Axial-bundle phases--new modes of 2D, 3D, and helical columnar self-assembly in liquid crystalline phases of bolaamphiphiles with swallow tail lateral chains

Two series of polyphilic molecules composed of a rigid and linear p-terphenyl core, terminated at both ends with polar glycerol groups capable of hydrogen bonding, and two branched swallow tail-type lateral chains, composed of a fluorinated and a nonfluorinated branch or two fluorinated branches, were synthesized and investigated by differential scanning calorimetry, polarizing microscopy, and X-ray diffraction (XRD) with respect to their self-assembly in thermotropic liquid crystalline (LC) phases. Hexagonal columnar phases were formed by all molecules, at least at the highest temperature. In these phases the columns are composed of a core of aromatic rods and an aliphatic shell. The aromatic rods form bundles which are rotationally averaged and lie parallel to the column long axis. This unique organization is proven by different optical and XRD methods. The aromatic and glycerol groups inside the rod bundles are segregated into alternating segments. Depending on temperature and molecular structure, long-range intercolumnar correlation of this periodicity could take place, leading to a 3D-ordered LC phase with rhombohedral R ?3m symmetry. The bundles are embedded in the matrix of the lateral chains, which is divided into fluoroalkyl- and aliphatic-rich regions. In the 2D columnar phase the fluorinated regions take the form of either straight columns running along the edges of the hexagonal Voronoi cells or, for compounds with a higher degree of fluorination, fuse to a hexagonal honeycomb enclosing the aromatic cores. In the R ?3m phase the fluorine-rich chains are preferentially found along right- and left-handed helices wound around the 3(1) screw axes between the main aromatic columns.     

基于聚环氧乙烷链和亚苯基的刚棒-线团分子自组装

含有亲水性聚环氧乙烷(PEO)链和多个亚苯基作为刚棒的刚棒-线团(rod-coil)分子,由于具有很强的π-π堆积作用和微相分离作用,可以在本体和溶液中产生有序排列形成各种组装体。材料的性能主要依靠分子聚集体来体现,因此研究分子聚集体的性质和用途成为当前超分子领域研究的重要内容。刚棒嵌段的形状影响着分子排列的方式、自组织性能和微结构,本文总结了Y型、T 型、O型、 K型、n型、螺旋桨型和哑铃型等不同形状分子的自组装行为,并展望了此类刚棒-线团分子未来研究的方向。     

Nanoscale molecular rods with a new building block for solubility enhancement

A new building block bearing a [1,3]dioxolo[4,5- f][1,3]benzodioxole core was developed to enhance the solubility of molecular rods by lateral alkyl chains. On incorporation in molecular rods with oligospiroketal structure, the straight geometry is retained, which was concluded from the X-ray crystal structure analysis of one of the rods. The determination of the solubility of a collection of rods bearing this building block revealed that already a butyl group efficiently hinders the aggregation of the rods and consequently causes a considerable enhancement of the solubility. Piperidine rings are located at the ends of the rods, which offer the opportunity for versatile functionalization. Thus, an N, N'-bis(azidoacetyl)-functionalized rod was prepared, which could serve as rigid linkage, initiated by a "Click" reaction.     

Molecular alignment of rigid rods in nonrigid spherical pores

We have investigated the orientation ordering of two shish-kebab chains confined by spherically harmonic potentials through Monte Carlo simulations and asymptotic analysis. The rigid rod is modeled as shish-kebab chains consisting of tangent hard spheres aligned in the same axis, and the harmonic potential is chosen to model nonrigid cavities. We first show that the interactions between a rod and the spherically harmonic potential are independent of chain orientation, indicating that the alignment of two confined rods arises from the excluded volume interactions alone. In the strong fields, the order parameter of two confined rods converges to different values, depending on the parity of chain length. From asymptotic order parameters, we find that the rods of odd-number beads rotate more freely even under the limiting strong confinement. However, the two rods of even-number beads are essentially trapped in a configuration of perpendicular alignment through intercalation of their central grooves. We attribute the dependence of the parity of chain length to the different locations of the center-of-mass in a rod for these two cases. Furthermore, we compare the shish-kebab chains with different rod models in the simulations, and utilize these models to explore the effect of the local rod smoothness on molecular alignment. Our findings suggest that increasing local rod smoothness enhances the rotational degree of freedom for confined rods, and the effect of local rod roughness emerges under strong enough applied potentials.     

长棒状T形三组分双亲分子相行为的耗散粒子动力学模拟

采用耗散粒子动力学方法建立了以侧链长度和温度为变量的相图,计算了相关结构参数及每个液晶相对应侧链的有效体积分数,重现了实验上观察到的五边形、六边形柱状相及刚棒在层内位置的有序堆垛层状结构.研究发现,长棒状T形三组分双亲分子相图中的相序列和相结构符合相关报道,并且侧链有效体积分数f L与实验值一致.表明粗粒化模型反映了真实T形分子的主要特性,如分子的拓扑结构、排除体积效应及分相趋势、刚棒液晶基元的长径比和侧链的空间效应.证实了侧链尺寸对主/侧链非线性连接体系的自组装结构及各组成单元的位置分布的重要作用.     

Effects of Nematic Ordering on the Relaxation Spectrum of a Polymer Network with Included rod‐like Particles: Mean‐Field Approximation

Summary: Equilibrium and local dynamic properties of ordered polymer networks with included rod‐like particles are considered using a simplified network model. Lagrange multipliers in the equations of motion of rigid rods are replaced by their averaged values. This approximation corresponds to modelling rod‐like particles by elastic Gaussian springs with mean‐square lengths independent of the orientational order. Nematic‐like interactions between network segments and rods are taken into account in terms of the Maier‐Saupe mean‐field approximation. Nematic ordering of rods induces network segments ordering and changes the relaxation spectrum of the network. The relaxation spectrum of the ordered network splits into two main branches for parallel and perpendicular components of chain segments with respect to the director. Relaxation times of a polymer network are calculated as functions of the wave number for the corresponding normal mode and of the order parameter taking into account both the dynamic factor (determined by friction effects) and the statistical factor (related to mean‐square fluctuations of segment projections). We compare the relaxation spectra of ordered unstretched polymer networks with fixed boundaries with those for polymer networks at free boundaries. A polymer network with free boundaries is stretched along the director. This produces additional fine structure of the two main branches in the relaxation spectrum.

Cell of a three‐chain network model with included rods.     

Conformational and dynamic effects of pendant-PEK chains on rigid-rod polymers by molecular dynamics simulation

Molecular dynamics simulations were performed with models of a single-component ‘hairy-rod’ molecular composite composed of flexible meta poly(aryl ether ketone) (mPEK) chains (the ‘hairs’) grafted to a poly(p-phenylene benzobisthiazole) (PBZT) rigid-rod backbone. The molecular-composite concept, conceived to circumvent solubility problems and improve compressive strength of PBZT, relies on an even distribution of rods in a coil-like matrix. Two-molecule simulations show that the pendant mPEK chains associate with the rods causing large bends in the rod backbone. Simulations of bulk systems imply that the mPEK chains greatly reduce the correlation of rod orientation without altering rod spacing. Radial distribution and orientation correlation functions as well as correlation volumes are computed.     

Are the continuum and the lattice representation of freely jointed chains equivalent?

The partition function and force elongation relation of freely jointed chains of rigid rod segments are calculated for arbitrary dimensions in the continuum and on a lattice. The qualitative differences between these models are discussed for polymers embedded in spaces of various dimensions. These investigations show that chains of rigid rod segments in higher dimensions are equivalent to chains of segments with internal degree of freedom in lower dimensions. Networks composed of these chains show a similar behaviour as uniaxial stretching experiments of rubber-like materials.     

Freely jointed chain with variable segment number and length

The configurational properties of a molecular chain are investigated. The chain consists of rigid rod segments, which are consecutively correlated. This correlation is described by a two-state model. In the first state, neighboring segments are freely hinged togethers whereas in the second state they are parallel. The partition function and the force-extension relation are calculated. They show significant differences as compared to the usual results for a freely jointed chain. Thus the experimentally observed force-extension relation of polymeric networks buil up of such chains might be affected.     

Ordered Arrangements of Semiflexible Polymers at the Interface with Solids

The molecular arrangements and conformations of dense systems of semiflexible polymer chains near solid surfaces have been investigated by Monte Carlo simulations. At variance with the results obtained for more flexible chains, the mean square end‐to‐end distance and the mean square radius of gyration of chains in contact with the surfaces are found to be much higher than in the bulk. This is related to the increased length of the surface trains and to the increased tendency of such trains to form rod‐like strands. As a result, the first layer of polymer units in contact with the surfaces consists of two‐dimensional domains of locally parallel chain segments. The width of these domains is several times the transverse diameter of the chains.     

Structure of nonionic surfactant micelles in the ionic liquid ethylammonium nitrate

The structure of micelles formed by nonionic polyoxyethylene alkyl ether nonionic surfactants, C n E m , in the room-temperature ionic liquid, ethylammonium nitrate (EAN), has been determined by small-angle neutron scattering (SANS) as a function of alkyl and ethoxy chain length, concentration, and temperature. Micelles are found to form for all alkyl chains from dodecyl through to octadecyl. Dodecyl-chained surfactants have high critical micelle concentrations, around 1 wt%, and form weakly structured micelles. Surfactants with longer alkyl chains readily form micelles in EAN. The observed micelle structure changes systematically with alkyl and ethoxy chain length, in parallel with observations in aqueous solutions. Decreasing ethoxy chain length at constant alkyl chain length leads to a sphere to rod transition. These micelles also grow into rods with increasing temperature as their cloud point is approached in EAN.     

Shape of fluid vesicles anchored by rigid rod

A method combined the self-consistent field theory (SCFT) for the rigid rod with the Helfrich curvature elasticity theory for the vesicle has been developed for studying the shape of vesicles anchored by rigid rod. Both the deformation of the vesicle and the density distribution of rod segments can be obtained. Because of the vesicle's impenetrability for the rod segments and the decrease of the available space for the rod orientational configurations, the anchored rod segments exert the inhomogeneous entropic pressure on the vesicle and induce the change of vesicle shape. The interaction between the rod segments and the vesicle membrane exerts an extra tension to the membrane. Thus the interaction between the vesicle membrane and the rod segments, the rod length, and the bending rigidity of vesicle are investigated as the important factors to the shape transformation of the vesicle and the density distribution of rod segments. This method can be extended to more complicated and real biological systems, such as polymers with different topological architectures/vesicle, multiple chains/vesicle, protein inclusions, etc.     

Breaking Parallel Orientation of Rods via a Dendritic Architecture toward Diverse Supramolecular Structures

Self‐assembled nanostructures of rod‐like molecules are commonly limited to nematic or layered smectic structures dominated by the parallel arrangement of the rod‐like components. Distinct self‐assembly behavior of four categories of dendritic rods constructed by placing a tri(hydroxy) group at the apex of dendritic oligo‐fluorenes is observed. Designed hydrogen bonding and dendritic architecture break the parallel arrangement of the rods, resulting in molecules with specific (fan‐like or cone‐like) shapes. While the fan‐shaped molecules tend to form hexagonal packing cylindrical phases, the cone‐shaped molecules could form spherical motifs to pack into various ordered structures, including the Frank–Kasper A15 phase and dodecagonal quasicrystal. This study provides a model system to engineer diverse supramolecular structures by rod‐like molecules and sheds new light into the mechanisms of the formation of unconventional spherical packing structures in soft matter.     

Ordered nanostructures from self‐assembly of H‐shaped coil–rod–coil molecules

A new class of π‐conjugated, skewed H‐shaped oligomers, consisting of biphenyl, phenylene vinylene, and phenylene ethynylene units as the rigid segment, were synthesized via Sonogashira coupling and Wittig reactions. The coil segments of these molecules were composed of poly(ethylene oxide) (PEO) or PEO with lateral methyl groups between the rod and coil segment, respectively. The experimental results revealed that the lateral methyl groups attached to the surface of the rod and coil segments dramatically influenced the self‐assembling behavior of the molecules in the crystalline phase. H‐shaped rod–coil molecules containing a lateral methyl group at the surface of the rod and PEO coil segments self‐assemble into a two‐dimensional columnar or a three‐dimensional body‐centered tetragonal nanostructures in the crystalline phase, whereas molecules lacking a lateral methyl group based on the PEO coil chain self‐organize into lamellar or hexagonal perforated lamellar nanostructures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 85–92     

Study of controllable aggregation morphology of ABA amphiphilic triblock copolymer in dilute solution by changing the solvent property

We have studied, both experimentally and theoretically, the aggregation morphology of the ABA amphiphilic triblock copolymer in dilute solution by changing the solvent property. Experimental results showed that the micellar morphology changed from spheres to rods and then to vesicles by changing the common solvent from N-N-dimethylformamide (DMF) to dioxane and then to tetrahydrofuran (THF). These controllable aggregates were also obtained by Monte Carlo simulation. The simulative results showed that the solvent property is a key factor that determines the copolymer aggregation morphology. The morphology changed from spheres to rods and then to vesicles by increasing the solvent solubility, corresponding to the change of stretched of the copolymer chains in the micellar cores. This result is in good agreement with the experimental one. Moreover, the simulative results revealed that the end-to-end distant of the ABA triblock copolymer in the vesicle was larger than that in the spheres and rods, indicating that the copolymer chains were more stretched in vesicles than in the spheres and rods. Furthermore, we gave the distribution of the fraction of the chain number with the end-to-end distance. The results indicated that the amount of folded chains is almost the same as that of stretched chains in the vesicle. Although most chains were folded, stretched chains could be found in the rod and sphere micelles.     

Calamitic bolaamphiphiles with (semi)perfluorinated lateral chains: polyphilic block molecules with new liquid crystalline phase structures

Novel bolaamphiphiles consisting of a rigid biphenyl unit, two terminal polar 1,2-diol units and laterally attached (semi)perfluorinated chains have been synthesized via palladium-catalyzed cross coupling reactions as the key step. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light optical microscopy, DSC, and X-ray scattering, and the influences of the length, number, structure, and position of the lateral chain on the mesomorphic properties were studied. A wide variety of unique liquid crystalline phases were found upon elongation of the lateral semiperfluorinated chains. For short- and medium-chain length a series of columnar phases were observed, and upon further elongation of the lateral chain a series of novel mesophases with layer structures were found. In the columnar phases, the nonpolar lateral chains segregate into columns, which are embedded in honeycomb-like networks of cylinders consisting of the biphenyl units. Strings of hydrogen-bonding networks of the diol groups provide cohesive forces, which maintain the overall structure. Changing the length of the lateral chains influences the diameter of the columns and thus determines the number of biphenyl units which are required to surround these columns. The number of these units [four (c2mm, p4mm), five (p2gg), six (p6mm), eight (c2mm) or 10 (p2gg)] defines the shape of the cylinders as well as the lattice type of the columnar phase. It is proposed that the columnar phases with a p2gg lattice result from the regular organization of pairs of cylinders which have a pentagonal cross sectional shape. In the mesophases with layer structure the aromatic rodlike cores are arranged parallel to the layer planes, and the onset of orientational and positional ordering of the biphenyl segments leads to a sequence of subtypes for these lamellar phases (Lam(Iso)-Lam(N)-Lam(X)).     

Self-assembly of coil-rod-coil molecules into bicontinuous cubic and oblique columnar assemblies depending on coil chain length

Coil-rod-coil molecules 1–3, consisting of four biphenyls and a p-terphenyl unit linked together with ether bonds as a rod segment and poly(propylene oxide) (PPO) with a degree of polymerization (DP) of 7, 12, 17 as coil segments were synthesized. These molecules contain lateral methyl groups at 2 and 5 positions of the middle benzene ring of p-terphenyl. The self-assembling behavior of molecules 1–3 was investigated by means of DSC, POM and SAXS in the bulk state. Molecule 1 self-organizes into a lamellar structure in the bulk state and transfers into a bicontinuous cubic structure in the liquid crystalline phase. While, molecules 2, 3 containing longer coil chains than 1 self-assemble into the hexagonal perforated lamellar (HPL) structures and the oblique columnar structures in the solid state and liquid crystalline phase, respectively. These results reveal that self-organizing behavior of such molecules is dramatically influenced by the length of the coil chains connected with the rod building block, as well as the lateral methyl groups incorporating in the middle of the rod segment.     

A self assembled molecular zipper based on a perfluorophenyl-phenyl diacetylene motif

The synthesis and characterization of a molecular rod consisting of a pentafluorophenyl and a para-dodecylphenyl subunit linked by a diacetylene and its large area self assembly into perfect parallel lines consisting of interlocked molecular rods are reported and discussed.     

Longitudinal diffusion of a polar tracer in the outer segments of rod photoreceptors from different species

Vertebrate rod photoreceptors are the ultimate light sensors, as they can detect a single photon. In darkness, rods maintain a high concentration of the intracellular messenger cyclic guanosine monophosphate (cGMP), which binds to and keeps open cationic channels on the plasma membrane of the outer segment. Absorption of a photon by the visual pigment of the rod, rhodopsin, initiates a biochemical amplification cascade that leads to a reduction in the concentration of cGMP and closure of the channels, thereby converting the incoming light to an electrical signal. Because the absorption of a photon and the ensuing reactions are localized events, the magnitude of the response of the rod to a single photon depends on the spread of the decrease in the cGMP concentration along the length of the outer segment. The longitudinal diffusion of cGMP depends on the structural parameters of the rod outer segment, specifically the area and the volume available for diffusion. To characterize the effect of rod outer segment cytoarchitecture on diffusion, we have used fluorescence recovery after photobleaching (FRAP) and examined the mobility of a fluorescent polar tracer, calcein, in the rod outer segments from three species with different outer segment structures: frog (Rana pipiens), mouse (Mus musculus domesticus) and gecko (Gekko gekko). We found that the diffusion coefficient is similar for all three species, in the order of 8-17 microm(2) s(-1), in broad agreement with the predictions by Holcman and Korenbrot (Biophys. J. 2004:86;2566-2582) based on the known cytoarchitecture of rod outer segments. Consequently, the results also support their prediction that the longitudinal spread of light excitation in rods is similar across species.     

Theory of Intrachain Relaxation Spectra for Polymer Networks Possessing a Short- or Long-Scale Ordering. Effects of the Nematic Ordering on the Relaxation Spectrum of a Polymer Network with Included Rods

We discuss the relaxation properties of polymer networks possessing either short-scale ordering caused by rigidity of network strands or long-scale liquid crystalline order. The main topics of the paper are the equilibrium and local dynamic properties of a polymer network ordered due to nematic-like interactions of the network segments with included rod-like particles. A simplified three chain network model is used. Lagrange multipliers in the equations of motion of hard rods are replaced by their averaged values. This approximation corresponds to modelling the rod-like particles by elastic Gaussian springs, their mean-square lengths independent of the ordering. Nematic-like interactions between network segments and rods are taken into account in terms of the Maier-Saupe mean-field approximation. Nematic ordering of rods induces ordering of the network segments. Relaxation spectrum of the ordered network splits into two main branches for the parallel and perpendicular components of the chain segments with respect to the director. We calculate the relaxation times of a polymer network as functions of the wave number. The relaxation spectrum of an isotropic network and that of the ordered network with included rods are compared.