Block Copolymer Particles Research Articles

Aspect Ratio-Controlled Synthesis of Uniform Colloidal Block Copolymer Ellipsoids from Evaporative Emulsions

Block copolymers (BCPs) confined in evaporative emulsions can assemble into ellipsoidal particles when solvent evaporation is faster than polymer diffusion within the droplet. Here, we report the synthesis of monodisperse, ellipsoidal polystyrene-block-1,4-polybutadiene (PS-b-PB) BCP particles with tunable aspect ratios (ARs) ranging from 1.0 to 2.2 and particle sizes ranging from 0.1 to 5 μm by membrane emulsification and subsequent solvent evaporation. The ratio of the propagation distance (lp) of ordered BCP domains perpendicular to the particle surface to the particle size (rBCP), or lp/rBCP, was found to be a critical parameter dictating the particle shape, where lp/rBCP > 1 yielded ellipsoids. We show that the AR of colloidal BCP ellipsoids can be precisely controlled by varying particle size (i.e., membrane pore size) and BCP molecular weight, as predicted by theoretical calculations of the free energy of particle elongation including three terms: (1) the interfacial energy between the two blocks o...

Inorganic Nanoparticle Induced Morphological Transition for Confined Self-Assembly of Block Copolymers within Emulsion Droplets.

Recently, it has been reported that the incorporation of functional inorganic nanoparticles (NPs) into the three-dimensional (3D) confined self-assembly of block copolymers (BCPs) creates the unique nanostructured hybrid composites, which can not only introduce new functions to BCPs but also induce some interesting morphological transitions of BCPs. In the current study, we systematically investigate the cooperative self-assembly of a series of size-controlled and surface chemistry-tunable gold nanoparticles (AuNPs) and polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer within the emulsion droplets. The influences of the size, content, and surface chemistry of the AuNPs on the coassembled nanostructures as well as the spatial distribution of AuNPs in the hybrid particles are examined. It is found that the size and content of the AuNPs are related to the entropic interaction, while the surface chemistry of AuNPs is related to the enthalpic interaction, which can be utilized to tailor the self-assembled morphologies of block copolymer confined in the emulsion droplets. As the content of PS-coated AuNPs increases, the morphology of the resulting AuNPs/PS-b-P2VP hybrid particles changes from the pupa-like particles to the bud-like particles and then to the onion-like particles. However, a unique morphological transition from the pupa-like particles to the mushroom-like particles is observed as the content of P4VP-coated AuNPs increases. More interestingly, it is observed that the large AuNPs are expelled to the surface of the BCP particles to reduce the loss in the conformational entropy of the block segment, which can arrange into the strings of necklaces on the surfaces of the hybrid particles.

Morphological Evolution of Block Copolymer Particles: Effect of Solvent Evaporation Rate on Particle Shape and Morphology.

Shape and morphology of polymeric particles are of great importance in controlling their optical properties or self-assembly into unusual superstructures. Confinement of block copolymers (BCPs) in evaporative emulsions affords particles with diverse structures, including prolate ellipsoids, onion-like spheres, oblate ellipsoids, and others. Herein, we report that the evaporation rate of solvent from emulsions encapsulating symmetric polystyrene-b-polybutadiene (PS-b-PB) determines the shape and internal nanostructure of micron-sized BCP particles. A distinct morphological transition from the ellipsoids with striped lamellae to the onion-like spheres was observed with decreasing evaporation rate. Experiments and dissipative particle dynamics (DPD) simulations showed that the evaporation rate affected the organization of BCPs at the particle surface, which determined the final shape and internal nanostructure of the particles. Differences in the solvent diffusion rates in PS and PB at rapid evaporation rates induced alignment of both domains perpendicular to the particle surface, resulting in ellipsoids with axial lamellar stripes. Slower evaporation rates provided sufficient time for BCP organization into onion-like structures with PB as the outermost layer, owing to the preferential interaction of PB with the surroundings. BCP molecular weight was found to influence the critical evaporation rate corresponding to the morphological transition from ellipsoid to onion-like particles, as well as the ellipsoid aspect ratio. DPD simulations produced morphologies similar to those obtained from experiments and thus elucidated the mechanism and driving forces responsible for the evaporation-induced assembly of BCPs into particles with well-defined shapes and morphologies.

Synthesis of fluorinated nanoparticles via RAFT dispersion polymerization-induced self-assembly using fluorinated macro-RAFT agents in supercritical carbon dioxide

Nano-sized fluorinated block copolymer particles are prepared by RAFT dispersion polymerization with polymerization-induced self-assembly proceeding in supercritical carbon dioxide.

Controlled spatial dispersion of CdSe tetrapod nanocrystals with amphiphilic block copolymer particles

Abstract We report, for the first time, a nanoscale control of the spatial distribution of semiconducting tetrapod (TP) nanocrystals within block copolymer particles. Polystyrene (PS) block copolymer (BCP)/CdSe TP nanocrystal hybrid particles were prepared by the nanoprecipitation of TP/BCP mixtures into methanol. The BCPs consisted of short, polar terminal block bearing methyl disulfide anchoring moiety which serves both to bind to CdSe TP surfaces as well as to drive self-assembly during nanoprecipitation into polar solvent systems. The resulting BCP/TP hybrid particles showed various spatial distribution and the number density of TPs with respect to individual polymer particles depending on the degree of polymerization of the block copolymer as well as the nature of the solvent in which nanoprecipitation was done.

RAFT-Mediated Polymerization-Induced Self-Assembly of Poly(Acrylic Acid)-b-Poly(Hexafluorobutyl Acrylate): Effect of the pH on the Synthesis of Self-Stabilized Particles.

This paper describes a very simple strategy towards self-stabilized poly(acrylic acid)-block-poly(hexafluorobutyl acrylate) (PAA-b-PHFBA) block copolymer particles via reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization-induced self-assembly. Hexafluorobutyl acrylate (HFBA) monomer conversion and number-average molar mass of PAA-b-PHFBA increased gradually with the increase in the pH value of the aqueous phase. When pH < 10, the molecular weight distributions of PAA-b-PHFBA were narrow, however, when the pH was raised to 11.55, PAA-b-PHFBA block copolymers had a broader distribution (ĐM = 1.82) with a serious trailing toward the low molecular weight. Furthermore, the morphology and size of PAA-b-PHFBA latex particles were measured by transmission electron microscopy and dynamic light scattering. The results indicated that the PAA-b-PHFBA latex particles had a clear spherical core-shell structure and the latex particles’ size increased with the increase of pH value.

Particles with Tunable Porosity and Morphology by Controlling Interfacial Instability in Block Copolymer Emulsions.

A series of porous block copolymer (BCP) particles with controllable morphology and pore sizes was fabricated by tuning the interfacial behavior of BCP droplets in oil-in-water emulsions. A synergistic adsorption of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) BCPs and sodium dodecyl sulfate (SDS) to the surface of the emulsion droplet induced a dramatic decrease in the interfacial tension and generated interfacial instability at the particle surface. In particular, the SDS concentration and the P4VP volume fraction of PS-b-P4VP were key parameters in determining the degree of interfacial instability, leading to different types of particles including micelles, capsules, closed-porosity particles, and open-porosity particles with tunable pore sizes ranging from 10 to 500 nm. The particles with open-porosity could be used as pH-responsive, high capacity delivery systems where the uptake and release of multiple dyes could be achieved.

Engineering the Shape of Block Copolymer Particles by Surface-Modulated Graphene Quantum Dots

Surface-engineered, 10 nm-sized graphene quantum dots (GQDs) are shown to be efficient surfactants for producing 3-pentadecyl phenol (PDP)-combined poly(styrene-b-4-vinylpyridine) (PS-b-P4VP(PDP)) particles that feature tunable shapes and internal morphologies. The surface properties of GQDs were modified by grafting different alkyl ligands, such as hexylamine and oleylamine, to generate the surfactant behavior of the GQDs. In stark contrast to the behavior of the unmodified GQDs, hexylamine-grafted GQDs and oleylamine-grafted GQD surfactants were selectively positioned on the PS and P4VP(PDP) domains, respectively, at the surface of the particles. This positioning effectively tuned the interfacial interaction between two different PS/P4VP(PDP) domains of the particles and the surrounding water during emulsification and induced a dramatic morphological transition to convex lens-shaped particles. Precise and systematic control of interfacial activity of GQD surfactants was also demonstrated by varying the ...

Toughening and Mechanical Properties of Epoxy Modified with Block Co-polymers and MWCNTs

The objective of this work was to systematically develop and understand novel polymeric hybrid nanocomposites that include block copolymers (BCP) with tailored morphologies in order to generate high toughness. Furthermore, rigid fillers in the form of multi-walled carbon nanotubes (MWCNT) were added systematically together with block copolymers to study the combined effect of rigid nanofillers and more ductile BCP particles. The resulting matrix was extensively and carefully characterized by standard methods. This included thorough characterization of mechanical, fracture mechanical and thermal properties. Results show that both fracture toughness, KIc, and critical energy release rate, GIc, were increased linearly to a maximum of 2.10 MPa.m1/2 and 1.46 kJ/m2 respectively by the addition of 12 wt. % BCP. Fractography studies reveal toughening mechanisms of the nanocomposites that were identified as both the cavitation of spherical micelles and enhanced plastic deformation and furthermore fiber pull-out in the case of hybrid nanocomposites.

Block copolymer synthesis by controlled/living radical polymerisation in heterogeneous systems.

Nanostructured soft materials open up new opportunities in material design and application, and block copolymer self-assembly is one particularly powerful phenomenon that can be exploited for their synthesis. The advent of controlled/living radical polymerisation (CLRP) has greatly simplified block copolymer synthesis, and versatility towards monomer types and polymer architectures across the different forms of CLRP has vastly expanded the range of functional materials accessible. CLRP-controlled synthesis of block copolymers has been applied in heterogeneous systems, motivated by the numerous process advantages and the position of emulsion polymerisation at the forefront of industrial latex synthesis. In addition to the inherent environmental advantages of heterogeneous routes, the incidence of block copolymer self-assembly within dispersed particles during polymerisation leads to novel nanostructured materials that offer enticing prospects for entirely new applications of block copolymers. Here, we review the range of block copolymers prepared by heterogeneous CLRP techniques, evaluate the methods applied to maximise purity of the products, and summarise the unique nanoscale morphologies resulting from in situ self-assembly, before discussing future opportunities within the field.

Phase Separated Block Copolymer Particles with Tuneable Morphologies: Striped, Onion, and Patchy Particles

The aqueous solution self-assembly of a series of poly(2-vinyl pyridine)-block-poly(dimethylsiloxane) (P2VP-b-PDMS) diblock copolymers is reported. The particles show a phase separated interior morphology which can be tuned from onion-like sphere to axially stacked lamella with dimensions in the nanoscale. The key to this transition is the control of the interfacial properties using a mixture of two surfactants with preferential wetting to the respective blocks (P2VP or PDMS). Influence of block weights on particle morphology was investigated with results showing that the volume fraction of PDMS plays a crucial role in self-assembly, and a non-conventional structure of a prolate spheroid nanoparticle with a unique phase separated interior morphology can be synthesised.

Additives Induced Structural Transformation of ABC Triblock Copolymer Particles.

Here we report the structural control of polystyrene-b-polyisoprene-b-poly(2-vinylpyridine) (PS-b-PI-b-P2VP) asymmetric ABC triblock copolymer particles under 3D confinement by tuning the interactions among blocks. The additives, including 3-n-pentadecylphenol, homopolystyrene, and solvents, which can modulate the interactions among polymer blocks, play significant roles in the particle morphology. Moreover, the structured particles can be disassembled into isolated micellar aggregates with novel morphologies or mesoporous particles with tunable pore shape. Interestingly, the formed pupa-like PS-b-PI-b-P2VP particles display interesting dynamic stretch-retraction behavior when the solvent property is changed after partial cross-linking of the P2VP block. We further prove that such dynamic behavior is closely related to the density of cross-linking. The strategies presented here are believed to be promising routes to rationally design and fabricate block copolymer particles with desirable shape and internal structure.

Monodipserse Nanostructured Spheres of Block Copolymers and Nanoparticles via Cross-Flow Membrane Emulsification

Monodisperse colloidal particles of polystyrene-b-polybutadiene (PS-b-PB) block copolymers (BCPs) were successfully prepared, in which uniform emulsions containing BCPs were first generated by cross-flow membrane emulsification using tubular Shirasu porous glass (SPG) membrane, and then unique internal nanostructures were developed by controlled evaporation of solvent inside emulsion. The diameter of those BCP particles could be controlled from 200 nm to 5 μm by tuning the pore diameter of the membrane. With symmetric BCPs, onion-like nanostructures inside particles were formed. Coiled cylinders in the BCP particles were also developed by adding homopolymers, in which the assembled BCP structure is strongly dependent on the particle size, demonstrating the importance of our membrane method in generating monodisperse BCP particles. Further investigation of process parameters showed that for a given pore diameter, the operation pressure (P) and surfactant concentration were critical parameters for narrow si...

Nanostructured Colloidal Particles by Confined Self-Assembly of Block Copolymers in Evaporative Droplets

Block copolymers (BCPs) can create various morphology by self-assembly in bulk or film. Recently, using BCPs in confined geometries such as thin film (one-dimension), cylindrical template (two-dimension), or emulsion droplet (three-dimension), nanostructured BCP particles have been prepared, in which unique nanostructures of the BCP are formed via solvent annealing process and can be controlled depending on molecular weight ratio and interaction parameter of the BCPs, and droplet size. Moreover, by tuning interfacial property of the BCP particles, anisotropic particles with unique nanostructures have been prepared. Furthermore, for practical application such as drug delivery system, sensor, self-healing, metamaterial, and optoelectronic device, functional nanoparticles can be incorporated inside BCP particles. In this article, we summarize recent progress on the production of structured BCP particles and composite particles with metallic nanoparticles.

Guidelines for the Synthesis of Block Copolymer Particles of Various Morphologies by RAFT Dispersion Polymerization.

This article presents the recent developments of radical dispersion polymerizaton controlled by reversible addition fragmentation chain transfer (RAFT) for the production of block copolymer particles of various morphologies, such as core-shell spheres, worms, or vesicles. It is not meant to be an exhaustive review but it rather provides guidelines for non-specialists. The article is subdivided into eight sections. After a general introduction, the mechanism of polymerization-induced self-assembly (PISA) through RAFT-mediated dispersion polymerization is presented and the different parameters that control the morphology produced are discussed. The next two sections are devoted to the choice of the monomer/solvent pair and the macroRAFT agent. Afterwards, post-polymerization morphological order-to-order transitions (i.e. morphological transitions triggered by extrinsic stimuli) or order-to-disorder transitions (i.e. disassembly of chains) are discussed. Assemblies based on more complex polymer architectures, such as triblock copolymers, are presented next, and finally the possibility to stabilize these structures by crosslinking is reported. The manuscript ends with a short conclusion and an outlook.

Dinonylphenyl end-capped poly(ethylene glycol)-b-polystyrene: synthesis and its unusual crystalline and self-assembly behaviors

Block copolymers, dinonylphenyl end-capped polyethylene glycol-b-polystyrene (DNPE-PEO-b-PSs) were synthesized in a one-step atom transfer radical polymerization (ATRP) of styrene. The PEO block in the DNPE-PEO-b-PS samples (volume fraction of PS: 66.8–93.2 %) was found to be amorphous, which contrasts with DNPE-PEO precursor, traditional methoxide end-capped polyethylene glycol-b-polystyrene (PEO-b-PS) and dinonylphenyl end-capped poly-(ethylene glycol)-b-poly(fluorinated methyl methacrylate)(DNPE-PEO-b-PFMAs). Meanwhile, DNPE-PEO-b-PSs display an intriguing self-assembly behavior in solution. Block copolymer particles with mesoporous internal structures are directly formed by self-assembly of DNPE-PEO-b-PS in tetrahydrofuran/water solutions. It is proposed that DNPE end group has an important effect on the crystallization of the block copolymers as well as their self-assembly behaviors in solution.

Highly Luminescent Polymer Particles Driven by Thermally Reduced Graphene Quantum Dot Surfactants.

We report the use of highly luminescent graphene quantum dots (GQDs) as efficient surfactants to produce Pickering emulsions and novel polymer particles. To generate the GQD surfactants, the surface properties of 10 nm sized, non-reduced GQDs (nGQDs), which have strong hydrophilicity, were synthesized and modified in a systematic manner by the thermal reduction of oxygen-containing groups at different treatment times. In stark contrast to the behavior of the nGQDs, thermally reduced GQDs (rGQDs) can produce highly stable Pickering emulsions of oil-in-water systems. To demonstrate the versatility of the rGQD surfactants, they were applied in a mini-emulsion polymerization system that requires nanosized surfactants to synthesize submicron-sized polystyrene particles. In addition, the use of rGQD surfactants can be extended to generating block copolymer particles with controlled nanostructures. Particularly, the polymer particles were highly luminescent, a characteristic produced by the highly fluorescent GQD surfactants, which has great potential for various applications, including bioimaging, drug delivery, and optoelectronic devices. To the best of our knowledge, this is the first report in which nanosized GQDs were used as surfactants.

Aspect ratio effect of nanorod surfactants on the shape and internal morphology of block copolymer particles

The effect of aspect ratio (AR) of nanorod surfactants on the shape and internal morphology of block copolymer (BCP) particles was systemically investigated using CuPt nanorods with different AR values ranging from 1 to 15. The positioning of the nanorods in the BCP particles was dependent strongly on their AR values, and it enabled the modulation of the interfacial properties at selective locations on the BCP particles, thus producing interesting morphological transition of the BCP particles.

Biomimetic block copolymer particles with gated nanopores and ultrahigh protein sorption capacity

The design of micro- or nanoparticles that can encapsulate sensitive molecules such as drugs, hormones, proteins or peptides is of increasing importance for applications in biotechnology and medicine. Examples are micelles, liposomes and vesicles. The tiny and, in most cases, hollow spheres are used as vehicles for transport and controlled administration of pharmaceutical drugs or nutrients. Here we report a simple strategy to fabricate microspheres by block copolymer self-assembly. The microsphere particles have monodispersed nanopores that can act as pH-responsive gates. They contain a highly porous internal structure, which is analogous to the Schwarz P structure. The internal porosity of the particles contributes to their high sorption capacity and sustained release behaviour. We successfully separated similarly sized proteins using these particles. The ease of particle fabrication by macrophase separation and self-assembly, and the robustness of the particles makes them ideal for sorption, separation, transport and sustained delivery of pharmaceutical substances.

Reversible Transformation of Nanostructured Polymer Particles

A reversible transformation of overall shape and internal structure as well as surface composition of nanostructured block copolymer particles is demonstrated by solvent-adsorption annealing. Polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) pupa-like particles with PS and P4VP lamellar domains alternatively stacked can be obtained by self-assembly of the block copolymer under 3D soft confinement. Chloroform, a good solvent for both blocks, is selected to swell and anneal the pupa-like particles suspended in aqueous media. Reversible transformation between pupa-like and onion-like structures of the particles can be readily tuned by simply adjusting the particle/aqueous solution interfacial property. Interestingly, poly(vinyl alcohol) (PVA) concentration in the aqueous media plays a critical role in determining the particle morphology. High level of PVA concentration is favorable for pupa-like morphology, while extremely low concentration of PVA is favorable for the formation of onion-like particles. Moreove...