Lamellar Microdomains Research Articles

Thin Films of Semifluorinated Block Copolymers Prepared by ATRP

AbstractA symmetric diblock copolymer, poly(pentafluorostyrene‐b‐styrene) (PPFS‐b‐PS), is synthesized by atom transfer radical polymerization (ATRP). The behavior of PPFS‐b‐PS thin films on silicon wafers is investigated. Lamellar microdomains oriented parallel to the film surface are formed when the thin films are heated to 220 °C for 24 h, due to preferential interfacial interactions and the low surface energy of the PPFS. An electric field is used to overcome the low surface energy of the fluorinated block and preferential interactions with the substrate so as to force the lamellar microdomains to orient normal to the surface.

Phase Behavior of Binary Blends of Block Copolymers Having Hydrogen Bonding

Phase behavior of binary blends consisting of high molecular weight polystyrene-block-poly(2-vinylpyridine) copolymer (PS-b-P2VP) and low molecular weight polystyrene-block-poly(4-hydroxystyrene) copolymer (PS-b-PHS) was investigated by using small-angle X-ray scattering and transmission electron microscopy. Both PS-b-P2VP and PS-b-PHS exhibited lamellar microdomain over the entire experimental temperatures up to 300 °C. When the weight fraction of PS-b-PHS in the blend was less than 0.1, the lamellar microdomains were maintained. However, with increasing amount of PS-b-PHS, the microdomains in the blends were transformed to hexagonally packed (HEX) cylindrical microdomains and body-centered cubic (BCC) spherical microdomains. On the other hand, when a relatively high molecular weight of PS-b-PHS was used, the BCC spherical microdomains were not observed even at a large weight fraction of PS-b-PHS in the blend, but HEX cylindrical microdomains were formed. The phase behaviors observed experimentally were rationalized by the results of the self-consistent mean-field theory. Based on the theoretical results, some of the PS-b-PHS dissolves into P2VP microdomains and thereby increases the effective volume of P2VP/PHS phase because of hydrogen bonding between P2VP and PHS blocks. In the case of smaller molecular weight of PS-b-PHS, the dissolved amount is significant because the favorable interactions of P2VP/PHS chains becomes dominant over the relatively small enthalpic penalty between PS/P2VP in the P2VP microdomains as well as the gain of the translational entropy of the PS-b-PHS chains. The increase in the effective volume of the PHS/P2VP phase leads to a transformation of the microdomains in the blends from lamellar to HEX cylindrical microdomains and BCC spherical microdomains. On the other hand, when a relatively higher molecular weight of PS-b-PHS was used, the effective volume of the P2VP/PHS phase was not increased greatly because of a large enthalpic loss arising from the contact between PS/P2VP as well as a small gain in the translational entropy. Consequently, only lamellar and HEX cylindrical microdomains are observed.

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-P tBMAs) 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 P tBMA 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-P tBMA film, leading to a modulated nonequilibrium structure. Interestingly, for lamella-forming PS- b-P tBMA 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.

Critical Analysis of the Crystal Orientation Behavior in Polyethylene-Based Crystalline−Amorphous Diblock Copolymer

Orientation of polyethylene (PE) crystals formed over a broad range of undercooling in a polyethylene-block-poly(D,L-lactide) (PE-b-PDLLA) diblock copolymer has been critically examined. Due to the large segregation strength and approximate 50/50 composition in this system, the crystallization took place within one-dimensionally confined lamellar microdomains without forming spherulites. A homogeneous crystal orientation with the PE crystalline stems orienting parallel to the lamellar interface was observed at crystallization temperatures (T(c)) between 45 and 102 °C. Once the sample was crystallized at T(c) ≤ 40 °C or directly quenched into liquid nitrogen from the melt, the isotropic WAXD pattern indicated that the PE crystals became randomly oriented. Considering that homeotropic orientation was not identified for the present system and other PE-based crystalline-amorphous diblocks, we concluded that PE crystals in the lamellar microdomains always show homogeneous orientation when there is a preferred orientational order. We further organized the thermodynamic and kinetic factors that may govern the preferred crystal orientation in diblock copolymers and concluded that the orientational order should be controlled by the competition between nucleation and crystal growth kinetics. The persistence of homogeneous orientation of PE was attributed to its excellent nucleating power.

Perpendicular orientation of microdomains in PS-b-PMMA thin films on the PS brushed substrates

The microdomain orientation in thin films of cylinder- and lamella-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) copolymers on polystyrene (PS) brushed (or grafted) substrates was studied using scanning force microscopy (SFM) and grazing incidence small-angle X-ray scattering (GISAXS), where the grafting density (σ) of the underlying PS layers are controlled. The perpendicular orientation for cylindrical and lamellar microdomains in PS-b-PMMA films was found when the grafting density range of the PS brush was adjusted, which is presumably caused by the surface heterogeneity of the effective PS brush chains on the PS brushed substrates, rather than the balanced interfacial interactions. These results suggest that the homopolymer brushed substrates of the same chemical identity with one component of BCP may serve as a facile means to achieve a desired microdomain orientation of BCP films by controlling σ of the underlying polymer layers.

Existence of microdomain orientation in thermoplastic elastomer through a case study of SEBS electrospun fibers

Although the microdomains of polymeric systems including the thermoplastic elastomers in the as-spun electrospun fiber were reported, the orientation of microdomains has not yet been well clarified. The present work shows an existence of microdomain orientation through a case study of a well-aligned electrospun fibers of polystyrene-block-poly(ethylene-co-1-butene)-block-polystyrene triblock copolymer (SEBS) obtained from an electrospinning unit equipped with a rotational disk fiber-collector. Two-dimensional small-angle X-ray scattering (2D-SAXS) patterns of the as-spun electrospining SEBS fibers show elliptical and four-point patterns suggesting an orientation of distorted and fragmented lamellar microdomains. The electrospun fibers obtained from a low rotational disk collector speed (31.5 m/min) exhibits a significant microdomain distortion whereas the fibers obtained from high take-up velocities (310 m/min, 620 m/min, and 1240 m/min) show higher fragmented-microdomain stretching. By annealing the electrospun fibers, the fibers develop an isotropic SAXS pattern with traces of the remained anisotropic orientation. Based on the above mentioned evidences in SEBS, the present work, for the first time, clarifies that the as-spun thermoplastic elastomers fibers show not just a simple microdomain as used to be observed by transmission electron microscope (TEM) but rather with orientation which can be confirmed by SAXS.

Transition behavior of PS- b-PMMA films on the balanced interfacial interactions

The thickness dependence of the order-to-disorder transition (ODT), measured by in situ grazing-incidence small-angle X-ray scattering (GISAXS), has been investigated in thin films of a symmetric polystyrene- b-poly(methyl methacrylate) (PS- b-PMMA) on a random copolymer (P(S- r-MMA)) grafted to the substrate where the interfacial interactions are balanced. With decreasing film thickness less than 25 L 0, the ODT significantly decreases to 193 °C for film of 10 L 0 in thickness, because the interfacial interactions by a random copolymer grafted to the substrate provide a surface-induced compatibilization toward two block components. However, a plateau of the ODT at ∼213 °C for films thicker than 25 L 0 was observed above the bulk value of 200 °C. The elevation of this ODT indicates a suppression of compositional fluctuations normal to the film surface, more than likely because the dominant orientation of the lamellar microdomains was found to be parallel to the film surface.

Birefringence Control of Semicrystalline Block Copolymers by Crystallization under Confinement

A series of semicrystalline block copolymers (BCPs), poly(4-vinylpyridine)-block-poly(ε-caprolactone) (P4VP-PCL), with lamellar phases have been synthesized. P4VP-PCL BCP thin films with large-scale, oriented lamellar microdomains were obtained by rimming coating process followed by oscillated shearing using a homemade shear device. Owing to the vitrified P4VP microdomains and strongly segregated microphase separation, specific PCL crystalline chain orientation can be formed from the growth of anisotropic PCL crystallites under confinement so as to uniformly increase the birefringence of the BCP thin films. The enhanced birefringence corresponds well with the increase of PCL crystallinity. Consequently, the birefringence of the P4VP-PCL thin-films can be fine-tuned by PCL crystallization. The variation on the birefringence of the BCP thin films attributed to crystallization and melting is a reversible process with respect to temperature. The BCP thin films can thus be used as temperature-stimulated materials with controllable birefringence via crystallization kinetics.

Crystal orientation of poly(ɛ-caprolactone) blocks confined in crystallized polyethylene lamellar morphology of poly(ɛ-caprolactone)-block-polyethylene copolymers

The crystal orientation of poly(ɛ-caprolactone) (PCL) blocks in PCL-block-polyethylene (PE) copolymers has been investigated using two-dimensional small-angle X-ray scattering (2D-SAXS) and 2D wide-angle X-ray diffraction (2D-WAXD) as a function of crystallization temperature Tc and thickness of PCL layers dPCL. The PCL blocks were spatially confined in the solid lamellar morphology formed by the crystallization of PE blocks (PE lamellar morphology), an alternating structure of crystallized PE lamellae and amorphous PCL layers. This confinement is expected to be intermediate between hard confinement by glassy lamellar microdomains and soft confinement by rubbery ones, because the crystallized PE lamellae consist of hard PE crystals covered with amorphous (or soft) PE blocks. The 2D-SAXS results showed uniaxial orientation of the PE lamellar morphology after applying the rotational shear to the sample. Therefore, it was possible to investigate crystal orientation of PCL blocks within the oriented PE lamellar morphology. The 2D-WAXD results revealed that the c axis of PCL crystals (i.e., stem direction of PCL chains) was parallel to the lamellar surface normal irrespective of Tc when 16.5 nm ≥ dPCL ≥ 10.7 nm. However, it changed significantly with changing Tc when dPCL = 8.8 nm; the c axis was perpendicular to the lamellar surface normal at 45 °C ≥ Tc ≥ 25 °C while it was almost random at 20 °C ≥ Tc ≥ 0 °C. These results suggest that the PE lamellar morphology plays a similar role to glassy lamellar microdomains regarding spatial confinement against subsequent PCL crystallization.

Microphase separation in thin films of supramolecular assemblies composed of a triblock copolymer and low‐molecular‐weight additive

AbstractThe phase behavior of supramolecular assemblies (SMAs) formed by poly(4‐vinylpyridine)‐block‐polystyrene‐block‐poly(4‐vinylpyridine) (P4VP‐b‐PS‐b‐P4VP) triblock copolymer with 2‐(4′‐hydroxybenzeneazo)benzoic acid (HABA) was investigated with respect to the molar ratio (X) between HABA and 4VP monomer unit in bulk as well as in thin films. The results were compared with SMAs formed by a PS‐b‐P4VP diblock copolymer of similar composition as the triblock but half the molecular weight to ascertain the effect of molecular architecture on microphase separation. In bulk, both the di‐ and triblock SMAs showed composition‐dependent morphological transitions, which could be tuned by HABA/4VP molar ratio. The domain spacing of the SMA was not significantly affected by the molecular architecture of the constituting block copolymers. In thin films also, both the di‐ and triblock SMAs showed more or less similar morphological transitions depending on X. Interestingly, the domain orientation of the cylindrical or lamellar microdomains in the SMAs was influenced by the molecular architecture of the block copolymer. After chloroform annealing, although the diblock SMAs showed in‐plane orientation of the domains, triblock SMAs showed perpendicular domain orientation. The perpendicular orientation of the microdomains in triblock was favored because it allowed the mid‐PS blocks to acquire normal distribution of loop and bridged conformations. Furthermore, the orientation of the lamellar and cylindrical microdomains of the diblock SMAs was found to switch to perpendicular orientation after annealing in 1,4‐dioxane vapors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1594–1605, 2010

Facile Alignment of Amorphous Poly(ethylene oxide) Microdomains in a Liquid Crystalline Block Copolymer Using Magnetic Fields: Toward Ordered Electrolyte Membranes

Large area microdomain alignment in poly(ethylene oxide-b-6-(4′-cyanobiphenyl-4-yloxy) hexyl methacrylate) block copolymers was successfully accomplished by the application of a 6 T magnetic field while cooling from elevated temperatures in the melt state. Small-angle X-ray scattering demonstrated that lamellar and cylindrical PEO microdomains aligned with their interfaces along the applied field, whereas the smectic layers of the liquid crystalline mesophase are perpendicular to the field. This is in agreement with the positive diamagnetic anisotropy of the cyano-biphenyl mesogen and a homogeneous anchoring condition at the intermaterial dividing surface (IMDS) between the two blocks. The alignment of the system is driven by the diamagnetic anisotropy of the smectic mesophase and not by the crystallization of PEO at lower temperatures. The addition of poly(acrylic acid) and LiClO4 salt result in the suppression of PEO crystallinity and stronger segregation between the polymer blocks leading to improved order in the material. The resulting films are well aligned over millimeter length scales of area and thickness. We use a novel continuous rotational annealing approach to break the degeneracy of the lamellar alignment, permitting facile directed assembly of the system during a single cooling step. Our experiments demonstrate the creation of well-aligned arrays of amorphous PEO domains over large length scales and offer a route to functional materials, in particular, for selective transport applications such as solid ionic electrolytes.

Bending of Lamellar Microdomains of Block Copolymers on Nonselective Surfaces

Symmetric diblock copolymers self-assemble into lamellar microdomains which can be used to create line and space patterns on substrates by controlling the orientation of microdomains. Previously, it has been reported that the orientation of lamellar microdomains can be controlled perpendicular to the surface by modifying the substrate surface to become nonselective to each domain. For practical use of the block copolymer based patterns for nanofabrications, of great interest is precise control over the lateral placement of lamellar microdomains. Commonly predefined guiding patterns containing chemical and/or topographical contrast have been used to direct the lateral placement of lamellae on substrates. In the absence of guiding patterns, the lamella microdomains produce fingerprint-like surface patterns on nonselective substrates as reported previously with a variety of model diblock copolymers, where the most well-studied diblock copolymer is poly(styrene-b-methyl methacrylate) (PS-b-PMMA). The fact that this fingerprint-like pattern contains many regions of high curvature of bent lamellae suggests the cost of lamellae bending may be relatively small. Previously, Nealey and co-workers reported that the lamellar microdomains of PS-bPMMA faithfully follow the lithographically predefined surface patterns of chemical contrast. They showed that bent block copolymer lamellae could be obtained when homopolymers were added to the copolymer. More recently, Wilmes et al. examined the extent to which patterns with different curvatures can be created within the same block copolymer thin films. They showed that the lamellae conform to patterns with radii of curvature equal to the equilibrium domain spacing. In this Note, we report our investigation on the bending characteristics of lamellar microdomains resulting from directed assembly using topographic guiding patterns. We designed the topographic guiding patterns as elbows with varying corner angles. By controlling the surface of guiding patterns nonselective to the microdomains, we rendered the orientation of lamellae perpendicular to the surfaces of bottom and sidewalls of the guiding patterns. Detailed experimental observation of the degree of bending and tilting of lamellae at the sidewall surface is reported along with a theoretical investigation using self-consistent-field calculations, which provide additional knowledge regarding the free energy and interfacial characteristics of the experimentally observed morphologies. We also extended our investigation to a block copolymer hybrid system which is a mixture of an organic diblock copolymer, poly(styrene-b-ethylene oxide) (PS-b-PEO), and an organosilicate (OS) resin. The effect of larger orientational correlation length of the hybrid system on the bending property of lamellar microdomains will be discussed.

Transition Behavior of Weakly Interacting PS-b-PMMA Films on Preferential Surfaces: A Direct Observation by GISAXS

The phase transitions for the films of a symmetric polystyrene-b-poly(methyl methacryltate) (PS-b-PMMA) on a modified surface, like the order-to-disorder transition (ODT) in the weak segregation regime, was investigated by in situ grazing incidence small-angle X-ray scattering (GISAXS). The selective interactions at the surface by PS-brush substrate that favors the preferential interactions with the PS component of the block copolymer enhance the parallel orientation of the lamellar microdomains to the film surface. This effect for weakly interacting PS-b-PMMA films leads to a gentle decrease of the transition temperature with increasing film thickness up to 50L0 (lamellar period). We discuss this thickness dependence of transition temperatures for PS-b-PMMA films on preferential surfaces in terms of the temperature dependence of χ between two block components.

Directed Self-Assembly of Lamellar Microdomains of Block Copolymers Using Topographic Guiding Patterns

Directed Self-Assembly of Lamellar Microdomains of Block Copolymers Using Topographic Guiding Patterns

Full Color Stop Bands in Hybrid Organic/Inorganic Block Copolymer Photonic Gels by Swelling−Freezing

We report a facile way of fabricating hybrid organic/inorganic photonic gels by selective swelling and subsequent infiltration of SiO(2) into one type of lamellar microdomain previously self-assembled from modest-molecular-weight block copolymers. Transparent, in-plane lamellar films were first prepared by assembly of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP), and subsequently the P2VP domains were swollen with a selective solvent, methanol. The swollen structures were then fixated by synthesizing SiO(2) nanoparticles within P2VP domains. The resulting frozen photonic gels (f-photonic gels) exhibited strong reflective colors with stop bands across the visible region of wavelengths.

Time-resolved SAXS studies of self-assembling process of palladium nanoparticles in templates of polystyrene- block-polyisoprene melt: Effects of reaction fields on the self-assembly

Nanocomposites composed of palladium (Pd) nanoparticles, (Pd) n, and a symmetric diblock copolymer (dibcp) of poly(styrene)- block-poly(isoprene) (PS- b-PI) were prepared via thermal reduction of palladium acetylacetonate, Pd(acac) 2, by isothermal heat treatments of the as-cast dibcp films containing Pd(acac) 2 by 1 wt%, at temperatures below and above the order–disorder transition temperature ( T ODT) of the dibcp. Effects of the reaction field on the thermal reduction process of Pd(acac) 2 were studied by setting the field either in the disordered state or the ordered state of the dibcp. The reduction process was studied in situ and at real time by time-resolved small-angle X-ray scattering. In the as-cast films, a larger fraction of Pd(acac) 2 is preferentially incorporated in PS lamellar microdomains, seemingly due to stronger interactions of Pd(acac) 2 with PS than with PI. The spatial concentration fluctuations of (Pd) n in the final stage of the reduction reflect the memory of the concentration fluctuations of the dibcp before the reduction, either in the disordered state or in the ordered state. We found that the scaled scattering form factor of (Pd) n is universal with time and temperature of the reduction. This reveals that the reduction mechanism of Pd(acac) 2 and the growth mechanism of (Pd) n do not depend on temperature; they are same below and above T ODT. However, the reduction rate and the growth rate as well as the concentration of (Pd) n in the final stage depend on temperature: the higher the temperature is, the larger are the rate and the concentration.

Dilute solutions and phase behavior of polydisperse A- b-(A- co-B) diblock copolymers

The effect of polydispersity on dilute solution properties and microphase separation of polydisperse high-molecular-weight ( M w > 10 5 g mol −1) polystyrene- block-poly(styrene- co-acrylonitrile) diblock copolymers, PS- block-P(S- co-AN), was studied in this work. For experiments, a series of diblock copolymers with variable weight fractions of acrylonitrile units ( w AN = 0.08–0.29) and length of block P(S- co-AN) was synthesized using nitroxide-mediated radical polymerization (NMP) technique, namely, by chain extension of nitroxide-terminated polystyrene (PS-TEMPO). According to light scattering and viscometry measurements in dilute tetrahydrofuran (THF) solutions the studied diblock copolymers assumed random coil conformation with the values of characteristic structure factor R g/ R h = 1.50–1.76. It was found that polydisperse diblock copolymers being in strong segregation limit (SSL) self-assembled into microphase-separated ordered morphologies at ordinary temperature. The long periods of lamellar microdomains were larger compared to theoretical predictions for hypothetical monodisperse diblock copolymers. It was demonstrated by means of SAXS and TEM that a transition from a lamellar (LAM) to irregular face-centered-cubic (FCC) morphology occurred with increasing volume fraction of P(S- co-AN) block.

Observation of surface corrugation-induced alignment of lamellar microdomains in PS-b-PMMA thin films

We report lateral alignment of lamellar microdomains in thin films of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) on corrugated surfaces where surface energy is controlled as non-selective to PS and PMMA by anchoring polymer brushes of a random copolymer of PS and PMMA. The orientational order determined by Herman's orientation function indicates that the lamellae align perpendicular to the direction of surface corrugation within a window of surface corrugation length scales used in this study. The effect of relative scales of periodicity and film thickness of PS-b-PMMA to those of surface corrugation on the alignment of lamellae is discussed in this paper.

Transition Behavior of Block Copolymer Thin Films on Preferential Surfaces

The phase transitions in block copolymers (BCPs), like the order-to-disorder transition, occur when the enthalpic term of free energy of mixing is equal to the entropic term. In thin films, interactions at the substrate/polymer and polymer/air interfaces influence this free energy balance, leading to a change in the transition behavior. Here, we report on the transition behavior of BCP thin films on a substrate modified by polymer chains that preferentially interact with one component of the BCP. The thickness dependence of the transition temperature shows that interfacial interactions enhance the orientation of the lamellar microdomain parallel to the film surface even in 40 L0 in thickness, where L0 is the equilibrium period of the BCP in the bulk. This phenomenon can be attributed to the fact that, in a thin film geometry with a preferential interaction of one component with the substrate, a high concentration of one component is pinned at the substrate leading to an amplification of a periodic variation in the composition and a shift of transition temperature.

Patterning sub-10 nm line patterns from a block copolymer hybrid

We report the formation and directed self-assembly of sub-10 nm half-pitch line patternsfrom lamellar microdomains of a block copolymer hybrid. The hybrid, which is a mixture ofpoly(styrene-b-ethylene oxide) (PS-b-PEO) and a low molecular weight organosilicate (OS), shows strong segregation between two phases (i.e. PSand PEO+OS) and forms lamellar microdomains of down to approximately 7 nm in half-pitch. Patternsapplicable to multifinger device layouts are created by self-assembling the hybrid ontopographic pre-patterns with a chemically non-selective surface. With careful designof the guiding topographic pattern geometry, well-controlled lateral placementincluding bent structures of lamellar microdomains can be obtained by this approach.