Three‐dimensional structural analysis of a block copolymer by scanning electron microscopy combined with a focused ion beam

A three‐dimensional (3D) lamellar structure of a poly(styrene‐block‐isoprene) block copolymer was observed at submicrometer and micrometer levels by scanning electron microscopy combined with a focused ion beam (FIB–SEM). The 3D lamellar structure with an exceptionally large periodicity, about 0.1 μm, was successfully reconstructed, and the size of the reconstructed image by FIB–SEM was 6.0 × 6.0 × 4.0 μm³, which was greater than the transmission electron microtomography data, 3.8 × 3.9 × 0.24 μm³, by a factor of about 40. This result indicates that 3D reconstruction using FIB–SEM is quite useful for direct 3D observations, especially analyses of polymeric materials at the submicrometer and micrometer levels. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 677–683, 2007     

The correlation between ion beam/material interactions and practical FIB specimen preparation.

The focused ion beam (FIB) tool has been successfully used as both a stand alone analytical instrument and a means to prepare specimens for subsequent analysis by SEM, TEM, SIMS, XPS, and AUGER. In this work, special emphasis is given to TEM specimen preparation by the FIB lift-out technique. The fundamental ion/solid interactions that govern the FIB milling process are examined and discussed with respect to the preparation of electron transparent membranes. TRIM, a Monte Carlo simulation code, is used to physically model variables that influence FIB sputtering behavior. The results of such computer generated models are compared with empirical observations in a number of materials processed with an FEI 611 FIB workstation. The roles of incident ion attack angle, beam current, trench geometry, raster pattern, and target-material-dependent removal rates are considered. These interrelationships are used to explain observed phenomena and predict expected milling behaviors, thus increasing the potential for the FIB to be used more efficiently with reproducible results.     

The effects of fabrication and annealing on the structure and hydrogen permeation of Pd–Au binary alloy membranes

The addition of gold to palladium membranes produces many desirable effects for hydrogen purification, including improved tolerance of sulfur compounds, reduction in hydride phase formation, and, for certain compositions, improved hydrogen permeability. The focus of this work is to determine if sequential plating can be used to produce self-supported alloy membranes with equivalent properties to membranes produced by conventional metallurgical techniques such as cold-working.Sequential electroplating and electroless plating were used to produce freestanding planar Pd–Au membranes with Au contents ranging from 0 to 20 wt%, consisting of Au layers on both sides of a pure Pd core. Membranes were characterized by single-gas permeation measurements, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), and high temperature, controlled-atmosphere XRD (HTXRD). Sequentially plated foils tested without any prior annealing had significantly lower H₂ permeabilities than either measured or literature values for homogeneous foils of equivalent composition. This effect appears to be due to the formation of stable gold-enriched surface layers. Pretreatment of membranes to 1023 K created membranes with hydrogen permeabilities equivalent to literature values, despite the fact that trace amounts of surface gold remained detectable with XRD.     

Interactions of zeatin with gold ions and biomimetic formation of gold complexes and nanoparticles

We report here a simple one-pot synthesis for the preparation of gold nanoparticles biomimetically using zeatin nanostructures. Zeatin, a plant phytohormone was self-assembled into nanospheres. Those nanospheres transformed into nanoribbons over a period of time upon formation of zeatin–gold (III) complexes in the presence of hydrogen tetrachloroaurate. Further, upon heating, gold nanoparticles were formed due to mineralization in the presence of zeatin nanofibers. The effect of pH on the self-assembly of zeatin and the formation of gold nanoparticles, was investigated. We also compared the preparation of gold nanoparticles in the presence of zeatin nanoribbons, using a known reducing agent such as hydrazine, which resulted in loss of morphology control and alignment of the gold nanoparticles. Thus zeatin nanoribbons act as templates which allow for size as well as alignment control for the gold nanoparticles. The materials obtained were analyzed using FTIR, absorbance spectroscopy as well as by transmission electron microscopy, EDX, SEM and AFM. The method involved here is a mild, green-synthetic process, which could be used for facile preparation of morphology controlled gold nanoparticles and may open up new avenues for device fabrications for a wide range of applications, particularly in optoelectronics and sensors.     

Polymerization and characterization of novel poly(acrylonitrile‐co‐styrene/pyrrole) nanoparticles: A comparison between microwave and conventional method

In this study, poly(acrylonitrile‐co‐styrene/pyrrole) or poly(AN‐co‐ST/Py) copolymer was successfully synthesized using microwave preparation technique, and its comparison with the conventional heating method is investigated. Different polymerization factors affecting on the preparation conditions and conversion yield such as monomer concentration, comonomers ratio, initiator concentration, cosolvent ratio, cosolvent type, polymerization temperature, and polymerization time have a considerable effect on the conversion yield %, functional groups, and molecular weight. The copolymerization process was approved by Fourier transform infrared, thermogravimetric analysis, ¹H NMR spectroscopy, and gel permeation chromatography. The formation of poly(AN‐co‐ST/Py) nanoparticles was confirmed by SEM, and their possible formation mechanisms were also proposed. The SEM images of poly(AN‐co‐ST/Py) prepared by the microwave method showed that the synthesized copolymer had spikes or rods with spherical structure of the produced copolymers than the poly(AN‐co‐ST/Py) nanoparticles prepared by the conventional heating method. Microwave method showed advantages for the produced copolymers compared to that prepared by conventional method, where it can offer a copolymer in short time, high yield, and more thermally stable copolymers, rather than conventional method.     

Formation of antifouling functional coating from deposition of a zwitterionic-co-nonionic polymer via “grafting to” approach

We develop a new process for the preparation of synergistic antifouling functional coatings on gold surfaces via a “grafting to” approach. The strategy includes a synthetic step of polymer brushes that consist of poly (ethylene glycol) (PEG) and zwitterionic side chains via a typical reversible-addition fragmentation chain transfer (RAFT) polymerization process, and a subsequent deposition of the polymer brushes onto a gold substrate. The presence of PEG and zwitterion chains on these polymer brush-coated gold surfaces has been proved to have a synergistic effect on the final antifouling property of the coating. PEG chains lower the electrostatic repulsion between zwitterionic polymer chains and increase their graft density on gold surfaces, while zwitterionic polymer effectively improves the antifouling property that is offered by PEG chains alone. Protein adsorption and cell attachment assays tests are conducted to confirm that this copolymer layer on gold surface has a pronounced resistance against proteins such as Bovine serum albumin and Lysozyme. Importantly, the antifouling property can be systematically adjusted by varying the molar ratio of PEG to zwitterionic chains in the final coating copolymer.     

New approach to fabricate nanoporous gold film

A simple preparation of ultrathin nanoporous gold film was described. Copper and gold were used to fabricate Cu-Au alloy film sthrough vacuum deposition. The formation of nanoporous gold films from the alloy films involved thermal process and chemical etch by hydrochloric acid or by nitric acid. The free-standing nanoporous gold films have been analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectrometer (XPS) and surface-enhanced Raman scattering (SERS). It was noted that the nanoporous gold film etched by hydrochloric acid is uniform with a cover of fog-like moieties.     

Polymer Cages as Universal Tools for the Precise Bottom‐Up Synthesis of Metal Nanoparticles

A template synthesis allows the preparation of monodisperse nanoparticles with high reproducibility and independent from self‐assembly requirements. Tailor‐made polymer cages were used for the preparation of nanoparticles, which were made of cross‐linked macromolecules with pendant thiol groups. Gold nanoparticles (AuNPs) were prepared in the polymer cages in situ, by using different amounts of cages versus gold. The polymer cages exhibited a certain capacity, below which the AuNPs could be grown with excellent control over the size and shape. Control experiments with a linear diblock copolymer showed a continuous increase in the AuNP size as the gold feed increased. This completely different behavior regarding the AuNP size evolution was attributed to the flexibility of the polymer chain depending on cross‐linking. Moreover, the polymer cages were suitable for the encapsulation of AgNPs, PdNPs, and PtNPs by the in situ method.     

Preparation of gold microparticles using halide ions in bulk block copolymer phases via photoreduction

Gold microparticles were prepared from the gold salt in the solid bulk phase of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer via a photoreduction process in the presence of halide ions. The shapes and sizes of the gold microparticles were found to be dependent on the types and amount of halide ions as well as the types of cations used due to the combined effects of the adsorption power and oxidative dissolution ability of the additives on gold surfaces. Gold nanorods were obtained when poly(ethylene oxide) was used instead of the block copolymer. This suggests that the poly(propylene oxide) (PPO) parts in the block copolymer are essential for the formation of gold microparticles, even though the degree of the direct interaction between the PPO blocks and gold salt is not significant.     

Formation of an Inorganic‐Organic Host‐Guest Material by Intercalation of Acetone Formaldehyde Sulfite Polycondensate into a Hydrocalumite Structure

An acetone formaldehyde sulfite based polycondensate (Mw ~ 64 kDa), which is commonly used as a superplasticizer in cement and concrete, was intercalated into a hydrocalumite type LDH structure. Preparation was done by controlled rehydration of tricalcium aluminate in the presence of the polymer. Formation of the LDH phase was confirmed by XRD, IR spectroscopic and TG measurements. Elemental composition of the organo‐mineral phase reveals charge balancing of the cationic LDH main layers by the polycondensate and OH^– ions. Low crystallinity observed by XRD and presence of LDH foils was verified by SEM images. There, the novel Ca‐Al‐LDH phase shows a morphology of intergrown platelets which is typical for layered calcium aluminate hydrates. Depending on the preparation method, ultra‐thin foils with 20 nm–50 nm thickness were observed. TEM images also support presence of a layered structure.     

Gold nanoparticle synthesis in graft copolymer micelles

 An amphiphilic poly(acrylic acid)/polystyrene graft copolymer (PAA-g-PS) has been used to form “nanoreactors” for the synthesis of gold clusters. Such copolymers tend to form stable micelles in non-polar organic solvents where the poly(acrylic acid) chains constitute the core, and the polystyrene chains, the shell. In the present study, the micellar structure of PAA-g-PS in toluene has been demonstrated by dynamic light scattering and transmission electron microscopy (TEM). The subsequent preparation of gold-graft copolymer composites involved the introduction of gold chloride (AuCl₃), either in powder form or previously dissolved in ether, into the micellar cores of the PAA-g-PS in toluene. The gold salt was then reduced by ultraviolet (UV) irradiation of the emulsion, or of dried cast films. TEM and ultraviolet-visible (UV/Vis) spectroscopy were used to characterize the resulting composites. Gold particles of less than 5 nm in diameter were observed in all cases, but the size distribution and the spatial arrangement of the clusters in the cast films were modified when diethyl ether was used to introduce AuCl₃ into the PAA-g-PS micellar cores. This was thought to be due to enhanced nucleation of the gold particles and partial disruption of the micellar cores in the presence of diethyl ether. Received: 21 January 1998 Accepted: 11 June 1998     

Imaging thin films of nanoporous low-k dielectrics: comparison between ultramicrotomy and focused ion beam preparations for transmission electron microscopy.

Ultramicrotomy, the technique of cutting nanometers-thin slices of material using a diamond knife, was applied to prepare transmission electron microscope (TEM) specimens of nanoporous poly(methylsilsesquioxane) (PMSSQ) thin films. This technique was compared to focused ion beam (FIB) cross-section preparation to address possible artifacts resulting from deformation of nanoporous microstructure during the sample preparation. It was found that ultramicrotomy is a successful TEM specimen preparation method for nanoporous PMSSQ thin films when combined with low-energy ion milling as a final step. A thick, sacrificial carbon coating was identified as a method of reducing defects from the FIB process which included film shrinkage and pore deformation.     

Synthesis and characterization of a novel hydroxypropyl chitosan-graft-β-Cyclodextrin copolymer as potential drug carrier

Abstract

One of the most attractive fields in drug delivery is the design of nanoparticles that could be used to transport drugs into the target place at appropriate time. This paper describes the preparation of a novel hydroxypropyl chitosan-graft-β-cyclodextrin (HPCS-g-β-CD) copolymer using the 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) promoted condensation reaction. Structural characterization was finished with FTIR and ¹H NMR spectra, which confirmed the grafting of β-cyclodextrin (β-CD) onto hydroxypropyl chitosan (HPCS). The thermal properties were studied based on DSC and TGA analyses, which revealed the fact that grafting of β-CD onto HPCS resulted in obvious enhancement of the decomposition temperature of the copolymer. Aqueous solution of HPCS-g-β-CD copolymer was stable at pH 7-8 based on the Zeta potential test. The morphological characteristics were investigated by SEM, TEM and AFM analysis, and the results indicated that this copolymer had three-dimensional structure which was suitable for loading drugs.     

Monolayer-protected gold nanoparticles by the self-assembly of micellar poly(ethylene oxide)-b-poly(epsilon-caprolactone) block copolymer

A study is presented of the preparation of gold nanoparticles incorporated into biodegradable micelles. Poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PEO-b-PCL) copolymer was synthesized by ring-opening polymerization, and the hydroxyl end group of the PCL block was modified with thioctic acid using dicyclohexyl carbodiimide as the coupling reagent. The PEO-b-PCL-thioctate ester (TE) thus obtained was used in a later step to form monolayer protected gold nanoparticles via the thioctate spacer. Gold nanoparticles stabilized with the PEO-b-PCL block (named Au/Block (x/y), where x/y is the mole feed ratio between HAuCl4 and PEO-b-PCL-TE) were prepared and analyzed. Au/Block (1/1), Au/Block (2/1), and Au/Block (3/1) nanoparticles were found to form stable dispersions in the organic solvents commonly used to dissolve the unlabeled block copolymer. The average diameter of the nanoparticles was determined by transmission electron microscopy (TEM) and found to be 6+/-2 nm. Au/Block (4/1) nanoparticle dispersions in organic solvents, on the other hand, were not stable and produced large gold clusters (50-100 nm). Cluster formation was attributed to the low grafting density of the block copolymer, which facilitates agglomeration. For Au/Block (12/1), along the same trend, only an insoluble product was isolated. Micelles in water were prepared by the slow addition of the dilute Au/Block solution in dimethylformamide into a large excess of water with vigorous stirring. Au/Block (1/1) and Au/Block (2/1) formed nanosized structures of 5-7 nm. TEM images of stained Au/Block (1/1) micelles, made in water, clearly showed the formation of core-shell structures. Au/Block (3/1) micelles, on the other hand, were not stable and large agglomerates a few microns in size were observed. The study focuses on the synthesis, characterization, and aggregation behavior of gold-loaded PEO-b-PCL block copolymer micelles, a potential system for drug delivery in conjunction with tissue and subcellular localization studies.     

Influence of metalation on the morphologies of poly(ethylene oxide)-block-poly(4-vinylpyridine) block copolymer micelles

Micellization of a poly(ethylene oxide)-block-poly(4-vinylpyridine) (PEO45-b-P4VP28) copolymer in water during metalation (incorporation of gold compounds and gold nanoparticle formation) with three types of gold compounds, NaAuCl4, HAuCl4, and AuCl3, was studied using dynamic light scattering (DLS) and transmission electron microscopy (TEM). The transformations of the PEO45-b-P4VP28 block copolymer micelles in water were found to depend on a number of parameters including the thermal history of the as-prepared block copolymer, the type of the metal compound, and the metal loading. For the HAuCl4-filled PE045-b-P4VP28 micelles, the subsequent reduction with hydrazine hydrate results in a significant fraction of rodlike micelles, suggesting that slow nucleation (confirmed by the formation of the large gold nanoparticles) and facilitated migration of gold ions yields the ideal conditions for sphere-to-rod micellar transition.     

Improved focused ion beam target preparation of (S)TEM specimen--a method for obtaining ultrathin lamellae

Specimen quality is vital to (scanning) transmission electron microscopy (TEM) investigations. In particular, thin specimens are required to obtain excellent high-resolution TEM images. Conventional focused ion beam (FIB) preparation methods cannot be employed to reliably create high quality specimens much thinner than 20 nm. We have developed a method for in situ target preparation of ultrathin TEM lamellae by FIB milling. With this method we are able to routinely obtain large area lamellae with coplanar faces, thinner than 10 nm. The resulting specimens are suitable for low kV TEM as well as scanning TEM. We have demonstrated atomic resolution by Cs-corrected high-resolution TEM at 20 kV on a FIB milled Si specimen only 4 nm thick; its amorphous layer measuring less than 1 nm in total.     

Double hydrophilic block copolymer monolayer protected hybrid gold nanoparticles and their shell cross-linking

This paper describes the syntheses of core/shell gold nanoparticles stabilized with a monolayer of double hydrophilic block copolymer and their stimuli responsiveness before and after shell cross-linking. The hybrid nanoparticles consist of gold core, cross-linkable poly(2-(dimethylamino)ethyl methacrylate) (PDMA) inner shell, and poly(ethylene oxide) (PEO) corona. First, diblock copolymer PEO-b-PDMA was prepared via the reversible addition-fragmentation chain transfer (RAFT) technique using a PEO-based macroRAFT agent. The dithioester end group of PEO-b-PDMA diblock copolymer was reduced to a thiol end group. The obtained PEO-b-PDMA-SH was then used to prepare diblock copolymer stabilized gold nanoparticles by the "grafting-to" approach. 1,2-Bis(2-iodoethoxy)ethane (BIEE) was utilized to selectively cross-link the PDMA residues in the inner shell. The stimuli responsiveness and colloidal stability of core/shell gold nanoparticles before and after shell cross-linking were characterized by laser light scattering (LLS), UV-vis transmittance, and transmission electron microscopy (TEM). At pH 9, the average hydrodynamic radius Rh of non-cross-linked hybrid gold nanoparticles starts to increase above 35 degrees C due to the lower critical solution temperature (LCST) phase behavior of the PDMA blocks in the inner shell. In contrast, Rh of the shell cross-linked gold nanoparticles were essentially independent of temperature. Core/shell gold nanoparticles before and after shell cross-linking exhibit reversible swelling on varying the solution pH. Compared to non-cross-linked core/shell gold nanoparticles, shell cross-linking of the hybrid gold nanoparticles leads to permanent core/shell nanostructures with much higher colloidal stability and physically isolates the gold core from the external environment.     

Formation of gold@polymer core-shell particles and gold particle clusters on a template of thermoresponsive and pH-responsive coordination triblock copolymer

Template synthesis of various morphological gold colloidal nanoparticles using a thermoresponsive and pH-responsive coordination triblock copolymer of poly(ethylene glycol)-b-poly(4-vinylpyridine)-b-poly(N-isopropylacrylamide) is studied. The template morphology of the thermoresponsive and pH-responsive coordination triblock copolymer, which can be tuned by simply changing the pH or temperature of the triblock copolymer aqueous solution, ranges from single chains to core-corona micelles and further to micellar clusters. Various morphological gold colloidal nanoparticles such as discrete gold nanoparticles, gold@polymer core-shell nanoparticles, and gold nanoparticle clusters are synthesized on the corresponding template of the triblock copolymer by first coordination with gold ions and then reduction by NaBH4. All three resultant gold colloidal nanoparticles are stable in aqueous solution, and their sizes are 2, 10, and 7 nm, respectively. The gold@polymer core-shell nanoparticles are thermoresponsive. The gold nanoparticle cluster has a novel structure, and each one holds about 40 single gold nanoparticles.     

Surface-catalyzed growth of polymethylene-rich copolymer films on gold

We report the preparation of a new class of copolymer films that consist of polymethylene (PM) with a low percentage ( approximately 1-4%) of randomly distributed ethyl ester side groups, consistent with poly(methylene-co-ethyl acetate). The films are prepared through a surface-catalyzed polymerization on gold surfaces upon exposure to a dilute solution of diazomethane (DM) and ethyl diazoacetate (EDA) in ether at 0 degrees C. While EDA alone does not polymerize at gold surfaces but DM does decompose on gold to grow PM films, the combined presence of EDA and DM results in dramatic enhancements in film growth and promotes an alternative mechanism for propagation as compared with that for the PM homopolymerization. The rate of copolymer film growth is constant over a approximately 24 h period, consistent with a controlled polymerization in which chain terminations are minimized. Carefully controlled experiments indicate that chain propagation does not occur at the outer film-solvent interface, but more likely, at the film-metal interface, suggesting a catalyzed insertion mechanism that extends the chain and pushes the outer chain termini further away from the metal-polymer interface. The results also suggest that adsorbed intermediates of EDA function as co-catalysts to promote the propagation reaction. Of particular importance for materials modification is that the ester side chains of these copolymer films can be hydrolyzed to carboxylate groups that exhibit pH-dependent wettability.