摘要:

AbstractThe positron lifetime measurement technique was applied to analyze the structural changes occurring in solid‐state polymerization of acrylamide and trioxane. Positronium (Ps) parameters, τ2andI2, which show a marked change as a function of the γ‐ray irradiation dose, signaled the presence of three clearly defined stages in the polymerization process: initiation, propagation, and saturation. The initial increase in τ2, may very well be the result of efficient trapping of Ps in vacancies created around the polymer nuclei. The complicated behavior ofI2may be explained as a chemical reaction between Ps precursors and the radiolysis products of this polymerization p

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300101

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractSolid‐state2H quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of2H spin lattice relaxation times have been obtained for films of poly(p‐phenylene vinylene) deuterated in phenylene ring positions (PPV‐d4). NMR line shapes show that all the phenylene rings of PPV undergo 180° rotational jumps about the 1,4 ring axis (“ring flips”) at 225°C. The temperature dependence of the2H line shapes show that the jump motion is thermally activated, with a median activation energy, Ea= 15 kcal/mol, and a distribution of activation energies of less than ±2 kcal/mol. The jump rate was also determined from the magnitude of the anisotropic T2relaxation associated with2H line shapes and from the curvature of inversion recovery intensity data. The experimental activation energy for jumps is comparable to the intramolecular potential barrier for rotation about phenylene vinylene bonds.2H NMR provides a method for determining the phenylene‐vinylene rotational barrier in pristine PPV, and may potentially be used to study conjugation in co

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300102

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe electrochemical polymerization of thiophene was carried out both in the presence and absence of ultrasonic waves. Without irradiation, the anode potential increased with increasing current density. Polymerization at high potential gave a low polymer yield and produced macroscopically heterogeneous films. Ultrasonic irradiation resulted in an improvement of polymer yield and in a lowering of the anode potential during polymerization. The effects were especially marked when the polymerizations were carried out at low temperature, low monomer concentration, and high current density. These results are explained by the formation of diffusion layer in the vicinity of the electrode during electrochemical reactions.

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300103

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractIn this second paper in a series on cold drawing of polymeric fibers and films, asymptotic formulae are derived for the tension in thin bodies of nonlinearly viscoelastic materials subject to inhomogeneous stretching. The formulae, which are valid to within an error of fourth order in sample thickness, give the tension in fibers and films as functionals of the histories of the local stretch and its first two spatial gradients. It is shown that the theory obtained by taking the formulae to be exact is consistent with thermodynamical principles. Material response for various types of histories is discussed, and the theory is employed to calculate the creep response of fibers under static loads. Numerical solutions of the evolution equation for the stretch field in creep show that for an appropriate class of materials with slowly fading memory there is a range of applied loads for which an initially homogeneous deformation evolves into a well defined neck whose edges advance at high speed along the fiber and in so doing transform moderately stretched material into highly stretched (i.e., drawn) material. The calculated fiber profiles and the predicted dynamics of neck formation and growth are in good accord with familiar observations. A description is given of the circumstances under which one can use instantaneous (i.e., high speed) response functions to calculate fiber profiles in rapidly growing necks.

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300104

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractMiscibility in blends of random copolymers ofo‐chlorostyrene andp‐chlorostyrene [P(oClSy‐co‐pClS1‐y)] with 8 atactic polystyrene (aPS) fractions has been studied at temperatures ranging from 150°C to 300°C. Miscibility windows whose size depends on the molecular weight of the PS and on the copolymer composition,y, were observed for each blend. From these data, the temperature dependence of the three segmental interaction parameters required to describe this system we

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300105

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe morphology of nonperiodic layer (NPL) crystallites appearing in three thermotropicrandomcopolyesters composed of 30/70, 58/42, and 75/25 monomer ratios ofp‐oxybenzoate (B) and 2,6‐oxynaphthoate (N) is investigated with transmission electron microscopy. Copolymer films, produced by shearing the materialsabovetheir melting points and subsequently annealing them above their glass transition temperature, exhibit ordered entities averaging no more than 20 nm thick and 40–60 nm long, as observed with dark‐field imaging. Selected‐area electron diffractograms exhibit sharp equatorial reflections, indicating ordered packing of random intramolecular sequences of the molecules. Microdensitometry of these diffraction patterns reveal that both the equatorial and first meridional reflections are radiation sensitive, decaying exponentially with dose. Saturation doses (v) for the crystallites depend on the copolymer composition and degree of polymerization (DP) and are estimated to be approximately 130–150C/m2for the high‐molecular‐weight copolymers (DP ≈ 150), with the N‐rich copolymer possessing greater radiation resistance, and about 430 C/m2for a low‐molecular‐wei

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300106

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractA new polyimide derived from 4,4′‐isophthaloyldiphthalic anhydride (IDPA) and 1,3‐bis(4‐aminophenoxy‐4′‐benzoyl)benzene (1,3‐BABB) having semicrystalline behavior was prepared at NASA Langley Research Center in 1987. The crystalline transitions of this thermoplastic polyimide have been studied. The differential scanning calorimetry (DSC) pattern of partially imidized film exhibited two distinct crystalline melt endotherms. For this study each crystalline phase was isolated and enhanced by controlled thermal treatment. A film containing approximately 50% of both phases and an amorphous film were also prepared. Evaluations of these films were performed by DSC, wide‐angle x‐ray scattering (WAXS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Three distinct crystalline morphologies; ellipsoid, cubic, and needlelike embedded in an amorphous matrix were observed as a function of various cur

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300107

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractA new technique for the determination of spatial density distributions is described. The density is derived from the refractive index gradient as determined with a quantitative Schlieren optical technique. The advantages of this technique are a high spatial resolution and sensitivity. Gapwise profiles can be measured without slicing the sample so that more reliable data are obtained quickly. This is demonstrated for the gapwise density distributions in quenched polycarbonate specimens. It is shown that the density distribution is determined by a competition between the local cooling rate variation and the cooling stress distribution. The residual density is predominantly determined by the coolant temperature. The density in the core is higher than at the surface for high coolant temperatures, whereas for low coolant temperatures the density in the core is lower than at the surface. The observed effects are explained semiquantitatively by a simple model using calculated cooling stresses and cooling rates.

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300108

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractA comparison of solid‐state13C nuclear magnetic resonance (NMR) spectra of virgin and vacuum γ‐irradiated poly (ethylene oxide) (PEO) evidences marked differences. The unirradiated PEO shows a well‐resolved amorphous resonance and a weak, broad envelope of crystalline resonances, while the irradiated PEO presents well‐resolved resonances for both the crystalline and amorphous carbons. Upon recrystallization from the melt both PEO samples yield solid‐state13C NMR spectra that are closely similar to that of the virgin, unheated sample. Observation of both melt‐recrystallized samples at −60°C yields similar spectra with well‐resolved crystalline resonances. Crosslinking is the predominant chemical change occurring during the γ‐irradiation of PEO under vacuum and produces a change in the motional character of the crystalline phase. This change is not the result of a reduction in crystallinity as evidenced by differential scanning calorimetry (DSC) observations. The most probable explanation is that the crosslinks are concentrated at the surface of the crystalline lamellae with a resultant change in the low frequency molecular motions of the crystalline chains. This motional change shifts theT1pHsuch that the crystalline carbon nuclei can now be cross‐polarized at room temperature and the resonance linewidth is reduced. Following melting and recrystallization the motional characteristics of the irradiated PEO are nearly identical to those of the unirradiated sample, probably as a result of a redistribution of the crosslinks throughout the amorphous phase du

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300109

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY

摘要:

AbstractThe formation of mechanoradicals under anaerobic conditions and their reactivity with oxygen at room temperature is described for several methacrylic vinyl polymers. Observed electron spin resonance (ESR) spectra of the mechanoradicals formed were all essentially identical and are clearly assigned to a respective endchain radical. The ESR kinetics of the mechanoradical formation of polymethylmethacrylate (PMMA) and polymethacrylamide (PMAAm) exhibit an interesting contrast; the progressive changes in the radical concentration in PMMA as a function of duration of milling gradually decrease after reaching a maximum value, while those of PMAAm show a parabolic increase. This discrepancy has been ascribed to mechanoradicals of PMAAm that are strongly stabilized by intermolecular and intramolecular doubly hydrogen‐bonded networks among the amide groups. Such interactions also are to lower the reactivity of the mechanoradicals with oxygen. Thus, the mechanoradicals of both PMAAm and PMAA do not give a single peroxy radical, but rather a mixture of the mechanoradical and peroxy radical even after exposure to air, while the mechanoradicals of other polymers are rapidly converted to the corresponding peroxy radicals. Such a difference was observed in experiments on the mechanical fracture of such polymers under aerobic condition

ISSN:0887-6266    

DOI:10.1002/polb.1992.090300110

出版商:John Wiley&Sons, Inc.    

年代:1992

数据来源: WILEY