摘要:

AbstractGiven a diffusant, modeled as a hard sphere, and a microstructure representative of a glassy polymer, represented as a rigid matrix of hard sphere atoms, a quantitative determination of the unoccupied space in the polymer was achieved by applying a Delaunay tessellation of space to available glassy microstructures of atactic polypropylene and bisphenol‐A‐polycarbonate. A description is also given of how local portions of unoccupied space are distributed and interconnected across a polymeric matrix. The shape and dimensions of clustered portions of unoccupied space were analyzed. A visualization of these clusters indicated significant differences, depending on the nature of the diffus

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300501

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractAn electron spin resonance (ESR) study confirms a free radical mechanism for aqueous vinyl polymerization initiated by three redox systems involving thiourea. The primary radical, NH2C (NH)S., could not be detected by ESR spectroscopy, perhaps because of its largeganisotropy and/or very short relaxation time. Propagating radicals are trapped from methyl‐methacrylate by 2‐methyl‐2‐nitroso propane (MNP) and their structures are found to be the same as those produced by either azo‐bis‐isobutyrnitrile or di‐t‐butyl peroxylate. The latter monomer, however, reacts with MNP in the absence of any initiator, giving

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300502

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe dielectric permittivity ε′ and loss ε″ of diglycidyl ether of bisphenol‐A thermosets cured with nonstoichiometric amounts of diamino‐diphenyl methane have been measured during the course of their chemical reactions from the sol to gel to glass‐formation regions. ε′ monotonically decreases with time and ε″ initially decreases, increases to a peak value, and finally decreases to extremely low values characteristic of the glassy state. The initial decrease in ε″ is due to the decrease in the dc conductivity, and the peak is due to the dipolar reorientation. The appearance of these features shifts to longer time when the thermoset is stoichiometrically starved by decreasing the amount of the curing agent and, at a molar ratio of 4 : 1 of the epoxide to diamine, the ε″ peak does not appear during the curing process. Complex plane plots of ε′ and ε″ have the shape of an arc in all cases except when the molar ratio of the epoxide to diamine is 4 : 1. The dielectric consequences of the chemical changes with time during the crosslinking of a thermoset are analogous to the frequency dependence of ε″ of a condensed phase. The time dependence of ε″ follows a stretched exponential decay, ϕ(t= exp − [(t/τ)γ], where 0<γ<1. The parameter decreases with decreasing amine content. ε″ has been analyzed to obtain the increase in the relaxation time as curing progresses. A representation of ε*in terms of electrical modulus M*shows the occurrence of, first a Maxwell relaxation due to dc conductivity, and second a dipolar relaxation, during the period of a typical isothermal cure. Changes in the features of the isothermal cure that occur on changing the amount of the curing agent are discussed in terms of network formation in the thermoset, and the change in the electrical conductivity with curing time has been analyzed in terms of both a power law for gel formation and by a new equation t

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300503

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe dielectric permittivity and loss of diglycidyl ether of bisphenol‐A (DGEBA) cured with greater than and less than the stoichiometric amounts of diaminodiphenyl methane (DDM) have been measured over a temperature range 77–350 K prior to curing and gelation, after curing at about 340 K and further aging for a predetermined period. The height of the γ‐relaxation peak monotonically decreases during the post‐cure period and becomes masked by the contributions from the β‐relaxation peak, whose height, in turn, first increases on postcuring to a same maximum value for both nonstoichiometric thermosets and then decreases. This decrease is attributed to physical aging effects. The β‐relaxation peak shifts towards higher temperature on postcuring. Comparison between the changes in the dielectric properties of the saturated and starved thermosets show that while the γ‐relaxation process may be attributed to the motion of the epoxide dipolar groups of the unreacted DGEBA, the β‐relaxation process is not attributable entirely to the motion of OH groups and of the unreacted amines in the thermoset. Explanations involving the chain and network packing in the structure of a thermoset are necessary for the observed behavior of the β‐relaxation process in amine saturat

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300504

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractA composite degradation methodology is extended to the conversion‐dependence function in order to explain the importance of multiple reaction mechanisms which might be considered to be involved in degradation processes. Based on two elementary reaction mechanisms, a specific form of the model equation is derived, which is capable of describing various types of degradation behavior showing sigmoidal rate as well as deceleratory rate. The conversion‐dependence function is derived to be independent of the Arrhenius‐type reaction constant or temperature, and thus the kinetic parameters are determined by analytic methods that have been developed for isothermal and dynamic‐heating experiments without any modification or additional assumptions. The developed model equation is tested by predicting the isothermal master curve of polyether‐ether‐ketone (PEEK), which is used as a model system in this study. The activation energies of the model system are analyzed using comparable methods for isothermal and dynamic experiments, which compare favorably in terms of the activation energy as a function of conversion. The resulting model equation, based on the kinetic parameters determined by isothermal experiments, can accurately predict both isothermal and dynamic‐heating thermogravimetry utilizing the same constants and identical reaction mechanisms without addition

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300505

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractBlends of an aromatic polyethersulfone (commercial name Victrex) and a polyimide (commercial name Matrimid 5218), the condensation product of 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride and 5(6)‐amino‐1‐(4′‐aminophenyl)‐1,3,3′‐trimethylindane, were studied by differential scanning calorimetry, dynamic mechanical analysis, and rheological techniques. The blends appeared to be miscible over the whole range of compositions when cast as films or precipitated from solution in a number of solvents. After annealing above the apparent phase boundary, located aboveTg, the blends were irreversibly phase separated indicating that the observed phase boundary does not represent a true state of equilibrium. Only a narrow “processing window” was found for blends containing up to 20 wt % polyimide. Rheological measurements in this range of compositions indicated that blending polyethersulfone with polyimide increases the complex viscosity and the elastic modulus of the blends. For blends containing more than 10 wt % polyimide, abrupt changes in the rheological properties were observed at temperatures above the phase boundary. These changes may be consistent with the formation of a network structure (due to phase separation and/or crosslinking). Blends containing less than 10 wt % polyimide exhibited stable rheological properties after heating at 320°C for 20 min, indicating the existence of

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300506

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe fatigue behavior of an ethylene‐hexene copolymer was investigated. The effects ofR, frequency, relative times under the maximum and minimum stress, and waveform were measured. The phenomenological aspects were related to the microscopic aspects of the failure process. The maximum stress produces damage by disentangling the molecules in the fibrils of the craze and the minimum stress produces damage by bending the fibrils. The net damage, which is a product of these two damage processes, has been represented by a simple equation which accounts for the phenomenological observation

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300507

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractSpecific infrared absorptions for each form of Nylon‐6 have been indentified. Quenched films were prepared by melt casting and converted in turn to the α‐crystalline and γ‐crystalline structures. The films were examined using Fourier transform infrared spectroscopy (FTIR) with the attenuated total reflection configuration and by X‐ray diffractometry. The FTIR spectrum of the pure mesomorphous component of the quenched film was calculated by subtracting away the contribution of α‐crystalline peaks present due to incomplete quenching. A curve‐fitting procedure was applied to the pure mesomorphous and gamma crystalline spectra. Several peaks in the mesomorphous spectrum are observed to develop a shoulder upon conversion to the γ‐crystalline form. This is due to the extragauchecharacter imparted to the CN bond between the methylene unit and the amide group. These and other band assignments were confirmed by analysis

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300508

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractSorption rate curves of CO2, N2, and He gases below 1 atm were measured for polyimide films prepared from benzophenone tetracarboxylic dianhydride (BTDA) with 3,5‐diaminotoluene trifluoride (DATF), 2,4‐diaminotoluene (DAT),m‐phenylenediamine (MPD), and diaminobenzoic acid (DABA). The molecular structures of these four polyimides differ only in the substituent groups of the diamine structure. These polyimides exhibit dualmode type sorption isotherms for carbon dioxide that are concave to the pressure axis, typical of glassy polymer/gas system. The apparent diffusion coefficients below 1 atm pressure of carbon dioxide for this series of compounds decrease in the order: BTDA‐DATF>BTDA‐DAT>BTDA‐MPD>BTDA‐DABA. A linear relation between the logarithm of the apparent diffusion coefficient and the reciprocal of free volume, calculated by the method of Bondi using density data, is found for these polyimides. However, this tendency is not observed for the other two gases. The activation energies of the apparent diffusion coefficients at 20 cmHg pressure of carbon dioxide increase with increasing cohesive energy density of the polyimides. The energy per mole of free volume elements in a liquidlike structure in each cohesive energy density may be equated to the activation energy and used to calculate the free volume. The values from the activation energy are almost the same as those from B

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300509

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractBlends of poly (butylene terephthalate) (PBT) and a liquid‐crystalline copolyester (60 mol % poly(p‐hydroxy benzoic acid)/40 mol % polyethylene terephthalate) (LCP) were prepared in the melt state. The investigation of mechanical properties indicated that, for the processing conditions used, neither the addition of up to 30 wt % LCP to PBT nor the cooling history affected significantly the tensile modulusE. For oriented specimens, a marked improvement ofEwas obtained for all the blends, and increased with the LCP content. This improvement was more marked for slowly cooled samples. X‐ray diffraction was used to quantify the orientation of the crystalline PBT and liquid‐crystalline LCP phases. It was shown that neither the thermal history nor the presence of up to 30 wt % LCP affected the orientation behavior of the PBT crystalline phase. For the LCP phase, measurements were not possible for concentrations lower than 10 wt %, and were more difficult and less precise than for PBT. Nevertheless, it was possible to show that a better orientation was obtained for the slowly cooled samples and for higher concentrations of LCP in the blends. This correlated with the enhancement of mechanical properties observed for the oriented

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300510

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY