ISSN:0887-6266    

DOI:10.1002/polb.1992.090301101

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractMonodisperse sized crosslinked polystyrene (PS) beads prepared by reaction of styrene (S) and divinylbenzene (DVB), in batch emulsion copolymerization in the absence of emulsifiers, arenotuniformly crosslinked, because DVB is more reactive than S. For copolymerization of 1 to 10 mol % DVB and S, within each crosslinked PS microbead, the crosslink density varies by a factor exceeding two and decreases with increased conversion. A semicontinuous copolymerization, involving incremental additions of DVB, produces uniformly crosslinked PS beads. For both copolymerization techniques,Tgcorrelates well with crosslink density and PS beads are spherical and monodisperse in size. © 1992 John Wiley&Sons, Inc

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301102

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAmorphous polyarylene ether ketones were examined in the glassy state by positron annihilation lifetime spectroscopy (PALS) and in the melt by standard rheological techniques. Specimens were well‐characterized fractions of two isomeric structures. PALS clearly shows that the polymer with meta linkages in its backbone contains larger voids (>0.25 nm radius). Thus despite their similar bulk densities, the two materials must pack very differently on a local scale. On the other hand, the free volumes inferred from the WLF treatment of melt viscosity data are practically identical in both materials ca. 4% atTg. The comparison between techniques sheds some light on the distribution of free volume. © 1992 John Wiley&Sons, I

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301103

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractA superconducting quantum interference device (SQUID) magnetometer suitable for measurements of the magnetic susceptibility up to 600 K has been used to follow the kinetics of realignment in the nematic phase of a side‐chain liquid–crystal polymer. Experiments are performed using monodomain and polydomain samples. Results obtained in monodomain samples are in quantitative agreement with the prediction of the continuum theory up to an angle of approximately 60°. Experiments conducted at higher starting angles give clear indication of backflow effects. Data obtained from experiments on polydomain samples have been compared with model calculations using the data on the monodomain samples. This comparison leads to the conclusion that in this case backflow effects play an important role, too. All results show that SQUID magnetometry allows monitoring of the kinetics of realignment with highest precision. © 1992 John Wiley&Sons

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301104

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe slow crack growth resistance was measured in an ethylene‐octene copolymer as a function of the morphological changes produced by varying the thermal history. Morphology was varied by annealing the quenched state at temperatures between 86°C and the melting point. The slow crack growth behavior was measured by the lifetime of a notched tensile specimen under a constant load. In general, the lifetime exhibited a maximum at a critical value of the annealing temperature. This critical annealing temperature decreased with a decrease in the temperature at which the lifetime was measured. The former result is understandable in terms of the increase in crystal strength as the annealing temperature is increased and the decrease in the number of tie molecules when more material is melted as the annealing temperature increases. The latter result depends on the relationship between crystal size and the effect of testing temperature. Differential scanning calorimetry data played a key part in analyzing the results. © 1992 John Wiley&Sons,

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301105

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractGas permeability and permselectivity are investigated for polybenzoxazoles frombis(3‐amino‐4‐hydroxyphenyl)‐1,1,1,3,3,3‐hexafluoropropane (BAHHP) and aromatic diacid chlorides. Effects of thermal cyclization on the permeation properties are also investigated. The polybenzoxazole from BAHHP and 4,4′‐(1,1,1,3,3,3‐hexafluoroisopropylidene)dibenzoyl chloride (HFDB) displays high performance for CO2/CH4separation (\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm P}_{{\rm CO}_2 } $\end{document}= 6.1 × 10−9cm3(STP) cm−1s−1cm‐Hg−1, and\documentclass{article}\pagestyle{empty}\begin{document}$ {{{\rm P}_{{\rm CO}_2 } } \mathord{\left/ {\vphantom {{{\rm P}_{{\rm CO}_2 } } {{\rm P}_{{\rm CH}_4 } }}} \right. \kern-\nulldelimiterspace} {{\rm P}_{{\rm CH}_4 } }} $\end{document}= 38 at 35°C). The polybenzoxazole from BAHHP and 2,6‐naphthalene dicarbonyl chloride displays high performance for H2/CO or H2/CH4separation (\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm P}_{{\rm H}_2 } $\end{document}= 2.4 × 10−9cm3(STP) cm−1s−1cm‐Hg−1,\documentclass{article}\pagestyle{empty}\begin{document}$ {{{\rm P}_{{\rm H}_2 } } \mathord{\left/ {\vphantom {{{\rm P}_{{\rm H}_2 } } {{\rm P}_{{\rm CO}} }}} \right. \kern-\nulldelimiterspace} {{\rm P}_{{\rm CO}} }} $\end{document}= 71, and\documentclass{article}\pagestyle{empty}\begin{document}$ {{{\rm P}_{{\rm H}_2 } } \mathord{\left/ {\vphantom {{{\rm P}_{{\rm H}_2 } } {{\rm P}_{{\rm CH}_{\rm 4} } }}} \right. \kern-\nulldelimiterspace} {{\rm P}_{{\rm CH}_{\rm 4} } }} $\end{document}= 250). Permeation properties for the polybenzoxazole from BAHHP and HFDB are close to those for a polyimide of similar chemical structure. The permeation properties are discussed in connection with packing density and local segment

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301106

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractSorption and diffusion of water vapor are investigated gravimetrically for polyimide films. The activity dependence of the solubility and diffusion coefficients,SandD, respectively, is classified under four types: (1) constantSandDtype, (2) dual‐mode sorption and transport type, (3) dual‐mode type followed by a deviation due to a plasticization effect at high vapor activity, and (4) constantSandDtype followed by a deviation due to water cluster formation at high activity. For the dual‐mode type, the Henry's law component is much larger than the Langmuir component except at low activity, and therefore deviation in behavior from the first type is small.Sis larger for polyimides with higher content of polar groups such as carbonyl, carboxyl, and sulfonyl.Dis larger for polyimides with a higher fraction of free space, with some exceptions. The polyimide from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride and dimethyl‐3,7‐diaminodibenzothiophene‐5, 5‐dioxide belongs to the third type and displays both largeSand largeD. The polyimide from 2,2‐bis(3,4‐dicarboxyphenyl) hexafluoropropane dianhydride and 4,4′‐oxydianiline belongs to the fourth type, and has the largestDbut rather smallSbecause of the hydrophobic C(CF3)2groups.

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301107

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAmphiphilic films and hydrogels have been prepared from ethanol/water solutions containing a hydrocarbon‐grafted water‐soluble cellulose ether. These materials are characterized by dispersed hydrophobic microdomains which form spontaneously in the solvent due to the inherent incompatibility of the side chains with water. At low applied shear stress, the microdomains behave as temporary linkages of finite lifetimes, imparting viscoelastic properties to the networks. The molecular weight between microdomains was found to be independent of the volume fraction of polymer in the gel, and the number of linkages per backbone ranges from 22.8 ± 1.3 to 26.2 ± 1.5 over the frequency range 30–50 rad/s. The behavior of the solutions and gels was characterized using fluorescence and dynamic rheological measurements. It was demonstrated that the microdomains are capable of sequestering water‐insoluble solutes. © 1992 John Wiley

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301108

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe interpretation of the mechanical and dielectric β relaxations of poly(cyclohexyl acrylate) by the coupling scheme suggests that the dielectric relaxation process is more complex than the mechanical one, contrary to what occurs in the glass–rubber relaxation. The distribution of activation energies for the dielectric process, obtained from isochronal and isothermal loss curves, increases with temperature and frequencies. The determination of the distribution of the activation‐free energy indicates a distribution of the activation entropy for both the mechanical and dielectric β process, suggesting that a distribution of preexponential factors in the Arrhenius equation also exists. © 1992 John Wiley&Son

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301109

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractProton nuclear magnetic resonance (NMR) spin‐spin relaxation measurements were made on three commercial‐grade polyethylenes in the melt state, free of solvent. All samples exhibit a three‐component relaxation behavior, with components being assigned to amorphous low‐molecular weight material (non‐network fraction), amorphous entangled network fraction, and an ordered or high‐segmental‐density fraction, in order of decreasing relaxation times. Sample thermal history is shown to have a considerable effect on the overall relaxation behavior, and therefore on the relative amounts of each of the three components in the melt. An adequate thermal treatment of samples produces an equilibrium melt with invariant composition of the three fractions. The effects of thermal history on the relative amount of high‐segment‐density regions in the melt parallels its effect on the fraction of crystalline material in the solid polymer. These results are evidence for the persistence of ordered regions in polyethylene at temperatures well above the crystalline melting point of the polymer. We further comment on the nature of the two slower relaxing components and present examples of how the components manifest themselves in other polymer characterization techniques. © 1992 Jo

ISSN:0887-6266    

DOI:10.1002/polb.1992.090301110

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY