摘要:

AbstractConsequences are explored of a hole size distribution in an amorphous polymer for theortho‐positronium (o‐Ps) lifetime (τ3) and intensity (I3), determined by positron annihilation lifetime spectroscopy. The disordered lattice model, with a vacancy fractionhas a central quantity, is used to represent the equation‐of‐state behavior of the polymer. By means of Monte Carlo simulations, we obtain the cluster size distribution as a function ofhand hence temperature. The predicted average cluster size and the cluster concentration are compared to τ3andI3data, respectively, for bisphenol‐a polycarbonate. Furthermore, the influence of an o‐Ps lifetime distribution on the experimental mean τ3is investigated. By mimicking the computational methods used in experimental analysis, agreement between experiment and theory in respect to τ3and toI3in the melt ensues. In the glass, however, the experimentalI3becomes increasingly smaller with decreasing temperature than is computed. These deviations may result from a distortion of the equilibrium free volume. © 1992 John

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301301

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractEmploying the laser‐induced holographic grating relaxation technique, we have measured tracer diffusion coefficients of a phtochromous dye, camphorquinone, in uniaxially drawn polycarbonate films as a function of stretch ratio. Anisotropy in the tracer diffusion coefficient has been observed withD∥greater thanD⟂by at least a factor of 4 for the film stretched to the stretch ratio δ = 2.3. The diffusion coefficient along the direction of stretchD∥increases significantly with increasing δ, whereasD⟂decreases slightly with increasing δ. The stretch ratio dependence ofD∥andD⟂is interpreted according to a modified free volume theory. The strain rate and stretch temperature dependence of the anisotropic tracer diffusion coefficient has also been investigated. © 1992 Joh

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301302

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractDielectric measurement on a polymer‐dispersed liquid crystal (PDLC) has been carried out in the frequency range from 10 Hz to 1 MHz and over the temperature range from 100 to 330 K. The PDLC sample was prepared by thermally induced phase separation of a 50% mixture by weight of commercially available liquid crystal E7 with PMMA and was sandwiched between two indium tin oxide glass plates separated by 40 μm spacers to form a “window.” The dielectric spectrum at low temperature (220–250 K) shows two distinct relaxation processes. Which occur at about 5 K lower than those in pure E7 havingTg≈ 209 K. From differential scanning calorimetry data, the nematic transition of LC droplets in the PDLC is at 258 K, about 6 K lower than that of pure E7. The Maxwell‐Wagner effect has been observed in the low‐frequency side as the temperature increases from 280 to 320 K. At room temperature, the loss peak associated with the Maxwell‐Wagner effect shows an amplitude dependence with excitation level but no frequency shift. The effect of different concentrations of E7 in PDLC samples at a given temperature shows the 50% mixture has the “fastest” relaxation frequency in such a dispersed heterogeneous system. © 1992

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301303

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe gel electrophoretic migration of streptavidin‐DNA complexes is severely altered by the phenomenon known as “trapping electrophoresis.” We present a first computer simulation study of this process. Our simulations use the very efficient biased reptation algorithm. The steady state is characterized by a large increase in band broadening and interband separation. However, we also find that for a narrow range of molecular sizes, the separation power of gel electrophoresis is greatly increased. We discuss the implications of our findings for the possible improvement of DNA sequencing technologies. © 1992 John Wiley&Son

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301304

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractReal‐time x‐ray scattering at elevated temperatures has been used to investigate the thermal expansion characteristics of poly(butylene terephthalate), PBT. Changes in the six lattice parameters of the α‐form of PBT were obtained from wide‐angle x‐ray scattering over the temperature range from 35 to 215°C. The linear thermal expansion coefficients relating the unit cell parameters at temperatureTto their values at 0°C are found to be\documentclass{article}\pagestyle{empty}\begin{document}$$\begin{array}{l}{a = 4.80(1 + 2.03 \times 10^{ - 4} T)\quad \alpha = 100.26(1 + 3.96 \times 10^{ - 5} T)} \\ {b = 5.98(1 + 1.22 \times 10^{ - 4} T)\quad \beta = 114.82(1 + 8.13 \times 10^{ - 5} T)} \\ {c = 11.55(1 + 2.13 \times 10^{ - 4} T)\quad \gamma = 111.43(1 - 3.92 \times 10^{ - 5} T)} \\ \end{array}$$\end{document}The temperature dependence of both the long period and the lamellar thickness of semicrystalline PBT were determined from real‐time small‐angle x‐ray scattering analysis of the one‐dimensional electron density correlation function. The long period, lamellar thickness, and degree of crystallinity increase as the temperature increases. We find an average linear thermal expansion coefficient of the bulk material to be αave= 5.0 × 10−4°C−1.

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301305

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractElectron diffraction from single crystal lamellae and whiskers of poly(2,6‐oxynaphthoate) reveals the presence of at least 3 unit cells. The equatorial reflections in the patterns from the whiskers correspond to the dominant (phase I) hk0 diffraction pattern from the lamellae; phase I is monoclinic with 2 chemical repeats per physical repeat. The intensity distributions in the hk0 patterns of phase I and II resemble those of the same phases in poly(p‐oxybenzoate). The hk0 reflections of phase III suggest a common internapthalene unit spacing, but variable lateral (and possibly axial) shifts; apparently related orthorhombic and monoclinic patterns, with variable γ*, are observed. At elevated temperature, above the crystalliquid crystal transition (ca. 330°C), quadrant reflections are retained; the change in the hk0 pattern from any given crystal is gradual, extending over some 40°C. Above the liquid crystal‐liquid crystal transition (ca. 460°C) the pattern can be interpreted in terms of nematic or possibly smectic A packing. © 1992 John Wiley

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301306

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractFolded chain single crystals 35 Å thick have been grown from the liquid crystal state of an aromatic‐aliphatic azomethine ether polymer (AZMEP‐n) having a 10‐carbon flexible segment (n= 10). Electron diffraction has permitted refinement of the triclinic unit cell. The molecular axes lie at an ca. 65° angle to the lamella normal and fold every third chemical repeat distance. For AZMEP‐1 and ‐8 extended chain lamellae are formed; for AZMEP‐7 both folded and extended chain lamellae are found. The observations of folded chain lamallae are in agreement with prior suggestions from our laboratory of chain folding in the liquid crystalline state in thin films. © 1992 John W

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301307

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe morphology of ca. equimolar random terpolymer liquid crystal polymers of an aliphatic segment of 4–7 carbon atoms, oxybenzoate, and dioxyphenyl crystallized from the nematic state in the form of thin films on glycerine by slow cooling and quenching has been characterized by electron microscopy (TEM) and diffraction (ED). In all cases a folded chain, lamellar structure is found. The ED studies suggest adjacent reentry, and despite a large ΔH, indicate no change in lateral molecular packing at the crystal‐“liquid crystal” transition with a transformation to the nematic state at a higher temperature “liquid crystal”‐liquid crystal transition. The results are interpreted as suggesting similar folded chain, lamellar morphology in the nematic state. © 1992 John

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301308

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractExtruded thin films of a liquid‐crystalline charge‐conjugated rigid‐rod polymer poly(p‐phenylenebenzobisthiazole), PBT, and a semicrystalline thermoplastic polyethylene‐terephthalate (Mylar) were fabricated and examined for film thickness, refractive index, and linear attenuation coefficient. Optical waveguide modes were successfully induced on the polymeric films using a prism coupler at λ = 633 and 1300 nm. Highly consistent thickness values were obtained for the polymeric films. In addition, the anisotropic nature of the optical properties was determined using TE and TM propagation modes. A refractive index as high as 2.3 was observed on PBT film. The refractive index data suggested that the PBT and Mylar films were optically anisotropic with refractive indicesn⟂ (out‐of‐plane) invariably smaller thann∥ (in‐plane). Large anisotropy was also discovered in the linear attenuation coefficient α, with α∥/α⟂≈ 50 for the Mylar films. Complementary polarimetric and spectroscopic interference measurements were also applied to investigate the optical anisotropy of the extruded polymeric films

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301309

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractBulk rigid‐rod molecular composites were successfully obtained by powder consolidation of a copolymer containing both the reinforcing rigid‐rod segments and the thermoplastic matrix. By chemically linking the reinforcing segments and the matrix molecule, the copolymer was designed to minimize phase separation in the molecular composite. The copolymer was an articulated rigid‐rod poly(p‐phenylenebenzobisthiazole), aPBT, with an aromatic poly(ether ketone), mPEK, thermoplastic pendant grafted at the points of articulation. The copolymer powder was pre‐formed and compression molded at an elevated temperature, which resulted in bulk rigid‐rod molecular composites with three‐dimensionally isotropic properties. Compared to the neat mPEK homopolymer, significant increases in glass transition temperatureTgand tensile properties have been realized for the aPBT‐g(mPEK) copolymers with low rod content. Taking into account the aspect ratio of the aPBT, the bulk rigid‐rod molecular composite showed a tensile modulus as predicted by the Halpin‐Tsai equation. In addition, x‐ray scattering revealed minimal rod aggregation. However, for the copolymer of higher rod content, significant phase separation was observed in the copolymer powder, which resulted in a decrease inTgas well as reinforcement efficiency of the bulk rigid‐rod molecular composite as compared to those derived from the copolymers of low rod content. © 1

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301310

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY