Influence of Smectic Liquid Crystallinity on Lamellar Microdomain Structure in a Main‐Chain Liquid Crystal Block Copolymer Fiber

The microlamellar and smectic liquid crystal (LC) structures of a block copolymer of a main‐chain LC polyester connected at both ends with poly(ethyl methacrylate) are investigated by fiber X‐ray scattering. In the as‐spun fiber, the lamellae are parallel to the fiber axis, while the smectic layers are perpendicular to it. Annealing the as‐spun fiber at a temperature higher than the isotropization temperature (T_i) of the LC segment preserves the lamellae, but the LC structure disappears. Further annealing the fiber at T < T_i improves the lamellar stacking coherence and aligns the smectic layers parallel to the lamellae. In contrast, annealing the as‐spun fiber at T < T_i conserves the smectic layers and arranges the lamellae in parallel to the smectic layers. Thus, the liquid crystallinity affects the lamellar ordering and orientation.

     

Directing the Smectic Layer Orientation by Shear Flow in Hierarchical Lamellar‐within‐lamellar Liquid Crystalline Diblock Copolymers

A self‐assembled lamellar‐within‐lamellar structure of a side chain liquid crystalline diblock copolymer was shear aligned to induce overall alignment and to direct the smectic layer orientation within the copolymer lamellae. The copolymer consisted of a polystyrene block and a poly(methyl methacrylate) block bearing cholesteryl mesogens with only short oxycarbonyloxyethyl spacers separating the mesogens from the backbone. Upon shearing, the copolymer lamellae exhibited uniaxial alignment whereas the smectic layers of the mesogens showed coexisting perpendicular and parallel orientations with respect to the copolymer lamellae. The fraction of the parallel oriented domains could be systematically increased by tuning the oscillation frequency and strain amplitude.

     

Lamellar Morphology of an ABA Triblock Copolymer with a Main‐Chain Nematic Polyester Central Block

A liquid crystal (LC) ABA triblock copolymer with poly(styrene) (PS) A blocks and a main‐chain nematic LC polyester B block is synthesized by atom‐transfer radical polymerization of styrene with an LC polyester macro‐initiator. The nematic LC and PS amorphous phases are segregated from each other to form lamellae with a spacing of 27 nm. The 16 nm‐thick nematic LC lamellae are significantly smaller than the contour length of the LC segment (63 nm), and the nematic director is parallel to the lamellae. The central LC segment is primarily more extended in the lamellar direction, but folds so as to meander through the LC lamella and bridges adjacent PS domains. The lamellar microstructure exhibits a reversible spacing increase of up to 31 nm with increasing temperature, suggesting a corresponding increase in the probability of the chain folding.

     

Effects of Smectic Layer Deformation on Mechanical Properties of Glassy Liquid Crystal Polymer Fibers

In this work, it is demonstrated that the stress response of glassy smectic liquid crystal (LC) polymer fibers is largely dependent on layer deformations. Fibers with smectic layers stacked along the fiber axis (fiber A), prepared by stretching a molten main-chain LC polyester, deform the layers by tensile deformation in the LC state, yielding fibers with layers tilted from the fiber axis (fiber B) and those with divided layers (fiber C). These fibers in the glassy LC state differ in Young's modulus (E), yield stress and strain (σ_y, εσ_y), and strain at break (ε_b). Fiber A has ε_b = 30%; however, fiber B is much more ductile (ε_b > 290%). Fiber C has higher E = 0.97 GPa and σ_y = 82 MPa than fiber A (E = 0.33 MPa and σ_y = 33 MPa). Annealing fiber C in the LC state yields fiber D with large-area layers, similar to fiber A. Fiber D has E = 0.74 GPa and σ_y = 25 MPa, comparable to those of fiber A. Thus, dividing layers improves E and σ_y. This strengthening of glassy smectic LC by dividing layers applies to other polymers with layered structures, including lamellar block copolymers and semi-crystalline polymers.     

Structural study of polybutadiene-poly(ethylene oxide) block copolymers. Influence of the nature of the amorphous block on the refolding of the poly(ethylene oxide) chains

Our earlier studies on block copolymers polystyrene-poly(ethylene oxide) has allowed to state the influence of the molecular weight of the copolymer and of its composition on the refolding of the crystallized poly(ethylene oxide) chains. To precise the effect of the nature of the amorphous block, we have studied by X-ray diffraction and differential scanning calorimetry, the lamellar crystalline structures exhibited by block copolymers polybutadiene-poly(ethylene oxide) (BEO) in the presence of a preferential solvent of one block: xylene for polybutadiene (PB), acetic acid or acrylic acid for poly(ethylene oxide) (PEO). In systems BEO copolymer/preferential solvent of PEO, the lamellar structure with crystallized PEO chains (LCC) exists at temperatures below about 45°C and for solvent concentrations ranging from 0 to a value characteristic of the copolymer. In the LCC structure, the chains of PEO crystallize in two layers; the solvent forms a third layer located between the PEO layers. We have studied the influence of the nature of the amorphous block on the variation of the structural parameters with the solvent concentration. In systems BEO copolymer/preferential solvent of PB, the lamellar structure with crystallized PEO chains (LC) appears at temperatures below about 50°C and for all solvent concentrations where the mesophase exists. In the LC structure, the chains of PEO crystallize folding in two superposed layers. We have established the influence of the solvent concentration and the nature of the amorphous block on the number of folds and the crystallinity of the PEO chains.     

Correlation between Molecular Architecture,Morphology, and Deformation Behaviour of Styrene/Butadiene Block Copolymers

The correlation between the molecular architecture, morphology, and micromechanical deformation behaviour of styrene/butadiene (SB) block copolymers with different architectures (linear and star block copolymers, total styrene content, Φ_ST = 0.74) was studied using dynamic mechanical analysis (DMA), uniaxial tensile testing, scanning force microscopy (SFM), and high voltage electron microscopy (HVEM). Deformation of the individual phases under uniaxial strain at the molecular level was monitored by Fourier transform infrared (FT‐IR) spectroscopy. It was demonstrated that the morphology and deformation behaviour of these block copolymers are strongly influenced by their molecular topology, block symmetry, and the nature of the interface between the component blocks. While the cylindrical morphology (hexagonal polybutadiene (PB) cylinders in polystyrene (PS) matrix) was observed in a symmetric SBS triblock copolymer with Φ_ST = 0.74, a “two‐component three‐phase” morphology was found in an asymmetric star block copolymer having an equivalent chemical composition and an SBS arm structure. Likewise, an SBS triblock copolymer with a composition identical to the former ones but with highly asymmetric PS end blocks revealed a lamellar morphology. While no locally confined deformation zones were observed in the lamellar block copolymers, the cylindrical block copolymer was found to deform through the formation of highly localised craze‐like deformation zones.

SFM phase images showing lamellae‐like morphology of the star block copolymer ST2, phase difference 25 degrees.     

A Smectic A Liquid Single Crystal Elastomer (LSCE): Phase Behavior and Mechanical Anisotropy

The analysis of the phase behavior of a smectic A (S_A) elastomer reveals a nematic phase existing within a small temperature range below the isotropic state. Stress‐optical measurements in the pretransformational regime of the isotropic state indicate smectic as well as nematic fluctuations yielding a critical exponent of γ = 0.65. The formation of the liquid single crystal elastomer (LSCE) at the isotropic to liquid crystalline phase transformation equals a nematic LSCE. At the nematic to S_A phase transformation, the orientation of the director remains constant while the tendency of the network strands towards an oblate equilibrium conformation is suppressed by the high modulus parallel to the smectic layer normal. The mechanical anisotropy of the S_A‐LSCE as a function of the temperature is characterized by entropy elasticity perpendicular to the smectic layer normal. Parallel to the layer normal the mechanical response is determined by the enthalpy elastic response of the smectic layers having a modulus larger by about two orders of magnitude. In this direction the modulus decreases linearly with increasing temperature and reflects the falling stability of the layers. Accordingly, above a deformation of about 2% the homogeneous layered structure breaks down at a threshold stress that also falls linearly with increasing temperature while the threshold strain remains constant at about 2% elongation.     

Thermotropic Liquid Crystals of Main‐Chain Polyesters having a Mesogenic 4,4′‐Biphenyldicarboxylate Unit, 14

Summary: Anisotropy in molecular motions of a glassy smectic CA (SmCA) liquid crystal of a main‐chain BB‐5(3‐Me) polyester was examined by dynamic mechanical analysis (DMA) for fibrous monodomain samples with two distinct orientations of smectic layers perpendicular and parallel to the fiber axis. The α‐process attributed to micro‐Brownian motion of the polymer on the glass transition shows clear anisotropy explainable by the nature of the smectic layer structure. Time‐temperature superposition is applicable to the α‐process, so that the frequency of the micro‐Brownian motion in the SmCA phase can be estimated despite the narrow frequency range in DMA. The results suggest that the decoupling of the molecular motions in two characteristic directions are parallel and perpendicular to the layer. The amplitude of the micro‐Brownian motion in the layer normal direction is 3 times smaller than that in the layer direction, and the frequency in the layer normal direction is 2.5 times higher than that in the layer direction.

Illustration of molecular packing structures: fiber A (left side) and fiber B (right side) with the orientations of polymers parallel and perpendicular to the fiber, respectively.     

Synthesis and characterization of non‐covalent liquid crystalline diblock copolymers

Poly(dimethylaminoethylmethacrylate)‐b‐poly(sodium methacrylate) diblocks, (polyDMAEMA‐b‐polyMA), were synthesized as precursors of liquid crystalline (LC) copolymers. These LC copolymers were prepared by proton‐transfer between a carboxylic acid‐containing mesogen (A) and the dimethylamino substituent of the polyDMAEMA block and by electrostatic interactions between the polyMA subunits and an ammonium‐containing mesogen (B). When mesogen A is complexed with DMAEMA units, a dramatic enhancement of the mesophase stability is noted. The mesogenic properties of these LC copolymers were compared to those ones of the parent LC homopolymers, in relation to the copolymer composition. The supramolecular organization of the LC diblock copolymers was studied by small‐angle X‐ray scattering (SAXS), and smectic mesophases were observed in some LC (co)polymers. For samples containing a major LC block, a smectic mesophase coexists with microphases formed by the second amorphous block. Different organizations, including a body‐centered lattice of spheres and a hexagonal array of cylinders, were observed, depending on the investigated copolymers and their composition. When the LC block is the minor component, it forms microdomains too small for a liquid crystalline order to emerge. Moreover, no supramolecular organization of these dispersed microdomains was detected by SAXS. A homogeneous smectic mesophase was observed when the two blocks are bearing liquid crystalline moieties.     

Ferroelectric liquid crystalline elastomers, 1. Variation of network topology and orientation

In order to evaluate structure‐property relations in ferroelectric LC‐elastomers concerning netpoint topology and netpoint density, three different elastomers were investigated. As far as the netpoint topology is concerned systems with a crosslinking within the smectic layers (intra‐layer) and between different layers (inter‐ layers) behave differently. Only the inter‐layer systems (elastomer E2 ) are able to stabilize the polar order of the smectic‐C* phase. Increasing the crosslinking density by stepwise crosslinking leads to a continuous shift of the ferroelectric hysteresis. Two switching times with and against the elastic field of the network are observed. The difference between these switching times increases with increasing crosslinking density. From this difference an electric field can be calculated, which is necessary for a compensation of the elastic field of the network. Crosslinking of elastomer E2 in the smectic A‐phase leads to a stabilization of a macroscopically untilted state. If a tilt is induced in the crosslinked smectic A‐phase by application of an electric field (electroclinic effect) the network keeps a memory of the polar state present during crosslinking.     

Mechanical field orientation of chiral smectic C polymer networks

Deformation experiments on chiral smectic C elastomers show a significant influence of the layer structure. In contrast to nematic systems a deformation of the smectic polydomain does not lead to a uniform director orientation. Obviously, the smectic layers lock in the polydomain structure in a transient network. Therefore, on experimental time scale, no equilibrium conditions can be achieved during the deformation process in the smectic state. Reducing the influence of the smectic layers by swelling the sample with toluene offers the possibility to prepare samples with uniform director orientation. In these samples the smectic layers are distributed on a cone around the director, preventing ferroelectric properties. Further deformation experiments on these aligned samples show a strong interaction of the layer orientation with the mechanical field in the smectic state. Layers which enclose large angles with the mechanical field realign to layer orientations with small angles to the deformation direction. This orientation behaviour leads to an anisotropic distribution of the layer on the cone causing a non-centrosymmetric phase structure. This anisotropic alignment can be locked in by chemical crosslinking, leading to a highly transparent non-centrosymmetric bulk material.     

Phase diagram and structural study of polystyrene — poly(ethylene oxide) block copolymers, 1. Systems polystyrene/poly(ethylene oxide)/diethyl phthalate

By means of low angle X-ray scattering and differential scanning calorimetry, we have drawn the phase diagrams of poly(ethylene oxide)-polystyrene block copolymers in the presence of diethyl phthalate as a preferential solvent of polystyrene. Such systems exhibit two liquid-cristalline structures in terms of temperature and solvent concentration. At temperatures below about 50°C, a lamellar structure (LC) with crystallized poly(ethylene oxide) chains exists. Between 50 and about 170°C, we find a structure with melted poly(ethylene oxide) chains. Like for amorphous block copolymers, the type of the latter structure is governed by the copolymer composition. We have shown that in the LC structure, the poly(ethylene oxide) chains crystallize folding in two superposed layers, and we have studied the number of folds and the crystallinity of the poly(ethylene oxide) blocks as a function of solvent concentration, composition and rel. mol. mass of the copolymer, and of the crystallization temperature.     

Enhancement of the Photoalignment Stability of Block Copolymer Brushes by Anchor Segments

It is known that the use of a homopolymer derived from a relatively high‐surface‐energy block moiety in a diblock copolymer as a substrate can facilitate phase separation of the block copolymer in the form of a layered structure parallel to the substrate. This is an effective approach for preparing a photoresponsive surface molecular brush for liquid‐crystal alignment. However, the alignment of the polymer brushes soon diminishes during elastic deformation of the polymeric substrate due to the weak interaction between the copolymer and polymeric substrate in the interfacial blend layer. In this study, rigid segments composed of N‐p‐anisylmaleimide were controllably introduced via copolymerization into a series of well‐defined diblock copolymers as “anchors” to resist the “drift” of the copolymer from a polymethyl methacrylate substrate at above its glass transition temperature. The anchoring mechanism and the stability of the aligned azobenzene polymeric brushes were systematically studied and compared.     

Synthesis and Crosslinking of a Series of Dimeric Liquid Crystalline Epoxy Resins Containing Imine Mesogens

Eight aromatic imine liquid crystalline (LC) diepoxides with dimeric architecture from readily available commercial reagents were synthesized and characterized by spectroscopic techniques. We characterized their liquid crystalline mesophases by differential scanning calorimetry (DSC), hot‐stage polarized‐light optical microscopy (POM) and wide‐angle X‐ray diffraction (WAXS). The mesomorphism of the products was related to their structure, which varies in the length of the central spacer. Odd‐membered spacer mesogens lead to the formation of smectic C mesophases, whereas even‐membered samples of this series lead to the formation of smectic A and/or nematic mesophases. By relating the calculated molecular parameters to the layer spacings obtained by WAXS, we deduced the organization of the mesogens in the smectic phases, since the dimeric compounds present a variety of possible arrangements. Liquid crystal thermosets (LCTs) were obtained from these monomers by isothermal curing with equimolar amounts of 2,4‐diaminotoluene (DAT). The curing process was studied by calorimetry and their thermal stability was evaluated by thermogravimetry (TGA). Most monomers produced nematic‐like networks, but in one case a smectic C mesophase was also locked.     

Smectic‐B Liquid Single Crystal Elastomers as Efficient Optical Mechanotransducers

Carbazole‐based liquid single‐crystal elastomers (LSCEs) are valuable materials to convert mechanical forces into optical signals, i.e., optical mechanotransduction. The identification and rationalization of the different structural factors governing the mechanotransduction process are essential to guide the future design of these smart sensing materials. Herein, how the type of mesophase displayed by the elastomer impacts its transducing capabilities is explored. Remarkably, smectic‐B LSCEs are significantly more efficient mechanotransducers than their nematic analogs, independent of the length of the flexible spacer that connects the fluorogenic monomers to the main polymeric backbone. In this instance, the fluorogenic moieties are located within the smectic lamellae formed by the mesogenic units, thereby forcing a stronger interaction between both the platforms and resulting in a much more effective fluorescence quenching upon deformation.     

Smectic-A liquid single crystal elastomers – strain induced break-down of smectic layers

The mechanical response of a smectic-A liquid-crystalline side-chain polymer network with macroscopically ordered layers (smectic-A liquid single crystal elastomer) to extensive stress applied parallel to the layer normal is described. By stress-strain measurements a characteristic threshold strain of about 3% is observed, at which the elastic modulus changes dramatically. In the regime of small deformation the elastomer remains optically transparent possessing a large Young modulus. The macroscopically ordered smectic-A structure is conserved, probably showing some layer undulation. Above the threshold strain, however, the monodomain structure breaks down and the elastomer becomes completely opaque showing a much smaller elastic modulus. By strain dependent X-ray scattering measurements the structure of the polydomain is analysed. A splitting of the small angle reflection into four maxima is observed, indicating a layer rotation. The intensities of these reflections, however, decrease drastically with strain suggesting a partial distortion of the smectic layers toward a nematic like structure.     

Structure and mechanical properties of Saxidomus purpuratus biological shells

The strength and fracture behavior of Saxidomus purpuratus shells were investigated and correlated with the structure. The shells show a crossed lamellar structure in the inner and middle layers and a fibrous/blocky and porous structure composed of nanoscaled particulates (∼100 nm diameter) in the outer layer. It was found that the flexure strength and fracture mode are a function of lamellar organization and orientation. The crossed lamellar structure of this shell is composed of domains of parallel lamellae with approximate thickness of 200–600 nm. These domains have approximate lateral dimensions of 10–70 μm with a minimum of two orientations of lamellae in the inner and middle layers. Neighboring domains are oriented at specific angles and thus the structure forms a crossed lamellar pattern. The microhardness across the thickness was lower in the outer layer because of the porosity and the absence of lamellae. The tensile (from flexure tests) and compressive strengths were analyzed by means of Weibull statistics. The mean tensile (flexure) strength at probability of 50%, 80–105 MPa, is on the same order as the compressive strength (∼50–150 MPa) and the Weibull moduli vary from 3.0 to 7.6. These values are significantly lower than abalone nacre, in spite of having the same aragonite structure. The lower strength can be attributed to a smaller fraction of the organic interlayer. The fracture path in the specimens is dominated by the orientation of the domains and proceeds preferentially along lamella boundaries. It also correlates with the color changes in the cross section of the shell. The cracks tend to undergo a considerable change in orientation when the color changes abruptly. The distributions of strengths, cracking paths, and fracture surfaces indicate that the mechanical properties of the shell are anisotropic with a hierarchical nature.     

Collagen texture and osteocyte distribution in lamellar bone

A comparative scanning and transmission electron microscopy study was carried out on collagen fiber texture and osteocyte lacunae distribution in human lamellar bone. The results show that bony lamellae are not made up of parallel-arranged collagen fibers, as classically maintained. They are instead made up of highly interlaced fibers, and the lamellation appears to be due to the alternation of collagen-rich and collagen-poor layers, namely of dense and loose lamellae. The present study additionally shows that osteocyte lacunae are only located inside loose lamellae. Such structural organization of lamellar bone is briefly discussed in terms of bone biomechanics and osteogenesis.     

A Prerequisite of the Poly(ε‐Caprolactone) Self‐Induced Nanohybrid Shish–Kebab Structure Formation: An Ordered Crystal Lamellae Orientation Morphology of Fibers

The shish–kebab structure has been extensively applied in many fields; however, the formation mechanism is still an open question. In this study, different electrospun poly(ε‐caprolactone) (PCL) fibers are applied as the shish material in a self‐induced crystallization process, and two different self‐induced crystal structures are obtained. The PCL fibers with an ordered crystalline morphology lead to an induced crystalline structure with the crystal lamellae perpendicular to the fiber axis. However, the PCL fibers with a disordered structure induce a complicated (less ordered) crystalline lamellae morphology. Investigation of the surface crystalline structure reveals that the self‐induced nanohybrid shish–kebab (SINSK) structure follows a lattice matching and epitaxial growth mechanism. The internal crystalline structure of PCL nanofibers plays a dominant role in the formation of the SINSK structure. This study may prove helpful in screening materials for formation of the SINSK structure.     

Orientation behavior of smectic polymer networks by uniaxial mechanical fields

Polymer networks showing different smectic polymorphism with either an interdigitated smectic A to undulated smectic A phase transformation or an interdigitated smectic A to double layer smectic C phase transformation are synthesized. Their orientation behavior in uniaxial mechanical fields is investigated by means of X-ray scattering techniques and thermal expansion measurements. By applying uniaxial extension to the smectic elastomers in the isotropic state ordered smectic networks result with the director uniformly aligned parallel to the strain axis. These networks are optically transparent. Uniaxial deformations of the ordered smectic networks in the direction parallel and perpendicular to the director are performed that also help to elucidate the structure of the smectic phases. We find a reorientation process of layers of the ordered smectic C network. The spontaneous elongation in the direction of the director at the isotropic to smectic A phase transformation of the smectic elastomers is observed by thermoelastic measurements. This indicates that the polymer backbone conformation in the smectic elastomers is prolate in contrast to previous observations of oblate chain conformation in linear smectic polymers.