A new, practical approach to a variety of highly electrooptically active polymers for device development is described. It involves the use of a new thermally cross-linkable, hyperbranched oligomer containing nonlinear optical (NLO) chromophores as a macromolecular dopant in a common host polymer. A series of NLO polymeric blends were readily formulated and showed large and stable electrooptic (EO) coefficients (up to 65 pm/V). In comparison with previously studied linear NLO polyimides and guest-host polymers doped with molecular chromophores and even linear NLO analogous oligomers, this new approach offers clear advantages for device development in terms of improved poling efficiency, larger EO coefficients, good temporal stability, and versatile material formulation.
We report the design, synthesis, and characterization of a series of mesogen-jacketed liquid crystalline polymers with bent-core liquid crystals (BCLCs). For the first time, BCLC mesogens were directly side-attached to the polymer backbone and bent-core mesogen-jacketed liquid crystalline polymers (BMJLCPs) were achieved. Both three-ring and five-ring mesogens were employed. The n-alkoxy substituent lengths for the three-ring and five-ring BMJLCPs were controlled as n ) 1-5 and n ) 6-16, respectively. Various characterization techniques such as differential scanning calorimetry, wide-angle X-ray diffraction, and polarized light microscopy were used to study their mesomorphic phase behavior. The monomers of five-ring BMJLCPs with relatively long tails showed mesophase behavior. Columnar liquid crystalline phase was observed in both three-ring and five-ring BMJLCPs. Columnar rectangular (Φ R ) phase was observed in the three-ring system. In the five-ring BMJLCPs, relatively short-tail homologues possess Φ R phase, while columnar hexagonal phase was observed in the longtail samples. The differences in the phase structures were attributed to the "softness" of the macromolecular BMJLCP column surface.
This paper reports two important results with cross-linked precipitation polymerization. (1) Acetonitrile, a substance harmful to human health, is the most commonly used solvent for the synthesis of cross-linked polymeric microspheres by precipitation polymerization. Here, the much safer acetic acid replaced acetonitrile as a solvent in the precipitation polymerization of monodisperse cross-linked poly(divinylbenzene) (PDVB-55) microspheres. Pumpkin-like particles and microspheres were obtained. XPS results displayed a significant amount of double bonds on the surface of the particles. The effect of monomer content, temperature, and initiator amount on the formed particles were studied. For a DVB loading below 1 vol % at 70 degrees C, monodisperse microspheres with smooth surfaces and narrow diameters were successfully obtained. With a DVB loading of 2 vol % and by observing the shapes of particles obtained with three different temperature(60, 70, and 80 degrees C), we found that more spherical particles were obtained at higher temperatures and pumpkin-like particles were obtained at lower temperatures. No significant differences in morphology or the coefficient of variation (CV) of the particles were obtained for different initiator loadings, whereas the particle diameters could be increased with increased initiator concentrations. (2) In order to obtain a better understanding of the formation mechanism of these particles, time-dependent experiments, for the first time, were conducted in a hydrophobic monomer system. By tracing the whole polymerization process, some important results were found. First, with the polymerization time at 70 degrees C, the particle diameters were found to increase from 800 nm to 3.0 microm, the CV displayed a decrease, and the amount of spheres and the spherical evenness of the particle surfaces improved. Second, by quantitatively calculating the particle number from the yields and diameters data, it is found that starting from 3.1% yield or two hours reaction time the total amount of particles in the system is almost a constant (about 9.6 x 10 (8)/L), which means that no homocoagulation occurred and no new particles were generated after nucleation, and there is a linear relation between cubic diameters and yields. These two results give us a distinct impression that particle growth almost comes from capturing of newly formed oligomers. Based on the above results, a scheme for the particle formation is proposed, which shows that that pumpkin-like particles are caused by a prolonged nucleation including the homocoagulation of primary nuclei. The growth of the particles includes two modes, an in situ surface polymerization of monomer and the adsorption of PDVB-55 oligomers. The differences between results in acetonitrile and in acetic acid (higher yields, smaller size, not spherical but pumpkin-like particles in acetic acid) were due to the lower solubilizability of acetic acid which is the so-called proton-containing solvent with the hydrogen bonding structure.
Organic solids and polymers that absorb in the near-infrared (NIR) region (1000-2000 nm) represent a class of emerging materials and show a great potential for use in photonics and telecommunications. The radical anions of stacked aromatic imides, fused phorphyrin arrays, polythiophenes, sandwich-type lanthanide bisphthalocyanines, semiquinones, and mixed-valence dinuclear metal complexes are a few known examples of NIR-absorbing organic materials. Most of these NIR-absorbing materials are also electrochemically active or electrochromic (EC). This brief review covers several types of NIR-absorbing organic materials and discusses their potentials for applications in EC variable optical attenuators (VOAs).
We report the design, synthesis, and characterization of side-chain liquid crystalline (LC) poly-(meth)acrylates with end-on bent-core liquid crystalline (BCLC) mesogens. Both conventional free radical polymerization and atom transfer radical polymerization have been used to synthesize these liquid crystalline polymers (LCP). The resulting polymers exhibit thermotropic LC behavior. Differential scanning calorimetry, thermopolarized light microscopy, wide-angle X-ray diffraction, and small-angle X-ray scattering were used to characterize the LC structure of both monomers and polymers. The electro-optic (EO) measurement was carried out by applying a triangular wave and measuring the LC EO response. SmCP (Smectic C indicates the LC molecules are tilted with respect to the layer normal; P denotes polar ordering) phases were observed for both monomers and polymers. In LC monomers, typical antiferroelectric switching was observed. In the ground state, SmCP A (A denotes antiferroelectric) was observed which switched to SmCP F (F denotes ferroelectric) upon applying an electric field. In the corresponding LCP, a unique bilayer structure was observed, which is different from the reported BCLC bilayer SmCG (G denotes generated) phase. Most of the LCPs did not switch upon applying electric field while weak AF switching was observed in a low molecular weight poly{(3′-[4-(4-n-dodecyloxybenzoyloxy)benzoyloxy]-4-(12-acryloyloxydodecyloxy)benzoyloxybiphenyl} sample.
This paper reports on two important results regarding the precipitation polymerization of poly(divinylbenzene) (PDVB) in acetic acid (HAc). (1) Acetic acid is a novel kind of solvent worthy of investigation because it is amphipathic and innoxious. Thus, two kinds of model solvents, methyl ethyl ketone (MEK) and n-heptane, were selected to investigate the solvent effect on the particle morphology of PDVB-55 during precipitation polymerization in acetic acid. Monodisperse PDVB-55 microspheres were obtained with an MEK content of 30 vol % and a DVB loading of 2 vol %. Odd-shaped particles were found to almost disappear when MEK was added. For MEK contents up to 90 vol %, space-filling macrogels consisting of small particles with diameters of around 10 nm were obtained. More homocoagulated particles were produced when n-heptane was added, for which concentrations up to 50 vol % gave rise to cauliflower-like particles. Thus, in the acetic acid system, microspheres, pumpkin-like particles, macrogels, and coagulum could be successfully obtained. (2) The preparation of nonpolar PDVB-55 particles could be more predictable. For the first time-based on the regulation of former studies--the regularity of the dispersive term (delta(d)) on the particle morphology for a PDVB precipitation polymerization system was reported. The three-dimensional Hansen solubility parameters were utilized to perfect the regularity of the Hildebrand solubility parameter. Microspheres or particles were formed in the range of moderate delta values for both parameters, i.e., delta = 20.2-24.3 MPa1/2 or delta = 16 MPa1/2. What was even more important, delta(d) was found to be around 15.4 MPa1/2, and delta(h) should be below 13.5 MPa1/2. Cyclohexane, cyclohexanone, n-butyl acetate, and 1,4-dioxane were used to verify this regularity, and positive results were obtained. Stable, uniform, and well-separated PDVB-55 microspheres and particles were produced as a result of interaction forces between oligomers, polymers, and solvent.
A series of nonlinear optical (NLO) polyimides were prepared by grafting 4-16 wt % zwitterionic pyridinium tricyanoquinonedimethane chromophore with a large negative hyperpolarizability (β 0 ) -590 × 10 -30 esu) onto the host polyimides through an ester bond formation. To enhance the temporal stability of the poled NLO polymers, a new thermally reactive group, i.e., 5-aminobenzocylcobutenone, was introduced into the NLO polymer systems to effect the cross-linking during the thermal poling process. The resulting NLO polyimides showed good solubility in polar aprotic solvents such as N,N-dimethylformamide and could form uniform films by casting or spin coating without any noticeable chromophore aggregation or phase separation as examined under an optical microscope. These NLO polyimides had glass transition temperatures in the range of 170-213 °C and were thermally stable up to 270 °C. The electrical resistivity of NLO polyimides was found to be on the order of 10 15 Ω‚cm at 80 °C and dependent on the applied voltages. As the temperature increased, the resistivity dropped monotonically to 2.4 × 10 11 Ω‚cm at 200 °C. With a 10 wt % chromophore loading (or a number density of 15 × 10 19 cm -3 ) in the polymer, the electrooptic (EO) coefficient (r 33 at 1550 nm) reached 45 pm/V. No substantial decrease in EO coefficients was observed after the test cell was kept in the dark at 85 °C under nitrogen after 1200 h nor when the test cell was irradiated with 1550 nm light (50 µW/∼4 mm 2 ) for 10 h at ambient temperature in air.
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