Control of Self-Assembled Structures in Binary Mixtures of A−B Diblock Copolymer and A−C Diblock Copolymer by Changing the Interaction between B and C Block Chains

Phase Behavior of Binary Blends of Diblock Copolymers: Progress and Opportunities

The phase behavior of binary blends of diblock copolymers has been examined extensively in the past decades. Experimental and theoretical studies have demonstrated that mixing two different block copolymers provides an efficient and versatile route to regulate their self-assembled morphologies. A good understanding of the principles governing the self-assembly of block copolymer blends has been obtained from the study of A1B1/A2B2 diblock copolymer blends. The second (A2B2) diblocks could act synergistically as fillers and cosurfactants to regulate the domain size and interfacial properties, resulting in the formation of ordered phases not found in the parent (A1B1 or A2B2) diblock copolymer melts. The study of A1B1/A2B2 block copolymer blends further provides a solid foundation for future research on more complex block copolymer blends.

Alternating Gyroid in Block Polymer Blends

Alternating gyroid is a lower symmetry variant of the double gyroid morphology, where the left-handed and right-handed chiral networks are physically distinct. This structure is of particular interest for photonic applications owing to predictions of a complete photonic band gap subject to the requirement of a large dielectric contrast between the individual networks and sufficient optical matching between one of the networks and the matrix. We provide evidence, via self-consistent field theory (SCFT), that stoichiometric blends of double-gyroid-forming AB and BC diblock copolymers with relatively immiscible A and C blocks should form an alternating gyroid morphology with complementary three-dimensional A and C networks that have a free energy that is nearly degenerate with two phase-separated double gyroid states. Solvent casting offers the potential for trapping this binary mixture of diblock copolymers in this metastable alternating gyroid phase. Theory further predicts that the addition of a minuscule amount (<1%) of ABC triblock terpolymer will open an alternating gyroid stability window in the resulting ternary-phase diagram. The surfactant-like stabilization produced by the triblock is relatively insensitive to its exact composition provided the B-block forms a sufficiently long bridge between the A-rich and C-rich networks. This blending strategy provides significant synthetic and material processing advantages compared to prevailing methods to produce an alternating gyroid phase in block polymers.

Molecular and Structure-Properties Comparison of an Anionically Synthesized Diblock Copolymer of the PS-b-PI Sequence and Its Hydrogenated or Sulfonated Derivatives

An approach to obtaining various nanostructures utilizing a well-studied polystyrene-b-poly(isoprene) or PS-b-PI diblock copolymer system through chemical modification reactions is reported. The complete hydrogenation and partial sulfonation to the susceptible carbon double bonds of the PI segment led to the preparation of [polystyrene-b-poly(ethylene-alt-propylene)] as well as [polystyrene-b-poly(sulfonated isoprene-co-isoprene)], respectively. The hydrogenation of the polyisoprene block results in enhanced segmental immiscibility, whereas the relative sulfonation induces an amphiphilic character in the final modified material. The successful synthesis of the pristine diblock copolymer through anionic polymerization and the relative chemical modification reactions were verified using several molecular and structural characterization techniques. The thin film structure–properties relationship was investigated using atomic force microscopy under various conditions such as different solvents and annealing temperatures. Small-angle X-ray scattering was employed to identify the different observed nanostructures and their evolution upon thermal annealing.

Pattern formation of metal-oxide hybrid nanostructures via the self-assembly of di-block copolymer blends

The templated self-assembly of block copolymers (BCPs) with a high Flory-Huggins interaction parameter (χ) can effectively create ultrafine, well-ordered nanostructures in the range of 5-30 nm. However, the self-assembled BCP patterns remain limited to possible morphological geometries and materials. Here, we introduce a novel and useful self-assembly method of di-BCP blends capable of generating diverse hybrid nanostructures consisting of oxide and metal materials through the rapid microphase separation of A-B/B-C BCP blends. We successfully obtained various hybridized BCP morphologies which cannot be acquired from a single di-BCP, such as hexagonally arranged hybrid dot and dot-in-hole patterns by controlling the mixing ratios of the solvents with a binary solvent annealing process. Furthermore, we demonstrate how the binary solvent vapor annealing process can provide a wide range of pattern geometries to di-BCP blends, showing a well-defined spontaneous one-to-one accommodation in dot-in-hole nanostructures. Specifically, we show clearly how the self-assembled BCPs can be functionalized via selective reduction and/or an oxidation process, resulting in the excellent positioning of confined silica nanodots into each nanospace of a Pt mesh. These results suggest a new method to achieve the pattern formation of more diverse and complex hybrid nanostructures using various blended BCPs.

Exploring the Self-Assembly Capabilities of ABA-Type SBS, SIS, and Their Analogous Hydrogenated Copolymers onto Different Nanostructures Using Atomic Force Microscopy

In this work, the self-assembled morphologies obtained for poly(styrene-b-butadiene-b-styrene) (SBS) and poly(styrene-b-isoprene-b-styrene) (SIS) ABA-type copolymers were investigated before and after hydrogenation of the polydiene block, which led to poly(styrene-b-ethylene)/poly(ethylene-b-styrene) (SEES) and poly(styrene-b-ethylene)/poly(propylene-b-styrene) (SEPS) copolymers, respectively. The evaluation of different morphologies was carried out using atomic force microscopy (AFM), analyzing the effect of various parameters such as the solvent and polymer concentrations employed for film casting (toluene, cyclohexane, or tetrahydrofurane with concentrations of 1 and 3 wt%), together with that of the annealing treatment (thermal annealing at room temperature, and 60, 80, and 100 °C). The effect of these parameters in combination with the chemical nature of the polydiene block led to different morphologies with different topographic aspects affecting the roughness (Ra) of the film.

Effect of Systematic Hydrogenation on the Phase Behavior and Nanostructural Dimensions of Block Copolymers

Unsaturated polydienes are frequently hydrogenated to yield polyolefins that are more chemically stable. Here, the effects of partial hydrogenation on the phase behavior and nanostructure of polyisoprene-containing block copolymers are investigated. To ensure access to the order-disorder transition temperature (T_ODT) over a wide temperature range, we examine copolymers with at least one random block. Dynamic rheological and scattering measurements indicate that T_ODT increases linearly with increasing hydrogenation. Small-angle scattering reveals that the temperature-dependence of the Flory-Huggins parameter changes and the microdomain period increases, while the interfacial thickness decreases. The influence of hydrogenation becomes less pronounced in more constrained multiblock copolymers.

Manipulating the ABCs of self-assembly via low-χ block polymer design

Block polymer self-assembly typically translates molecular chain connectivity into mesoscale structure by exploiting incompatible blocks with large interaction parameters (χ_ij). In this article, we demonstrate that the converse approach, encoding low-χ interactions in ABC bottlebrush triblock terpolymers (χ_AC [Formula: see text] 0), promotes organization into a unique mixed-domain lamellar morphology, which we designate LAM_P Transmission electron microscopy indicates that LAM_P exhibits ACBC domain connectivity, in contrast to conventional three-domain lamellae (LAM₃) with ABCB periods. Complementary small-angle X-ray scattering experiments reveal a strongly decreasing domain spacing with increasing total molar mass. Self-consistent field theory reinforces these observations and predicts that LAM_P is thermodynamically stable below a critical χ_AC, above which LAM₃ emerges. Both experiments and theory expose close analogies to ABA' triblock copolymer phase behavior, collectively suggesting that low-χ interactions between chemically similar or distinct blocks intimately influence self-assembly. These conclusions provide fresh opportunities for block polymer design with potential consequences spanning all self-assembling soft materials.

Host-guest self-assembly in block copolymer blends

Ultrafine, uniform nanostructures with excellent functionalities can be formed by self-assembly of block copolymer (BCP) thin films. However, extension of their geometric variability is not straightforward due to their limited thin film morphologies. Here, we report that unusual and spontaneous positioning between host and guest BCP microdomains, even in the absence of H-bond linkages, can create hybridized morphologies that cannot be formed from a neat BCP. Our self-consistent field theory (SCFT) simulation results theoretically support that the precise registration of a spherical BCP microdomain (guest, B-b-C) at the center of a perforated lamellar BCP nanostructure (host, A-b-B) can energetically stabilize the blended morphology. As an exemplary application of the hybrid nanotemplate, a nanoring-type Ge2Sb2Te5 (GST) phase-change memory device with an extremely low switching current is demonstrated. These results suggest the possibility of a new pathway to construct more diverse and complex nanostructures using controlled blending of various BCPs.

PLLA Morphology Controlled by Dry-cast Process

The influence of solvent evaporation rate, in the dry-casting process, on crystallinity and morphology of crystallizable polymeric membrane was studied. Poly-l-lactic acid (PLLA) was taken as an example. The membranes were characterized by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). By controlling the solvent evaporation rate, samples with various crystallinities were prepared. From higher to lower crystallinity the membranes exhibit three types of surface morphology in sequence: granule, stripe and smooth.

Decoupling Bulk Thermodynamics and Wetting Characteristics of Block Copolymer Thin Films

The consequences on certain physical properties of controlled levels of epoxidation of the poly(isoprene) blocks in poly(styrene-b-isoprene) (PS-PI) diblock copolymers and poly(isoprene) (hPI) homopolymers have been studied, where the products after epoxidation are denoted PS-PIxn and hPIxn, respectively. The effective interaction parameters χ_eff between the PS and the PIxn blocks were estimated by applying mean-field theory to the lamellar periodicities identified by small-angle X-ray scattering and to the order-to-disorder transition temperatures determined by dynamic mechanical spectroscopy. These results were fit to a binary segment-segment interaction parameter model indicating a nonlinear change in χ_eff with percent epoxidation. In contrast, contact angle measurement on hPIxn and lamellar orientations of thin-film PS-PIxn suggest that the surface energy of PIxn increases linearly with epoxidation. This decoupling of bulk and thin-film thermodynamic behaviors is attributed to the different roles that a random copolymer architecture plays in establishing three-dimensional order versus wetting at a two-dimensional surface.

Hierarchically structured materials from block polymer confinement within bicontinuous microemulsion-derived nanoporous polyethylene

The self-assembly behavior of block polymers under strong two-dimensional and three-dimensional confinement has been well-studied in the past decade. Confinement effects enable access to a large suite of morphologies not typically observed in the bulk. We have used nanoporous polyethylene, derived from a polymeric bicontinuous microemulsion, as a novel template for the confinement of several different cylinder-forming block polymer systems: poly(isoprene-b-2-vinylpyridine), poly(styrene-b-isoprene), and poly(isoprene-b-dimethylsiloxane). The resultant materials exhibit unique hierarchical arrangements of structure with two distinct length scales. First, the polyethylene template imparts a disordered, microemulsion-like periodicity between bicontinuous polyethylene and block polymer networks with sizes on the order of 100 nm. Second, the block polymer networks display internal periodic arrangements produced by the spontaneous segregation of their incompatible constituents. The microphase-separated morphologies observed are similar to those previously reported for confinement of block polymers in cylindrical pores. However, at present, the morphologies are spatially variant in a complex manner, due to the three-dimensionally interconnected nature of the confining geometry and its distribution in pore sizes. We have further exploited the unique structure of the polyethylene template to generate new hierarchically structured porous monoliths. Poly(isoprene-b-2-vinylpyridine) is used as a model system in which the pyridine block is cross-linked, post-infiltration, and the polyethylene template is subsequently extracted. The resultant materials possess a three-dimensionally continuous pore network, of which the pore walls retain the unique, microphase-separated morphology of the confined block polymer.