Self-assembly of diblock copolymer mixtures in confined states: A Monte Carlo study

The self-assembly of diblock copolymer mixtures (A-b-B/A-b-C or A-b-B/B-b-C mixtures) subjected to cylindrical confinement (two-dimensional confinement) was investigated using a Monte Carlo method. In this study, the boundary surfaces were configured to attract blocks A but repel blocks B and C. Relative to the structures of the individual components, the self-assembled structures of mixtures of the diblock copolymers were more complex and interesting. Under cylindrical confinement, with varying cylinder diameters and interaction energies between the boundary surfaces and the blocks, we observed a variety of interesting morphologies. Upon decreasing the cylinder's diameter, the self-assembled structures of the A(15)B(15)/A(15)C(15) mixtures changed from double-helix/cylinder structures (blocks B and C formed double helices, whereas blocks A formed the outer barrel and inner core) to stacked disk/cylinder structures (blocks B and C formed the stacked disk core, blocks A formed the outer cylindrical barrel), whereas the self-assembled structures of the A(15)B(7)/B7C15 mixtures changed from concentric cylindrical barrel structures to screw/cylinder structures (blocks C formed an inside core winding with helical stripes, whereas blocks A and B formed the outer cylindrical barrels) and then finally to the stacked disk/cylinder structures.

Formation of ordered microphase-separated pattern during spin coating of ABC triblock copolymer

In this paper, the authors have systematically studied the microphase separation and crystallization during spin coating of an ABC triblock copolymer, polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-b-P2VP-b-PEO). The microphase separation of PS-b-P2VP-b-PEO and the crystallization of PEO blocks can be modulated by the types of the solvent and the substrate, the spinning speed, and the copolymer concentration. Ordered microphase-separated pattern, where PEO and P2VP blocks adsorbed to the substrate and PS blocks protrusions formed hexagonal dots above the P2VP domains, can only be obtained when PS-b-P2VP-b-PEO is dissolved in N,N-dimethylformamide and the films are spin coated onto the polar substrate, silicon wafers or mica. The mechanism of the formation of regular pattern by microphase separation is found to be mainly related to the inducement of the substrate (middle block P2VP wetting the polar substrate), the quick vanishment of the solvent during the early stage of the spin coating, and the slow evaporation of the remaining solvent during the subsequent stage. On the other hand, the probability of the crystallization of PEO blocks during spin coating decreases with the reduced film thickness. When the film thickness reaches a certain value (3.0 nm), the extensive crystallization of PEO is effectively prohibited and ordered microphase-separated pattern over large areas can be routinely prepared.

Motion of integrated CdS nanoparticles by phase separation of block copolymer brushes

A new method of reversibly moving US nanoparticles in the perpendicular direction was developed on the basis of the phase separation of block copolymer brushes. Polystyrene-b-(poly(methyl methaerylate)-co-poly(cadmium dimethacrylate)) (PS-b-(PMMA-co-PCdMA)) brushes were grafted from the silicon wafer by surface-initiated atom transfer radical polymerization (ATRP). By exposing the polymer brushes to H2S gas, PS-b-(PMNlA-co-PCdNlA) brushes were converted to polystyrene-b-(poly(methyl methacrylate) -co-poly(methacrylic acid)(CdS)) (PS-b-(PMMA-co-PMAA(CdS))) brushes, in which US nanoparticles were chemically bonded by the carboxylic groups of PMAA segment. Alternating treatment of the PS-b-(PMMA-co-PMAA(CdS)) brushes by selective solvents for the outer block (a mixed solvent of acetone and ethanol) and the inner PS block (toluene) induced perpendicular phase separation of polymer brushes, which resulted in the reversible lifting and lowering of US nanoparticles in the perpendicular direction. The extent of movement can be adjusted by the relative thickness of two blocks of the polymer brushes.

Development of nanodomain and fractal morphologies in solvent annealed block copolymer thin films

We have systematically studied the thin film morphologies of asymmetric polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer subjected to solvent vapors of varying selectivity for the constituent blocks. Upon a short treatment in neutral or PS-selective vapor, the film exhibited a highly ordered array of hexagonally packed, cylindrical microdomains. In the case of PEO selective vapor annealing, such ordered cylindrical microdomains were not obtained. instead, fractal patterns on the microscale were observed and their growth processes investigated. Furthermore, hierarchical structures could be obtained if the fractal pattern was exposed to neutral or PS selective vapor.

Direct formation of giant vesicles from synthetic polypeptides

This report describes direct formation of giant vesicles from a series of poly(L-lysine)-block-poly(L-phenylalanine) (PLL-b-PPA) block copolymers from their water solution. These polymers are prepared by successive ring-opening polymerization (ROP) of the two alpha-amino acid N-carboxyanhydrides and then removing the side chain protecting groups by acidolysis. The structures of the copolymers are confirmed by nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and size exclusion chromatography ( SEC). The vesicles are studied by atomic force microscopy (AFM), field emission scanning electron microscopy (ESEM), and confocal laser scanning microscopy (CLSM). Rhodamine B is used as a fluorescent probe to confirm the existence of the vesicle with an aqueous interior. The vesicle size is in the range 0.55-6 mu m, depending on the absolute and relative lengths of the two blocks, on initial polymer concentration, and on solution pH. The vesicles are still stable in water for 2 months after preparation. Addition of the copolymer to DNA solution results in complex formation with it. The complex assumes the morphology of irregular particles of less than 2 mu m. It is expected to be used in drug and gene delivery.

RGD peptide grafted biodegradable amphiphilic triblock copolymer poly(glutamic acid)-b-poly(L-lactide)-b-poly(glutamic acid): Synthesis and self-assembly

A novel amphiphilic biodegradable triblock copolymer (PGL-PLA-PGL) with polylactide (PLA) as hydrophobic middle block and poly(glutamic acid) (PGL) as hydrophilic lateral blocks was successfully synthesized by ring-opening polymerization (ROP) Of L-lactide (LA) and N-carboxy anhydride (NCA) consecutively and by subsequent catalytic hydrogenation. The results of cell experiment of PGL-PLA-PGL suggested that PGL could improve biocompatibility of polyester obviously. The copolymer could form micelles of spindly shape easily in aqueous solution. The pendant carboxyl groups of the triblock copolymer were further activated with N-hydroxysuccinimide and combined with a cell-adhesive peptide GRGI)SY Incorporation of the oligopeptide further enhanced the hydrophilicity and led to formation of spherical micelles. PGL-PLAPGL showed better cell adhesion and spreading ability than pure PLA and the GRGDSY-containing copolymer exhibited even further improvement in cell adhesion and spreading ability, indicating that the copolymer could find a promising application in drug delivery or tissue engineering.

Synthesis and characterization of novel poly(ester carbonate)s based on pentaerythritol

Novel poly(ester carbonate)s were synthesized by the ring-opening polymerization Of L-lactide and functionalized carbonate monomer 9-phenyl-2,4,8,10-tetraoxaspiro[5,5]undecan-3-one derived from pentaerythritol with diethyl zinc as an initiator. H-1 NMR analysis revealed that the carbonate content in the copolymer was almost equal to that in the feed. DSC results indicated that T-g of the copolymer increased with increasing carbonate content in the copolymer. Moreover, the protecting benzylidene groups in the copolymer poly(L-lactide-co-9-phenyl-2,4,8,10-tetraoxaspiro[5,5]undecan-3-one) were removed by hydrogenation with palladium hydroxide on activated charcoal as a catalyst to give a functional copolymer, poly(L-lactide-co-2,2-dihydroxylmethyl-propylene carbonate), containing pendant primary hydroxyl groups. Complete deprotection was confirmed by H-1 NMR and FTIR spectroscopy. The in vitro degradation rate of the deprotected copolymers was faster than that of the protected copolymers in the presence of proteinase K. The cell morphology and viability on a copolymer film evaluated with ECV-304 cells showed that poly(ester carbonate)s derived from pentaerythritol are good biocompatible materials suitable for biomedical applications.

Self-assembly of polypeptide-containing ABC-type triblock copolymers in aqueous solution and its pH dependence

Self-assembling of novel biodegradable ABC-type triblock copolymer poly(ethylene glycol)-poly(L-lactide)-poly(L-glutamic acid) (PEG-PLLA-PLGA) is studied. In aqueous media, it self-assembles into a spherical micelle with the hydrophobic PLLA segment in the core and the two hydrophilic segments PEG and PLGA in the shell. With the lengths of PEG and PLLA blocks fixed, the diameter of the micelles depends on the length of the PLGA block and on the volume ratio of H2O/dimethylformamide (DMF) in the media. When the PLGA block is long enough, morphology of the self-assembly is pH-dependent. It assembles into the spherical micelle in aqueous media at pH 4.5 and into the connected rod at or below pH 3.2. The critical micelle concentration (cmc) of the copolymer changes accordingly with decreasing solution pH. Both aggregation states can convert to each other at the proper pH value. This reversibility is ascribed to the dissociation and neutralization of the COOH groups in the LGA residues. When the PLGA block is short compared to the PEG or PLLA block, it assembles only into the spherical micelle at various pH values.

Competition of lamellar orientation in thin films of a symmetric poly(styrene)-b-poly(L-lactide)diblock copolymer in melt state

We have studied the lamellar orientation in thin films of a model diblock copolymer, symmetric poly(styrene)-b-poly(L-lactide) (PS-PLLA), in the melt state on supported silicon wafer surface. In this system, while the PLLA block prefers to wet the polymer/substrate interface, the polymer/air as well as polymer/polymer interface is neutral for both blocks due to the similar surface energies of PS and PLLA in melt state. Our results demonstrate that the interplay of the interfaces during phase separation results in a series of structures before approaching the equilibrium state. Lamellar orientation of thin films with different initial film thicknesses at different annealing stages has been investigated using atomic force microscopy (AFM), transmission electronic microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It is found that in the early stage (annealing time t < 10 min), the polymer/substrate interface dominates the structure evolution, leading to a parallel lamellar structure with holes or islands formed depending on the initial film thickness. Later on, the neutral air interface becomes important and leads to a transition of lamellar orientation from parallel to perpendicular. It is interesting to see that for films with thickness h > 2L, where L is the bulk lamellar period, the lamellar orientation transition can occur independently in different parallel lamellar domains due to the neutrality of polymer/polymer interface.

High-dimensional assembly depending on polyoxoanion templates, metal ion coordination geometries, and a flexible bis(imidazole) ligand

By introducing the flexible 1,1'-(1,4-butanediyl)bis(imidazole) (bbi) ligand into the polyoxovanadate system, five novel polyoxoanion-templated architectures based on [As8V14O42](4-) and [V16O38Cl](6-) building blocks were obtained: [M(bbi)(2)](2)[As8V14O42(H2O)] [M = Co (1), Ni (2), and Zn (3)], [Cu(bbi)](4)[As8V14O42(H2O)] (4), and [Cu(bbi)](6)[V16O38Cl] (5). Compounds 1-3 are isostructural, and they exhibit a binodal (4,6)-connected 2D structure with Schlafli symbol (3(4)center dot 4(2))(3(4)center dot 4(4)center dot 5(4)center dot 6(3))(2), in which the polyoxoanion induces a closed four-membered circuit of M-4(bbi)(4). Compound 4 exhibits an interesting 3D framework constructed from tetradentate [As8V14O42](4-) cluster anions and cationic ladderlike double chains. There exists a bigger M-8(bbi)(6)O-2 circuit in 4. The 3D extended structure of 5 is composed of heptadentate [V16O38Cl](6-) anions and flexural cationic chains; the latter consists of six Cu(bbi) segments arranged alternately. It presents the largest 24-membered circuit of M-24(bbi)(24) so far observed made of bbi molecules and transition-metal cations. Investigation of their structural relations shows the important template role of the polyoxoanions and the synergetic interactions among the polyoxoanions, transition-metal ions, and flexible ligand in the assembly process.

Luminescent supramolecular polymers: Cd2+-directed polymerization and properties

A family of supramolecular polymers was prepared via Cd2+-directed self-assembly polymerization of his (2,2':6',2 ''-terpyridine)-based ligand monomers, using oligofluorenes and triphenylamine as bridges under mild conditions. The polymers were fully characterized using thermogravimetric analysis, inherent viscosity, electrochemical measurements, UV-visible spectroscopy, photoluminescence (PL) and electroluminescence (EL). Polymers with oligofluorenes as spacers exhibited blue emission (434-442 nm) in dimethyl acetamide (DMAc) solution, while polymers with triphenylamine as spacer presented an emission peak at 494 nn in DMAc solution. Complexation polymerization of bis(2,2':6',2 ''-terpyridine)-based ligand monomers with cadmium(II) improved fluorescence quantum yields dramatically, and the film PL quantum yields of these polymers were about 0.38-0.54. Single-layer light-emitting diodes were fabricated with the configuration indium tin oxide (ITO)/polymer/Ca/Al; the EL showed green emission and the onset voltages of the devices were 8-11 V.

Crystallization and ring-banded spherulite morphology of poly(ethylene oxide)-block-poly(epsilon-caprolactone) diblock copolymer

The crystallization behavior of crystalline-crystalline diblock copolymer containing poly(ethylene oxide) (PEO) and poly(epsilon-caprolactone) (PCL), in which the weight fraction of PCL is 0.815, has been studied via differential scanning calorimeter (DSC), wide-angle X-ray diffraction (WAXD), and polarized optical microscopy (POM). DSC and WAXD indicated that both PEO and PCL blocks crystallize in the block copolymer. POM revealed a ring-banded spherulite morphology or the PEO-b-PCL diblock copolymer.