Isotropic Morphology Research Articles

Control of Crystal Morphology in Poly(l-lactide) by Adding Nucleating Agent

ADVERTISEMENT RETURN TO ISSUEPREVCommunication to the...Communication to the EditorNEXTControl of Crystal Morphology in Poly(l-lactide) by Adding Nucleating AgentHongwei Bai, Weiyi Zhang, Hua Deng*, Qin Zhang, and Qiang Fu*View Author Information College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China*Tel/Fax +86 28 8546 1795, e-mail [email protected] (Q.F.); e-mail [email protected] (H.D.).Cite this: Macromolecules 2011, 44, 6, 1233–1237Publication Date (Web):February 14, 2011Publication History Received26 October 2010Revised14 January 2011Published online14 February 2011Published inissue 22 March 2011https://doi.org/10.1021/ma102439tCopyright © 2011 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views3165Altmetric-Citations177LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (2 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Crystal structure,Crystallization,Crystals,Morphology,Nanofibers Get e-Alerts

Endothelial cell scaffolds generated by 3D direct writing of biodegradable polymer microfibers

The engineering of large (thickness > 100 μm) tissues requires a microvascular network to supply nutrients and remove waste. To produce microvasculature in vitro, a scaffold is required to mechanically support and stimulate endothelial cell (EC) adhesion and growth. Scaffolds for ECs are currently produced by patterning polymers or other biomaterials into configurations which often possess isotropic morphologies such as porous films and fibrous mats. We propose a new “direct-write” process for fabricating scaffolds composed of suspended polymer microfibers that are precisely oriented in 3D, providing directional architecture for selectively guiding cell growth along a desired pathway. The diameters of the fibers produced with this process were predictably and repeatably controlled through modulation of the system parameters, enabling production of fibers with microvascular-scale diameters (5–20 μm) from a variety of biodegradable polymers. These scaffolds were successfully seeded with ECs, which conformed to the geometry of the fibers and proliferated over the course of one week.

Mesoscale modeling of the influence of morphology on the mechanical properties of proton exchange membranes

The nano-scale morphology of a proton exchange membrane (PEM) strongly influences its proton conductivity and mechanical performance. In this paper, a multi-scale modeling approach has been developed to obtain the morphologies of hydrated perfluorosulfonic acid (PFSA) membranes and then to predict their mechanical properties based on the simulated morphology. Two representative ionic domain morphologies were compared, spherical and cylindrical, to represent cast and extruded membranes. The calculated overall elastic Young’s moduli are very close for both morphologies and agree well with experiments. The nano-scale phase segregation in hydrated PFSA induces a non-uniform distribution of local stress. The peak stress is localized at the smeared water/PFSA interface. The cylindrical morphology develops much lower peak stress than the isotropic spherical morphology under the same level of strain. These results explain why the extruded membranes show more than 10 times longer life in durability tests than recast membranes, despite having similar bulk moduli.

Super Proton Conductive High-Purity Nafion Nanofibers

In this paper, we report the high proton conductivity of a single high-purity Nafion nanofiber (1.5 S/cm), which is an order of magnitude higher than the bulk Nafion film ( approximately 0.1 S/cm). We also observe a nanosize effect, where proton conductivity increases sharply with decreasing fiber diameter. X-ray scattering provides a rationale for these findings, where an oriented ionic morphology was observed in the nanofiber in contrast to the isotropic morphology in the bulk film. This work also demonstrates the successful fabrication of high-purity Nafion nanofibers ( approximately 99.9 wt %) via electrospinning and higher humidity sensitivity for nanofibers compared to the bulk. These results should have a significant impact on fuel cells and sensors.

Smoothed local histogram filters

Local image histograms contain a great deal of information useful for applications in computer graphics, computer vision and computational photography. Making use of that information has been challenging because of the expense of computing histogram properties over large neighborhoods. Efficient algorithms exist for some specific computations like the bilateral filter, but not others. Here, we present an efficient and practical method for computing accurate derivatives and integrals of locally-weighted histograms over large neighborhoods. The method allows us to compute the location, height, width and integral of all local histogram modes at interactive rates. Among other things, it enables the first constant-time isotropic median filter, robust isotropic image morphology operators, an efficient "dominant mode" filter and a non-iterative alternative to the mean shift. In addition, we present a method to combat the over-sharpening that is typical of histogram-based edge-preserving smoothing. This post-processing step should make histogram-based filters not only fast and efficient, but also suitable for a variety of new applications.

A morphological study of porous polylactide scaffolds prepared by thermally induced phase separation

Polylactide (PLA) has been widely used as a three-dimensional scaffold for cell transplantation and as a drug carrier for controlled release due to its superior biocompatibility and biodegradability. The technique of thermally induced phase separation (TIPS) is favorable for the fabrication of a porous scaffold due to its various advantages. A diverse range of porous structures can be obtained by TIPS through adjustment of process parameters. In this study, PLA scaffolds were prepared from a ternary PLA–dioxane–water system by the TIPS method. The effects of fabrication parameters, such as quenching route and the composition of the ternary system, on the macroscopic and microscopic morphologies of the scaffolds were studied. The phase diagram of this system showed that the presence of water non-solvent is essential for a liquid–liquid phase separation to occur. Accordingly, a binary PLA–dioxane system (100/0) exhibited only a solid–liquid phase separation. For a ternary PLA–dioxane–water system, the cloud-point temperature corresponding to a liquid–liquid phase separation was highly dependent on its water content and polymer concentration, while the freezing point corresponding to a solid–liquid phase separation was nearly independent of the polymer concentration and water content. Given a slight increase in the water content of the solvent mixture of 5%, i.e. a change in dioxane/water ratio from 90/10 to 85/15, or an increase in the polymer concentration of 2–30%, there was a substantial 40–50 °C increase in cloud-point temperature, in contrast with a nearly constant freezing point of between −10 and −20 °C. Morphology of the scaffold prepared via solid–liquid phase separation of a binary dioxane–PLA system gave a highly anisotropic tubular morphology with an internal ladder-like structure, while scaffolds prepared via liquid–liquid phase separation of a ternary PLA–dioxane–water system exhibited isotropic morphology. The ultimate pore structure of a scaffold was controlled by various processing parameters such as polymer concentration, water content, quench route and molecular weight of the polymer. Liquid–liquid de-mixing is a highly dynamic process that is subject to the effects of coarsening at the intermediate or later stage of the de-mixing process. For scaffolds prepared from solutions with polymer concentration of less than 7.5 wt%, the pore size decreases with the increase of polymer concentration. The coarsening effect was minimal and the final pore structure was found to be mainly determined by the liquid–liquid de-mixing mechanism, attributable to the smaller liquid–liquid de-mixing gap. For polymer concentrations higher than 7.5 wt%, the coarsening effect was more important, and a bimodal pore-size distribution was observed; in addition to the uniform small pores caused by instantaneous spinodal decomposition, large pores formed during the subsequent coarsening process. SEM micrographs showing that sedimentation occurred by aging time of about 5 and 60 min for scaffolds prepared from a 7.5 wt% PDLLA solution with dioxane/water ratio of 90/10 at −5 and −12 °C, respectively. Although this result suggests that such scaffolds rapidly lost macroscopic uniformity, porosity data indicates that the specimens did not lose integrity by aging for up to 120 min.

Synthesis of SrTiO<sub>3</sub> Nanoparticles by a pH-Varying Sol-Gel Process

SrTiO3 is one of the most technologically important dielectric ceramics. In this paper, SrTiO3 nanoparticles were synthesized by a simple sol-gel route using NaOH to adjust the pH of the system. Three different strontium-containing starting precursors (Sr(NO3, SrCl2.6H2O and Sr(CH3COO)2.0.5H2O) were employed. Thermogravimetric analysis (TGA) showed complete decomposition of organics at approximately 800°C; this temperature was chosen as a calcination temperature. At 800°C, 6 hours, only the nitrate route with pH = 10 gives single-phase SrTiO3. Other conditions yield various impurities, most likely due to the incompletion of the reaction although they still contain very broad XRD peaks corresponding to SrTiO3, signifying the probable phase formation capability at these conditions. The obtained SrTiO3 powders are in the nanoscale range of 10 to 40 nm with isotropic morphology.

Surface morphology alignment of block copolymers induced by injection molding

Block copolymers are of increasing interest because of their nanometer-scale morphologies, which can be utilized in a range of applications, including nanolithography. Orientation of the domains can be controlled by part design and processing conditions in injection molding. In this work the surface morphology and alignment of block copolymers by mechanical flow fields from injection molding was investigated using a styrene–ethylene/butylene–styrene triblock copolymer (SEBS) and compared with the morphology induced by spin coating. Compared with the isotropic morphology found by spin coating and annealing, the surface domains were oriented in the flow direction. Increasing mold temperature and injection velocity enhanced the degree of orientation, whereas melt temperature had little effect. Smaller characteristic lengths were produced with higher mold temperatures and injection velocities.

Comparison of diamond film adhesion on molybdenum substrates with different surface morphologies

To evaluate the effect of substrate morphology on the adhesion of diamond film, two types of substrate morphology of molybdenum (Mo) were compared. The two morphology types were formed by polishing a Mo substrate with SiC abrasive paper along one direction (anisotropic morphology) and by polishing the Mo substrate with diamond powder in a random direction (isotropic morphology). Ultrasonic cavitation tests were conducted to evaluate the adhesion of the diamond films on these Mo substrates. In the case of low surface roughness, there was very little difference between the effects of SiC abrasive paper polishing and diamond powder polishing. In the case of high surface roughness, the adhesion of the diamond film on the SiC paper polished Mo substrate was larger than that of the diamond film on the diamond powder polished Mo substrate. Detachment of the diamond film from the SiC paper polished Mo substrate progressed along the polishing direction; while detachment of the diamond film from the diamond powder polished Mo substrate progressed in a random direction. It was thought that the detachment of the diamond film from a Mo substrate having an anisotropic polishing trace was suppressed because the anisotropic grooves restricted the formation of connections between the points of detachment at right angles to the groove direction. Therefore, the anisotropic surface morphology of the Mo substrate is effective for improving the adhesion of diamond film.

Predicted electric-field-induced hexatic structure in an ionomer membrane

Coarse-grained molecular-dynamics simulations were used to study the morphological changes induced in a Nafion-like ionomer by the imposition of a strong electric field. We observe the formation of structures aligned along the direction of the applied field. The polar head groups of the ionomer sidechains aggregate into clusters, which then form rodlike formations which assemble into a hexatic array aligned with the direction of the field. Occasionally these lines of sulfonates and protons form a helical structure. Upon removal of the electric field, the hexatic array of rodlike structures persists and has a lower calculated free energy than the original isotropic morphology.

Numerical methods for the quantification of the mechanical properties of crystal aggregateswith morphologic and crystallographic texture

The influence of an anisotropic distribution of crystal orientations and an anisotropic average grain shape is analysed using finite element simulations. By the numerical approach, which is based on a statistical volume element with periodic microstructure and periodic boundary conditions, the influence of the crystallographic and the morphologic texture can be separated by combining (an)isotropic orientation distributions with (an)isotropic grain morphologies.

Effect of die cavity constraint on foaming behavior and pore morphology of sheet type porous aluminum

Foaming behavior of 6061 aluminum sheet-type precursor by free foaming or by foaming in die cavity was examined. Projected area of the precursor during foaming process was recorded by two video cameras to analyze the foaming behavior of both vertical (thickness) and horizontal (in-plane) directions. The free foaming behavior of sheet type precursor was extremely unidirectional along the compressed direction of precursor. The specimen expanded only along the vertical direction, and did not expand along the horizontal direction of the sheet type precursor, while pore morphology in the aluminum foam was quite isotropic. Two separate parallel plates were used to investigate the effect of die constraint. The precursor expanded along the in-plane direction when foamed in between the parallel plates. The porosity was not significantly affected by the constraint. The isotropic pore morphology was maintained after foamed in the parallel plates. The mean size of the pores decreased by decreasing the die spacing.

Microwave absorption properties and the isotropic antenna mechanism of ZnO nanotrees

In this paper, ZnO nanowires and ZnO nanotrees have been prepared and their microwave absorption properties have been investigated in detail. Complex permittivity and permeability of the ZnO nanostructures and paraffin composites have been measured in a frequency of 0.1–18 GHz. Excellent microwave absorption performances have been observed in ZnO nanotree composite compared to ZnO nanowire composite, and the maximum absorption is enhanced as the concentration of the nanotrees increases in the composite. The value of minimum reflection loss for the composites with 60 vol % ZnO nanotrees is −58 dB at 4.2 GHz with a thickness of 4.0 mm. Such strong absorption is attributed to the unique isotropic antenna morphology of the ZnO nanotrees in the composite.

Unusual Aqueous-Phase Behavior of Cationic Amphiphiles with Hydrogen-Bonding Headgroups

Two cationic surfactants with hydroxyl and carbamate hydrogen-bonding sites at their headgroups were synthesized. Both surfactants are ionic liquids (one of them at room temperature). Samples are isotropic solutions over the entire 0-100% concentration range, which is highly unusual for ionic surfactants. Surface tension, NMR, and conductivity measurements indicate classical micelle formation in aqueous solutions with CMCs below 10 mM. Pulse-gradient spin-echo (PGSE) NMR confirms micelle formation and provides micellar hydrodynamic radii of about 3.8 nm. Because this value is larger than the length of the extended surfactant molecules, about 2.7 nm, it appears that hydrogen-bonded water of hydration contributes substantially to the effective micelle size. At higher concentrations (above 25 wt %), surfactant solutions become viscous, but line broadening in the NMR is small relative to that found with a conventional cationic surfactant (CTAB). Thus, long rod formation, the source of line broadening in the latter, is absent with the new surfactants. Finally, PGSE NMR data show a 5-fold decrease in the diffusion coefficient between 5 and 20 wt %, above which the diffusion coefficients remain constant. The results are best explained by micelle clustering that is likely aided by intermicellar hydrogen bonding. The possibility of an isotropic liquid crystal (LC) phase with cubic symmetry is discussed and dismissed, demonstrating that LC formation of ionic surfactants at high concentrations, the usual behavior in past work, need not occur. Nor is there a definite connection between ionic liquid behavior and isotropic morphology.

Flow Induced Crystallization in Isotactic Polypropylene−1,3:2,4-Bis(3,4-dimethylbenzylidene)sorbitol Blends: Implications on Morphology of Shear and Phase Separation

Nucleation is the limiting stage in the kinetics of polymer crystallization. In many applications of polymer processing, nucleation is enhanced with the addition of nucleating agents. 1,3:2,4-Bis(3,4-dimethylbenzylidene)sorbitol or DMDBS is a nucleating agent tailored for isotactic polypropylene (iPP). The presence of DMDBS changes the phase behavior of the polymer. For high enough temperatures, the system iPP−DMDBS forms a homogeneous solution. However, in the range of concentration spanning from 0 to 1 wt % of DMDBS, the additive can phase separate/crystallize above the crystallization temperature of the polymer, forming a percolated network of fibrils. The surface of these fibrils hosts a large number of sites tailored for the nucleation of iPP. The aim of this paper is to investigate the combined effect of flow and DMDBS phase separation on the morphology of iPP. To this end, we studied the rheology of phase separated iPP−DMDBS systems and its morphology with time-resolved small-angle X-ray scattering (SAXS). The effect of flow is studied combining rheology, SAXS, and a short-term shear protocol. We found that, with phase separation, DMDBS forms fibrils whose radius (∼5 nm) does not depend on the DMDBS concentration. The growth of these fibrils leads to a percolated network with a mesh size depending on DMDBS concentration. Compared to the polymer, the relaxation time of the network is quite long. A shear flow, of 60 s-1 for 3 s, is sufficient to deform the network and to produce a long-lasting alignment of the fibrils. By design, lateral growth of iPP lamellae occurs orthogonally to the fibril axis. Therefore, with crystallization, the preorientation of DMDBS fibrils is transformed into the orientation of the lamellae. This peculiarity is used here to design thermomechanical histories for obtaining highly oriented iPP morphologies after shearing well above the melting point of the polymer (i.e., without any undercooling). In contrast, when shear flow is applied prior to DMDBS crystallization, SAXS showed that iPP crystallization occurs with isotropic morphologies.

Effect of polarization and morphology on the optical properties of absorbing nanoporous thin films

This paper investigates the possibility of tuning the optical properties of thin films by introducing nanopores with different shape, size, and spatial distribution. The complex index of refraction of nanoporous thin films with various morphologies is determined for normally incident transverse magnetic (TM) and transverse electric (TE) absorbing electromagnetic waves by numerically solving the two-dimensional Maxwell's equations. The numerical results are compared with predictions from widely used effective medium approximations. For thin films with isotropic morphology exposed to TM waves, good agreement is found with the parallel model. For thin films with anisotropic morphology, the numerical results for TE waves are independent of the morphology and agree well with the Volume Averaging Theory model. By contrast, for incident TM waves, the retrieved effective optical properties depend on both porosity and film morphology. These results can be used to design nanocomposite materials with tunable optical properties and to determine their porosity and pore's spatial arrangement.

Novel fabrication techniques to produce microspheres by thermally induced phase separation for tissue engineering and drug delivery

A novel application of thermally induced phase separation (TIPS) is described enabling the rapid formation of monodisperse porous microspheres. By taking advantage of TIPS processing parameters, the porosity, the pore morphology (bimodal/channel-like/radial towards the centre) and the presence of an open-pore or dense skin region can be tailored. Achievable sizes range from 10 to 2000 μm in diameter. The technique facilitates the homogeneous inclusion of particulate fillers and drugs. Moreover, the combined TIPS/oil-in-water emulsion technique allows for the production of microspheres with isotropic pore morphology with interconnected spherical pores of 30–70 μm and well-formed porous microspheres of 10–200 μm in diameter with an open porous surface. This method is advantageous over existing techniques by avoiding the use of long-term exposure to an aqueous continuous phase as used in oil-in-water or water-in-oil-in-water processing and therefore drug encapsulation efficiencies will be higher.

Micelles-mediated synthesis of magnetic Fe 3Pt nanoparticles of cubic morphology and their characterisation

Development of a new synthetic route for the evolution of Fe 3Pt particles with cubic morphology has been discussed here. These particles are expected to be applied for hyperthermia therapy due to their soft magnetic character because of a sharp fall of magnetic moment in the neighbourhood of 320 K. The soft magnetic property is due to their isotropic cubic morphology and structure.

Control of ordered structure and morphology of cubic mesoporous silica SBA-1 via direct synthesis of thiol-functionalization

Well-ordered cubic mesoporous silicas SBA-1 functionalized with thiol groups have been synthesized via co-condensation of tetraethoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) templated by cetyltriethylammonium bromide (CTEABr) under strongly acidic conditions. Various synthesis parameters such as HCl concentration, synthesis temperature, and time for hydrothermal treatment were systematically investigated as a function of MPTMS contents. The materials thus obtained were characterized by a variety of techniques including powder X-ray diffraction (XRD), solid-state 13C and 29Si NMR spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and nitrogen sorption measurements. Direct evidence of the presence of chemically attached thiol moieties was provided by solid-state 29Si and 13C NMR spectroscopy. A high degree of structural ordering was still retained when MPTMS loading was incorporated up to 20 mol% in the synthesis mixture. Increasing the synthesis temperature and the time for the hydrothermal treatment not only increased the degree of framework cross-linking, but also did not lead to the undesirable phase transformation as often observed in the conventional synthesis of pure silica SBA-1. SEM results reveal that the morphology of thiol-functionalized SBA-1 depends on the HCl concentration used in the synthesis. The sample prepared with a low acid concentration exhibits a highly isotropic morphology with more facets than that of pure silica SBA-1, whereas it changes to a spherical shape as the acid concentration is increased. The maximum content of the attached thiol group (–SH) in the mesoporous framework is 2.39 mmol/g. The thiol-functionalized SBA-1 mesoporous materials are efficient Hg 2+ adsorbents.

Effect of the nature of the precursor on the morphology of MoO 3 thin films spin-coated on Si (100)

Thin films of α-MoO 3 were efficiently deposited on Si (100) by spin-coating of various precursor solutions i.e. ammonium heptamolybdate (HMA), citrate (CIT) and oxalate (OXA) complexes of Mo 6+, and peroxo-polymolybdic (PER) solutions. The effect of the nature and concentration of the precursor and of the spin-coating rate were studied by X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The nature of the precursor used for the deposition influences the morphology of the crystals. For films obtained from CIT precursor solutions, an anisotropic morphology of MoO 3 crystals with predominant basal faces is obtained, while MoO 3 crystals spin-coated from PER precursor solutions possess a more isotropic morphology. Films deposition from OXA and HMA precursors possess an intermediate morphology. Homogeneity and thickness of the films can be adjusted by tuning the concentration of the precursor solution and the spin-coating rate. Whatever the precursor used, the larger the crystals formed, the more developed were the basal faces obtained.