Microphase separation in multiblock copolymer melts: Nonconventional morphologies and two-length-scale switching

The phase behavior of AfmN(BN2AN2)B(1-fmN) multiblock copolymer melts is studied within the weak segregation theory. The interplay between ordering on different length scales is shown to cause dramatic changes both in the ordered phase symmetry and periodicity upon small variation of the architectural parameters of the macromolecules. Phase diagrams are presented in the (f,chiN) plane (chi is the Flory-Huggins parameter) for various values of the architecture parameters n and m. Near the critical surface, i.e., for (f-0.5)2<1, such nonconventional cubic phases as the face-centered cubic (FCC), simple cubic (SC), (double) gyroid, and the so-called BCC(2) (single gyroid) are found to be stable. The lamellar morphology is shown to be replaced by BCC2, FCC, or SC (depending on the structural parameters) as the most stable low-temperature phase.     

Double gyroid structures made of asymmetric dimers

Gyroid cubic phases are interesting for both scientists and engineers due to possible applications in electronic devices. New series of dimeric molecules, despite their flexible molecular structure, can display double gyroid cubic phase with Ia3d symmetry and lattice parameter corresponding to double molecular length. The cubic phase is structurally related to columnar phase and both phases often coexist in the same temperature window. Apparently, for studied compounds stronger molecular asymmetry promotes cubic structure. Interestingly, for the examined compounds the transition between two isotropic liquids was observed.     

Effect of molecular architecture on phase behavior of graft copolymers

Influence of molecular architecture on phase behavior of graft copolymer melts was studied by using a reciprocal-space self-consistent filed theory (SCFT). The phase diagrams were examined as functions of the architectural parameters describing the graft copolymers (i.e., the number of grafts and the position of first junction). In comparison with the well-known phase diagram of diblock copolymers, the phase diagrams of the graft copolymers are asymmetric. When the number of grafts or the position of first junction varies, the boundaries of order-order transitions have shifts due to the variation in the chain stretching energy. The change in molecular architecture also significantly alters the domain spacing of ordered structures but has weak impact on the density distributions of graft copolymers. For comparison of the theoretical predictions with the existing experimental results, the phase diagrams of graft copolymers were also calculated at strong segregation. The SCFT calculations can accurately capture the characteristics of the phase behavior of graft copolymer melts.     

Calculations of and evidence for chain packing stress in inverse lyotropic bicontinuous cubic phases

Inverse bicontinuous cubic lyotropic phases are a complex solution to the dilemma faced by all self-assembled water-amphiphile systems: how to satisfy the incompatible requirements for uniform interfacial curvature and uniform molecular packing. The solution reached in this case is for the water-amphiphile interfaces to deform hyperbolically onto triply periodic minimal surfaces. We have previously suggested that although the molecular packing in these structures is rather uniform the relative phase behavior of the gyroid, double diamond, and primitive inverse bicontinuous cubic phases can be understood in terms of subtle differences in packing frustration. In this work, we have calculated the packing frustration for these cubics under the constraint that their interfaces have constant mean curvature. We find that the relative packing stress does indeed differ between phases. The gyroid cubic has the least packing stress, and at low water volume fraction, the primitive cubic has the greatest packing stress. However, at very high water volume fraction, the double diamond cubic becomes the structure with the greatest packing stress. We have tested the model in two ways. For a system with a double diamond cubic phase in excess water, the addition of a hydrophobe may release packing frustration and preferentially stabilize the primitive cubic, since this has previously been shown to have lower curvature elastic energy. We have confirmed this prediction by adding the long chain alkane tricosane to 1-monoolein in excess water. The model also predicts that if one were able to hydrate the double diamond cubic to high water volume fractions, one should destabilize the phase with respect to the primitive cubic. We have found that such highly swollen metastable bicontinuous cubic phases can be formed within onion vesicles. Data from monoelaidin in excess water display a well-defined transition, with the primitive cubic appearing above a water volume fraction of 0.75. Both of these results lend support to the proposition that differences in the packing frustration between inverse bicontinuous cubic phases play a pivotal role in their relative phase stability.     

Self-assembly in block polyelectrolytes

The self-consistent field theory (SCFT) complemented with the Poisson-Boltzmann equation is employed to explore self-assembly of polyelectrolyte copolymers composed of charged blocks A and neutral blocks B. We have extended SCFT to dissociating triblock copolymers and demonstrated our approach on three characteristic examples: (1) diblock copolymer (AB) melt, (2) symmetric triblock copolymer (ABA) melt, (3) triblock copolymer (ABA) solution with added electrolyte. For copolymer melts, we varied the composition (that is, the total fraction of A-segments in the system) and the charge density on A blocks and calculated the phase diagram that contains ordered mesophases of lamellar, gyroid, hexagonal, and bcc symmetries, as well as the uniform disordered phase. The phase diagram of charged block copolymer melts in the charge density--system composition coordinates is similar to the classical phase diagram of neutral block copolymer melts, where the composition and the Flory mismatch interaction parameter χ(AB) are used as variables. We found that the transitions between the polyelectrolyte mesophases with the increase of charge density occur in the same sequence, from lamellar to gyroid to hexagonal to bcc to disordered morphologies, as the mesophase transitions for neutral diblocks with the decrease of χ(AB). In a certain range of compositions, the phase diagram for charged triblock copolymers exhibits unexpected features, allowing for transitions from hexagonal to gyroid to lamellar mesophases as the charge density increases. Triblock polyelectrolyte solutions were studied by varying the charge density and solvent concentration at a fixed copolymer composition. Transitions from lamellar to gyroid and gyroid to hexagonal morphologies were observed at lower polymer concentrations than the respective transitions in the similar neutral copolymer, indicating a substantial influence of the charge density on phase behavior.     

Phase behavior of amphiphilic lipid molecules at air-water interfaces: an off-lattice self-consistent-field modeling

We introduce an extended application of the off-lattice self-consistent-field theory (SCFT) to model lipid monolayers at air-water interfaces. The off-lattice SCFT is used without a priori symmetry assumptions on equilibrium morphologies. This enables us to capture asymmetric lipid membranes at air-water interfaces which are otherwise unattainable with a conventional SCF model. Equilibrium morphologies in systems containing lipid molecules, fractions of air, and water are studied as a function of the relative amount of lipid molecules. The corresponding Langmuir isotherms are analyzed to reveal possible phase transitions. We consider both saturated and unsaturated lipid molecules with a branched structure. For saturated lipids, we find two distinct morphological phases, i.e., micellar and lamellar, showing a pronounced first-order phase transition with a well-defined region of phase coexistence. This region is sensitive to the hydrophilicity of lipid molecules and the miscibility of air with water molecules. The phase coexistence is also influenced by the size of hydrophilic and hydrophobic parts of lipid molecules. In contrast, membranes of unsaturated lipids have developed a continuous range of smooth structural transformations from a circular to an ellipsoidal micellar morphology and eventually to a lamellar structure. The shape of the lamella changes from a slightly undulated to a vigorously curved. Unlike saturated lipid membranes, there is no apparent first-order phase transition or a region of phase coexistence for unsaturated lipid membranes. We interpret this as a result of a higher flexibility of unsaturated lipid membranes which enables them to adopt a wider range of conformations in comparison with saturated lipid membranes.     

New lipid family that forms inverted cubic phases in equilibrium with excess water: molecular structure-aqueous phase structure relationship for lipids with 5,9,13,17-tetramethyloctadecyl and 5,9,13,17-tetramethyloctadecanoyl chains

With a view to discovering a new family of lipids that form inverted cubic phases, the aqueous phase behavior of a series of lipids with isoprenoid-type hydrophobic chains has been examined over a temperature range from -40 to 65 degrees C by using optical microscopy, DSC (differential scanning calorimetry), and SAXS (small-angle X-ray scattering) techniques. The lipids examined are those with 5,9,13,17-tetramethyloctadecyl and 5,9,13,17-tetramethyloctadecanoyl chains linked to a series of headgroups, that is, erythritol, pentaerythritol, xylose, and glucose. All of the lipid/water systems displayed a "water + liquid crystalline phase" two-phase coexistence state when sufficiently diluted. The aqueous phase structures of the most diluted liquid crystalline phases in equilibrium with excess water depend both on the lipid molecular structure and on the temperature. Given an isoprenoid chain, the preferred phase consistently follows a phase sequence of an H II (an inverted hexagonal phase) to a Q II (an inverted bicontinuous cubic phase) to an L alpha (a lamellar phase) as A* (cross-section area of the headgroup) increases. For a given lipid/water system, the phase sequence observed as the temperature increases is L alpha to Q II to H II. The present study allowed us to find four cubic phase-forming lipid species, PEOC 18+4 [mono- O-(5,9,13,17-tetramethyloctadecyl)pentaerythritol], beta-XylOC 18+4 [1- O-(5,9,13,17-tetramethyloctadecyl)-beta- d-xylopyranoside], EROCOC 17+4 [1- O-(5,9,13,17-tetramethyloctadecanoyl)erythritol], and PEOCOC 17+4 [mono- O-(5,9,13,17-tetramethyloctadecanoyl)pentaerythritol]. The values of T K (hydrated solid-liquid crystalline phase transition temperature) of the cubic phase-forming lipids are all below 0 degrees C. Quantitative analyses of the lipid molecular structure-aqueous phase structure relationship in terms of the experimentally evaluated "surfactant parameter" allow us to rationally select an optimum combination of hydrophilic/hydrophobic part of a lipid molecule that will form a desired phase in a desired temperature range.     

蔗糖对MO/水立方液晶体系流变性质的影响

主要研究了蔗糖对甘油单油酸脂(monoolein, MO)/水立方液晶体系的流变学性质及其相行为的影响. 根据体系流变性质的变化和偏光显微镜照片, 得出随着蔗糖含量的增加, MO/水体系发生了由反相立方液晶到反相六角状液晶的相转变. 蔗糖与MO分子通过氢键相互作用, 减弱了两亲分子间的静电斥力. 当蔗糖含量增加到一临界值时, 体系的立方结构被破坏, 继续增加蔗糖的含量, 体系就会形成新的反相六角状液晶.     

Fluorescence probing of the temperature-induced phase transition in a glycolipid self-assembly: hexagonal ↔ micellar and cubic ↔ lamellar

Water-driven self-assembly of lipids displays a variety of liquid crystalline phases that are crucial for membrane functions. Herein, we characterize the temperature-induced phase transitions in two compositions of an aqueous self-assembly system of the octyl β-D-glucoside (βGlcOC(8)) system, using steady-state and time-resolved fluorescence measurements. The phase transitions hexagonal ? micellar and cubic ? lamellar were investigated using tryptophan (Trp) and two of its ester derivatives (Trp-C(4) and Trp-C(8)) to probe the polar headgroup region and pyrene to probe the hydrophobic tail region. The polarity of the headgroup region was estimated to be close to that of simple alcohols (methanol and ethanol) for all phases. The pyrene fluorescence indicates that the pyrene molecules are dispersed among the tails of the hydrophobic region, yet remain in close proximity to the polar head groups. Comparing the present results with our previously reported one for βMaltoOC(12), increasing the tail length of the hexagonal phase from C(8) to C(12) leads to less interaction with pyrene, which is attributed to the more random and wobbling motion of the longer alkyl tail. We measured a reduction (more hydrophobic) in the ratio of the vibronic peak intensities of pyrene (I(1)/I(3)) for the lamellar phase compared to that of the cubic phase. The higher polarity in the cubic phase can be correlated to the nature of its interface, which curves toward the bulk water. This geometry also explains the slight reduction in polarity of the headgroup region compared to the other phases. Upon the addition of Trp-C(8), the fluorescence lifetime of pyrene is reduced by 28% in the lamellar and cubic phases, whereas the I(1)/I(3) value is only slightly reduced. The results reflect the dominant role of dynamic interaction mechanism between the C(8) chain of Trp-C(8) and pyrene. This mechanism may be important for these two phases since they participate in the process of membrane fusion. Both lipid compositions show completely reversible temperature-induced phase transitions, reflecting the thermodynamic equilibrium structures of their mesophases. Probing both regions of the different lipid phases reveals a large degree of heterogeneity and flexibility of the lipid self-assembly. These properties are crucial for carrying out different biological functions such as the ability to accommodate various molecular sizes.     

Lamellar-non-lamellar phase transitions in synthetic glycoglycerolipids studied by time-resolved X-ray diffraction

The phase sequences of eight fully hydrated synthetic, stereochemically pure glycoglycerolipids with saturated alkyl chains 12-18 carbon atoms long and a glucose, galactose or mannose head group are followed in real time during heating and cooling scans using synchrotron X-ray diffraction. One of them, 1,2-di-O-hexadecyl-3-O-β-D-glucosyl-sn-glycerol, has been characterized by X-ray diffraction for the first time. A summary of the lamellar-non-lamellar transition sequences and reversibility for all eight glycoglycerolipids studied is provided. It includes also observations of intermediate phases, previously not detected. Lattice parameters of the various phases have been determined as functions of chain length in monoglucosides. While the repeat periods of the lamellar phases increase linearly with chain length, an anomalously high lattice spacing of the inverted hexagonal phase is observed at a chain length of 14 carbon atoms. This maximum coincides with the disappearance of the cubic phases from the phase sequence upon chain elongation from 12 to 14 carbon atoms. It thus appears that the expanded H_II phase in 14-Glc retains structural characteristics of the anticipated cubic phases. Upon heating to high temperatures, its high lattice spacing gradually approaches that of the 'normal' hexagonal phase. A direct transition from lamellar subgel to inverted hexagonal phase has been observed to proceed without intermediate structures, but with an extended phase coexistence region, in 1,2-di-O-tetradecyl-3-O-β-D-galactosyl-sn-glycerol and 1,2-di-O-octadecyl-3-O-β-D-galactosyl-sn-glycerol. This transition is not reversible on cooling when lamellar phases skipped in the heating scan intervene. By contrast, the direct lamellar gel-inverted hexagonal phase transitions are fully reversible with minor or absent temperature hysteresis.     

Effect of temperature and water content on the structure of 1,2-propanediol and 1,3-propanediol: Near-infrared spectroscopic study

Effect of temperature and water content on the structure of 1,2-propanediol (12PD) and 1,3-propanediol (13PD) in the liquid phase has been studied by Fourier-transform near-infrared (FT-NIR) spectroscopy. In addition, the spectra of both diols in CCl₄ solutions at various concentrations were measured. The experimental spectra were analyzed by two-dimensional (2D) correlation approach and chemometric methods. The present results give no evidence that 12PD form the intramolecular hydrogen bonding. In contrast, significant amounts of 13PD molecules in diluted CCl₄ solution is involved in the intramolecular hydrogen bonding. At higher concentrations the intramolecular hydrogen bonds are broken and replaced by the intermolecular ones. The structure of pure liquid propanediols is determined by the intermolecular hydrogen bonding. Unlike for monohydroxyl alcohols, addition of water to propanediols leads to faster temperature-induced breaking of the hydrogen-bonded associates. However, variation of water content at constant temperature does not influence the structure of both diols. In this respect behavior of propanediols is similar to that of the monohydric alcohols. The molecules of water in the mixtures are hydrogen bonded to the diols and act as a double proton donor. This bonding appears to be stronger than that in bulk water.     

Hydrophilic interactions between organic and water molecules as models for monolayers at the gas/water interface

Model clusters of surfactant prototypes with small number of water molecules are calculated at different levels of theory. All approaches used yield correct trends in the variation of the dipole moment upon tail elongation or polar headgroup variation. Models including one, two, or more water molecules are optimized. The most stable structures are those with maximum number of atoms involved in hydrogen bonding. The normal components of the dipole moment prove to be less sensitive to the nature (aliphatic or aromatic) of the hydrophobic tail, in accord with findings from the phenomenological models. Values of the dipole moment approaching the experimental estimates required inclusion of sufficient aqueous environment (>20 water molecules per hydrophilic head) and of lateral intersurfactant interactions into the model.     

Chirality Induction through Nano-Phase Separation: Alternating Network Gyroid Phase by Thermotropic Self-Assembly of X-Shaped Bolapolyphiles

The single gyroid phase as well as the alternating double network gyroid, composed of two alternating single gyroid networks, hold a significant place in ordered nanoscale morphologies for their potential applications as photonic crystals, metamaterials and templates for porous ceramics and metals. Here, we report the first alternating network cubic liquid crystals. They form through self-assembly of X-shaped polyphiles, where glycerol-capped terphenyl rods lie on the gyroid surface while semiperfluorinated and aliphatic side-chains fill their respective separate channel networks. This new self-assembly mode can be considered as a two-color symmetry-broken double gyroid morphology, providing a tailored way to fabricate novel chiral structures with sub-10 nm periodicities using achiral compounds.     

Chirality Induction through Nano‐Phase Separation: Alternating Network Gyroid Phase by Thermotropic Self‐Assembly of X‐Shaped Bolapolyphiles

The single gyroid phase as well as the alternating double network gyroid, composed of two alternating single gyroid networks, hold a significant place in ordered nanoscale morphologies for their potential applications as photonic crystals, metamaterials and templates for porous ceramics and metals. Here, we report the first alternating network cubic liquid crystals. They form through self‐assembly of X‐shaped polyphiles, where glycerol‐capped terphenyl rods lie on the gyroid surface while semiperfluorinated and aliphatic side‐chains fill their respective separate channel networks. This new self‐assembly mode can be considered as a two‐color symmetry‐broken double gyroid morphology, providing a tailored way to fabricate novel chiral structures with sub‐10 nm periodicities using achiral compounds.     

α-生育酚在模型生物膜中的分子动力学模拟

用分子动力学方法模拟了280, 310和350 K下α-生育酚在二豆蔻酰磷脂酰胆碱、二豆蔻酰磷脂酰乙醇胺、二硬脂酰磷脂酰胆碱和二硬脂酰磷脂酰乙醇胺双层膜中的性质, 包括了空间位置、氢键、取向和动力学性质, 取得了如下的结论. 第一, 生育酚头部的羟基一般位于脂双层亲疏水界面的下方, 升高温度将促进羟基向膜双层的中心移动, 在350 K时观察到了在上下两个单层间的翻转. 第二, 生育酚主要与磷脂的酯基形成氢键, 几乎不与磷脂酰乙醇胺的氨基形成氢键; 比较生育酚与磷脂酰胆碱和乙醇胺形成的氢键后发现, 后者更稳定. 第三, 生育酚的头部在膜中取向多变, 与膜的法线夹角不固定, 尾部的构象也很复杂. 第四, 在温度较低时, 生育酚的侧向扩散系数与磷脂的相当, 但在350 K时其扩散速度明显加快; 在垂直方向生育酚的扩散速度很慢.     

Redox activity and diffusion of hydrophilic, hydrophobic, and amphiphilic redox active molecules in a bicontinuous cubic phase

The objective was to examine how a bicontinuous cubic phase influences the diffusion and electrochemical activity of dissolved molecules. The cubic phase is a structure with three-dimensional continuous channels of water separated by an apolar membrane. A redox active molecule can dissolve in three different environments. A hydrophobic molecule will prefer the interior of the membrane, a hydrophilic molecule will prefer the water channels, and an amphiphilic molecule will be situated with its headgroup at the surface of the membrane and its tail in the interior. The electrochemical activity was measured with cyclic voltammetry and the transport behavior with chronocoulometry. All the molecules were redox active in the cubic phase; that is, all the molecules could reach the surface of the electrode and react. The cubic phase made the kinetics of the charge transfer slower, showing a quasi-reversible behavior. The reason may be that a layer of the membrane adheres to the hydrophobic electrode surface. The diffusion experiment showed that the diffusion was slower than in solution. The molecules that were restricted to diffuse within the membrane gave particularly low mass transport rates.     

Pure silica chabazite molecular spring: a structural study on water intrusion-extrusion processes

Water intrusion-extrusion isotherms performed at room temperature on hydrophobic pure silica chabazite show that the water-Si-CHA system displays real spring behavior. However, differences in pressure-volume diagrams are observed between the first and the other intrusion-extrusion cycles, indicating that some water molecules interact with the inorganic framework after the first intrusion. (29)Si and especially (1)H solid-state NMR showed the creation of new defect sites upon the intrusion-extrusion of water and the existence of two kinds of water molecules trapped in the supercage of Si-CHA: a first layer of water strongly hydrogen bonded with the silanols of the framework and a subsequent layer of liquidlike physisorbed water molecules undergoing interaction with the first layer. This hydrogen bonding scheme is also supported by X-ray powder diffraction.     

The Temperature‐Dependent Hydrogen‐Bonding Signature of Lipids Monitored in the Far‐Infrared Domain

Phospholipids are studied by means of Fourier transform infrared (FTIR) spectroscopy in the mid‐ and far‐infrared spectral ranges, thereby establishing the hydrogen‐bonding continuum as a function of the temperature. The well‐known mid‐infrared spectrum of the phospholipid layer clearly shows a temperature‐dependent phase transition. In the far‐infrared region (from 300 to 50 cm^?1), an alternation of the interaction between the phospholipids and water molecules is found. The hydrogen‐bonding network ensemble and bound water molecules can be monitored in this spectral region. The lipid structure is found to strongly influence the intermolecular hydrogen‐bonding interplay. Thus, studies in the far‐infrared region provide significant information—at the molecular level—about the intermolecular hydrogen‐bonding signature of self‐assembled phospholipids.     

Lamellar to inverted hexagonal mesophase transition in DNA complexes with calamitic, discotic, and cubic shaped cationic lipids

In this study, we report on the lipid tail molecular shape/size effect on the mesophase self-assembly behaviors of various cationic lipids complexed with double-stranded DNA. The molecular shape of the cationic lipids was tailored from rodlike (a cyanobiphenyl imidazolium salt) to discotic (a triphenylene imidazolium salt), and finally to cubic [a polyhedral oligomeric silsesquioxane (POSS) imidazolium salt]. An increase in the cross-sectional area of the hydrophobic tails with respect to the hydrophilic imidazolium head induced a negative spontaneous curvature of the cationic lipids. As a result, a morphological change from lamello-columnar (L(C)(alpha)) phase for the DNA-cyanobiphenyl imidazolium salt (DNA-rod) and DNA-triphenylene imidazolium salt (DNA-disk) complexes to an inverted hexagonal columnar (H(C)(II)) phase for the DNA-POSS imidazolium salt (DNA-cube) complex was observed. The DNA-rod complex had a typical smectic A (SmA) L(C)(alpha) morphology, whereas the DNA-disk complex had a double lamello-columnar liquid crystalline phase. However, when the lipid tail changed to POSS, an H(C)(II) morphology was achieved. These morphological changes were successfully characterized by X-ray diffraction and transmission electron microscopy. We expect that these liquid crystalline and crystalline DNA hybrid materials may become potential functional materials for various applications such as organic microelectronics and gene transfection.     

Aqueous self-assembly of phytantriol in ternary systems: effect of monoolein, distearoylphosphatidylglycerol and three water-miscible solvents

The aqueous phase behavior of phytantriol (PT) in mixtures of monoolein (MO), distearoylphosphatidylglycerol (DSPG), propylene glycol (PG), polyethylene glycol 400 (PEG 400) and 2-methyl-2,4-pentanediol (MPD) was investigated by visual inspection, polarized light microscopy and small angle X-ray diffraction at room temperature. The phase diagrams of PT and MO in water are qualitatively very similar and PT/MO mixtures in excess water form one cubic phase of space group Pn3m irrespective of mixing ratio. The addition of the charged membrane lipid DSPG to the PT system gives rise to a considerable water swelling of the cubic phases as well as the occurrence of a cubic phase of space group Im3m. Whereas all three solvents studied give rise to a sponge (L3) phase in the MO-water system, this phase was only found when MPD was added to the PT-water system. The results are discussed with respect to the chemical differences between PT and MO.