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.     

Hierarchical nanoparticle/block copolymer surface features via synergistic self-assembly at the air-water interface

This work demonstrates the first example of the controlled organization of semiconducting nanoparticles (NPs) using amphiphilic block copolymer self-assembly at the air-water interface. Preferential interactions between polystyrene-functionalized NPs and the polystyrene block of an amphiphilic polystyrene-b-poly(ethylene oxide) block copolymer result in synergistic self-assembly at the air-water interface, forming a range of highly stable one-dimensional NP/polymer surface features, including branched nanowires, nanocables up to 100 microm in length, and nanowires with nanoring connectors. This strategy offers new routes to hierarchical hybrid assemblies with potential photonics applications because the nanoscale organization of NPs is coupled to features with dimensions that are commensurate with optical wavelengths.     

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.     

Interfacial behaviors of PMMA-PEO block copolymers at the air/water interface

Amphiphilic block copolymers are attracting con-siderable attention because they exhibit unique self- assembly properties in selective organic solvents[1―4]. Semicrystalline poly(ethylene oxide) (PEO), having many interesting physicochemical properties s…     

2-Dimensional Network Formation from a Graft Copolymer at the Air-Water Interface

Summary: The UV induced cross-linking of a well defined graft copolymer polynorbornene-g-poly(ethylene oxide) at the air-water interface has been investigated. Network formation has been monitored qualitatively by observing changes in surface pressure with UV exposure time working under constant area conditions. Surface film concentration has been used as the design parameter to manipulate initial film organization, and consequently position of the functional groups. Exposure of the copolymer film to UV light at different surface concentrations in the liquid condensed region shows the closer the molecules pack together the faster the cross-linking reaction, while no cross-linking occurs when the molecules are far apart in the liquid expanded state.     

Aggregation behavior of star-like PEO-PPO-PEO block copolymer in aqueous solution

The aggregation behavior of a star-like amphiphilic block copolymer (denoted as AP432, which was synthesized via anionic polymerization), in aqueous solutions was investigated by surface tension, steady-state fluorescence, dynamic light scattering (DLS) and transmission electron microscopy (TEM). For comparison, a commercially available linear amphiphilic PEO-PPO-PEO block copolymer, Pluronics L64, which has a similar PEO fraction to AP432, was also studied. It is found that the different molecular structure of AP432 and L64 leads to a significant difference on their behavior both at the air/water interface and in bulk aqueous solutions. The results of surface tension measurements indicate that the surface activity of AP432 is much more pronounced than that of L64. The formation of AP432 and L64 aggregates are identified by DLS, fluorescence and TEM measurement.     

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.     

Thermally induced changes in amphiphilicity drive reversible restructuring of assemblies of ABC triblock copolymers with statistical polyether blocks

ABC triblock copolymers in which a block with stimulus-dependent solvophilicity resides between solvophilic and solvophobic end blocks can undergo reversible transitions between different thermodynamically stable assemblies in the presence or absence of stimulus. As a new example of such a copolymer system, thermoresponsive poly(ethylene oxide)-b-poly(ethylene oxide-stat-butylene oxide)-b-poly(isoprene) (E-BE-I) triblock copolymers with narrow molecular weight distributions (M(w)/M(n): 1.05-1.18) were prepared by sequential living anionic and nitroxide-mediated radical polymerizations. The specific copolymers examined (9.0 ≤ M(n) ≤ 14.4 kg/mol, 14% ≤ wt % isoprene ≤35%) form near-spherical aggregates with narrow size distributions at 25 °C. The thermoresponsive behavior of these polymers was studied by applying cloud point, DLS, and TEM measurements to a representative polymer, E(2.3)BE(5.3)I(2.3). The transformation of polymer aggregates from spherical micelles to vesicles (polymersomes) at elevated temperatures was detected by DLS and TEM studies, both with and without cross-linking of polymer assemblies. The rate of transformation with E-BE-I systems is more rapid than that observed for poly(ethylene oxide)-b-poly(N-isopropylacrylamide)-b-poly(isoprene) assemblies, suggesting that interchain hydrogen bonding of responsive blocks after dehydration plays an important role in the kinetics of aggregate rearrangement.     

Thin film assembly of spider silk-like block copolymers

We report the self-assembly of monolayers of spider silk-like block copolymers. Langmuir isotherms were obtained for a series of bioengineered variants of the spider silks, and stable monolayers were generated. Langmuir-Blodgett films were prepared by transferring the monolayers onto silica substrates and were subsequently analyzed by atomic force microscopy (AFM). Static contact angle measurements were performed to characterize interactions across the interface (thin film, water, air), and molecular modeling was used to predict 3D conformation of spider silk-like block copolymers. The influence of molecular architecture and volume fraction of the proteins on the self-assembly process was assessed. At high surface pressure, spider silk-like block copolymers with minimal hydrophobic block (f(A) = 12%) formed oblate structures, whereas block copolymer with a 6-fold larger hydrophobic domain (f(A) = 46%) formed prolate structures. The varied morphologies obtained with increased hydrophobicity offer new options for biomaterials for coatings and related options. The design and use of bioengineered protein block copolymers assembled at air-water interfaces provides a promising approach to compare 2D microstructures and molecular architectures of these amphiphiles, leading to more rationale designs for a range of nanoengineered biomaterial needs as well as providing a basis of comparison to more traditional synthetic block copolymer systems.     

聚苯乙烯-聚(4-乙烯基吡啶)嵌段共聚物LB膜结构形态的研究

采用氯仿作为铺展溶剂,将嵌段共聚物聚苯乙烯-聚(4-乙烯基吡啶)(PS-b-P4VP)稀溶液铺展于空气与水界面上,利用Langmuir-Blodgett(LB)膜技术转移至固体基底.研究了不同的嵌段比、表面压和小分子1-芘丁酸(PBA)的加入对嵌段共聚物气液界面聚集组装的影响.研究发现随着亲水段(P4VP)的增加,聚集组装结构由纳米片状、带状转变成纳米条状、纳米点状结构.表面压对纯PS-b-P4VP聚集组装产生影响,表面压增大,组装体排列紧密;随着表面压的继续增大,单层聚集结构遭到破坏,发生堆叠.加入PBA小分子后,PBA与PS-b-P4VP形成氢键,形态发生明显变化,原来的片状结构转变为条状或点状结构.     

Chain-growth polycondensation for well-defined aramide. Synthesis of unprecedented block copolymer containing aramide with low polydispersity

Poly(p-benzamide) with a defined molecular weight and a low polydispersity and a block copolymer containing this well-defined aramide was synthesized. Phenyl 4-aminobenzoate, which would yield poly(p-benzamide), did not polymerize under the conditions of chain-growth polycondensation. However, phenyl 4-(4-octyloxybenzylamino)benzoate (1b) polymerized at room temperature in the presence of base and phenyl 4-nitrobenzoate (2) as an initiator in a chain-growth polycondensation manner to give well-defined aromatic polyamides having the 4-octyloxybenzyl groups as a protecting group on nitrogen in an amide. It was confirmed by a model reaction that deprotection of this protecting group proceeded completely with trifluoroacetic acid (TFA) without breaking the amide linkage. The utility of this approach to poly(p-benzamide) with a low polydispersity was demonstrated by the synthesis of block copolymers. Thus, phenyl 4-(octylamino)benzoate (1a) polymerized in the presence of 2 and base, followed by addition of 1b and base to the reaction mixture of the prepolymer to yield the block copolymer of 1a and 1b with a controlled molecular weight and a low polydispersity. The block copolymer was treated with TFA, resulting in a soluble block copolymer of poly(N-octyl-p-benzamide) and poly(p-benzamide). The SEM images of the supramolecular assemblies of the block copolymer showed mum-sized bundles and aggregates of flake structures.     

两亲性嵌段聚醚酯在空气-水界面上的Langmuir单分子膜

聚醚酯是一种新型弹性材料,目前已成为工业化产品[1].对这种嵌段聚醚酯的合成和弹性行为[2]、熔体的流变性能[3]、以及纤维在拉伸状态下的聚集态结构和分子运动[4]已有一些报道.     

Non-surface activity and micellization of ionic amphiphilic diblock copolymers in water. Hydrophobic chain length dependence and salt effect on surface activity and the critical micelle concentration

We reported previously (Macromolecules 2003, 36, 5321; Langmuir, 2004, 20, 7412) that amphiphilic diblock copolymers having polyelectrolytes as a hydrophilic segment show almost no surface activity but form micelles in water. In this study, to further investigate this curious and novel phenomenon in surface and interface science, we synthesized another water-soluble ionic amphiphilic diblock copolymer poly(hydrogenated isoprene)-b-sodium poly(styrenesulfonate) PIp-h2-b-PSSNa by living anionic polymerization. Several diblock copolymers with different hydrophobic chain lengths were synthesized and the adsorption behavior at the air/water interface was investigated using surface tension measurement and X-ray reflectivity. A dye-solubilization experiment was carried out to detect the micelle formation. We found that the polymers used in this study also formed micelles above a certain polymer concentration (cmc) without adsorption at the air-water interface under a no-salt condition. Hence, we further confirmed that this phenomenon is universal for amphiphilic ionic block copolymer although it is hard to believe from current surface and interface science. For polymers with long hydrophobic chains (more than three times in length to hydrophilic chain), and at a high salt concentration, a slight adsorption of polymer was observed at the air-water interface. Long hydrophobic chain polymers showed behavior "normal" for low molecular weight ionic surfactants with increasing salt concentration. Hence, the origin of this curious phenomenon might be the macroionic nature of the hydrophilic part. Dynamic light scattering analysis revealed that the hydrodynamic radius of the block copolymer micelle was not largely affected by the addition of salt. The hydrophobic chain length-cmc relationship was found to be unusual; some kind of transition point was found. Furthermore, very interestingly, the cmc of the block copolymer did not decrease with the increase in salt concentration, which is in clear contrast to the fact that cmc of usual ionic small surfactants decreases with increasing salt concentration (Corrin-Harkins law). These behaviors are thought to be the special, but universal, characteristics of ionic amphiphilic diblock copolymers, and the key factor is thought to be a balance between the repulsive force from the water surface by the image charge effect and the hydrophobic adsorption.     

Study of the Air-Water Interfacial Properties of Biodegradable Polyesters and Their Block Copolymers with Poly(ethylene glycol)

It has been reported that the surface pressure-area isotherm of poly(d,l-lactic acid-ran-glycolic acid) (PLGA) at the air-water interface exhibits several interesting features: (1) a plateau at intermediate compression levels, (2) a sharp rise in surface pressure upon further compression, and (3) marked surface pressure-area hysteresis during compression-expansion cycles. To investigate the molecular origin of this behavior, we conducted an extensive set of surface pressure and AFM imaging measurements with PLGA materials having several different molecular weights and also a poly(d,l-lactic acid-ran-glycolic acid-ran-caprolactone) (PLGACL) material in which the caprolactone monomers were incorporated as a plasticizing component. The results suggest that (i) the plateau in the surface pressure-area isotherm of PLGA (or PLGACL) occurs because of the formation (and collapse) of a continuous monolayer of the polymer under continuous compression; (ii) the PLGA monolayer becomes significantly resistant to compression at high compression because under that condition the collapsed domains become large enough to become glassy (such behavior was not observed in the nonglassy PLGACL sample); and (iii) the isotherm hysteresis is due to a coarsening of the collapsed domains that occurs under high-compression conditions. We also investigated the monolayer properties of PEG-PLGA and PEG-PLGACL diblock copolymers. The results demonstrate that the tendency of PLGA (or PLGACL) to spread on water allows the polymer to be used as an anchoring block to form a smooth biodegradable monolayer of block copolymers at the air-water interface. These diblock copolymer monolayers exhibit protein resistance.     

Novel two-dimensional "ring and chain" morphologies in Langmuir-Blodgett monolayers of PS-b-PEO block copolymers: effect of spreading solution concentration on self-assembly at the air-water interface

A polystyrene-b-poly(ethylene oxide) (PS-b-PEO) (MW = 141k, 11.4 wt% PEO) diblock copolymer in the hydrophobic regime was spread from chloroform solutions of various concentrations at the air-water interface, and the resultant monolayers were transferred to glass substrates and imaged using atomic force microscopy. Monolayers prepared under identical conditions were also characterized at the air-water interface via Langmuir compression isotherms. The effects of spreading solution concentration on surface features, compressibility, and limiting mean molecular area were determined, revealing several interesting trends that have not been reported for other systems of PS-b-PEO. Spreading solutions > or = 0.50 mg/mL resulted almost exclusively in dot and spaghetti morphologies, with no observed continent features, which have been commonly found in more hydrophobic systems. For lower spreading solutions, < or = 0.25 mg/mL, we observed a large predominance of two novel surface morphologies, nanoscale rings and chains. The surface pressure (pi)-area (A) isotherms also exhibited a unique dependence on the spreading solution concentration, with limiting mean molecular areas and isothermal compressibilities of PS-b-PEO monolayers increasing below a critical concentration of spreading solution, suggesting a greater contribution from the PEO blocks. These results suggest that PS chain entanglement prior to solvent evaporation plays an important kinetic role in the extent of PEO adsorption at the air-water interface and in the morphologies of the resulting self-assembled surface aggregates.     

Interfacial micellar structures from novel amphiphilic star polymers

An amphiphilic heteroarm star polymer containing 12 alternating hydrophobic/hydrophilic arms of polystyrene (PS) and poly(acrylic acid) (PAA) connected to a well-defined rigid aromatic core was studied at the air-water and the air-solid interfaces. At the air-water interface, the molecules spontaneously form pancakelike micellar aggregates which measure up to several microns in diameter and 5 nm in thickness. Upon reduction of the surface area per molecule to 7 nm2, the two-dimensional micelles merged into a dense monolayer. We suggest that confined phase separation of dissimilar polymer arms occurred upon their segregation on the opposite sides of the rigid disklike aromatic core, forcing the rigid cores to adopt a face-on orientation with respect to the interface. Upon transfer onto solid supports the PS chains face the air-film interface making it completely hydrophobic, and the PAA chains were found to collapse and form a thin flattened underlayer. This study points toward new strategies to create large 2D microstructures with facial amphiphilicity and suggests a profound influence of star molecular architecture on the self-assembly of amphiphiles at the air-water interface.     

本体与界面条件下MAPTMS水解-缩合过程中簇集行为比较研究

以甲基丙烯酰氧丙基三甲氧基硅烷(MAPTMS)为前驱体,利用荧光光谱、在线衰减全反射红外光谱(In-situ ATR-FT-IR),比较性地研究了在本体及气液界面条件下MAPTMS在酸性介质中的水解-缩合过程.借助于LB膜技术,测定了MAPTMS在气液界面水解-缩合反应过程中的π-A曲线.不论在本体相还是气液界面相,荧光探针芘均表现出明显的芘分子完全重叠及部分重叠的激基缔合物的荧光发射特征.结果表明:MAPTMS水解产物在水相中发生簇集,而且簇集体疏水端相互作用存在一定刚性.芘的荧光光谱随反应进行表现出显著变化,但本体相荧光光谱变化较气液界面相剧烈.ATR-FT-IR光谱在线检测表明:本体水解缩合产物具有较高的聚合度,而气液界面水解缩合产物聚合度较低.上述结果说明体系中存在簇集行为的显著变化,且在气液界面MAPTMS水解产物受限程度比本体相大.分子在气液界面上的聚集行为也由-πA曲线的结果得到进一步证实.文中依据MAPTMS水解产物具有类似表面活性剂的特性对上述实验结果给予了解释.     

Self-assembly of short amyloidogenic peptides at the air-water interface

Short peptide stretches in amyloidogenic proteins can form amyloid fibrils in vitro and have served as good models for studying amyloid fibril formation. Recently, these amyloidogenic peptides have gained considerable attention, as non-amyloid ordered structures can be obtained from these peptides by carefully tuning the conditions of self-assembly, especially pH, temperature and presence of organic solvents. We have examined the effect of surface pressure on the self-assembled structures of two amyloidogenic peptides, Pβ(2)m (Ac-DWSFYLLYYTEFT-am) and AcPHF6 (Ac-VQIVYK-am) at the air-water interface when deposited from different solvents. Both the peptides are surface-active and form Thioflavin T (ThT) positive structures at the air-water interface. There is considerable hysteresis in the compression and expansion isotherms, suggesting the occurrence of structural rearrangements during compression. Preformed Pβ(2)m fibrillar structures at the air-water interface are disrupted as peptide is compressed to lower molecular areas but restored if the film is expanded, suggesting that the process is reversible. AcPHF6, on the other hand, shows largely sheet-like structures at lower molecular areas. The solvents used for dissolution of the peptides appear to influence the nature of the aggregates formed. Our results show that like hydrostatic pressure, surface pressure can also be utilized for modulating the self-assembly of the amyloidogenic and self-assembling peptides.     

Adsorption characteristics of zwitterionic diblock copolymers at the silica/aqueous solution interface

The adsorption of a zwitterionic diblock copolymer, poly(2-(diethylamino)ethyl methacrylate)-block-poly(methacrylic acid) (PDEA59-PMAA50), at the silica/aqueous solution interface has been characterised as a function of pH. In acidic solution, this copolymer forms core-shell micelles with the neutral PMAA chains being located in the hydrophobic cores and the protonated PDEA chains forming the cationic micelle coronas. In alkaline solution, the copolymer forms the analogous inverted micelles with anionic PMAA coronas and hydrophobic PDEA cores. The morphology of the adsorbed layer was observed in situ using soft-contact atomic force microscopy (AFM): this technique suggests the formation of a thin adsorbed layer at pH 4 due to the adsorption of individual copolymer chains (unimers) rather than micelle aggregates. This is supported by the remarkably low dissipation values and the relatively low degrees of hydration for the adsorbed layers, as estimated using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). In alkaline solution, analysis of the adsorption data suggests a conformation for the adsorbed copolymers where one block projects normal to the solid/liquid interface; this layer consists of a hydrophobic PDEA anchor block adsorbed on the silica surface and an anionic PMAA buoy block extending into the solution phase. Tapping mode AFM studies were also carried out on the silica surfaces after removal from the copolymer solutions and subsequent drying. Interestingly, in these cases micelle-like surface aggregates were observed from both acidic and alkaline solutions. The lateral dimension of the aggregates seen is consistent with the corresponding hydrodynamic diameter of the copolymer micelles in bulk solution. The combination of the in situ and ex situ AFM data provides evidence that, for this copolymer, micelle aggregates are only seen in the ex situ dry state as a result of the substrate withdrawal and drying process. It remains unclear whether these aggregates are caused by micelle deposition at the surface during the substrate withdrawal from the solution or as a result of unimer rearrangements at the drying front as the liquid recedes from the surface.