摘要:

Two-dimensional small-angle x-ray scattering (SAXS) from a highly drawn and annealed (120° 1044min) polyethylene sample (Lupolen 6001 H) by using a pinhole camera was fitted theoretically by a new adaptation of Hosemann's general paracrystalline model and a computer program developed for this purpose. The structure model consists of microparacrystallites (mPC's) whose centers build up bundles of mPC, which are called macroparacrystallites (MPC's). They have a lamellarlike structure in drawn and annealed samples with specific lateral grain boundaries, which are overlooked in the two-phase model. Results obtained were compared with those obtained by ČaČkoviČ et al. from the same specimen using the so-called restricted paracrystalline model. The general model eliminates the possibility of physical overlapping in the drawing direction of amorphous and crystalline regions. This removes constraints encountered in the restricted model on the mPC-thickness dispersion parameter gH, and inter-mPC displacement dispersion parameter g33, generated by overlapping effects. Nevertheless the two models give nearly the same results. Fit accuracy parameters have been introduced in a twelve-dimensional parameter space to give an indication of the accuracy of a given fitting and of the upper and lower limits or approximate error brackets of the physical parameters used in the fitting procedure. Because of the diffuse character of the SAXS, only half of the twelve parameters of the paracrystalline model could be obtained with a relatively high accuracy of 1-3%. The density of the “amorphous” phase is 0.89 g/cm3instead of the 0.85 g/cm3of a supercooled melt.

ISSN:0022-2348    

DOI:10.1080/00222347808212242

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Combined wide-and small-angle x-ray scattering studies (WAXS and SAXS) of line profiles of polymers prove the existence of small mosaic blocks (50-1500 Å in size). They all have liquid like distortions quantitatively defined by a g value and are identified therefore as microparacrystallites (mPC's). Unlike crystals, they can crystallize only in coherent units of not more than N net planes with N = (α*/g)2. Electron micrographs of strain-crystallized transpolyisoprene show directly the grain boundaries between adjacent mPC's within one lamella which appear in temperature range III. The free lateral surfaces of mPC's at the lamella ends are mostly responsible for the long-period growth during annealing in temperature range II. Annealing experiments with poly-1-butene prove that no premelting occurs but solely a solid-state diffusion. This apparently occurs mostly starting from the lateral grain boundaries on account of their larger excess free energy. In stretched isotactic polypropylene films, these lateral boundaries are so pronounced that equatorial interference maxima occur in SAXS. The lateral grain boundaries in mats of polyethylene single crystals are responsible for many phenomena, for instance degradation experiments with HNO3and gel-permeation chromatographs after Co60irradiation and further degradation with O3.

ISSN:0022-2348    

DOI:10.1080/00222347808212243

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The influence of intrachain double bonds on the melt crystallization kinetics of some linear polymers such as polyethylene, aliphatic polyesters, and polyamides is discussed. The analysis of the experimental results carried out in terms of the kinetic crystallization theory agrees, for all the polymers investigated, with a growth process of spherulites controlled by a surface two-dimensional coherent secondary nucleation. The presence oftransdouble bonds along the chain of the polymer affects the values of some kinetic and thermodynamic parameters related to the process of melting and crystallization. The dependence of Δ Φ* and σeon the percentage of unsaturation shows different trends for the condensation copolymers and for ethylene-butadiene copolymers. This behavior is interpreted in term of both different molecular and crystallographic structures of the copolymers.

ISSN:0022-2348    

DOI:10.1080/00222347808212244

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

We have carried out a set of experiments to test the accuracy of two procedures, one suggested by Curro and Salazar and the other by Moacanin, Aklonis, and Landel, in predicting the behavior of polymer networks which are simultaneously chemically reacting and physically relaxing. The chemical reaction utilized was a photoinduced scission of disulfide linkages which had been incorporated into the network; thus the reaction could be turned on and off at will. The necessary input data to support the calculations were measured directly on the experimental samples under conditions where no chemical reactions occurred. Our results show that the behavior predicted by the technique of Moacanin et al. is in good agreement with that observed experimentally.

ISSN:0022-2348    

DOI:10.1080/00222347808212245

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

A study of the morphology of uniaxially drawn spherulites of TMPS-DMS block copolymers with TMPS contents ranging from 90 to 30 wt % was made especially by electron and optical microscopy. For the entire range of polymer compositions, the copolymers, like the TMPS homopolymer, form negatively birefringent spherulites when crystallized from solution, or from the supercooled melt. A banded spherulitic morphology is observed. The deformability of copolymer spherulites increases as the DMS amorphous component increases. The DMS component coexists with, but is physically excluded from the ordered TMPS “crystalline” phase. In samples having the higher TMPS compositions, deformation occurs through crack formation, mechanical slip, tilting and/or twisting of the lamellae that help comprise the spherulites. At high DMS contents this “soft” component is undoubtedly responsible for the flexible character of these copolymers. Sample clarity also increases with DMS content as the crystallinity decreases.

ISSN:0022-2348    

DOI:10.1080/00222347808212246

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Single crystals of poly (γ-methyl D-glutamate) (PMDG) were obtained in a form of hexagonal platelet by isothermal crystallization from a mixed solution of N, N-dimethylformamide (DMF) and propionic acid (1:1 in volume). The electron diffraction pattern revealed that the single crystal is composed of α-helical molecular chains oriented perpendicularly to the plane surface. Comparison of the single crystal thickness with the length of the α-helical molecular chain calculated from the viscosity-average molecular weight showed that the molecular chains might fold at the lamellar surface. The fold formation was supported by the observation that fibrils were pulled out from the fracture surface when the single crystal was drawn on a substrate. Since the α-helical molecular chain cannot fold itself due to the helical conformation with intramolecular hydrogen bonds, it seems that the fold region is composed of non-α-helical segments. The surface of PMDG single crystals was considerably rougher than that of polyethylene single crystals. The surface state of PMDG single crystals was investigated on the basis of annealing and gold decoration. Thickening of single crystals was not observed when they were annealed up to 513°K but the lamellar thickness decreased with increasing annealing temperature upon annealing above 453°K. This annealing effect on the lamellar thickness was explained by the rearrangement of molecular chains caused by the transition from the α-helix to the non-α-helix on the surface and edge region of the single crystal.

ISSN:0022-2348    

DOI:10.1080/00222347808212247

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

A differential radial distribution function (DRDF) of molten natural rubber (NR) was derived from its wide-angle x-ray scattering (WAXS) data. Three peaks with Bragg spacings 4.85, 2.31, and 1.23 Å corresponding, respectively, to interchain, C1-C3and C1-C2distances are present in the corrected WAXS curve. The derived DRDF, which is shown to be greatly improved than the ones published so far, contains six peaks located at 1.51, 2.48, 3.98, 5.68, 10.65, and 15.25 Å. Maximum contributions of intramolecular atomic distances to the radial distribution peaks were estimated and compared with the experimental results. The comparison shows that the first three peaks are intramolecular in origin and the remaining peaks are predominantly due to intermolecular regularities. A periodicity of an approximately constant interval of 5.26 Å is found between the intermolecular peaks and is attributable to the organizations of more or less parallel chain segments in the material. This finding provided additional evidence for the presence of local lateral ordering to the extent of about 30 Å in molten-state natural rubber.

ISSN:0022-2348    

DOI:10.1080/00222347808212248

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

Wide-angle x-ray scattering (WAXS) of glassy PET and two samples annealed at 190°C for 15 and 30 min were analyzed by differential radial distribution function (DRDF) methods. The corrected intensity profile of the glassy sample show three peaks at 4.19, 2.13, and 1.19 Å. The sample annealed for 15 min shows splitting of the 4.19 Å peak into three peaks at 3.50 (d100), 3.96 (d110), and 5.21 (mainly d010and unresolved d011-and d-111) Å. Longer annealing for 30 min produces two more peaks at 4.11 (d-111) and 5.40 (d011) Å. Annealing does not affect the peaks at 1.19 and 2.13 Å (which are caused by first and second nearest atoms in the chain). The DRDFs of all three samples are similar in some respects as all show intrachain peaks at ∼ 1.45, 2.45, and 4.05 Å. From theoretical calculations of the intrachain distances and their contributions to the DRDF, the peak at the 4.90 Å in the glassy sample was found to be mainly due to intermolecular distances. Since the peaks above 4.9 Å are relatively weak, only nematic lateral ordering (intermolecular correlations) is present and is limited to 20-40 Å. It is suggested that very distorted paracrystalline lattice is present in the glassy state of PET, which is consistent with the previous conclusions based on dark-field electron microscopy studies. Annealed samples showed that all peaks originally weak in the glassy sample become stronger and slightly shifted to lower r values. In addition new peaks are observed with an intermolecular periodicity of ∼ 5 Å for distances up to 40-50 Å for a total chain to chain correlated distance of 80-100 Å, indicating the presence of a markedly improved ordering of laterally aligned chain segments as a result of improved packing of atoms within the lattice.

ISSN:0022-2348    

DOI:10.1080/00222347808212249

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The room-temperature, tensile mechanical properties and fracture topographies of a range of polyamide-cured bisphenol-A-diglycidyl ether epoxies with differing physical characteristics are reported. Epoxies of different physical structure were prepared by varying the initial physical state of the epoxy monomer (crystalline versus noncrystalline), the cure conditions, and the initial epoxy:polyamide curing agent ratio. Epoxy monomer crystals (mp 41.5°C) present in the unreacted epoxy remain embedded in the room-temperature partially cured glass. Exposure of this epoxy to 48°C produces liquidus islands of unreacted epoxy monomer which locally enhance crazing. At higher temperatures, the unreacted epoxy monomer escapes, leaving microvoids which collapse as the glass softens. Unreacted epoxy monomer also recrystallizes between microgel regions of high crosslink density at room temperature. The resultant liquid produced by melting these crystals causes homogeneous plasticization. Volatilization of the unreacted monomer produces a network structure of poor integrity between the original microgel regions which causes a brittle mechanical response. Fracture topography studies indicate that epoxies generally fail by void growth and coalescence through a simultaneously growing craze. It is suggested that flow occurs during failure by islands of high cross link density moving past one another.

ISSN:0022-2348    

DOI:10.1080/00222347808212250

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor

摘要:

The modification of a rate process by externally introduced conditions is well known. An equation of the same general form has been found applicable to such diverse rate phenomena as wear, friction, voltage breakdown, mechanical degradation and fracture. As applied to fracture rates, the equation takes the form, τ = τ0exp {(E−γσ)/RT}, where T is the time to fracture, τ0is a constant, E is an energy, γ is a stress concentration factor, σ is the applied stress, R is the gas constant, and T is the absolute temperature. This equation can only be an approximation, since it predicts a finite time to break when σ is zero. Further, although E is taken to be the energy of the breaking bond, it varies over a large range for many polymers having the same carbon backbone. In this paper, a two-position barrier model is developed, in which the barrier is that which arises from the energy for polymer chain motion. Assuming bond breaking subsequent to the rate-determining step, the stress term in the equation is more correctly represented by a hyperbolic cosecant term. Since the hyperbolic cosecant approaches infinity for small values and an exponential for moderate values, the recast equation indicates an infinite time to break at zero σ and reduces to the original equation at moderate values of σ. The meanings of E and γ are clarified, and implications of the equation are discussed. In addition, correlations with T values obtained at constant strain rate and under cyclic loading are indicated.

ISSN:0022-2348    

DOI:10.1080/00222347808212251

出版商:Taylor & Francis Group    

年代:1978

数据来源: Taylor