Self-Assembled Smectic Phases in Rod-Coil Block Copolymers

The dependence of nanostructures on the molecule rigidity of A2(B4)2-type miktoarm block copolymer

Using the dissipative particle dynamics simulation technique, we have studied the influence of the molecule rigidity on the nanostructures of the A2(B4)2-type miktoarm block copolymers. A typical spherical micellar ordered structure is obtained for a coil-coil miktoarm block copolymer in melt. By introducing a bond angle potential in our model to enhance the molecule rigidity systematically, we find, respectively, a hexagonal cylindrical structure and a parallel ellipsoid in lamellae structure which is discovered for the first time.

Ordering on multiple lengthscales in a series of side group liquid crystal block copolymers containing a cholesteryl-based mesogen

Hierarchical ordering in a side group liquid crystal block copolymer is investigated by differential scanning calorimetry, polarized optical microscopy, small-angle X-ray and neutron scattering (SAXS and SANS) and transmission electron microscopy (TEM). A series of block copolymers with a range of compositions was prepared by atom transfer radical polymerization, comprising a polystyrene block and a poly(methyl methacrylate) block bearing chiral cholesteryl mesogens. Smectic ordering is observed as well as microphase separation of the block copolymer. Lamellar structures were observed for far larger volume fractions than for coil-coil copolymers (up to a volume fraction of liquid crystal block, = 0.8). A sample with = 0.86 exhibited a hexagonal-packed cylinder morphology, as confirmed by SAXS and TEM. The matrix comprised the liquid crystal block, with the mesogens forming smectic layers. For the liquid crystal homopolymer and samples with high , a smectic-smectic phase transition was observed below the clearing point. At low temperature, the smectic phase comprises coexisting domains with monolayer S coexisting with interdigitated S domains. At high temperature a S phase is observed. This is the only structure observed for samples with lower . These unprecedented results point to the influence of block copolymer microphase separation on the smectic ordering.

Phase behavior of triblock copolymers varying in molecular asymmetry

The transformation from A(1)B diblock copolymer to A(1)BA(2) triblock copolymers varying in molecular asymmetry is investigated as the A(2) end block is progressively grown via chemical synthesis. Dynamic rheological measurements show that the order-disorder transition (ODT) temperatures of two copolymer series differing in composition and molecular weight decrease when the A(2) block is short relative to the A(1) block, and then increase as the length of the A(2) block is increased further. The resultant ODT minimum, predicted by mean-field theory, is attributed to mixing between long B and short A(2) blocks.

Nanostructure formation in poly(γ-benzyl-l-glutamate)-poly(ethylene glycol)-poly(γ-benzyl-l-glutamate) triblock copolymers in the solid state

The morphology in the solid state of a series of triblock copolymers comprising a poly(ethylene glycol)(PEG) midblock and symmetric poly(γ-benzyl--glutamate)(PBLG) end blocks has been studied using X-ray scattering and microscopy techniques. Transmission electron microscopy (TEM) on samples selectively stained with uranyl acetate provided clear assignment of morphologies for as-cast and annealed samples. The thickness of both PEG and PBLG domains was in good agreement with calculations based on the conformations of the respective chains, allowing for the crystal or amorphous state of PEG and the α-helical or β-sheet structure of the PBLG. Atomic force microscopy provided complementary information on surface morphology for several samples that was in good agreement with the structure observed by TEM. A morphology diagram was constructed. Cylindrical structures were observed for ordered samples with low , whereas at higher there was evidence for broken lamellar and "hockey puck" nanostructures. Regular lamellae were observed for intermediate compositions.

Amphiphilic treelike rods at interfaces: layered stems and circular aggregation

Amphiphilic dendron-rod molecules with three hydrophilic poly(ethylene oxide) (PEO) branches attached to a hydrophobic octa-p-phenylene rod stem were investigated for their ability to form two-dimensional micellar structures on a solid surface. A treelike shape of the molecules was reported to be a major factor in the formation of nonplanar micellar structures in solution and in the bulk state (cylindrical and spherical). We observed that in these treelike amphiphilic molecules the hydrophilic terminated dendron branches assemble themselves in surface monolayers with the formation of two-dimensional layered or circular micellar structures. We suggested the formation of the planar ribbon-like structures with interdigitated layering within the loosely packed monolayers and circular, ringlike structures (2D circular aggregates) in the precollapsed state.

Soluble Oligoaramide Precursors?A Novel Class of Building Blocks for Rod-Coil Architectures

A new synthetic route is described that allows the reversible conversion of the inherently insoluble oligo-p-benzamides into soluble materials through the formation of imidoyl chlorides. Syntheses of the corresponding dimer, trimer, and tetramer are reported; these compounds can easily be purified by crystallization and are accessible on the multigram scale. Structural proof was obtained by single-crystal X-ray structures of the trimer and tetramer precursors. They can be selectively functionalized into amides or esters at the terminal carboxylic acid group followed by hydrolysis of the imidoyl chlorides to the parent amides. This new class of compounds gives access to strongly aggregating rigid rodlike materials in few synthetic steps, as is demonstrated by the preparation of poly(ethylene glycol)-co-oligo(p-benzamide) rod-coil block copolymers.

Unprecedented Control over Polymerization ofn-Hexyl Isocyanate using an Anionic Initiator Having Synchronized Function of Chain-End Protection

The living anionic polymerization of isocyanates carried out using conventional initiators is associated with side reactions due to rapid initiation rates as well as back-biting by the growing chain, resulting in a lack of control on the molecular weight (MW) and molecular weight distribution (MWD) of the polymers. Successful control over the reaction was possible by using additives that could prevent back-biting. We find an initiator in sodium benzanilide (Na-BA), which has a slow initiation rate combined with additive function, so that use of an external additive is eliminated. The initiator has resulted in polymers with high yields and an unprecedented control over the MW and MWD. It is possible to introduce a number of functionalities at the termini of the polymer by using Na-BA derivatives as well as suitable terminating agents, leading to macromonomer, reactive and chiral polymers, and chiral macromonomer in approximately 100% yields. In the process, the finding has expanded the scope of polyisocyanates in diverse applications.

Length-Controlled Rodlike Self-Assemblies in Binary Mixed Langmuir−Blodgett Monolayers on Mica

In this paper, atomic force microscopy (AFM) has been used to investigate the morphology of monolayers of the amphiphilic rod-coil diblock molecule (EO7OPV) containing oligo(phenylene vinylene) dimer (OPV) and poly(ethylene oxide) (PEO) as well as the morphology of mixed monolayers of EO7OPV and palmitic acid (PA) deposited onto mica by the Langmuir-Blodgett technique. At surface pressures higher than 3 mN/m, EO7OPV forms regular-shaped aggregates with a monomolecular layer structure, where the hydrophilic PEO blocks are adsorbed onto the mica substrate and the hydrophobic OPV blocks form an ordered crystalline OPV layer on the top of the PEO layer through the strong pi-pi stacking interaction. In the mixed LB monolayers of EO7OPV and PA, the phase separation occurs. At a certain mixed ratio, EO7OPV molecules form rodlike domains with regular shape and uniform size at surface pressures higher than 3 mN/m. With the increase of the molar fraction of PA, the rodlike domains consisting of EO7OPV are elongated. The length of the rodlike domains can be tuned easily in a large range by altering the molar ratio of EO7OPV and PA. In addition, the rodlike domains are oriented to specific directions, corresponding to the directions of the potassium ion array on the mica surface having 6-fold symmetry. We demonstrate the possible formation mechanism and the elongation origin of rodlike domains in mixed LB monolayers and propose the two-step formation process of oriented rodlike domains deposited onto the mica substrate.

Multiple self-assembled nanostructures from an oligo(p-phenyleneethynylene) containing rod–coil–rod triblock copolymer

Induced by systematic variation of the initial polymer concentration in toluene, various morphologies of aggregates including vesicles, spheres, onion-like structures, and worm-like fibers from a rod-coil-rod triblock copolymer, oligo(p-phenyleneethynylene)-polystyrene-oligo(p-phenyleneethynylene), were observed by transmission electron microscopy.

Liquid-crystalline ordering in rod-coil diblock copolymers studied by mesoscale simulations

Using mesoscale dissipative particle dynamics (DPD) simulations, which ignore all atomistic details, we show the formation of lamella mesophases by cooling a fully disordered system composed of symmetric (A7B7) rod-coil diblock copolymers. Equilibration is achieved very rapidly using DPD, and isotropic, smectic A and crystalline phases of the rod-like blocks can be observed either by heating or cooling. An interesting pseudo-smectic phase can be characterized when the order-disorder transition temperature is above the clearing temperature. This phase gradually fades into a normal microphase-separated structure as the system is heated through the clearing temperature. Simulations of pure rods, however, show the formation of isotropic, nematic, smectic A and crystalline phases.

Melting of lamellar phases in temperature sensitive colloid-polymer suspensions

We prepare a novel suspension composed of rodlike fd virus and thermosensitive polymer poly(N-isopropylacrylamide) whose phase diagram is temperature and concentration dependent. The system exhibits a rich variety of stable and metastable phases, and provides a unique opportunity to directly observe melting of lamellar phases and single lamellae. Typically, lamellar phases swell with increasing temperature before melting into the nematic phase. The highly swollen lamellae can be superheated as a result of topological nucleation barriers that slow the formation of the nematic phase.

Morphology of microphase separated domains in rod–coil copolymer melts

We investigate the morphology of microphase separated domains in diblock copolymers where each chain consists of a stiff rod block and a flexible coil block. A simplified phenomenological model system is introduced, which is coarse-grained in terms of the local concentration difference between the two blocks and the local director field of the rod part. Computer simulations of this set of time-evolution equations in two dimensions show in the weak segregation regime that the elastic energy in the rod-block rich domains affects drastically the structures of microphase separated domains. A coil-to-rod transition is incorporated into the model system to examine the elastic and anisotropic effects. The effects of the external electric field are also investigated to control the domain morphology.

Organization of branched rod–coil molecules into a 3-D tetragonally perforated lamellar mesophase

Tetramerization of coil-rod-coil ABC triblock copolymers to a tetrabranched molecule induces an unusual 3-D tetragonally perforated layered liquid crystalline phase as an intermediate structure between 1-D lamellar and 2-D hexagonal columnar phases.

Rapamycin inhibits GM-CSF-induced neutrophil migration

The molecular mechanisms that govern cell movement are the subject of intense study, as they impact biologically and medically important processes such as leukocyte chemotaxis and angiogenesis, among others. We demonstrate that leukocyte chemotaxis is prevented by the macrolide immunosuppressant rapamycin, a specific inhibitor of the mammalian target of rapamycin (mTOR)/ribosomal p70-S6 kinase (p70S6K) pathway. Both neutrophil chemotaxis and chemokinesis elicited by granulocyte-macrophage colony-stimulating factor (GM-CSF) were strongly inhibited by rapamycin with an IC(50) of 0.3 nM. Inhibition, although at a higher dose, was also observed when the chemoattractant was interleukin-8. As for the mechanism, rapamycin targeted the increase of phosphorylation of p70S6K due to GM-CSF treatment, as demonstrated with specific anti-p70S6K immunoprecipitation and subsequent immunoblotting with anti-T(421)/S(424) antibodies. Rapamycin also inhibited GM-CSF-induced actin polymerization, a hallmark of leukocyte migration. The specificity of the effect of rapamycin was confirmed by the use of the structural analog FK506, which did not have a significant effect on chemotaxis but effectively rescued rapamycin-induced p70S6K inhibition. This was expected from a competitive effect of both molecules on FK506-binding proteins (FKBP). Additionally, GM-CSF-induced chemotaxis was completely (>90%) blocked by a combination of rapamycin and the MAPK kinase (MEK) inhibitor PD-98059. In summary, the results presented here indicate for the first time that rapamycin, at sub-nanomolar concentrations, inhibits GM-CSF-induced chemotaxis and chemokinesis. This serves to underscore the relevance of the mTOR/S6K pathway in neutrophil migration.

Ordering of quantum dots using genetically engineered viruses

A liquid crystal system was used for the fabrication of a highly ordered composite material from genetically engineered M13 bacteriophage and zinc sulfide (ZnS) nanocrystals. The bacteriophage, which formed the basis of the self-ordering system, were selected to have a specific recognition moiety for ZnS crystal surfaces. The bacteriophage were coupled with ZnS solution precursors and spontaneously evolved a self-supporting hybrid film material that was ordered at the nanoscale and at the micrometer scale into approximately 72-micrometer domains, which were continuous over a centimeter length scale. In addition, suspensions were prepared in which the lyotropic liquid crystalline phase behavior of the hybrid material was controlled by solvent concentration and by the use of a magnetic field.

A surface-reactive rod-coil diblock copolymer: nano- and micropatterned polymer brushes

A diblock copolymer consisting of poly(3-(triethoxysilyl)propylisocyanate) (PIC) and polystyrene(PS) was synthesized by anionic polymerization. A polymeric monolayer of the block copolymer was formed on silica substrates by various grafting techniques such as immersion, casting, or contact-printing. The PIC block adheres covalently to Si substrates in an in-plane fashion due to its extended rodlike conformation and reactivity to the silica. The polystyrene blocks aggregate to form mounds on the surface resulting in a new type of nanopatterned polymer brush. The self-limiting adsorption of the rod coils results in a thickness of about 5 nm regardless of the solution concentration and coating method. This particular property allowed microcontact printing directly onto silicon or glass substrates. The resulting surface morphology consisted of nanoscale domains given by the block copolymer and uniform thickness micropatterns transferred from the stamp within the printed area. This study offers a simple new method to prepare a covalent polymeric monolayer with nano- and micropatterns, which can be performed directly onto various silicon-based substrates.