induced crystallization behavior of isotactic polypropylene (iPP) at low shear rate and low temperature was investigated by in situ optical microscopy, time-resolved small-angle light scattering (SALS) and ex situ atomic force microscopy (AFM). Some new details and insight of the cylindrites were observed, which are important to delineate the formation mechanism of the cylindrite structures. Optical microscopic measurements showed that the core of cylindrite was formed rapidly after shear cessation, at late stage the quantity of the newly formed crystals increased and these crystals impinged into each other rapidly due to the space limitation. Measurement of the length of cylindrite core from optical microscopic observation showed that the length of cylindrite core did not change or changed very slightly with crystallization time. SALS results indicated that immediately after shear flow, the highly oriented structure emerged and the scattering intensity increased with crystallization time, and eventually the scattering patterns became circularly symmetric and the anisotropic scattering decreased, indicating the impingement of different cylindrite structures. AFM observation showed that the crystalline lamellae grew both perpendicular to and along the cylindrite core. In addition, a much wider cylindrite core was observed. Compared with the radius of gyration of a molecule, the process of shear-induced crystallization may have involved a large number of entangled molecules under a low shear rate. These evidences clearly indicate that the cylindrite core comes from the stretched bundles of the entangled network strands but not from the extended crystals of stretched single chains. A model based on the above observations has been proposed to explain the mechanism of the cylindrite developing process at low shear rate and low temperature.