Volume Fraction Of Block Research Articles

Structure−Properties Relationship in Proton Conductive Sulfonated Polystyrene−Polymethyl Methacrylate Block Copolymers (sPS−PMMA)

We report on the dependence of proton conductivity on the morphologies of sulfonated polystyrene−poly(methyl methacrylate) (sPS−PMMA) diblock copolymers. Three different diblock copolymers of varying molecular weight and block volume fraction were studied, and for each one several sulfonation degrees of the PS block were considered. The investigation of the morphologies of the self-assembled sPS−PMMA diblocks was carried out in both dry samples and samples saturated with water, by means of small angle neutron scattering (SANS), transmission electron microscopy (TEM) and cryoTEM. Depending on molecular weight and sulfonation degrees, isotropic phase (ISO), lamellar phases (LAM), cylindrical hexagonal phase (HEX) and hexagonally perforated lamellae (HPL) were observed. The lamellar morphologies underwent marked volume expansion upon water pick up, while negligible swelling was detected for the other morphologies. Proton conductivity was measured in both the dry and wet states, the latter conditions resultin...

Nanophase-Separated Structures of AB Block Copolymer/C Homopolymer Blends with Complementary Hydrogen-Bonding Interactions

We report on the morphological variations of SP/H blends containing a poly(styrene-block−2-vinylpyridine) diblock copolymer (SP) (Mw = 130K) and poly(4-hydroxystyrene) (H) (Mw = 14K) with different blend compositions. The structures were observed by transmission electron microscopy (TEM) as well as by small-angle X-ray scattering (SAXS). The results were compared with those obtained from SP/P blends, where P denotes the poly(2-vinylpyridine) homopolymer. The neat SP diblock copolymer, with an S block volume fraction, ϕs, of 0.67, has a hexagonally packed cylindrical structure. As P (Mw of 6.6K) is added to the copolymer, morphology of the blend transits to a lamellar structure. Further addition of the homopolymer causes macrophase separation of P homopolymer chains. In contrast, macrophase separation of H homopolymer has never been observed in SP/H blends, even upon addition of large amounts of H. At first, it was noted that the unusual morphological transition from the cylindrical structure to the spherical structure occurs upon addition of H. Subsequently transitions to cylindrical, lamellar and, finally to inverse cylindrical structures occur with successive additions of H. This result is due to the miscibility of P and H via hydrogen-bonding interactions which appear to serve as the driving force for production of uniform nanophase-separated structures covering a wide composition range.

Morphological and Scattering Studies of Binary Mixtures of Block Copolymers: Cosurfactant Effects Observed in the Parameter Space of Temperature, Blend Composition, and Molecular Weight Ratio

Thermoreversible order-order transitions or “order−distorted order” transitions (defined “OOT” for simplicity) between various morphologies occurring on binary mixtures of diblock copolymers were investigated by using small-angle X-ray scattering. The mixtures were composed of a long asymmetric polystyrene-block-polyisoprene (SI) having a spherical morphology and a short symmetric SI, which are hereafter denoted as as and s3, respectively. The symmetric diblock, s3, is short enough to be in a disordered state at room temperature. Upon raising temperature from ambient temperature toward order−disorder or “distorted order-disorder” transition temperature, a unique re-entrant bicontinuous-cylinder- bicontinuous OOT was observed for as/s3 = 85/15 and 82/18 (w/w). Other types of OOT, such as cylinder-sphere and lamella-bicontinuous, were observed on the as/s3 mixtures of different compositions. These OOTs were characterized by monitoring the evolution of the inverse peak scattered intensity, the square of the half-width at half-maximum of the peak, and the characteristic period of the microdomains, as a function of temperature, T. The complex morphological behaviors observed here can be understood on the basis of two key features of the binary blends: the cosurfactant effect and the possibility for the chemical junctions of the shorter diblock to migrate away from the interface as segregation power decreases. As a summary of this series of studies, a three-dimensional phase diagram was constructed in the parameter space of overall volume fraction of PS block in the mixture, the ratio of the degree of polymerization of as to that of s3 and T. Such a phase diagram illustrates that the cosurfactant effect opens perspectives in tailoring morphologies of self-assembled, nanostructured materials.

Synthesis and Characterization of Novel Linear PB-b-PS-b-PEO and PE-b-PS-b-PEO Triblock Terpolymers

Novel polybutadiene-block-polystyrene-block-poly(ethylene oxide) (PB-b-PS-b-PEO) linear triblock terpolymers have been synthesized by sequential living anionic polymerization. Further catalytic hydrogenation led to polyethylene-block-polystyrene-block-poly(ethylene oxide) (PE-b-PS-b-PEO), a triblock terpolymer with two crystallizable blocks and a glassy middle block. Bulk morphologies have been studied by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) for different compositions. Thermal properties of the PEO block, as determined by differential scanning calorimetry (DSC), showed dependence with the block volume fraction (φPEO) and its polymerization degree (NPEO). The corresponding properties for the PE block are also functions of the polymerization degree (NPE) and the volume fraction of the PEO block (φPEO). Since the PEO block is the first to segregate from solution in toluene, its volume fraction determines the overall morphology and consequently the thermal properties of the studied terpolymers.

A Self-Consistent Field Theory Study on the Morphologies of Linear ABCBA and H-Shaped (AB)2C(BA)2Block Copolymers

By using a combinatorial screening method based on the self-consistent field theory, we investigate the equilibrium morphologies of linear ABCBA and H-shaped (AB)(2)C(BA)(2) block copolymers in two dimensions. The triangle phase diagrams of both block copolymers are constructed by systematically varying the volume fractions of blocks A, B, and C. In this study, the interaction energies between species A, B, and C are set to be equal. Four different equilibrium morphologies are identified, i.e., the lamellar phase (LAM), the hexagonal lattice phase (HEX), the core-shell hexagonal lattice phase (CSH), and the two interpenetrating tetragonal lattice phase (TET2). For the linear ABCBA block copolymer, the reflection symmetry is observed in the phase diagram except for some special grid points, and most of grid points are occupied by LAM morphology. However, for the H-shaped (AB)(2)C(BA)(2) block copolymer, most of the grid points in the triangle phase diagram are occupied by CSH morphology, which is ascribed to the different chain architectures of the two block copolymers. These results may help in the design of block copolymers with different microstructures.

Block Copolymer Film with Sponge-Like Nanoporous Strucutre for Antireflection Coating

We prepared nanoporous films by spin-coating of polystyrene-block-poly(methyl methacrylate) copolymers (PS-b-PMMA) to a glass and irradiating by ultra-violet source followed by selective removal of PMMA blocks with acetic acid. When spin-coated PS-b-PMMA was no longer annealed at high temperatures, microphase separation between two blocks occurred only in the short-range scale. The porous films prepared from PS-b-PMMA with the volume fraction of PMMA block of 0.69 exhibited a spongelike nanoporous structure over the entire film thickness and showed excellent antireflection with a minimum reflection less than 0.1% at visible and near-infrared wavelengths. The observed reflectances were in good agreement with the predictions based on the characteristic matrix theory.

Comparing the Morphology and Phase Diagram of H-Shaped ABC Block Copolymers and Linear ABC Block Copolymers

By using a combinatorial screening method based on the self-consistent field theory (SCFT) for polymers, we have investigated the morphology of H-shaped ABC block copolymers (A2BC2) and compared them with those of the linear ABC block copolymers. By changing the ratios of the volume fractions of two A arms and two C arms, one can obtain block copolymers with different architectures ranging from linear block copolymer to H-shaped block copolymer. By systematically varying the volume fractions of block A, B, and C, the triangle phase diagrams of the H-shaped ABC block copolymer with equal interactions among the three species are constructed. In this study, we find four different morphologies (lamellar phase (LAM), hexagonal lattice phase (HEX), core-shell hexagonal lattice phase (CSH), and two interpenetrating tetragonal lattice (TET2)). Furthermore, the order-order transitions driven by architectural change are discussed.

Solvent-Induced Novel Morphologies in Diblock Copolymer Blend Thin Films

We report the morphology and phase behaviors of blend thin films containing two polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers with different blending compositions induced by a selective solvent for the PMMA block, which were studied by transmission electron microscopy (TEM). The neat asymmetric PS-b-PMMA diblock copolymers employed in this study, respectively coded as a1 and a2, have similar molecular weights but different volume fractions of PS block (fPS=0.273 and 0.722). Another symmetric PS-b-PMMA diblock copolymer, coded as s, which has a PS block length similar to that of a1, was also used. For the asymmetric a1/a2 blend thin films, circular multilayered structures were formed. For the asymmetric a1/symmetric s blend thin films, inverted phases with PMMA as the dispersed domains were observed, when the weight fraction of s was less than 50%. The origins of the morphology formation in the blend thin films via solvent treatment are discussed. Combined with the theoretical prediction by Birshtein et al. (Polymer 1992, 33, 2750), we interpret the formation of these special microstructures as due to the packing frustration induced by the difference in block lengths and the preferential interactions between the solvent and PMMA block. Results obtained here suggest that diblock copolymer blend thin films treated with a selective solvent offer an alternative and attractive approach to control the self-organization of polymers.

Lattice Monte Carlo Simulations of the Gyroid Phase in Monodisperse and Bidisperse Block Copolymer Systems

Lattice Monte Carlo (MC) simulations in the NVT ensemble together with a coarse-grained model of the block copolymer chains are used to explore the phase diagram of pure and bidisperse diblock copolymer melts as a function of temperature and block volume fraction. In the pure systems, we found that the gyroid phase is stable in only a narrow region of the phase diagram. Through the analysis of the structure of the channels and nodes formed by the minority component in the gyroid phase, we found evidence of “packing frustration” of the chains inside each such node, manifested as a central low-density region. The use of chain-length bidispersity was then investigated as a way to reduce such packing frustration in the gyroid phase. We found that the longer chains in such systems tend to segregate preferentially inside the gyroid nodes. For a system with components with a 2:1 ratio of chain lengths, we observed an increased range of temperatures where the gyroid phase is stable.

Interfacial Behavior of Anionically Synthesized Amphiphilic Star Block Copolymers Based on Polybutadiene and Poly(ethylene oxide) at the Air/Water Interface

We describe the preparation and the surface properties of monolayers of a new set of (PB-b-PEO)4 amphiphilic four-arm star block copolymers. A divergent anionic polymerization method yielded copolymers with well-defined molecular weights, block volume fractions, and architecture. Initially, tri- and tetracarbanionic initiators were used to initiate the polymerization of butadiene followed by end-capping of living chains with ethylene oxide. The tri- and tetrafunctionalities of the resulting hydroxyl-terminated star polydienes were confirmed by 1H NMR spectroscopy. The four-arm star polymeric alcohols were then titrated with diphenylmethylpotassium to yield the analogous potassium alkoxides. The resulting macroinitiators were used to polymerize corona blocks of ethylene oxide. Different samples of well-defined (PB-b-PEO)4 amphiphilic star block copolymers exhibiting narrow molar mass distribution were prepared with poly(ethylene oxide) over a range of volume fractions. Isotherm experiments at the air/water...

Interplay between Smectic Ordering and Microphase Separation in a Series of Side-Group Liquid-Crystal Block Copolymers

Hierarchical ordering in a series of side-group liquid-crystal block copolymers was investigated in the bulk via differential scanning calorimetry (DSC), polarized light microscopy, small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The diblock copolymers comprise a polystyrene block and a block of poly(methyl methacrylate) bearing a chiral biphenyl ester mesogenic unit linked to the backbone by a dodecyloxy spacer. A series of copolymers with different volume fractions of mesogenic block were prepared by atom transfer radical polymerization. Ordering of mesogens into a smectic phase is characterized by a period 3.5 nm. Glass transition temperatures and the clearing temperature for each sample were determined by DSC. Additional ordering occurs due to microphase separation of the block copolymer at a length scale of 22−27 nm, as confirmed by SAXS and SANS. The order−disorder transition was found to be coincident with the smectic−isotropic transition for a sample comprising PS cylinders. A hexagonal morphology was determined for samples with both a minority and a majority liquid-crystal block. Remarkably, the morphology comprising liquid-crystal (LC) cylinders in a polystyrene matrix could be oriented by slow cooling through the clearing temperature, in the presence of a strong magnetic field. The inverse morphology of cylinders formed by the PS block in an LC matrix was not oriented in this way. This is ascribed to the nucleation of defects around the nanorods in the LC matrix. The thin film nanostructure was investigated by atomic force microscopy (AFM) and X-ray reflectivity for a sample comprising PS cylinders. AFM confirmed a hexagonal-packed cylinder morphology in thin films with coexisting parallel and perpendicular orientations of rods with respect to the substrate. The presence of Bragg peaks in specular X-ray reflectivity intensity profiles indicates a proportion of smectic layers lying parallel to the substrate, with a spacing similar to that in bulk. Our results provide a comprehensive picture of hierarchical ordering in the bulk and in thin films.

Interplay between domain microstructure and nematic order in liquid crystalline/isotropic block copolymers

The domain microstructure and the nematic LC mesophase in a series of side-chain liquid crystalline/isotropic (LC/I) diblock copolymers with systematically varied block volume fractions were studied in a broad temperature range (25–170 °C) by DSC, polarized microscopy, and wide and small angle X-ray scattering. At all temperatures the block copolymers are microphase separated. The PSLC block copolymers exhibit order at two length-scales: on one hand, a nematic LC mesophase with characteristic length-scale of 0.43 nm (intermesogen distance); on the other hand, lamellar, hexagonal or cubic domain microstructures with characteristic length-scales of 27–44 nm (lattice parameter). The LC block was either located in the matrix or confined inside the microdomains. The thermotropic behavior is characterized by the sequence g/~35 °C/n/~115 °C/i and is not affected by the domain microstructure and/or dimensions. Analysis of the lamellar dimensions showed that the LC chain is stretched. With increasing temperature, a thermal expansion of both blocks takes place followed by a retraction of the LC chain above TNI. The phase diagram is asymmetric and does not alter above TNI. No order-to-order transitions triggered by the nematic-isotropic transition are observed. It was shown that domain microstructures of low interfacial curvature (lamellar and hexagonal) are energetically favored over the geometrically expected ones of high interfacial curvature (micellar cubic) due to the presence of nematic LC mesophase in the matrix or in the microdomains. By comparison to theory a Kuhn segment length of the LC block b LC =0.86 nm was derived from the location of the lamellar/hexagonal phase boundaries.

Confinement Size Effect on Crystal Orientation Changes of Poly(ethylene oxide) Blocks in Poly(ethylene oxide)-b-polystyrene Diblock Copolymers

A series of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymers were designed and synthesized to study the change in crystal orientation of PEO blocks under different confinement sizes. The volume fraction of PEO blocks (fPEO) in these copolymers was kept almost identical (the fPEO values were between 0.45 and 0.48) but with different number-average molecular weights for the PS and PEO blocks (MnPEO and MnPS). Therefore, the phase morphology of these copolymers was a lamellar structure with different PEO and PS layer thicknesses (dPEO and dPS) detected by synchrotron small-angle X-ray scattering (SAXS) experiments. Since the melting temperature of these PEO crystals is lower than the glass transition temperature of the PS layers, the PEO block crystals could be melted and recrystallized under one-dimensional (1D) confinement at different crystallization temperatures (Tc). The PEO block crystal orientation changes were monitored using synchrotron wide-angle X-ray diffraction (WAXD) experimen...

Crystallization-Induced Microdomain Coalescence in Sphere-Forming Crystalline−Amorphous Diblock Copolymer Systems: Neat Diblock versus the Corresponding Blends

Crystallization in the microdomains of crystalline−amorphous diblock copolymers may induce domain coalescence when the crystallization temperature lies above the Tg of the amorphous matrix. In this study, the perturbation of microdomain structure driven by isothermal crystallization and postannealing in a poly(ethylene oxide)-block-poly(1,4-butadiene) (PEO-b-PB) and PEO-b-PB/PB homopolymer (h-PB) blends consisting of spherical PEO domains in the melt state is systematically investigated to reveal whether the presence of homopolymer in the amorphous matrix would hinder or facilitate the microdomain coalescence driven by the crystallization. Three samples with nearly the same volume fraction and length of PEO block are adopted for the study; the first is an asymmetric PEO-b-PB that contains no h-PB, the second is a blend containing 12 wt % of h-PB, and the third is a blend consisting of 63 wt % of h-PB. Characterization of the domain structure after isothermal crystallization and postannealing treatment by ...

Dielectric Relaxation in Liquid Crystalline/Isotropic Block Copolymers: Effect of Nanoscale Confinement on the Segmental Dynamics

We have synthesized a series of liquid crystalline/isotropic block copolymers with narrow molecular weight distribution and with well-defined chemical structure. Block volume fractions were varied ...

Microdomain Structures and Phase Transitions in Binary Blends Consisting of a Highly Asymmetric Block Copolymer and a Homopolymer

Microdomain structures and phase transitions in binary blends consisting of a highly asymmetric block copolymer and a homopolymer were investigated using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and oscillatory shear rheometry. For the study, a polystyrene-block-polyisoprene-block-polystyrene copolymer (Vector 4111) was mixed with a low molecular weight polystyrene (PS), where Vector 4111 has a 0.16 volume fraction of PS blocks. TEM images show that during heating from room temperature each of the binary blends prepared (86/14, 73/27, 62/38, and 49/51 Vector 4111/PS) exhibits “distorted microdomains” that have lost long-range order and yet have retained a distinct interface before reaching the micelle-free (or microdomain-free) homogeneous phase. SAXS measurements show that, during heating, each of the binary blends first undergoes lattice disordering transition and then demicellization transition. Following the terminologies introduced in our recent paper (Macromolecul...

Mesoscopic surface patterns formed by block copolymer micelles

The lateral organization of block copolymer micelles into monolayers is investigated using transmission electron microscopy. Depending on the relative block length and volume fraction, block copolymers organize into ordered spherical, stripe, and inverse spherical domains. Phase structure and phase kinetics can be also adjusted by loading the micelles with a functional inorganic component. The stability and orientation of these laterally segregated domains is attributed to its evolution from a micellar monolayer. The domain surfaces can be selectively functionalized either by chemical reactions or adsorption, giving access to organized polarity patterns or caralysts arrays on the length scale of some nanometers. Micellar layers adsorbed onto the first layer show reorganized surface structures, which is known for low molecular weight adsorption layers, but has not been described for colloidal structures.

Nanostructures from POSS-Grafted Block Copolymer Precursors

ABSTRACTA new method to prepare novel nanocomposites has been studied in which a polyhedral oligomeric silsesquioxane (POSS) monomer is grafted to a polyisoprene-block-polystyrene copolymer to yield an inorganic-organic hybrid copolymer (POSS-g-PI-block-PS). PS-block-PI copolymers (SI) were synthesized via anionic polymerization in tetrahydrofuran (THF) as solvent. The polyisoprene block of the SI diblock copolymers in this study featured a very high vinyl group fraction (65 mole % of 3,4- addition and 35 mole % of 1,2-addition) available to react with hydride-substituted POSS (isobutyldimethyl silane-POSS) to form a grafted block copolymer. Due to the immiscibility between POSS-grafted PI block and PS block, the grafted block copolymer developed a microdomain structure. The volume fraction of PS block in the grafted block copolymer was varied in the range of 0.45 to 0.37 in order to yield cylindrical PS microdomains in the matrix of POSS-grafted PI. After exposure to an oxygen plasma, it is expected that preferential erosion of the grafted block copolymer will occur for the organic potion (PS microdomains) thus leaving a well-defined nanoporous surface structure for nonlithographic functionalization of coated substrates.

Ultra-small-angle x-ray scattering studies on order-disorder transition in diblock copolymers

The phase behavior near the order-disorder transition (ODT) in diblock copolymers was characterized by ultra-small-angle x-ray scattering (USAXS) method for two polystyrene-block-polyisoprene (PS-b-PI) copolymers having about equal block volume fraction f=0.45 (f is the volume fraction of PS) but different molecular weights. By using USAXS method, the peak width and peak intensity were found to change by about 2 orders of magnitude across the ODT temperature for both PS-b-PI. Furthermore, in a narrow temperature region very close to the ODT, it is revealed that the coexistence of the ordered and the disordered phases occurs at thermal equilibrium for both PS-b-PI. The systematic results in the present study clearly show the thermal fluctuation effects on the phase diagram in the low molecular weight diblock copolymer systems.

Thermally induced morphological transition from lamella to gyroid in a binary blend of diblock copolymers

We report an experimental result of the thermally induced morphological transition from lamellar to gyroid phase in a binary blend of polystyrene-block-polyisoprene (SI) diblock copolymers. Two SI copolymers employed in this study have almost the same molecular weights but different volume fractions of polystyrene (PS) block (φPS=0.26 and 0.65). The blend was prepared by these two SI diblock copolymers and the overall volume fraction of PS block in the blend, φPS¯, was 0.58. Time-resolved small-angle x-ray scattering (SAXS) experiments were conducted to reveal morphological structures as a function of temperature from 120 to 205 °C during the heating with a rate of 2 °C/min. It was observed from SAXS results that the lamellar phase (L) was found below 164 °C, the gyroid phase (G) above 188 °C, and the coexistence of L and G phases between these temperatures. The gyroid phase was confirmed by transmission electron micrograph. The transition from L to G phase is different from a thermally induced transition of kinetically locked morphology formed by the vitrification of the PS matrix, because it was also found by a separate SAXS experiment for the specimen annealed for long times in which the lamellar phase was stable up to 140 °C, which is well above the glass transition temperature of the PS block. A temperature sweep of SAXS experiment with a smaller heating rate was a useful method to roughly estimate the transition temperature. Moreover, it was found from this experiment that the position of the first-order peak does not change remarkably upon the transition. Although the thermoreversibility of the transition between L and G phases has not yet been confirmed, it does not necessarily mean that the lamellar morphology is not stable at lower temperatures.