Lamellar Microdomain Structure Research Articles

Tailoring the Microstructure of Lamellar Ti3C2Tx MXene Aerogel by Compressive Straining.

Aerogels are attracting increasing interest due to their functional properties, such as lightweight and high porosity, which make them promising materials for energy storage and advanced composites. Compressive deformation allows the nano- and microstructure of lamellar freeze-cast aerogels to be tailored toward the aforementioned applications, where a 3D nanostructure of closely spaced, aligned sheets is desired. Quantitatively characterizing their microstructural evolution during compression is needed to allow optimization of manufacturing, understand in-service structural changes, and determine how aerogel structure relates to functional properties. Herein we have developed methods to quantitatively analyze lamellar aerogel domains, sheet spacing, and sheet orientation in 3D and to track their evolution as a function of increasing compression through synchrotron phase contrast X-ray microcomputed tomography (μCT). The as-cast domains are predominantly aligned with the freezing direction with random orientation in the orthogonal plane. Generally the sheets rotate toward flat and their spacing narrows progressively with increasing compression with negligible lateral strain (zero Poisson’s ratio). This is with the exception of sheets close to parallel with the loading direction (Z), which maintain their orientation and sheet spacing until ∼60% compression, beyond which they exhibit buckling. These data suggest that a single-domain, fully aligned as-cast aerogel is not necessary to produce a post-compression aligned lamellar structure and indicate how the spacing can be tailored as a function of compressive strain. The analysis methods presented herein are applicable to optimizing freeze-casting process and quantifying lamellar microdomain structures generally.

Control of crystal orientation of spatially confined PCL homopolymers by cleaving chain-ends of PCL blocks tethered to nanolamella interfaces

We have examined the crystalline nanomorphology (i.e., crystallinity, melting temperature (reflecting crystal thickness), and crystal orientation) of poly(ε-caprolactone) (PCL) blocks and PCL homopolymers spatially confined in lamellar microdomain structures (nanolamellae) using two polystyrene (PS)-b-PCL diblock copolymers (with either PCL chain-end anchored to nanolamella interfaces) and one PS-b-PCL-b-PS triblock copolymer (with both chain-ends anchored). These copolymers had photocleavable o-nitrobenzyl groups at all the block junctions, so the PCL blocks could be converted into PCL homopolymers by UV irradiation with keeping the nanolamellar geometry. The crystallization of photocleaved PCL homopolymers was achieved by two procedures; (1) the amorphous PCL blocks were first converted into PCL homopolymers and they were crystallized at room temperature (RT) (one-step crystallization) or (2) the PCL blocks were first crystallized at selected temperatures to form some crystalline nanomorphology and then converted into PCL homopolymers to further crystallize and/or anneal at RT (two-step crystallization). The crystallinity and melting temperature of PCL homopolymers were similar for both procedures, suggesting that a certain amount of PCL homopolymers formed lamellar crystals with a comparable thickness. However, the lamellar crystals showed significantly different orientations between two procedures, because some crystal orientation was made by the advance crystallization of PCL blocks and remained unchanged after UV irradiation/subsequent crystallization in two-step crystallization, yielding a substantial difference in the crystal orientation of PCL homopolymers in nanolamellae.

Combined effects of confinement size and chain-end tethering on the crystallization of poly(ε-caprolactone) chains in nanolamellae

The crystallization behavior of poly(ε-caprolactone) (PCL) chains spatially confined in a lamellar microdomain structure (nanolamella) with the PCL layer thickness d of 15.0 nm (space confinement) was examined using synchrotron small-angle X-ray scattering (SR-SAXS), differential scanning calorimetry (DSC), and infra-red spectroscopy (IR). The PCL chains were covalently tethered to nanolamella interfaces at both chain-ends, either chain-end, or no chain-end (chain confinement) to bring an additional change in their molecular mobility. The individual effect of space confinement and chain confinement on the crystallization of PCL chains was elucidated by comparing this result with that of PCL chains confined in a smaller nanolamella (i.e., d = 10.9 nm) derived in our previous study and of bulk PCL homopolymers without any confinements. Though the crystallization rate depended significantly on the state of chain-end tethering for PCL chains confined in the nanolamella with d = 10.9 nm, it converged toward bulk PCL with the difference considerably reduced when confined in the nanolamella with d = 15.0 nm. The melting temperature and crystallinity showed similar results. We tried to convert chain confinement into space confinement on the basis of crystallization rates experimentally obtained to understand their combined effect on the crystallization.

Effects of crystal structure of poly(β-propiolactone) blocks on the cooperative crystallization of a polyethylene-block-poly(β-propiolactone) diblock copolymer

The crystalline morphology of a double crystalline polyethylene-block-poly(β-propiolactone) diblock copolymer (PE-b-PPL) was examined using synchrotron small-angle X-ray scattering (SR-SAXS), where the PPL block crystallized into δ- or β-form according to the thermal history during sample preparation. Furthermore, the isothermal crystallization process of PE-b-PPL was pursued using simultaneous time-resolved SR-SAXS and wide-angle X-ray diffraction (SR-WAXD). The crystallization behavior of δPPL blocks (i.e., PPL blocks to crystallize into δ-form) was similar to that of PE blocks without any detectable induction time, so simultaneous crystallization of PE and δPPL blocks was observed on quenching from a lamellar microdomain structure (LMS), yielding a largely distorted LMS (with the standard deviation of each lamella thickness ∼ 36%). On the other hand, the βPPL blocks (i.e., PPL blocks to crystallize into β-form) started to crystallize after an appreciable induction time, showing sequential crystallization (i.e., advance crystallization of PE blocks and subsequent crystallization of βPPL blocks) to form a slightly distorted LMS (18%). Consequently, the final crystalline morphology was significantly different depending on the crystal structure of PPL blocks, though the crystallization started from an identical LMS. The crystallization mechanism of PE-b-PPL was discussed by considering the difference in crystallization kinetics between PE blocks and PPL (δPPL or βPPL) blocks.

Crystallization behavior of poly(β-propiolactone)-block-polyethylene copolymers with varying polyethylene crystallinities

The crystallization behavior of poly(β-propiolactone)-block-polyethylene (PPL-b-PE) copolymers with high PE crystallinities χPE (>0.30) has been examined using time-resolved synchrotron small-angle X-ray scattering and Fourier transform infrared spectroscopy, where the PE block crystallized first and subsequently the PPL block crystallized on quenching from a strongly segregated melt. The crystallization of PE blocks destroyed the lamellar microdomain structure (LMS) existing in the melt to form the crystalline lamellar morphology (CLM), and then PPL blocks crystallized within CLM. This morphology formation was compared to our previous results for the crystallization of PPL-b-PE copolymers with low χPE (0.12 < χPE < 0.26), where the crystallizability of PE blocks was not sufficiently large to destroy LMS. As a result, PE blocks crystallized promptly within LMS to reinforce and stabilize it against the subsequent crystallization of PPL blocks, yielding the confined crystallization of both blocks within LMS. We summarize these results including the case of χPE = 0, and propose three mechanisms of morphology formation occurring in PPL-b-PE copolymers according to χPE (i.e., high, low, or zero).

Crystallization of double crystalline block copolymer/crystalline homopolymer blends: 1. Crystalline morphology

The crystalline morphology formed in binary blends of poly(ε-caprolactone)- block-polyethylene (PCL-b-PE) copolymers and PCL homopolymers has been examined using synchrotron small-angle X-ray scattering (SR-SAXS) and differential scanning calorimetry (DSC) as a function of the homopolymer fraction in the blend. The PE block crystallized first on quenching from a lamellar microdomain structure to set a hard lamellar morphology (PE lamellar morphology) in the blend, followed by the crystallization of PCL chains (i.e., PCL homopolymers + PCL blocks). Two binary blends were studied by considering the miscible state of PCL homopolymers in the microdomain structure: when the PCL homopolymers were uniformly mixed with PCL blocks, they formed a mixed crystal. When the PCL homopolymers were localized between PCL blocks in the microdomain structure, DSC results suggested the possible formation of separate PCL crystals in the PE lamellar morphology. The effect of the advance crystallization of PE blocks on the subsequent crystallization of PCL chains was discussed as compared with the crystalline morphology formed in PCL-block-polybutadiene copolymer/PCL homopolymer blends, where the crystallization of PCL chains started directly from a microdomain structure without forming the hard lamellar morphology.

Isothermal crystallization of poly(β-propiolactone) blocks starting from lamellar microdomain structures of double crystalline poly(β-propiolactone)-block-polyethylene copolymers

The isothermal crystallization behavior of poly(β-propiolactone) (PPL) blocks starting from lamellar microdomain structures (LMS) formed in molten PPL-block-polyethylene (PPL-b-PE) copolymers has been examined using time-resolved synchrotron small-angle X-ray scattering and Fourier transform infrared spectroscopy as a function of the crystallinity of PE blocks χPE. For getting PPL-b-PE copolymers with varying χPE, the amount of ethyl branches in PE blocks was controlled during polymerization. The crystallizable temperature of PE blocks was slightly higher than that of PPL blocks and the crystallization rate of PE blocks was extremely fast, so the crystallization of PE blocks occurred first on quenching to form the crystallized LMS, in which the PPL block crystallized. The time evolution for the crystallinity of PPL blocks χPPL showed an exponential-type increase with no induction time. In contrast, LMS was replaced with the crystallized lamellar morphology and χPPL showed a sigmoidal time evolution in PPL-b-PE with a higher amount of ethyl branches, where the PE block did not crystallize (i.e., χPE = 0) prior to the crystallization of PPL blocks. It was concluded that the prompt crystallization of PE blocks just after quenching reinforced the existing LMS to make it stable against subsequent crystallization, yielding the confined crystallization of PPL blocks.

Phase Behavior of a Binary Mixture of a Block Copolymer with Lower Disorder-to-Order Transition and a Homopolymer

The phase diagrams for a binary mixture of symmetric polystyrene-block-poly(n-butyl methacrylate) copolymer (PS-b-PnBMA) exhibiting lower disorder-to-order transition (LDOT) and PS homopolymer and a binary mixture of PS-b-PnBMA and PnBMA homopolymer were investigated by using polarizing optical microscopy, turbidity measurement, small-angle X-ray scattering, rheology, and transmission electron microscopy. Emphasis was placed on the effect of the molecular weight of homopolymers on the phase behavior of the mixtures. We found that when the molecular weight of a homopolymer was smaller than the half of molecular weight of the corresponding block, LDOT of the mixture increased (thus miscibility was enhanced). However, the changes of microdomains for the mixtures containing PS homopolymer are different from those for the mixtures containing PnBMA homopolymer, especially when the molecular weight of homopolymer becomes large. For instance, for the mixture of PS-b-PnBMA and PnBMA with a molecular weight of 65000, lamellar microdomains structure was destroyed even at the addition of 20 wt % of PnBMA homopolymer. On the other hand, for the mixture of PS-b-PnBMA and PS with a molecular weight 68000, the lamellar microdomain structure was maintained up to 40% of PS homopolymer. The phase diagrams of the mixtures were compared with predictions developed based on the compressible random phase approximation.

Temperature–composition phase diagrams of binary blends of block copolymer and homopolymer

The temperature–composition phase diagrams for six pairs of diblock copolymer and homopolymer are presented, putting emphasis on the effects of block copolymer composition and the molecular weight of added homopolymers. For the study, two polystyrene- block-polyisoprene (SI diblock) copolymers having lamellar or spherical microdomains, a polystyrene- block-polybutadiene (SB diblock) copolymer having lamellar microdomains, and a series of polystyrene (PS), polyisoprene (PI), and polybutadiene (PB) were used to prepare SI/PS, SI/PI, SB/PS, and SB/PB binary blends, via solvent casting, over a wide range of compositions. The shape of temperature–composition phase diagram of block copolymer/homopolymer blend is greatly affected by a small change in the ratio of the molecular weight of added homopolymer to the molecular weight of corresponding block ( M H,A/ M C,A or M H,B/ M C,B) when the block copolymer is highly asymmetric in composition but only moderately even for a large change in M H,A/ M C,A ratio when the block copolymer is symmetric or nearly symmetric in composition. The boundary between the mesophase (M 1) of block copolymer and the homogeneous phase (H) of block copolymer/homopolymer blend was determined using oscillatory shear rheometry, and the boundary between the homogeneous phase (H) and two-phase liquid mixture (L 1+L 2) with L 1 being disordered block copolymer and L 2 being macrophase-separated homopolymer was determined using cloud point measurement. It is found that the addition of PI to a lamella-forming SI diblock copolymer or the addition of PB to a lamella-forming SB diblock copolymer gives rise to disordered micelles (DM) having no long-range order, while the addition of PS to a lamella-forming SB diblock copolymer retains lamellar microdomain structure until microdomains disappear completely. Thus, the phase diagram of SI/PI or SB/PB blends looks more complicated than that of SI/PS or SB/PS blends.

Intermediate Segregation Type Chain Length Dependence of the Long Period of Lamellar Microdomain Structures of Supramolecular Comb−Coil Diblocks

A characteristic intermediate segregation type chain length dependence of the long period D of the lamellar microdomain structure of a class of comb−coil supramolecules is reported. The supramolecular comb−coil diblock copolymers studied consist of a polystyrene (PS) “coil” block and a “comb” block of poly(4-vinylpyridine) (P4VP) either hydrogen bonded to pentadecyl phenol (PDP) (i.e., P4VP(PDP)-b-PS) or first protonated with methanesulfonic acid (MSA) and then hydrogen bonded to PDP (i.e., P4VP(MSA)(PDP)-b-PS). In both cases we find a scaling D ∼ , δ ≅ 0.8, where Ntot denotes the total number of monomers of the P4VP-b-PS backbone. In the case of diblock copolymers this would correspond to a characteristic intermediate segregation regime behavior. Pure PS-b-P4VP, on the other hand, shows the expected strong segregation behavior D ∼ , δ ≅ 0.7.

One-Dimensionally Periodic Dielectric Reflectors from Self-Assembled Block Copolymer−Homopolymer Blends

Introduction. The well-ordered structures of block copolymer microdomains formed by self-assembly are potentially interesting optical materials, specifically as photonic crystals. Photonic crystals are structures with a periodic variation of refractive index.1-3 For light of the appropriate frequency, such crystals can be used to bend and steer the light, opening up many possible applications such as broad-band low loss waveguides for communication and efficient laser mirrors. The first photonic crystals were made by machining overlapping holes into a slab (millimeter length scales for microwave frequencies).4 Current work employs lithographic processing (at the micron and submicron length scale for IR and visible frequencies) to make dielectric structures.5 Very recent work has introduced self-assembly as a method to make photonic crystals with periodicities suitable for frequencies in the visible part of the spectrum. Materials used include colloidal crystal arrays,6 artificial opals and inverse opals,7,8 and hollow spherical micelles.9 Block copolymers can self-assemble into a wide variety of periodic structures including 1-, 2-, and 3-dimensional structures.10 There are several essential challenges to overcome in order to achieve desirable photonic crystal properties employing block copolymers. These include obtaining the correct size of domains for the optical frequencies of interest, attainment of long-range domain order and appropriate orientation, as well as providing sufficient dielectric contrast between the domains. Accessing large periodicities in block copolymers is of course possible using very high molecular weight blocks, but attaining well-ordered structures is difficult due to the very high viscosity of such systems. An alternative approach is to utilize a liquid crystalline block for which the domain thickness varies linearly with block molecular weight. In this way, spacings into the micron range can be produced.11 Another approach is to blend block copolymers with homopolymers and thus swell the microdomains. Block copolymer/homopolymer blends have been investigated both experimentally12-14 and theoretically.15,16 Such blends offer tailorable domain spacings, as well as the processing advantage afforded by somewhat lower molecular weight components. The dependence of the lamellar repeat in bulk cast blends of homopolystyrene and polystyreneb-polyisoprene block copolymers has been analyzed as a function of the percent homopolymer added and its molecular weight.14 The lamellar thickness depends on the many factors including homopolymer volume fractions and the block composition and molecular weight. A regular lamellar morphology having a difference in the index of refraction between the respective domains is similar to a layered optical filter or quarter-wave stack. In this article we demonstrate that a diblock copolymer/homopolymer blend, which can self-assemble into a lamellar microdomain structure, can exhibit a limited angular range stop band at visible frequencies.

Researches on Polymer Surfaces: Structure and Dynamics. Non-linear Macroscopic Pattern Formation on Polymer Film Surface.

This article reviews non-linear macroscopic pattern formation on polymer film surfaces. As a result of solvent evaporation from a dilute polymer solution heated from below, the free surface of the as-cast polymer film undulates and exhibits a two-dimensional pattern due to the undulation. For an ordinary polymer, a hexagonal (or polygonal) pattern was obtained. This pattern, due to surface undulation, was found to be ascribed to convection existing in the solution. On the other hand, a very peculiar concentric-ring pattern was observed for an SBS film, where SBS designates polystyrene-block-polybutadiene-block-polystyrene triblock copolymers. Note that this sample forms lamellar microdomain structures with a ca. 22 nm identity period. In this pattern, a set of concentric rings contained in a cellular compartment is a unit constituent and was found to be a set of wrinkles on the surface. The mechanism of formation of the concentricring pattern is presented in this article, and moreover, control of the morphology of the pattern is discussed.

Blends of amorphous-crystalline block copolymers with amorphous homopolymers. Morphological studies by electron microscopy and small angle scattering

A morphological study was performed with symmetric diblock ethylene-propylene copolymer (DEP) and the binary blends made from DEP and atactic polypropylene (APP) by use of small angle X-ray, light and neutron scattering, and also scanning and transmission electron microscopy. DEP contains a crystallizable polyethylene block and an amorphous atactic polypropylene block. Quenching the blends in liquid nitrogen preserved the morphology in the melt state. This quenching technique revealed that DEP forms a lamellar microdomain structure and blending DEP and APP includes morphological changes in the microdomain structures as well as macrophase separation. When the APP chain was shorter than the APP block, the addition of APP changed the morphology from a lamellar to a bicontinuous cylindrical and then a discrete cylindrical and finally to a spherical structure. On the other hand, when the APP chain was longer than the APP block, macrophase separation was observed and only a transition from a lamella to a bicontinuous cylinder occurred. These morphological transitions in the melt state can be correlated to differences in the crystallization kinetics of the blends.

Microphase separation in poor‐solvent polyelectrolyte solutions: Phase diagram

AbstractMicrophase separation in solutions of weakly charged polyelectrolytes in poor solvents is studied in the weak segregation limit within the framework of the mean field approximation using a method first developed by Leibler. As a result a complete phase diagram of the solution near the critical point is obtained. The regions of the stability of the disordered, homogeneous phase and of body‐centered cubic (bcc), triangular and lamellar microdomain structures, as well as the phase separation regions are determined. The most striking difference in comparison with the corresponding diagram for block‐copolymer melts is the existence of broad phase separation regions even for monodisperse systems. As the quality of solvent becomes poorer, the triangular microdomain structure remains the most stable among microdomain phases of other symmetry.

Spatial Distribution of a Midblock-Associating Homopolymer Blended into a Triblock Copolymer

The spatial distribution of midblock associating homopolymer confined within the lamellar microdomain structure of a triblock copolymer is probed with small angle neutron scattering experiments. The materials examined are poly(styrene-b-[saturated 1,2-butadiene]-b-styrene) triblock copolymers to which a low molecular weight poly(saturated 1,2-butadiene) homopolymer has been added. The butadienes are saturated with either hydrogen, deuterium, or mixtures of the two gases in order to vary their neutron scattering contrast with respect to polystyrene. The results of contrast matching experiments demonstrate that there is a strong tendency for the homopolymer to localize at the center of the midblock microdomain. Experimental scattering profiles are modeled using one-dimensional scattering density profiles in order to obtain a quantitative description of the blend morphologies. This modeling indicates that two distinct scenarios exist for homopolymer localization in a triblock copolymer: one wherein the microdomain structure contracts and a second wherein there is an expansion of the microdomain. Possible origins of this behavior are proposed on the basis of the consideration of the configuration available to the midblock sequence, that is, tie chains that traverse the midblock domain, or loops that enter and exit the midblock microdomain through the same interface.

Charge mosaic membrane with a microstructure of poly(styrene‐b‐4‐vinylpyridine)

AbstractWell defined AB and BAB type poly[styrene(ST)‐b‐4‐vinylpyridine(4VP)]s were prepared by anionic living polymerization in tetrahydrofuran (THF) at −78°C. Casting AB and BAB type poly(ST‐b‐4VP)s with a composition of about 50 mol% PST from 1,1,2‐trichloroethane (TCE) produced specimens with lamellar microdomain structures. Quaternization of P4VP and sulfonation of PST domains (lamellar and spherical structures), accompanied by each domain fixing, were carried out under various reaction conditions. The piezodialysis of the prepared charge mosaic membrane with its lamellar microdomain structure was measured from a sodium chloride (NaCl) aqueous solution. The results are discussed in some detail.