The influence of confinement and curvature on the morphology of block copolymers

In the bulk, at equilibrium, diblock copolymers microphase separated into nanoscopic morphologies ranging from body-centered cubic arrays of spheres to hexagonally packed cylinders to alternating lamellae, depending on the volume fraction of the components. However, when the block copolymers are forced into cylindrical pores, where the diameter of the pores are only several repeat periods of the copolymer morphology or less, then commensurability of the copolymer period and the pore diameter can impose a frustration on the microdomain morphology. In addition, due to the small pore diameter, a curvature is forced on the microdomain morphology. In combination with interfacial interactions between the blocks of the copolymer and the pore walls, the preferential segregation of one component to the walls, spatial confinement and forced curvature are shown to induce transitions in the fundamental morphology of the copolymers seen in the bulk. Lamellar morphologies transformed into torus-type morphologies, cylinders are forced into helices, and body-centered cubic arrays of spheres are force into helical arrays of spheres due to these restraints. The novel morphologies, not accesssible in the bulk, open a large array of nanoscopic structures that can be used as templates and scaffolds for the fabrication of inorganic nanostructured materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3377–3383, 2005     

Amphiphilic liquid‐crystalline 4‐miktoarm star copolymers with a siloxane junction leading to cylindrically nanostructured templates for a siloxane‐based nanodot array

Well‐defined A₃B‐, A₂B₂‐, and AB₃‐type 4‐miktoarm star copolymers (M_n = 10,500–16,200, M_w/M_n = 1.16–1.18) consisting of poly(ethylene oxide) (PEO) and polymethacrylate bearing an azobenzene mesogen (PMA(Az)) as the arms and cyclotetrasiloxane as the core unit were synthesized using a combined route composed of a thiol‐ene click reaction and atom transfer radical polymerization. Microphase‐separated structures of the star copolymers in thin films with a thickness of approximately 100 nm were investigated by GISAXS and TEM. The A₃B‐type star‐(PEO)₃[PMA(Az)]₁ copolymer formed a more highly ordered PEO cylinder array with perpendicular alignment in the PMA(Az) matrix than that of the corresponding linear‐type block copolymer. The center‐to‐center distance of the PEO cylinders and the cylinder diameter were 13 and 4 nm, respectively. The highly ordered star‐(PEO)₃[PMA(Az)]₁ thin film was directly transferred to a siloxane‐based nanodot array by oxygen reactive ion etching. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1175–1188     

Microphase Separation of Block Copolymer Thin Films

Today, high‐ordered micro‐ and nano‐patterned surfaces are widely used in many areas, such as in the preparation of super‐thin dielectric films, photonic crystals, antireflective films, super‐non‐wetting surfaces, bio‐compatible surfaces and microelectric devices. Considering the critical fabrication conditions and the irreducible high cost of the photolithography technique in patterning nano‐scale structures (<100 nm), the development of other micro‐ and nano‐patterning techniques that can be used to fabricate long‐range ordered features – especially nanoscale arrays – is a promising subject in surface science. In contrast to the traditional photolithography patterning technique, block copolymers can spontaneously phase separate into arrays of periodic patterns with length‐scales of 10–50 nm, which provides an efficient pathway to pattern nanoscale features. Today, preparing long‐range ordered arrays by block copolymer microphase separation is one of the most promising techniques for the fabrication of nanoscale arrays, not only being a simple process but also having a lower cost than traditional methods. In this feature article, we first summarize the many techniques developed to induce ordering in the microphase separation of the block copolymer thin films. Then, evolution, order–order transitions and reversible switching microdomains are considered, since they are very important in the ordered engineering of microphase separation of the block copolymer thin films. Finally, the outlook of this research area will be given.

     

Phase separation in thin films of end‐grafted block copolymers

An atomic force microscopy investigation was carried out on various thick (30–120 nm) polymethyl methacrylate‐b‐polystyrene and poly(2‐(dimethyl amino)ethyl methacrylate)‐b‐polystyrene films prepared via a grafting‐from method. The structure of the films was examined with both topographic and phase imaging. Several different morphologies were observed including a perforated lamellar phase with irregular perforations. In addition, complementary small‐angle X‐ray scattering and reflectometry results measurements on a non‐grafted polymer are presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Microindentation Hardness of Nanostructured Thermoplastic Materials

Summary: The micromechanical behaviour of various thermoplastics based nanocomposites was investigated with the aid of the microindentation technique. The materials studied were microphase separated styrene-butadiene block copolymer systems and several thermoplastics reinforced with nano-sized fillers having variable dimensionality. It was found that the microhardness behaviour of the nanocomposites studied was strongly influenced by the dimensionalities of the filler. Due to large surface to volume-ratio one- and two-dimensional fillers exhibit a far better reinforcing behaviour than the three dimensional ones. In case of nanostructured block copolymers, the microhardness is not determined by the total polystyrene (PS)/polybutadiene (PB) composition alone but diminishes gradually in presence of freely standing dangling polybutadiene chains even if the morphology of the systems remains unaltered.     

Crystallization‐dominated and microphase separation/crystallization‐coexisted structure of all‐conjugated phenylene–thiophene diblock copolymers

The crystallization‐dominated and microphase separation/crystallization‐coexisted structure of the all‐conjugated diblock copolymers poly(2,5‐dihexyloxy‐p‐phenylene)‐block‐(3‐hexylthiophene) (PPP‐b‐P3HT, denoted as BmTn) with different block compositions was affected by the aggregation state of the diblock copolymers in solvents with different solubilities. For B34T66, B62T38, and B75T25, the coexistence of microphase separation and crystallization was obtained in good solvent with few crystalline aggregates. For B34T66 with a longer P3HT block, densely stacked fiber crystal structures in thin films were found by using marginal solvents with crystalline aggregations in solutions. As for B62T38 and B75T25 with shorter P3HT block and longer PPP block, crystal structures were obtained by the use of solvents with a much larger solubility difference of the two blocks. Thus, microphase‐separated structures are prone to form from solutions with coil conformation and fiber crystals from solutions with larger aggregates, which resulted in the increased crystallinity. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1718–1726     

Polymerization-induced phase separation in gradient copolymers

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Weak segregation in molten statistical copolymers

We present a qualitative argument suggesting that a statistical (AB) copolymer should display a certain form of weak segregation, with A rich and B rich regions of size comparable to the coil radius. This should occur if the Flory parameter χ is larger than unity (much larger than for the phase separation of A and B of homopolymers) in agreement with detailed calculations by Frederikson, Milner and Leibler. We achieve a certain qualitative insight for the resulting microphase.     

Microphase separation of block copolymer solutions in a flow field

Films of polystyrene–polybutadiene–polystyrene (PS/PB/PS) triblock copolymer and polystyrene-poly(ethylene/propylene) (PS/PEP) diblock copolymer were cast from toluene solutions subjected to hydrodynamic flow at room temperature using a device based on a novel casting method we term ‘roll-casting.’ Polymer solutions were rolled between two corotating eccentric cylinders while at the same time the solvent was removed at a controlled rate. As the solvent evaporated, the block copolymers microphase separated into globally oriented structures. A discussion of the flow field that develops during roll-casting is presented and specific attention is given to the importance of the shear and elongation rates present. For the triblock and diblock, respectively, the processed structures consisted of polystyrene cylinders assembled on a hexagonal lattice in a polybutadiene matrix, and unidirectional lamellae of alternating polystyrene and polyethylene/propylene. Small-angle x-ray scattering (SAXS) and transmission electron microscopy (TEM) indicated the near single-crystal structure both types of films. SAXS also showed the styrene cylinders and the alternating lamellae to be packed closer together in roll-cast films than in simple quiescently cast films. A molecular orientation mechanism is proposed to describe both these results as well as the changes in packing and in macroscopic sample dimensions measured after complete solvent evaporation and after sample annealing. © 1993 John Wiley & Sons, Inc.     

Nanoparticle arrangements in block copolymer particles with microphase‐separated structures

In this study, metal‐polymer particles with microphase‐separated structures were prepared by self‐organized precipitation, where a good solvent is evaporated from a solution that also contains block copolymer, Au NPs, and a poor solvent. Control of the microphase‐separated structure in composite particles consisting of Au NPs and block copolymer was accomplished by changing the Au NP size, the mix ratio, and the copolymerization ratio of the block copolymer. The morphology of the inner structures was changed from a lamellar phase to a spherical phase by increasing the Au NP concentration. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

嵌段序列对线型ABC三嵌段高分子微相分离动力学的影响

用动态密度泛函理论研究了嵌段序列对线型ABC三嵌段高分子微相分离动力学机理的影响. 针对一个典型的线型ABC三嵌段高分子, 通过系统地改变各嵌段的体积分数, 我们给出了嵌段序列为ABC 和BAC时, 关于微相分离机理的三元相图. 发现除各嵌段的平均组分、相互作用能外, 嵌段序列也影响其微相分离的机理和最终的相结构. 此外, 嵌段序列的变化还导致了三元相图对称性的破缺.     

Highly Asymmetric Phase Diagram of a Poly(1,2‐octylene oxide)–Poly(ethylene oxide) Diblock Copolymer System Comprising a Brush‐Like Poly(1,2‐octylene oxide) Block

The phase diagram of a series of poly(1,2‐octylene oxide)–poly(ethylene oxide) (POO–PEO) diblock copolymers is determined by small‐angle X‐ray scattering. The Flory–Huggins interaction parameter was measured by small‐angle neutron scattering. The phase diagram is highly asymmetric due to large conformational asymmetry that results from the hexyl side chains in the POO block. Non‐lamellar phases (hexagonal and gyroid) are observed near f_PEO = 0.5, and the lamellar phase is observed for f_PEO ≥ 0.5.

     

Synthesis and dielectric properties of novel liquid crystalline triblock copolymers with cyanobiphenyl moieties and poly(n‐butyl acrylate) segments

In this work, self‐assembly method was used to improve the dielectric constant of triblock copolymers. A series of ABA triblock copolymers with a defined length of poly(n‐butyl acrylate) (PBA, B block) segment and different lengths of liquid crystalline (LC) poly[11‐(4‐cyano‐4′‐biphenoxy)undecyl methacrylate] (P11CBMA, A block) segments were synthesized by using the atom transfer radical polymerization method. The well‐defined triblock copolymers P11CBMA_m‐b‐PBA_n‐b‐P11CBMA_m possess three different B/A ratios (n = 50, m = 17, 43, 53). Due to the supramolecular cooperative motion effect, the copolymers can form worm‐like microstructure (W_LC = 52.8%), cylinder‐like nanostructure with P11CBMA phase embedded in PBA matrix (W_LC = 73.9%), and wide stripe structure with LC domains distributed unevenly in a continuous PBA matrix (W_LC = 77.7%) after annealed at 160°C (above T_i) under N₂ for 24 h, respectively. In order to study the influence of microphase separated morphology of triblock copolymer on the dielectric properties, solvent annealing was also used to develop various nanostructures. After thermal or solvent annealing, the dielectric constants of block copolymers increased dramatically while their loss factors remained the same. For different block copolymers, the dielectric constants increased with the increase of the LC block length. For diverse treatments, dielectric permittivities of samples varied widely with different nanostructures. The results show that the dielectric constants of block copolymers could be tuned by the block ratios and the self‐assembled microstructures. These findings will inspire researchers using self‐assembly method to design and develop novel flexible materials with high dielectric permittivity. Copyright © 2014 John Wiley & Sons, Ltd.     

On the microphase separation kinetics of symmetric diblock copolymers

Time-resolved small-angle x-ray scattering studies were performed on symmetric diblock copolymers of polystyrene and poly(methyl methacrylate), P(S-b-MMA). Freeze-dried powders of P(S-b-MMA) having a molecular weight of 8.4×10⁴ were rapidly heated to temperatures above the glass transition temperature to initiate the microphase separation. The microphase separation process was found to consist of a rapid, local microphase separation followed by a long-term coarsening process. The period characterizing the lamellar microphase separated structure was found to increase initiallv and then saturate at longer times. These results are discussed in light of recent theoretical developments.In celebration of his 65th birthday, this article is dedicated to Prof. E. W. Fischer whose methodic and thorough approach to research has been and continues to be a model for us to follow. Es freut mich, daß ich mit Herrn Fischer gearbeitet habe. Ich habe vieles von ihm gelernt. Ich hoffe, daß auch ich so fleißig sein werde, wenn ich so jung bin wie er.     

Microphase Separation within Disk Shaped Aggregates of Triblock Bottlebrushes

Well‐defined AbBA triblock bottlebrush with poly(N,N‐dimethyl acrylamide) (PAm) as A block and polyacrylate, densely grafted with poly(tert‐butyl acrylate)‐block‐polystyrene (PBA‐b‐PS), as brush bB block is prepared by controlled radical polymerization and click chemistry. The triblock copolymer with a composition of PAm₂₀₀‐b‐b(PBA₁₄‐b‐PS₄₇)₁₆₇‐b‐PAm₂₀₀ is obtained and is further transformed into PAm₂₀₀‐b‐b(PAA₁₄‐b‐PS₄₇)₁₆₇‐b‐PAm₂₀₀ by hydrolysis of the PBA segment into poly(acrylic acid) (PAA). In a mixture of N,N‐dimethylformamide (DMF) and methanol, a poor solvent of bB block, PAm₂₀₀‐b‐b(PAA₁₄‐b‐PS₄₇)₁₆₇‐b‐PAm₂₀₀ self‐assembled into disk‐like platelets, which have an internal lamellar structure by further microphase‐separation of PAA‐b‐PS branches in 2D. Moreover, Ag nanoparticles are aligned by PAA segments along the disk to form a pattern.

     

Thermoreversible gels of polyacrylonitrile

Polyacrylonitrile (PAN) is soluble at room temperature in some ionic aqueous media and in many polar organic liquids such as dimethyl sulfoxide, but the polymer will not crystallize from these solvents. It is known, however, that PAN will from single crystals from dilute propylene carbonate (PC) solutions. It was found that on cooling concentrated PAN-PC solutions, thermoreversible gels were formed. The gels showed x-ray diffraction peaks and calorimetry indicated first-order dissolution endotherms and crystallization exotherms. Thus, on cooling, crystallization of the polymer from this solvent was identified as the cause for gelation of concentrated solutions. © 1992 John Wiley & Sons, Inc.     

Synthesis and Microphase Separation Behavior of Random,Mixed Cylindrical Brush Copolymers Bearing Polystyrene and Poly(ε-caprolactone) Side Chains

A series of mixed, random cylindrical brush copolymers bearing polystyrene(PS) and poly(ε-caprolactone)(PCL) side chains were synthesized via the combination of ring-opening polymerization(ROP) and atom transfer radical polymerization(ATRP). These novel cylindrical brush copolymers have been characterized by means of nuclear magnetic resonance(NMR) spectroscopy, gel permeation chromatography(GPC) and differential scanning calorimetry(DSC). It was found that the mikto-armed cylindrical brush copolymers were microphase-separated in bulks and that the morphologies were dependent on the mass ratios of PS to PCL side chains. One of the cylindrical brush copolymers was employed to incorporate into epoxy thermoset to investigate effect of the mikto-armed cylindrical brush architecture on the reaction-induced microphase separation behavior. Depending on the concentration of the cylindrical brush in epoxy, the thermosets can display the morphologies with the spherical, worm-like and lamellar PS microdomains dispersing in continuous thermosetting matrices.     

Three-Dimensional Nanoparticle Arrays Templated by Self-Assembled Block-Copolymer Gels

Water-soluble triblock copolymers (PEO-PPO-PEO) are utilized to provide thermoreversible micellar templates for three-dimensional nanoparticle arrays. The triblock forms a cubic micellar structure with typical dimensions of tens of nanometers. The temperature-dependent amphiphilic nature of the block copolymers provides increased structure control and allows the use of pre-made silica and gold nanoparticles as well as globular proteins. Using rheology and small-angle neutron scattering (SANS), we characterize the influence of the nanoparticles on the local particle structure and on the macroscopic mechanical properties. We are able to incorporate significant quantities of nanoparticles into the block copolymer gel without destroying the ordered structure. Contrast matching SANS demonstrates that some level of the template structure is transferred to the nanoparticles. This study demonstrates the feasibility and potential of using this simple approach to generate novel nanoparticles-polymer composites.     

Roll-Casting of block copolymers and of block copolymer-homopolymer blends

An improved technique for casting highly oriented films of block copolymers from solutions subjected to flow is presented. Polymer solutions were rolled between two counter-rotating adjacent cylinders while at the same time the solvent was allowed to evaporate. As the solvent evaporated, the block copolymers microphase separated into globally oriented structures. Using this method known as ‘roll-casting’ we present in this paper a study of the morphology of polystyrene-polybutadiene-polystyrene (PS/PB/PS) triblock copolymer cast with and without additional high molecular weight homopolymers. The pure copolymer films consisted of polystyrene cylinders assembled on a hexagonal lattice in a polybutadiene matrix in a near single-crystal structure. Blends of copolymer with high molecular weight polystyrene and/or polybutadiene, phase separated into ellipsoidal regions of homopolymer embedded in an oriented block copolymer matrix. Annealing the films resulted in conversion of the homopolymer regions to spheres accompanied by some misalignment of the copolymer microdomains. The morphology of these films as revealed by TEM is discussed. A brief discussion of the flow field that develops in the experimental system is also presented and its similarity to the flow field of our previous work is shown. © 1994 John Wiley & Sons, Inc.