The thermally stimulated current technique applied to side-chain liquid-crystal polymers

The thermally stimulated current (t.s.c.) technique is applied to three side-chain liquid-crystal polymers (LCP) and an LCP that contains a highly dipolar dye. The t.s.c. trace is shown to be very sensitive to a number of LCP properties including glass transition, degree of mesogenic alignment and the amount of polarization that can be stored. A strong correlation between such properties and the results of high-frequency dielectric spectra is established. These parameters are directly relevant for the application of these materials as optical storage media and in non-linear optics. In particular, the technique is able to define the optimum conditions for poling these LCP systems. The technique can also be used to determine a number of fundamental molecular properties such as activation energy, molecular relaxation distribution and polymer flexibility and, as such, allows fundamental examination of the relationships between main-chain and mesogenic motions.     

Polymer bonded magnets. II. Effect of liquid crystal polymer and surface modification on magneto‐mechanical properties

We studied the effect of liquid crystal polymer (LCP) and surface modification of neodymium‐iron‐boron (Nd‐Fe‐B) magnetic alloy on the magneto‐mechanical behavior of poly (phenylene sulfide) (PPS) bonded Nd‐Fe‐B magnets to accelerate efforts to develop useful thermoplastic magnets with optimal performance. The results indicate that blending the LCP with PPS provides the required balance of properties for the targeted applications. These properties include superior magneto‐mechanical performance at elevated temperatures, minimal melt viscosity at optimal LCP volume fraction, high stiffness, and improved dimensional stability, making the thermoplastic magnets suitable for use at elevated temperatures and in chemically corrosive environments where commercial rare earth alloy magnets are not useable. Enhanced wetting of the magnetic Nd‐Fe‐B powders by the polymers, formation of reinforcing LCP domains, and interactions between the polymers and the magnetic powders are thought to be responsible for the beneficial function of the LCP and Nd‐Fe‐B surface modifier in the PPS bonded Nd‐Fe‐B magnets.     

Permeation and separation of organic liquid mixtures through liquid-crystalline polymer networks

A side-chain liquid-crystalline polymer (LCP) was synthesized by the addition of the mesogenic monomer to poly(methyl siloxane) in presence of a Pt-catalyst. When an aqueous solution of 10wt% ethanol was permeated through a LCP membrane by pervaporation at various temperatures, the permeation rate increased with increasing temperature and drastically changed at glass-nematic (Tg) and nematic-isotropic (TNI) transition temperatures of the LCP membrane. The LCP membrane exhibited the waterpermselectivity in the glassy and liquid-crystalline states. The ethanol concentration in the permeate increased with increasing permeation temperature and the LCP membrane changed from the waterpermselectivity to the ethanol-permselectivity around TNI. These results suggested that the permselectivity was influenced by the change of the LCP membrane structure, that is, its state transformation. It was found that a balance of the orientation of mesogenic groups and flexibility of siloxane chains is very important for the permeability and selectivity.     

Silicone‐based cholesteric liquid crystalline polymers: Effect of crosslinking agent on phase transition behavior

Silicone‐based cholesteric liquid crystalline polymers (ChLCP) were fabricated with variable clearing temperatures as controlled by their chemical compositions. The chemical structures of the mesogenic monomers and ChLCP were confirmed by FTIR and ¹H‐NMR spectroscopy. The mesogenic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. The experimental results demonstrated that the glass transition temperatures and the clearing points of the liquid crystalline polymers decreased with increasing proportion of mesogenic crosslinking agent up to 12.50 mol % (LCP‐3), and at higher proportion of crosslinking agent, the clearing points disappeared, indicating that the network chains have less chance to orient themselves. Thermogravimetric analysis showed that the LCP‐3 was the most stable up to 230°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Study on a series of main-chain liquid–crystalline ionomers containing sulfonate groups

A series of main-chain liquid–crystalline ionomers containing sulfonate groups pendant on the polymer backbone were synthesized by an interfacial condensation reaction of 4,4′-dihydroxy-α,α′-dimethyl benzalazine, a mesogenic monomer, with brilliant yellow (BY), a sulfonate-containing monomer, and a 1/9 mixture of terephthaloyl and sebacoyl dichloride. The structures of the polymers were characterized by IR and UV spectroscopies. DSC and thermogravimetric analysis were used to measure the thermal properties of those polymers, and the mesogenic properties were characterized by a polarized optical microscope, DSC, and wide-angle X-ray diffraction. The ionomers were thermally stable to about 310 °C. They were thermotropic liquid–crystalline polymers (LCPs) with high mesomorphic-phase transition temperatures and exhibited broad nematic mesogenic regions of 160–170 °C, and they were lyotropic LCPs with willowy leaf-shaped textures in sulfuric acid. However, the thermotropic liquid–crystalline properties were somewhat weakened because the concentration of BY was more than 8%. The inherent viscosity in N-methyl-2-pyrrolidone suggested that intramolecular associations of sulfonate groups occurred at low concentration, and intermolecular associations predominated at higher concentration. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2210–2218, 2001     

Side chain polysiloxane liquid crystals and their behaviour in electric fields

Optical and electro-optical properties of side chain liquid crystalline polymers are described. The polymers studied were homopolysiloxanes with mesogenic side groups separated from the polymer backbone by flexible polymethylene spacers of varying length. The effect of the flexible spacer on the phase properties and electric field effects will be discussed. Low frequency a.c. fields induced a turbulent scattering texture, whilst at higher frequencies (> 1 kHz) a homeotropic alignment was obtained. The latter effect leads to high contrast displays which are durably stored at temperatures well above the glass transition. Various parameters affecting the response of the polymers to applied fields will also be discussed. These include temperature, frequency and magnitude of the electric field, film thickness and the length of the flexible spacer.     

Optical biaxiality of nematic LC-side chain polymers with laterally attached mesogenic groups

Summary Liquid crystalline polymers with mesogenic groups laterally attached to the polymer backbone were investigated by polarizing microscopy with respect to their optical properties. The nematic polymers were macroscopically oriented in a magnetic or electric field. Polarization microscopic investigations with convergent polarized light show optical biaxial behavior of the nematic phase.     

Thermal properties and enthalpy relaxation of tyrosine-derived polyarylates

Sixteen degradable, tyrosine-derived polyarylates with well-defined chemical structures were used to study the effect of polymer structure on the glass transition temperature and enthalpy relaxation kinetics (physical aging). These polyarylates compose a model system where the number of methylene groups present in either the pendent chain or the polymer backbone can be altered independently and in a systematic fashion. Quantitative differential scanning calorimetry was employed to measure the glass transition temperature and the enthalpy relaxation kinetics. Correlations between these material properties and the polymer structure were established. The glass transition temperature of this family of polymers ranged from 13 to 78°C. The addition of methylene groups to either the pendent chain or the polymer backbone made a fairly constant contribution to lowering the glass transition temperature. The rate of enthalpy relaxation increased with an increasing number of methylene groups in the polymer backbone, but was independent of the number of methylene groups in the pendent chain. This observation indicated that the rate of enthalpy relaxation in these polymers was limited by the mobility of the polymer backbone. The enthalpy relaxation data was fitted to the Cowie-Ferguson model and the relaxation times obtained ranged from 44 min to about 100 min. Although these structure-property correlations facilitate the design of new materials with predictable thermal properties, they are rarely investigated for biomedical polymers. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1441–1448, 1997     

Synthesis and properties of liquid crystalline polymers with low Tm and broad mesophase temperature ranges

Novel thermotropic liquid crystalline polymers (TLCP) with improved processability and lower cost than Vectra^® were synthesized by esterification and melt polycondensation. Aromatic compounds, 6-hydroxyl-2-naphthoic acid, 2,6-naphthlenedicarboxylic acid, 4-hydroxybenzoic acid or terephthalic acid, were used as polycyclic stiff rod-like mesogenic groups. Aliphatic diol, ethylene glycol or 1,4-butanediol was used as flexible spacers to form semi-flexible main chain TLCP. The LCP products were characterized by FT-IR and ¹H NMR to identify their chemical structures. The analytical results showed the polymers had inherent viscosities of 0.35-0.54 dL/g. The crank shaft structure of the naphthalene decreased the melting temperature (T_m) of LCP (181-272 °C) to facilitate extrusion or injection molding, and maintained relatively high heat deflection temperature for high performance engineering plastics applications. The 5% weight loss decomposition temperatures were above 400 °C. LCPs have good solvent resistance and low hygroscopicity. The SEM morphology showed strong orientation on the surface in the flow direction and many micro-fibers structure in a sectional drawing. The optical textures of polymers observed by POM revealed a strong birefringence in the melts and indicated that they form nematic mesophase. All polymers have broad mesophase temperature (ΔT_meso) ranges.     

Synthesis of side-chain liquid crystalline polyacrylates,polymethacrylates and polysiloxanes containing 4-cyano-biphenyl 4-alkanyloxybenzyl ether side groups

Summary The synthesis and characterization of side-chain liquid crystalline polyacrylates, polymethacrylates and polysiloxanes containing 4-cyanobiphenyl 4-alkanyloxybenzyl ether side groups are presented. All polymers display respectively a smectic A mesophase. The influence of the polymer backbone flexibility on the phase transition temperatures of the synthesized side-chain liquid crystalline polymers is discussed. The results demonstrate that flexible backbones enhance the decoupling of the motions of the side chain and main chain. Therefore, among three kinds of polymer backbones which contain the same mesogenic side groups, the most flexible one, i.e., polysiloxane, reveals a highest isotropization temperature and a widest temperature range of mesophase.     

Studies on compatibilization of blends of polypropylene and a thermotropic liquid crystalline polymer

Thermotropic liquid crystalline polymers, LCPs, are frequently blended with thermoplastics to achieve an in situ composite structure. Significant mechanical reinforcement is obtained for the matrix polymer in the direction of the LCP fibers, but the transversal properties are often inferior because of the incompatibility of the components. Blends of LCP with polypropylene, and with three related matrix polymers, and PP/LCP blends with added potential compatibilizers were prepared and studied for their mechanical properties and morphology. A notable improvement in impact strength was achieved when a small amount of ethylene-based terpolymer was added as compatibilizer. © 1993 John Wiley & Sons, Inc.     

Immiscible polymer blend electrorheological fluids: Composition dependence

We investigated the composition dependence of the electrorheological properties of immiscible polymer blends which consist of liquid crystalline polymers (LCPs) and polydimethylsiloxane (DMS). We used two different kinds of LCPs, designated as A and B polymers. We observed that for a fixed ratio of an LCP and DMS (LCP:DMS = 2:1) the electrorheological properties change from type I to type II as the fraction of the A polymer is reduced. Microscopic observations indicate that the change in the electrorheological properties is associated with the structural change; in type I, LCP droplets are dispersed in DMS, while in type II, DMS droplets are dispersed and, furthermore, that the structural change is associated with the miscibility between DMS and the LCPs; the A polymer is partially miscible with DMS, while the B polymer is hardly miscible with DMS. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3673–3680, 2002     

Self‐reinforced polypropylene/LCP extruded strands and their moldings

Polypropylenes (PP) of various molecular weights were mixed with a thermotropic liquid crystal polymer (LCP) and strands were prepared by extrusion and stretching. The strands were subsequently pelletized and then injection molded at temperatures below the melting point of LCP. The mechanical properties and the morphology of the strands and injection‐molded specimens were investigated as a function of draw ratio, LCP concentration, and PP molecular weight. The results for strands show that an increase in the draw ratio, LCP concentration and matrix molecular weight in general enhance the modulus and tensile strength. However, the tensile properties of injection‐molded specimens are found to be reduced compared with those of the original strands, in particular at high LCP concentration. The morphology of LCP changes from spherical or ellipsoidal droplets to elongated fibrils in the strands as the draw ratio increases, but this aligned LCP fibrillar morphology was not transferred to the injection‐molded specimens because of the disorientation of fibrils during injection molding. Compatibilization of PP/LCP blends was also studied by using various polymers. Maleic anhydride and acrylic acid modified PPs improved the tensile properties modestly, but maleic anhydride modified EPDM reduced the tensile properties.     

Wettability of Highly Fluorinated Polymer Coatings

Wettability of a linear polymer is a function of backbone structure, side-chain substitution, and fluorination. For a polymer to be suitable as a thin coating, a convenient method of application is mandatory to insure continuity and adherence to the respective substrate. Wetting properties of highly fluorinated ethylenic and methacrylic polymers are examined. The latter have proven to be effective barrier films for the prevention of liquid spreading in certain applications, and the effect of solvents on the wetting and adhesion of such films and on their stability with a variety of liquids are discussed.     

Chemoenzymatic Isolation and Characterization of High Purity Mammalian Melanin

Although melanin is one of the most ubiquitous polymers in living systems, our understanding of its molecular structure, biosynthesis and biophysical properties has been limited to only a small number of organisms other than humans. This is in part due to the difficulty associated with isolating pure melanin. While purification methods exist, they typically involve harsh treatments with strong acid/base conditions combined with elevated temperatures that can lead to the polymer backbone degradation. To be successful, a viable isolation method must deliver a selective, yet complete degradation of non-melanin biopolymers as well as remove small molecule metabolites that are not integrative to the melanin backbone. Here, we demonstrate the use of chemoenzymatic processing guided by fluorescent probes for the purification and isolation of native mammalian melanin without significant induction of chemical degradation. This multi-step purification-tracking methodology enables quantitative isolation of pure melanin from mammalian tissue for spectroscopic characterization.     

液晶热性能的研究

讨论了液晶聚合物及相应的液晶单体的热性能,指出液晶聚合物的液晶温度区间总量宽于液晶单体。通常在侧链液晶聚合物中第二种单体的引 导致液晶温度区间变宽。     

Fundamental water and salt transport properties of polymeric materials

Fundamental water and salt transport properties of polymers are critical for applications such as reverse osmosis (RO), nanofiltration (NF), forward osmosis (FO), pressure-retarded osmosis (PRO), and membrane capacitive deionization (MCDI) that require controlled water and salt transport. Key developments in the field of water and salt transport in polymer membranes are reviewed, and a survey of polymers considered for such applications is provided. Many polymers considered for such applications contain charged functional groups, such as sulfonate groups, that can dissociate in the presence of water. Water and ion transport data from the literature are reviewed to highlight the similarities and differences between charged and uncharged polymers. Additionally, the influence of other polymer structure characteristics, such as cross-linking and morphology in phase separated systems, on water and salt transport properties is discussed. The role of free volume on water and salt transport properties is discussed. The solution–diffusion model, which describes the transport of water and ions in nonporous polymers, is used as a framework for discussing structure/property relations in polymers related to water and salt transport properties. Areas where current knowledge is limited and opportunities for further research are also noted.     

Electrorheological effect of liquid crystalline polymers

Large increases in shear stress upon application of a 2.0 kV/mm electric field were observed in homogeneous fluids composed of polysiloxane-based liquid crystalline polymers (LCPs) in dimethyl silicone at a shear rate of 200 s^?1. The increase was largest (about 3,000 Pa at 50°C) with LCP consisting of a polysiloxane bearing mesogenic groups as side chains. With LCP having the mesogenic groups within the main chain, the maximum increase was about 1,300 Pa at 90°C. It was about 400 Pa at 30°C with LCP having the mesogenic groups at both ends only (biterminal), and several Pa at 30°C with LCP having the mesogenic group at one end only (monoterminal). The increases were smaller with mesogenic groups of lower positive dielectric anisotropy in the side chain LCP. The side chain, biterminal, and monoterminal LCPs exhibited Newtonian flow in the electric field and shear stress yield at low shear rates in its absence. The complex dynamic modulus and viscosity of the side chain LCP in the electric field showed no dependence on strain at deformation displacements approaching 5°, but in its absence were generally strain-dependent, and suggest the strong electrorheological effect of these homogeneous LCP fluids is related to a flexible-chain linkage between their crystalline domains. © 1995 John Wiley & Sons, Inc.     

Design of semiconducting indacenodithiophene polymers for high performance transistors and solar cells

The prospect of using low cost, high throughput material deposition processes to fabricate organic circuitry and solar cells continues to drive research towards improving the performance of the semiconducting materials utilized in these devices. Conjugated aromatic polymers have emerged as a leading candidate semiconductor material class, due to their combination of their amenability to processing and reasonable electrical and optical performance. Challenges remain, however, to further improve the charge carrier mobility of the polymers for transistor applications and the power conversion efficiency for solar cells. This optimization requires a clear understanding of the relationship between molecular structure and both electronic properties and thin film morphology. In this Account, we describe an optimization process for a series of semiconducting polymers based on an electron rich indacenodithiophene aromatic backbone skeleton. We demonstrate the effect of bridging atoms, alkyl chain functionalization, and co-repeating units on the morphology, molecular orbital energy levels, charge carrier mobility, and solar cell efficiencies. This conjugated unit is extremely versatile with a coplanar aromatic ring structure, and the electron density can be manipulated by the choice of bridging group between the rings. The functionality of the bridging group also plays an important role in the polymer solubility, and out of plane aliphatic chains present in both the carbon and silicon bridge promote solubility. This particular polymer conformation, however, typically suppresses long range organization and crystallinity, which had been shown to strongly influence charge transport. In many cases, polymers exhibited both high solubility and excellent charge transport properties, even where there was no observable evidence of polymer crystallinity. The optical bandgap of the polymers can be tuned by the combination of the donating power of the bridging unit and the electron withdrawing nature of co-repeat units, alternating along the polymer backbone. Using strong donors and acceptors, we could shift the absorption into the near infrared.