A study of the versatile micropatterns of diblock copolymer micelles: the effect of copolymer concentration, substrate, film thickness and micelles morphology

We report the novel use of polystyrene-block-poly(acrylic acid) (PS-b-PAA) diblock copolymer micelles as the nano-building blocks in fabricating orderly aligned three-dimensional micropatterns with high regularity through a one-step evaporation-induced cracking process. Crack patterns of square, rectangular, stripe-like and mesh-like structures in micron scale were obtained. The effect of the concentration of diblock copolymer, the properties of the substrates, the thickness of the drying layer, and the morphology of the micelles on the regularity of the crack patterns was studied. By regulating the above factors, we achieved micropatterns of various structures. We further developed a cheap, fast, and simple method for fabricating micromolded structures using the crack patterns as templates.     

The vesicle formation in a binary amphiphilic diblock copolymer/homopolymer solution

The formation of vesicles in a binary blend of an amphiphilic diblock copolymer AB/homopolymer C was studied in a dilute solution using the real space two-dimensional self-consistent field theory (SCFT). Special attention was played to the role played by the homopolymer C in controlling the vesicle formation. In the simulations, it was found that as the averaged volume fraction of homopolymer C was decreased while keeping the total averaged volume fraction of block copolymer AB and homopolymer C unchanged, there was a morphological transition from the bilayer vesicles to rod-like/circle-like micelles. The compound vesicle structure was observed in the simulations. When the averaged volume fraction of block copolymer AB was further increased while the total averaged volume fraction of AB and C remained unchanged, the compound vesicle structure became less favored entropically than the unilamellar vesicle structure. The effect of the degree of polymerization of the homopolymer C on the vesicle formation of the amphiphilic system was examined. By reducing the degree of polymerization of the homopolymer C to unity, the component C became a small solvent molecule immiscible with the bulk solvent. It was found that the small solvent C exerted no influence on the morphological stability of the vesicles.     

Excluded volume effect on the self-assembly of amphiphilic AB diblock copolymer in dilute solution

The self-assembly of amphiphilic AB diblock copolymer in solution with different molecular size was investigated by using the self-consistent field theory (SCFT). By continuously varying the solvent size and the polymer concentration, the phase diagram is constructed. The aggregate concentration of the amphiphilic AB diblock copolymer decreases in solution with larger solvent size. The phase will transit in solution with different solvent size due to the excluded volume interaction effect. This widely existing excluded volume effect is very useful for the separation of polymers and helpful for understanding the crowding effect in bio-molecules.     

TEM characterization of diblock copolymer templated iron oxide nanoparticles: Bulk solution and thin film surface doping approach

The morphology of a novel diblock copolymer, poly(norbornene methanol)-b-poly(norbornene dicarboxylic acid), was investigated before and after metal oxide doping by transmission electron microscopy (TEM) using a novel iodine vapor staining method to image the undoped polymer. A lamellar morphology was observed by TEM after staining the undoped diblock copolymer with iodine vapor. Thin film surface doping resulted in a confinement of the iron oxide nanoparticles within the lamellar domains. Spherical nanoparticle aggregates were observed through a bulk solution doping method. It was observed that the particles were templated by the underlying lamellar structure of the copolymer when the thin film surface doping method was used.     

The nature of phase transitions of symmetric diblock copolymer melts under confinement

The effects of confinement, both the structure frustration and the surface field, on phase transitions of symmetric diblock copolymer melts are investigated within several theoretical methods on the mean-field level. Confinements are applied by restricting polymer chains in the finite spaces of slabs. The surface can be neutral or preferential depending on the strength of the surface field. Within the one-dimensional self-consistent mean-field theory, for the neutral surface case, an oscillative behavior is observed for the size dependence of the order-disorder transition (ODT) point (χN)_t due to the structure frustration. The spinodal (χN)_s for this corresponding confined system is also calculated using the Gaussian fluctuation theory and the Landau-Brazovskii theory, and (χN)_s coincides exactly with (χN)_t. On the other hand, the surface effect plays the role to decrease (χN)_t due to the surface-induced spatial oscillation for the preferential surface case. In all confined systems considered, the ODT for symmetric diblock copolymer melts is a continuous second-order phase transition in the present mean-field calculation.     

Synthesis, photocatalytic and antibacterial activities of ZnO particles modified by diblock copolymer

Various shapes of ZnO particles were synthesized through a precipitation method as a low cost technique. The morphological changes due to the use of poly(ethylene oxide)-b-poly(propylene oxide) copolymer as a stabilizer and precipitation from pH solutions of 8, 10 and 12 were investigated. The ZnO powder that was prepared from pH 10 showed the highest efficiency for degrading the dyes methylene blue, rhodamine B and reactive orange under backlight fluorescent tubes. This may be due to the highest surface area obtained from precipitation at this pH. These prepared ZnO particles also exhibited antibacterial effects that were stronger on Staphylococcus aureus than on Escherichia coli.     

Functionalized organic nanoparticles from core-crosslinked poly(4-vinylbenzocyclobutene-b-butadiene) diblock copolymer micelles

Surface-functionalized polymeric nanoparticles were prepared by: a) self-assembly of poly(4-vinylbenzocyclobutene-b-butadiene) diblock copolymer (PVBCB-b-PB) to form spherical micelles (diameter: 15-48 nm) in decane, a selective solvent for PB, b) crosslinking of the PVBCB core through thermal dimerization at 200-240 °C, and c) cleavage of the PB corona via ozonolysis and addition of dimethyl sulfide to afford aldehyde-functionalized nanoparticles (diameter: ∼16-20 nm), along with agglomerated nanoparticles ranging from ∼30 to ∼100 nm in diameter. The characterization of the diblock copolymer precursors, the intermediate micelles and the final surface-functionalized crosslinked nanoparticles was carried out by a combination of size exclusion chromatography, static and dynamic light scattering, viscometry, thermogravimetric analysis, ¹H NMR and FTIR spectroscopy and transmission electron microscopy.     

Preparation of diblock copolymer based on poly(4-n-butyltriphenylamine) via palladium coupling polymerization

A self-condensing monomer 4-(4′-bromophenyl)-4″-n-butyldiphenylamine (1) was synthesized, and successfully converted to poly(4-n-butyltriphenylamine) (PBTPA) by arylamination using palladium catalyst. PBTPAs can be functionalized at both terminals separately by adding an aryl bromide or arylamine derivatives as a terminator, which enabled us to prepare the diblock copolymer PBTPA-block-PEO. Polymer characterization was performed by ¹H NMR, ¹³C NMR, and DSC, which confirmed that the PEO segment was successfully introduced at the terminal of PBTPA. The surface morphology in a thin film of PBTPA-block-PEO was examined by AFM, revealing that a microphase-separated structure or cup-shaped structure with PEO sphere domains formed when the film was spin-cast from 1,1,2,2-tetrachloroethane solutions of different concentrations.     

Regular and irregular micelles formed by A LEL triblock copolymer in aqueous solution

Laser light scattering (LLS) techniques were used to characterize the micellization of poly(d,l-lactide)-poly(ethylene glycol)-poly(d,l-lactide) (LEL) triblock copolymer (MW 1K-2K-1K) in aqueous solution. We observed the existence of both thermodynamically stable flower-like micelles (regular micelles) and large, less soluble nanoparticles (irregular micelles) in dilute aqueous solutions with the same preparation procedure. Both kinds of micelles were found to co-exist with single copolymer chains. The initial copolymer concentration determines the nature of the micelles. The regular core-shell micelle formation follows a closed association mechanism, resulting in flower-like micelles. The hydrophobicity of a L unit is estimated as ∼0.5-0.6 B (polyoxybutylene) units from the micellization parameters, which is quite consistent with earlier estimations obtained from EL diblock copolymers.     

Micropatterning from drying micelle solution of diblock copolymers: strap-structure, quadrate farmland-structure and biomimetic structure

Micropatterns of strap and farmland structures of polystyrene-block-poly(acrylic acid) (PS-b-PAA) copolymer were obtained by drying the micelle solutions. The drying process of the solution was monitored by dynamic optical microscopy (OM) with a digital video camera. The surface structure of the micelle aggregations was studied by OM, scanning electron microscopy and Alpha-Step 200 profiler system. It is believed that solvent evaporation and micelle migration account for the observed micelle micropatterns. The potential application of the micropatterns is also addressed.     

Effect of interfacial tension on micellization of a polystyrene-poly(ethylene oxide) diblock copolymer in a mixed solvent system

The micellization of a polystyrene-poly(ethylene oxide) (PS-PEO) diblock copolymer in a mixed solvent of water and tetrahydrofuran (THF) was studied in terms of an aggregation number as measured by laser light scattering. The results show the aggregation number (f) of the micelle decreases monotonically as the amount of THF is increased. These experimental results are compared with a theoretical model that includes the variations in the interfacial tension. Experimentally, the interfacial tension is adjusted with the addition of relatively low levels of THF. The theory indicates that the aggregation number decreases as the interfacial tension is decreased. The theoretical prediction is in very good agreement with the experimental results. The main conclusion of this study is that the interfacial tension is the key factor in determining the aggregation number of the diblock copolymer micelle in a water-THF solvent system.     

Crystallization, melting, and morphology of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene}

A poly(ethylene oxide) diblock copolymer containing a short block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PEO-b-PMPCS) has been successfully synthesized via atom transfer radical polymerization (ATRP) method. The number average molecular weights (M_n) of the PEO and PMPCS blocks are 5300 and 2100 g/mol, respectively. Combining the techniques of differential scanning calorimetry (DSC), optical microscopy (OM), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), we have found that the PMPCS blocks, which are tablet-like, can significantly affect the crystallization and melting of the diblock copolymer. The sample studied can form the crystals with a monoclinic crystal structure identical to that of the homo-PEO. The melting temperature (T_m) of the diblock copolymer increases monotonically with crystallization temperature (T_c), which is remarkably similar to the behavior of long period. On the basis of Gibbs-Thomson relationship, the equilibrium T_m of the diblock copolymer is estimated to be 65.4 °C. In a wide undercooling (ΔT) range (14 °C<ΔT<30 °C), the isothermal crystallization leads to square-shaped crystals. The PEO-b-PMPCS crystallization exhibits a regime I→II transition at ΔT of 19 °C. The PEO blocks are non-integral folded (NIF) in the crystals, and the PMPCS blocks rejected to lamellar fold surfaces prevent the NIF PEO crystals from transforming to integral folded (IF) ones. Furthermore, the PMPCS tablets may adjust their neighboring positions up or down with respect to the lamellar surface normal, forming more than one PMPCS layer to accompany the increase in the PEO fold length with increasing T_c.     

Synthesis, characterization and modification of azide-containing dendronized diblock copolymers

A series of dendronized diblock copolymers having rigid backbone and reactive surface were synthesized by ring-opening metathesis polymerization (ROMP) from dendronized norbornene derivatives using the second generation Grubb's catalyst. The bromine-terminated block of those rigid nanostructures has been converted to more reactive azide groups in one straightforward step. The resulting polymers were then functionalized by post-polymerization reaction with fullerene C₆₀ (electron acceptor) using thermal [3 + 2] cycloaddition reaction or with porphyrin (electron donor) using copper-catalyzed “click chemistry”, the ultimate goal being the preparation of efficient polymeric materials for photovoltaic applications. While fullerene addition was not complete (approximately 50%) because of cross-linking reactions and steric hindrance on the dendrimers surface, Zn-porphyrin introduction went to completion clearly demonstrating the usefulness of click chemistry for polymer functionalization.     

Crystallization of double crystalline symmetric diblock copolymers

We report dynamic Monte Carlo simulation results on the crystallization of double crystalline symmetric A-B diblock copolymer, wherein the melting temperature of A-block is higher than B-block. Crystallization of A-block precedes the crystallization of B-block upon cooling from a homogeneous melt. The morphological development is controlled by the interplay between crystallization and microphase separation. With increasing segregation strength, we observe a gradual decrease in crystallinity accompanying with smaller and thinner crystals. During crystallization, A-block crystallizes first and creates confinement for the crystallization of B-block. Thus, crystallization of B-block slows down influencing the overall crystal morphology. At higher segregation strength, due to the repulsive interaction between blocks, block junction is stretched out, which is reflected in the increased value of mean square radius of gyration. As a result, a large number of smaller size crystals form with less crystallinity. The onset of microphase separation shifts towards higher temperature with increasing segregation strength. Isothermal crystallization reveals that the transition pathways strongly depend on segregation strength. The value of Avrami index shows the formation of two dimensional lamellar crystals of both the blocks. Two-step (sequential), compared to one-step (coincident) isothermal crystallization, produces higher crystallinity in A-block, however, the crystallinity of B-block is almost identical in both the cases.     

Solvent vapor induced dewetting in diblock copolymer thin films

The dewetting pattern development of thin film of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer has been studied after ‘annealing’ in the PMMA block selective solvent vapor. Initially, typical circular dewetted holes are observed. Further annealing, however, results in the formation of fractal-like holes. The heterogeneous stress induced by the residual solvent remaining in the film after spin-coating induces the anisotropy of the polymer mobility during the annealing process, which triggers the formation of the intriguing surface patterns.     

Concentric lamella structures of symmetric diblock copolymers confined in cylindrical nanopores

We applied a real-space self-consistent field theory to investigate the concentric lamella structures of symmetric diblock copolymers confined in the cylindrical nanopores with the preferential surfaces. The symmetric diblock copolymers are selected to locate in the very weak and strong segregation regions where the lamellae obviously exhibit the “soft” and “rigid” characteristics, respectively. For the soft lamellae, the cylindrical confinement induces the soft concentric lamella structure with the same thickness as the bulk lamellar period, except that the thickness of the innermost layer depends on the confinement degree. For the rigid lamellae, the cylindrical confinement not only induces the rigid concentric lamella structure having the linear dependence on the confinement degree, but also results in several novel morphologies, such as the connective concentric lamella and the broken concentric lamella structures. The results are quantitatively discussed in a wide range of confinement degree and can be reasonably understood based on symmetry breaking and structural frustration. In addition, our results are quantitatively compared to the available observations from the simulations and experiments, which are in good agreements and may be helpful to experimentally fabricate the ordered nanostructures on the large scale.     

Surface-induced phase transitions in dense nanoparticle arrays of lamella-forming diblock copolymers

The surface-induced phase transitions in dense nanoparticle arrays of lamella-forming diblock copolymers are investigated by using the real-space self-consistent field theory. The dense nanoparticle array provides a distinct and strong confinement environment where there are incomplete confinements in three spatial directions. Several complicated phases with cubic symmetries, such as the monocontinuous and bicontinuous phases, are identified in the dense nanoparticle arrays. Through adjusting the strength of surface preference, the disorder phases are induced into the complicated order phases in the nanoparticle arrays with small periods while the order-order transitions occurs in those with large periods. Investigations on the free and entropic energies indicate that these surface-induced phase transitions are of first-order and the stabilities of order phases are intimately correlated to the strength of surface preference. Our results enrich the knowledge about the phase behaviors of macromolecules in confined systems, which may be helpful to fabricate the novel nanomaterials based on the block copolymers.     

Phase behaviors of bidisperse nanoparticle/block copolymer mixtures in dilute solutions

The complex microstructures of bidisperse nanoparticles/diblock copolymer mixtures in dilute solutions have been investigated by a theoretical approach which combines the self-consistent field theory (SCFT) and the density functional theory (DFT). Special attention is payed to the role played by the block ratio and the interaction parameters between each component in the mixture. It is shown that the conformational entropy of the polymer chains, the block ratio of the diblock copolymer, the chemical difference between two kinds of particles and the steric packing effect of the particles play important roles in determining the morphologies of the systems. It is found that with the increase of the block ratio, the mixture undergoes a morphological transition from compound micelles to spherelike micelles. The increase of chemical difference between the two kinds of particles can promote the formation of “a jujube set in a cake”. When the selectivity of the particles is changed, another type of micelle emerges. Specifically, in the case where the particles are nonselective to the A- and B-blocks, ordered structures from the phase separation between the two types of particles emerge inside the micelles formed by the amphiphilic diblock copolymers in solutions.     

Synthesis of an amphiphilic copolymer bearing rhodamine moieties and its self-assembly into micelles as chemosensors for Fe in aqueous solution

We report on the fabrication of an amphiphilic random copolymer-based colorimetric and fluorescent chemosensor for Fe³⁺ ions that was prepared by free radical polymerization of a novel rhodamine-based Fe³⁺-recognizing monomer, R6GEM, with N-vinylpyrrolidone (NVP). Because of its amphiphilic property, the copolymer P(NVP-co-R6GEM) can self-assemble into micelles, which allows it to be used as a chemosensor in aqueous solution. Upon addition of Fe³⁺ ions to the micelle solution, visual color change and fluorescence enhancement were observed. Moreover, other metal ions did not induce obvious changes to the absorption and fluorescence spectra. The water dispersibility and biocompatibility of these polymer micelles could provide a new strategy for detecting analytes in environmental and biological systems.