摘要:

The difference between the dynamic and equilibrium tension in a macromolecular chain is expressed as a ``functional'' of the variation of the nonlinear strain function &fgr;(r*) developed in the kinetic theory of elasticity, with respect to time through the interval (0,t)r*≡r/rmwhereris the vectorial end‐to‐end distance of the molecular chain andrmis the maximum separation). The ``functional'' is expanded in an integral series analogous to Taylor's series and higher terms are neglected to obtain a linear integral equation for the viscously retarded response of the network chain. The equation obtained is a generalized one‐dimensional Boltzmann's superposition equation. It is then shown that the time‐dependent response of the molecular chain is independent of the magnitude of the deformation and, consequently, is of the same analytical form whether the deformation is infinitesimal or finite. From this it necessarily follows that there cannot be an inconsistency at finite stress and strain which is not allowed at infinitesimal excitations. Thus the response at finite excitations can be treated generally by employing the ``generalized'' superposition equation and the same techniques which have been utilized in the linear theories. Employing the usual kinetic theory assumptions, equations are developed for the macroscopic response of a well‐vulcanized rubber. Experimental data obtained in creep, stress relaxation, and dynamic stress‐strain for three different elastomers are presented which support the approach outlined. Some consequences of the theory are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1702913

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The elastic and anelastic properties (in the megacycle frequency range), viscosity, and electrical resistivity of vitreous selenium near its glass transition temperature were measured as a function of the quench (or melt) temperature between 540° and 720°K. The measurements reveal a correlation between the various properties and the internal molecular structure of the material at the quench temperature. Ultrasonic attenuation below the glass transition temperature of vitreous selenium cannot originate from simple mechanical losses due to viscous flow. A phonon‐phonon scattering mechanism is suggested.

ISSN:0021-8979    

DOI:10.1063/1.1702914

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

In an attempt to grow single crystals of synthetic polypeptides, polyglycine, poly‐l‐alanine, and poly‐l‐tyrosine have been precipitated from dilute solution and also cast as films by the slow evaporation of solvent. Polyglycine precipitated from solutions containing equal parts of trifluoroacetic acid (TFA) and trifluoroethanol (TFE) by the addition of water, crystallizes with the polyglycine II structure in the form of lamellar platelets with folded molecular chains. In films of polyglycine cast from these solutions, however, sheaves of ribbonlike crystals are produced, also chain folded and having the same structure. Selected‐area electron diffraction from these various crystals yields corroborative evidence for some of the structural features of polyglycine II. Poly‐l‐alanine cast from solution in TFA and TFE, on the other hand, crystallizes to give platelets and sheaves side by side in the same preparations. Both have the crystal structure of the &agr; form of poly‐l‐alanine, and indirect evidence indicates that chain folding probably occurs in both instances. There is a suggestion that, as grown, the crystals contain solvent complexed chemically with the polymer. Poly‐ltyrosine can be precipitated from solution in mixtures ofN,N‐dimethylformamide (DMF) andn‐heptanol in the form of hexagonal platelets and these too probably consist of folded molecules complexed with solvent. A tentative correlation between conformation in solution and crystalline morphology is suggested in the case of poly‐l‐tyrosine, but no definitive conclusion can be reached without further study of the conformation of the polymer held in solution under the conditions prevailing during crystallization.

ISSN:0021-8979    

DOI:10.1063/1.1702915

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Uniaxial tensile data from tests at different rates of extension over a wide temperature range are considered for butyl, silicone, Viton B, SBR, and natural rubber vulcanizates (series A) and for six Viton A‐HV vulcanizates (series B) of differing crosslink densities. For series A and for A‐6 in series B, equilibrium stress‐strain data were obtained at large deformations by an indirect method. The ultimate tensile properties of all vulcanizates were previously characterized by a time‐ and temperature‐independent failure envelope. The failure envelope's maximum extension ratio, (&lgr;b)max, is shown to be equal to or less than (&lgr;∞)max, the maximum extension ratio (hypothetical) in the absence of rupture and also the maximum extension ratio of network models. Failure and equilibrium data for series A vulcanizates are represented by a specific function of the equilibrium modulus and the maximum extensibility; except for SBR and possibly Viton B, equilibrium and failure data are sensibly identical; thus,(&lgr;b)max≅(&lgr;∞)max. For series B vulcanizates, qualitative considerations indicate that (&lgr;∞)max/(&lgr;b)maxis greater than unity and possibly dependent on crosslink density. Consideration of network models suggests that (&lgr;∞)maxshould be directly proportional toMc½and inversely proportional to (⟨r2⟩0/M)½. For series A, no correlation between (&lgr;b)maxand (⟨r2⟩0/M)½was found. For series B, it is shown that(&lgr;b)max∝Mc&bgr;, where &bgr; is a constant in the neighborhood of 0.7. For all vulcanizates,(&sgr;b)max≅104(&lgr;b)max−1, where (&sgr;b)maxis the stress in psi (based on the undeformed cross section) at (&lgr;b)max.

ISSN:0021-8979    

DOI:10.1063/1.1702916

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The variation of the real parts of the dynamic modulusE′, the dynamic strain‐optical coefficientK′, the mechanical loss tangent tan &dgr;, and the optical loss tangent tan &agr; have been measured as a function of temperature between −40° and +120°C at constant frequency of 1 cps for low‐density polyethylene, high‐density polyethylene, polypropylene, and nylon‐6. The results are interpreted in terms of both phenomenological theory and molecular mechanisms. The results for these polymers may be understood in terms of differing contributions by the processes of (a) spherulite or superstructure deformation, (b) delayed crystal reorientation within the deformed superstructure, and (c) relaxation of crystal orientation. For low‐density polyethylene, processes (b) and (c) are the principal contributors while for high‐density polyethylene, (a) becomes important at the lower temperatures. For polypropylene, (a) dominates and (b) begins to contribute at the higher temperatures, while for nylon‐6, (a) gives way to (c) without any appreciable region of important contribution of (b).

ISSN:0021-8979    

DOI:10.1063/1.1702917

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Direct determination of the rate of homogeneous spherulite initiation requires the subdivision of the melt into a large enough number of droplets so that all undesired nucleation catalysts (heterogeneities, motes, etc.) will be confined to a relatively small number of them. If the undesired heterogeneities nucleate very rapidly, the droplets free of them will freeze at a rate proportional to their volumes and the inherent (homogeneous) nucleation frequency per unit volune. In many cases, however, the time‐dependent freezing process is pseudohomogeneous, i.e., the result of concurrent homogeneous and heterogeneous (but sporadic or nearly sporadic) nucleation. A method is presented for analyzing data corresponding to this case and for determining the limits within which the droplet nucleation experiment can be expected to yield fruitful results.

ISSN:0021-8979    

DOI:10.1063/1.1702918

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The ``Avrami'' equation applies rigorously only to a system which does not change volume on transformation and where the developing regions are spheres. The consequences of the impossibility of achieving these conditions in real systems is discussed. It is shown that excessive approximation combined with the use of weight rather than volume fractions can lead to errors in the exponentnof as much as 0.3. It is emphasized that only at very low conversions can meaningful interpretations be made of the exponent.

ISSN:0021-8979    

DOI:10.1063/1.1702919

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Polypropylene aggregates, consisting of interwoven arrays of single‐crystal lamellae, have been obtained by crystallization of samples of isotactic polypropylene from dilute solutions held at constant temperatures between 85° and 115°C. The solvent used was &agr;‐chloronaphthalene with polymer concentrations ranging from 0.4% to 0.01% by weight. The resulting crystalline aggregates were examined by means of phase contrast and electron microscopy. Similar crystal aggregates have been studied by Khoury and found to be incipient spherulites. These aggregates tend to occur in the form of open weave structures, with characteristic cusps. Lath‐like lamellar crystals, with well‐defined faces, are found at the edges of these structures. These edge lamellae produce a single‐crystal electron‐diffraction pattern corresponding to the monoclinic structural form of isotactic polypropylene. On the basis of the experimental findings, it is suggested that folding occurs in monoclinic polypropylene only along a given set of parallel fold planes, viz. {010}. Preliminary results of a study of the fracture characteristics of the crystal lamellae tend to support the suggested hypothesis.

ISSN:0021-8979    

DOI:10.1063/1.1702920

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Polyethylene films were studied by photographic light scattering and photomicroscopy at successive increments of strain, usually 5%. Photomicrographs were taken at times sufficiently long after sample deformation so that the light scattering patterns were constant with respect to intensity and shape. Analysis of such scattering patterns and photomicrographs indicated that upon spherulite deformation, the ring spacing (hence the twist‐distance) increased in the direction of tensile strain and decreased at right angles to the strain direction, as noted in earlier experiments on samples subjected to tensile stress. At about 30% strain the wide‐angle parallel polarizer scattering pattern (polarization and strain directions horizontal) split into a set of 4 arcs nearly parallel to the meridian. With increasing strain (analyzer and strain directions vertical) the arcs of the 4‐arc wide‐angle crossed polarizer scattering pattern, originally at 45° to the polarizer and analyzer axes, occurred nearer the meridian and became nearly parallel to the equator. Photomicrographs indicated that at 30% strain the ring spacing at right angles to the strain direction was extremely small or in some cases not resolvable and that discontinuities occurred in the rings. Immediately adjacent to the axis of sample deformation in many spherulites there was a discontinuity in the rings such that they appeared to cross the axis at an angle of 50° to 60°. The relationship between the changes observed by light scattering and the changes observed by microscopy is shown. Photomicrographs and light scattering patterns are presented and a brief analysis of the changes is given in terms of mechanisms of spherulite deformation.

ISSN:0021-8979    

DOI:10.1063/1.1702921

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Measurements of melting curves, density, and long period of undrawn and drawn PE as a function of the draw ratio and the annealing temperature between 118° and 129.8°C for different annealing times were performed. Melting experiments show a linear increase of melting point and a reduction of heat content of the noncrystalline component with increasing draw ratio. As a consequence of plastic deformation during drawing, the tied macromolecules partly incorporated in more than one crystal are under high strain. This reduces not only the entropy but also the intra‐ (transinstead ofgaucheconformation) and interchain (aligned extended chains are more closely packed) energy of the amorphous regions. Annealing nearly instantaneously removes the strain on tied chains, restores the enthalpy and entropy to the values in a nearly completely relaxed supercooled melt, and increases crystallinity and long period. Subsequent prolonged annealing produces only slight additional changes.The variation of enthalpy and entropy rather well explains the observed drastic changes in sorption and diffusion of solvent molecules as a consequence of drawing and annealing.

ISSN:0021-8979    

DOI:10.1063/1.1702922

出版商:AIP    

年代:1965

数据来源: AIP