Phase separation of polystyrene-b-(ethylene-co-butylene)-b-styrene (SEBS) deposited on polystyrene thin films

Phase separation of a triblock copolymer, polystyrene-b-(ethylene-co-butylene)-b-styrene (SEBS) on the thin films of a homopolymer, polystyrene (PS), was studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The final morphology after phase separation was found to be greatly dependent on the relation between the molecular weight of the PS block and homo-PS. Dispersed spherical and worm-like micelles of SEBS were observed when the molecular weight of homo-PS is smaller than the PS block in SEBS, while large structures with inner micro-phase separation of SEBS was found when the molecular weight of homo-PS was much higher than that of the PS block. The origin of such a change in morphology is attributed to the difference of structure and interfacial tension at the interface between the matrix homo-PS and the PS block in SEBS triblock copolymer assembly.     

Nano-phase structures and mechanical properties of epoxy/acryl triblock copolymer alloys

Phase structures and mechanical properties of epoxy/acryl triblock copolymer alloys using several curing agents were studied. PMMA-b-PnBA-b-PMMA triblock copolymers synthesized by living anionic polymerization were applied as the toughening modifiers for the epoxy resins. An aromatic amine, an acid anhydride and an anionic polymerization catalyst as curing agents resulted in macro-phase separation in the epoxy/triblock copolymer blends during the cure process. However, a phenol novolac as the curing agent created nano-phase structures in the epoxy blends. The size of the spherical phases or cylindrical phases was about 40 nm in diameter, and the main component in the nano-phases was the PnBA of the triblock copolymer. The fracture toughness of the epoxy/triblock copolymer alloys with the nano-cylindrical phases reached 2530 J/m². The fracture toughness was more than twenty fold relative to the unmodified epoxy resin, and was equivalent to the toughness of polycarbonates.     

Effect of composition of added random copolymer on the phase behavior of block copolymer

Blends of styrene-butadiene diblock copolymer (S-B, 52 wt% styrene content) and styrene-butadiene random copolymer (SBR) of various styrene compositions were studied by small-angle X-ray scattering, light scattering, and transmission electron microscopy. The composition of random copolymer plays an important role in the solubilization of SBR in S-B. The order-disorder transition temperature, T_ODT, decreases linearly with the addition of SBR. T_ODT decreases as the symmetry in SBR composition increases and shows the highest value in the case of homopolymers. Asymmetric butadiene-rich SBR dissolves mostly into PB microdomain of S-B to increase lamella microdomain spacing, D, and its addition makes the overall microdomains of S and B in the mixture more asymmetrical. Symmetric SBR is localized into the interface of S-B microdomain to reduce unfavorable S-B contact at the interface. The phase diagram for S-B containing asymmetric SBR shows a succession of mixed mesophases of different morphologies from lamellae and cylinder to disordered liquid phases, whereas the phase diagram containing symmetric SBR shows two homogeneous phases and one region of two-phase coexistence, where macroscopically separated phases coexist together.     

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter E_mB_n block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E₂₂₄B₁₁₄ exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.     

Lamella reorientation in thin films of a symmetric poly(l-lactic acid)-block-polystyrene upon crystallization at different temperatures

The thin films of a symmetric crystalline-coil diblock copolymer of poly(l-lactic acid) and polystyrene (PLLA-b-PS) formed lamellae parallel to the substrate surface in melt. When annealed at temperatures well above the glass transition temperature of PLLA block (T_g^PLLA), the PLLA chains started to crystallize, leading to reorientation of lamellae. Such reorientation behavior exhibited dependence on the correlation between the crystallization temperature (T_c), the glass transition temperature of PS (T_g^PS), the peak melting point of PLLA crystals (T_m^PLLA), and the end melting point of PLLA crystals (T_m,end^PLLA). When annealed at (T_c=) 80 °C (T_c < T_g^PS < T_ODT, order-disorder transition temperature), 123 °C (T_g^PS < T_c < T_m^PLLA < T_ODT), 165 °C (T_g^PS < T_m^PLLA < T_c < T_m,end^PLLA < T_ODT), the parallel lamellae became perpendicular to the substrate surface, exclusively starting at the edge of surface relief patterns. Meanwhile, the corresponding lamellar spacing was significantly enhanced. The PLLA crystallization between PS layers was hypothesized to account for the lamella reorientation during annealing. The crystallization, chain conformation, and possible chain folding mechanisms were discussed, based on detailed analysis of the lamellar structure before and after crystallization.     

Nine-fold density multiplication of hcp lattice pattern by directed self-assembly of block copolymer

Block copolymer assembly directed by electron beam (EB) lithography enhances both resolution and throughput of the EB-generated patterns and provides a feasible path to fabricating master molds of nanometer scale patterns over macroscopic areas. In our previous paper [27], we demonstrated that the self-assembly process can interpolate points in between the EB-generated pattern, thus attaining four-fold density multiplication. Here, we report a nine-fold feature density multiplication can be attained by the directed block copolymer assembly. The equilibrium formation of perpendicular cylindrical domains in registration with the pre-patterned surface is confined within a narrow thickness range once all other parameters are fixed as found in a four-fold feature density multiplication. The tolerance of the lattice mismatch between chemical pattern and d spacing of domains for nine-fold feature density multiplication is smaller than that for four-fold feature density multiplication. We also found that the critical dimension formed by the block copolymer domains is independent of that defined by the EB pre-patterned features.     

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.     

Perpendicular lamellae induced at the interface of neutral self-assembled monolayers in thin diblock copolymer films

We obtained perpendicular lamellar orientations in thin films of symmetric polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, on self-assembled monolayers (SAMs) of 3-(p-methoxyphenyl)propyltrichlorosilane (MPTS) prepared on silicon wafers. In contrast to completely parallel lamellae on silicon wafers having a native oxide layer, perpendicular lamellae at the MPTS interface with parallel lamellae at the air interface were directly observed by transmission electron microscopy (TEM) in cross-sectional view. The perpendicular lamellae at the MPTS interface were attributed to the non-preferential (neutral) MPTS-covered substrate to both PS and PMMA blocks. The neutrality of the SAMs of MPTS was confirmed by the similar interfacial tension values of the SAMs of MPTS with PS and PMMA, estimated by contact angle measurements.     

Synthesis and characterization of block copolythiophene with hexyl and triethylene glycol side chains

A well-defined diblock copolythiophene, poly(3-hexylthiophene)-b-poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methylthiophene) (P3HT-b-P3TEGT) was successfully synthesized by a Grignard metathesis (GRIM) polymerization. The structure of the block copolymer was confirmed by size exclusion chromatography (SEC), ¹H NMR spectroscopy, differential scanning calorimetry (DSC), ultraviolet-visible (UV-vis) spectroscopy and fluorescence spectroscopy. The self-assembled structure of the block copolythiophene was investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and synchrotron grazing incidence X-ray scattering (GIXS). These analyses found that the block components in films form vertically oriented lamellar structure via phase separation. Furthermore, both the phases were found to consist of molecular multi-layers respectively, in which the layers stack in the out-of-plane of the film. The P3HT phase exhibited crystalline, which is originated from the π−π stacked thiophene backbones. In contrast, the poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methylthiophene) (P3TEGT) phase revealed amorphous. Overall, the amphiphilic nature of P3HT-b-P3TEGT successfully demonstrated to lead to well-defined self-assembly structure in films.     

Ordering and microdomain orientation in block copolymer films by thermal deprotection

Ordering and microdomain orientation for the films of symmetric polystyrene-b-poly(tert-butyl methacrylate)s (PS-b-PtBMAs) was investigated by in-situ grazing incidence small-angle X-ray scattering (GISAXS) and the electron microscopy. During thermal deprotection at higher temperature (200 °C), functional tert-butyl ester units in the PtBMA block component are integrated into inter- or intra-molecular anhydride linkages. It was observed that this process causes an increase in the Flory-Huggins interaction parameter (χ) between the two block components for disordered PS-b-PtBMA film, leading to a modulated nonequilibrium structure. Interestingly, for lamella-forming PS-b-PtBMA film, a significant chain stretching in lateral direction during thermal deprotection resulted in a characteristic strain-induced perpendicular orientation in the middle of the film confined between two parallel orientations of lamellar microdomains.     

Time-resolved GISAXS and cryo-microscopy characterization of block copolymer membrane formation

Time-resolved grazing-incidence small-angle X-ray scattering (GISAXS) and cryo-microscopy were used for the first time to understand the pore evolution by copolymer assembly, leading to the formation of isoporous membranes with exceptional porosity and regularity. The formation of copolymer micelle strings in solution (in DMF/DOX/THF and DMF/DOX) was confirmed by cryo field emission scanning electron microscopy (cryo-FESEM) with a distance of 72 nm between centers of micelles placed in different strings. SAXS measurement of block copolymer solutions in DMF/DOX indicated hexagonal assembly with micelle-to-micelle distance of 84–87 nm for 14–20 wt% copolymer solutions. GISAXS in-plane peaks were detected, revealing order close to hexagonal. The d-spacing corresponding to the first peak in this case was 100–130 nm (lattice constant 115–150 nm) for 17 wt% copolymer solutions evaporating up to 100 s. Time-resolved cryo-FESEM showed the formation of incipient pores on the film surface after 4 s copolymer solution casting with distances between void centers of 125 nm.     

Self-Assembly structures through competitive interactions of miscible crystalline-amorphous diblock copolymer/homopolymer blends

A series of miscible crystalline-amorphous diblock copolymers, (poly(?-caprolactone)-b-(vinyl phenol), PCL-b-PVPh) were prepared through sequential ring-opening and controlled living free radical (nitroxide-mediated) polymerizations and then blended with poly(vinyl pyrrolidone) (PVP) homopolymer. Specific interactions, miscibility, and self-assembly morphologies mediated by hydrogen bonding interactions of this new A-B/C type blend, were investigated in detail. Micro-phase separation of these miscible PCL-b-PVPh diblock copolymers occurs by blending with PVP through competitive hydrogen bonding interaction in this A-B/C blend. FTIR, XRD, and DSC analyses provide positive evidences that the carbonyl group of PVP is a significantly stronger hydrogen bond acceptor than PCL, thus the PCL block is excluded from the PVPh/PVP miscible phase to form self-assembly structure. ¹³C CP/MAS solid-state NMR spectra provide additional evidence confirming that micro-phase separation occurs in the blend system because of the presence of more than two T_1ρ(H) values for this A-B/C blend system. According to the result of the FTIR and SAXS results, the smaller molecular weight system contains a greater fraction of the hydrogen-bonded carbonyl group, cause indirectly the high degree of phase separation among these blends. In addition, the SAXS profiles possess a sharp primary peak and highly long range ordered reflections q/q^∗ ratios of 1:2:3 at lower PVP content, an indication of the lamellar structure in the blend which is consistent with TEM image. The phase behavior and morphology shifts from lamellar to cylinder structure with further increase in the PVP content.     

Microphase separation and crystallization in mixtures of polystyrene-poly(methyl methacrylate) diblock copolymer and poly(vinylidene fluoride)

This study examined the microdomain structures and the crystallization behavior in binary blends consisting of an asymmetric block copolymer and a homopolymer using small-angle X-ray scattering, optical microscopy and differential scanning calorimetry. A polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) was mixed with a low molecular weight poly(vinylidene fluoride) (PVDF), where the PS-b-PMMA had a 0.30 wt fraction of the PMMA block. At a PVDF concentration of <13.0 wt%, the PVDF was completely miscible with the PMMA microdomains, and the blends had a cylindrical structure. The addition of PVDF altered the morphology from a PMMA-cylindrical structure to a lamellar structure and finally to a PS-cylindrical structure. When the PVDF concentration was <23.0 wt%, the PVDF was distributed uniformly within the PMMA microdomains. After adding more PVDF, some of the PVDF was locally dissolved in the middle of the PMMA microdomains. The addition of PVDF also affected the ordered microstructure in the blends, leading to a well-defined microdomain structure. However, PVDF crystallization significantly disturbed the pre-existing microdomain structure, resulting in a poorly ordered morphology. In the blends, PVDF had unique crystallization behavior as a result of the space constraints imposed by the microdomains.     

(Polystyrene-g-polyisoprene)-b-polystyrene comb-coil block copolymer in selective solvent

Comb-coil block copolymers consisting of two components polystyrene (PS) and polyisoprene (PI) were synthesized through combining TEMPO living free radical polymerization (LFRP) and anionic polymerization using “grafting-onto” strategy. Two typical samples with the same backbone but grafted with different numbers and lengths of branches, forming lamellar and cylinder phases, respectively, were investigated. As the selective solvent was added into these block polymers, the micro-structures transformed from disordered or weakly ordered structure into well-organized lamellar structure in the intermediate polymer concentration. The power law behavior of the lamellae space with respect to polymer concentration indicates that the two samples form the well-organized structure through different paths. The sample with longer branch length thus less branch points forms lamellae phase with smaller lamellae space. This difference at lamellae spacing is attributed to the two different ways that the chain assembles.     

Micro- or nanoseparated phases in thermoset blends of an epoxy resin and PEO-PPO-PEO triblock copolymer

Diaminodiphenylmethane (DDM) curing at several temperatures of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin modified with a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) block copolymer has been investigated in order to characterize the miscibility and morphological features. Two distinct phases are present for every blends studied except for DGEBA/DDM modified with 10 wt% PEO-PPO-PEO and cured at low temperature. Depending on the curing condition, phase separation takes place at micro or nanoscale due to competition among kinetic and thermodynamic factors. The mechanistic approach used for modeling the curing reactions shows that the formation of epoxy-hydroxyl complex and the auto catalytic process are slightly decreased whilst the noncatalytic process is favoured upon copolymer addition. Modifier addition delays curing process as the influence of both formation of epoxy-hydroxyl complex and catalytic process on reaction rate is higher than the influence of noncatalytic process. A thermodynamic model describing a thermoset/block copolymer considered as only one entity system is proposed. The LCST behaviour allows to elucidate nano or micro separated structures obtained at low and high curing temperatures, respectively.     

Block copolymer nanoshells

With the help of cell dynamics simulation we investigate morphology of thin block copolymer film around a nanoparticle. The obtained structures include: parallel, perpendicular, mixed and perforated lamellae, parallel and perpendicular cylinders and spheres. Analogy and difference with planar films are discussed. Our simulation suggests that novel porous nanocontainers can be formed by the coating of a sacrificial nano-bead by a block copolymer layer with a well controlled nanostructure.     

Construction of the state diagram of polymer blend thin films using differential AC chip calorimetry

The phase separation behavior of polymer blend thin films of 100-150 nm was studied using differential AC Chip calorimetry. By taking advantage of the low sensor and sample mass inherent to chip calorimetry, a new methodology based on temperature jumps was developed. This methodology allowed the construction of the state diagram of polymer blend thin films as evidenced for two model systems (PVME/PS and PVME/Phenoxy) displaying a lower critical solution temperature behavior.The state diagram in thin films was compared to the one obtained in bulk using Modulated Temperature DSC. In comparison with bulk, a lower phase separation temperature and a broadening of the homogeneous glass transition temperatures is observed for both model systems. This might be an indication of a surface induced ‘destabilization’ by composition gradients which are not present in bulk.     

Diblock copolymer healing

The issue of self-healing materials is of paramount importance due to its intrinsic scientific value, as well as potential applications in a wide variety of fields, such as manufacturing, medicine, and electronics. We have investigated the behavior of poly(styrene-b-methylmethacrylate) diblock copolymer, following deformation performed by a silicon atomic force microscopy tip. We observed the changes in the polymer as it was subsequently heated in situ, and found how diblock “scars” can heal. These observations give important guidance to efforts that seek to create nanostructures using these methods, while also revealing fundamental insights into the mechanisms of polymer repair on the nanoscale.     

Crystallization and coalescence of block copolymer micelles in semicrystalline block copolymer/amorphous homopolymer blends

Crystallization of two oxyethylene/oxybutylene block copolymers (E₇₆B₃₈ and E₁₅₅B₇₆) from micelles in block copolymer/amorphous homopolymer blends was studied by differential scanning calorimetry (DSC) and time-resolved small angle X-ray scattering (SAXS). Unlike the simultaneous crystallization and formation of superstructure in crystallization from an ordered structure, crystallization of block copolymer from micelles can be divided into two steps. The core of the micelles firstly crystallizes individually, with first-order crystallization kinetics and homogeneous nucleation mechanism. The SAXS revealed that crystallization-induced deformation occurs for the micelles, which strongly depends on microstructure of the block copolymers. For the shorter block copolymer E₇₆B₃₈, larger deformation induced by crystallization was observed, leading to coalescence of the micelles after crystallization, while for the longer block copolymer E₁₅₅B₇₆ the micelles show little deformation and the morphology of micelle is retained after crystallization.     

Flow-induced stripe pattern formation in phase-separating fluids

We demonstrated that the introduction of a temperature gradient along the free surface induces a particular stripe pattern in phase-separating fluids. The horizontal temperature gradient drove lateral-periodic spiral liquid motion flowing from warmer to cooler places due to thermocapillarity. Properly chosen polymer compositions and initial film thicknesses in ternary solutions allowed us to promote a phase separation in the presence of spiral flow, which assembled the demixed polymer droplets along the flow-stagnation lines. The resulting assembled phases aligned in the temperature gradient direction and eventually formed periodic polymer stripes involving the same spacing as that of the flow axis. The critical condition for the stripe pattern formation was given by the ratio of two relevant film thicknesses, i.e. the thickness for the onset of the phase separation and that for the cessation of liquid motion.