Anisotropic Polymer Particles Research Articles

Linear growth of colloidal dumbbells into three-lobed polymer nanoparticles mediated by a gradient in surface wettability

Anisotropy is a deciding factor in determining the hydrodynamics and self-assembly of colloidal particles. Linking particle morphology to said behaviors promoted the development of strategies to obtain anisotropic particles exhibiting defined shapes and symmetries. Dumbbell-shaped polymer particles made by phase separation during seeded polymerization are prominent examples. Phase separation among monomer and seed particle yields a liquid protrusion of monomer on the seed. This protrusion is then polymerized, becoming solid and yielding a solid spherical lobe. When this process is performed with spherical seeds, two-lobed particles, known as colloidal dumbbells, are obtained. Repeating this process of lobe formation one or more times could pave the way to tailored particle morphologies. Given the higher degree of anisotropy, multi-lobed particles can expand the rich phase behavior already found for dumbbells. We propose a new route in making anisotropic polymer particles by directing phase separation in a linear direction, thus permitting linear growth. Colloidal particles composed of three individual polymer lobes with the potential for site-specific modifications are obtained. Triggering of the phase separation is done complementary to prior efforts in fabricating three-lobed polymer particles based on cross-linked precursor particles. We will show that tailored surface properties of anisotropic seed particles can prove as an effective tool not only to promote the monomer-polymer phase separation, but also to guide it in a linear direction. Such gradients in surface functionalization open perspectives for making polymer colloids on a large scale in whose custom-tailored shapes their phase behavior and superstructure formation are already established.Graphical

Shape-Tunable Biconcave Disc-Like Polymer Particles by Swelling-Induced Phase Separation of Seeded Particles with Hydrophilic Shells.

Anisotropic shape-tunable polymer particles have gained significant attention for their wide applications, and their performances are usually strongly correlated to their shapes. In contrast to convex particles, the synthesis of highly uniform concave polymer particles remains a great challenge. Here, we present a facile and effective route to synthesize biconcave polystyrene (PS) discs by swelling-induced phase separation of hydrophilically modified PS microspheres and report an unexpected finding that even a tiny amount of hydrophilic units that were incorporated into PS microspheres can significantly change the shape of phase interfaces, resulting in the transformation of disc shapes from convex to flat to concave. This is realized by several typical hydrophilic monomers, such as sodium styrene sulfonate (NaSS), acrylic acid (AA), or (2-(methacryloyloxy)ethyl)trimethylammonium chloride (METAC). The effect of the distribution of hydrophilic units in microspheres was investigated, and the mechanism of shape tuning has been discussed. The curvatures of the bottom surfaces of discs show a strong correlation to the content of hydrophilic units. In particular, we emphasize that the shape control method is general since it does not depend on specific hydrophilic units. This research paves the way for precisely structuring polymer particle shapes, which is important for polymer particles to be used for self-assembly, diffusion, rheology, transport, filler, and many other applications.

Emulsion-Guided Controllable Construction of Anisotropic Particles: Droplet Size Determines Particle Structure.

Anisotropic particles have attracted significant attention due to their alluring features that distinguish them from isotropic particles. One of the most appealing strategies for the synthesis of anisotropic particles is the emulsion-guided method. However, morphological control and the understanding of formation mechanisms have remained a major challenge. Based on a novel mechanism, here, a facile one-pot emulsion-templating method for the tunable construction of anisotropic polymeric particles (APPs) with different defined structures is reported. Three types of monocomponent APPs with new morphologies and sizes in the range of 240-650nm, including Janus mushroom-like mesoporous poly(m-phenylenediamine) (PmPD) particles, wheel-shaped particles, and acorn-like PmPD particles, are obtained by controlling the average size of the oil droplets in the emulsion. Furthermore, the APPs demonstrate the ability for conversion to nitrogen-doped anisotropic carbon particles (ACPs) by pyrolysis at 800°C under a N2 atmosphere, thereby inheriting their structures. These novel ACPs show appreciable potential as metal-free electrocatalysts for use in oxygen reduction reactions. Compared to their isotropic counterpart, these ACPs exhibit remarkable advantages such as enhanced specific surface area and pore volume, reduced stacking density, and easy fabrication of continuous and uniform membrane electrodes.

Structurally Anisotropic Janus Particles with Tunable Amphiphilicity via Polymerization of Dynamic Complex Emulsions.

A facile one-step approach for the synthesis of physically and chemically anisotropic polymer particles with tunable size, shape, composition, wettability, and functionality is reported. Specifically, dynamically reconfigurable oil-in-water Janus emulsions containing photocurable hydrocarbon or fluorocarbon acrylate monomers as one of the droplet phases are used as structural templates to polymerize them into precision Janus particles with highly uniform anomalous morphologies including (hemi-) spheres, lenses, and bowls. During polymerization, each interface is exposed to a different chemical environment, yielding particles with an intrinsic Janus character that can be amplified via side-selective postfunctionalization. The fabrication method allows to start with various common emulsification techniques, thus generating particles in the range of 200 nm –150 μm, also at a technical scale. The anisotropic shape combined with the asymmetric wettability profile of the produced particles promotes their directed self-assembly into colloidal clusters as well as their directional alignment at fluid interfaces. We foresee the application of such Janus particles in technical emulsions or oil recovery, for the manufacturing of programmed self-assembled architectures, and for the engineering of microstructured interfaces.

Biocompatible Anisotropic Polymeric Particles: Synthesis, Characterization, and Biomedical Applications.

Recently, biocompatible anisotropic particles have emerged as a powerful platform in the biomedical field, especially in the areas such as therapeutics, theranostics, biosensor, targeted drug delivery, triggered drug delivery, cellular uptake, targeted cellular internalization, etc., because of their unique characteristics and superior performances over their isotropic analogues. Macromolecular anisotropic particles can primarily be classified into three different categories, such as shape anisotropy, compositional anisotropy, and surface anisotropy, i.e., heterogeneous surface chemistry leading to anisotropic surface patches. In this review, we have attempted to give an overview on the biocompatible anisotropic polymeric particles related to their synthesis and applications in biomedical arena with emphasis on their fascinating properties over spherical particles. A brief summary of their future developments is also highlighted.

Sequential Selective Solvent On-Film Annealing: Fabrication of Monolayers of Ordered Anisotropic Polymer Particles.

Although various strategies have been developed to prepare anisotropic polymeric particles, it remains challenging to fabricate monolayers of anisotropic polymeric particles, which can extend the applications of anisotropic particles. Here, we develop a novel and facile approach to fabricate monolayers of anisotropic polymeric particles. Monolayers of polystyrene (PS) microspheres with a mean diameter of 10 μm are deposited on glass substrates coated with poly(methyl methacrylate) films, followed by sequential selective solvent on-film annealing processes. Monolayers of anisotropic polymeric particles, such as the snowman-like PS particles, are successfully fabricated. Such unique structures possess the long-range ordering of monolayers (the structure factor) and the anisotropic geometry of individual particles (the form factor). The nanomechanical properties of the PS particles are also characterized using atomic force microscopy force volume measurements, showing a decrease in the Young's moduli of the PS particles owing to the looser packing of the polymer chains. This work provides the most facile and versatile strategy by far to fabricate monolayers of ordered anisotropic polymeric particles, which are inaccessible by other traditional means.

Hollow polymer microrods of tunable flexibility from dense amphiphilic block copolymer brushes.

Polymer microrods of aspect ratio ∼10, and tunable flexibility are attractive model systems to study density and index matched liquid crystalline phases. However, the synthesis of anisotropic polymer particles is arduous, due to the lack of directional polymer growth mechanisms. In this work, non-cross-linked, hollow polymer microrods are developed from a dense block copolymer brush grown from the surface of micron-sized silica rods. The copolymer brush, comprising a hydrophobic inner block and a hydrophilic outer layer, is synthesized by surface-initiated atom transfer radical polymerization, and is exploited in the preparation of robust polymer rod particles in water, following etching of the inorganic core. The solvent-incompatible inner block is crucial for the synthesis of the rod-like polymer particles, in the absence of chemical cross-links, and the block copolymer composition affects the colloidal stability and flexibility of the hollow anisotropic colloids. For shorter hydrophobic block lengths, well-defined, yet flexible, hollow rods are obtained, whereas increasing the hydrophobic content of the copolymer results in rigid, tube-like particles. The approach is generic and could be easily employed to obtain polymer rod particles in any solvent medium, upon the appropriate selection of the solvent-incompatible inner block and the solvent-compatible outer block.

Shape-Anisotropic Diblock Copolymer Particles with Varied Internal Structures.

Anisotropic polymer particles have promising applications in various fields, whereas their preparation usually suffers from tedious procedures. Here, we introduce a facile strategy to fabricate novel shape-anisotropic particles with varied internal structures via self-assembly of block copolymers (BCPs), with perfluorooctane (PFO) as the liquid template in emulsion droplets. By increasing the volume ratio of PFO to polystyrene- block-poly(4-vinylpyridine) (PS- b-P4VP) or decreasing the initial concentration of the BCPs, the self-assembled polymer particles change from spherical core-shell structures to anisotropic particles. Moreover, the anisotropic shape and internal structure of the polymer particles, including cone-like particles with alternative PS and P4VP lamellas, crescent-shaped particles with cylindrical P4VP domains, and plate-like particles with spherical P4VP domains, can be obtained by changing the block ratio or molecular weight or by adding a hydrogen-bonding agent. Based on the in situ optical microscopy investigation of the morphology evolution of the emulsion droplet, we conclude that both kinetic and thermodynamic factors during emulsion evolution determine the formation of shape-anisotropic polymeric particles with controllable internal structures.

Sunny-Side-Up Egg-Shaped Structures: Surface Modification To Form Anisotropic Polymer Particles Driven by the Plateau–Rayleigh Instability as Fluorescence Manipulation Platforms

The Plateau–Rayleigh instability is a phenomenon driven by the reduction of the surface energies, in which a liquid cylinder transforms to a series of droplets with distinct spacing. This instability phenomenon is also applied broadly for the transformation of cylindrical materials to spherical particles in homogeneous environments. The demonstration of the instability in heterogeneous environments, however, has been less studied. Here, we study the Plateau–Rayleigh instability of electrospun polystyrene (PS)/poly(methyl methacrylate) (PMMA) core–shell fibers annealed on n-octadecyltrichlorosilane (ODTS)-modified glass substrates with toluene vapors. After the annealing procedures, the PS/PMMA core–shell fibers transform to anisotropic sunny-side-up egg-shaped polymer structures, driven by the reduction of the total surface and interfacial energies. Quantitative analyses are performed on the sizes of the structures. Fluorescent fibers are also prepared by electrospinning and annealed on the ODTS-modified ...

Two-Step Solvent On-Film Annealing (2-SOFA) Method: Fabrication of Anisotropic Polymer Particles and Implications for Colloidal Self-Assembly

In recent years, anisotropic polymer particles have gained increased interest owing to their special properties and broader applications, such as drug delivery, optical traps, and e-paper display. Most strategies to produce anisotropic polymer particles, however, require sophisticated instruments or additional surfactants. Here, we develop a simple and versatile method, the two-step solvent on-film annealing (2-SOFA) technique, to make anisotropic polymer particles with different shapes. Polystyrene (PS) microspheres spin-coated on poly(methyl methacrylate) (PMMA) films are chosen as model materials. By sequentially annealing the PS/PMMA composites in different solvent vapors, anisotropic polymer particles with distinctive and diverse shapes can be produced, such as half-eaten-peach-shaped, snowman-shaped, and bowler-hat-shaped morphologies. An exquisite selective removal strategy is applied to check the morphologies of the PS/PMMA composite films and to comprehend the transformation mechanism at differen...

Solvent-Induced Shape Recovery of Anisotropic Polymer Particles Prepared by a Modified Thermal Stretching Method.

Anisotropic polymer particles have attracted great attention because of their unique properties and potential applications in various areas, such as microelectronics, drug delivery, and medical imaging. The fabrication and morphology control, especially the shape recovery, of anisotropic polymer particles, however, remains a challenging task. In this work, we develop a novel strategy to fabricate anisotropic polymer particles by thermally stretching poly(vinyl alcohol) (PVA) films embedding polystyrene (PS) microspheres using a weight. Depending on the preannealing condition, anisotropic PS particles with two different shapes, sharp-headed and blunt-headed PS particles, can be obtained. The PVA films can be selectively removed by isopropanol/water, releasing the anisotropic PS particles. By adding tetrahydrofuran (THF), a good solvent for PS, into the PS particle-containing solutions, the anisotropic particles gradually transform back to spheres to reduce the total interfacial energies. The shape recovery rates of the polymer particles can be controlled by the amount of the added THF. This work not only provides a simple and feasible route to fabricate anisotropic polymer particles but also contributes to a deeper understanding in the solvent-induced shape recovery process from anisotropic polymer particles to polymer spheres.

Measurement of Anisotropic Particle Interactions with Nonuniform ac Electric Fields.

Optical microscopy measurements are reported for single anisotropic polymer particles interacting with nonuniform ac electric fields. The present study is limited to conditions where gravity confines particles with their long axis parallel to the substrate such that particles can be treated using quasi-2D analysis. Field parameters are investigated that result in particles residing at either electric field maxima or minima and with long axes oriented either parallel or perpendicular to the electric field direction. By nonintrusively observing thermally sampled positions and orientations at different field frequencies and amplitudes, a Boltzmann inversion of the time-averaged probability of states yields kT-scale energy landscapes (including dipole-field, particle-substrate, and gravitational potentials). The measured energy landscapes show agreement with theoretical potentials using particle conductivity as the sole adjustable material property. Understanding anisotropic particle-field energy landscapes vs field parameters enables quantitative control of local forces and torques on single anisotropic particles to manipulate their position and orientation within nonuniform fields.

Thermal-Annealing-Induced Self-Stretching: Fabrication of Anisotropic Polymer Particles on Polymer Films.

Designing anisotropic particles of various shapes draws great attention to scientists nowadays. We develop a facile and simple method to fabricate anisotropic polymer particles from spherical polymer particles. Poly(vinyl alcohol) (PVA) films spin-coated with polystyrene (PS) microspheres are confined on both sides using binder clips and are heated above the glass-transition temperatures of the polymers. During the thermal annealing process, the PS particles sink into the PVA films and transform to anisotropic particles. Depending on the distances to the bound regions, oblate spheroid PS particles or prolate spheroid particles with different aspect ratios can be obtained. The transformation of the particles is mainly driven by the stretching forces and the squeezing forces. The main advantage of this method is that anisotropic particles with different shapes can be fabricated simultaneously on a single film. We expect that this novel method can be helpful to various fields including colloids science, suspension rheology, and drug delivery.

Solvent On-Film Annealing (SOFA): Morphological Evolution of Polymer Particles on Polymer Films via Solvent Vapor Annealing

Over the past few decades, anisotropic polymer particles are of significant interest because of their unique properties which can be applied in various areas, such as drug delivery, biotechnology, and electronics. Most approaches to synthesize anisotropic polymer particles, however, are complicated, and the three-dimensional shapes of the anisotropic particles are usually difficult to be controlled. In this work, we develop a solvent on-film annealing (SOFA) method to fabricate anisotropic polymer particles by studying the morphological evolution of polystyrene (PS) microspheres on poly(methyl methacrylate) (PMMA) films annealed in toluene vapor. At different annealing stages, the isotropic PS microspheres gradually transform to anisotropic PS particles with different morphologies, such as UFO-, cymbal-, peanut-, and bowl-shaped particles. The morphology evolution is driven by the surface and interfacial tensions during the annealing processes and can be confirmed by a selective removal technique. Acetic ...

Swelling-Induced Deformation of Spherical Latex Particles

Experimental evidence is presented showing that the direct formation of anisotropic colloidal polymer particles via aqueous heterophase polymerization is essentially controlled by the entropy gain of the linear fraction in the semi-interpenetrating network of the seed particles during swelling. Anisotropic particles are produced via photoinitiated polymerization, allowing swelling and polymerization to take place at the same temperature. These experiments prove that the temperature effect on rubber elasticity is, if at all, only of minor importance for the formation of anisotropic polymer particles. The major significance of swelling of the seed particles for the whole process is underlined by additional studies with optical microscopy and model simulations.

Asymmetric Polymer Particles with Anisotropic Curvatures by Annealing Polystyrene Microspheres on Poly(vinyl alcohol) Films.

Anisotropic polymer particles such as Janus particles have attracted significant attention in recent years because of their unique properties and unusual self-assembly behavior. Most anisotropic polymer particles synthesized so far, however, only have different chemical regions compartmentalized on the particles. It remains a great challenge to fabricate anisotropic polymer particles with different shapes within a single particle. A novel approach is developed to prepare anisotropic polymer particles that contain two hemispheres with different curvatures by annealing polystyrene microspheres on poly(vinyl alcohol) films. During the annealing process, the polymer microspheres gradually sink into the polymer films and transform to asymmetric polymer particles, driven by the surface and interfacial tensions of the polymers. Selective removal techniques are also used to confirm the morphologies of the asymmetric particles.

High-Throughput Contact Flow Lithography.

High-throughput fabrication of graphically encoded hydrogel microparticles is achieved by combining flow contact lithography in a multichannel microfluidic device and a high capacity 25 mm LED UV source. Production rates of chemically homogeneous particles are improved by two orders of magnitude. Additionally, the custom-built contact lithography instrument provides an affordable solution for patterning complex microstructures on surfaces.

An automated multidimensional thin film stretching device for the generation of anisotropic polymeric micro- and nanoparticles.

Anisotropic polymeric particles are of growing interest for biomaterials applications due to their unique properties. These include the ability for these particles to evade nonspecific cellular uptake and to have enhanced targeted cellular uptake and interaction. One of the most widely used methods for generating anisotropic polymeric particles is the thin film stretching procedure. Despite its theoretical simplicity, this procedure, as it has been implemented to date, can be difficult due to the inconsistent nature of the manual operation of machinery used to stretch the film. We have constructed an automated thin film stretcher for control over biomaterials via thin film stretching in 1D and 2D and as a result, have enabled precise generation of anisotropic polymeric particles. We demonstrate that this device can be utilized to produce anisotropic biodegradable particles of different size, shape, and material consistency. Furthermore, we show that this machine has enabled the scaled up and rapid production of anisotropic polymeric particles, including polymeric microparticles that mimic the shape of red blood cells. Further application of this automated thin film stretching device could allow for significant impact to diverse biomaterial and biomedical applications such as biomimetic particles for immunoengineering and long-circulating particles for controlled release of drugs.

Size and rigidity of cylindrical polymer brushes dictate long circulating properties in vivo.

Studies of spherical nanoengineered drug delivery systems have suggested that particle size and mechanical properties are key determinants of in vivo behavior; however, for more complex structures, detailed analysis of correlations between in vitro characterization and in vivo disposition is lacking. Anisotropic materials in particular bear unknowns in terms of size tolerances for in vivo clearance and the impact of shape and rigidity. Herein, we employed cylindrical polymer brushes (CPBs) to answer questions related to the impact of size, length and rigidity on the in vivo behavior of PEGylated anisotropic structures, in particular their pharmacokinetics and biodistribution. The modular grafting assembly of CPBs allowed for the systematic tailoring of parameters such as aspect ratio or rigidity while keeping the overall chemical composition the same. CPBs with altered length were produced from polyinitiator backbones with different degrees of polymerization. The side chain grafts consisted of a random copolymer of poly[(ethylene glycol) methyl ether methacrylate] (PEGMA) and poly(glycidyl methacrylate) (PGMA), and rendered the CPBs water-soluble. The epoxy groups of PGMA were subsequently reacted with propargylamine to introduce alkyne groups, which in turn were used to attach radiolabels via copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC). Radiolabeling allowed the pharmacokinetics of intravenously injected CPBs to be followed as well as their deposition into major organs post dosing to rats. To alter the rigidity of the CPBs, core-shell-structured CPBs with polycaprolactone (PCL) as a water-insoluble and crystalline core and PEGMA-co-PGMA as the hydrophilic shell were synthesized. This modular buildup of CPBs allowed their shape and rigidity to be altered, which in turn could be used to influence the in vivo circulation behavior of these anisotropic polymer particles. Increasing the aspect ratio or altering the rigidity of the CPBs led to reduced exposure, higher clearance rates, and increased mononuclear phagocytic system (MPS) organ deposition.

Nanopressing: Toward Tailored Polymer Microstructures and Nanostructures

A simple and versatile method is developed for preparing anisotropic polymer particles by pressing polymer microspheres at elevated temperatures. Polystyrene (PS) microspheres are used to demonstrate this approach. Depending on the mechanical deformation and wetting of the polymers on the substrates, polymer structures with special shapes such as barrel-like or dumbbell-like shapes can be prepared. The morphology of polymer structures can be controlled by the experimental parameters such as the pressing pressure, the pressing temperature, and the pressing time. The wetting of the polymers on the substrates dominates when the samples are annealing at higher temperatures for longer times.