9 research outputs found

    Interplay between Liquid Crystalline Order and Microphase Segregation on the Self-Assembly of Side-Chain Liquid Crystalline Brush Block Copolymers

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    Herein we investigate the influence of competing self-organizing phenomena on the hierarchical self-assembly of liquid crystalline brush block copolymers (LCBBCs). A library of LCBBCs are synthesized using ring-opening metathesis polymerization (ROMP) of norbornene side-chain functionalized monomers comprising (1) cholesteryl mesogen with nine methylene spacer and (2) semicrystalline poly­(ethylene glycol) (PEG). The self-assembly of LCBBCs with variations in LC block content (7–80 wt %) are investigated in their melt state. All LCBBCs show two distinct thermal transitions corresponding to PEG semicrystalline phase and LC mesophases. Interestingly, the LCBBCs display a multilevel hierarchical structure evidenced by the results from X-ray scattering and transmission electron microscopy (TEM): (1) smectic A (SmA) mesophases (<i>d</i> = 3–7 nm) by the assembly of cholesteryl side chains and (2) microphase segregation into lamellar or cylinder (<i>d</i> = 40–75 nm) resulting from the incompatibility between LC moieties and PEG side chain. Surprisingly, the presence of microphase-segregated domains in LCBBCs prevents the formation of cholesteric mesophase in sharp contrast to side-chain liquid crystalline homopolymer (SCLCP) bearing the same mesogen and the flexible spacer. This could be attributed to very high surface to volume ratio at intermaterial dividing surface (IMDS) in LCBBCs, by which only LC layers (i.e., SmA mesophase) are favored to form at the IMDS. On the fundamental side, these LCBBCs are an interesting scaffold to explore the impact of interactions between LC order and microphase segregation of side-chain polymeric brushes on the self-assembly of LCBBCs. Moreover, these new LCBBC scaffolds will serve as a tool box for rational design of hierarchically organized functional materials for stimuli responsive applications

    Surface Aligned Main-Chain Liquid Crystalline Elastomers: Tailored Properties by the Choice of Amine Chain Extenders

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    A promising way to induce shape transformation in soft materials is via spatial variation in the orientation of the alignment of liquid crystalline elastomers (LCEs). Here, we improve the nascent thermomechanial shape transformation in main-chain LCEs prepared via aza-Michael addition reactions. Specifically, increasing the alkyl length in the <i>n</i>-alkylamine chain extender effectively reduces the actuation temperature by destabilizing the nematic phase as well as reduces the glass transition temperature (<i>T</i><sub>g</sub>) by increasing the free volume. In addition, incorporating a hydroxyl end-group in the amine chain extender (i.e., <i>n</i>-alkanolamine) increases the actuation strain and improves the film quality by preventing side-chain aggregates of <i>n</i>-alkylamine-functionalized LCEs. Interestingly, uniaxially aligned <i>n</i>-alkanolamine-functionalized LCEs exhibit an unprecedentedly large elongation and an enhanced toughness even along the loading direction likely due to hydrogen bonding between chains. Thus, our study highlights that the choice of amine chain extender during LCEs synthesis can be an efficient strategy to tailor the properties as well as to provide a new functionality in the LCEs which may expand their range of applications in shape morphing devices, smart coatings, and dynamic substrates

    Tunable Encapsulation Structure of Block Copolymer Coated Single-Walled Carbon Nanotubes in Aqueous Solution

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    Nanosized and shape-tunable molecular building blocks can provide great opportunities for the fabrication of precisely controlled nanostructures. In this work, we have fabricated a molecular building block of single-walled carbon nanotubes (SWNTs) coated by PPO–PEO–PPO block copolymers whose encapsulation structure can be controlled via temperature or addition of small molecules. The structure and optical properties of SWNT block copolymers have been investigated by small-angle neutron scattering (SANS), ultraviolet–visible (UV–vis) spectroscopy, atomic force microscopy (AFM), and molecular dynamics (MD) simulation. The structure of the hydrated block copolymer layer surrounding SWNT can be controlled reversibly by varying temperature as well as by irreversibly adding 5-methylsalicylic acid (5MS). Increasing hydrophobicity of the polymers with temperature and strong tendency of 5MS to interact with both block copolymers and π orbitals of the SWNTs are likely to be responsible for the significant structural change of the block copolymer encapsulation layer, from loose corona shell to tightly encapsulating compact shell. Our result shows an efficient and simple way to fabricate and manipulate carbon-based nano building blocks in aqueous systems with tunable structure

    Hierarchically Self-Assembled Photonic Materials from Liquid Crystalline Random Brush Copolymers

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    Here we report a general methodology to attain novel hierarchical nanostructures using new polymer scaffolds that self-assemble to form cholesteric 1D photonic mesophases existing in conjunction with microphase segregated domains. To achieve this, a series of liquid-crystalline random brush copolymers (LCRBC) consisting of cholesteryl liquid crystalline (LC) mesogen and brushlike PEG as side chain functionality are synthesized. At room temperature, all LCRBCs exhibits microphase segregation of PEG side chains on length scale of 10–15 nm, whereas LC domain forms smectic mesophase (3–7 nm LC layers). Interestingly, upon heating a cholesteric mesophase is exclusively observed for copolymer containing 78 and 85 wt % of LC content (LCRBC78 and LCRBC85, respectively) existing along with microphase segregated PEG domains. Moreover, the phase behavior of these copolymers studied by temperature-controlled small-angle X-ray scattering (SAXS) suggests the order–disorder transition for the microphase segregated structure coincides with the cholesteric–isotropic transition. Remarkably, LCRBC78 and LCRBC85 quenched from cholesteric mesophase exhibits nanoscale hierarchical order consisting of (1) smectic LC ordering with 3–7 nm periodicity, (2) microphase segregation of PEG side chain on 10–12 nm length scale, and (3) periodicities from helical mesophase (cholesteric phase) on optical length scales of 150–200 nm. Thus, by exploiting LCRBC molecular architecture and composition, hierarchical nanostructure can be obtained and preserved which allows for the creation of unique 1D-photonic materials

    Atomistic Structure of Bottlebrush Polymers: Simulations and Neutron Scattering Studies

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    We have used small angle neutron scattering (SANS) measurement and atomistic molecular dynamics (MD) simulations to investigate the conformation of bottlebrush polymers with poly­(norbornene) (PNB) backbone and different sizes of poly­(lactide) (PLA) side chains (PNB<sub>25</sub>-<i>g</i>-PLA<sub>5</sub>, PNB<sub>25</sub>-<i>g</i>-PLA<sub>10</sub>, and PNB<sub>25</sub>-<i>g</i>-PLA<sub>19</sub>). At early stage of simulations, stretched side chains with visible spatial-correlations of about 30 Å were observed. The experimentally measured SANS data, on the other hand, does not exhibit any correlation peaks in the corresponding length scale indicating a compact form rather than a stretched-hairy polymer conformation. As the simulation continued, the spatial correlations between side chains disappeared after about 40 ns of chain relaxation, and the scattering intensity calculated for the simulated structure becomes reasonably close to the measured one. Statistical approach is used to overcome the time scale limitation and search for optimal conformation space, which also provides a good agreement with the experimental data. Further coarse-grained simulation results suggest that the side chain conformation strongly depends on the solubility competition among side chain, backbone, and solvent. Significant changes of backbone dynamics due to the side chain encapsulation have been revealed and discussed

    Molecular Design of Liquid Crystalline Brush-Like Block Copolymers for Magnetic Field Directed Self-Assembly: A Platform for Functional Materials

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    We report on the development of a liquid crystalline block copolymer with brush-type architecture as a platform for creating functional materials by magnetic-field-directed self-assembly. Ring-opening metathesis of <i>n</i>-alkyloxy cyanobiphenyl and polylactide (PLA) functionalized norbornene monomers provides efficient polymerization yielding low polydispersity block copolymers. The mesogenic species, spacer length, monomer functionality, brush-chain length, and overall molecular weight were chosen and optimized to produce hexagonally packed cylindrical PLA domains which self-assemble and align parallel to an applied magnetic field. The PLA domains can be selectively removed by hydrolytic degradation resulting in the production of nanoporous films. The polymers described here provide a versatile platform for scalable fabrication of aligned nanoporous materials and other functional materials based on such templates

    Azobenzene Molecular Machine: Light-Induced Wringing Gel Fabricated from Asymmetric Macrogelator

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    To develop light-triggered wringing gels, an asymmetric macrogelator (1AZ3BP) was newly synthesized by the chemically bridging a photoisomerizable azobenzene (1AZ) molecular machine and a biphenyl-based (3BP) dendron with a 1,4-phenylenediformamide connector. 1AZ3BP was self-assembled into a layered superstructure in the bulk state, but 1AZ3BP formed a three-dimensional (3D) network organogel in solution. Upon irradiating UV light onto the 3D network organogel, the solvent of the organogel was squeezed and the 3D network was converted to the layered morphology. It was realized that the metastable 3D network organogels were fabricated mainly due to the nanophase separation in solution. UV isomerization of 1AZ3BP provided sufficient molecular mobility to form strong hydrogen bonds for the construction of the stable layered superstructure. The light-triggered wringing gels can be smartly applied in remote-controlled generators, liquid storages, and sensors

    Structural Evolution of Polylactide Molecular Bottlebrushes: Kinetics Study by Size Exclusion Chromatography, Small Angle Neutron Scattering, and Simulations

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    Structural evolution from poly­(lactide) (PLA) macromonomer to resultant PLA molecular bottlebrush during ring opening metathesis polymerization (ROMP) was investigated for the first time by combining size exclusion chromatography (SEC), small-angle neutron scattering (SANS), and coarse-grained molecular dynamics (CG-MD) simulations. Multiple aliquots were collected at various reaction times during ROMP and subsequently analyzed by SEC and SANS. These complementary techniques enable the understanding of systematic changes in conversion, molecular weight and dispersity as well as structural details of PLA molecular bottlebrushes. CG-MD simulation not only predicts the experimental observations, but it also provides further insight into the analysis and interpretation of data obtained in SEC and SANS experiments. We find that PLA molecular bottlebrushes undergo three conformational transitions with increasing conversion (i.e., increasing the backbone length): (1) from an elongated to a globular shape due to longer side chain at low conversion, (2) from a globular to an elongated shape at intermediate conversion caused by excluded volume of PLA side chain, and (3) the saturation of contour length at high conversion due to chain transfer reactions

    Poly(3-hexylthiophene) Molecular Bottlebrushes via Ring-Opening Metathesis Polymerization: Macromolecular Architecture Enhanced Aggregation

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    We report a facile synthetic strategy based on a grafting through approach to prepare well-defined molecular bottlebrushes composed of regioregular poly­(3-hexylthiophene) (<i>rr</i>-P3HT) as the conjugated polymeric side chain. To this end, the <i>exo</i>-norbornenyl-functionalized P3HT macromonomer was synthesized by Kumada catalyst transfer polycondensation (KCTP) followed by postpolymerization modifications, and the resulting conjugated macromonomer was successfully polymerized by ring-opening metathesis polymerization (ROMP) in a controlled manner. The P3HT molecular bottlebrushes display an unprecedented strong physical aggregation upon drying during recovery, as verified by several analyses of the solution and solid states. This remarkably strong aggregation behavior is attributed to a significant enhancement in the number of π–π interactions between grafted P3HT side chains, brought about due to the bottlebrush architecture. This behavior is qualitatively supported by coarse-grained molecular dynamics simulations
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