Dielectric Chain Dynamics of Side-Chain Liquid Crystalline Polymer

Both chain dynamics and statistics of side-chain liquid crystalline polymer are experimentally explored. Dielectric measurements over a wide range of frequencies and temperature windows demonstrate that the chain dynamics in the liquid crystalline phase (SmA) is retarded and has higher apparent activation energy compared to that in isotropic melt. The molecular weight dependence of the normal mode relaxation time in the isotropic melt conforms to the Zimm model with excluded volume effect, while it shows Rouse behavior in the liquid crystalline phase. The mean squared end-to-end distance of polymer chain in the liquid crystalline phase decreases compared to that in the isotropic melt. The main chain takes a self-avoiding walk with ⟨<i>R</i>_e²⟩ ∼ <i>N</i>^2ν, ν = 0.54–0.6 in the isotropic melt, whereas a random walk between smectic layers with ν ≅ 0.5, consistent with the results from chain dynamics

Hierarchical Dynamics of Nonsticky Molecular Nanoparticle-Tethered Polymers: End and Topology Effects

Segmental and chain dynamics of the type-A polyethers tethered with nonsticky polyhedral oligomeric silsesquioxane molecular nanoparticles (MNPs) have been investigated experimentally by a combination of broadband dielectric spectroscopy, rheology, and pulsed-field-gradient (PFG) nuclear magnetic resonance (NMR) techniques, to provide a fundamental understanding of the effects of chain ends and topology on the hierarchical dynamics of polymers. Series of poly­(1,2-butylene oxide)­s of a narrow molecular weight distribution with the MNPs tethered at one or two chain ends (denoted as P-B and P-B-P) or tethered at the side (denoted as P-s-B) have been elaborately designed and synthesized. Both dielectric segmental and chain relaxations of P-B and P-B-P were found to be retarded by the attachment of the MNPs at the ends. In contrast to traditional polymer glass transition behaviors, their dielectric glass transition temperatures (Tg) decrease with an increase in the molecular weight until saturated at the high-molecular weight limit, which has been ascribed to the relatively low mobility of the MNPs in comparison to those of the usual polymer end segments. The Flory–Fox equation has been modified to depict the tendency of Tg versus molecular weight for P-B and P-B-P, which yields the equivalent number of segments for chain extension to account for the end effects of the MNPs on the segmental level. On the chain length scale, we found that the dielectric and rheological behaviors of the unentangled P-B and P-B-P complied with the predictions of the Rouse model in terms of the scaling law of the longest relaxation time versus the effective molecular weight and the relaxation spectra resolved in the low-p mode region, whereas a crossover from the Rouse to the reptation behavior was also observed. 29Si PFG NMR spectroscopy has been utilized to selectively probe the diffusion of the MNPs attached to the polymers. The unique role of the MNP and its influence on the dynamics of polymers were further discussed in the context of hierarchically branched polymers. In contrast, the rheological behaviors of P-s-B with side-tethered MNPs revealed gradual disentangling of the chains with an increase in the number of MNPs and accelerated chain relaxation in comparison with that of P-B-P, reflecting the distinct topology effects of the MNPs on the polymer dynamics

Chiral Photonic Liquid Crystalline Polyethers with Widely Tunable Helical Superstructures

Liquid crystalline polymers with tunable structures on the scale of visible wavelength are important in optical technology due to their enhanced mechanical stability, processability, and structural integrity. Herein, we report a series of cholesteric liquid crystalline (CLC) polyethers with a widely tunable pitch length and a broad CLC phase window through a bottom-up structural design. The well-defined multicomponent polyethers were successfully synthesized by utilizing monomer-activated anionic ring-opening polymerization. Through adjustment of the composition of chiral cholesteryl (Ch) and photochromic azobenzene (Az) mesogenic moieties, rich phase behaviors have been discovered, and a phase boundary diagram was constructed consequently, wherein cholesteric helical superstructures in a broad composition range and temperature window straight down to the glassy state at room temperature were achieved. Particularly, the planar oriented helical superstructures can exhibit widely tunable and switchable reflections over the entire visible range across red, green, and blue colors through temperature and light control, which are closely related to the extraordinary flexibility of the polyether backbone. Their thermo-light dual-responsive properties provide an alternative opportunity to fabricate smart and switchable polymeric LC materials for optical applications

Cholesteric Liquid Crystalline Polyether with Broad Tunable Circularly Polarized Luminescence

Strong circularly polarized luminescence (CPL) with high purity and broad tunability was achieved in a new type of polyether-based cholesteric liquid crystalline (CLC) copolymers comprising chiral cholesteryl, nematic mesogens, and cross-linkable moieties. The phase boundary diagram of the copolymers was constructed, wherein the CLC phase in a wide composition and temperature window down to room temperature was achieved. Furthermore, reflection colors across the infrared and visible light regions can be continuously tuned by altering composition or temperature, which can be further fixed in the flexible CLC elastomer by photo-cross-linking. Introducing achiral dyes in the CLC thin films can generate strong CPL with distinct handedness and high dissymmetry factors (glum). Particularly, the left-handed full-color CPL is obtained by selective circularly polarized scattering in the spectral region outside the band gap of the CLC thin film, and the right-handed CPL with glum up to −1.05 is achieved within the band gap of the CLC thin film following the selective circularly polarized reflection mechanism. This type of CPL active material is expected to have potential applications in liquid crystal display and photonics

Assembly, Structure and Optical Response of Three-Dimensional Dynamically Tunable Multicomponent Superlattices

We report the successful fabrication of optically active three-dimensional (3D) superlattices that incorporate DNA-encoded components, metallic nanoparticles, and molecular chromophores in well-defined positions. A DNA linker with three distinct binding sites serves as an assembly agent and dynamically tunable structural element for the superlattice. Using small angle X-ray scattering we have revealed the organization of particle-chromophore 3D arrays and monitored their reversible contractions and expansions that were modulated by ionic strength changes. As the distance between the molecular chromophores and plasmonic nanoparticles in the superlattice was regulated in situ, we were able to uncover the relationship between experimentally determined structure and optical response of the system. This dynamical tunability of superlattice results in a dramatic optical response: nearly a three times change of emission rate of the chromophore. The evolution of lifetime with structural changes reasonably agrees with the calculations based on a cumulitative coupling of chromophores with metallic nanoparticles in different coordination shells

Hierarchical Nanostructures and Self-Assemblies in Smectic-Nematic Liquid Crystalline Diblock Copolymers

Coexistence of smectic and nematic orders in 3D curvaceous bicontineous cubic or hexagonal hierarchical structures is observed in a novel class of nanophase separated, flexible double liquid crystalline (LC) diblock copolymers of different molecular weights (MWs) but similar compositions, obtained via sequential anionic polymerization. The diblock copolymer of higher MW exhibits an exceptional order–order transition (OOT) from lamellae (Lam) to hexagonal-packed cylinder (HPC) upon nematic ordering. In contrast, the polymer with lower MW forms a thermodynamically stable, ordered gyroid structure, interwining with LC defects on nanoscale. Delicate balance of collective LC interactions and geometric frustration dictates this unique behavior, which offers a genuine way to fine-tune 2D and 3D complex structures with sub-10 nm feature sizes

Statistics and Dynamics of Polymer Melt in Neutral Diblock Copolymer Single-Crystal Platelets

Polymer single-crystal (SC) platelets of poly­(butylene oxide)-b-poly­(lactic acid) (PBO-b-PLLA) of a well-defined shape, size, and grafting density have been fabricated and embedded into PBO melt for the study of the statistics and dynamics of the host polymers. The colloidal liquid-crystalline order of SCs above a threshold concentration of ∼2.2 vol % provides a confining environment for the molten PBO. Meanwhile, the peculiar type-A characteristics of PBO allow us to simultaneously probe the dielectric chain dimensions and the hierarchical dynamics of polymers under confinement. We observe negligible changes to the mean-square end-to-end distance of the polymer melt as well as the chain and segment dynamics, even the interlayer distance approaches the length scale comparable to the size of the host polymers. Our results provide direct evidence of the impacts of neutral walls on both the statistics and the dynamics of confined polymer melts, which can be also enlightening for the field of polymer nanocomposites

DNA Linker-Mediated Crystallization of Nanocolloids

Biofunctionalized nanocolloids offer a promising platform for creation of novel materials using addressable interactions. Crystalline phases are of especial interest for the development of novel plasmonic, magnetic, and catalytic metamaterials. When flexible single-stranded linker DNAs are added to the mixture of two types of dispersed, ssDNAs capped gold nanocolloids which are noncomplementary to each other but complementary to the respective ends of the linker DNA, a crystalline phase of body-centered cubic unit cell is formed at the premelting temperature of the system. An evolution of the structure, crystal formation, and thermodynamic path toward equilibrium state have been studied in details using in-situ small-angle X-ray scattering for different DNA linker designs