Cooperative surface-induced self-assembly of symmetric diblock copolymers confined films with embedded nanorods

The self-assembly of symmetric diblock copolymers confined films with embedded nanorods is investigated by the self-consistent field (SCF) theory. We obtain some phase diagrams as a function of film thickness H and nanorod diameter D. The increase in preferential nanorod diameter D can promote the formation of incomplete cylindrical and spherical structures near the film surfaces, and can also induce complete lamellar, cylindrical and spherical structures in the interior. The formation of these induced self-assembled structures is due to the competition between inner surface confinement (two parallel surfaces) and outer surface confinement (nanorods). This investigation can provide some insights into the self-assembly of diblock copolymers with complex confinements.     

Self-assembly of linear ABC coil-coil-rod triblock copolymers

Self-assembly of linear ABC coil-coil-rod triblock copolymer melt is studied by applying self-consistent-field lattice techniques in three-dimensional (3D) space. In contrast to rod-coil diblock copolymers, our results reveal the effect of the broad parameter space on the self-assembly of the linear ABC coil-coil-rod triblock copolymers. Seven stable structures are found stable, including “two-color” lamella, “three-color” lamella, “two-color”-perforated lamella, “three-color”-perforated lamella, core-shell hexagonal lattice phase, strip, and micelle. When the two coil blocks have equal lengths (f_A = f_B), the lamellar structure dominates the majority of the phase diagram. The effects of the two coil blocks on the self-assembly are explored by tuning the relative length of the A and B coil blocks in terms of keeping the length of the block C (rod). Moreover, by switching the position of the blocks B and C, the influence of the block sequencing on the self-assembly is studied.     

Cylindrical phase of diblock copolymers confined in thin films. A real-space self-consistent field theory study

We have used real-space self-consistent field theory to search possible morphology of an asymmetric AB diblock copolymer thin film confined between two homogeneous hard walls. The volume fraction of the A block is fixed to be f=0.3, as expected, a cylindrical phase is stable without confinement (in the bulk). Our simulation reveals that under confinement, in addition to parallel and perpendicular cylinders, other phases, such as flat lamellae, perforated lamellae, undulated cylinders and undulated lamellae, are also stable due to the block-substrate interactions. Three new structures, i.e. undulated lamellae, undulated cylinders and parallel cylinders with non-integer period, are observed to be stable with suitable film thickness and block-substrate interaction. By systematically varying the film thickness and the interaction parameters between the two blocks, phase diagrams are constructed for typical block-substrate interactions. We compare the phase diagrams for weak and strong substrate preference and discuss the effects of confinement and substrate preference on the stability of various structures.     

Micromechanical simulation of molecular architecture and orientation effect on deformation and fracture of multiblock copolymers

Multiblock copolymers containing a large number of blocks have distinct microstructures and mechanical responses that are different from that of conventional diblock and triblock copolymers. A combined simulation method that utilized MesoDyn for morphologies and probabilistic lattice spring model (LSM) for mechanical properties was adopted in this work. Simulation results show that tensile strength increases dramatically with an increase in the number of blocks within “hard-soft” multiblock copolymers. This phenomenon can be described by the occurrence of bridging and looping chain conformations in experiment. One-dimensional lamellae were built to provide an ideal morphology for studying the influence of lamellar orientation on multiblock copolymer mechanical properties. During tensile tests different failure processes were observed with two kinds of interface strength that corresponded to a difference in chain structures (diblock, triblock or multiblock copolymers). These studies provide an efficient method for correlating the complex morphologies to the mechanical response of multiblock copolymers.     

Microstructures of lamella-forming diblock copolymer melts under nanorod-array confinements

The microstructures of lamellae-forming diblock copolymer melts confined in nanorod arrays are investigated using the real-space self-consistent field theory. The nanorod array leads to the incomplete confinement at each direction so that the confinement-dimension is fractional between zero and two. This incomplete confinement can yield a rich variety of mixture microstructures by varying its fractional confinement-dimension, such as the mixture of concentric lamellae and parallel lamellae and the mixture of the concentric lamellae and cylinders, as well as a series of continuous network mixtures. By comparing with the available simulations and experiments, these novel microstructures can be understood based on the symmetry competition and structural frustration that originated from the incomplete confinement. Our theoretical predictions may be helpful to the design of nanomaterials.     

Internal morphologies of diblock copolymer nanorods fabricated from regular and irregular pores of anodized aluminum oxide templates

Internal nanostructures in nanorods of polystyrene-poly(4-vinyl pyridine) (PS-PVP) diblock copolymers fabricated from pores having regular or irregular contours in AAO templates were investigated by cross-sectional transmission electron microscopy. When nanorods of PS-PVP copolymers were produced from pores with regular contours, a typical morphology of concentric cylinders was observed due to the strong affinity of the PVP block to the surface of the AAO pores. In the case of PS-PVP nanorods obtained from pores with irregular contours, a concentric cylindrical morphology was not induced. Instead, a nanostructure of lamellae mostly parallel to the axis of the nanorods was observed with a similar period of lamellae to the bulk lamellar period. In addition, nanorods having a functional coaxial nanostructure were fabricated by synthesizing Au nanoparticles in concentric cylinders of the nanorods to demonstrate the utilization of self-assembled internal nanostructures in nanorods.     

Self-assembly of rod-terminally tethered three-armed star-shaped coil block copolymer: Investigation of the presence of the branching in the coil to the self-assembled behavior

Self-assembled behavior of rod-terminally tethered three-armed star-shaped coil block copolymer melts was studied by applying self-consistent-field lattice techniques in three-dimensional (3D) space. Similar to rod-coil diblock copolymers, five morphologies were observed, i.e., lamellar, perforated lamellar, gyroidlike, cylindrical and sphericallike structures, while the distribution of the morphologies in the phase diagram was dramatically changed with respect to that of rod-coil diblock copolymers. The perforated lamella was replaced by the cylinder when f_rod = 0.45, and the lamella was replaced by the perforated lamella when f_rod = 0.5 when the arms A₁ and A₂ had an equal length and the volume fraction of A₃ arm was low enough. Simulations were also performed when the arms A₁ and A₂ had unequal lengths. These results demonstrate that simple branching in the coil induces interesting microphase transitions.     

Macroscopically oriented lamellar microdomains created by “cold zone-heating” method involving OOT

The zone-heating method, which involves ordering of materials under moving temperature gradient ∇T, has been widely used as a technique to create macroscopically oriented ordered structures of various kinds of materials. We applied this method to a large molecular weight symmetric polystyrene-block-polyisoprene diblock copolymer (dibcp) at a temperature where the ∇T field exists below its order-disorder transition temperature T_ODT of the dibcp. In this method we first prepared the solvent-cast bulk films of the dibcp having a nonequilibrium morphology of hexagonally packed cylindrical microdomains (hex-cyl) by using a solvent selectively good for polyisoprene blocks. Then the zone-heating method was applied to the order-order transition process from the nonequilibrium hex-cyl to equilibrium lamellae. The “cold zone-heating” method, “cold” in the sense of the ∇T field existing below T_ODT, successfully created macroscopically oriented lamellae with their normals preferentially oriented parallel to the ∇T axis and their edges preferentially standing with respect to the bulk film surfaces. It was also found that the initial orientation of the (100) plane of hex-cyl normal to the ∇T axis prefers to that parallel to the ∇T axis for a better macroscopic alignment of lamellae. A possible model for the cold zone-heating-induced lamellar orientation will be discussed in the text.     

Nanoscale localization of poly(vinylidene fluoride) in the lamellae of thin films of symmetric polystyrene-poly(methyl methacrylate) diblock copolymers

We employed thin film blends of diblock copolymers with functional homopolymers as a simple strategy to incorporate organic functional materials into nanodomains of diblock copolymers without serious synthesis. A blend pair of polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymers and poly(vinylidene fluoride) (PVDF) was selected as a model demonstration because PVDF is a well-known ferroelectric polymer and completely miscible with amorphous PMMA. Thin films of symmetric PS-PMMA copolymers provided the nanometer-sized PMMA lamellae, macroscopically parallel to the substrate, in which PVDF chains were dissolved. Thus, amorphous PVDF chains were effectively confined in the PMMA lamellae of thin film blends. The location of PVDF chains in the PMMA lamellae was investigated by the dependence of the lamellar period on the volume fraction of PVDF, from which we found that PVDF chains were localized in the middle of the PMMA lamellae. After the crystallization of PVDF, however, some of PVDF migrated to the surface of the film and formed small crystallites.     

Soft nanoconfinement effects on the crystallization behavior of asymmetric poly(ethylene oxide)‐block‐poly(ε‐ caprolactone) diblock copolymers

The time‐ and temperature‐related crystallization process for the structure transitions of asymmetric crystalline‐crystalline diblock copolymers from the melt to crystallites was investigated with synchrotron simultaneous small‐angle/wide‐angle X‐ray scattering. Two asymmetric poly(ethylene oxide)‐poly(ε‐caprolactone) diblock copolymers were chosen. It is found in the course of the copolymer crystallization that the shorter blocks are uncrystallizable in both of the asymmetric diblock copolymers and final lamellar structures are formed in both of them. The final lamellar structure was confirmed from atomic force microscopy observations. The small‐angle X‐ray scattering data collected were analyzed with different methods for the early stage of crystallization. Guinier and Debye‐Bueche plots indicate that there are neither isolated domains nor correlated domains formed before the formation of lamellae in the asymmetric diblock copolymers during the crystallization process. The structure evolution was calculated according to the correlation function, and the soft nanoconfined crystallization behavior is discussed. Copyright © 2012 Society of Chemical Industry     

NanoSIMS: Insights into the Organization of the Proteinaceous Scaffold within Hexactinellid Sponge Spicules

The giant basal spicules (GBS) from Monorhaphis chuni (Porifera [sponges], Hexactinellida) represent the largest biosilica structures on Earth and can reach lengths of 300 cm (diameter of 1.1 cm). The amorphous silica of the inorganic matrix is formed enzymatically by silicatein. During this process, the enzyme remains trapped inside the lamellar‐organized spicules. In order to localize the organic silicatein scaffold, the inside of a lamella has been analyzed by nano‐secondary ion mass spectrometry (NanoSIMS). It is shown that the GBSs are composed of around 245 concentrically arranged individual siliceous lamellae. These surround an internal siliceous axial cylinder. The lamellae adjacent to the cylinder are thicker (10–30 μm) than the more peripheral lamellae (2–10 μm). One lamella of a thickness of 18 μm has been selected for further analysis. This lamella itself is composed of three sublamellae with an individual thickness of 2–6 μm each, which are then further organized into three cylindrical slats (thickness: 1.6–1.8 μm). Other than the main lamellae, the sublamellae are not separated from each other by gaps. The element analysis of the sublamellae by NanoSIMS revealed that the siliceous matrix is embedded in an organic matrix that consists of up to 6–10 wt/% of C. The pattern of C distribution reflects a distinct zonation of the organic material within the solid intralamellar biosiliceous material. A growth model for the lamella starting from nanosized silica particles is proposed: During formation of a lamella nanosized silica particles fuse, through biosintering processes, to slats that build the individual sublamellae, which then finally form the lamellae. In turn, those lamellae may form the higher structural entity, the axial cylinder.     

Spontaneous origination of chirality in melts of diblock copolymers with rigid and flexible blocks

A self-consistent mean-field computer simulation of ordering in melts of diblock copolymers consisting of flexible and rigid rodlike blocks is performed. A three-dimensional model is considered, and a corresponding algorithm for solving mean-field equations in sequential and parallelized versions is developed. The coexistence of microphase separation and orientational ordering gives rise to the appearance of new types of spatial arrangements. In particular, phases with the cubic symmetry and the morphology of hexagonally arranged chiral cylinders are found. The transition of achiral cylinders to chiral cylinders in the melt of achiral diblock copolymers consisting of rigid and flexible blocks is revealed for the first time. The origination of chirality is due to the presence of rigid blocks in the system and orientational interactions between them. With a decrease in temperature, microphase separation caused by incompatibility of chemically different blocks initially occurs in these systems. As a result, the hexagonally ordered structure in which rigid blocks are concentrated in cylindrical microdomains arises. A further decrease in temperature results in the involution of cylindrical microdomains and the formation of a helical structure. To quantify the degree of chirality, a new pseudoscalar index, depending on the linear-scale parameter for which the chirality is studied, is suggested.     

Formation of lamellar structure by competition in crystallization of both components for crystalline-crystalline block copolymers

Crystallization and structure formation of poly(ethylene oxide)-poly(?-caprolactone) block copolymers (PEG-PCL) in which the melting temperatures of the components are close to each other were elucidated using differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. The diblock copolymers with 33, 46 and 59 wt% of PEG composition formed ordinary single spherulites similar to those of PCL homopolymers, while concentric double-circled spherulites appeared for the PCL-PEG-PCL triblock copolymer with 66 wt% PEG composition as observed previously. For the diblock copolymers, despite of the ordinary appearance of the single spherulites, the DSC thermograms and the WAXD patterns indicated the crystallization of PEG as well as PCL. The time-resolved SAXS profiles for the diblock copolymers showed that long spacings of the crystal lamellae decreased stepwise in the crystallization process. Synthesizing these results for the single spherulites, it was concluded that PCL crystallized first followed by the crystallization of PEG with preservation of the PCL crystal lamellar structure. This means that PEG must crystallize within confined space between the formerly formed PCL crystal lamellae. Such confined crystallization of PEG caused the suppressed melting temperature, crystallinity and crystallization rate especially in the smaller PEG compositions. In the melting process of the diblock copolymers, it was observed that the PEG component first melted with a stepwise increase in the long spacing.     

Morphologies of diblock copolymer confined in a slit with patterned surfaces studied by dissipative particle dynamics

Diblock copolymers with ordered mesophase structures have been used as templates for nano-fabrication. Unfortunately, the ordered structure only exists at micromete rscale areas, which precludes its use in many advanced applications. To overcome this disadvantage, the diblock copolymer confined in a restricted system with a patterned surface is proved to be an effective means to prohibit the formation of defects and obtain perfect ordered domains. In this work, the morphologies of a thin film of diblock copolymer confined between patterned and neutral surfaces were studied by dissipative particle dynamics. It is shown that the morphology of the symmetric diblock copolymer is affected by the ratio of the pattern period on the surface to the lamellar period of the symmetric diblock copolymer and by the repulsion parameters between blocks and wall particles. To eliminate the defects in the lamellar phase, the pattern period on the surface must match the lamellar period. The difference in the interface energy of different compartments of the pattern should increase with increasing film thickness. The pattern period on the surface has a scaling relationship with the chain length, which is the same as that between the lamellar period and the chain length. The lamellar period is also affected by the polydispersity of the symmetric diblock copolymer. The total period is the average of the period of each component multiplied by the weight of its volume ratio. The morphologies of asymmetric diblock copolymers are also affected by the pattern on the surface, especially when the matching period of the asymmetric diblock copolymer is equal to the pattern period, which is approximately equal to the lamellar period of a symmetric diblock copolymer with the same chain length.     

Synthesis, characterization and modification of azide-containing dendronized diblock copolymers

A series of dendronized diblock copolymers having rigid backbone and reactive surface were synthesized by ring-opening metathesis polymerization (ROMP) from dendronized norbornene derivatives using the second generation Grubb's catalyst. The bromine-terminated block of those rigid nanostructures has been converted to more reactive azide groups in one straightforward step. The resulting polymers were then functionalized by post-polymerization reaction with fullerene C₆₀ (electron acceptor) using thermal [3 + 2] cycloaddition reaction or with porphyrin (electron donor) using copper-catalyzed “click chemistry”, the ultimate goal being the preparation of efficient polymeric materials for photovoltaic applications. While fullerene addition was not complete (approximately 50%) because of cross-linking reactions and steric hindrance on the dendrimers surface, Zn-porphyrin introduction went to completion clearly demonstrating the usefulness of click chemistry for polymer functionalization.     

Polystyrene-poly(2-ethylhexylmethacrylate) block copolymers: Synthesis,bulk phase behavior,and thin film structure

Anionic polymerization was employed to synthesize well-defined diblock copolymers of polystyrene and poly(2-ethylhexylmethacrylate), PS-PEHMA. Diblock morphologies in bulk and in substrate-supported thin films were characterized by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM), respectively. PS-PEHMA diblocks exhibited thermotropic order-disorder transitions; one diblock showed a thermoreversible transition between lamellae and a higher-temperature morphology assigned as perforated lamellae. Unlike PS-poly(alkylmethacrylate) diblocks where the alkyl group is n-butyl or n-pentyl, PS-PEHMA diblocks showed a typical decreasing Flory interaction parameter with increasing temperature. Thin films of PS-cylinder-forming PS-PEHMA diblocks showed a strong preference for the cylinders to lie in the plane of the film; films of incommensurate thickness readily formed terraces. Films of commensurate thickness were easily aligned over macroscopic areas through the application of mechanical shear.     

Syntheses and characterizations of the multiple morphologies formed by the self-assembly of the semicrystalline P4VP-b-PCL diblock copolymers

A series of semicrystalline diblock copolymers of poly(4-vinylpyridine-b-?-caprolactone) (P4VP-b-PCL) have been synthesized by the living ROP of CL followed by the TEMPO polymerization of 4-VP. Depending on the relative block length and different solvent compositions, these copolymers self-assemble into different supramolecular structures in toluene/dichloromethane (DCM) solution, including spherical micelles, bowl-shaped vesicles, multilayer vesicles, porous spheres, and large compound micelles. In methanol/DCM solution system, the crystalline PCL core disturbs the balance of free energy, thus results in a series of morphological changes including spherical micelles, worm-like rods, vesicles, coexisted vesicles and lamella, and finally platelet lamella.     

The participation of block copolymers in the increase in the number of tie molecules

Three-block copolymers polyamide-block-polyX-block-polyamide (where X is styrene, ethylene-co-1-butene, and others) are able to improve substantially the toughness of poly(ϵ-caprolactam). Outer blocks of these copolymers are able to take part in folding lamellae of semi-crystalline polyamide, inner blocks are not. They operate as a spacer, meaning that the end blocks of copolymers capable of folding become parts of different lamellae, or that the unfoldable part of a block copolymer is pushed out of a folding lamella with a certain probability of the remaining outer block taking part in folding another lamella. This increases the number of tie-molecules.     

Sphere-forming diblock copolymers confined in a square well

By means of the mean-field dynamic density functional theory (DDFT), the self-assembly behavior of the sphere-forming diblock copolymers confined in the square well has been studied systematically. Various novel structures are predicted, such as a string of spheres, stacked square bricks, a row of elongated potato-like micelles rotated with respect to each other, a single helix, two staggered helices, well-ordered square array of cylinders, highly ordered body-center-cubic (BCC) spheres, highly ordered face-center-cubic (FCC) spheres and concentric layers. Those morphologies provide the theoretical evidence for the experimental observations. Moreover, some interesting phenomena are found, such as the decrease of the number of layers with the increase of the surface field and the formation of the soft spheres, which can be observed in the experiments. Furthermore, a new order parameter is defined to characterize the degree of micro-phase separation for each kind of microscopic structure, and the key factor in affecting micro-phase separation is discussed.     

Self‐assembled nanostructures of diblock copolymer films under homopolymer topcoats

To mitigate the interface energies of block copolymers with high interaction parameters, a topcoat strategy has been experimentally proposed to produce perpendicular oriented domains on a length scale of sub‐10 nm. However, the origin of perpendicular oriented domains and the effect of topcoats on the self‐assembled nanostructures remain to be uncovered. Herein, we use the dynamic self‐consistent field theory to explore the self‐assembly behaviors of symmetric block copolymer films under homopolymer topcoats. It is clearly demonstrated that the introduction of homopolymer topcoats enables a wide formation range of perpendicular oriented lamellae, originating from the fluctuating diblock copolymer/homopolymer interfaces in the process of in‐plane microphase separation of block copolymer films. Our simulation results also demonstrate that the formation range of perpendicular oriented lamellae can be tuned by changing the wetting properties of homopolymer topcoats and the thickness of block copolymer films, but is weakly dependent upon the chain length of homopolymers. Our theoretical findings have wide implications for understanding the formation of perpendicular oriented domains of block copolymer films, which are important for the rational design of self‐assembled nanostructures with new horizons for block copolymer lithography. © 2020 Society of Chemical Industry