Effect of composition of added random copolymer on the phase behavior of block copolymer

Blends of styrene-butadiene diblock copolymer (S-B, 52 wt% styrene content) and styrene-butadiene random copolymer (SBR) of various styrene compositions were studied by small-angle X-ray scattering, light scattering, and transmission electron microscopy. The composition of random copolymer plays an important role in the solubilization of SBR in S-B. The order-disorder transition temperature, T_ODT, decreases linearly with the addition of SBR. T_ODT decreases as the symmetry in SBR composition increases and shows the highest value in the case of homopolymers. Asymmetric butadiene-rich SBR dissolves mostly into PB microdomain of S-B to increase lamella microdomain spacing, D, and its addition makes the overall microdomains of S and B in the mixture more asymmetrical. Symmetric SBR is localized into the interface of S-B microdomain to reduce unfavorable S-B contact at the interface. The phase diagram for S-B containing asymmetric SBR shows a succession of mixed mesophases of different morphologies from lamellae and cylinder to disordered liquid phases, whereas the phase diagram containing symmetric SBR shows two homogeneous phases and one region of two-phase coexistence, where macroscopically separated phases coexist together.     

Block copolymer nanoshells

With the help of cell dynamics simulation we investigate morphology of thin block copolymer film around a nanoparticle. The obtained structures include: parallel, perpendicular, mixed and perforated lamellae, parallel and perpendicular cylinders and spheres. Analogy and difference with planar films are discussed. Our simulation suggests that novel porous nanocontainers can be formed by the coating of a sacrificial nano-bead by a block copolymer layer with a well controlled nanostructure.     

Crystallization and coalescence of block copolymer micelles in semicrystalline block copolymer/amorphous homopolymer blends

Crystallization of two oxyethylene/oxybutylene block copolymers (E₇₆B₃₈ and E₁₅₅B₇₆) from micelles in block copolymer/amorphous homopolymer blends was studied by differential scanning calorimetry (DSC) and time-resolved small angle X-ray scattering (SAXS). Unlike the simultaneous crystallization and formation of superstructure in crystallization from an ordered structure, crystallization of block copolymer from micelles can be divided into two steps. The core of the micelles firstly crystallizes individually, with first-order crystallization kinetics and homogeneous nucleation mechanism. The SAXS revealed that crystallization-induced deformation occurs for the micelles, which strongly depends on microstructure of the block copolymers. For the shorter block copolymer E₇₆B₃₈, larger deformation induced by crystallization was observed, leading to coalescence of the micelles after crystallization, while for the longer block copolymer E₁₅₅B₇₆ the micelles show little deformation and the morphology of micelle is retained after crystallization.     

Metallo-supramolecular complexes mediated thermoplastic elastomeric block copolymer

Orthogonal multi-phase strategy has been applied to generate a novel type of thermoplastic elastomeric block copolymer which was obtained via anionic polymerization. Functional terpyridine groups which can potentially form metallo-supramolecular complexes with a variety of metal ions has been introduced to the polystyrene-b-polyisoprene (PS-PI) living chain ends through the termination reaction in an optimized condition. Iron(II), cobalt(II) and zinc(II) metal ions have been used to form metal-ligand complexes with the terpyridine end groups, which can phase separate from the polymer matrix to form hybrid clusters. The formation of metallo-supramolecular hybrid clusters have dramatic effects on the micro-phase separated structures of the PS-PI diblock copolymer, which have been characterized by transmission electron microscopy, small-angle X-ray scattering and atomic force microscopy. This type of hybrid material containing hard PS nanophase and metal-ligand clusters exhibit distinct mechanic properties such as increased modulus, higher yield strength and improved toughness, which is further discussed in light of nature of the metal-ligand bonds and the liability of the clusters. The utilization of metallo-supramolecular complex and its high tunability is potential to fabricate new types of supramolecular nanocomposite materials.     

Two-dimensional block copolymer photonic crystals

A high molecular weight polystyrene-block-polyisoprene copolymer was synthesized by anionic polymerization of styrene and isoprene monomers. Polystyrene cylindrical domains in a hexagonal lattice with relatively large periodicity and controlled orientation have been produced through roll casting of the polystyrene-block-polyisoprene copolymer with a total molecular weight of 1.0×10⁶ g/mol. The large periodicity and effective processing lead to reflectivity of about 70% in the visible regime, and generate a two-dimensional photonic crystal with a partial band gap.     

Characterization of a sulfonated pentablock copolymer for desalination applications

Water and salt transport properties were determined in a family of sulfonated pentablock copolymers to characterize their potential as chlorine-tolerant desalination membrane materials. The degree of sulfonation, block molecular weight, and casting conditions can be independently varied to tune the transport properties of these materials. Data for water uptake, water permeability, salt permeability, and apparent surface charge are presented. Apparent diffusion coefficients of water in these materials were calculated using the solution-diffusion theory. Generally speaking, water sorption, water diffusivity, water permeability, and salt permeability increase with increasing degree of sulfonation. As ion exchange capacity increases from 0.4 to 2.0 meq/g (dry polymer), water uptake values vary between 21% and 118%, and water permeability values, in units of cm² s⁻¹, vary over 4 orders of magnitude. Salt permeability depends on both the upstream sodium chloride concentration, between 0.01 and 1.0 mol L⁻¹, and the degree of sulfonation. Both water permeability and salt permeability are sensitive to the conditions used when casting the polymer films. Apparent surface charge, as characterized by zeta potential, has been shown to be related to the fouling tendency of several membrane materials. In these materials, zeta potential is most negative in samples with low levels of sulfonation and is near neutral in samples with the highest level of sulfonation.     

Theoretical study on morphology of ABCD 4-miktoarm star block copolymer

A real-space implementation of the self-consistent field theory (SCFT) has been used to study the morphologies of ABCD 4-miktoarm star block copolymers. For the sake of numerical tractability, the morphologies and the phase diagrams of ABCD 4-miktoarm star block copolymers are investigated in two dimensions (2D) by varying the volume fractions of the blocks and the interaction parameters. Many interesting and complex morphologies occur and compared with ABCD linear block copolymers; ABCD 4-miktoarm star block copolymers have more regular disciplines. We found that systems with similar components have similar morphologies and at the weaker segregation, the minority components always cannot separate from the other blocks and they easily dissolve to form one phase with other block(s), but with the increase of the segregation degree (large ), the ordered phases can be well separated. With the help of our computational prediction, experimental researchers can work more purposefully and efficiently.     

Self-assembly of an A-B diblock copolymer blended with a C homopolymer and a C-D diblock copolymer through hydrogen bonding interaction

In this study, we investigated the miscibility, phase behavior, and self-assembled nanostructures formed from the immiscible crystalline-amorphous diblock copolymer poly(?-caprolactone-b-4-vinyl pyridine) (PCL-b-P4VP, A-B) when blended with the homopolymer poly(vinyl phenol) (PVPh, C) and the diblock copolymer poly(vinyl phenol-b-styrene) (PVPh-b-PS, C-D). Long-range-ordered microphase separation was difficult to achieve in the PCL-b-P4VP/PVPh (A-B/C) blend system because PVPh interacted with both the P4VP and PCL blocks simultaneously through hydrogen bonding interactions. In contrast, we observed sharp, multiple orders of diffraction in the SAXS profiles of the PCL-b-P4VP/PVPh-b-PS (A-B/C-D) blend system, indicating that perfect microphase separation occurred because the incorporation of the PS block induced the PVPh block to hydrogen bond preferentially with the P4VP block. This simple A-B/C-D (PCL-b-P4VP/PVPh-b-PS) diblock copolymer mixture exhibited self-assembly behavior (a three-lamella phase) similar to that of a corresponding ABC triblock copolymer.     

Silica reinforced triblock copolymer gels

The effect of silica and polymer coated silica particles as reinforcing agents on the structural and mechanical properties of polystyrene-poly(ethylene/butylene)-polystyrene (PS-PEB-PS) triblock gel has been investigated. Different types of chemically modified silica have been compared in order to evaluate the influence of the compatibility between gel and filler.Time-resolved SANS and small-angle X-ray scattering (SAXS) shows that the presence of silica particles affects the ordering of the polystyrene domains during gelsetting. The scattering pattern of silica-reinforced gels reveals strong scattering at very low q, but no structure and formfactor information. However, on heating above the viscoelastic to plastic transition, the ‘typical’ scattering pattern of the copolymer gel builds-up. All reinforced gels are strengthened by the addition of the reinforcing agent. The transitions from a viscoelastic rubber to a plastic fluid and from a plastic fluid to a viscoelastic liquid are shifted to more elevated temperatures when silica is added to the triblock copolymer gel.     

Asymmetric morphology from an organic/organometallic block copolymer

Block copolymer self-assembly is a burgeoning subject in polymer and materials science driven by both fundamental and applied inspirations. Whereas the vast majority of block copolymer studies have focused on highly symmetric morphologies, here we report the first observation of an unusual asymmetric cylindrical phase in thick films of an organic/organometallic block copolymer, poly(styrene-block-ferrocenyldimethylsilane) (PS-b-PFS). Microscopy and X-ray scattering data establish the lack of symmetry in this structure and reveal an unusual 3-D network organization. Following selective removal of the PS matrix, the remaining nanoporous film has characteristics of potential value in separation applications such as substantial interconnection (mechanical strength), uniform pore size, and chemical and physical stability.     

Dynamic formation of SEBS copolymer submicrometric structures

Highly ordered A-B-A block copolymer arrangements in the submicrometric scale, resulting from dewetting and solvent evaporation of thin films, have inspired a variety of new applications in the nanometric world. Despite the progress observed in the control of such structures, the intricate scientific phenomena related to regular patterns formation are still not completely elucidated. SEBS is a standard example of a triblock copolymer that forms spontaneously impressive pattern arrangements. From macroscopic thin liquid films of SEBS solution, several physical effects and phenomena act synergistically to achieve well-arranged patterns of stripes and/or droplets. That is, concomitant with dewetting, solvent evaporation, and Marangoni effect, Rayleigh instability and phase separation also play important role in the pattern formation. These two last effects are difficult to be followed experimentally in the nanoscale, which render difficulties to the comprehension of the whole phenomenon. In this paper, we use computational methods for image analysis, which provide quantitative morphometric data of the patterns, specifically comprising stripes fragmentation into droplets. With the help of these computational techniques, we developed an explanation for the final part of the pattern formation, i.e. structural dynamics related to the stripes fragmentation.     

Design and stabilization of block copolymer micelles via phenol-pyridine hydrogen-bonding interactions

A new approach for the preparation of block copolymer micelles in non-selective solvent is introduced. Phenol-pyridine hydrogen-bonding interactions are used for the first time to prepare core-shell micelles in non-selective solvents using block copolymers and bifunctional low-molecular-weight hydrogen-bonding crosslinkers. Poly(styrene-b-4-vinylphenol)/Bis-pyridyl ethane and poly(styrene-b-4-vinylpyridine)/Bisphenol A were investigated as micelle formation due to phenol-pyridine hydrogen bond crosslinking. The influence of several factors such as temperature, concentration, solvent and pH in micellization-demicellization process was analyzed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), dynamic light scattering (DLS) and atomic force microscopy (AFM). This method opens new possibilities to the generation of block copolymer micelles in non-selective solvents.     

Visualization of block copolymer distribution on a sheared drop

We have visualized a fluorescently-labeled poly(styrene-b-methylmethacrylate) (NBD-PS-b-PMMA) block copolymer on the surface of a polymethylmethacrylate (PMMA) drop in a polystyrene (PS) matrix. Confocal microscopy revealed that the block copolymer distributed uniformly on the drop surface before deformation. However, in shear flow the copolymer concentration was higher at the tips and edges of the drop. Visualization of drop deformation using a counter-rotating apparatus showed enhanced drop deformation for a drop with block copolymer resulting in larger area generation. Drops with block copolymer showed widening even for shear strains exceeding 10, in contrast to bare drops, which first widened and then shrank. These results agree qualitatively with the observed distribution of fluorescent block copolymer. Copolymer concentration is highest in the regions of high curvature, where lowering interfacial tension should be most effective in retarding drop retraction. Block copolymer on these highly curved surfaces is found to be very effective since the exact theory for zero interfacial tension by Cristini fits our drop widening results well.     

Influence of block lengths and symmetries of block copolymers on phase behavior of polymer A/polymer B/block copolymer ternary blends

Monte Carlo simulations were used to investigate the compatibilizing effects of diblock copolymers in A/B/A-B diblock copolymer ternary blends and triblock copolymers in A/B/triblock copolymer ternary blends, respectively. The volume fraction of homopolymer A was 19% and was the dispersed phase. The simulation results show that diblock copolymers with longer A-blocks are more efficient as compatibilizers, and symmetric triblock copolymers with a shorter middle block length are easily able to bridge each other through the association of the end blocks. This kind of triblock copolymers have relatively high ability to retard phase separation as compatibilizers.     

Nano-phase structures and mechanical properties of epoxy/acryl triblock copolymer alloys

Phase structures and mechanical properties of epoxy/acryl triblock copolymer alloys using several curing agents were studied. PMMA-b-PnBA-b-PMMA triblock copolymers synthesized by living anionic polymerization were applied as the toughening modifiers for the epoxy resins. An aromatic amine, an acid anhydride and an anionic polymerization catalyst as curing agents resulted in macro-phase separation in the epoxy/triblock copolymer blends during the cure process. However, a phenol novolac as the curing agent created nano-phase structures in the epoxy blends. The size of the spherical phases or cylindrical phases was about 40 nm in diameter, and the main component in the nano-phases was the PnBA of the triblock copolymer. The fracture toughness of the epoxy/triblock copolymer alloys with the nano-cylindrical phases reached 2530 J/m². The fracture toughness was more than twenty fold relative to the unmodified epoxy resin, and was equivalent to the toughness of polycarbonates.     

Synthesis and characterization of perfluorocyclobutyl aryl ether-based amphiphilic diblock copolymer

Perfluorocyclobutyl aryl ether-based amphiphilic diblock copolymer containing hydrophilic poly(ethylene glycol) segment was synthesized by atom transfer radical polymerization (ATRP). Perfluorocyclobutyl-containing methacrylate-based monomer, 4-(4′-p-tolyloxyperfluorocyclobutoxy)benzyl methacrylate, was prepared firstly, which can be polymerized by ATRP in a controlled way to obtain well-defined homopolymers with narrow molecular weight distributions (M_w/M_n ≤ 1.30). The molecular weights increased linearly with the conversions of monomer and the apparent polymerization rate exhibited first-order relation with respect to the concentration of monomer. ATRP of 4-(4′-p-tolyloxyperfluorocyclobutoxy)benzyl methacrylate was initiated by PEG-based macroinitiators with different molecular weights to obtain amphiphilic diblock copolymers with narrow molecular weight distributions (M_w/M_n < 1.35) and the number of perfluorocyclobutyl linkage can be tuned by the feed ratio and the conversion of the fluorine-containing methacrylate monomer. The critical micelle concentrations of these amphiphilic diblock copolymers in water and brine were determined by fluorescence probe technique. The morphologies of the micelles were found to be spheres by TEM.     

Tailoring the morphology of self-assembled block copolymer hollow fiber membranes

Isoporous asymmetric polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) hollow fiber membranes were successfully made by a dry-jet wet spinning process. Well-defined nanometer-scale pores around 20–40 nm in diameter were tailored on the top surface of the fiber above a non-ordered macroporous layer by combining block copolymer self-assembly and non-solvent induced phase separation (SNIPS). Uniformity of the surface-assembled pores and fiber cross-section morphology was improved by adjusting the solution concentration, solvent composition as well as some important spinning parameters such as bore fluid flow rate, polymer solution flow rate and air gap distance between the spinneret and the precipitation bath. The formation of the well-organized self-assembled pores is a result of the interplay of fast relaxation of the shear-induced oriented block copolymer chains, the rapid evaporation of the solvent mixture on the outer surface and solvent extraction into the bore liquid on the lumen side, and gravity force during spinning. Structural features of the block copolymer solutions were investigated by small-angle X-ray scattering (SAXS) and rheological properties of the solutions were examined as well. The scattering patterns of the optimal solutions for membrane formation indicate a disordered phase which is very close to the disorder-order transition. The nanostructured surface and cross-section morphology of the membranes were characterized by scanning electron microscopy (SEM). The water flux of the membranes was measured and gas permeation was examined to test the pressure stability of the hollow fibers.     

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter E_mB_n block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E₂₂₄B₁₁₄ exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.     

Mechanisms of ordering in block copolymer sub-monolayer films upon selective solvent annealing

The solvent annealing induced two-dimensional ordering in poly(styrene–ethylene/butylenes–styrene) (SEBS) triblock copolymer sub-monolayer films with a thickness of 16 nm equaling half of the bulk domain thickness l₀ was investigated by time-resolved ex-situ atomic force microscopy (AFM). Cyclohexane, a selective solvent for majority poly(ethylene/butylenes) block was used. The detailed pathway information on the ordering of highly regular hexagonal spheres was obtained by repeatedly taking images of the same marked area on the sample surface after ex-situ annealing treatments. Two different ordering mechanisms were observed under two different solvent annealing conditions: 1) under a well-sealed environment with a slow cyclohexane evaporation rate, poorly-ordered short cylinders first break into spheres, then the evolution of spherical phase takes place to gradually improve the orderliness of spheres, and finally well-ordered hexagonal spheres are formed; 2) under a poorly-sealed environment with a fast cyclohexane evaporation rate, “holes” and “islands” are instantaneously formed on the sample surface first, and then the surface gradually get even annihilating hole and island areas. Within the hole areas, the transition from short cylinders to hexagonal spheres takes place via a not well-defined microphase separation structure, while in the island areas, the transitions between cylinders and spheres take place to improve the orderliness of cylinders, and finally the enough-ordered cylinders transform into hex-spheres.     

Time-resolved GISAXS and cryo-microscopy characterization of block copolymer membrane formation

Time-resolved grazing-incidence small-angle X-ray scattering (GISAXS) and cryo-microscopy were used for the first time to understand the pore evolution by copolymer assembly, leading to the formation of isoporous membranes with exceptional porosity and regularity. The formation of copolymer micelle strings in solution (in DMF/DOX/THF and DMF/DOX) was confirmed by cryo field emission scanning electron microscopy (cryo-FESEM) with a distance of 72 nm between centers of micelles placed in different strings. SAXS measurement of block copolymer solutions in DMF/DOX indicated hexagonal assembly with micelle-to-micelle distance of 84–87 nm for 14–20 wt% copolymer solutions. GISAXS in-plane peaks were detected, revealing order close to hexagonal. The d-spacing corresponding to the first peak in this case was 100–130 nm (lattice constant 115–150 nm) for 17 wt% copolymer solutions evaporating up to 100 s. Time-resolved cryo-FESEM showed the formation of incipient pores on the film surface after 4 s copolymer solution casting with distances between void centers of 125 nm.