摘要:

AbstractVelocity and attenuation of longitudinal acoustic waves in a frequency range of 5–16 MHz have been measured during the curing of diglycidyl ether of bisphenol‐A with ethylene diamine. The velocity monotonically increases and reaches a limiting value and the attenuation reaches a maximum and thereafter decreases as curing proceeds. Both kinds of data have been transformed into a complex‐longitudinal‐modulus formalism. The complex‐plane plots of the longitudinal modulus show an arc‐skewed shape at the long (curing) time end. It is fitted, for the initial phase of curing, to a stretched exponential decay function with an exponent γ = 0.22 to 0.28. These results then are considered in terms of a process with a negative feedback between molecular diffusion and chemical reactions to obtain the increase in the relaxation time with the curing time. This increase is sigmoidal. Calculations of the complex longitudinal modulus show the consistency of o

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300801

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractModel networks of poly(dimethylsiloxane) (PDMS) were prepared by tetrafunctionally endlinking hydroxyl‐terminated chains of various molecular weights. Some networks were prepared from mixtures of chains so as to yield a bimodal distribution of network chain lengths and, in some cases, these networks were prepared in solution. The stress–strain behavior of these unimodal and bimodal networks was studied in pure shear, which was imposed by stretching a sheet of the network having a large ratio of width to length in the direction perpendicular to the width. The pure‐shear moduli of both types of networks generally were found to depend markedly on strain. Stress–strain isotherms for unimodal networks prepared from chains of one or the other of two molecular weights were well interpreted using the constrained‐junction model of Flory and Erman. The bimodal networks showed large increases in the pure‐shear modulus at high strains which were similar to those reported for uniaxial extension and compression. Endlinking in solution decreases the modulus in general and its upturn in particular, presumably because of diminished chain‐juncti

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300802

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAn equilibrium theory is proposed for crystallization of (A, B) binary copolymers whose comonomeric unit sequences are statistically described by conditional pair probabilitiesPAA, PAB, PBA, andPBB. These are linked to the product of the reactivity ratios byr=rArB=(PAAPBB)/(PABPBA). Three cases are considered here, (i) B units are rejected from the crystals, (ii) cocrystallization of A and B comonomeric units is possible in the full range of compositions within a single crystal structure (copolymer isomorphism), (iii) cocrystallization takes place either in a poly(A)‐type or in a poly(B)‐type structure, depending on composition (copolymer isodimorphism). For case (i) crystallization the theory demonstrates, according to expectation, that alternating copolymers (r= 0) produce the largest melting point depression, whereas in case (ii) they give rise to the smallest composition difference between the crystals and the liquid. The theory developed here further illustrates that for binary copolymers which are isodimorphic (case iii), a phase diagram is obtained similar to that for a classical binary system of small molecu

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300803

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractGas transport properties are reported for two series of films prepared from copolyesters of 73 mol % hydroxybenzoic acid (HBA) and 27 mol % 2,6‐hydroxynaphthoic acid (HNA) which systematically vary the degree of orientation and annealing time. Scanning electron microscopic (SEM) photomicrographs of the liquid‐crystalline polymer (LCP) films showed evidence of a skin‐core structure and polydomain texture. The degree of orientation in the films was quantified by analyzing the azimuthal intensity of the x‐ray reflection associated with the lateral packing of the nematic mesophase. Using heat of fusion data from differential scanning calorimetry (DSC), the films were found to contain low levels of crystallinity estimated to be in the range of 5 to 15 wt %, which increased with annealing time. Permeability measurements were made for He, H2, O2, N2, Ar, and CO2at 35°C and the diffusivities were computed from time‐lag data. The films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. A moderate reduction in permeability was observed with increased orientation, which could be attributed directly to a decrease in the effective diffusivity. The effect of increased crystallinity from annealing on the permeability coefficients was smaller than would be expected for similar changes in the crystallinity of conventional polymers. Analysis using a simple two‐phase model suggests that a mechanism dominated by transport in a small volume fraction of boundary regions possibly could account for the bulk transport properties of the

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300804

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractGas transport properties are reported for a series of compression‐molded films prepared from copolyesters of hydroxybenzoic acid (HBA) and 2,6 hydroxynaphthoic acid (HNA) having 30/70, 58/42, 73/27, 75/25, and 80/20 mol % HBA/HNA. The mesomorphic and crystalline morphology of the materials was characterized using dynamic mechanical thermal analysis (DMTA), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X‐ray diffraction As evidenced by DMTA, the phenyl and naphthyl moieties of the HBA/HNA materials exhibit a significant degree of segmental mobility below the glass transition temperature. The nonlinear nature of the naphthyl unit leads to a more hindered rotation about the chain axis. Permeability measurements were made for He, H2, O2, N2, Ar, and CO2at 35°C and the diffusivities were computed from time‐lag data. As previously observed in these materials, the films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. The liquid‐crystalline copolyester: (LCP) materials with the highest HNA content exhibit the best barrier properties. It appears that the more hindered motions of the naphthyl unit restrict penetrant mobility. The reduction in permeability with increased naphthyl unit content is accompanied by a very dramatic increase in selectivity between gas pairs. Fractional free volume analysis was used to correlate the transport properties of the LCP materials and other conventional polymers. A “two‐phase” modification of the free volume correlation suggests that transport may likely occur in a small volume fraction of a less dense

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300805

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe structure and phase transitions in poly[bis‐(2,2,3,3‐tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x‐ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 Å, b = 9.96 Å, c = 4.96 Å, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis‐trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in ori

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300806

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractLinear low‐density polyethylene (LLDPE) and polypropylene (PP) were charged by sliding under a cylindrical aluminium contact. The surface charge accumulation on the polymer films was time dependent, and a function of the sliding velocity, metal contact force, and film type. The surface potential increased linearly with velocity in the range 0.33–0.75 m/s, and showed a square‐root dependence on the contact force up to 6.5 N. Thermally stimulated charge decay (TSCD) studies showed longer charge lifetimes in samples friction‐charged on one side than in corona‐charged samples. Friction charging creates deeper near‐surface traps than are normally present in the polymers. Charge stability is further imporved by simultaneously friction charging both surfaces of the films, increasing the half‐value charge decay temperature (T1/2) by 27°C and 37°C over that in corona‐charged samples (i.e., to 98°C and 120°C for LLDPE and PP, respectively). These monopolarity electrets should prove useful in cha

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300807

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe Rayleigh‐Gans‐Debye approximation is used to investigate the small‐angle, cross‐polarized light scattering by a sphere of radiusa, birefringence Δμ and relative index μ. If θmaxis the polar scattering angle of the intensity maxima, the quantityÛmax= 4πa/λ × sin (θmax/2) behaves in two different ways according to the signs of Δμ and μ − 1. When Δμ>0, μ>1 or Δμ<0, μ<1, thenÛmaxvaries from 2.8 to 4.1 as Δμ increases from zero. If Δμ0, μ<1, thenÛmaxgoes from 2.8 to about 6, thereafter decreasing to 4.1. Another interesting result is that the value ofÛmaxfor a highly briefringent sphere is 4.1 only for large diameters. It decrea

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300808

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe mechanical and electrical properties of solution‐processed [or gel‐spun] blends of poly(3‐octylthiophene) and ultrahigh molecular weight polyethylene are discussed. Tensile drawing at elevated temperatures of the phase‐separated blends resulted in significant improvements of the mechanical properties, in comparison with those of the neat conducting polymer, with values of the Young's modulus reaching>40 GPa and tensile strengths in excess of 2 GPa. Doping of the undrawn polyblend fibers with iodine vapor or FeCl3resulted in materials of useful levels of electrical conductivity covering the full range of 10−15to 10 S/cm. A distinct percolation threshold for electrical conductivity was not observed, even at poly(3‐octylthiophene) concentrations as low as 0.5 w/w %; the electrical conductivity of the latter blend, after doping with iodine vapor, was 8

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300809

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have studied simple empirical equations to estimate light absorption loss αvdue to harmonics of molecular vibrations of transparent amorphous polymers used in plastic optical fibers (POFs). In the visible region, absorption involves two losses. One is αv, and the other is the electronic transition absorption loss, αe. Of the two, αvis considerably larger than αein the wavelength region used for optical communication with POFs. We have clarified relationships between chemical structure of repeat units of polymers and αv. We find that αvis proportional to the concentration of specific chemical bonds (CH, NH, and Obond;H bonds) in the polymer solid, and we propose empirical equations to estimate αvfrom the polymer density and the chemical structure of the repeat unit. These equations are used to estimate αvof several polymers [i.e., poly(methyl methacrylate), polystyrene, and polycarbonate]. The estimated values are nearly equal to the experimental or reference values. Furthermore, to minimize the attenuation in the POF, we conclude that the POF core polymer should have no NH, OH, or aliphatic CH bonds

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300810

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY