AFM study of micelle chaining in surface films of polystyrene-block-poly(ethylene oxide) stars at the air/water interface

Linear and Star Poly(ionic liquid) Assemblies: Surface Monolayers and Multilayers

The surface morphology and organization of poly(ionic liquid)s (PILs), poly[1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] are explored in conjunction with their molecular architecture, adsorption conditions, and postassembly treatments. The formation of stable PIL Langmuir and Langmuir-Blodgett (LB) monolayers at the air-water and air-solid interfaces is demonstrated. The hydrophobic bis(trifluoromethylsulfonyl)imide (Tf₂N^-) is shown to be a critical agent governing the assembly morphology, as observed in the reversible condensation of LB monolayers into dense nanodroplets. The PIL is then incorporated as an unconventional polyelectrolyte component in the layer-by-layer (LbL) films of hydrophobic character. We demonstrate that the interplay of capillary forces, macromolecular mobility, and structural relaxation of the polymer chains influence the dewetting mechanisms in the PIL multilayers, thereby enabling access to a diverse set of highly textured, porous, and interconnected network morphologies for PIL LbL films that would otherwise be absent in conventional LbL films. Their compartmentalized internal structure is relevant to molecular separation membranes, ultrathin hydrophobic coatings, targeted cargo delivery, and highly conductive films.

Interface-enforced complexation between copolymer blocks

Binary diblock copolymers and corresponding ternary miktoarm stars are studied at oil-water interfaces. All polymers contain oil-soluble poly(propylene oxide) PPO, water-soluble poly(dimethylaminoethyl methacrylate) PDMAEMA and/or poly(ethylene oxide) PEO. The features of their Langmuir compression isotherms are well related to the ones of the corresponding homopolymers. Within the Langmuir-trough, PEO-b-PPO acts as the most effective amphiphile compared to the other PPO-containing copolymers. In contrast, the compression isotherms show a complexation of PPO and PDMAEMA for PPO-b-PDMAEMA and the star, reducing their overall amphiphilicity. Such complex formation between the blocks of PPO-b-PDMAEMA is prevented in bulk water but facilitated at the interface. The weakly-interacting blocks of PPO-b-PDMAEMA form a complex due to their enhanced proximity in such confined environments. Scanning force microscopy and Monte Carlo simulations with varying confinement support our results, which are regarded as compliant with the mathematical random walk theorem by Pólya. Finally, the results are expected to be of relevance for e.g. emulsion formulation and macromolecular engineering.

Interfacial rheology and aggregation behaviour of amphiphilic CBABC-type pentablock copolymers at the air–water interface: effects of block ratio and chain length

The interfacial rheology, aggregation behaviour and packing model of the structure evolution of three amphiphilic CBABC-type pentablock copolymers were investigated at the air–water interface.

Structure-rheology relationship in weakly amphiphilic block copolymer Langmuir monolayers

The linear viscoelastic behavior in the low-frequency regime at the water/air interface of three different polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) copolymer monolayers, with block length ratio varying from 66-33 to 50-50 and 25-75 in molecular units, was studied and related to the interfacial behavior, characterized by means of Langmuir isotherms, and their structure, characterized by means of the atomic force microscopy technique. The two monolayers with the highest PMMA amount showed a single phase transition at about 12 mN/m, the viscoelastic behavior changing from a predominantly elastic to a viscoelastic one. This change in the viscoelastic properties was ascribed to the beginning of entanglement among the PMMA coronas of the predominantly circular quasi-2D micelles formed by the two copolymer systems. Conversely, the polymer with the lowest PMMA amount, despite having the same PMMA block length of the PS-PMMA 50-50 block copolymer, was found to behave as a viscoelastic system at any surface pressure value. This characteristic behavior cannot therefore be simply related to the molecular weight difference, but it has been put in connection to the irregular micelle structure observed in this case, consisting of a mixture of spherical and wormlike micelles, and to the different conformation adopted by the PMMA block. By blending this copolymer with an immiscible elastic homopolymer, namely poly(2-vinylpyridine), it was possible to tune the micelle nanostructure, obtaining regular circular quasi-2D micelles, with viscoelastic properties as expected for the PMMA-rich copolymer monolayers. To the best of our knowledge, this study shows for the first time the explicit dependence upon the relative block length and, in turn, upon the nanostructure of the quasi-2D micelles, of the viscoelastic properties of Langmuir monolayers and suggests that molecular weight and intermolecular interactions are not the only parameters governing the polymer conformation and, in turn, the polymer rheology and dynamics in quasi-2D confined systems.

Polymeric Surfactants: Novel Agents with Exceptional Properties

This article presents recent progress in the field of polymeric surfactants made of permanently amphiphilic block copolymers or of stimulus-sensitive ones. We highlight key points in the design of amphiphilic macromolecules, to yield polymer surfactants with tailor-made properties, as well as recently developed and still challenging application fields for this new class of surfactants. The efficiency boosting of amphiphilic block copolymers as co-surfactants in microemulsions is discussed, as are surface modification by polymer surfactants, and stabilization of dispersions. Moreover, the use of block copolymers in nanosciences is presented, for instance as a tool for nanomaterial fabrication, or for biomedical and cosmetic applications in bio-nanotechnology. Finally, self-assembly and applications of some newly developed “exotic” amphiphilic block copolymer structures as new surface-active materials will be highlighted.

Polystyrene-poly(ethylene oxide) diblock copolymer: the effect of polystyrene and spreading concentration at the air/water interface

Polystyrene-block-poly(ethylene oxide) (PS-PEO) is an amphiphilic diblock copolymer that undergoes microphase separation when spread at the air/water interface, forming nanosized domains. In this study, we investigate the impact of PS by examining a series of PS-PEO samples containing constant PEO (~17,000 g·mol(-1)) and variable PS (from 3600 to 200,000 g·mol(-1)) through isothermal characterization and atomic force microscopy (AFM). The polymers separated into two categories: predominantly hydrophobic and predominantly hydrophilic with a weight percent of PEO of ~20% providing the boundary between the two. AFM results indicated that predominantly hydrophilic PS-PEO forms dots while more hydrophobic samples yield a mixture of dots and spaghetti with continent-like structures appearing at ~7% PEO or less. These structures reflect a blend of polymer spreading, entanglement, and vitrification as the solvent evaporates. Changing the spreading concentration provides insight into this process with higher concentrations representing earlier kinetic stages and lower concentrations demonstrating later ones. Comparison of isothermal results and AFM analysis shows how polymer behavior at the air/water interface correlates with the observed nanostructures. Understanding the impact of polymer composition and spreading concentration is significant in leading to greater control over the nanostructures obtained through PS-PEO self-assembly and their eventual application as polymer templates.

Interfacial behavior of OEG-linear dendron monolayers: aggregation, nanostructuring, and electropolymerizability

We report on the interesting interfacial behavior of oligoethylene glycol or OEGylated linear dendron monolayers at the air-water interface as a function of (a) carbazole dendron generation, (b) the length of the OEG units, and (c) the surface pressure applied upon compression. Surface pressure-area isotherms, hysteresis studies, and isobaric creep measurement revealed a structure-property relationship consistent with the hydrophilic-lipophilic balance of a linear dendron with the OEG group serving as the surface anchor to the water subphase. AFM studies revealed that all the OEGylated carbazole dendrons self-assemble into spherical morphology at low surface pressures but form ribbonlike structures as the surface pressure is increased. This nanostructuring is primarily imparted by the increase in van der Waals forces with increasing amount of carbazole units per dendron generation on a hydrophilic mica surface. Further, electrochemical cross-linking of the carbazole molecules by cyclic voltammetery (CV) on doped Si wafer has enabled the formation of an LB film monolayer with a secondary level of organization in the monolayer imparted by the inter- and intramolecular cross-linking among the carbazole units. This study should provide a basis for monolayer film materials based on combining the LB technique and electrochemical cross-linking for nanostructuring superstructures at the air-water interface.

Strands, networks, and continents from polystyrene dewetting at the air-water interface: implications for amphiphilic block copolymer self-assembly

We demonstrate that nanoscale aggregates similar to those formed via amphiphilic block copolymer self-assembly at the air-water interface, including strands, networks, and continents, can be generated by the simple spreading of PS homopolymer solutions on water. Two different PS homopolymers of different molecular weight (PS-405k, M(n) = 405 000 g mol(-1) and PS-33k, M(n) = 33 000 g mol(-1)) are spread at the air-water interface at various spreading concentrations ranging from 0.25 to 3.0 mg/mL. Aggregate formation is driven by PS dewetting from water as the spreading solvent evaporates. We propose that a high spreading concentration or a high molecular weight lead to chain entanglements that restrict macromolecular mobility in the solution, enabling the kinetic trapping of nanostructures associated with early and intermediate stages of PS dewetting. Comparison of PS-405k with a mainly hydrophobic PS-b-PEO block copolymer of similar molecular weight (PSEO-392k, M(n) = 392 000 g mol(-1), 2.0 wt % PEO) allows the effect of a relatively short surface active block on aggregate formation to be investigated. We show that whereas the PEO block is not a required component for the formation of strands and other nonglobular aggregates, it does increase the number of these aggregates at a given spreading concentration and decreases the minimum spreading concentration at which these aggregates are observed, along with decreasing the dimensions and polydispersity of specific surface features. The results provide supporting evidence for the role of PS dewetting in the generation of multiple PS-b-PEO aggregate morphologies at the air-water interface, as originally described in earlier paper from our group.

Bulk and surface assembly of branched amphiphilic polyhedral oligomer silsesquioxane compounds

This study probes the behavior of two series of organic-functionalized core-shell silsesquioxane (POSS-M)p-(x/y) derivatives with various hydrophobic-hydrophilic terminal group compositions in the bulk state and within mono- and multilayered films at the air-water interface and on solid surface. POSS-M refers to mixed silsesquioxane cores, in contrast to the geometrically specific POSS compounds. It is composed of polyhedra, incompletely condensed polyhedra, ladder-type structures, linear structures, and all the possible combinations thereof and attracts great interest because of its facile preparation, low polydispersity, high yield, and low cost. The two series of (POSS-M)p-(x/y) molecules are different in hydrophobic-hydrophilic balance of their terminal groups, with x and y respectively referring to the molar percent of -OCONH-C(18)H(37) tails and -OH for p = 1 and the percent of -OCONH-C(18)H(37) tails and -OCO-C(6)H(4)COOH terminal groups for p = 2. In the bulk state, the presence of aromatic rings in (POSS-M)2-(x/y) series resulted in a lower symmetry crystal structure than the (POSS-M)1-(x/y) series. Moreover, the (POSS-M)p-(x/y) molecules that contain a sufficient amount of -OCONH-C(18)H(37) tails exhibit double endothermic transition, which attributed to the melting of alkyl chains followed by the melting of the unit cells of (POSS-M) cores. The surface morphologies for the various hydrophobic-hydrophilic combinations at surface pressure p = 0.5 mN/m are similar to that observed for the classical amphiphilic star polymers. However, at higher surface pressure (p > or = 5 mN/m), the POSS-M compounds with lower content of hydrophilic groups form a uniform monolayer.

Pressure- and temperature-induced association of arborescent polystyrene-graft-poly(ethylene oxide) copolymers at the air-water interface

The influence of surface pressure and subphase temperature on the association of arborescent polystyrene- graft-poly(ethylene oxide) (PS- g-PEO) copolymers at the air-water interface was investigated using the Langmuir balance and atomic force microscopy (AFM) techniques. These dendritic molecules form stable condensed monolayers with surface compressional moduli >250 mN/m. The variation in film thickness observed as a function of surface pressure suggests that at low surface pressures (gaslike phase) the PEO chains remain adsorbed at the air-water interface. At higher surface pressures (condensed phase), the PEO chains partially desorb into the subphase and adopt a more brushlike conformation. Large islandlike clusters with a broad size distribution were observed for samples with PEO contents of up to 15% by weight. In contrast, copolymers with PEO contents of 22-43% displayed enhanced side-by-side association into ribbonlike superstructures upon compression. The same effect was observed even in the absence of compression when the subphase temperature was increased from 12 to 27 degrees C. The temperature-induced association was attributed to increased van der Waals attractive forces between the PS cores relative to the steric repulsive forces between PEO chains in the coronas because the solvent quality for the PEO segments decreased at higher temperatures. The restricted number of superstructures observed for arborescent copolymers as compared with linear- and star-branched PS-PEO block copolymers is attributed to the enhanced structural rigidity of the molecules due to branching.

Effect of temperature on the interfacial behavior of a polystyrene-b-poly(methyl methacrylate) diblock copolymer at the air/water interface

Monolayers of a polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymer at the air-water interface were studied by measuring the surface pressure-area isotherms at several temperatures. Langmuir film balance experiments and atomic force microscopy showed that the diblock copolymer molecules formed surface micelles. In the plot of the surface pressure versus surface area per repeating unit, the monolayer changed from the gas phase to the liquid expanded phase at lower surface pressure for systems at low temperature compared to those at high temperature. In addition, a plateau, corresponding to the transition from the liquid expanded to liquid condensed phase, appeared in that plot at lower surface pressure for systems with a higher subphase (water) temperature. Hysteresis was observed in the compression-expansion cycle process. Increasing the subphase temperature alleviated this hyteresis gap, especially at low surface pressures. The minimum in the plot of the surface pressure versus surface area per repeating unit in the expansion process (which arises from the transition) and the transition plateau appeared more vividly at higher water temperature. These dynamic experimental results show that PS-PMMA diblock copolymers, in which both blocks are insoluble in water, do not form complicated entanglements in two-dimensional space. Although higher water temperature provided more entropy to the chains, and thus more conformational freedom, it did not change the surface morphology of the condensed film because both blocks of PS-PMMA are insoluble in water.

Experimental evidence for a surface concentration-dependent mechanism of formation of hemimicelles in Langmuir monolayers of semi-fluorinated alkanes

We show that formation of surface hemimicelles by a series of molecular semi-fluorinated alkanes CFCH (F8Hm diblocks; = 14, 16, 18, 20) in Langmuir-Blodgett monolayers is not promoted by surface pressure, but depends on the surface area available before transfer, hence on a critical surface concentration. Evidence is provided for the presence of isolated micelles at zero surface pressure (very large molecular area) for certain FnHm diblocks. It is the molecular structure of the diblock that essentially determines the morphology of the hemimicelles, independently of compression conditions.

Two-dimensional polymeric nanomaterials through cross-linking of polybutadiene-b-poly(ethylene oxide) monolayers at the air/water interface

Two-dimensional polymeric nanomaterials consisting of a continuously cross-linked polybutadiene (PB) two-dimensional network with poly(ethylene oxide) (PEO) domains of controlled sizes trapped within the PB network were synthesized. To reach that goal, novel (PB(Si(OEt)3)-b-PEO)3 star block copolymers were designed by hydrosilylation of the pendant double bonds of (PB-b-PEO)3 star block copolymer precursors with triethoxysilane. The (PB(Si(OEt)3)-b-PEO)3 star block copolymers were characterized by 1H NMR and IR spectroscopy. Self-condensation of the triethoxysilane pendant groups under acidic conditions led to a successful cross-linking of the polybutadiene blocks directly at the air/water interface without any additives or reagents. This strategy was found more efficient than radical cross-linking of (PB-b-PEO)3 with AIBN to get a homogeneously cross-linked monolayer of controlled and fixed morphology as demonstrated by the easy mechanical removal of the cross-linked Langmuir film from the water surface. As shown by AFM imaging, this strategy allows the accurate control of the PEO "pore" size depending on the monolayer surface pressure applied during the cross-linking reaction. The subphase pH and surface pressure influence on the cross-linking kinetics and monolayer morphologies were investigated by Langmuir trough studies (isotherm and isobar experiments) and AFM imaging.

Blends of poly(epsilon-caprolactone) and intermediate molar mass polystyrene as Langmuir films at the air/water interface

Poly(epsilon-caprolactone)/polystyrene (PCL/PS) blends, where nonamphiphilic PS is glassy in the bulk state at the experimental temperature of 22.5 degrees C, are immiscible as Langmuir films at the air/water (A/W) interface. Surface pressure-area per monomer isotherm analyses indicate that the surface concentration of amphiphilic PCL is the only factor influencing the surface pressure below the collapse transition. For PS-rich blends, Brewster angle microscopy (BAM) studies at the A/W interface and atomic force microscopy studies on Langmuir-Schaefer films reveal that PS nanoparticle aggregates formed at very low surface pressures can form networks upon further compression. The morphologies seen in PS-rich blends (networklike rings) are consistent with a recent study of a nonamphiphilic polyhedral oligomeric silsesquioxane (POSS), octaisobutyl-POSS, blended with amphiphilic poly(dimethylsiloxane), suggesting that the nonamphiphilic PS aggregates at the A/W interface produce domains with dipole densities that differ from that of pure PCL. In all composition regimes, the amphiphilic PCL phase tends to spread and form a continuous surface layer at the A/W interface, while simultaneously improving the dispersion of nonamphiphilic PS domains. During film expansion, BAM images show a gradual change in the surface morphology from highly continuous networklike structures (PS-rich blends) to broken ringlike structures (intermediate composition) to small discontinuous aggregates (PCL-rich blends). This study provides valuable information on the morphological evolution of semicrystalline PCL-based polymer blends confined in a "two-dimensional" geometry at the A/W interface and fundamental insight into the influence of microstructure (domain size, phase-separated structures, crystalline morphology, etc.) on the interfacial properties of blends as Langmuir films.

Self-assembly of polystyrene-block-poly(ethylene oxide) copolymers at the air-water interface: is dewetting the genesis of surface aggregate formation?

Block copolymer self-assembly at the air-water interface is commonly regarded as a two-dimensional counterpart of equilibrium block copolymer self-assembly in solution and in the bulk; however, the present analysis of atomic force microscopy (AFM) and isotherm data at different spreading concentrations suggests a nonequilibrium mechanism for the formation of various polystyrene-b-poly(ethylene oxide) (PS-b-PEO) aggregates (spaghetti, dots, rings, and chainlike aggregates) at the air-water interface starting with an initial dewetting of the copolymer spreading solution from the water surface. We show that different spreading concentrations provide kinetic snapshots of various stages of self-assembly at the air-water interface as a result of different degrees of PS chain entanglements in the spreading solution. Two block copolymers are investigated: MW = 141k (11.4 wt % PEO) and MW = 185k (18.9 wt % PEO). Langmuir compression isotherms for the 185k sample deposited from a range of spreading concentrations (0.1-2.0 mg/mL) indicate less dense packing of copolymer chains within aggregate cores formed at lower spreading concentrations due to a competition between the interfacial adsorption of PEO blocks and the kinetic restrictions of PS chain entanglements. From AFM analysis of the transferred Langmuir-Blodgett films, it is clear that PS chain entanglements in the spreading solution also affect the morphological evolution of surface aggregates for both samples, with earlier structures being trapped at higher concentrations. At the highest spreading concentration for the 141k copolymer, the coexistence of long spaghetti aggregates with cellular arrays of holes, along with various transition structures, indicates that various surface aggregates evolve from networks of rims formed as a result of dewetting of the evaporating spreading solution from the water surface.

Equilibrium and dynamics of Langmuir monolayers when the interface is a selective solvent: Polystyrene-b-poly(t-butyl acrylate) block copolymers

The surface pressure of monolayers of insoluble diblock copolymers has been measured. One of the blocks is made of poly(t-butyl acrylate) (PtBA), and the other one by polystyrene (PS). The interface is a good solvent for PtBA, while it is a poor solvent for PS. For the sake of comparison, monolayers of a PtBA homopolymer (good solvent conditions) and of poly(4-hydroxy styrene) (P4HS) (poor solvent conditions) have been also measured. It has been found that the relative length of the blocks plays an important role on the shape of the surface pressure Pi versus surface concentration Gamma curves and also on the shape of the equilibrium compressibility versus Gamma curves. However, it does not affect the maximum value of Pi reached at high Gamma's. Surprisingly, the ellipsometric thickness of the copolymer monolayers is almost independent of the relative length of the blocks. The dynamics of the monolayers has been studied by step compression and by surface-light scattering techniques. When M(w,PtBA) >> M(w,PS) single exponential relaxations are observed. However, stretched exponentials are obtained for M(w,PS) > or = M(w,PtBA). The relaxation times decrease with increasing Gamma for all the copolymers studied. This is the behavior usually found for poor solvent conditions (P4HS) and opposite to that found for homopolymers under good solvent conditions [PtBA, poly(vinylacetate)]. This means that the solvent quality of the interface does not determine the pressure dependence of tau. The elasticity modulus of the monolayers in the kilohertz range takes values that are similar to those of the high-frequency limit of the relaxation experiments. This means that the relaxation processes have characteristic frequencies below 1 Hz.

Surface morphologies of Langmuir-Blodgett monolayers of PEOnPSn multiarm star copolymers

Star polymers composed of equal numbers of poly(ethylene oxide) (PEO) and polystyrene (PS) arms with variable lengths and a large (up to 38 total) number of arms, PEO(n)PS(n), have been examined for their ability to form domain nanostructures at the air-water and air-solid interfaces. All PEO(n)PS(n) star polymers formed stable Langmuir-Blodgett (LB) monolayers transferable to a solid substrate. A range of nanoscale surface morphologies have been observed, ranging from cylindrical to circular domains to bicontinuous structures as the weight fraction of the PEO block varied from 19% to 88% and n from 8 to 19. For the PS-rich stars and at elevated surface pressure, a two-dimensional supramolecular netlike nanostructure was formed. In contrast, in the PEO-rich star polymer with the highest PEO content, we observed peculiar dendritic superstructures caused by intramolecular segregation of nonspherical core-shell micellar structures. On the basis of Langmuir isotherms and observed monolayer morphologies, three different models of possible surface behavior of the star polymers at the interfaces were proposed.

Polystyrene-block-poly(ethylene oxide) stars as surface films at the air/water interface

Star diblock copolymers containing polystyrene (PS) and poly(ethylene oxide) (PEO) were investigated as surface films at the air/water interface. Both classic and dendritic-like stars were prepared containing either a PS core and PEO corona or the reverse. The investigated polymers, consisting of systematic variations in architectures and compositions, were spread at the air/water interface, generating reproducible surface pressure-area isotherms. All of the films could be compressed to higher pressures than would be possible for pure PEO. For stars containing 20% or more PEO, three distinct regions appeared. At higher areas, the PEO absorbs in pancakelike structures at the interface with PS globules sitting atop. Upon compression, a pseudoplateau transition region appeared. Both regions strongly depended on PEO composition. The pancake area and the pseudoplateau width and pressure increased in a linear fashion with an increasing amount of PEO. In addition, minimum limits of PEO chain length and mass percentage were determined for observing a pseudoplateau. At small areas, the film proved less compressible, producing a rigid film in which PS dominated. Here, the film area increased with both molecular weight and the amount of PS. Comparison with pure linear PS showed the stars spread more, occupying greater areas. Among the stars, the PEO-core stars were more compact while the PS-core stars spread more. The influence of architecture in terms of the core/corona polymers and branching were also examined. The effects of architecture were subtle, proving less important than PEO chain length or mass percentage.