摘要:

AbstractThe mechanical and dielectric relaxation behavior of strained and unstrained networks, prepared from hydroxyl‐terminated poly(diethylene glycol‐trans‐1,4‐cyclohexane dicarboxylate) (PDGC), is studied over a wide interval of frequencies and temperatures. The mechanical relaxation spectrum exhibits a glass‐rubber absorption, designated β, located in the vicinity of 0°C at 0.1 Hz, followed by a β relaxation which appears to be the result of two overlapping peaks centered at −80°C (β1) and −110°C(β2). These two peaks coalesce into a single peak in the case of strained networks. The dielectric relaxation spectrum also exhibits an α absorption followed by a subglass β relaxation whose width decreases as the elongation ratio λ increases. The activation energy associated with the mechanical β1appears to increase as λ increases. However, the activation energy of the dielectric β process does not show a clear dependence on the elongation ratio. The analysis of the conformational characteristics of PDGC chains indicates that rotational transitions through the CcyC* bonds of the acid residue would give rise to high dielectric activity. Conformational changes about the CH2CH2bonds of the glycol residue would produce significant mechanical activity but, comparatively, low dielectric activity. The glass‐rubber absorption is slightly displaced toward the high‐temperature side as the elongation ration increases, suggesting that the entropic effects overcome the volume effects. The glass‐rubber transition is interprete

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290112

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractRaman spectra of the CC stretching region of strained gel‐spun ultradrawn ultrahighmolecular weight polyethylene fibers were recorded. On the low‐wavenumber side of both the asymmetric and symmetric band, an additional broad band appeared during straining of the fiber, indicating the presence of CC bonds exhibiting high strain. The fraction of such CC bonds was estimated to be 40%. At 4% strain, the shifts of the CC stretching bands were found to increase linearly with the initial Young's modulus. From Raman spectra, information about strain distribution can be obtained. Considering the observed differences in strain distribution between fibers produced in a different way, it can be understood why one sample can be drawn to a higher extent without rupture of CC bonds. The results were discussed in conjunction with results of X‐ray measurements reported by Prasad and Grubb. It is suggested that the fibers can be described by two crystalline fractions mixed on the molecular level and an additional small amount of a third fraction with a low

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290113

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractMeasurement of the molecular weight, radius, and molecular weight distribution of polyimide precursors (amic acids and easters) is quite difficult due to strong polymer–solvent interactions, electrostatic interactions, and the possibility of partial imidization in solution. The large quantities of solvent and high dilutions necessary in liquid chromatography make this technique the most sensitive to these effects. The distortions in the chromatogram from these effects can make assessment of the molecular weight distribution from a polystyrene calibration very tenuous. Use of a light‐scattering detector permits direct measurement of the scattering intensity (and thus the molecular weight), as the sample elutes from the column. For the esters, anomalous peaks and unexpectedly large apparent values for both the molecular weight and molecular weight distribution are shown to be due primarily to partial imidization in solution catalyzed by basic solvent impurities. Chromatographic results obtained in neutralized solvent are in agreement with light‐scattering results obtained at higher concentra

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290114

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractPoly(oxy‐1,3‐phenylenecarbonyl‐1,4‐phenylene), mPEK, was synthesized in a methane sulfonic acid/phosphorus pentoxide medium through the polycondensation of 3‐phenoxybenzoic acid. The polymer was soluble at room temperature in strong acids and common organic solvents. Differential scanning calorimetry on mPEK indicated a glass transition temperature of 132°C and no detectable crystal melting behavior. The polymer was thermo‐oxidatively stable up to 420°C in air as determined by thermogravimetric analysis. Thermal degradation resulted in a crosslinked polymer. Compression molding was successfully applied to consolidate mPEK powder into bulk specimens without causing degradation. Tensilefractured surfaces of the consolidated specimens were examined by scanning electron microscopy which strongly indicated adequate fusion of mPEK powder but also disclosed microvoids of uniform size, shape, and distribution. X‐ray scattering revealed that mPEK powder was amorphous and that the consolidated mPEK specimens were structurally isotropic as well as amorphous. The bulk specimen density was determined to be 1.29 ± 0.01 g/cm3and the three‐dimensionally isotropic bulk tensile properties were shown to be 4.10 ± 0.10 GPa (modulus), 0.07 ± 0.01 GPa (strength), and 0.021 ± 0.006 (elongation at break). Tensile properties of the amorphous mPEK specimens compared favourably with literature values of state‐of‐the‐art semicrystalline thermoplastics. Crystaline phases could be realized for mPEK by melt‐processing, or

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290115

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

摘要:

AbstractA theoretical equation has been developed to described the rate of slow crack growth in an ethylene‐hexene copolymer in terms of the basic morphological parameters. These parameters are spacing of the butyl branches, number of tie molecules, and the thickness of the lamellar crystal. Experimentally, the thickness of the lamellae and the long period were determined as functions of the branch density. The calculation of the number of tie molecules is based on the values of the molecular weight and the long period. The model of slow crack growth is based on the rate of disentanglement of the tie molecules. The rate of disentanglement varies inversely with the number of tie molecules and directly with the number of tie molecules that are not pinned by the branche

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290116

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY

ISSN:0887-6266    

DOI:10.1002/polb.1991.090290101

出版商:John Wiley&Sons, Inc.    

年代:1991

数据来源: WILEY