Statistical thermodynamics of ABA block copolymers. II

A microstructural model for the phase-separated states of ABA triblock copolymers is proposed. It postulates the simultaneous existence of three phases: pure A, pure B, and a mixed region. Incorporation of the mixed region distinguishes this treatment from all other theories and is responsible for considerable flexibility in the model without increased numbers of parameters. Calculations of free energy changes required to establish specific microstructures permit the prediction of a favored one from among five possibilities: discrete spheres of A (or B), discrete cylinders of A (or B), and lamellae.     

Synthesis and properties of ABA propylene-ethylene block copolymers

Experimental data, which includes catalyst lifetimes, thermal analyses, fractionation by urea complexation, x-ray diffraction, and ¹³C-NMR spectroscopy, are presented to confirm the successful synthesis of ABA propylene-ethylene block copolymers. A dry catalyst system of DEAC-TiCl₃(AA) and a gas-phase polymerization technique was used to prepare the copolymers. PRP-and ERE-type copolymers (P-isotactic polypropylene, E-polyethylene, and R-random propylene-ethylene copolymer block) were prepared. Some preliminary physical property data are given which indicate that PRP-type copolymers can behave as elastomeric fibers. The stress-strain behavior also indicates block copolymer formation.     

Hybrid thermotropic liquid-crystalline block copolymers

A new approach to the preparation of two classes of block copolymers containing liquid-crystalline segments is reported. Copolymers 1 and 2 are constituted by polytetrahydrofuran and side-chain liquid-crystalline polymethacrylate blocks, whereas block copolymers 3 consist of polystyrene and main-chain liquid-crystalline polyester blocks. The synthetic procedures leading to copolymers 1-3 are extremely versatile and can be used to prepare a great variety of block copolymer architectures. In both copolymer classes the chemically different blocks are strongly segregated in the solid and melt phases and undergo individual phase transitions. The mesophase transition temperatures of the liquid-crystalline blocks are very similar to those of the corresponding homopolymers, and their enthalpies are directly proportional to the content of the liquid-crystalline block.     

Phase behavior in thin films of cylinder-forming ABA block copolymers: experiments

We experimentally establish a phase diagram of thin films of concentrated solutions of a cylinder forming polystyrene-block-polybutadiene-block-polystyrene triblock copolymer in chloroform. During annealing the film forms islands and holes with energetically favored values of film thickness. The thin film structure depends on the local thickness of the film and the polymer concentration. Typically, at a thickness close to a favored film thickness parallel orientation of cylinders is observed, while perpendicular orientation is formed at an intermediate film thickness. At high polymer concentration the cylindrical microdomains reconstruct to a perforated lamella structure. Deviations from the bulk structure, such as the perforated lamella and a wetting layer are stabilized in films thinner than approximately 1.5 domain spacings.     

Phase behavior in thin films of cylinder-forming ABA block copolymers: mesoscale modeling

The phase behavior of cylinder-forming ABA block copolymers in thin films is modeled in detail using dynamic density functional theory and compared with recent experiments on polystyrene-block-polybutadiene-block-polystyrene triblock copolymers. Deviations from the bulk structure, such as wetting layer, perforated lamella, and lamella, are identified as surface reconstructions. Their stability regions are determined by an interplay between surface fields and confinement effects. Our results give evidence for a general mechanism governing the phase behavior in thin films of modulated phases.     

Synthesis and properties of high strength high modulus ABA block copolymers

A series of novel ABA block copolymers were synthesized containing a rigid-rod (B) block for reinforcement and a flexible coil (A) block as the matrix. Poly[(benzo[1, 2d: 4, 5d′]bisthiazole-2,6-diyl)-1,4-phenylene] (PBT) was the rigid-rod (B) block utilized in this study and was polymerized in such a way as to provide carboxylic acid end-groups. The carboxy-terminated PBT was copolymerized with the AB monomers, 3,4-diaminobenzoic acid and 4-amino-3-mercaptobenzoic acid, which generates a benzimidazole or benzthiazole (A) block, as well as grafts the blocks together. Composition of the blocks could be varied by the weight of AB monomer used in the copolymerizations. Solution behavior of the copolymers in methanesulfonic acid was determined, and fibers were obtained by wet spinning techniques. The block copolymers exhibited typical tenacities of 200 ksi, 16 Msi modulus, and an elongation to break of 1.4%. Critical concentration values for fabrication increased approximately 3% over mechanical mixtures of the same heterocyclic components.     

Unexpected consequences of block polydispersity on the self-assembly of ABA triblock copolymers

Controlled/"living" polymerizations and tandem polymerization methodologies offer enticing opportunities to enchain a wide variety of monomers into new, functional block copolymer materials with unusual physical properties. However, the use of these synthetic methods often introduces nontrivial molecular weight polydispersities, a type of chain length heterogeneity, into one or more of the copolymer blocks. While the self-assembly behavior of monodisperse AB diblock and ABA triblock copolymers is both experimentally and theoretically well understood, the effects of broadening the copolymer molecular weight distribution on block copolymer phase behavior are less well-explored. We report the melt-phase self-assembly behavior of SBS triblock copolymers (S = poly(styrene) and B = poly(1,4-butadiene)) comprised of a broad polydispersity B block (M(w)/M(n) = 1.73-2.00) flanked by relatively narrow dispersity S blocks (M(w)/M(n) = 1.09-1.36), in order to identify the effects of chain length heterogeneity on block copolymer self-assembly. Based on synchrotron small-angle X-ray scattering and transmission electron microscopy analyses of seventeen SBS triblock copolymers with poly(1,4-butadiene) volume fractions 0.27 ≤ f(B) ≤ 0.82, we demonstrate that polydisperse SBS triblock copolymers self-assemble into periodic structures with unexpectedly enhanced stabilities that greatly exceed those of equivalent monodisperse copolymers. The unprecedented stabilities of these polydisperse microphase separated melts are discussed in the context of a complete morphology diagram for this system, which demonstrates that narrow dispersity copolymers are not required for periodic nanoscale assembly.     

Tandem interactions in the self-assembly of ionic block copolymers

The aim of the present review is to show how the phenomena of block copolymer self-assembly and interactions of ionic (or ionizable) groups in polymer systems can be combined to produce materials with versatile and unique behavior. In our discussion, we consider two classes of tandem interactions. First, block copolymers containing short ionic blocks and long nonionic blocks are investigated in organic media. In systems of this type, block copolymer self-assembly and short-range electrostatic interactions act in tandem, forming regular and highly-stable spherical structures. Next, we consider ionic (or ionizable) block copolymers dissolved in aqueous media. In this case, block copolymer self-assembly acts in tandem with the hydrophilic nature of the soluble blocks, resulting in a wide range of unique morphologies.     

Controlling interfacial interactions for directed self assembly of block copolymers

Over the past 15 years, block copolymer lithography has emerged as its own research field within the broader block copolymer and polymer thin film communities. This distinction is associated with the unique requirements set by the semiconductor device industry, such as low-defect densities, precise feature registration, and complex pattern layouts. To achieve perfection in block copolymer lithography, the surface and substrate interactions must be carefully tuned to control domain ordering in three dimensions. This perspective discusses recent modeling efforts that underline the challenges of predicting interfacial interactions and the resulting block copolymer structures. We emphasize studies that facilitate the design and interpretation of experiments, including materials selection, guiding pattern geometry, and selecting tools for three-dimensional metrology. Finally, we propose that translation of block copolymer lithography to semiconductor manufacturing will require integrated experimental and modeling efforts to interrogate the vast parameter space that controls both lateral and out-of-plane ordering. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 96–102     

Statistical thermodynamics of ABA block copolymers. III. Microstructural transitions and model verification

The predictions of a model presented previously are tested with data on five styrenebutadiene–styrene copolymers. Differential scanning calorimetry and laser light transmission are the primary tools, spanning 180–630°K, with some evidence supplied also by electron microscopy and mechanical properties. The existence of a first-order phase transition, characterized by a separation temperature T_s, is verified and found to be predictable by the model. Details of morphology, including transitions in microstructure in certain temperature ranges, are also reported and shown to be consistent with the theory.     

Hybrid metal–polymer composites from functional block copolymers

The combination of metals and polymers in hybrid materials is a research area of great current interest. A number of methods for controlling the positioning of metallic species within polymer matrices on the nanometer scale have been developed. This highlight focuses on the use of functional block copolymers for the localization of metal species, especially nanoparticles, on the nanometer scale through block copolymer phase segregation. Research from the author's group on the use of alkyne‐functional block copolymers for the preparation of cobalt‐containing materials is discussed in this context. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4323–4336, 2005     

Interaction of liposomes with silica nanocapsules: from lipid bilayer coating to multi-liposomal composites

A new delivery system has been developed that combines liposomes and silica nanocapsules. It was found that in the presence of cationic unilamellar liposomes, silica nanocapsules are instantly coated with a lipid bilayer. In contrast, cationic stealth liposomes modified with polyethylene glycol can adsorb on the surface of silica to form multi-liposomal composites in which the liposomes remain intact.     

Glass transition temperatures of ABA poly(styrene-b-isoprene) block copolymers by differential scanning calorimetry

The glass transition behavior of two sets of ABA poly(styrene-b-isoprene) block copolymers was examined by differential scanning calorimetry. In one series, the triblock copolymers had different total molecular weights and the same (30 wt %) polyisoprene content, in the other, the molecular weight was constant (30,000 g/mol) and the elastomer content was the variable. For all triblock copolymers studied, the data show an inward shift for the glass transition temperatures T_g of the corresponding homopolymers. This shift increases for the rigid-phase T_g as the polystyrene block length decreases. Depending on their molecular characteristics, two, three, or only one T_g were found. The third T_g was interpreted in terms of the existence of an interphase. Some of these conclusions could be confirmed by transmission electron microscopy.     

Synthesis of AB and ABA block copolymers as compatibilizers in nylon 6/polycarbonate blends

Nylon 6 (Ny6) and Bisphenol A polycarbonate (PC) are immiscible and form biphasic blends. To improve the compatibility of Ny6 and PC several ABA and AB Ny6/PC block copolymers were synthesized, and their compatibilizing behavior on the blends were tested. Block copolymers were prepared by reacting monoamino- or diamino-terminated Ny6 homopolymers with high molecular weight PC at 130°C in anhydrous DMSO. The reaction of diamino- and monoamino-terminated Ny6 with polycarbonate produces block copolymers of the type PC-Ny6-PC (ABA) and PC-Ny6 (AB), respectively, plus a certain amount of unconverted PC degradated to lower molecular weights. To separate the block copolymer from the unconverted PC, a selective fractionation with tetrahydrofuran (THF) and trifluoroethanol (TFE) was carried out. Three different fractions were obtained: THF-soluble fraction, TFE-soluble fraction, and the TFE-insoluble fraction. The scanning electron microscopy (SEM) analysis of a 75/25 (wt/wt) Ny6/PC blend added with 2% of ABA or AB block copolymers, showed the presence of smaller PC particles more adherent to the polyamide matrix, with respect to the same blend nonadded, which is clearly biphasic. The size of the PC particles decreases from ABA to AB compatibilized blends and the adhesion with the matrix is increases in the same way. © 1996 John Wiley & Sons, Inc.     

Charge transfer interactions between conjugated block copolymers and reduced graphene oxides

The charge transfer interactions between reduced graphene oxides and conjugated block copolymers were confirmed by various spectroscopic methods, giving rise to manipulation of the electrical properties of the former.     

Nanocrystal self-assembly with rod-rod block copolymers

Two distinct morphologies of hexylselenophene-hexylthiophene rod-rod block copolymer films can be prepared depending on the molecular weight of the sample (see picture: left M(n) =12.9, right M(n) =3.9?kg?mol(-1) ). These polymers can be used to organize spherical CdSe nanocrystals (yellow) into either dispersed or aligned hierarchical structures. Scale bars: 200?nm.     

Model block copolymers with complex architecture

The synthesis of non linear block copolymers of the type (BA)₂B (3-miktoarm star copolymer), (BA)₃B (4-miktoarm star copolymer), (BA)₃B(AB)₃ (super H-shaped), B₂AB₂ (H-shaped) and (B,A)A(B,A) (π-shaped), where A is polyisoprene 1,4 and B is polystyrene was performed using anionic polymerization techniques and suitable chlorosilane chemistry. Characterization data showed that the samples are molecularly and compositionally homogeneous. TEM, SAXS and SANS were used to study the microphase behavior of the copolymers. For all samples, the results were analyzed in the frame of the theoretical predictions given by Milner and taking into account the results from previous studies on the A₂B and A₃B miktoarm star copolymers.     

Functional block copolymers: nanostructured materials with emerging applications

Recent advances in polymer synthesis have significantly enhanced the ability to rationally design block copolymers with tailored functionality. The self-assembly of these macromolecules in the solid state or in solution allows the formation of nanostructured materials with a variety of properties and potential functions. This Review illustrates recent progress in the field of block copolymer materials by highlighting selected emerging applications.