Amphiphile micelles formed by polystyrene/poly(2-vinyl pyridine) heteroarm star copolymers in toluene

The Micellization of Well-Defined Single Graft Copolymers in Block Copolymer/Homopolymer Blends

A series of well-defined (polyisoprene)2(polystyrene), I2S, single graft copolymers with similar total molecular weights but different compositions, fPS, were blended with a low molecular weight polyisoprene homopolymer matrix at a constant concentration 2 wt%, and the micellar characteristics were studied by small-angle x-ray scattering. To investigate the effect of macromolecular architecture on the formation and characteristics of micelles, the results on the single graft copolymers were compared with those of the corresponding linear polystyrene-b-polyisoprene diblock copolymers, SI. The comparison reveals that the polystyrene core chains are more stretched in the case of graft copolymer micelles. Stretching turned out to be purely a result of the architecture due to the second polyisoprene block in the corona. The micellization of a (polystyrene)2(polyisoprene), S2I, graft copolymer was also studied, and the comparison with the results of the corresponding I2S and SI copolymers emphasizes the need for a critical core volume rather than a critical length of the core-forming block, in order to have stable micelles. Finally, the absence of micellization in the case of the I2S copolymer with the highest polystyrene volume fraction is discussed. For this sample, macrophase separation occurs, with polyisoprene cylinders formed in the copolymer-rich domains of the phase-separated blends.

Comparison between novel star-like redox-sensitive amphiphilic block copolymer and its linear counterpart copolymer as nanocarriers for doxorubicin

Linear and star-like redox-sensitive amphiphilic block copolymers have been studied as anticancer drug delivery systems. However, few reports directly compared the properties of those two structures especially when they are used as nanocarriers for antitumor drugs. To address this, a novel star-like copolymer and its linear counterpart were synthesized with a hydrophobic/redox-responsive/hydrophilic structure. The overall molecular weight of the star-shaped copolymer was nearly equal to that of the linear counterpart. The star-like micelles exhibit size of 90 nm, which was smaller than that of linear copolymers (151.6 nm) and critical micelle concentration of 1 mg/L, which was lower than that of the linear micelles (8.9 mg/L). The disassembly behaviors and the redox-sensitivity of the nanoparticles to reductive stimuli of glutathione was evaluated from the changes of the micellar size and morphology. Furthermore, doxorubicin was physically loaded into the hydrophobic part of the copolymers. The drug-loading capacities in the star-like and linear micelles were 15.94 and 7.53 wt%, respectively. Drug release studies carried out at two different glutathione concentrations. A cytotoxicity study of the micelles was performed by MTT assay. The prepared star copolymer showed no significant toxicity against HDF cells while enhanced cytotoxicity of the DOX-loaded micelles against MCF-7 cells was observed. Therefore, developing sucrose-PCL-SS-PEG copolymer reported in this paper as an effective reduction-responsive carrier with excellent properties and cell biocompatibility is promising for the efficient intracellular delivery of hydrophobic chemotherapeutic drugs. This work also indicates that modification of the nanocarrier structure is a potential strategy for optimizing drug delivery.

Identification of the key steps in the self-assembly of homogeneous gold metal nanoparticles produced using inverse micelles

The self-assembly of gold nanoparticles (Au NPs) using polymer-encapsulated inverse micelles was studied using a set of advanced X-ray techniques (i.e. XAFS, SAXS) in addition to DLS, UV-vis spectroscopy and TEM. Importantly the combination of these techniques with the inverse micelle approach affords us detailed insight and to rationalize the evolving molecular chemistry and how this drives the formation of the Au NPs. We observe that the mechanism comprises three key steps: an initial fast reduction of molecular Au(iii) species to molecular Au(i)Cl; the latter species are often very unstable during the self-assembly process. This is followed by a gradual reduction of these molecular Au(i) species and the formation of sub-nanometric Au clusters which coalesce into nanoparticles. It was also found that addition of small amounts of HCl can accelerate the formation of the Au clusters (the second phase) without affecting the final particle size or its particle size distribution. These findings would help us to understand the reaction mechanism of Au NP formation as well as providing insights into how NP properties could be further tailored for a wide range of practical applications.

Thermo-Resistant Soft Glassy Suspensions of Polymeric Micellar Nanoparticles in Ionic Liquid

We report the rheological and structural properties of a suspension comprising poly(ethylene oxide)-polystyrene-poly(ethylene oxide) core-shell micellar nanoparticles dispersed in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid. A liquid to soft solid transition was observed at a copolymer concentration of 10 wt % above which an elastic soft material was formed, which was composed of non-ordered jammed core-shell micellar nanoparticles. In the soft solid state, a significant reduction in the size of the nanoparticles, approaching hard sphere behavior, was observed by small-angle X-ray scattering which is attributed to compression of the soft poly(ethylene oxide) coronas. The nonvolatile ionic liquid-based glassy soft solid formed exhibited remarkable thermal stability with a melting temperature of 141 °C at 20 wt % copolymer, which renders it suitable for applications involving elevated temperatures and/or reduced pressure where water-based formulations are inappropriate.

Coarse-grained simulation study on the self-assembly of miktoarm star-like block copolymers in various solvent conditions

The self-assembly processes of miktoarm star-like block copolymers (Ax1)y-C-(Bx2)y, in which y homopolymer chains of type A with chain length x1 as well as y chains of type B with chain length x2 are connected to a center core C, are investigated by using Brownian dynamics simulations. We focus on the selective solvent condition, i.e., the solvent is poor for components B and C, but varies from good to poor for component A. The miktoarm star-like block copolymers with A and B arms of equal length can form spherical micelles when the solvent is good for component A. In the same solvent conditions, the micelles contain fewer miktoarm star-like block copolymers as the arm length increases. When the solvent is poor for component A, the miktoarm star-like block copolymers can self-assemble into cylindrical or disk-like micelles with decreasing solvent quality. For the miktoarm star-like block copolymers with longer B arms but shorter A arms, self-assembly into spherical vesicles is possible in a wide range of solvent conditions.

Surface morphologies of Langmuir-Blodgett monolayers of PEOnPSn multiarm star copolymers

Star polymers composed of equal numbers of poly(ethylene oxide) (PEO) and polystyrene (PS) arms with variable lengths and a large (up to 38 total) number of arms, PEO(n)PS(n), have been examined for their ability to form domain nanostructures at the air-water and air-solid interfaces. All PEO(n)PS(n) star polymers formed stable Langmuir-Blodgett (LB) monolayers transferable to a solid substrate. A range of nanoscale surface morphologies have been observed, ranging from cylindrical to circular domains to bicontinuous structures as the weight fraction of the PEO block varied from 19% to 88% and n from 8 to 19. For the PS-rich stars and at elevated surface pressure, a two-dimensional supramolecular netlike nanostructure was formed. In contrast, in the PEO-rich star polymer with the highest PEO content, we observed peculiar dendritic superstructures caused by intramolecular segregation of nonspherical core-shell micellar structures. On the basis of Langmuir isotherms and observed monolayer morphologies, three different models of possible surface behavior of the star polymers at the interfaces were proposed.

Synthesis and Characterization of Six-Arm Star Poly(δ-valerolactone)-block-Methoxy Poly(ethylene glycol) Copolymers

Novel amphiphilic six-arm star diblock copolymers based on biocompatible and biodegradable poly(delta-valerolactone) (PVL) and methoxy poly(ethylene glycol) (MePEG) were synthesized by a two-step process. First, the hydrophobic star-shaped PVL with hydroxyl terminated functional groups was synthesized using a multifunctional alcohol, dipentaerythritol (DPE), as the initiator and fumaric acid as the catalyst. The amphiphilic six-arm star copolymer of poly(delta-valerolactone)-b-methoxy poly(ethylene glycol), (PVL-b-MePEG)(6), was then synthesized by coupling the hydroxyl terminated six-arm PVL homopolymer with alpha-methoxy-omega-chloroformate-poly(ethylene glycol) (MePEG-COCl). (1)H NMR and GPC analyses confirmed the successful synthesis of star-shaped copolymers with predicted compositions and narrow molecular weight distributions. DSC analysis revealed that the glass transition temperatures of the star PVL homopolymers with M(n) between 5000 and 49 000 are not dependent on their molecular weights, whereas the melting temperatures of both the PVL homopolymers and the amphiphilic (PVL-b-MePEG)(6) copolymers increase with an increase in the PVL molecular weight. Micelles were prepared from the (PVL-b-MePEG)(6) copolymers via the dialysis method and found to have effective mean diameters ranging from 10 to 45 nm, depending on the copolymer composition. In addition, the (PVL-b-MePEG)(6) copolymers having lower PVL content were found to form micelles with a narrow monomodal size distribution, whereas the copolymers having higher PVL content tended to form aggregates with a bimodal size distribution. The noncytotoxicity of the copolymers was also confirmed in CHO-K1 fibroblast cells using a cell viability assay, indicating that the (PVL-b-MePEG)(6) copolymers are suitable for biomedical applications such as drug delivery.

Interfacial micellar structures from novel amphiphilic star polymers

An amphiphilic heteroarm star polymer containing 12 alternating hydrophobic/hydrophilic arms of polystyrene (PS) and poly(acrylic acid) (PAA) connected to a well-defined rigid aromatic core was studied at the air-water and the air-solid interfaces. At the air-water interface, the molecules spontaneously form pancakelike micellar aggregates which measure up to several microns in diameter and 5 nm in thickness. Upon reduction of the surface area per molecule to 7 nm2, the two-dimensional micelles merged into a dense monolayer. We suggest that confined phase separation of dissimilar polymer arms occurred upon their segregation on the opposite sides of the rigid disklike aromatic core, forcing the rigid cores to adopt a face-on orientation with respect to the interface. Upon transfer onto solid supports the PS chains face the air-film interface making it completely hydrophobic, and the PAA chains were found to collapse and form a thin flattened underlayer. This study points toward new strategies to create large 2D microstructures with facial amphiphilicity and suggests a profound influence of star molecular architecture on the self-assembly of amphiphiles at the air-water interface.