Fabrication of fibril like aggregates by self-assembly of block copolymer mixtures via interpolymer hydrogen bonding

This paper describes the formation of fibril like aggregates from the self-assembly of block copolymer mixture (polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) and polystyrene-b-poly(acrylic acid) (PS-b-PAA)) via interpolymer hydrogen bonding in nonselective solvent. The hydrogen bonding between P4VP and PAA in chloroform leads to the formation of complex. When all the pyridine units in P4VP were all hydrogen bonded to acrylic acid in PAA, the formed complex is insoluble, resulting in the formation of spherical micellar aggregates and nanorods. However, two kinds of supramolecules with insoluble or soluble complex are formed in the solution when PS-b-P4VP and PS-b-PAA are mixed with equal mole ratio. The fibril like aggregates can be formed from the self-assembly of supramolecule with soluble complex during spin-coating process. The effects of evaporation rate of solvent and solution concentrations on the formation of fibril like aggregates were investigated. The results prove that the kinetic factors play an important role in the formation of the fibril like aggregates.     

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.     

A new approach in the study of tethered diblock copolymer surface morphology and its tethering density dependence

A poly(l-lactic acid)-block-polystyrene-block-poly(methyl methacrylate) (PLLA-b-PS-b-PMMA) triblock copolymer was synthesized with a crystalline PLLA end block. Single crystals of this triblock copolymer grown in dilute solution could generate uniformly tethered diblock copolymer brushes, PS-b-PMMA, on the PLLA single crystal substrate. The diblock copolymer brushes exhibited responsive, characteristic surface structures after solvent treatment depending upon the quality of the solvent in relation to each block. The chemical compositions of these surface structures were detected via the surface enhanced Raman scattering technique. Using atomic force microscopy, the physical morphologies of these surface structures were identified as micelles in cyclohexane and “onion”-like morphologies in 2-methoxyethanol, especially when the PS-b-PMMA tethered chains were at low tethering density.     

Deposition of silver nanoparticles on single wall carbon nanotubes via a self assembled block copolymer micelles

We demonstrate a facile route to decorate the surface of networked single walled carbon nanotubes (SWNTs) with silver nanoparticles (Ag NPs). The method is based on utilization of either spherical poly(styrene-b-4vinylpyridine) (PS-b-P4VP) or cylindrical poly(styrene-b-acrylic acid) (PS-b-PAA) copolymer micelles capable of stabilizing nanotubes in solution and subsequently forming a thin and uniform block copolymer/SWNTs composite film upon spin coating. The selective doping of silver acetate into either P4VP or PAA domains in a thin composite film, followed by thermal treatment, results in the formation of Ag NPs in the cores of micelles. Further heat treatment at 500 °C sufficiently high for degrading both block copolymers allows us to fabricate a thin SWNTs network in which Ag NPs are efficiently deposited on the surface of nanotubes. A sharp surface plasmon absorption band around 400 nm of the networked SWNTs with Ag NPs confirms the presence of Ag NPs with narrow distribution in their size.     

Synthesis of Amphiphilic Diblock Copolymers by DPE Method

Amphiphilic diblock copolymers, poly(methyl methacrylate)-b-poly(acrylic acid) (PMMA-b-PAA) and polystyrene-b-poly(acrylic acid) (PS-b-PAA), were prepared by 1,1-diphenylethene (DPE) method under mild conditions. Firstly, free radical polymerization of tert-butyl acrylate (tBA) was carried out with AIBN as initiator in the presence of DPE, giving a DPE-containing precursor, PtBA, with controlled molecular weight. Secondly, methyl methacrylate and styrene were polymerized in the presence of PtBA precursor, and PS-b-PtBA and PMMA-b-PtBA diblock copolymers with controlled molecular weights were obtained respectively. Finally, amphiphilic diblock copolymers, PMMA-b-PAA and PS-b-PAA, were prepared by hydrolysis of PS-b-PtBA and PMMA-b-PtBA. The formation of PS-b-PAA and PMMA-b-PAA was confirmed by ¹H NMR. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) were used to detect the self-assembly behavior of the amphiphilic diblock polymers in tetrahydrofuran (THF).     

Micromolding of PDMS scaffolds and microwells for tissue culture and cell patterning: A new method of microfabrication by the self-assembled micropatterns of diblock copolymer micelles

We introduce an innovative fabrication of the polymer scaffolds for tissue culture by utilizing the evaporation induced self-assembled micropatterns of polystyrene-block-poly(acrylic acid) (PS-b-PAA) diblock copolymer micelles. The microstructures were used as templates for micromolding a silicon elastomer, poly(dimethylsiloxane) (PDMS), into tissue scaffolds and microwells for cell patterning purpose. Cultivation of human epithelial cells (Calu-3 cell line) on the PDMS scaffolds demonstrates potential applications in tissue engineering and cell-based biosensors. The reported method is rapid, simple, economical, and versatile comparing with the existing microfabrication techniques.     

Water-induced morphology evolution of block copolymer micellar thin films

Morphology evolution of diblock copolymer polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) micellar thin film in the presence of water was investigated. Surface holes with nanoscale cavities in hexagonal order could be induced by water treatment for certain periods. The nanoscale surface cavities could be transformed into isolated nanospheres in a dry environment or back to protruding densely packed spheres by toluene (a selective solvent for PS coronae) treatment. The morphology evolution of micellar thin film strongly depended on the slow evaporation of toluene solvent, the swelling of P4VP cores in the humid environment, and the subsequent movement of PS chains induced by air and toluene. The incompatibility between solvent and block, and that between the unlike blocks also played an important role in the morphology evolution.     

Morphology of polystyrene/polystyrene-block-poly(methyl methacrylate)/poly(methyl methacrylate) composite particles

Polystyrene/polystyrene-block-poly(methyl methacrylate)/poly(methyl methacrylate) (PS/PS-b-PMMA/PMMA) composite particles were prepared by releasing toluene from PS/PS-b-PMMA/PMMA/toluene droplets dispersed in a sodium dodecyl sulfate aqueous solution. The morphology of the composite particles was affected by release rate of toluene, the molecular weight of PS-b-PMMA, droplet size, and polymer composition. ‘Onion-like’ multilayered composite particles were prepared from toluene droplets of PS-b-PMMA and of PS/PS-b-PMMA/PMMA, in which the weights of PS and PMMA were the same. The layer thicknesses of the latter multilayered composite particles increased with an increase in the amount of the homopolymers. PS-b-PMMA/PS composite particles had a sea-islands structure, in which PMMA domains were dispersed in a PS matrix. On the other hand, PS-b-PMMA/PMMA composite particles had a cylinder-like structure consisting of a PMMA matrix and PS domains.     

Study of the time evolution of the surface morphology of thin asymmetric diblock copolymer films under solvent vapor

The time development of the surface morphology of asymmetric polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) thin films ‘annealing’ in methanol vapor, a selective solvent for minority P4VP block, was investigated by atomic force microscopy(AFM). For PS-b-P4VP with cylindrical structure in bulk, as annealing time progressed, the surface morphology underwent structural transitions from featureless topography to hybrid morphology of cylindrical and spherical pits, to cylinders, to nanoscale depressions, back to cylinders again. The different film thickness made the number of the transitions observed, at any given annealing time, different. The thicker the film is the more transitions at a given annealing time can be observed. If the film was not thick enough, depressions appeared. For PS-b-P4VP with spherical structure in bulk, it displayed nanoscale depressions with the annealing time increasing. A possible mechanism of the transition of morphologies during solvent annealing was proposed.     

Controlled organization of self-assembled rod-coil block copolymer micelles

Spherical micelles of a series of poly(styrene-block-(2,5-bis[4-methoxyphenyl]oxycarbonyl)styrene) (PS-b-PMPCS) rod-coil diblock copolymers in a selective solvent can organize into large mono-layered films with a well-ordered hexagonal packing of the spheres after solvent evaporation. Organized domains in the spherical micelle film were observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The core-shell structure of the spherical micelle remained after solvent evaporation. The micelle diameter in the ordered film as observed by TEM and AFM agree. The size of the spherical micelles can be controlled by the length of PMPCS when the length of the PS is fixed. The sphere diameters were varied from several tens of nanometers to more than one hundred nanometers. Solutions of smaller micelle spheres formed less ordered films than those from larger micelle particles. Additionally, monolayer films of cylindrical worm-like micelles were also prepared. Those cylindrical micelles were observed to be end-capped by spherical micelles. The monolayer micelle film from the largest spherical micelles appeared red when observed in optical microscopy in the reflection mode. A broad adsorption peak with a maximum adsorption wavelength of 545 nm was observed via UV-Vis spectroscopy.     

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.     

Synthesis and characterization of polystyrene-block-polyisoprene nanofibers with different crosslinking densities

Polystyrene-block-polyisoprene or PS-b-PI with 74 styrene and 230 isoprene units was prepared by anionic polymerization. The diblock formed cylindrical micelles with PI core and PS corona in a PS-selective solvent N,N-dimethylacetamide. Nanofibers with different crosslinking densities were obtained after PI core crosslinking with differing amounts of sulfur monochloride. The nanofibers were characterized by elemental analysis, FTIR, transmission electron microscopy, and light scattering.     

Vesicle formation of polystyrene-block-poly (ethylene oxide) block copolymers induced by supercritical CO2 treatment

A novel route for a preparation of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) block copolymer vesicles induced by supercritical carbon dioxide (scCO₂) is demonstrated. When PS-b-PEO block copolymer solutions in tetrahydrofuran (THF) are treated with scCO₂ at 70 °C for different times, PS-b-PEO copolymers first assemble into aggregated spheres; then aggregated spheres change into large compound micelles and finally evolve into vesicles. The possible formation mechanism of the vesicles is discussed.     

Tuned phase behavior of weakly interacting polystyrene-block-poly(n-pentyl methacrylate) by selective solvent

We investigated, via small angle X-ray scattering, depolarized light scattering, rheometry, and transmission electron microscopy, the phase behavior of the mixture of a symmetric polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS-b-PnPMA) showing the closed-loop phase behavior and excellent baroplasticity, and dodecanol, a PnPMA-selective solvent. We found that the addition of a selective solvent is simple, but very effective to obtain various microdomains including hexagonally packed cylinders and gyroids. Also, with increasing temperature, the mixtures showed multiple ordered-to-ordered transitions (OOTs) in addition to upper ordered-to-disordered transition (UODT). The first observation of gyroid microdomains in PS-b-PnPMA is very important, although they have been widely reported in many block copolymers, for instance, PS-block-polyisoprene copolymer (PS-b-PI) and PS-block-poly(d,l-lactide) copolymer (PS-b-PLA). Since the gyroid microdomains of PS-b-PnPMA show excellent baroplasticity, external pressure instead of temperature could easily change the microdomains.     

Effects of additives on the morphologies of thin titania films from self-assembly of a block copolymer

The effects of additives of poly(methyl methacrylate) (PMMA) and HAuCl₄ on the morphologies of hybrid titania films formed via co-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymers, titania sol-gel precursor in a selective solvent were investigated. The results show that addition of PMMA or HAuCl₄ has an important influence on the morphologies of hybrid titania films. Addition of PMMA or HAuCl₄ can induce the morphology transition of the PS-b-PEO/titania sol-gel mixture from spherical micelles to vesicles. Therefore, the morphologies of the hybrid films formed on silicon substrate surfaces by spin-coating can be controlled by the addition of homopolymer (PMMA) or inorganic precursor (HAuCl₄) into the PS-b-PEO/titania sol-gel mixtures, allowing access to nanoparticles or nanoporous films. After removing the polymer matrix, nanoparticle aggregates or nanobowl-like structures are left behind on the substrate surfaces.     

Morphology change of asymmetric diblock copolymer micellar films during solvent annealing

The morphology change of an asymmetric polystyrene-block-poly(2-vinyl pyridine) (PS-b-PVP) diblock copolymer micellar film was investigated during solvent vapor annealing in chloroform. Initially, smaller islands in nanometer-length scale form at the film surface. Further annealing results in the growth of the islands composed of the PS-b-PVP cylinders above the bottom brush layer. For comparison, a film of the block copolymer prepared from THF solution (without micellar structure) was also studied. The surface morphology of the film from THF evolves via spinodal dewetting mechanism during solvent vapor annealing. At a long time solvent vapor annealing, the two kinds of the films display the same surface morphologies, which are determined by the interplay between the surface field and autodewetting.     

Block copolymer photonic mesogels exhibiting dual volume phase transitions

Photonic mesogel films exhibiting brilliant photonic colors were prepared by selective swelling of polystyrene-b-quaternized poly(2-vinyl pyridine) (PS-b-QP2VP) block copolymers, and their volume phase transition behaviors were investigated in various solvent mixtures. The swollen PS-b-QP2VP lamellae segregate into mesophased gels where the highly swollen QP2VP gel layers are alternating with the glassy PS layers and exhibit strong responsive photonic colors in visible regime. Utilizing the changes of photonic stop bands, the swelling behaviors of the photonic mesogels were able to be monitored with sub-nanometer accuracy. Unusual dual volume phase transitions were observed at certain conditions where hydrogen bonding became significantly strong.     

Effects of substrate roughness on the orientation of cylindrical domains in thin films of a polystyrene-poly(methylmethacrylate) diblock copolymer studied using atomic force microscopy and cyclic voltammetry

This paper describes the orientation of cylindrical domains in thin films of a polystyrene-poly(methylmethacrylate) diblock copolymer (PS-b-PMMA; 0.3 as the PMMA volume fraction) on gold and oxide-coated Si substrates having different surface roughness. Atomic force microscopy images of PS-b-PMMA films having thickness similar to the domain periodicity permitted us to study the effects of substrate roughness and block affinity on domain orientation. PS-b-PMMA films on gold substrates showed metastable vertical domain orientation that was attained more slowly on rougher substrates. In contrast, the domains were horizontally oriented on oxide-coated Si regardless of surface roughness and the annealing conditions examined. In addition, cyclic voltammetry data for PS-b-PMMA films on gold substrates whose PMMA domains were etched suggested that the metastable vertically oriented domains reached the underlying substrates. These results indicate that PS-b-PMMA films containing vertically oriented cylindrical domains can be obtained by using rough gold substrates upon annealing under controlled conditions.     

Structure and phase transition in thin films of block copolymer micelles complexed with inorganic precursors

Block copolymer micelle can be used as nano-reactor where separated domains serve as a compartment for the production of nanomaterials, ultimately creating nanocomposite materials. In this work, thin nanocomposite films generated from polystyrene-b-poly(acrylic acid) (PS-b-PAA) micellar solution in which small amount of inorganic precursor was added were investigated. The films were prepared by spin coating onto silicon substrate, and then solvent-annealed. As-spun films exhibit typical micellar structure with spherical shape along which inorganic nanoparticles are dispersed. Such morphology remains unchanged after solvent annealing for micellar films with small amount of inorganic precursor. However, further increase in the amount of inorganic precursors brings about the morphological changes, producing different organization of inorganic nanoparticles in composite films. This behavior was found to strongly depend on the types of precursors and solvents used for annealing. These results illustrate a simple yet useful route to generate the polymeric nanocomposites with diverse structure and composition.     

Control over the surface plasmon band of block copolymer/Ag/Au nanoparticles composites by the addition of single walled carbon nanotubes

In this study, we demonstrate control over the localized surface plasmon band (SPB) of a micellar poly(styrene-block-4vinylpyridine) (PS-b-P4VP) copolymer thin film composite that includes Ag and Au nanoparticles (NPs) in the presence of single walled carbon nanotubes (SWCNTs). Ag and Au NPs are preferentially located in the P4VP core and the PS corona of ordered spherical PS-b-P4VP copolymer micelles, respectively. This structure gave rise to a single SPB due to the coupling of Ag and Au SPBs. The non-covalent addition of SWCNTs in the block copolymer micelles shifts the coupled SPB to a lower wavelength. The maximum shift in the coupled SPB of approximately 30 nm was achieved in the PS-b-P4VP/Ag/Au NPs composite. The carbon nanotube induced modulation of the coupled SPB stems from the charge accumulation effect of the SWCNTs placed between the two types of nanoparticles.