Perforated Lamellae Research Articles

Phase Behavior of Sphere-Forming Triblock Copolymers in Films

The self-assembly of sphere-forming triblock copolymers confined between two thin homogeneous surfaces is investigated based on mean-field dynamic density functional theory. The morphologies deviating from the bulk sphere-forming phase are revealed, including cylinders oriented perpendicular to the surface, cylinders oriented parallel to the surface, perforated lamellae and lamellae by varying film thickness and surface field strength. The phase diagram of surface reconstruction is also constructed. By comparing the present phase diagram with the other relevant phase diagram for the cylinder-forming triblock copolymer film, the difference between the sphere-forming and the cylinder-forming triblock copolymer thin film is discussed.

ORDERED MICROPHASES OF SYMMETRIC DIBLOCK COPOLYMER IN SELECTIVE SOLVENT: DISSIPATIVE PARTICLE DYNAMICS SIMULATION AND RADIAL DISTRIBUTION FUNCTION ANALYSIS

The self-assembly process of a symmetric diblock copolymer A5B5 in a selective solvent is studied using the dissipative particle dynamics method. Four types of morphologies can form, including lamellae, perforated lamellae, cylinders and spheres. Three phase diagrams are presented to demonstrate the effect of the concentration and interaction parameters on the ordered microstructures. Radial distribution function (RDF) characterizing the spatial correlations of the blocks is calculated. RDF not only confirms the morphological observations, but also provides more quantitative details. The evolution process of hexagonally packed cylinder and lamellae is also illustrated.

Highly Tunable Self-Assembled Nanostructures from a Poly(2-vinylpyridine-b-dimethylsiloxane) Block Copolymer

An extraordinarily large degree of tunability in geometry and dimension is demonstrated in films of a self-assembled block copolymer. A poly(2-vinylpyridine-b-dimethylsiloxane) block copolymer with highly incompatible blocks was spun-cast on patterned substrates and treated with various solvent vapors. The degree of selective swelling in the poly(2-vinylpyridine) matrix block could be controlled over an extensive range, leading to the formation of various microdomain morphologies such as spheres, cylinders, hexagonally perforated lamellae, and lamellae from the same block copolymer. The systematic control of swelling ratio and the choice of solvent vapors offer the unusual ability to control the width of very well-ordered linear features within a range between 6 and 31 nm. This methodology is particularly useful for nanolithography based on directed self-assembly in that a single block copolymer film can form microdomains with a broad range of geometries and sizes without the need to change molecular weight or volume fraction.

Solid-state nanostructured materials from self-assembly of a globular protein-polymer diblock copolymer.

Self-assembly of three-dimensional solid-state nanostructures containing approximately 33% by weight globular protein is demonstrated using a globular protein-polymer diblock copolymer, providing a route to direct nanopatterning of proteins for use in bioelectronic and biocatalytic materials. A mutant red fluorescent protein, mCherryS131C, was prepared by incorporation of a unique cysteine residue and site-specifically conjugated to end-functionalized poly(N-isopropylacrylamide) through thiol-maleimide coupling to form a well-defined model protein-polymer block copolymer. The block copolymer was self-assembled into bulk nanostructures by solvent evaporation from concentrated solutions. Small-angle X-ray scattering and transmission electron microscopy illustrated the formation of highly disordered lamellae or hexagonally perforated lamellae depending upon the selectivity of the solvent during evaporation. Solvent annealing of bulk samples resulted in a transition toward lamellar nanostructures with mCherry packed in a bilayer configuration and a large improvement in long-range ordering. Wide-angle X-ray scattering indicated that mCherry did not crystallize within the block copolymer nanodomains and that the β-sheet spacing was not affected by self-assembly. Circular dichroism showed no change in protein secondary structure after self-assembly, while UV-vis spectroscopy indicated approximately 35% of the chromophore remained optically active.

Perforated Lamellae Morphology in Novel P2VP(PDMS-b-PI-b-PS)2 3-Miktoarm Star Quarterpolymer

The melt morphology of a novel P2VP(PDMS-b-PI-b-PS)2 3-miktoarm star quarterpolymer is investigated. The melt morphology is determined using rheology and small-angle neutron and X-ray scattering data in conjunction with dissipative particle dynamics simulations of a model molecule matching the volume fractions of the experimental system. The simulations are used to interpret the scattering data and identify the melt morphology as a new perforated lamellae structure. In this structure the P2VP forms hexagonally perforated lamellae containing close-packed protrusions of PS connecting adjacent lamellae layers of PS. The PI part forms a lining between the P2VP and PS domains constrained by the molecular architecture.

Synthesis and morphology of new asymmetric star polymers of poly[4-(9,9-dihexylfloren-2-yl)styrene]-block-poly(2-vinylpyridine) and their non-volatile memory device applications

We report the synthesis, morphology, and properties of new 3-arm AB22 and 5-arm AB44 asymmetric amphiphilic star polymers of poly[4-(9,9-dihexylfloren-2-yl)styrene]-block-poly(2-vinylpyridine) (P(St-Fl)-b-P2VP), with narrow molecular weight distributions (Mw/Mn < 1.11). The asymmetric star polymers were synthesized by living anionic polymerization using 1,1-diphenylethylene (DPE)-functionalized P(St-Fl) with two or four benzyl bromide moieties, followed by linking reaction with a living anionic P2VP. Depending on the arm numbers of P2VP, lamellar and hexagonally perforated lamellae morphologies were observed for the AB22 and AB44P(St-Fl)-b-P2VP from the results of SAXS, WAXS, and TEM. The [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) incorporated in the polymer matrix increased the order degree of the liquid-crystal-like morphology in the P(St-Fl) block. The prepared AB22 and AB44P(St-Fl)-b-P2VP could be blended with PCBM for electrical write-once-read-many times (WORM) memory devices application. The threshold voltage (−2.8 to −3.9 V) and ON/OFF ratio (103 to 106) of the WORM characteristic were obtained by tuning the PCBM content. The ON or OFF state of the WORM memory devices could remain over 104 s without any degradation. The optical absorption and photoluminescence results indicated the formation of charge transfer complexation between PCBM and the 2-vinylpyridine or pendent fluorene chromophores. The switching behavior was further explained by the charge injection dominated thermionic emission in the OFF state and field-induced charge transfer in the ON state. These experimental results provide a new strategy to prepare a conjugated moiety containing asymmetric star polymers and their potential electrical memory device applications.

Self-Assembly of ABC Star Triblock Copolymer Thin Films Confined with a Preferential Surface: A Self-Consistent Mean Field Theory

The microphase separation and morphology of a nearly symmetric A(0.3)B(0.3)C(0.4) star triblock copolymer thin film confined between two parallel, homogeneous hard walls have been investigated by self-consistent mean field theory (SCMFT) with a pseudospectral method. Our simulation experiments reveal that under surface confinement, in addition to the typically parallel, perpendicular, and tilted cylinders, other phases such as lamellae, perforated lamellae, and complex hybrid phases have been found to be stable, which is attributed to block-substrate interactions, especially for those hybrid phases in which A and B blocks disperse as spheres and alternately arrange as cubic CsCl structures, with a network preferred structure of C block. The results show that these hybrid phases are also stable within a broad hybrid region (H region) under a suitable film thickness and a broad field strength of substrates because their free energies are too similar to being distinguished. Phase diagrams have been evaluated by purposefully and systematically varying the film thickness and field strength for three different cases of Flory-Huggins interaction parameters between species in the star polymer. We also compare the phase diagrams for weak and strong preferential substrates, each with a couple of opposite quality, and discuss the influence of confinement, substrate preference, and the nature of the star polymer on the stability of relatively thinner and thick film phases in this work.

Nanostructuration of self-assembled poly(styrene-b-isoprene-b-styrene) block copolymer thin films in a highly oriented pyrolytic graphite substrate

Highly oriented pyrolitic graphite (HOPG) is a useful substrate to visualize epitaxial formation due to its crystallographic structure. The morphology of a poly(styrene-b-isoprene-b-styrene) block copolymer thin film on a HOPG substrate was investigated by atomic force microscopy. Block copolymer domains generated a morphology with triangular regularity. This arrangement was induced by the HOPG substrate structure due to van der Waals attraction between the HOPG π-conjugated system and aromatic ring of polystyrene domains. However, increasing the film thickness, the substrate effect on the surface morphology decreased. As a consequence, film surfaces showed the coexistence of different structures such as highly aligned cylinders and perforated lamellae. When film thickness exceeded a threshold value, the substrate did not have effect in the surface arrangements and the surface showed a similar morphology to that existing in bulk.

Microcapsules with Tailored Nanostructures by Microphase Separation of Block Copolymers

Colloidal microcapsules of thin nanoshell membranes were prepared by the evaporation-induced self-assembly of poly(styrene)-block-poly(butadiene)-block-poly(styrene) (SBS) triblock copolymers inside the oil phase of a water-in-oil-in-water (W/O/W) double emulsion, which was prepared via a two-step sequential emulsification process. To see the nanoconfinement effect, the shell thickness was kept very thin and comparable to the feature scale of self-organized nanostructure. By exploiting the density mismatch between the microcapsules and the continuous medium, single-cored microcapsules with thin shells were selectively separated from mixtures of multicored microcapsules or thick-shelled microcapsules. The internal morphology of the membrane could be modulated by controlling the volume fractions of styrene and butadiene blocks, which was achieved simply by adding polystyrene homopolymer of relatively low molecular weight. By varying the amount of homopolymer, various morphologies were produced in the microcapsule membranes, including lamellae, perforated lamellae, and cylindrical and spherical phases. Moreover, the addition of polystyrene homopolymer could be used to control the mechanical characteristics of the microcapsules.

Self-Assembly of Lamellar- and Cylinder-Forming Diblock Copolymers in Planar Slits: Insight from Dissipative Particle Dynamics Simulations

We present a dissipative particle dynamics simulation study on nanostructure formation of symmetric and asymmetric diblock copolymers confined between planar surfaces. We consider symmetric and slightly asymmetric diblock copolymers that form lamellar nanostructures in the bulk, and highly asymmetric diblock copolymers that form cylindrical nanostructures in the bulk. The formation of the diblock copolymer nanostructures confined between the planar surfaces is investigated and characterized by varying the separation width and the strength of the interaction between the surfaces and the diblock copolymers. Both the slit width and the surface interaction strongly influence the phase diagram, especially for the asymmetric systems. For the symmetric and slightly asymmetric diblock copolymer systems, the confinement primarily affects the orientation of the lamellar domains and only marginally influences the domain morphologies. These systems form parallel lamellar phases with different number of lamellae, and perpendicular and mixed lamellar phases. In a narrow portion of the phase diagram, these systems exhibit a parallel perforated lamellar phase, where further insight into the appearance of this phase is provided through free-energy calculations. The confined highly asymmetric diblock copolymer system shows, in addition to nanostructures with parallel and perpendicular cylinders, noncylindrical structures such as parallel lamellae and parallel perforated lamellae. The formation of the various confined nanostructures is further analyzed by calculating structural characteristics such as the mean square end-to-end distance of the diblock copolymers and the nematic order parameter.

Self-Assembly of Star-Polymer-Attached Nanospheres for Polymer Nanocomposites

Self-assembly is a versatile approach for the preparation of polymer nanocomposites with prescribed morphologies in the high-technology applications. Here, we use a Brownian dynamics (BD) method to explore the self-assembly of star-polymer-attached nanospheres and propose that a star polymer attached to the nanosphere brings anisotropy and induces the formation of polymer nanocomposites with different morphologies. These morphologies include hollow sphere, porous structure, lamella, perforated lamella with star polymer through nanosphere, and cylinder-like nanosphere, core−shell micelle, gyroid-like network, and star-polymer-formed cylindrical structure, depending on temperature, concentration, and nanosphere size. To give a framework of these mesostructures, a temperature versus concentration phase diagram is presented for each case of nanospheres. In order to better understand the self-assembly of star-polymer-attached nanospheres, we also explore the mechanism of the formation of these morphologies by ...

Control of gyroid forming block copolymer templates: effects of an electric field and surface topography

The control over 10 nm scale porosity derived from self-assembly of copolymers is an extremely promising method for the synthesis of organic–inorganic hybrid materials applied, for example, in solar cells. Here, we report the thin film behaviour of a poly(4-fluorostyrene)-b-poly(D,L-lactide) PFS-b-PLA block copolymer which adopts the bicontinuous gyroid phase in the bulk and may be used to form a porous template suitable for patterning functional materials by selective degradation of the minority PLA domains. The response of the copolymer morphology to DC electric fields is probed at temperatures where the bulk copolymer adopts either the gyroid or the cylindrical phase. At 150 °C electric field alignment results in vertical arrays of cylinders, lamellae, and perforated lamellae while at 180 °C the gyroid phase coexists with a standing perforated lamellar phase. We show that both polymer–substrate interactions and substrate topography are critical factors determining substrate reconstruction of the gyroid phase. Spontaneous cross-film percolation of the minority network phase on a given substrate, a prerequisite for electrochemical replication, is dependent on surface topology at the scale of the gyroid unit cell. Importantly, under suitable processing conditions all these complex copolymer morphologies can be electrochemically replicated to produce highly ordered freestanding nanostructured arrays over large areas.

Self-assembly of end-tethered nanorods in a neat system and role of block fractions and aspect ratio

We report a computational study of the self-assembly of end-tethered nanorods in a neat system (no solvent). We present morphological phase diagrams for low and moderate aspect ratio rods as a function of inverse temperature vs. relative tether fraction. Our simulations predict that the end-tethered rods self-assemble into hexagonally arranged chiral cylinders, hexagonally perforated lamellae, monolayer and bilayer arrowhead structures and wavy lamellae. For high aspect ratio tethered nanorods and small tether fractions, we observe that the tethered nanorods self-assemble into smectic and zig-zag lamellar morphologies.

Confined Sphere-Forming Block Copolymers: Phase Behavior and the Role of Chain Architecture

We have considered the phase behavior of sphere-forming block copolymers in confinement by means of dynamic density functional theory (DDFT). We have in particular focused on the role of the chain architecture by choosing two equivalent AB and ABA block copolymers. The role of surface fields (EM) and film thickness (H) on the phase behavior is elucidated. For both systems, we identified surface reconstructions such as a wetting layer, parallel cylinders, perforated lamellae and lamellae, that were previously also found in confined cylinder-forming systems. In contrast to cylinder-forming systems, we observed that the part of the film that is affected by the surface field depends on the nature of the surface layer. In particular, cylindrical and lamellar surface reconstructions connect to the bulk structure in the interior of the film via cylindrical layers. The origin of this extended surface field range lies in the restricted chain conformations. As a direct consequence, confinement plays a role also in relatively thick films for stronger surface fields. We show in detail the complex interplay between the surface field and confinement, and how this introduces a certain amount of asymmetry even for this setup with symmetric wetting conditions. We identified the role of the chain architecture, which is only striking in neutral surfaces, where a small repulsive interaction balances the entropie gain of positioning the shortest block next to the surface. The AB diblock forms a layered spheres structure for most H, while for the ABA triblock a new structure, perpendicular cylinders that span the film, is observed for all considered H. © 2009 American Chemical Society.

Microphase Separation in Sulfonated Block Copolymers Studied by Monte Carlo Simulations

The underpinnings of microphase separation in symmetric poly(styrenesulfonate-block-methylbutylene) (PSS−PMB) copolymer melts were examined by Monte Carlo lattice simulations. The main challenge is understanding the effect of ion pairs in the PSS block on thermodynamics. We assume that experimentally determined Flory−Huggins interaction parameters are adequate for describing intermonomer interactions. Our model does not account for either electrostatic or dipolar interactions. This enables comparisons between simulated and experimentally observed microphases reported by Park and Balsara [Macromolecules 2008, 41, 3678] without resorting to any adjustable parameters. The PSS block in both experiments and theory is partially sulfonated. We quantified the effect of sequence distribution on phase behavior by using alternating and blocky PSS chains in the simulations. Depending on temperature and sequence distribution, simulations show perforated lamellae, gyroid, and hexagonally packed cylinders in addition to...

Synthesis and Self-Assembly Behavior of Poly(fluorenylstyrene)-block-poly(2-vinylpyridine) Block Copolymers and Their Blends with Single Wall Carbon Nanotubes (SWCNTs)

We have synthesized a series of poly[4-(9,9-dihexylfluoren-2-yl)styrene]-block-poly(2-vinylpyridine) (P(St-F1)-b-P2VP) and poly[4-di(9,9-dihexylfluoren-2-yl)styrene]-block-poly(2-vinylpyridine) (P(St-F12)-b-P2VP) comblike block copolymers and investigated their self-assembly behavior. The block composition, molecular weight, and the length of the fluorene side chain were varied to control the final morphologies. Although it was found that side chains composed by up to two fluorene units were not sufficient to induce any liquid crystalline structure, the phase diagrams of the resulting block copolymers greatly depended on fluorene content. Depending on the number of pendant fluorene units, the block volume ratio, the solvent used to cast the samples, and thermal annealing history, spherical, hexagonally packed cylinders, lamellar, gyroidal, and hexagonally perforated lamellae could be found. The resulting phase diagram, compared to the reference PS-b-P2VP system, was interpreted in terms of the conformatio...

First detection of lamella-gyroid-cylinder phase transition of neat polyethylene-poly(ethylene oxide) diblock copolymers on the basis of synchrotron WAXD/SAXS and infrared/Raman spectral measurements

The phase transition behaviour of polyethylene-b-poly(ethylene oxide) (PE-b-PEO) diblock copolymer with relatively short chain lengths has been studied on the basis of temperature dependent infrared and Raman spectral measurements and synchrotron WAXD/SAXS simultaneous measurements, from which the concrete structural changes were deduced successfully from the various levels of molecular chain conformation, chain packing mode and higher-order structure. The higher-order structure has been found to transform between lamella, perforated lamella, gyroid, cylinder and sphere structures. The inner structural changes occurring in the polyethylene and poly(ethylene oxide) parts have been related with these morphological changes. The morphological transition from lamella to gyroid occurs with keeping the crystalline state of polyethylene parts. This apparently curious transition can be interpreted reasonably by assuming the thermally-activated chain motion in the crystal lattice, which may play an important role as a trigger to induce the morphological change from lamella to gyroid. This idea was supported by the measurement of half-width of Raman anti-symmetric CH2 stretching band sensitive to the thermal mobility of alkyl chains.

Size of Bicelle Defects Probed via Diffusion Nuclear Magnetic Resonance of PEG

Diffusion of various poly(ethylene glycol) (PEG) tracers of well-defined molecular weight and narrow polydispersity confined within the aqueous interstices between positively magnetically aligned bicelles was measured using pulsed-field-gradient 1H nuclear magnetic resonance. The bicelles consisted of mixtures of dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG), and dihexanoylphosphatidylcholine (DHPC) in the molar ratios q = [100 DMPC +5 DMPG]/[DHPC] = 3.5, 4.5, and 5.5, to which Yb3+ had been added in the ratio 1:75 Yb3+/phospholipid. The field gradients were applied such that diffusion was measured in the direction parallel to the normal to the bicelles' planar regions, thereby rendering the experiment sensitive to the ability of PEG to traverse lamellar defects within the bicelles. The pulsed-field-gradient nuclear magnetic resonance diffusive intensity decays were diffusion-time-independent in all cases, with diffusive displacements corresponding to many hundreds of bicellar lamellae. This permitted a description of such diffusive decays in terms of a mean behavior involving a combination of straight obstruction effects common to all PEG, with hindrance to diffusion proportional to the relative size of a given PEG with respect to the size of the lamellar defects. Across the range of PEG molecular weights (200–4600) and bicelle compositions examined, the apparent radial dimension of the lamellar defects decreased from 165 Å with q = 3.5 to 125 Å with q = 5.5. This is opposite to the trend predicted from static geometric models of either bicelle disks or perforated lamellae. Qualitatively, the observed trend suggests that mobility of the obstructions to diffusion will need to be considered to reconcile these differences.

Sphere-forming diblock copolymers in slit confinement: A dynamic density functional theory study

With mean-field dynamic density functional theory, we study the morphologies of sphere-forming diblock copolymers confined between two homogeneous surfaces. The effects of the film thickness and the surface field strength on the phase behavior of sphere-forming copolymer film are investigated. The morphologies deviating from the bulk sphere-forming structure are revealed, including cylinders oriented perpendicular to the surface, cylinders oriented parallel to the surface, perforated lamellae and lamellae by varying the film thickness, and surface field strength. We also construct the phase diagram of surface reconstruction, in which some interesting phase transitions are presented. Besides, we compare the present phase diagram with the relevant phase diagram of cylinder-forming block copolymer film.

Relationship between Morphological Change and Crystalline Phase Transitions of Polyethylene−Poly(ethylene oxide) Diblock Copolymers. 3. Dependence of Morphological Transition Phenomena on the PE/PEO Segmental Lengths and Its Possible Origins

By measurement of the small-angle and wide-angle X-ray scatterings and infrared and Raman spectra and thermal data, microphase separation phenomena have been investigated for a series of polyethylene-poly(ethylene oxide) diblock copolymer (PE-b-PEO) in both the heating and cooling processes and compared with the structural changes occurring inside the PE and PEO domains. The complicated morphological changes between lamella, perforated lamella, gyroid, cylinder, and sphere phases were detected for the copolymer with relatively short PE segments. The orthorhombic crystalline structure of PE was kept unchanged in the lamella-to-gyroid transition. When the PE orthorhombic phase transformed to the pseudohexagonal or rotator phase, the gyroid morphology changed to the cylinder. On the other hand, the diblock copolymer with relatively long PE segment was found to show only the lamellar morphology, in which the order-disorder structural transition between the orthorhombic and pseudohexagonal phases occurred in the PE crystal region. As a possibility, the large difference in morphological change between the copolymers with short and long PE segments has been ascribed to the difference in thermal mobility of PE segments, which is controlled by the conformation of chains and their packing mode, i.e., an extended chain or a folded chain. The extended chains may move thermally and actively along the interfacial boundary in addition to the librational motion around the chain axis, resulting in a variety of morphological changes, whereas the thermal motion of the folded chains may be suppressed because of the geometrical constraint and does not cause such a large-scale morphological change from the lamellar structure. This concept, a thermal activity and geometrical constraint, is considered to be quite important in the interpretation of complicated morphological changes observed for many crystalline-amorphous and crystalline-crystalline diblock copolymers when viewed from the molecular level.