Adsorbed and Spread Layers of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide) Block Copolymers at the Air−Water Interface Studied by Sum-Frequency Vibrational Spectroscopy and Tensiometry

Generation of maghemite nanocrystals from iron–sulfur centres

Electron beam induced generation of maghemite nanocrystals from polymer-encapsulated iron–sulfur centres.

Adsorption of poly(ethylene oxide)-containing amphiphilic polymers on solid-liquid interfaces: Fundamentals and applications

The adsorption of amphiphilic molecules of varying size on solid-liquid interfaces modulates the properties of colloidal systems. Nonionic, poly(ethylene oxide) (PEO)-based amphiphilic molecules are particularly useful because of their graded hydrophobic-hydrophilic nature, which allows for adsorption on a wide array of solid surfaces. Their adsorption also results in other useful properties, such as responsiveness to external stimuli and solubilization of hydrophobic compounds. This review focuses on the adsorption properties of PEO-based amphiphiles, beginning with a discussion of fundamental concepts pertaining to the adsorption of macromolecules on solid-liquid interfaces, and more specifically the adsorption of PEO homopolymers. The main portion of the review highlights studies on factors affecting the adsorption and surface self-assembly of PEO-PPO-PEO block copolymers, where PPO is poly(propylene oxide). Block copolymers of this type are commercially available and of interest in several fields, due to their low toxicity and compatibility in aqueous systems. Examples of applications relevant to the interfacial behavior of PEO-PPO-PEO block copolymers are paints and coatings, detergents, filtration, and drug delivery. The methods discussed herein for manipulating the adsorption properties of PEO-PPO-PEO are emphasized for their ability to shed light on molecular interactions at interfaces. Knowledge of these interactions guides the formulation of novel materials with useful mesoscale organization and micro- and macrophase properties.

Influence of the linkage type between the polymer backbone and side groups on the surface segregation of methyl groups during film formation

Although poly(vinyl acetate) (PVAc) differs from poly(methyl acrylate) (PMA) only in the reversed position of the ester group, a large difference in the concentration dependence of the casting solution on the corresponding surface structure of the cast films of PVAc, PMA and poly(methyl methacrylate) (PMMA) was observed. The hydrophobicity of both PMA and PMMA films increased with increasing concentration of the corresponding polymer solution, whereas cast PVAc films showed the reverse trend. The surface structure of the cast films prepared with different concentrations of the casting solution, characterized by sum frequency generation (SFG) vibrational spectra, showed that the order of the methylene groups increased while that of the acetyl methyl group decreased on the surface of cast PVAc film with increasing concentration of casting solution. However, the order of the ester methyl group increased and that of methylene groups did not change for cast PMA films with increasing concentration of casting solution. The cast PMMA film showed a reverse trend compared with the corresponding PMA film. It is apparent that well-ordered ester or acetyl methyl groups on the surface, which are oriented away from the polymer film, rather than methylene groups, play an important role in determining surface hydrophobicity, as the latter shield the OC[double bond, length as m-dash]O groups of PVAc, PMA and PMMA film surfaces from being exposed, resulting in low surface free energy. The reason for this difference is attributed to the relatively low energy for ester methyl group reorientation, an ester group structure nearer to the trans state and more regular local configuration of segments in concentrated solutions of PMA and PMMA compared to that of PVAc.

Phase transitions in polymer monolayers: Application of the Clapeyron equation to PEO in PPO-PEO Langmuir films

In this paper we investigate the application of the two-dimensional Clapeyron law to polymer monolayers. This is a largely unexplored area of research. The main problems are (1) establishing if equilibrium is reached and (2) if so, identifying and defining phases as functions of the temperature. Once this is validated, the Clapeyron law allows us to obtain the entropy and enthalpy differences between two coexisting phases. In turn, this information can be used to obtain insight into the conformational properties of the films and changes therein. This approach has a wide potential for obtaining additional information on polymer adsorption at interfaces and the structure of their monolayer films. The 2D Clapeyron law was applied emphasizing polyethylene oxide (PEO) in polypropylene oxide (PPO)-PEO block copolymers, based on new well-defined data for their Langmuir films. Values for enthalpy per monomer of 0.12 and 0.23 kT were obtained for the phase transition of two different PEO chains (Neo of 2295 and 409, respectively). This enthalpy was estimated to correspond to 1.2±0.4 kT per EO monomer present in train conformation at the air/water interface.

Bilayer Charge Reversal and Modification of Lipid Organization by Dendrimers as Observed by Sum-Frequency Vibrational Spectroscopy

Polyamidoamine (PAMAM) dendrimers are hyperbranched, nanosized polymers with promising biomedical applications as nanocarriers in targeted drug delivery and gene therapy. For the development of safe dendrimer-based biomedical applications it is necessary to gain an understanding of the detailed mechanism of the interactions of both cationic and anionic dendrimers with cell membranes. To characterize dendrimer-membrane interactions we applied solid-supported lipid bilayers as biomembrane models and utilized infrared-visible sum-frequency vibrational spectroscopy to independently probe the interactions of cationic G5-NH2 and anionic G4.5-COONa dendrimers with the two leaflets of the lipid bilayers. Interaction with both dendrimers led to changes in the interfacial water structure and charge density as evidenced by the changes in the OH band intensities in the sum-frequency spectra of the bilayers. Interaction with the G5-NH2 dendrimer also led to a unique inversion of the sign of the OH-stretch amplitudes, in addition to a decrease in their absolute values. We suggest that the positively charged amino groups on the G5-NH2 dendrimer surface bind to the negatively charged bilayer, while uncompensated positive charges not involved in the binding cause a reversal of the electric field and thus an opposite orientation of the interfacial water molecules. More subtle but nonetheless significant changes were seen in the relative magnitudes of the CH amplitudes. The methyl antisymmetric to symmetric stretch amplitude ratios are altered, implying changes in the tilt angles of the phospholipid alkyl chains. The conformational order of the phospholipid alkyl chains of both leaflets is also influenced by the G5-NH2 dendrimer while G4.5-COONa has no effect on the alkyl chain conformation.

Synthesis and controlled growth of osmium nanoparticles by electron irradiation

Defined-size osmium nanoparticles (1.5–50 nm) were synthesized on a B- and S-doped turbostratic graphitic structure from an organometallic osmium complex encapsulated in self-spreading polymer micelles and characterised by (HR)TEM, AFM and XPS.

Molecular arrangement of symmetric and non-symmetric triblock copolymers of poly(ethylene oxide) and poly(isobutylene) at the air/water interface

The behavior of a series of amphiphilic triblock copolymers of poly(ethylene oxide) (PEO) and poly(isobutylene) (PIB); including both symmetric (same degree of polymerization (DP) of the terminal PEO blocks) PEOm-b-PIBn-b-PEOm and non-symmetric (different DP of the terminal PEO blocks) PEOm-b-PIBn-b-PEOz, is investigated at the air/water interface by measuring surface pressure vs mean molecular area isotherms (π vs mmA), Langmuir-Blodgett (LB) technique, and infrared reflection-absorption spectroscopy (IRRAS). The block copolymer (PEO32-b-PIB160-b-PEO32) with longer PEO segments forms a stable monolayer and the isotherm reveals a pseudo-plateau starting at π∼5.7 mN/m, also observed in the IRRAS, which is assigned to the pancake-to-brush transition related to the PEO dissolution into the subphase and subsequent PEO brush dehydration. Another plateau is observed at π∼40 mN/m, which is attributed to the film collapse due to multilayer formation. The pancake-to-brush transition could not be observed for samples with smaller PEO chains. The isotherms for block copolymers, with short PEO chains, both symmetric (PEO3-b-PIBn-b-PEO3) and non-symmetric (PEO12-b-PIBn-b-PEO3), reveal another transition at π∼20-25 mN/m. This is interpreted to be due to the conformational transition from a folded state where the middle PIB block is anchored to the water surface at both ends by the terminal hydrophilic segments to an unfolded state with PIB anchored to the water surface at one end. It is assumed that this transition involves the removal of PEO3 chains from the water surface in case of non-symmetric PEO12-b-PIB85-b-PEO3 and in case of symmetric, probably one PEO3 of each PEO3-b-PIB85-b-PEO3 chain. Because of the weaker interaction of the short PEO3 chains with the water surface as compared with the relatively longer PEO12 chains, the film of PEO3-b-PIB85-b-PEO3 collapses at much lower surface pressure after the transition as compared with the PEO12-b-PIB85-b-PEO3. The AFM images reveal the formation of microdomains of almost uniform height (6-7 nm) in LB films of PEO3-b-PIB85-b-PEO3 and PEO12-b-PIB85-b-PEO3 after transferring onto silicon surfaces. These domains are assumed to be the mesomorphic domains of ordered and folded PIB chains.

Fabrication of crystals from single metal atoms

Metal nanocrystals offer new concepts for the design of nanodevices with a range of potential applications. Currently the formation of metal nanocrystals cannot be controlled at the level of individual atoms. Here we describe a new general method for the fabrication of multi-heteroatom-doped graphitic matrices decorated with very small, ångström-sized, three-dimensional (3D)-metal crystals of defined size. We irradiate boron-rich precious-metal-encapsulated self-spreading polymer micelles with electrons and produce, in real time, a doped graphitic support on which individual osmium atoms hop and migrate to form 3D-nanocrystals, as small as 15 Å in diameter, within 1 h. Crystal growth can be observed, quantified and controlled in real time. We also synthesize the first examples of mixed ruthenium–osmium 3D-nanocrystals. This technology not only allows the production of ångström-sized homo- and hetero-crystals, but also provides new experimental insight into the dynamics of nanocrystals and pathways for their assembly from single atoms.

Metal nanocrystals are used in an ever growing list of applications, but precise control and understanding of their formation can be difficult. Here, the authors show a route that allows the controlled formation of metal nanocrystals to be carried out and observed at an atom-by-atom level.

Adsorbed and spread films of amphiphilic triblock copolymers based on poly(2,3-dihydroxypropyl methacrylate) and poly(propylene oxide) at the air-water interface

The adsorption behavior of the novel type of water-soluble amphiphilic triblock copolymers PGMA-b-PPO-b-PGMA at the air-water interface is studied by tensiometry and monolayer techniques. In particular, (PGMA(14))(2)-PPO(34) is found to be strongly surface active (Pi(max) approximately 38.1 mN/m, cmc approximately 50 microM), in spite of having a relatively short hydrophobic PPO middle block. Time-dependent adsorption measurements exhibit two different types of adsorption kinetics depending on concentration. Monolayers deposited by spreading form pseudo-Langmuir films, in spite of (PGMA(14))(2)-PPO(34) high water solubility. The transition from a dilute to a semidilute regime during compression of the monolayer occurs at a mean molecular area around 4424 A(2)/molecule (Pi = 0.03 mN/m). Above Pi = 2.1 mN/m (1291 A(2)/molecule) PGMA segments begin to change from a flat two-dimensional conformation to loops and tails protruding into the subphase. The onset of the conformational change for PO segments takes place at a mean molecular area of approximately 625 A(2)/molecule (Pi = 15.5 mN/m). In the range Pi approximately 21.4-23.8 mN/m the PPO blocks adopt a three-dimensional conformation. A new methodology for the estimation of the amount of polymer absorbed at the interface (Gamma) as a function of the solution bulk concentration from tensiometry measurements in pseudo-Langmuir spread films is proposed. The obtained adsorption isotherm suggests the occurrence of a conformational change of the PPO block segments toward conformations having smaller molar areas for actual bulk concentrations above 6.0 x 10(-8) M. The surface tension versus polymer bulk concentration curve (gamma vs ln C) exhibits a sharp break at low concentrations, which is probably due to a conformational change within the adsorbed layer. A theoretical multiple conformation model reproduces satisfactorily the experimental dependency of surface pressure on polymer adsorption (Pi vs Gamma) at low and intermediate ranges of polymer concentrations in the solution bulk. A ratio of approximately eight between the average molecular areas at low coverage and at full coverage confirms that (PGMA(14))(2)-PPO(34) chains are highly flexible and able to adopt very different conformations during the transition of the adsorbed polymer film from a highly diluted to a nearly saturated state. There are some experimental indications that nonequilibrium effects might determine a bimodal conformational distribution within the adsorbed (PGMA(14))(2)-PPO(34) layer, which departs substantially from the equilibrium adsorption picture.

Self-associative behavior and drug-solubilizing ability of poloxamine (tetronic) block copolymers

The incidence of the structural features on the self-assembly of different poloxamines (the conventional sequential Tetronic 304, 901, 904, 908, 1107, 1301, and 1307; a reverse-sequential counterpart Tetronic 150R1; and a chemically modified derivative, N-methylated Tetronic 1107) was thoroughly studied in 10 mM HCl by means of pi-A isotherm, surface tension, and pyrene fluorescence measurements. The size and size distribution of the aggregates were investigated by dynamic and static light scattering, and the morphology was probed by transmission electron microscopy. The abilities of the different derivatives to solubilize the drug simvastatin were also evaluated. Poloxamines with both higher PO/EO ratio and molecular weight (T1301 and T150R1) led to micelles with larger and more hydrophobic cores, particularly adequate for hosting hydrophobic molecules and protecting the labile lactone form of simvastatin from hydrolysis. On the other hand, the hydroxy acid form of simvastatin interacted with the central ethylenediamine group under alkaline pH (T304) or when a permanent positive charge due to methylation was present. Micelles of long poloxamine molecules containing large PPO blocks (with 23-29 units, namely, T1301, T1307, and T150R1), particularly the one that also has long PEO blocks, were the most physically stable toward dilution.

Protein interaction with a Pluronic-modified poly(lactic acid) Langmuir monolayer

Interaction of bovine serum albumin (BSA) with poly(lactic acid) (PLA) layers mixed with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Pluronic) at air/solution interfaces was studied by the Langmuir balance technique. Wettability of the mixed PLA-Pluronic system was characterized in the form of a transferred one-layer Langmuir-Blodgett film, and considerable hydrophilization was obtained for all of the Pluronics (6400, 6800, 10500, and 12700) applied here. The density of PEO chains in the monolayer and hence the coverage of PLA was controlled by the composition and the compression of the mixed monolayers. Tensiometric investigations revealed that a significant reduction of BSA adsorption/penetration was achieved by applying the Pluronic 6800 and 12700 with long PEO blocks for hydrophilization of PLA. Interaction of BSA with the modified PLA monolayer depended on the density and length of the PEO chains. The surface morphological characteristics of the films determined by atomic force microscopy were in good correlation with the results of BSA interaction. The average roughness of the polymer LB layer was high due to BSA penetration into the PLA film, while smooth surfaces with small roughness were obtained when the PLA layer was modified by Pluronic 6800.