Morphological Change of Aggregates of Polystyrene-block-poly (2-vinyl pyridine)/HAuCl4 Complexes in Solution

The self-assembly of polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP)/HAuCl₄ complexes in solutions is described. A morphological change from spheres to aggregated spheres, and then to vesicles, all of which contain Au precursor within the micellar corona or vesicle shell, can be prepared from an identical diblock copolymer by controlling the solvent selectivity. The possible mechanisms leading to the morphological transitions are also discussed. This may be a simple, facile route for preparation of a desired gold-containing aggregate.     

Effect of added homopolymer on structures of thin films of PS-b-PDMS/PS mixture under solvent vapor annealing

Self assembly of poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) followed by reactive ion etching technique is a promising method for fabricating periodical silica nanopatterns and can be applicable for device fabrication on nanoscale. We demonstrated a technologically useful way to control the inorganic silica nanostructures in thin films by directly mixing asymmetric (PS-b-PDMS) diblock copolymer with homopolymers of majority component, polystyrene (PS) under solvent vapor annealing followed by UV/O₃ treatment. The effects of molecular weight and volume fraction of added homopolymer (PS) on morphology and size of the nanostructure of blends have been carefully investigated by atomic force microscopy. Different morphology transitions observed on the ordering film surface by atomic force microscopy (AFM) are associated with kinetics of phase evolution with respect to homo-PS with different molecular weight. The periodic spacings and dimensions of the microdomains were readily tuned at the same time, just by adjusting the molecular weight and volume fraction of the blended homopolymer.     

Catalytic properties of gold nanoparticles immobilized on the surfaces of nanocarriers

Gold nanoparticles are immobilized in the hydrophilic coronas of spherical micelle carriers for high catalytic activity. The micelle is formed by self-assembly of block copolymer, polystyrene-b-poly (acrylic acid), in basic aqueous solution (pH 10) and has a polystyrene core and a poly (acrylic acid) corona. The gold nanoparticles are anchored into the poly (acrylic acid) corona by in situ reduction of the mixture of HAuCl₄ and micelle with NaBH₄. The sizes of the gold nanoparticles can be adjusted by changing the content of the HAuCl₄. In the process of catalyzing p-nitrophenol to p-aminophenol, the reaction shows one-order kinetics, furthermore, the reaction rate increases with the concentration of composites as well as reaction temperature. Comparing the composites with polystyrene as core and poly (4-vinylpyridine)/Au as corona, the catalytic activity of the present composites is higher, which is ascribed to their hydrophilic corona structure.     

Self-assembled block polymer templates as high resolution lithographic masks

Diblock copolymer thin films have recently received more attention due to their ability to organize into nanometric structures under thermal annealing. This phenomenon was studied for an asymmetric poly(styrene-block-methyl methacrylate) (PS-b-PMMA) diblock copolymer with PMMA weight fraction of 0.3 and MW = 67,100 g mol⁻¹. First, the surface chemistry of the substrate was modified to favor the formation of vertical PMMA cylinders surrounded by a PS matrix. We have also found that the mean pore area of cylinders increases with their coordination number. Finally, these films were used as a deposition or etching mask to produce well-organized arrays of holes, dots and nanopillars.     

The effect of polystyrene-block-poly(4-vinylpyridine) prepared by a RAFT method in the dispersion polymerization of styrene

The dispersion polymerization of styrene has been carried out using polystyrene-block-poly(4-vinylpyridine) copolymer [P(S-b-4VP)], prepared by a reversible addition-fragmentation chain transfer (RAFT) method, as a steric stabilizer in alcohol media. These block copolymer contains a long poly(4-vinylpyridine) block and a short polystyrene block. The stable spherical particles were obtained when the block copolymer concentrations increased from 2 to 20 wt.% relative to the monomer and the average particle sizes decreased from 340 to 200 nm with increasing concentration of the block copolymer. Alcoholic solvents, from methanol to n-hexanol, are responsible for the particle size. These results indicate that the poly(S-b-4VP) block copolymer is effective for providing polystyrene nano-sized particles with a low content of it working as a good stabilizer in any kind of alcoholic medium.     

Crystallization Behavior of PEO in Nano‐Structured PEO‐b‐PS/PPO Blends

Blends of poly (ethylene oxide)‐b‐polystyrene (PEO‐b‐PS) diblock copolymer and poly (2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) homopolymer were obtained by solution blending, and the morphologies of PEO dispersed nanoparticles in PPO/PS matrix were observed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The isothermal crystallization kinetics was studied using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Nonisothermal crystallization kinetics was studied using DSC. The results showed that PEO segments were easier to crystallize in the blend than in the copolymer probably due to the interfaces of PPO acting as nucleation sites to promote the crystallization of PEO. The crystallization of PEO blocks destroyed the pre‐existing microdomain structure even though the glass transition temperature of the matrix was much higher than the crystallization temperature.     

Fluorescence enhancement of the conjugated polymer films based on well-ordered Au nanoparticle arrays

In this paper, well-ordered Au nanoparticle arrays on silicon substrates were employed as efficient metal-enhanced fluorescence (MEF) substrates for investigating the fluorescence properties of the conjugated polymer poly(3-hexylthiophene) (P3HT). The ordered Au nanoparticle arrays were fabricated by block copolymer self-assembly technology, and the particle sizes were controlled by adjusting the molar ratios of HAuCl₄ precursor to vinyl pyridine units. The approach is economical and suitable to fabricate large-area MEF substrates. The results about fluorescence properties of P3HT showed that the fluorescence intensities of the P3HT films were improved on ordered Au nanoparticle arrays compared to those on bare silicon substrate and were significantly enhanced with the Au nanoparticle sizes increasing. The mechanism is based on localized surface plasmon resonances, coupling and propagating surface plasmons, and the emission enhancement mainly resulted from the increase of the excitation rate. This work provides a new way to prepare efficient MEF substrates for high-performance fluorescence-based devices.     

A simple pathway to ordered silica nanopattern from self-assembling of block copolymer containing organic silicon block

Self-assembly of block copolymer is an effective strategy to prepare periodic structures at nanoscale. In this paper an unique and very simple method to prepare inorganic silica nanopattern is demonstrated from self-assembling of poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) on the surface of silicon wafer. To simplify the patterning process, at first we obtain highly ordered PDMS microdomains, which are covered with PS layer by controlling solvent vapor annealing conditions. Following exposure to UV/O₃ irradiation, nanopatterned surface consisting of silicon oxide is fabricated directly via selectively etching PS phase and converting PDMS phase into silicon oxide. As tuning the composition of the block copolymer, hexagonally packing dot and straight stripe pattern can be obtained. Finally, the time evolution from spheres morphology to aligned long cylinders is discussed. These results hold promise for nanolithography and the fabrication of nanodevices.     

Polyblends of poly(styrene-b-butadiene-b-styrene) and polystyrene. II. Electron microscopy

Electron micrographs of a series of polyblends of poly(styrene-b-butadiene-b-styrene (SBS) and polystyrene (PS) are presented. These poly blends were cast from three different solvents, i.e., tetrahydrofuran—methyl ethyl ketone (THF/ MEK), benzene—heptane, and carbon tetrachloride. PS of four different molecular weights were used. It is shown that when the molecular weight of PS exceeds that in the block copolymer, a third phase in the form of “islands” is formed. Lamellarlike structures are formed in the block domains upon the addition of homopolymer. Stretching perturbs the morphology of the block domains somewhat, and produces craze marks in the PS islands. The craze marks are removed upon annealing. These results are discussed in conjunction with the observed mechanical behavior of the same polyblends reported in Part I of this series.     

Electrocatalytically Active Hollow Carbon Nanospheres Derived from PS‐b‐P4VP Micelles

A facile method using polystyrene‐b‐poly(4‐vinyl pyridine) (PS‐b‐P4VP) micelles is demonstrated to synthesize N/FeN₄‐doped hollow carbon nanospheres (N/FeN₄‐CHNS) with high electrocatalytic activity for oxygen reduction reactions (ORRs). Uniform spherical micelles with PS core and P4VP shell are prepared by exposing PS‐b‐P4VP in a mixture of ethanol/tetrahydrofuran. Pyridinic N in shell cooperates with Fe³⁺ to induce an in situ polymerization of pyrrole. Tuning molecular composition of PS‐b‐P4VP can form hollow carbon spheres with controlled size down to sub‐100 nm that remains challenge using traditional hard template strategies. N/FeN₄‐CHNS possesses a series of desirable properties as electrode materials, including easy fabrication, high reproducibility, large surface area, and highly accessible porous surface. This electrocatalyst exhibits excellent ORR activity (onset potential of 0.976 V vs reversible hydrogen electrode (RHE) and half‐wave potential of 0.852 V vs RHE), higher than that of commercial Pt/C (20 wt%) in an alkaline media, and shows a good activity in an acidic media as well. In addition to its higher stability and methanol tolerance than Pt/C in both alkaline and acidic electrolytes, highly competitive single cell performance is achieved in a proton exchange membrane fuel cell. This work provides a general approach to preparing functionalized small hollow nanospheres based on self‐assembly of block copolymers.     

Influence of Grafting Density of Diblock Copolymers on Interfacial Assembly Behavior and Interfacial Shear Strength of Glass Fiber/Polystyrene Composites

To investigate the influence of the grafting density and the molecular structure of block copolymers on the interfacial assembly behavior and interfacial shear strength, macromolecular coupling agents, hydroxyl-terminated poly(n-butyl acrylate-b-styrene) (HO-P(BA-b-S)) were synthesized by atom transfer radical polymerization, and then chemically anchored on the glass fiber surfaces to form a well-defined monolayer. The phase separation and 'hemispherical' domain morphologies of diblock copolymer brushes at the polystyrene/glass fiber interface were observed. The interfacial assembly morphology differs with changes in the grafting density of diblock copolymers. When the grafting density is greatest, the highest height difference of the hemispherical domain and the largest surface roughness are achieved, as well as the best interface shear strength. It was also found that the copolymer brush with a PBA block of the polymerization degree (Xn) about 77 is the optimal option for the interfacial adhesion of PS/GF composites. Thus, the grafting density and molecular structure of diblock copolymers determines the interfacial assembly behavior of copolymer brushes, and therefore the interfacial shear strength.     

Morphology of Polystyrene Core–dPoly(ethylene oxide) Shell Arborescent Copolymer Micelles from Small-Angle Neutron Scattering Analysis

Arborescent (dendrigraft) copolymers with a branched polystyrene (PS) core grafted at the chain termini with deuterated poly(ethylene oxide) segments (PS-dPEO) were characterized in benzene and acetone by small-angle neutron scattering measurements using the contrast matching technique. While copolymers incorporating a G1 (twice-grafted) PS core aggregated to some extent, the portion of the scattering curve corresponding to non-aggregated copolymer molecules could still be analyzed to determine the shape and segment radial density profile for core and shell. These were derived from the pair distance distribution function P(r) and the scattering length density contrast profile Δρ(r) = ρ(r) ? ρ(solvent), obtained by the indirect Fourier transformation and deconvolution methods. The profiles obtained for the G1 copolymer are consistent with a well-defined PS core–dPEO shell morphology, only observed previously for upper generation (G4) polymers with deuterated PS (dPS) chains grafted randomly on arborescent PS substrates. Detailed morphological analysis could not be carried out for an analogous G3 arborescent PS copolymer terminally grafted with dPEO segments due to extensive aggregation in both solvents.     

One-dimensional optical reflectors based on self-organization of polymeric comb-shaped supramolecules

We demonstrate that complexation of dodecylbenzenesulphonic acid, DBSA, to a diblock copolymer of polystyrene- block-poly(4-vinylpyridine), PS- block-P4VP, leads to polymeric supramolecules PS- block-P4VP(DBSA)y (y = 1.0, 1.5, and 2.0), which self-organize with a particularly large lamellar periodicity in excess of 1000 A. The structures consist of alternating PS and P4VP(DBSA)y layers, where the latter contains smaller internal structure, probably lamellar. The DBSA side chains are bonded to the pyridines by protonation and hydrogen bonding and they effectively plasticize the material. In this way relatively well-developed structures are obtained even without annealing or macroscopic alignment. Transmission and reflectance measurements show that a relatively narrow and incomplete bandgap exists for supramolecules of high molecular weight block copolymer at ca. 460 nm.     

Intermacromolecular complexation due to specific interactions. 6. Miscibility and complexation between Poly{Styrene-co-[p-(2-Hydroxypropan-2-yl)Styrene]}and poly[n-butyl Methacrylate-co-(4-vinylpyridine)]

In recent years, considerable attention has been focused on polymer miscibility and Complexation due to hydrogen bonding. Monodisperse, proton-donating polystyrene (PS), that is, poly{styrene-co-[p-(2-hydroxypropan-2-yl)styrene]} [PS(t-OH)], was synthesized via chemical modification of polystyrene. Poly[n-butyl methacrylate-co-(4-vinylpyridine)] (BVPy), as a proton acceptor, was prepared by free-radical copolymerization of the corresponding monomers at low conversion. In organic solutions of PS(t-OH)/BVPy blends, viscometry was employed to study the Complexation behavior. Solvents with different proton-accepting abilities were used and hence proved to exert distinctive effects on solution complexation. In very dilute solutions, the complex aggregate was observed by static and dynamic light scattering (LS). Differential scanning calorimetry (DSC) investigation proved that miscibility could be greatly enhanced when a small amount of hydrogen bonding was introduced into the originally immiscible PS/PBMA (polybutyl methacrylate) system. Although it was found that the T _g of the hydrogen-bonding polymer complexes occurred at higher temperatures with respect to linear weight-average value, DSC measurement alone could not accurately distinguish polymer complexes from ordinary miscible blends. Based on the data of miscibility from DSC and complexation from vis-cometry in 1,2-dichloroethane for a few tens of blends, which cover broad ranges of the contents of interaction sites, a map showing the immiscibility-miscibility-complexation transitions by strengthening the hydrogen bonding for the system of PS(t-OH)/BVPy was successfully constructed.     

Block copolymer adsorption from a homopolymer melt to an amine-terminated surface

Using neutron reflectometry, the adsorption of diblock copolymers from a neutral polystyrene (PS) matrix is studied as a function of substrate type and non-adsorbing block degree of polymerization. The block copolymer is poly(deutero styrene)-block-poly(methyl methacrylate) and the substrates are silicon oxide, SiO_x, and SiO_x functionalized with (3-aminopropyl)triethoxysilane (APTES). We have determined the equilibrium volume fraction-depth profiles for such films, and compared them with volume fraction profiles generated by self-consistent mean-field (SCMF) theory and find good agreement between the experimental and theoretical data. SCMF calculations show that the segmental interaction energy between PS matrix chains and APTES is two orders of magnitude stronger than that between PS and SiO_x.     

Patterning of gold nano-octahedra using electron irradiation combined with thermal treatment and post-cleaning process

A novel approach to pattern nanocrystalline gold (Au) octahedra is presented based on electron irradiation combined with thermal treatment and post-cleaning process using HAuCl₄-loaded poly(styrene-b-2-vinyl pyridine) (PS-b-P2VP) block copolymer (BCP) as a precursor material. The BCP tends to cross-link under electron irradiation, and thus a patterned film can be prepared by selectively irradiating an electron beam onto a precursor film using a shadow mask. A post-thermal treatment leads to the formation of crystalline Au nano-octahedra inside the patterned film with a help of the BCP acting as a capping agent. Subsequently, the BCP can be removed by O₂ plasma etching combined with oxidative degradation, with the Au nanoparticles remaining. As a result, a patterned film consisting of high-purity nanocrystalline Au octahedra is fabricated. The sizes of the Au octahedral nanoparticles can be readily controlled from 49 to 101 nm by changing the thickness of the precursor film. The patterned Au nano-octahedra films exhibit excellent surface-enhanced Raman scattering behavior with the maximum enhancement factor of ~10⁶.     

End group effect on surface and interfacial segregation in PS-PMMA blend thin films

Thin films of polystyrene (PS)/poly (methyl methacrylate) (PMMA) blends with different end groups were investigated using ToF-SIMS and AFM. PS with -OH and -NH₂ end groups were blended in toluene solvent with pure PMMA homopolymer, and PMMA having anhydride end group. The ToF-SIMS spectra of PS-OH/PMMA resembled that of pure PS-PMMA blends showing an increase of PMMA intensity after annealing. On the contrary, the PS-NH₂ blended with PMMA showed an increase in PS intensity on the surface after annealing. The ToF-SIMS spectra were similar to that of a pure PS-PMMA di-block copolymer. These results indicate copolymer formation at the surface. The PS-NH₂ with PMMA-anhydride blend spectra showed very slight changes in spectra before and after annealing and the AFM images revealed spinodal bi-continuous structures on the surface before and after annealing. The copolymer formation is found to occur in the as-cast film itself and not after thermal treatment.     

Model for UV induced formation of gold nanoparticles in solid polymeric matrices

UV irradiation of polymeric PMMA films containing HAuCl₄ followed by annealing at 60-80 °C forms gold nanoparticles directly within the bulk material. The kinetics of nanoparticle formation was traced by extinction spectra of nanocomposite film changes vs annealing time. We propose that UV irradiation causes HAuCl₄ dissociation and thus provides a polymeric matrix with atomic gold. The presence of an oversaturated solid solution of atomic gold in the polymeric matrix leads to Au nanoparticle formation during annealing. This process can be understood as a phase transition of the first order. In this paper we apply several common kinetic models of the phase transition for describing Au nanoparticle formation inside the solid polymer matrix. We compare predictions of these models with the experimental data and show that these models cannot describe the process. We propose that the stabilization effect of the matrix on the growing gold nanoparticles is important. The simplest model introducing some probability for the transition from growing nanoparticle to the non-growing, stabilized form is suggested. It is shown that this model satisfactorily describes the experimentally observed evolution of the extinction spectrum of Au nanoparticles forming in a polymer matrix.     

Confinement and processing effects on glass transition temperature and physical aging in ultrathin polymer films: Novel fluorescence measurements

Fluorescence intensity measurements of chromophore-doped or -labeled polymers have been used for the first time to determine the effects of decreasing film thickness on glass transition temperature, T _g, the relative strength of the glass transition, and the relative rate of physical aging below T _g in supported, ultrathin polymer films. The temperature dependence of fluorescence intensity measured in the glassy state of thin and ultrathin films of pyrene-doped polystyrene (PS), poly(isobutyl methacrylate) (PiBMA), and poly(2-vinylpyridine) (P2VP) differs from that in the rubbery state with a transition at T _g. Positive deviations from bulk T _g are observed in ultrathin PiBMA and P2VP films on silica substrates while substantial negative deviations from bulk T _g are observed in ultrathin PS films on silica substrates. The relative difference in the temperature dependences of fluorescence intensity in the rubbery and glassy states is usually reduced with decreasing film thickness, indicating that the strength of the glass transition is reduced in thinner films. The temperature dependence of fluorescence intensity also provides useful information on effects of processing history as well as on the degree of polymer-substrate interaction. In addition, when used as a polymer label, a mobility-sensitive rotor chromophore is demonstrated to be useful in measuring relative rates of physical aging in films as thin as 10 nm. Received 21 August 2001