摘要:

AbstractPossible effects of cyclic stress on physical aging in polycarbonate were investigated using differential scanning calorimetry (DSC) measurements. When the enthalpy overshoot by DSC of specimens of different previous thermophysical aging histories is measured as a function of the cyclic stress amplitudes, two characteristic regimes are observed. By correlating with optical microscopic observations, these regimes are identified as the incubation and crazing stages (denoted regimes I and II, respectively). The enthalpy relaxation behavior in Regime I is similar to thermophysical aging, indicating that the glassy structure as a whole is initially shifted to one where molecular mobility is retarded by relatively low amplitude cyclic stress. A strong interaction is also seen between the enthalpy overshoot and previous physical aging. That is, the more the material is previously aged, and the shorter the incubation period, the longer the crazing region is. As a result, brittle failure occurs over a wider load range compared with less aged specimens.

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300301

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractPositron annihilation lifetime spectroscopy (PALS) is used to probe structural changes in glassy polycarbonate in terms of changes in the hole volume and the number density of holes during fatigue (cyclic stress) aging. Theortho‐positronium (o‐Ps) pickoff annihilation lifetime τ3, as well as the intensityI3, were measured as a function of cyclic stresses and various previous thermophysical aging histories. It is found that τ3, the longest of the three lifetime components resolved in the PALS of glassy polycarbonate, increases when a cyclic stress is applied. These results indicate that there is a structural change during fatigue aging. The “holes” whereo‐Ps can localize become larger upon fatigue aging. These results also suggest that a significant distinction exists between structural changes induced by thermophysical aging and fatigue aging. Theo‐Ps annihilation intensity, which is a relative measure of the hole density in a material, showed a continuous decrease upon fatigue aging, indicating the possibility of hole coalescence, which could be a precursor of crazing. The interaction between thermophysical aging and fatigue aging corresponds very well with the enthalpy relaxation behavior as reported previously,viz., a well‐aged sample is much more sensitive to cyclic stress. More importantly, it is hypothesized that fatigue failure initiation is probably closely related to hole size and dens

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300302

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe dielectric relaxation behavior of poly(phenylene sulfide), PPS, has been investigated from room temperature to 180°C. This study was undertaken to examine the mobility of the amorphous phase through the glass transition region, to determine the contribution that rigid amorphous phase material makes to the relaxation process. Semicrystalline samples contain a fraction of the rigid amorphous phase, which was determined from the heat capacity increment at the glass transition, using degree of crystallinity determined from x‐ray scattering. In the dielectric experiment, we measured the temperature and frequency dependence of the real and imaginary parts of the dielectric function. ε″ vs. ε′ was used to determine the dielectric relaxation intensity, δε = εs–ε∞, at temperatures above the glass transition. For amorphous PPS, δε decreases as temperature increases, while for all semicrystalline PPS, δε increases with temperature. The ratio of semicrystalline intensity to amorphous intensity determines the total fraction of dipoles which are already relaxed at a given temperature. Results indicate that more and more rigid amorphous phase material relaxes as the temperature is increased. This provides the first evidence that rigid amorphous phase material in PPS contains chains that possess different levels of molecular mobility. Finally, to the temperature of the loss peak maximum, at a given frequency, we assign the value of the dielectricTg. For both melt and cold crystallization, the dielectricTgsystematically decreases as the crystallization temperature increases, and as the fraction of rigid amorp

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300303

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe amount and structure of the crystals formed by the solvent‐induced crystallization (SIC) following a sorption‐desorption cycle of five fluids (benzene, toluene, chloroform, methylene chloride, and carbon disulfide) in amorphous PEEK was determined by wideangle x‐ray scattering (WAXS). The SIC crystal structure was compared with that produced by thermal methods, both those formed at low temperature by heating the amorphous material 10–20°C aboveTgor by cooling from the melt. Although smaller in size, the SIC crystals are tighter and more organized than those produced thermally. The WAXS data indicates that all five fluids produce approximately 35% crystallinity in PEEK. Gravimetric data suggest that a low‐density region, consisting of either microvoids or highly disordered amorphous region, surrounds th

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300304

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe molecular weight for a dilute solution ofcis‐polybenzoxazole (PBO) in polyphosphoric acid (PPA) was determined by fitting the rheological data with a semiempirical polydisperse hybrid theory. The hybrid theory models a semiflexible rigid rod as an elastic cylinder. The cylinder has both a rotational relaxation spectrum given by an ideal rigid rod and an internal bending relaxation spectrum spaced in accord with the relaxation time spectra of a flexible coil with fully developed hydrodynamic interactions. The model was fitted to rheological data collected for a 0.05 weight percent solution with intrinsic velocity (one‐point determination) of 320 ± 10 cc/g. The model predicts a number‐average molecular weight near 12800 ± 400 g/mol with a polydispersity index of 2.5 ± 0.1. By using the Yamakawa‐Yoshizaki equation the intrinsic viscosity is calculated for the model molecular weight distribution a

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300305

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe effect of heating on four chemically synthesized polyanilines, three of which were doped by HCI, H2SO4, HCIO4, and one sample which was dedoped, was investigated. The structural variations during thermal decomposition were studied by real‐time synchrotron radiation diffractometry and correlated with mass spectrometry (MS), thermal gravimetry (TG), and differential thermal analysis (DTA). These macroscopic and microscopic techniques were combined in order to shed more light on the role played by the water and the counteranion molecules on the structure of doped polyaniline. Evidence for two H2O fractions was found for HCI‐doped polyaniline. Chain degradation was observed for HCIO4and H2SO4‐doped polyaniline. The results indicate that intermediate sulphonation of polymer chains occurs in the latter

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300306

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractPoly(ether ketone) and poly(ether‐ether ketone) are characterized by valence band x‐ray photoelectron spectroscopy. The valence band region appears to be resolved into six regions. Peak assignments were made by comparing the observed spectra with simulations from extended Hückel type band calculations. Results of the band calculations agreed fairly well with the experimental

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300307

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe synthesis, morphology, and mechanical properties of sulfonated poly(butylene terephthalate) (PBT) and its unsulfonated analogs were studied. The morphology of these copolymers crystallized from the melt were examined by a combination of wide‐angle x‐ray scattering (WAXS), polarized light microscopy, and small‐angle light scattering (SALS). Stress‐strain measurements are correlated with the morphological results. Spherulitic morphology, with a maltese cross at 45°C with respect to the crossed polars, is formed at low sulfonate levels (≤ 5.0 mol %). At a higher ion content, the maltese cross rotates 45° to form a cross pattern. At still higher sulfonate contents, typically 13 mol %, the light scattering pattern disappears completely. Microscopic and WAXS examination of these functionalized PBT copolymers confirms that the crystallinity level decreases with increasing ion content and is eliminated completely at the higher sulfonation level. The spherulite radius, however, remains invariant until the highest functionalization level. On the contrary, the morphology and properties of the unsulfonated isophthalate copolymer analogs remain relatively constant over the entire composition range examined. In several compositions clearly inferior properties are noted compared with the ion‐containi

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300308

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe two‐dimensional crystal structure of poly(p‐phenylene) is investigated by linkedatom Rietveld analysis of the x‐ray diffraction powder profile. Two molecular chains are packed in a rectangular pgg unit cell (a= 0.779 nm;b= 0.551 nm) with a paracrystalline shift distortion along the chain axis. The molecular conformation is not rigidly planar; rotations between adjacent phenyl‐ring planes in a molecule alternate with an angle of about 20°. The setting angle between the mean molecular plane and theaaxis

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300309

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300310

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY