Side-chain Liquid Crystalline Block Copolymers Research Articles

Synthesis and Characterization of Side-Chain Liquid-Crystalline Block Copolymers Containing Cyano-Terminated Phenyl Benzoate Moieties

Block copolymers, known for their capacity to undergo microphase separation, spontaneously yield various periodic nanostructures. These precisely controlled nanostructures have attracted considerable interest due to their potential applications in microfabrication templates, conducting films, filter membranes, and other areas. However, it is crucial to acknowledge that microphase-separated structures typically exhibit random alignment, making alignment control a pivotal factor in functional material development. To address this challenge, researchers have explored the use of block copolymers containing liquid-crystalline (LC) polymers, which offer a promising technique for alignment control. The molecular structure and LC behavior of these polymers significantly impact the morphology and alignment of microphase-separated structures. In this study, we synthesized LC diblock copolymers with cyano-terminated phenyl benzoate moieties and evaluated the microphase-separated structures and molecular alignment behaviors. The LC diblock copolymers with a narrow molecular weight distribution were synthesized by atom transfer radical polymerization. Small angle X-ray scattering measurements revealed that the block copolymers exhibit smectic LC phases and form cylinder structures with a lattice period of about 18 nm by microphase separation. The examination of block copolymer films using polarized optical microscopy and polarized UV-visible absorption spectroscopy corroborated that the LC moieties were uniaxially aligned along the alignment treatment direction.

Random Planar Orientation in Liquid-Crystalline Block Copolymers with Azobenzene Side Chains by Surface Segregation.

Rodlike liquid-crystalline (LC) mesogens preferentially adopt a homeotropic orientation by excluded volume effects at the free surface in side-chain LC (SCLC) polymer films. The homeotropic orientation is not advantageous for in-plane LC alignment processes. Surface segregation of polymers is the phenomenon in which one component with a low surface free energy covers the surface in a mixture of two or more polymers or a block copolymer film. In SCLC block copolymer films, the surface segregation structure induces a random planar orientation due to the formation of a microphase-separated interface parallel to the substrate via the covering of one of the segregated polymer blocks. This feature article focuses on the unique, random planar orientation induced by the surface segregation of SCLC block copolymer films with the photoresponsive azobenzene (Az) mesogenic group. A transition moment of the Az mesogens is parallel to the molecular long axis, and light irradiation is conducted perpendicular to the film surface in general photoreaction processes. Therefore, the homeotropic molecular orientation in the SCLC polymer systems with Az mesogenic units inhibits efficient photoreaction reorientations in thin films. The random planar orientations by the surface segregation of a coil block in SCLC block polymers provide efficient in-plane photoreorientation and photoswitching with LC hierarchical mesostructures, such as microphase-separated structures of SCLC block copolymers and laminated LC polymer films. On the other hand, surface-segregated SCLC blocks form a high-density polymer LC brush layer with a random planar orientation by self-assembly, which exhibits efficient angular selective photoreactions. These approaches using the surface segregation of SCLC block copolymers are expected to offer new concepts for the LC photoalignment process for LC polymer devices.

Synthesis and characterization of side-chain liquid-crystalline block-copolymers containing laterally attached photoluminescent quinquephenyl units via ATRP

A series of novel side-chain liquid-crystalline polymers (SCLCPs) consisting of laterally attached photoluminescent p-quinquephenyl (QQP) pendants with different flexible terminal- and/or side-alkoxy chains were synthesized via atom transfer radical polymerization (ATRP). Homopolymers (HP1–HP3) and block-copolymers (PSP1–PSP3 and PEOP1–PEOP3), where QQP units were copolymerized with styrene or ethylene oxide monomers, possessed the number average molecular weights (Mn) of 8.7–26.0×103 with narrow PDI values of 1.08–1.26. Various characterization techniques of polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) were used to investigate their mesomorphic properties, and all homopolymers and block-copolymers exhibited the nematic phase affected by the flexible terminal- and/or side-alkoxy chains of the conjugated rod-like pendants. In addition, the photophysical properties of these polymers were measured by UV–vis and photoluminescence (PL) spectroscopies, which showed blue PL emissions with rather high fluorescence quantum yields in solutions.

Preparation and Characterization of Linear and Miktoarm Star Side-Chain Liquid Crystalline block Copolymers with p-Methoxyazobenzene Moieties via a Combination of ATRP and ROP

One linear and two miktoarm star side-chain liquid crystalline (LC) block copolymers with p-methoxyazobenzene moieties were prepared by a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP) techniques. First, ROPs of ϵ -caprolactone (ϵ -CL) were carried out catalyzed by Sn(Oct)2 using three multifunctional initiators, hydroxyethyl 2-bromoisobutyrate (AB type), 3-hydroxy-2-(hydroxymethyl)-2-methylpropyl 2-bromo-2-methylpropanoate (A2B type) and 2,2-bis(hydroxymethyl)propane-1,3-diyl bis(2-bromo-2-methylpropanoate) (A2B2 type), at 110°C in toluene, respectively. Second, the previously obtained poly(ϵ -caprolactone)s (PCLs) with bromines functionalities were used as the macroinitiators to conduct ATRP of 6-(4-methoxy-4-oxy-azobenzene) hexyl methacrylate (MMAZO) with CuBr/PMDETA as the catalyst systems at 85°C in anisole to prepare the linear and miktoarm side-chain LC block copolymers (PCL-b-PMMAZO, (PCL)2-(PMMAZO) and (PCL)2-(PMMAZO)2). The produced polymers were well-controlled with the controlled molecular weights and the relatively narrow molecular weight distributions (M w/M n ≤ 1.35). The structures of the obtained polymers were all characterized by NMR, FT-IR and GPC analysis. Furthermore, the LC properties of the linear and miktoarm star block copolymers were also investigated by differential scanning calorimetry (DSC) and thermal polarized optical microscopy (POM).

Influence of LC Content on the Phase Structures of Side-Chain Liquid Crystalline Block Copolymers with Bent-Core Mesogens

We report the phase structures of a series of poly(styrene-block-{3′-[4-(4-n-dodecyloxybenzoyloxy)benzoyloxy]-4-(12-methacryloyloxydodecyloxy)benzoyloxybiphenyl}) (PS-b-PMAC) side-chain liquid crystalline block copolymers (SC LCBCP). The SC liquid crystalline polymer was formed by side attaching a bent-core mesogen to the polymer backbone using a 12-carbon spacer. The phase structure of the high and low fPMAC samples were investigated using differential scanning calorimetry, small-angle and wide-angle X-ray scattering, and transmission electron microscopy techniques. The PS coil block and PMAC LC block phase separate into a lamellar morphology in all of the samples investigated (volume fraction of PMAC fPMAC ∼ 0.31−0.65). However, both the LC phase and the orientation of the hierarchical structure under mechanical shear showed strong dependence on the LC content. Samples having a high fPMAC (0.5−0.65) showed a SmC2 LC phase (Smectic C denotes the LC molecules are tilted with respect to the layer normal, and 2 represents a bilayered structure), similar to that observed in PMAC homopolymers. Upon mechanical shear, these smectic layers oriented parallel to the shear plane and the BCP lamellae oriented perpendicular to the shear plane with the layer normal parallel to the vorticity direction. In samples having a lower fPMAC, the BCP lamellae laid parallel to the shear plane and the LC phase structure in these samples was columnar rectangular. A detailed structural and morphological study will be reported.

Controlling the morphology of side chain liquid crystalline block copolymer thin films through variations in liquid crystalline content.

In this paper, we describe methods for manipulating the morphology of side-chain liquid crystalline block copolymers through variations in the liquid crystalline content. By systematically controlling the covalent attachment of side chain liquid crystals to a block copolymer (BCP) backbone, the morphology of both the liquid crystalline (LC) mesophase and the phase-segregated BCP microstructures can be precisely manipulated. Increases in LC functionalization lead to stronger preferences for the anchoring of the LC mesophase relative to the substrate and the intermaterial dividing surface. By manipulating the strength of these interactions, the arrangement and ordering of the ultrathin film block copolymer nanostructures can be controlled, yielding a range of morphologies that includes perpendicular and parallel cylinders, as well as both perpendicular and parallel lamellae. Additionally, we demonstrate the utilization of selective etching to create a nanoporous liquid crystalline polymer thin film. The unique control over the orientation and order of the self-assembled morphologies with respect to the substrate will allow for the custom design of thin films for specific nanopatterning applications without manipulation of the surface chemistry or the application of external fields.

Synthesis of Double Side-Chain Liquid Crystalline Block Copolymers Using RAFT Polymerization and the Orientational Cooperative Effect

We report on the synthesis and characterization of a new type of diblock copolymer that is composed of two side-chain liquid crystalline polymers (SCLCP1-block-SCLCP2). The diblock copolymer of poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate}-block-poly{6-[4-(4-cyanophenyl)phenoxy]hexyl methacrylate} (PAzoMA-b-PBiPMA) was successfully synthesized using the reversible addition−fragmentation chain transfer (RAFT) polymerization. Low-polydispersity samples of various compositions can be prepared using either PAzoMA or PBiPMA macromolecular chain transfer agent. We show that the orientational cooperative effect can be effective even in microphase-separated samples due to the interaction of the two mesogens via the interface. Indeed, the photoinduced orientation of azobenzene groups on the PAzoMA block can bring biphenyl groups on the PBiPMA block to orient in the same direction inside their respective lamellar domains. While RAFT can be used to access to double or all-SCLC block copolymers, this study also demonstrates the potential interest of exploring the interplay of the LC order of different SCLCPs within a microphase-separated system.

A Model for Self-Assembly in Side Chain Liquid Crystalline Block Copolymers

We present a new model based on self-consistent field theory (SCFT) approach and complement it by strong segregation theory (SST) based calculations to characterize the self-assembly behavior in side-chain liquid crystalline block copolymers. Our model considers a micromechanical representation of flexible coil−coil diblock copolymers, with rodlike units grafted to one of the blocks. We present results which elucidate self-assembly arising from the interplay between block copolymer microphase separation and the orientational ordering of the rod segments. We determine the morphological phase diagram for this system by assuming two dimensional variations of composition profiles. Our numerical results are in very good agreement with reported experimental observations. Many of the traditional flexible diblock copolymer microphases are also predicted to occur for side chain liquid crystalline polymers, with smectic ordering accompanying within the microphases. The equilibrium phase morphologies are observed to depend on the molecular weight of the copolymer, the length of the rod units, the relative volume fractions of each block, and the energetic and orientational interactions between different components. Moreover, for the parameters considered in this article, microphase separation was observed to be a requisite for developing orientational ordering between mesogenic units. The results of SST provide a physical explanation for the observations and are in good agreement with that of the SCFT calculations.

Directing self-assembly in macromolecular systems: Hydrogen bonding in ordered polymers

Abstract Hydrogen-bonded side-chain liquid-crystalline block copolymers (SCLCBCPs) containing block segments with proton donors were combined with functionalized mesogenic pendent groups. Studies of a wide range of mesogen and polymer combinations were carried out to study the relationship between morphology and mesophase behavior. The thermal behaviors of the resulting self-assembled microstructures were also examined. A model of the assembly process leading to mesogenic group organization corresponding to different compositions was proposed.

Microphase Separation Transition and Rheology of Side-Chain Liquid-Crystalline Block Copolymers

Microphase separation was induced from homogeneous polystyrene-block-polyisoprene (SI diblock) copolymer by grafting of a liquid-crystalline (LC) monomer onto the polyisoprene (PI) block. The origin of the formation of microdomain structure in the side-chain LC block copolymer from a homogeneous SI diblock copolymer is explained in terms of a dramatic increase in repulsive segmental interactions between the constituent blocks. For the investigation, three homogeneous SI diblock copolymers with varying block length ratios (SI-5/6, SI-10/6, and SI-14/3) were synthesized via anionic polymerization in tetrahydrofuran as solvent. This resulted in PI blocks having the following microstructures: 34% 1,2-addition, 59% 3,4-addition, and 7% 1,4-addition. Subsequently, the PI blocks in each of the SI diblock copolymers were first hydroxylated via hydroboration/oxidation reactions to yield hydroxylated SI diblock copolymers (SI-5/6-OH, SI-10/6-OH, and SI-14/3-OH), and then a liquid-crystalline (LC) monomer, 6-[(4-cyano-4'-biphenyl)oxy]hexanoic acid (5CN-COOH), synthesized in our laboratory, was grafted onto the hydroxylated PI blocks (PI-OH), yielding side-chain LC diblock copolymers (SI-5/6-5CN, SI-10/6-5CN, and SI-14/3-5CN). The disappearance of the -COOH and -OH groups after coupling reaction was confirmed using 1 H nuclear magnetic resonance spectroscopy and infrared spectroscopy. Transmission electron microscopy revealed that at room temperature SI-5/6-5CN has spherical microdomains, SI-10/ 6-5CN has hexagonally packed cylindrical microdomains, and SI-14/3-5CN has lamellar microdomains. The LC phase of the side-chain block copolymers was identified using polarizing optical microscopy. The clearing temperature (T cl ), determined by differential scanning calorimetry, of the side-chain LC block copolymers is found to be ca. 84 °C, whereas the T cl of LC monomer 5CN-COOH is 170 °C. Oscillatory shear rheometry indicates that the sphere-forming block copolymer SI-5/6-5CN undergoes a lattice disordering/ordering transition at 140 °C and a demicellization/micellization transition at 155 °C, while both the cylinder-forming block copolymer SI-10/6-5CN and lamella-forming block copolymer SI-14/3-5CN undergo microphase separation transition at temperatures much higher than 240 °C, the highest experimental temperature employed. Binodal curves exhibiting upper critical solution temperature were constructed, via cloud point measurement, for four pairs of polystyrene (PS) and PI grafted with 5CN-COOH (PI-5CN). An expression for the temperature dependence of the Flory-Huggins interaction parameter X for the PS/(PI-5CN) pair was determined and compared with the X expression for the PS/PI pair. The origin of the formation of microdomain structure in the side-chain LC block copolymers SI-5CN from a homogeneous SI diblock copolymer is explained in terms of molecular weight, block composition, and X for the PS/(PI-5CN) pair.

Side-Chain Liquid Crystalline Block Copolymers with Well Defined Structures Prepared by Living Anionic Polymerization IV. Microphase Morphology in Blends with Coil Homopolystyrenes

The microdomain structure in the blends of liquid crystal (LC)-coil block copolymer with coil homopolymers was studied. The LC-coil block copolymer used is composed of poly(6-[4-(4’-methoxyphenyl) phenoxy] hexyl methacrylate) as a LC segment and polystyrene as a coil segment. It has the molecular weight Mn=47000 with the LC segment fraction of 45 wt%. As a coil homopolymer, three polystyrenes with different molecular weights of 37000, 26000, and 8200 were used. In all the blends with LC segment fractions of 35% to 10%, the microphase segregation is clearly recognized and LC segment in resulting microdomain undergoes the well defined crystal-SmA-isotropic phase transitions. The microdomain structure depends on both the molecular weight of homopolystyrene blended and the phase structure of LC segment. When the highest molecular weight polystyrene with Mn=37000 was blended (so called in a “dry-brush” regime), a lamellar type of morphology is invariably observed even if the composition and temperature are varied. In contrast, when the lowest molecular weight polystyrene with Mn=8200 was used (in a “wet-brush” regime), the type of morphology is significantly altered. At the isotropic temperatures of LC segment, the lamellar morphology is observed for the blends with LC contents from 45% to 35%, but the cylindrical or spherical domain becomes predominant with a decrease in the LC content. On the other hand, the lamellar morphology is commonly observed at the crystal temperatures. In some blends with the lower weight fractions of LC segment, hence, there can be seen the order–order transition from sphere or cylinder to lamella on decreasing temperature. This morphological transformation is caused by the formation of the layered structure that tends to orient perpendicularly to the interface of microdomain. The results show that the structural order of LC segment affects both the microdomain morphology and the solubility style of the homopolymer into the block copolymer.

Synthesis of polystyrene-polysiloxane side-chain liquid crystalline block copolymers

The synthesis of a new liquid crystalline block copolymer consisting of a polystyrene block and a side-chain liquid crystalline siloxane block is reported. The synthetic approach described is based on the anionic polymerization of styrene and cyclic trimethyltrivinyltrisiloxane monomers, followed by functionalization of the siloxane block with side chain mesogens. The siloxane block has a Tg well below 25°C and is designed to exhibit a chiral smectic C* phase at room temperature. These block copolymers are the first side-chain liquid crystalline block copolymers which contain both a high Tg glassy block and a low Tg liquid crystalline block.

Synthesis of polystyrene-polysiloxane side-chain liquid crystalline block copolymers

The synthesis of a new liquid crystalline block copolymer consisting of a polystyrene block and a side-chain liquid crystalline siloxane block is reported. The synthetic approach described is based on the anionic polymerization of styrene and cyclic trimethyltrivinyltrisiloxane monomers, followed by functionalization of the siloxane block with side chain mesogens. The siloxane block has a Tg well below 25°C and is designed to exhibit a chiral smectic C* phase at room temperature. These block copolymers are the first side-chain liquid crystalline block copolymers which contain both a high Tg glassy block and a low Tg liquid crystalline block.