Block Copolymer Aggregates Research Articles

In My Element: Selenium

<i>In My Element</i>: Selenium

Synthesis, aggregation and responsivity of block copolymers containing organic arsenicals

Block copolymers containing an organic arsenical (AsAm) have been synthesised by aqueous SET-LRP.

Semi-crystalline polymethylene-b-poly(acrylic acid) diblock copolymers in selective solutions: Morphological and crystallization evolution dependent on calcium chloride

Although crystallization-driven self-assembly (CDSA) of semi-crystalline block copolymers (BCPs) affords one of the most promising approaches to tailor novel asymmetric nanostructures, crystalline BCPs’ response to external interactive additives remains less understanding. In the present work, the interactive disturbance effect of CaCl2 additive on CDSA of semi-crystalline polymethylene-b-poly(acrylic acid) diblock copolymers (PM-b-PAA) was systematically studied. It was found that pre-formed crystalline disklike BCP micelles obtained from CDSA could be drawn closely and bridged by Ca2+ cations, leading to merging and crystallization of PM cores as well as BCP morphological evolution. Particularly, an increasing epitaxial crystallization together with morphological transition from disklike BCP micelles to worm-like micelles, incomplete networks, and reticulate aggregates was successively observed as the stoichiometric ratio of [Ca2+]/[COOH] increased from 0 to 2. The complexation between Ca2+ and ionized carboxylic groups on the PAA segments was evidenced by Fourier transform infrared (FTIR) measurement, and could be tuned by manipulating solvent quality, solution pH, and block composition. In addition to interactive disturbance effect, pronounced saltingout effects, displayed at excessive amount of CaCl2 salt, resulted in dehydration and serious aggregation of BCPs.

Shape transition in ABC triblock copolymer micelles complexed with SDS through quaternized polyvinyl pyridine central block

Polystyrene-b-poly-2-vinylpridine-b-polyethyleneoxide (PS13-PVPy62-PEG47) block copolymer forms spherical core-shell-corona micelles (~50 nm) in water. At acidic pH~3, the central PVPy block gets quaternized (hydrophilic) and gets complexed with sodium dodecyl sulfate (SDS), which strongly interacts with PVPy+ electrostatically as reflected from marked increase in turbidity. The charge neutralization of the central block and accordingly favored hydrophobicity induces interesting structural transitions. Cryo-TEM images for micellar solution of block copolymer in the presence of 10 mM SDS at pH~3 show presence of micelles of ribbon type and threadlike morphology. SANS measurements support the micelle growth and shape transition for the block copolymer aggregates as fitted by core-shell model.

PEE–PEO Block Copolymer Exchange Rate between Mixed Micelles Is Detergent and Temperature Activated

We examine the kinetics of polymer chain exchange between mixed block copolymer–detergent micelles, a system relevant to the synthesis of protein-containing biomimetic membranes. While chain exchange between block copolymer (BCP) aggregates in water is too slow to observe, and detergent molecules exchange between micelles on a time scale of nanoseconds to microseconds, BCP chains exchange between mixed detergent–polymer micelles on intermediate time scales of many minutes to a few days. We examine a membrane-protein-relevant, vesicle-forming, ultrashort polymer, poly(ethyl ethylene)20-poly(ethylene oxide)18 (PEE20–PEO18). PEE20–PEO18 was solubilized in mixed micelles with the membrane protein compatible nonionic detergent octyl-β-d-glucoside (OG). Using cryo-TEM and small-angle neutron scattering (SANS), we demonstrate complete solubilization of the polymer into micelles. Using time-resolved SANS (TR-SANS), we provide the first direct evidence that detergents activate BCP chain exchange and determine kine...

Solubilization of Phenol Derivatives in Polymer Micelles Formed by Cationic Block Copolymer

The aggregation of cationic block copolymers formed by polystyrene (PS) and poly(ethyl-4-vinylpyridine) (PS-b-PE4VP) was studied in aqueous solution. Diblock copolymers of PS and poly(4-vinylpyridine) were synthesized by sequential anionic polymerization using BuLi as initiator. Subsequently, the 4-vinylpyridine units were quaternized with ethyl bromide to obtain cationic PS-b-PE4VP block copolymers with different quaternization degree. The self-aggregation of cationic block copolymers was studied by fluorescence probing, whereas the morphology and size of polymer micelles were determined by transmission electronic microscopy. Results indicate that spherical micelles with sizes lower than 100 nm were formed, whereas their micropolarity decreases with increasing quaternization degree. The partition of phenols between the micellar and aqueous phase was studied by using the pseudo-phase model, and the results show that the partition coefficients increase with increasing length of the side alkyl chain and are larger for star micelles. These results are discussed in terms of three-region model.

Photoresponsive amphiphilic block macrocycles bearing azobenzene side chains

The cyclic architecture has an impact on the photoisomerization and packing behavior of micellar aggregates of amphiphilic block copolymers bearing pendant azobenzene and carboxyl groups as compared to their linear counterparts.

Drug governs the morphology of polyalkylated block copolymer aggregates.

Polyalkylated copolymers based on mPEG-b-(AGE-C6,12 or 18)25 have been used to formulate clinically relevant concentrations of doxorubicin (DOX) and the impact of drug incorporation on copolymer aggregation behaviour was examined. The copolymer aggregates were analyzed by various microscopy techniques (TEM, cryo-TEM and AFM) and scattering methods (SANS, DLS). In the absence of the drug, the copolymers formed largely non-spherical aggregates (i.e. cylinders, vesicles). Drug incorporation during copolymer aggregate formation directed the formation of only spherical aggregates. As well, the nature of the core-forming block was found to influence drug release and cytotoxicity of the formulations.

Controlling the melting transition of semi-crystalline self-assembled block copolymer aggregates: controlling release rates of ibuprofen

Thermoresponsive bicontinuous and multi-lamellar micelles were self-assembled from poly[ethylene oxide]-block-(poly[octadecyl methacrylate]-random-poly[docosyl methacrylate]).

Constructing a molecular theory of self-assembly: Interplay of ideas from surfactants and block copolymers

Low molecular weight surfactants and high molecular weight block copolymers display analogous self-assembly behavior in solutions and at interfaces, generating nanoscale structures of different shapes. Understanding the link between the molecular structure of these amphiphiles and their self-assembly behavior has been the goal of theoretical studies. Despite the analogies between surfactants and block copolymers, models predicting their self-assembly behavior have evolved independent of one another, each overlooking the molecular feature considered critical to the other. In this review, we focus on the interplay of ideas pertaining to surfactants and block copolymers in three areas of self-assembly. First, we show how improved free energy models have evolved by applying ideas from surfactants to block copolymers and vice versa, giving rise to a unitary theoretical framework and better predictive capabilities for both classes of amphiphiles. Second we show that even though molecular packing arguments are often used to explain aggregate shape transitions resulting from self-assembly, the molecular packing considerations are more relevant in the case of surfactants whereas free energy criteria are relevant for block copolymers. Third, we show that even though the surfactant and block copolymer aggregates are small nanostructures, the size differences between them is significant enough to make the interfacial effects control the solubilization of molecules in surfactant micelles while the bulk interactions control the solubilization in block copolymer micelles. Finally, we conclude by identifying recent theoretical progress in adapting the micelle model to a wide variety of self-assembly phenomena and the challenges to modeling posed by emerging novel classes of amphiphiles with complex biological, inorganic or nanoparticle moieties.

Graphene Oxide and Pluronic Copolymer Aggregates–Possible Route to Modulate the Adsorption of Fluorophores and Imaging of Live Cells

In recent years, amphiphilic triblock copolymers have attracted increasing attention due to their tunable properties and biocompatible nature, and the degree of hydrophobicity of these block copolymers can be modulated by varying the hydrophobic poly(propylene oxide) (PPO) blocks and hydrophilic poly(ethylene oxide) (PEO) moieties. Beside these, compared to the conventional micelles, block copolymer aggregates are more heterogeneous. For this reason, we have chosen two different fluorophores with different hydrophobicity so that we can monitor the different regions into the aggregates. We have shown the effect of theses Pluronic block copolymer aggregates on the adsorption of two fluorophores on the graphene oxide (GO) surface. The PPO segment of the block copolymer strongly interact with the hydrophobic basal plane of GO. Thus, in the presence of these aggregates the interaction between the GO and fluorophores is restricted depending on their location into the aggregates. The adsorption of the fluorophor...

Insights into Noncovalently Core Cross-Linked Block Copolymer Micelles by Fluorescence Resonance Energy Transfer (FRET) Studies

Amphiphilic polymers have emerged as an important class of materials owing to their ability to produce a diverse range of self-assembled structures with container properties that can be used to address growing challenges in biomedical applications. Thus, in-depth understanding on their aggregation properties is important fundamentally as well as from an application viewpoint. In this article we describe utilization of fluorescence resonance energy transfer (FRET) as a powerful tool to elucidate various physical properties of amphiphilic block copolymer aggregates at very low polymer concentration (∼10–7 M) which otherwise are difficult to achieve using other commonly used techniques such as microscopy, scattering, or external probe based spectroscopic techniques. We synthesized a prepolymer based on PEO-b-PMMA-co-PHEMA and subsequently utilized the hydroxy groups of the HEMA units to covalently attach with either a green (D) or a red (A) fluorescent dye (D–A pair suitable for FRET) to get D- and A-labeled...

Cytosol-Specific Fluorogenic Reactions for Visualizing Intracellular Disintegration of Responsive Polymeric Nanocarriers and Triggered Drug Release

Supramolecular aggregates of stimuli-responsive block copolymers are increasingly utilized as drug nanocarriers. Although in situ tracking their triggered disintegration and drug release processes at the cellular level is highly desirable, it remains a considerable challenge. We report the fabrication of double hydrophilic block copolymers covalently conjugated with α,β-unsaturated ketone-caged coumarin functionalities in the thermoresponsive block. Upon thermo-induced micellization and cellular uptake, Michael addition reaction of unsaturated ketone moieties with thiol compounds (GSH and Cys) in the reductive subcellular compartments leads to micelle-to-unimer transition. This is accompanied by concomitant fluorescence emission turn-on and triggered drug release, allowing for the process visualization.

Polymerization-induced thermal self-assembly (PITSA).

Polymerization-induced self-assembly (PISA) is a versatile technique to achieve a wide range of polymeric nanoparticle morphologies. Most previous examples of self-assembled soft nanoparticle synthesis by PISA rely on a growing solvophobic polymer block that leads to changes in nanoparticle architecture during polymerization in a selective solvent. However, synthesis of block copolymers with a growing stimuli-responsive block to form various nanoparticle shapes has yet to be reported. This new concept using thermoresponsive polymers is termed polymerization-induced thermal self-assembly (PITSA). A reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from a hydrophilic chain transfer agent composed of N,N-dimethylacrylamide and acrylic acid was carried out in water above the known lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAm). After reaching a certain chain length, the growing PNIPAm self-assembled, as induced by the LCST, into block copolymer aggregates within which dispersion polymerization continued. To characterize the nanoparticles at ambient temperatures without their dissolution, the particles were crosslinked immediately following polymerization at elevated temperatures via the reaction of the acid groups with a diamine in the presence of a carbodiimide. Size exclusion chromatography was used to evaluate the unimer molecular weight distributions and reaction kinetics. Dynamic light scattering and transmission electron microscopy provided insight into the size and morphologies of the nanoparticles. The resulting block copolymers formed polymeric nanoparticles with a range of morphologies (e.g., micelles, worms, and vesicles), which were a function of the PNIPAm block length.

Control of morphology and corona composition in aggregates of mixtures of PS-b-PAA and PS-b-P4VP diblock copolymers: effects of solvent, water content, and mixture composition.

The morphologies and corona compositions in aggregates of mixtures of PS-b-PAA and PS-b-P4VP diblock copolymers are influenced by controllable assembly parameters such as water content, block copolymer molar ratios, and solvent effects as well as the hydrophilic block lengths and block length ratios. All these factors can affect the morphology of the aggregates as well as their corona composition, the latter especially in vesicles, where two interfaces are involved. The morphologies and corona compositions of the aggregates were investigated by transmission electron microscopy and electrophoretic mobility, respectively. They depend, to a large extent, on the solubility of P4VP and PAA in the given organic solvent (e.g., DMF, THF, or dioxane), which influences the coil dimensions of the hydrophilic chains. The water content affects both the size and the shape of the block copolymer aggregates as well as the corona composition. Water acts as a precipitant for the hydrophobic block in the common solvent and, therefore, its progressive addition to the solution changes the interaction parameter with the hydrophobic block. The block copolymer molar ratio has an effect on both the morphology and the corona composition of the aggregates. With increasing PS-b-P4VP content in the mixture, the morphology transforms gradually from large compound micelles (LCMs), through coexistence of LCMs and small spherical micelles (SSMs), and eventually to vesicles. As expected, the corona composition of the aggregates is also affected by the block copolymer molar ratio, and changes progressively from pure PAA to a mixture of PAA and P4VP and to pure P4VP with increasing PS-b-P4VP content. It is clear that the use of mixtures of the soluble chains offers the opportunity of fine-tuning the corona composition in block copolymer aggregates under assembly conditions.

Aggregation of polythiophene homopolymer and block copolymer in solution utilizing the characteristics of pyridine at the side chain

ABSTRACTPoly[3‐(5′‐hexylpyridine‐2′‐yl)thiophene] (P3PT) (Mn = 13900, H‐T content = 90%) was prepared by the regioselective Grignard metathesis reaction and the subsequent Kumada coupling polymerization. Likewise, poly(3‐hexylthiophene)‐b‐poly[3‐(5′‐hexylpyridine‐2′‐yl)thiophene] (P3HT‐b‐P3PT) (Mn = 17,300) was synthesized in the one‐pot and successive monomer addition protocol, in which the segment ratio was calculated to be 56 (P3HT)/44 (P3PT) base on the 1H NMR spectrum. The absorption and emission spectra of homopolymer P3PT(H), obtained by the protonation of the pyridine nitrogen, in THF/cyclohexane shifted to the longer wavelength as compared with those collected in THF, suggesting the aggregation in poor solvent. The aggregation of P3PT induced by the addition of Sc(OTf)3 could be controlled by the molar ratio of pyridine and scandium complex. The protonated block copolymer P3HT‐b‐P3PT(H) was also subjected to the aggregate formation. The absorption maximum in THF/CH3OH showed a bathochromic shift and the fluorescence emission was almost quenched. From the 1H NMR spectra and DLS measurements, P3HT‐b‐P3PT(H) forms nanometer scale aggregates particularly with the insolubility and stacking of non‐ionic P3HT in alcohol as the driving force. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3383–3389

Block copolymers in the synthesis of gold nanoparticles. Two new approaches: Copolymer aggregates as reductants and stabilizers and simultaneous formation of copolymer aggregates and gold nanoparticles

ABSTRACTIn this article, innovative applications of amphiphilic triblock and pentablock copolymers in the synthesis of gold nanoparticles are reported. The synthesis of gold nanoparticles is performed using two methods. In the first method, micellar aggregates of block copolymers and AuCl4− ions directly react in water; the nanoparticles obtained by this method are variable in size and are associated with copolymer aggregates. In the second method, two processes take place simultaneously: the aggregation of block copolymers and the reduction of Au (III) by the copolymers to form nanoparticles. In contrast with the first method, in this case, the nanoparticles obtained are located inside the copolymer aggregates. In both methods of synthesis, the block copolymers act simultaneously as reducing and stabilizing agents. To understand the role of copolymer aggregates in the synthesis of nanoparticles, molecular simulation methods are used. The gold nanoparticles, copolymer aggregates, and nanocomposite systems are characterized using transmission electron microscopy and dynamic light scattering. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3069–3079

Self-organization and aggregation of amphiphilic block copolymers of N-vinylpyrrolidone—block-2,2,3,3-tetrafluoropropylmethacrylate at interfaces

The physicochemical characteristics of monolayer formation of amphiphilic block copolymers of N-vinylpyrrolidone—block-2,2,3,3-tetrafluoropropylmethacrylate (PVP—block-PFMA) with different molecular weights (Mw) at the water-air interface and on the solid support (quartz, silicon) were studied. It was shown by UV and IR spectroscopy, analysis of the θ—S compression isotherms, and imaging by atomic force microscopy (AFM) that the state of PVP—block-PFMA monolayers on the aqueous subphase and solid support is determined by the ratio of Mw of comonomer blocks Mw(PFMA)/Mw(PVP), the aggregation of copolymers in solution and in the surface layer (surface micellation and phase isolation). Gibb’s surface energy of the films calculated in the Wendt—Good—Kaelbly—Dan—Fowkes approximation increases up to 30–33 mJ m−2 from 21 mJ m−2 (PFMA). The values of adhesion work Wadh(monolayer—water droplet—air) and Wadh(monolayer—water in octanol) and AFM imaging characterize the lipophilicity and surface topology of the films as optimal for thromboresistant materials.

Spherical Blackberry-type Capsules Containing Block Copolymer Aggregates

The design, preparation, and properties of nanosized blackberry-like structures are described. These capsules are composed of two layers of individual block copolymer aggregates, relatively large core vesicles onto which is deposited a layer of smaller vesicles or micelles. The composition of the adjacent layers is such as to ensure strong electrostatic interactions between them. The core vesicles are typically composed of either PS-b-P4VP with a positively charged corona or of PS-b-PAA with a negatively charged corona, and are surrounded by a layer of smaller, oppositely charged block copolymer vesicles or micelles. These composite structures bear a strong resemblance to blackberries, hence the proposed name. The blackberry structures can be prepared in solution or on a flat surface, for example, a silicon wafer. Four compositional possibilities for the blackberries structures were studied, in which the positively or negatively charged core vesicles are covered either by a layer of oppositely charged micelles or by vesicles. These structures represent the earliest stage of a layer-by-layer approach of small spherical aggregates onto a larger spherical hollow core. The strong interaction between the contacting layers is achieved by electrostatic interactions or by complementary acid-base properties, for example, H-bonding. These multicompartmented capsules could be used potentially as delivery vehicles for multiple components; each layer of the capsules could be loaded with hydrophobic (in the core of the micelles or vesicle wall) or hydrophilic molecules (in the vesicle cavity). The overall size of such structures can vary, but in any case can be kept below 1 μm.

Planar Multilayer Assemblies Containing Block Copolymer Aggregates

The design, preparation, and properties of planar multilayer structures composed of various combinations of sequentially deposited polyelectrolyte (PE) chains and self-assembled layers of individual block copolymer aggregates (vesicles, micelles, or large compound micelles (LCMs)) are described. The aggregates contain negatively or positively charged corona chains while the PE multilayers contain alternating polyanionic or polycationic chains deposited on silicon wafers. The final structures consist of combinations of layers of various charged species: multilayers of alternating PEs of poly(allyl hydrochloride) (PAH) and poly(acrylic acid) (PAA) as well as vesicles, micelles, or large compound micelles of ionized poly(styrene)-b-poly(4-vinylpyridine) (PS-b-P4VP) or of poly(styrene)-b-poly(acrylic acid) (PS-b-PAA). Two types of layer-by-layer (LbL) multilayer structures were studied: individual aggregate layers sandwiched between PE multilayers and layers of individual aggregates of various morphologies and of different corona chain charges, deposited on top of each other without intermediate multilayers or individual layers of PEs. The strong interactions between the successive layers are achieved mainly by electrostatic attraction between the oppositely charged layers. The planar LbL multilayers containing block copolymer aggregates could, potentially, be used as carriers for multiple functional components; each aggregate layer could be loaded with hydrophobic (in the core of the micelles, LCMs, or vesicle walls) or hydrophilic functional molecules (in the vesicular cavities). The overall thickness of such planar LbL multilayers can be controlled precisely and can vary from tens of nanometers to several micrometers depending on the number of layers, the sizes of the aggregates, and the complexity of the structure.