Morphology Of Block Copolymers Research Articles

Morphology controlled open circuit voltage in polymer solar cells

AbstractWe report a strong dependence of open circuit voltage on the altered morphology of block copolymer (P3HT‐b ‐PPerAcr) based solar cells. The open circuit voltage increases dramatically by about 300 mV by increasing the amount of acceptor homopolymer within the block copolymer/homopolymer blends. The change in open circuit voltage is found to be in correlation with the enrichment of acceptor moiety at the film surface as identified by Atomic force microscopy (AFM) and X‐ray photoelectron spectroscopy (XPS). Based on this fact, an additional increase in open circuit voltage to its maximum values is achieved by introducing an acceptor buffer layer at the cathode interface. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Supramolecular Complex of Poly(styrene)‐b‐Poly(4‐vinylpyridine) and 1‐Pyrenebutyric Acid in Thin Film

AbstractSummary: Here, we have described a novel supramolecular complex (SMC) between poly(styrene)‐b‐poly(4‐vinylpyridine) (PS‐b‐P4VP) and 1‐pyrenebutyric acid (PBA) and studied of its self assembly in thin film. PBA will make supramolecular complex with the P4VP block due to strong hydrogen bonding between the carboxylic group of 1‐pyrenebutyric acid and pyridine ring of P4VP. The formation of supramolecular complex between PS‐P4VP and PBA through hydrogen bonding is investigated through FTIR study. The supramolecular complex of PS‐b‐P4VP and 1‐pyrenebutyric acid changed the block copolymer morphology from cylindrical to lamella in thin film due to the increase of the volume fraction of P4VP (PBA). In both cases (parent block copolymer and SMC), the microdomains are oriented normal to the substrate after annealing in a selective solvent. Pure block copolymer shows cylindrical morphology with a periodicity of ∼26 nm, whereas the SMC shows lamellar morphology with a periodicity of ∼ 29 nm. After fabricating the thin film from SMC, 1‐pyrenebutyric acid can be easily removed by dissolving the thin film in ethanol to transform the block copolymer thin film into nanotemplate or membrane.

Macromol. Chem. Phys. 7/2011

Cover: Exotic morphologies of block copolymers in solution have been reported based on TEM imaging. This raises important questions: Do these morphologies really exist in solution? Or are they formed only during the TEM sample preparation? The current study brings attention to the importance of combining different methods including LLS, freeze-drying, and TEM to confirm block copolymer micelle morphologies in solution when cryogenic TEM is not available. Further details can be found in the article by H. Zhao, Q. Chen, L. Hong, L. Zhao, J. Wang,* and C. Wu* on page 663.

A toroidal tube solution to a problem involving mean curvature and Newtonian potential

The Ohta–Kawasaki theory for block copolymer morphology and the Gierer–Meinhardt theory for morphogenesis in cell development both give rise to a nonlocal geometric problem. One seeks a set in ℝ^3 which satisfies an equation that links the mean curvature of the boundary of the set to the Newtonian potential of the set. An axisymmetric, torus shaped, tube like solution exists in ℝ^3 if the lone parameter of the problem is sufficiently large. A cross section of the torus is small and the distance from the center of the cross section to the axis of symmetry is large. The solution is stable in the class of axisymmetric sets in a particular sense. This work is motivated by the recent discovery of a toroidal morphological phase in a triblock copolymer.

Breakdown of Inverse Morphologies in Charged Diblock Copolymers

Brownian Dynamics simulations are carried out to understand the effect of temperature and dielectric constant of the medium on microphase separation of charged-neutral diblock copolymer systems. For different dielectric media, we focus on the effect of temperature on the morphology and dynamics of model charged diblock copolymers. In this study we examine in detail a system with a partially charged block copolymer consisting of 75% neutral blocks and 25% of charged blocks with 50% degree of ionization. Our investigations show that due to the presence of strong electrostatic interactions between the charged block and counterions, the block copolymer morphologies are rather different than those of their neutral counterpart at low dielectric constant, however at high dielectric constant the neutral diblock behaviors are observed. This article highlights the effect of dielectric constant of two different media on different thermodynamic and dynamic quantities. At low dielectric constant, the morphologies are a direct outcome of the ion-counterion multiplet formation. At high dielectric constant, these charged diblocks behavior resembles that of neutral and weakly charged polymers with sustainable long-range order. Similar behavior has been observed in chain swelling, albeit with small changes in swelling ratio for large changes in polarity of the medium. The results of our simulations agree with recent experimental results and are consistent with recent theoretical predictions of counterion adsorption on flexible polyelectrolytes.

What Morphologies Do We Want? - TEM Images from Dilute Diblock Copolymer Solutions

The formation of various exotic morphologies of block copolymers in dilute solutions has often been claimed on the basis of TEM imaging. Do they really exist in solutions or are they just some kinetically frozen structures arising from TEM sample preparation? It is shown that different morphologies are formed due to a competition between different time scales. More and more people are realizing that it is necessary to use cryo-TEM to ascertain a novel morphology in polymer solutions, but not everyone is able to access a modern cryo-TEM. Our current study serves as an alarm to those who are interested in morphologies in block copolymer solutions that it is necessary to combine different methods, such as LLS, freeze-drying, and TEM, in their studies.

Phase behavior of binary mixtures of block copolymers and a non-solvent in miniemulsion droplets as single and double nanoconfinement

Nanoparticles consisting of different molecular weight poly(styrene-block-methyl methacrylate) (P(S-b-MMA)) copolymers and nanocapsules consisting of the same copolymers, but additionally with hexadecane as liquid core material were prepared by the miniemulsion process. The dependence of the morphology of block copolymer assemblies on the nanoconfinement was investigated. We introduced two nanoconfinement parameters, that are the nanoparticle diameter D and the shell thickness d; D was controlled by varying the concentration of surfactant in the miniemulsion, while d was controlled by the ratio hexadecane/copolymer. As the diameter D of the high molecular weight (Mw ∼ 203,700 g mol−1) P(S-b-MMA) nanoparticles increased, first Janus-particles (at D 1800 nm) were obtained. Nanocapsules with 0 < d < D also showed an onion-like structure. In both cases the outmost layer was PMMA as identified by XPS and the lamellar thickness was in agreement with theoretical considerations. Nanoparticles and nanocapsules prepared with a low-molecular weight (Mw ∼ 19,500 g mol−1) P(S-b-MMA) displayed patchy structures. This is the first time that the morphology of block copolymers was studied under double nanoconfinement in colloids.

Block copolymer – a versatile template tool for nanomaterials

made up of blocks of different monomers. For example, polystyrene-b-poly(methyl methacrylate) is usually made by first polymerizing styrene, and then subsequently polymerizing methyl methacrylate from the reactive end of the polystyrene chains. Because the different blocks are covalently bonded to each other, block copolymer can only microphase separate (or self-assemble) to form various types of ordered nanostructures ranging from 5 to 50 nm either in the bulk or solution depending on the ratio of different blocks, like hexagonal cylinder array, lamellae, and micelle nanostructures. Block copolymer provides a perfect platform for fabricating novel nanostructural materials for potential applications in advanced technologies such as nanocomposites, information storage, and bio-related nanomaterials. Due to the microphase separation of block copolymer, the morphology of block copolymer can be tuned from particles in the matrix, hexagonal cylinder to the lamellae and then inverse phases by continuously changing the length of blocks. These nanostructures are extensively used as templates for fabrication of mesoporous nanomaterials for example mesoporous SiO2, TiO2 and carbon. The mesoporous materials with high surface area have many important applications in catalyst carrier and membrane separation fields. Block copolymers are also used as template to organize the functional molecules, precursors, or nanoparticles due to the chemical difference of blocks. For example, metal ions can be loaded in one phase region; nanoparticles or nanowires can form within this phase region after reduction. On the other hand, the ordered nanostructures, especial for the vertical alignment array structure, can be directly or indirectly used as a mask for nanolithography such as Si, Ge and other semiconductor quantum dots or nanowires array. Nanostructure semiconductor and magnetic array structures are potentially possible for high efficiency information process and storage. Finally, block copolymer forms all kinds of micelles in the solution due to the solubility difference of different blocks, like spherical micelles, rodlike micelles, and vesicles. The functional units like drug molecules, fluorescent agent or magnetic agents can be co-assembled into the micelles in the solution and transferred to the target area for cure or diagnosis. Recently, block copolymer micelles have been extensively used for carrier of the drug delivery, image contrast agents, and fluorescence agents. In brief summary, block copolymer as a robust tool can be extensively using for the synthesis and fabrication of nanomaterials and biomaterials.

Morphological Investigations of Organic/Inorganic Nanocomposites Fabricated to Achieve Controlled Dispersion at High Loadings

ABSTRACTRealization of property enhancements inherent to the presence of nanoparticles continues to be a challenge for the production of bulk nanocomposite materials with commercially available techniques. This study combines twin-screw compounding with surface modification of SiO2 nanoparticles to enable targeted dispersion in a SEBS block copolymer. Production of these composites with high levels of well-dispersed particulates aims to leverage aggregation for production of hierarchical structure. The aggregation state of the particles as well as the level of order in the block copolymer morphology was determined through USAXS and TEM. Particles coated with ligands miscible with the end-blocks of the BCP (minority component) increased dispersion at all loading levels observed up to 10 vol%. Ligands employed to increase miscibility of the nanoparticle with the mid-block (majority component) resulted in large aggregates for all loadings without disturbance of the BCP morphology.

Effects of Polystyrene Block Content on Morphology and Adhesion Property of Polystyrene Block Copolymer

Effects of polystyrene block content on adhesion property and phase structure of polystyrene block copolymers were investigated. Polystyrene-block-polyisoprene-block-polystyrene triblock and polystyrene-block-polyisoprene diblock copolymers with different polystyrene block contents in the range from 13 to 35 wt% were used. In the case of the low polystyrene block content (below 16 wt%), a sea-island structure was observed: near-spherical polystyrene domains having a mean diameter of about 20 nm were dispersed in polyisoprene matrix. The phase structure changed from a sea-island structure to a cylindrical structure with an increase of polystyrene block content (over 18 wt%). Peel strength decreased with an increase of polystyrene block content and the pure triblock copolymers had lower peel strength than their blends with the diblock copolymers. Pulse nuclear magnetic resonance studies indicated that molecular mobility of polyisoprene phase decreased with an increase of polystyrene block content, and the molecular mobility was lower in the pure triblock than in the blend. Thus, the peel strength was found to be related to molecular mobility. The adhesion strength of the block copolymer depended on the molecular mobility: high molecular mobility can promote interfacial adhesion.

Shear ordered diblock copolymers with tuneable optical properties

A series of high molecular weight poly(styrene-b-isoprene) block copolymers with optical properties defined by composition in a non-selective solvent were studied using simultaneous ultra small angle X-ray scattering (USAXS) and optical spectrometry. A small magnitude shear produces ordered and oriented states in the copolymer solutions that persist for extended periods of time, and also have superior optical properties that are directly attributable to the mesoscopic block copolymer (BCP) morphology. We have demonstrated that the optical transmission of these materials can be tuned by the addition of low molecular weight poly(isoprene) and poly(styrene) to swell their respective domains within the diblock copolymer. The optical transmission peak for the diblocks could be tuned; from 380 nm-440 nm for the 670k diblock, 425 nm-540 nm for the 850k diblock and 541 nm-625 nm for the 1 million diblock by altering the solution concentration and composition. The full width at half maximum that can be achieved for the optical transmission peaks is as small as 15 nm at 473 nm with a Δλ/λ of 0.03, highlighting the high quality ordering in these systems. Also a small shift in the transmission peak wavelength was observed across a wide angle of view (15 nm at 30°) suggesting that these materials could be used for large area narrow band optical filters.

Computational and experimental investigation of morphology in thermoplastic elastomer gels composed of AB/ABA blends in B-selective solvent

Thermoplastic elastomer gels (TPEGs) are physically associating gels composed of ABA triblock copolymers that are highly swollen with a B selective solvent. A unique feature of TPEGs, when compared to typical chemically crosslinked gels, is that the properties are dictated by a combination of nano-scale block copolymer morphology and chain level structure. In this paper, we use dissipative particle dynamics (DPD) simulations to examine TPEGs composed of a binary mixture of AB and ABA block copolymers in a B-compatible solvent. The diblock and triblock differ in the block length, where the A and B block of the diblock chains are larger than those of the triblock. The DPD simulations illustrated numerous morphologies of these AB/ABA gels characterized by formation of pure and mixed micelles of various compositions, structures, and sizes. In addition, the ABA/AB copolymer ratio and processing conditions impacted the structural and mechanical properties of these blends. The simulation results were utilized to establish factors that affect an intermicellar distance and bridging chain fraction. The DPD-based morphology predictions were compared with experimental data based on atomic force microscopy (AFM) with good agreement.

ENGINEERING ORIENTATION IN BLOCK COPOLYMERS FOR APPLICATION TO PROSTHETIC HEART VALVES

This study demonstrates how the mechanical performance of polymeric material can be enhanced by morphology and phase orientation of block copolymers to achieve desired anisotropic mechanical properties. The material used was a new Kraton block copolymer consisting of styrene-isoprene-butadiene-styrene blocks having cylindrical morphology. We report a method of achieving long range uniaxial as well as biaxial orientation of block copolymer. Each microstructural organization results in a specific mechanical performance, which depends on the direction of the applied deformation. The method of tailoring mechanical properties by engineering microstructure may be successfully utilized to applications requiring anisotropic mechanical response, such as prosthetic heart valves.

Block Copolymers for Fuel Cells

Ion-containing block copolymers hold promise as next-generation proton exchange membranes in hydrogen and methanol fuel cells. These materials’ self-assembled ordered nanostructures facilitate proton transport over a wide range of conditions, a requirement for robust fuel cell performance. In this perspective, we will present an overview of the morphology and transport properties of ion-containing block copolymers that have been studied to gain insight into the fundamental behavior of these materials and, in some cases, are targeted toward applications in fuel cells and other electrochemical devices. We will discuss the challenges associated with predicting and obtaining well-ordered morphologies in block copolymers with high ion content, particularly those with chemistries that can withstand the chemical and mechanical stresses of the fuel cell, such as aromatic backbone block copolymers. New opportunities for ion-containing block copolymers in alkaline membrane fuel cells will also be reviewed.

Ordering in Mixtures of a Triblock Copolymer with a Room Temperature Ionic Liquid

Well-ordered block copolymer (BCP) morphologies are obtained by blending PEO−PPO−PEO triblock copolymer surfactants with the room temperature ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate. The selective association of the IL with the PEO blocks increases the segregation strength by raising the effective interaction parameter between the PEO and PPO blocks. Therefore, the copolymer/IL blends form well-ordered microdomains in the melt, whereas the neat copolymers are phase mixed. The IL is shown to interact with the PEO chains of the copolymers by a depression in the melting point of the PEO crystals with increasing IL concentration. Wide-angle X-ray diffraction and polarized optical microscopy also show a disruption of PEO crystallization in the copolymer/blends. Infrared spectroscopy indicates a favorable enthalpic interaction between the PEO blocks and the IL, which does not exist between PPO and the IL. Small-angle X-ray scattering confirms the enhanced ordering of the copolymers upon addition of the IL, the formation of well-ordered microdomains, and a change in the characteristic spacing with increasing IL concentration.

Single chain distribution analysis near a substrate using a combined method of three-dimensional imaging and SCF simulation

The effect of preferential wetting of one of the constituent block chains and corresponding block copolymer morphologies to a carbon substrate is studied from a molecular level. The single chain distribution of the block copolymer was estimated as a function of the distance from the substrate by a combined method of transmission electron microtomography (TEMT) and self-consistent field (SCF) simulation. The former provides three-dimensional (3D) morphological information of cylindrical microdomains near the surface, while the latter utilizes the 3D morphology to quantitatively determine the interaction between the block chains and substrate, which is further used to estimate the single chain distribution of one of the block chains, i.e., the subchain, of the matrix. It was found that the subchains in the vicinity of the wetting layer are substantially compressed, while the radius of gyration of the subchain at a distance L (L is the interlayer distance of the cylindrical microdomains from the substrate) has already reached the same value as that in the bulk, indicating that the propagation of the surface interaction is limited to one layer. The methodology developed in this study can be used not only to estimate the surface effect on polymer chains for a variety of different surfaces, but also to provide a means to understand complicated block copolymer morphologies from a molecular level.

Thermo-Responsive Behavior of Block and Random Copolymers of N-Isopropylamide/N,N-Dimethylacrylamide Synthesized via Reversible Addition—Fragmentation Chain Transfer Polymerization

Block and random copolymers of N-isopropylacrylamide (NIPAM) and N,N-dimethylacrylamide (DMA) with similar compositions and molecular weights were prepared by the reversible addition-fragmentation chain transfer (RAFT) polymerization with a trithiocarbonate (2-dodecylsulfanylthiocarbonyl- sulfanyl-2-methyl propionic acid) as the RAFT reagent. The aggregation behavior and morphology of block copolymers P(NIPAM-b-DMA) and random copolymers P(NIPAM-co-DMA) was investigated by dynamic light scattering (DLS) and atomic force microscopy (AFM). Although they have similar compositions and molecular weight, their solution properties and morphology differ greatly due to their different chemical sequence. The P(NIPAM-co-DMA) exhibits much higher lower critic solution temperature (LCST) at ∼46ºC than that of P(NIPAM-b-DMA) block copolymers (36ºC). The AFM results showed that P(NIPAM-b-DMA) molecules tend to coil and aggregate to form elliptical particles with average diameter 20 nm below the LCST (25ºC), while P(NIPAM-co-DMA) molecules form network configuration on SiO2 substrate. Above their LCST (50ºC), the strong hydrophobic aggregating transition takes place between the P(NIPAM-b-PDMA) segments, while the P(NIPAM-co-DMA) segments only show some evidence of entangling. These indicate that the distribution of DMA in the DMA/NIPAM copolymer chains can strongly interfere the hydrophobic assembling of NIPAM segments.

Supramolecular Assembly of Poly(styrene)-b-poly(4-vinylpyridine) and 1-Pyrenebutyric Acid in Thin Film and Their Use for Nanofabrication

Supramolecular assembly (SMA) between poly(styrene)-b-poly(4-vinylpyridine) (PS-b-P4VP) and 1-pyrenebutyric acid (PBA) is investigated in thin films after annealing in a 1,4-dioxane atmosphere. The PBA selectively enriches with the P4VP block through hydrogen bonding between the carboxylic group of 1-pyrenebutyric acid and pyridine ring of P4VP. The supramolecular assembly of PS-b-P4VP with 1-pyrenebutyric acid resulted in changing the cylindrical morphology of pure block copolymer into lamellar morphology in thin film due to the increase of the block compositions of P4VP (PBA). Pure block copolymer after solvent annealing shows cylindrical microdomains normal to the substrate with a periodicity ∼26 nm, whereas the SMA complex shows perpendicularly oriented lamella with a periodicity ∼29 nm. These thin films were further immersed in ethanol, a good solvent for P4VP/P4VP (PBA) and a nonsolvent for PS to generate nanotemplates. These nanotemplates are effectively used for the fabrication of arrays of nanowires of different functional materials like metal or semiconductor.

Determination of polymer morphology from the intensity measurement in the reciprocal space

Reciprocal space techniques based on scattering have been widely used for structure analysis of polymer materials. It would be more useful if the reciprocal methods can determine simultaneously the morphology of the polymer as those in the real space. However, the inverse problem, i.e., determining polymer morphology from the available scattering pattern, is very difficult because of the absence of the relative phase spectrum in the experimental measurements. Therefore, the retrieval of the missing phase spectrum plays a crucial role in this problem. Herein we theoretically propose that this can be accomplished by using phase-spectrum retrieval technique. We combined the Gerchberg–Saxton and hybrid input–output algorithms to determine the morphology of block copolymers from the corresponding scattering patterns in the reciprocal space. Numerical computations have been carried out to demonstrate the concept. It has also been shown that the algorithm is immune to noise. The demonstrated method could provide an extensive methodology for the investigation of polymer behaviors in the micro- and nano-scales by interconnecting real and reciprocal spaces.

The Pattern of Multiple Rings from Morphogenesis in Development

Under certain conditions the problem of morphogenesis in development and the problem of morphology in block copolymers may be reduced to one geometric problem. In two dimensions two new types of solutions are found. The first type of solution is a disconnected set of many components, each of which is close to a ring. The sizes and locations of the rings are precisely determined from the parameters and the domain shape of the problem. The solution of the second type has a coexistence pattern. Each component of the solution is either close to a ring or to a round disc. The first-type solutions are stable for certain parameter values but unstable for other values; the second-type solutions are always unstable. In both cases one establishes the equal area condition: the components in a solution all have asymptotically the same area.