Polymerization-induced self-assembly and disassembly during the synthesis of thermoresponsive ABC triblock copolymer nano-objects in aqueous solution

Polymerization-induced self-assembly (PISA) has been widely utilized as a powerful methodology for the preparation of various self-assembled AB diblock copolymer nano-objects in aqueous media. Moreover, it is well-documented that chain extension of AB diblock copolymer vesicles using a range of hydrophobic monomers via seeded RAFT aqueous emulsion polymerization produces framboidal ABC triblock copolymer vesicles with adjustable surface roughness owing to microphase separation between the two enthalpically incompatible hydrophobic blocks located within their membranes. However, the utilization of hydrophilic monomers for the chain extension of linear diblock copolymer vesicles has yet to be thoroughly explored; this omission is addressed for aqueous PISA formulations in the present study. Herein poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (G-H) vesicles were used as seeds for the RAFT aqueous dispersion polymerization of oligo(ethylene glycol) methyl ether methacrylate (OEGMA). Interestingly, this led to polymerization-induced disassembly (PIDA), with the initial precursor vesicles being converted into lower-order worms or spheres depending on the target mean degree of polymerization (DP) for the corona-forming POEGMA block. Moreover, construction of a pseudo-phase diagram revealed an unexpected copolymer concentration dependence for this PIDA formulation. Previously, we reported that PHPMA-based diblock copolymer nano-objects only exhibit thermoresponsive behavior over a relatively narrow range of compositions and DPs (see Warren et al., Macromolecules, 2018, 51, 8357–8371). However, introduction of the POEGMA coronal block produced thermoresponsive ABC triblock nano-objects even when the precursor G-H diblock copolymer vesicles proved to be thermally unresponsive. Thus, this new approach is expected to enable the rational design of new nano-objects with tunable composition, copolymer architectures and stimulus-responsive behavior.

Chain extension of linear AB diblock copolymer vesicles by seeded RAFT aqueous dispersion polymerization using a hydrophilic monomer C leads to polymerization-induced disassembly to form lower-order thermoresponsive ABC triblock copolymer nano-objects.     

ATRP of 3-(triethoxysilyl)propyl methacrylate and preparation of “stable” gelable block copolymers

ATRP of a gelable monomer, 3-(triethoxysilyl)propyl methacrylate (TESPMA), mediated by CuBr/N,N,N’,N’’,N”-pentamethyldiethylenetriamine (PMDETA) using ethyl 2-bromoisobutyrate (2-EBiB) as initiator was studied. The results indicate that polymerization follows the first-order kinetic. PolyTESPMA (PTESPMA) is much more stable to moisture which is important for exploring the properties of its block copolymer. A series of PEO-b-PTESPMA block copolymers with different composition were prepared. Self-assembly of PEO-b-PTESPMA has also been explored in a mixture of methanol and water and polymeric vesicles have been obtained. By introducing the gelation catalyst, the block copolymer vesicles can be stabilized by the silica networks.     

Hybridization of poly(4-vinyl pyridine)-b-polystyrene-b-poly(4-vinyl pyridine) aggregates in dioxane/water solution

Self-assembly of binary blends of two triblock copolymers of poly(4-vinyl pyridine)-b-polystyrene-b-poly(4-vinyl pyridine), i.e., P4VP₄₃-b-PS₂₆₀-b-P4VP₄₃ (P1) and P4VP₄₃-b-PS₃₆₆-b-P4VP₄₃ (P2), in dioxane/water solution was studied. These two triblock copolymers individually tend to form vesicles (P2) and cylindrical micelles (P1) in dilute solution. It was found that copolymer components in the blend, sample preparation method, and annealing time had significant effect on hybridization aggregate morphology. By increasing P1 content in the copolymer blends, fraction of looped and stretched cylinders increased, while fraction of bilayers decreased. Nearly no bilayer was observed when P1 content was above 85 wt%. On the other hand, fraction of cylinders decreased while fraction of bilayers increased with the increase of P2 content in copolymer blends. Lamellar structures were obtained, when P2 content was 60 wt% in the copolymer blends, whereas cylinders were seldom found when P2 content was above 80 wt%. These results indicate that P1 and P2 copolymer molecules cooperatively participate in the formation of cylinders and vesicles. Some exotic structures, such as lamellae with protruding cylinders (LPC), incomplete vesicles with protruding cylinders (VPC), and cylindrical bilayers, have been kinetically trapped. These structures may result from intramicellar fusion processes in cylindrical micelles. The striking structures represent a compromise between bilayer and cylindrical geometries.     

Silica nanoparticle-loaded thermoresponsive block copolymer vesicles: a new post-polymerization encapsulation strategy and thermally triggered release

A thermoresponsive amphiphilic diblock copolymer that can form spheres, worms or vesicles in aqueous media at neutral pH by simply raising the dispersion temperature from 1 °C (spheres) to 25 °C (worms) to 50 °C (vesicles) is prepared via polymerization-induced self-assembly (PISA). Heating such an aqueous copolymer dispersion from 1 °C up to 50 °C in the presence of 19 nm glycerol-functionalized silica nanoparticles enables this remarkable ‘shape-shifting’ behavior to be exploited as a new post-polymerization encapsulation strategy. The silica-loaded vesicles formed at 50 °C are then crosslinked using a disulfide-based dihydrazide reagent. Such covalent stabilization enables the dispersion to be cooled to room temperature without loss of the vesicle morphology, thus aiding characterization and enabling the loading efficiency to be determined as a function of both copolymer and silica concentration. Small-angle X-ray scattering (SAXS) analysis indicated a mean vesicle membrane thickness of approximately 20 ± 2 nm for the linear vesicles and TEM studies confirmed encapsulation of the silica nanoparticles within these nano-objects. After removal of the non-encapsulated silica nanoparticles via multiple centrifugation–redispersion cycles, thermogravimetric analysis indicated that vesicle loading efficiencies of up to 86% can be achieved under optimized conditions. Thermally-triggered release of the silica nanoparticles is achieved by cleaving the disulfide bonds at 50 °C using tris(2-carboxyethyl)phosphine (TCEP), followed by cooling to 20 °C to induce vesicle dissociation. SAXS is also used to confirm the release of silica nanoparticles by monitoring the disappearance of the structure factor peak arising from silica–silica interactions.

A loading efficiency of up to 86% is achieved for silica nanoparticles encapsulated within crosslinkable redox-sensitive thermoresponsive diblock copolymer vesicles in water at 50 °C; triggered release is also demonstrated for this system.     

In situ SAXS studies of a prototypical RAFT aqueous dispersion polymerization formulation: monitoring the evolution in copolymer morphology during polymerization-induced self-assembly

Small-angle X-ray scattering (SAXS) is used to characterize the in situ formation of diblock copolymer spheres, worms and vesicles during reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate at 70 °C using a poly(glycerol monomethacrylate) steric stabilizer. ¹H NMR spectroscopy indicates more than 99% HPMA conversion within 80 min, while transmission electron microscopy and dynamic light scattering studies are consistent with the final morphology being pure vesicles. Analysis of time-resolved SAXS patterns for this prototypical polymerization-induced self-assembly (PISA) formulation enables the evolution in copolymer morphology, particle diameter, mean aggregation number, solvent volume fraction, surface density of copolymer chains and their mean inter-chain separation distance at the nanoparticle surface to be monitored. Furthermore, the change in vesicle diameter and membrane thickness during the final stages of polymerization supports an ‘inward growth’ mechanism.

In situ small-angle X-ray scattering is used to monitor the formation of diblock copolymer spheres, worms and vesicles during reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate.     

LHRH/TAT dual peptides-conjugated polymeric vesicles for PTT enhanced chemotherapy to overcome hepatocellular carcinoma

Combination therapy such as photothermal therapy (PTT) enhanced chemotherapy is regarded as a promising strategy for cancer treatment. Herein, we developed redox-responsive polymeric vesicles based on the amphiphilic triblock copolymer PCL-ss-PEG-ss-PCL. To avoid the limited therapeutic effect of chemotherapeutic drugs caused by systemic exposures and drug resistance, the redox-sensitive polymeric vesicles were cargoed with two chemotherapeutics: doxorubicin (DOX) and paclitaxel (PTX). Besides, indocyanine green (ICG) was encapsulated, and cell-penetrating peptides and LHRH targeting molecule were modified on the surface of polymeric vesicles. The results indicated that the polymeric vesicles can load different kinds of drugs with high drug loading content, trigger drug release in responsive to the reductive environment, realize high cellular uptake via dual peptides and laser irradiation, and achieve higher cytotoxicity via chemo-photothermal combination therapy. Hence, the redox-responsive LHRH/TAT dual peptides-conjugated PTX/DOX/ICG co-loaded polymeric micelles exhibited great potential in tumor-targeting and chemo-photothermal therapy.     

Morphological transformations of nonequilibrium assemblies of amphiphilic diblock copolymer

Amphiphilic diblock copolymer polystyrene-block-poly(ethylene oxide) (PS-PEO) assembled into nonequilibrium bicontinuous structures or mixture of vesicles, bilayers and nanorods upon rapid micellization induced by rapid addition of selective solvent (water) into the PS-PEO solutions in a common solvent (dimethyl formamide) with different concentrations. These kinetically trapped assemblies were unstable and slowly evolved into thermodynamically favorable spheres and vesicles. The addition of non-ionic surfactant Pluronic P123 upon rapid micellization generated novel nanocages and flower-like vesicles. The nanocages spontaneously transformed into tubules capped with vesicles. These novel assemblies are beyond the classic phase diagram of block copolymer self-assemblies, especially for those primarily based on thermodynamics.     

Giant vesicles prepared by nitroxide-mediated photo-controlled/living radical polymerization-induced self-assembly

Giant vesicles with several-micrometer diameters were prepared by self-assembly induced by the nitroxide-mediated photo-controlled/living radical polymerization. The random block copolymerization of methyl methacrylate (MMA) and methacrylic acid (MAA) were performed using poly(methacrylic acid) (PMAA) as the prepolymer in an aqueous methanol solution to produce a PMAA-block-poly(MMA-random-MAA) random block copolymer (PMAA-b-P(MMA-r-MAA)). PMAA₁₉₅-b-P(MMA_0.817-r-MAA_0.183)₂₂₄ formed spherical vesicles with a 4.74 μm diameter and 0.108 μm wall thickness. A differential scanning calorimetry analysis demonstrated that the vesicles had a bilayer structure consisting of a hydrophilic PMAA surface and hydrophobic P(MMA-r-MAA) interface. The wet vesicles before air-drying were flexible and easily transformed by stress, whereas the dry vesicles were fragile and cracked. The vesicles in the solution were dissociated into much smaller vesicles by increasing the temperature. They were also transformed by a further temperature increase into hollow fibers and finally into membranes retaining the bilayer structure.     

Transparent titanium dioxide/block copolymer modified epoxy-based systems in the long scale microphase separation threshold

Microphase separated epoxy-based materials modified with an amphiphilic poly(styrene-block-ethylene oxide) diblock copolymer (PS-b-PEO) with low amount of PEO-block as well as ternary systems modified with this block copolymer and containing via sol–gel in situ synthesized TiO₂ nanoparticles were prepared and characterized. The obtained results indicate that block copolymer had enough amount of PEO-block in order to achieve microphase separated materials for a high range of PS-b-PEO contents, morphologies changing from spherical micelles to long wormlike micelles passing through vesicles upon increasing copolymer amounts. In the case of 20 wt.% inorganic/organic epoxy-based materials, addition of synthesized TiO₂ nanoparticles into PS-b-PEO-(DGEBA/MCDEA) system led to location of the nanoparticles in PEO-block/epoxy-rich confined between two microphase separated PS-block-rich phases. Designed highly transparent multiphase inorganic/organic epoxy-based materials possess interesting specific properties such as high UV shielding efficiency and high water repellence.     

Preparation of block copolymer vesicles in solution

Block copolymer vesicles can be prepared in solution from a variety of different amphiphilic systems. Polystyrene‐block‐poly(acrylic acid), polystyrene‐block‐poly(ethylene oxide), and many other block copolymer systems can produce vesicles of a wide range of sizes; those in the range of 100–1000 nm have been explored extensively. Different factors, such as the absolute and relative block lengths, the presence of additives (ions, homopolymers, and surfactants), the water content in the solvent mixture, the nature and composition of the solvent, the temperature, and the polydispersity of the hydrophilic block, provide control over the types of vesicles produced. Their high stability, resistance to many external stimuli, and ability to package both hydrophilic and hydrophobic compounds make them excellent candidates for use in the medical, pharmaceutical, and environmental fields. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 923–938, 2004     

Novel organotin-containing diblock copolymer with tunable nanostructures: Synthesis, self-assembly and morphological change

Interesting self-assembly behavior and morphological change of a novel organotin-containing diblock copolymer were firstly reported. The organotin-containing diblock copolymer, poly(methyl methacrylate)-block-poly(acetoxydibutyltin methacrylate) (PMMA-b-PADBTMA), was prepared via RAFT polymerization of ADBTMA with PMMA as the macroCTA and AIBN as the initiator in toluene. Both the FT-IR and TG analysis revealed an incorporation of both co-monomers in the resulted polymer backbone. The ratio of two segments was determined indirectly by TG analysis, gravimetric method and derivative process. All results from the different methods were well matched. And it was found that the morphology of the diblock copolymer could be changed easily from vesicles to nano-particle or cross-linked nano-composite under the ultrasonication or additional Ph₂SnCl₂, respectively. All the morphologies were analyzed by SEM, TEM and DLS. The self-assembly and the morphological change attributed to the strong coordination action between tin atoms and the carbonyl groups among PADBTMA segments.     

A Simple and Effective Approach to Vesicles and Large Compound Vesicles via Complexation of Amphiphilic Block Copolymer With Polyelectrolyte in Water

This work reports for the first time a simple and effective approach to trigger a spheres‐to‐ vesicles morphological transition from amphiphilic block copolymer/polyelectrolyte complexes in aqueous solution. Vesicles and large compound vesicles (LCVs) were prepared via complexation of polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) with poly(acrylic acid) (PAA) in water and directly visualized using cryo‐TEM. The complexation and morphological transitions were driven by the hydrogen bonding between the complementary binding sites on the PAA and PEO blocks of the block copolymer. The findings in this work suggest that complexation between amphiphilic block copolymer and polyelectrolyte is a viable approach to vesicles and LCVs in aqueous media.     

Structures of “crew-cut” aggregates of polystyrene-b-poly(acrylic acid) diblock copolymers

“Crew-cut” aggregates of polystyrene-b-poly(acrylic acid) block copolymers can be prepared by dissolving the copolymers in N,N-dimethylformamide (DMF) and adding water to the solution to induce aggregation of the styrene segments of the copolymer chains. The aggregates are formed at near-equilibrium conditions, and their structures are subsequently frozen by isolating them into aqueous solution by dialysis. Aggregates of a number of different morphologies have been prepared. The morphologies, identified by transmission electron microscopy, consist of spheres, rods, vesicles, lamellae, large compound vesicles, large compound micelles, etc. The formation of aggregates of different morphologies can be controlled by varying the copolymer composition, by changing the initial copolymer concentration in DMF, by adding ions (e.g. NaCl, CaCl₂, HCl and NaOH, etc), or by adding homopolystyrene.     

Hydrophobic energy estimation for giant vesicle formation by amphiphilic poly(methacrylic acid)-block-poly(alkyl methacrylate-random-mathacrylic acid) random block copolymers

The self-assembly induced by the photocontrolled/living radical polymerization mediated by 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl was performed for amphiphilic poly(methacrylic acid)-block-poly(alkyl methacrylate-random-methacrylic acid) containing ethyl, n-propyl, and n-butyl methacrylates in order to control the morphology based on the hydrophobic-hydrophilic balance. The morphology transformation from films to spherical vesicles via the transition was well-controlled by adjusting the ratio of the alkyl methacrylate unit to the methacrylic acid in the hydrophobic random copolymer block. The copolymers formed the respective morphologies at different ratios dependent on the alkyl chain length of the methacrylates; the ratio for the formation of the respective morphologies decreased as the alkyl chain length increased. The hydrophobic energy estimation of these copolymers demonstrated that the respective morphologies had definite hydrophobic energies independent of the alkyl chain length, indicating that the morphologies were determined only by the hydrophobic magnitude of the random copolymer block.     

Preparation of polymeric vesicles through ultrasonic treatment of poly(styrene‐block‐2‐vinylpyridine) micelles under alkaline conditions

An approach for the preparation of block copolymer vesicles through ultrasonic treatment of polystyrene‐block‐poly(2‐vinyl pyridine) (PS‐b‐P2VP) micelles under alkaline conditions is reported. PS‐b‐P2VP block copolymers in toluene, a selective solvent for PS, form spherical micelles. If a small amount of NaOH solution is added to the micelles solution during ultrasonic treatment, organic‐inorganic Janus‐like particles composed of the PS‐b‐P2VP block copolymers and NaOH are generated. After removal of NaOH, block copolymer vesicles are obtained. A possible mechanism for the morphological transition from spherical micelles to vesicles or Janus‐like particles is discussed. If the block copolymer micelles contain inorganic precursors, such as FeCl₃, hybrid vesicles are formed, which may be useful as biological and chemical sensors or nanostructured templates. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 953–959     

Morphological evolution of aggregates induced by the hydrophobic effect and hydrogen bonding interaction

The aggregation behavior of poly(ethylene oxide)-block-poly(methylmethacrylate) (PEO-b-PMMA) in toluene without and with the addition of polar small-molecules has been characterized using transmission electron microscopy (TEM). The preparation method involved copolymer dissolution in toluene which is a selective solvent for polymethylmethacrylate at room temperature, followed by the addition of polar small-molecules. By increasing polarity of the small-molecules at the same copolymer concentrations, these aggregates were found to undergo morphological transformations from spherical aggregates to multiple morphologies, including bean-pod liked structures (the morphologies are biomimetic), spheres and vesicles. A possible mechanism for the formation of this aggregate is proposed.     

A Vesicle‐to‐Worm Transition Provides a New High‐Temperature Oil Thickening Mechanism

Diblock copolymer vesicles are prepared via RAFT dispersion polymerization directly in mineral oil. Such vesicles undergo a vesicle‐to‐worm transition on heating to 150 °C, as judged by TEM and SAXS. Variable‐temperature ¹H NMR spectroscopy indicates that this transition is the result of surface plasticization of the membrane‐forming block by hot solvent, effectively increasing the volume fraction of the stabilizer block and so reducing the packing parameter for the copolymer chains. The rheological behavior of a 10 % w/w copolymer dispersion in mineral oil is strongly temperature‐dependent: the storage modulus increases by five orders of magnitude on heating above the critical gelation temperature of 135 °C, as the non‐interacting vesicles are converted into weakly interacting worms. SAXS studies indicate that, on average, three worms are formed per vesicle. Such vesicle‐to‐worm transitions offer an interesting new mechanism for the high‐temperature thickening of oils.     

Biocompatible polymer vesicles from biamphiphilic triblock copolymers and their interaction with bovine serum albumin

The self-assembly of the biamphiphilic triblock copolymer poly(ethylene oxide)-b-poly(caprolactone)-b-poly(acrylic acid) into polymer vesicles is studied. The vesicles provide both biocompatibility and biodegradability. Moreover, the biamphiphilic nature of the triblock copolymer provides different surface properties in the interior and in the outer interface of the vesicles. Preparation of the aggregates by direct dissolution of the copolymer in a solution of albumin does not alter the morphology of the aggregates, and thus, they have the potential to immobilize protein molecules. Since a part of the protein is encapsulated in the interior of the vesicles, they can be used as nanocontainers. A further fraction of the protein is bound to the outer interface, which is primarily composed of the poly(acrylic acid) tails. Immobilization of protein on the outer interface can stabilize the colloidal particles and also provide them with a biofunctional component.     

Facile synthesis of low-polydispersity block copolymer vesicles by azide-zwitterion cycloaddition

Polycaprolactone with azide (PCL-N₃) and polyethylene glycol with α,β-unsaturated ester (maPEG) as chain-end functional group were synthesized, respectively. Then, facile synthesis of polycaprolactone-block-polyethylene glycol (PCL-b-PEG) amphiphilic copolymer by azide-zwitterion cycloaddition of PCL-N₃ and maPEG, as the first example, was reported. The self-assembly of this amphiphilic copolymer was carried out in deionized water, and then the formation of vesicles was proven by the result of transmission electron microscope (TEM). Grainsize analyzer suggested that the diameter of the vesicles is 305 nm in average and the polydispersity of the vesicles is 0.128. The azide-zwitterion cycloaddition provides a powerful method for the post-functionalization of α,β-unsaturated ester or azide end-functionalized polymers.