Diameter-Dependence of the Morphology of PS-b-PMMA Nanorods Confined Within Ordered Porous Alumina Templates

Shell with Striped, Helical, and Bipolar Lamellae Structures from Soft Confinement-Induced Self-Assembly of AB Diblock Copolymers on a Nanocylinder

The soft confinement-induced self-assembly of AB diblock copolymers on a nanocylinder is studied via a simulated annealing method. The formation of multiple copolymer shells was predicted by varying the interfacial interaction, the size of confinement, and the height and diameter of the nanocylinder. The competition between solvent repulsion and nanocylinder attraction determined the degree of encapsulation of the copolymer shell. The formation of a helical copolymer shell was induced by the maximization of conformational entropy. The preferential distribution position of copolymers on anisotropic nanocylinder surfaces was induced by interfacial energy minimization. Our study contributes to the understanding of the formation mechanism of the helical structure in block copolymer aggregates and the fabrication of copolymer shells with predesigned morphologies.

Controlling the Formation of Polyhedral Block Copolymer Nanoparticles: Insights from Process Variables and Dynamic Modeling

This study delves into the formation of nanoscale polyhedral block copolymer particles (PBCPs) exhibiting cubic, octahedral, and variant geometries. These structures represent a pioneering class that has never been fabricated previously. PBCP features distinct variations in curvature on the outer surface, aligning with the edges and corners of polyhedral shapes. This characteristic sharply contrasts with previous block copolymers (BCPs), which displayed a smooth spherical surface. The emergence of these cornered morphologies presents an intriguing and counterintuitive phenomenon and is linked to process parameters, such as evaporation rates and initial concentration, while keeping other variables constant. Using a system of coupled Cahn-Hillard (CCH) equations, we uncover the mechanisms driving polyhedral particle formation, emphasizing the importance of controlling relaxation parameters for shape variable u and microphase separation v. This unconventional approach, differing from traditional steepest descent method, allows for precise control and diverse polyhedral particle generation. Accelerating the shape variable u proves crucial for expediting precipitation and aligns with experimental observations. Employing the above theoretical model, we achieve shape predictions for particles and the microphase separation within them, which overcomes the limitations of ab initio computations. Additionally, a numerical stability analysis discerns the transient nature versus local minimizer characteristics. Overall, our findings contribute to understanding the complex interplay between process variables and the morphology of polyhedral BCP nanoparticles.

Spatially Confined Fabrication of Polar Poly(Vinylidene Fluoride) Nanotubes

Polar poly(vinylidene fluoride) (PVDF) nanotubes have attracted significant attention due to their excellent piezoelectric and ferroelectric properties, yet a tunable fabrication of homogeneous polar PVDF nanotubes remains a challenge. Here, a simple method is reported to fabricate polar PVDF nanotubes using anodize aluminum oxide (AAO) membranes as templates that are removed by etching in a potassium hydroxide (KOH) solution and then ageing at room temperature. PVDF nanotubes originally crystallized in the AAO membrane are pure α-crystals with very low crystallinity, yet after being released from the templates, the crystallinity of the nanotubes markedly increases with ageing at room temperature, leading to the formation of β-PVDF crystals in a very short time, with the formation of γ crystals after longer ageing times. A large amount of γ crystals formed when the released PVDF nanotubes are heated to ≈130 °C. The formation of polar PVDF nanotubes released from the AAO templates treated with higher concentrations of alkaline solution results from the reaction of the surface of the PVDF nanotubes with the alkaline solution and structure reorganization under confined conditions. This large-scale preparation of β- and γ-PVDF opens a new pathway to produce polar PVDF nanomaterials.

A decade of innovation and progress in understanding the morphology and structure of heterogeneous polymers in rigid confinement

When confined in nanoscale domains, polymers generally encounter changes in their structural, thermodynamics and dynamics properties compared to those in the bulk, due to the high amount of polymer/wall interfaces and limited amount of matter. The present review specifically deals with the confinement of heterogeneous polymers (i.e. polymer blends and block copolymers) in rigid nanoscale domains (i.e. bearing non-deformable solid walls) where the processes of phase separation and self-assembly can be deeply affected. This review focuses on the innovative contributions of the last decade (2010-2020), giving a summary of the new insights and understanding gained in this period. We conclude this review by giving our view on the most thriving directions for this topic.

Phase behavior of disk-coil block copolymers under cylindrical confinement: Curvature-induced structural frustrations

In this paper, we explore the self-assembly behavior of disk-coil block copolymers (BCPs) confined within a cylinder using molecular dynamics simulations. As functions of the diameter of the confining cylinder and the number of coil beads, concentric lamellar structures are obtained with a different number of alternating disk-rich and coil-rich bilayers. Our paper focuses on the curvature-induced structural behavior in the disk-rich domain of a self-assembled structure, which is investigated by calculating the local density distribution P(r) and the orientational distribution G(r,θ). In the inner layers of cylinder-confined disk-coil BCPs, both P(r) and G(r,θ) show characteristic asymmetry within a bilayer which is directly contrasted with the bulk and slab-confined disk-coil BCPs. We successfully explain the structural frustration of disks arising from the curved structure due to packing frustration of disks and asymmetric stretching of coils to the regions with different curvatures in a bilayer. Our results are important to understand the self-assembly behavior of BCPs containing a rigid motif in a confined structure, such as a self-assembled structure of bacteriochlorophyll molecules confined by a lipid layer to form a chlorosome, the photosynthetic antennae complex found in nature.

Curved block copolymer nanodiscs: structure transformations in cylindrical nanopores using the nonsolvent-assisted template wetting method

In this work, we study the structure transformations of cylinder-forming polystyrene-block-polydimethylsiloxane (PS31k-b-PDMS14.5k) confined in cylindrical nanopores. PS-b-PDMS nanotubes, nanospheres, and curved nanodiscs are ingeniously prepared by a facile template wetting strategy using anodic aluminum oxide (AAO) templates. Quantitative analyses of the structure transformations from nanospheres to curved nanodiscs are also conducted, showing that the lengths of the curved nanodiscs can be controlled by adjusting the annealing temperature and time. Furthermore, the PDMS domains of the nanostructures can be selectively etched using HF solutions, generating porous PS nanostructures. This work not only offers versatile routes to prepare block copolymer nanostructures with controlled shapes but also provides a deeper understanding of the structure transformation of block copolymers in confined geometries.

Oriented crystallization of PEG induced by confinement in cylindrical nanopores: structural and thermal properties

Nanoporous ion track-etched polycarbonate is ideally suited for the study of confined polymers via small angle X-ray scattering (SAXS) due to the strictly parallel orientation of the pores as well as their uncorrelated lateral distribution. Nanopores with radii ranging from 17 to 213 nm are prepared and coated with SiO2via atomic layer deposition in order to obtain a well-defined and homogeneous surface. A low molecular weight polyethylene glycol (PEG) homopolymer with a semicrystalline lamellar bulk structure is introduced into the nanopores via melt infiltration. At high temperatures SAXS measurements confirm a uniform filling of the pores with amorphous polymer. Upon cooling below the melting point of PEG, a concentrical structure of semicrystalline lamellae is revealed for large pore radii. We introduce models which successfully describe the combined scattering from nanopores and semicrystalline or amorphous PEG inside. DSC measurements of the confined polymer show a decrease of melting temperature and heat of fusion per gram polymer upon reduction of the pore radius and hint at a change in the lamellar configuration.

Microwave-annealing-induced nanowetting of block copolymers in cylindrical nanopores

Block copolymers have attracted great attention because of their abilities to self-assemble into well-ordered microphase-separated structures. To generate nanopatterns of block copolymers with long-range ordering and low-defect densities in shorter time scales, microwave annealing has recently been applied. Microwave annealing, however, has so far only been used for block copolymer bulks and thin films. In this work, we discover that microwave annealing can be successfully applied to three-dimensional block copolymer nanostructures by studying the infiltration and microphase separation of block copolymers in cylindrical nanopores upon microwave irradiation. Cylinder-forming and lamella-forming poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) are introduced into the nanopores of anodic aluminum oxide (AAO) templates. In addition, AAO templates with different pore sizes are used to study the effect of the commensurabilities between the pore diameters and the repeating periods of the block copolymers on the morphologies of the block copolymer nanostructures.

Semicrystalline Block Copolymers in Rigid Confining Nanopores

We have investigated PLLA crystallization in lamellae-forming PS-b-PLLA confined to straight cylindrical nanopores under weak confinement (nanopore diameter D/equilibrium PS-b-PLLA period L0 ≥ 4.8). Molten PS-b-PLLA predominantly forms concentric lamellae along the nanopores, but intertwined helices occur even for D/L0 ≈ 7.3. Quenching PS-b-PLLA melts below TG(PS) results in PLLA cold crystallization strictly confined by the vitrified PS domains. Above TG(PS), PLLA crystallization is templated by the PS-b-PLLA melt domain structure in the nanopore centers, while adsorption on the nanopore walls stabilizes the outermost cylindrical PS-b-PLLA shell. In between, the nanoscopic PS-b-PLLA melt domain structure apparently ripens to reduce frustrations transmitted from the outermost immobilized PS-b-PLLA layer. The onset of PLLA crystallization catalyzes the ripening while transient ripening states are arrested by advancing PLLA crystallization. Certain helical structure motifs persist PLLA crystallization even if PS is soft. The direction of fastest PLLA crystal growth is preferentially aligned with the nanopore axes to the same degree as for PLLA homopolymer, independent of whether PS is vitreous or soft.

Order-order transitions of diblock copolymer melts under cylindrical confinement

The self-assembly behavior of AB diblock copolymers under cylindrical confinement is investigated using the self-consistent field theory. We focus on the impact of the confinement on the order-order transitions of three-dimensional morphologies by constructing two types of phase diagrams with continuously varying block compositions. One type is with respect to the block composition and the immiscibility parameter for various pore sizes, in which the order-order transitions are shown to be strongly impacted by the pore curvature and thus largely different from the bulk ones. Note that the morphologies are categorized by the intrinsical geometry of their domains, i.e., that helical morphologies are regarded as one type of cylindrical phase. Another type of phase diagram is with respect to the block composition and the pore diameter, which exhibits a number of interesting order-order transitions, especially the transition sequence from a straight line of spheres, to one straight cylinder and stacked disks as the pore diameter increases. A critical point is observed at which the stability region of the straight cylinder vanishes and thereby the spheres transform into the stacked disks continuously. The mechanism of these phase transitions is rationalized in the context of the bulk factors as well as an additional factor, i.e., the competition between the spontaneous curvature of the copolymer and the imposed curvature by the nanopore.

Blending Homopolymers for Controlling the Morphology Transitions of Block Copolymer Nanorods Confined in Cylindrical Nanopores

The microphase separation of block copolymers in confined geometries has been widely investigated over the last few decades. The controllability and versatility of the confinement-induced morphologies, however, are still difficult to be achieved because of the limited experimental parameters in the process of fabricating the confined nanostructures. In this work, we study the morphology transitions of lamellae-forming polystyrene-block-polydimethylsiloxane (PS-b-PDMS) nanorods confined in the nanopores of anodic aluminum oxide (AAO) templates. The nanorods are formed by solvent-assisted template wetting, and the morphologies are compared to those in the bulk state. By blending PS-b-PDMS with homopolystyrene (hPS), the morphologies of the nanorods can be controlled because of the changes of the effective volume fractions. Special morphology transitions from concentric lamellar morphology, to multihelical morphology, and finally to spherical-like morphology are observed by increasing the weight ratios of hPS. hPS with different molecular weights is also applied to investigate the effect of hPS on the morphologies of the PS-b-PDMS/hPS blend nanostructures. The unusual morphologies are further confirmed by a selective removal process, which also generates nanochannels for possible refilling with functional materials.

Selective solvent-induced reconstruction in confined space: one-dimensional mesoporous block copolymer structures in cylindrical nanopores

The formation of polymer nanostructures confined in cylindrical nanopores via a novel selective solvent-induced reconstruction process is investigated.

Morphology control of three-dimensional nanostructures in porous templates using lamella-forming block copolymers and solvent vapors

The microphase separation behavior of block copolymers confined in cylindrical nanopores has been extensively investigated. Recently, the solvent-annealing-induced nanowetting in templates (SAINT) method has been demonstrated to be a versatile approach for the infiltration of block copolymers into the nanopores of porous templates. The function of the annealing solvents, however, is still not well understood, especially in the morphology control of the fabricated block copolymer nanostructures. In this work, we elucidate the function of the annealing solvents in the SAINT method using a lamella-forming block copolymer, polystyrene-block-polydimethylsiloxane (PS-b-PDMS), and anodic aluminum oxide (AAO) templates. By changing the composition of the annealing solvents, different morphologies such as the concentric lamellar morphology, the winding cylinder morphology, and the irregular hybrid morphology are observed, mainly caused by the annealing-solvent-induced volume change. The morphology of the block copolymer nanostructures can be further confirmed using an HF solution to remove the PDMS domain selectively.

Frustrated phases under three-dimensional confinement simulated by a set of coupled Cahn-Hilliard equations

We numerically study a set of coupled Cahn-Hilliard equations as a means to find morphologies of diblock copolymers in three-dimensional spherical confinement. This approach allows us to find a variety of energy minimizers including rings, tennis balls, Janus balls and multipods among several others. Phase diagrams of confined morphologies are presented. We modify the size of the interface between microphases to control the number of holes in multipod morphologies. Comparison to experimental observation by transmission electron microtomography of multipods in polystyrene-polyisoprene diblock copolymers is also presented.

Crystallization behaviour of poly(ethylene oxide) under confinement in the electrospun nanofibers of polystyrene/poly(ethylene oxide) blends

We have studied the confined crystallization behaviour of poly(ethylene oxide) (PEO) in the electrospun nanofibers of the phase-separated blends of polystyrene (PS) and PEO, where PS was present as the major component. The size and shape of PEO domains in the nanofibers were considerably different from those in the cast films, presumably because of the nano-dimensions of the nanofibers and the extensional forces experienced by the polymer solution during electrospinning. The phase-separated morphology in turn influenced the crystallization behaviour of PEO in the blend nanofibers. At a PEO weight fraction of ≥0.3, crystallization occurred through a heterogeneous nucleation mechanism similar to that in cast blend films. However, as the PEO weight fraction in the blend nanofibers was reduced from 0.3 to 0.2, an abrupt transformation of the nucleation mechanism from the heterogeneous to predominantly homogenous type was observed. The change in the nucleation mechanism implied a drastic reduction of the spatial continuity of PEO domains in the nanofibers, which was not encountered in the cast film. The melting temperature and crystallinity of the PEO crystallites developed in the nanofibers were also significantly lower than those in the corresponding cast films. The phenomena observed were reconciled by the morphological observation, which revealed that the phase separation under the radial constraint of the nanofibers led to the formation of small-sized fibrillar PEO domains with limited spatial connectivity. The thermal treatment of the PS/PEO blend nanofibers above the glass transition temperature of PS induced an even stronger confinement effect on PEO crystallization.

The phase transition behavior of poly(butylene adipate) in the nanoporous anodic alumina oxide

PBA nanotubes with different diameters have been prepared.

Self-assembly of diblock copolymer confined in an array-structure space

The combination of top-down and bottom-up technologies is an effective method to create the novel nanostructures with long range order in the field of advanced materials manufacture. In this work, we employed a polymeric self-consistent field theory to investigate the pattern formation of diblock copolymer in a 2D confinement system designed by filling pillar arrays with various 2D shapes such as squares, rectangles, and triangles. Our simulation shows that in such confinement system, the microphase structure of diblock copolymer strongly depends on the pitch, shape, size, and rotation of the pillar as well as the surface field of confinement. The array structures can not only induce the formation of new phase patterns but also control the location and orientation of pattern structures. Finally, several methods to tune the commensuration and frustration of array-structure confinement are proposed and examined.

Patchy nanoparticles self-assembled from linear triblock copolymers under spherical confinement: a simulated annealing study

The self-assembly of linear ABC triblock copolymers confined in spherical nanopores is studied using a simulated annealing technique. Morphological phase diagrams as a function of the pore diameter, the selectivity of the pore-wall to the terminal blocks, and the copolymer composition are constructed. A variety of patchy nanoparticles and multiple morphological transitions are identified. Janus nanoparticles, which can be regarded as particles with one patch, are observed inside small nanopores. With increasing the pore diameter, the number of patches on a nanoparticle surface increases from one to two, four, five, six, and seven. The size of each patch increases periodically. The number of patches also increases with increasing the wall selectivity. The distribution of the patches on the surface of a given particle is highly symmetric. The interior structures of the patchy nanoparticles and the morphological transition are investigated by calculating the bridging fraction, the mean square end-to-end distance and the average contact number between different components. A series of entropy-driven morphological transitions is predicted. Furthermore, it is found that the overall patchy morphology is largely controlled by the volume fraction of the middle B-block, while the internal structure is largely controlled by the volume fraction ratio of the two terminal blocks. Our study demonstrates that the size of nanopores, the pore-wall selectivity, and the copolymer composition could be utilized as effective means to tune the structure and properties of the anisotropic nanoparticles.

Honeycombs in honeycombs: complex liquid crystal alumina composite mesostructures

Small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM) were used to study orientation patterns of two polyphilic liquid crystals (LC) confined to cylindrical pores of anodic aluminum oxide (AAO). The hierarchical hybrid systems had the LC honeycomb (lattice parameter 3.5-4 nm) inside the pores of the AAO honeycomb (diameters 60 and 400 nm). By conducting complete reciprocal space mapping using SAXS, we conclude that the columns of both compounds align in planes normal to the AAO pore axis, with a specific crystallographic direction of the LC lattice aligning strictly parallel to the pore axis. AFM of LC-containing AAO fracture surfaces further revealed that the columns of the planar anchoring LC (compound 1) formed concentric circles in the plane normal to the pore axis near the AAO wall. Toward the pore center, the circles become anisometric "racetrack" loops consisting of two straight segments and two semicircles. This mode compensates for slight ellipticity of the pore cross section. Indications are, however, that for perfectly circular pores, circular shape is maintained right to the center of the pore, the radius coming down to the size of a molecule. For the homeotropically anchoring compound 2, the columns are to the most part straight and parallel to each other, arranged in layers normal to the AAO pore axis, like logs in an ordered pile. Only near the pore wall the columns splay somewhat. In both cases, columns are confined to layers strictly perpendicular to the AAO pore axis, and there is no sign of escape to the third dimension or of axial orientation, the latter having been reported previously for some discotic LCs. The main cause of the two new LC configurations, the "racetrack" and the "logpile", and of their difference from those of confined nematic LC, is the very high splay energy and low bend energy of columnar phases.

Segmented helical structures formed by ABC star copolymers in nanopores

Self-assembly of ABC star triblock copolymers confined in cylindrical nanopores is studied using self-consistent mean-field theory. With an ABC terpolymer forming hexagonally-arranged cylinders, segmented into alternative B and C domains, in the bulk, we observe the formation in the nanopore of a segmented single circular and non-circular cylinder, a segmented single-helix, and a segmented double-helix as stable phases, and a metastable stacked-disk phase with fourfold symmetry. The phase sequence from single-cylinder, to single-helix, and then to double-helix, is similar as that in the cylindrically-confined diblock copolymers except for the absence of an equilibrium stacked-disk phase. It is revealed that the arrangement of the three-arm junctions plays a critical role for the structure formation. One of the most interesting features in the helical structures is that there are two periods: the period of the B/C domains in the helix and the helical period. We demonstrate that the period numbers of the B/C domains contained in each helical period can be tuned by varying the pore diameter. In addition, it is predicted that the period number of B/C domains can be any rational in real helical structures whose helical period can be tuned freely.

Phase behavior of binary blends of diblock copolymer/homopolymer confined in spherical nanopores

Binary blends of a diblock copolymer (AB) and an incompatible homopolymer (C) confined in spherical cavities are studied using a simulated annealing technique. The phase behavior of the blends is examined for four typical cases, representing the different selectivity of the pore surface to the A, B, and C species. The internal morphology of the spherical polymeric particles is controlled by the homopolymer volume fraction, the degree of confinement, and the composition of the copolymer. Inside a particle, the homopolymers segregate to form one or, under some conditions, two domains; thus, the homopolymers may act as an additional controlling parameter of the shape and symmetry of the copolymer domain. A rich array of confinement-induced novel diblock copolymer morphologies is predicted. In particular, core-shell particles with the copolymers as the shell wrapping around a homopolymer core or a copolymer-homopolymer combined core and Janus-like particles with the copolymers and the homopolymers on different sides are obtained.