摘要:

AbstractThe dilution effect on the crystallization kinetics of PDMS/toluene solutions, ranging from a polymer volume fraction of ϕ = 1.00 (pure PDMS) to ϕ = 0.32, was studied using1H high‐power NMR. Spin‐spin magnetic response was analyzed into relaxation components, arising from the different phases of the semicrystalline sample, through a spin‐echo technique. The intensity and shape of the amorphous component provide relevant information concerning (1) the global crystallization process and (2) the state of hindrance of the amorphous chains induced by the growing crystalline domains. It was shown that, in solutions, the main effect on the crystallization kinetics of changing concentration is to depress the equilibrium melting temperature of the system. However, a radically distinct crystallization rate between the pure and the more concentrated system must be explained in terms of the activation energy for interphase chain transport. Thermodynamic parameters of PDMS crystallites were also deduced from a model. Comparison between the isothermal development of the overall crystallinity and the variation of a characteristic relaxation time of the amorphous PDMS proton response gives an insight into the relative predominance of nucleation or growth rates in the crystallization mec

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270101

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThree samples of semiflexible thermotropic liquid‐crystalline polyesters based on mesogenic aromatic triads and a decamethylene spacer have been investigated by IR spectroscopy at various temperatures between room temperature and 290°C. The crystal‐to‐liquid crystal (either nematic or smectic) phase transition was accompanied by fairly strong spectral variations, whereas slight, but significant, changes in the IR profiles were detected at the liquid crystal‐to‐isotropic transition. By comparing the results obtained with the spectral behavior recorded for corresponding structural analogs of low molar mass, it was possible to attribute the spectral variations observed in the polymer samples to a decrease in intermolecular interactions rather than to conformational changes. The thermal transitions indicated by IR spectra were in good agreement with the analogous data obtained by calorimetric or optical microscopy

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270102

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe frequency, temperature, and hydrostatic pressure dependences of the dielectric properties, molecular relaxations, and phase transitions in PVDF and a copolymer with a 30 mol% trifluoroethylene were investigated. The β‐relaxation peak temperatureTβand the melting temperatureTmof both polymers, and the ferroelectric transition temperatureTcof the copolymer, are strong increasing functions of pressure. The magnitudes of the pressure derivatives ofTβ,Tc, andTmincrease in the order of the “transition” temperatures, i.e.,Tβ(P)

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270103

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractWe have studied the growth kinetics of {110} twins and single crystals of polyethylene in dilute solution of tetrachlorethylene. In terms of {110} twins, we succeeded in obtaining twins without {100} sectors, using a relatively high molecular weight fractionMw>104. It is confirmed that the growth is enhanced at the reentrant corner of the twins, and the enhanced growth face inclines to the {110} face because of consecutive generation of steps at the corner. These facts are strong evidence for nucleation‐controlled growth of single crystals. The growth rates and obliquity are measured at various supercoolings and concentrations. From consideration of kinetics of steps on the growth face, the following rates and velocity are independently determined from the experimental data: nucleation rate on a flat face, velocity of step propagation, and generation rate of steps at the reentrant corner. The supercooling dependence strongly supports regime II growth. The results on concentration dependence show that the velocity of steps is proportional to concentration over the whole range examined, and the nucleation rate is independent of it in the usual range and becomes proportional to it in the lower range. This concentration dependence of nucleation rate is attributed to the density of adsorbed polymer on the growth face. From this evidence, it is suggested that the rate of travel of steps is limited by volume diffusion of solute polymer, whereas the growth face is saturated with adsorbed polymer at ordinary concentrations. This contradictory situation could be explained by the hypotheses that the saturation density is rather low and that surface diffusion of adsorbed polymer is much slower than volume diffusion of solute polymer. The lower limit of the rate of folding is also determined for the first time from the velocity of step propagation. As regards the single crystals, it is found that the habit maintains a lozenge shape with sharpened points, even at very high supercooling (δT<50°C) if the concentration is very dilute. Diffusion‐limited growth is verified for the first time at the higher supercoolings, where the growth rate is almost independent of supercooling. The growth rate becomes almost equivalent to the velocity of steps determined in the experiments with twins, and this fact will support the accuracy of the evaluation of the step velocity. The order of magnitude of the growth rate obtained agrees with the value which is calculated from the balance between the flux of solute polymer to the growth face and the rate of growth of single cry

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270104

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractThe transport of oxygen through oriented PET at 25 and 60°C has been studied using the dynamic diffusion method. It did not prove possible to determine the diffusion and solubility coefficients with reasonable accuracy, and only the permeability coefficients are discussed in terms of the structure of the samples. The results suggest that the lowering of the oxygen permeability on drawing is related to the production of additional material with thetransconformation of the glycol residue, either in crystallites or in amorphous regions, rather than solely to the overall development of orientation or crystallinity

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270105

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractIn order to investigate the viscoelastic‐plastic properties of polymer solids and construct a constitutive equation for them, torsional tests were performed on polypropylene (PP) hollow cylinders. Testing was conducted under constant strain rate, abrupt change of strain rate, stress relaxation, creep, and cyclic loading conditions. The experimental data were compared with the numerical results derived from an overstress model. It is shown that the overstress theory explains the viscoelastic‐plastic properties of PP well, provided that the current strain is not below the maximum previous str

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270106

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractTransport of CO2across polycarbonate films has been studied using a diffusion cell technique employing a radioactively labelled tracer (14CO2). Because the14CO2driving force could be established independently of the unlabelled CO2driving force, several classes of experiments not possible with conventional techniques were performed. These different classes of experiments showed measurably different time lags. Formally, these experiments all are limiting cases of the more general mixed‐gas permeation problem; however, simplifying assumptions in the dual sorption theory are possible because the tracer concentration approaches zero and because the two species in this special mixed‐gas problem exhibit the same dual sorption parameters. These simplifications allow derivation of analytical expressions for the time lag for both the unlabelled and labelled gas species. The experimental measurements are in good agreement with the dual sorption model formulated for the mixed‐gas tracer diffusion pr

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270107

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractA study of size effects in the ultimate mechanical properties of crystalline and semicrystalline polymeric materials (fibers, single crystals) is conducted. The concept of size effect and its importance are discussed. A statistical/stochastic approach is adopted and is shown to yield analytical predictions which are at least as accurate as other modeling schemes (mainly based on fracture mechanics theory) previously proposed in the literature. Within this framework, we propose a possible scheme for simultaneous interpretation of both longitudinal (gauge length) and transversal (diameter) size effects, which provides some interesting information regarding the type of flaw population present in a given polymeric material. The statistical scheme used is based on the Poisson/Weibull model, and variants of it, since this model is relatively well established from both physical and experimental viewpoints. Some unsolved problems inherent to the Weibull/weakest link model for failure are discussed, and a new distribution function for the strength of solids is derived. This is illustrated through an analysis of available experimental data for ultrahigh‐molecular‐weight polyethylene, poly‐p‐phenylene terephthalamide (Kevlar), polydiacetylene, and polyoxymethylene. A maximum likelihood approach is used for the first time for interpreting the effect of diameter variability on the mechanical strength of polymeric fibers. We propose a procedure by which the Weibull shape parameter is maximized over a continuous range of values of the size exponent. Finally, we conjecture on the relative roles of defect populations distributed over the boundary and in t

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270108

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractA model of polymer melt diffusion recently proposed by Skolnick, Yaris, and Kolinski and which does not invoke reptation as the dominant mechanism of polymer melt motion is used to fit polymer self‐diffusion constant data measured by Antonietti, Fölsch, and Sillescu for a homopolymeric melt as well as for a probe in a larger molecular weight matrix. The quality of fit of the data indicates that reptation theory cannot be verified by simple comparison to diffusion constant measuremen

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270109

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY

摘要:

AbstractCrystals of poly(ε‐caprolactone) thicken at a measurable rate at the crystallization temperature. Melting points have been converted into lamellar thicknesses for three molecular weights and the lamellar thickening behavior analyzed using the Sanchez‐Eby approach of a relaxation time for thickening. Isothermal thickening curves are S shaped and can be superposed through horizontal shifts for a given molecular weight. Data for different molecular weights can be superposed using a horizontal shift combined with a small vertical shift. Shift factors have been treated using an Arrhenius approach to give an activation energy for thickening of 100 kcal/mol. Analysis using the Sanchez‐Eby approach leads to the conclusion that the active unit in thickening is approximately one stem

ISSN:0887-6266    

DOI:10.1002/polb.1989.090270110

出版商:John Wiley&Sons, Inc.    

年代:1989

数据来源: WILEY