摘要:

When oriented polystyrene monofilaments are heated to temperatures in the neighborhood of the transition temperature they retract to an unoriented state at conveniently measurable rates. Curves of length as a function of time have been measured at five different temperatures (75°–95°C, at 5° intervals) for two filaments with different initial amounts of orientation.When the retraction curves are plotted as lengthvslog time, it is found that the curves at different temperatures are not superposable by lateral shift along the logarithmic time scale, as would be true for simple viscoelastic behavior. Activation energies calculated from these curves are a function of time as well as of temperature; the activation energy at any given temperature tends to level out to a final constant value with increasing time, however. The rate of change of activation energy with time increases with increasing temperature, so that the final value is attained more quickly; the final value also decreases systematically with increasing temperature.Final activation energy values increase rapidly with decreasing temperature in this region and are very high (300 kcal at 75°). It is postulated that specific volume changes are responsible for the time‐dependent activation energies observed in these experiments. It is also of interest that these experiments appear to isolate the viscoelastic mechanism involving molecular configuration change from that involving local molecular distortions.

ISSN:0021-8979    

DOI:10.1063/1.1722151

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

In general, extension of an elastomer results in a degree of preferred orientation of the molecular chains composing the amorphous phase. Therefore the amorphous fraction of a partially crystalline elastomer must be related to the integrated intensity of the amorphous diffraction halo rather than to the intensity at any one azimuth. A noteworthy exception is natural rubber, for which simple meridional measurements suffice.A Geiger‐counter apparatus with beam monitor and temperature‐controlling accessories is described for making accurate measurements of the x‐ray intensities scattered at any azimuth and at small or moderate Bragg angles. Measurements of crystallinity in natural rubber are in essential agreement with the findings of previous workers. When polybutadiene is extended at room temperature, molecular orientation occurs, but little if any crystallization. Measurements at lowered temperatures show that the crystalline fraction becomes appreciable at about 0°C and that it increases with further reduction in temperature and with increasing extension ratio. Preferred orientation of the crystalline regions in extended polybutadiene has been measured quantitatively with the object of providing jointly with birefringence measurements a value of the birefringence of a single crystal of polybutadiene.

ISSN:0021-8979    

DOI:10.1063/1.1722152

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The semitheoretical expression,fc=&Dgr;n0−csF&Dgr;nc0−cs,wherefc=volume fraction crystalline, &Dgr;n0=observed birefringence,c=the stress‐optical coefficient,s=the stress on actual cross section,F=a known function of crystallite orientation, and &Dgr;nc0=the birefringence of a single polymer crystal, is proposed for relating birefringence, stress, and degree of crystallinity in polycrystalline elastomers under tensile stress. This expression is shown to yield results in substantial agreement with x‐ray results for the degree of crystallinity of 5°C polybutadiene when &Dgr;nc0=0.157. Birefringencedensity data on natural rubber (due to L. R. G. Treloar) are shown to be a special case of the foregoing expression, yielding a value &Dgr;nc0=0.218 for natural rubber which is in good agreement with the theoretical value for this quantity.

ISSN:0021-8979    

DOI:10.1063/1.1722153

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

Evidence is given showing that the equation, log&eegr;=3.4 logZ+K, represents an empirical law of flow which holds generally for sufficiently long flexible chain molecules in bulk or in solution. Here &eegr; is the viscosity,Zis the number of atoms in the chain, andKis a constant which is dependent on the polymer type and on the temperature. All of the available data which cover wide molecular weight ranges for linear (and branched) molecules in bulk or in solution, including results on polyesters, polyamides, polystyrene, polyisobutylene, polydimethyl siloxane, and polymethyl methacrylate, support this conclusion. For chains having fewer than a critical number,Zc, of chain atoms (Zcbeing characteristic of the polymer species) the dependence of &eegr; onZis less severe but more complex. These results are in semiquantitative agreement with the recent theory of F. Bueche. On the basis of this theory, values for the concentration of chain entanglements in various polymers are obtained from the observed values ofZc.

ISSN:0021-8979    

DOI:10.1063/1.1722154

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The problem considered is that of two bodies coming into normal contact over smooth curved surfaces. The initial relative velocity and the total kinetic energy involved is low. Contact is, however, confined to such small volumes of the objects involved that very high concentrations of energies are obtained at those places. The rates of application of stress are correspondingly high. The Hertz solution to this type of problem provides a useful approximation in the case of elastic objects.In the present treatment one of the impinging bodies is of viscoelastic material. Two viscoelastic bodies may also be treated if they are of the same material. The Laplace transform method is used to obtain the viscoelastic expression for the force developed between the two surfaces. This expression is then applied to the impact case. The expression can also be applied to other truly static cases; e.g., contact between gear tooth surfaces.The results are of technological interest, since it is not possible to say if a plastic is suitable for a certain category of impact applications, unless the rates of straining or stressing obtained in those applications can be estimated.

ISSN:0021-8979    

DOI:10.1063/1.1722155

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The low coefficient of friction for Teflon sliding on Teflon, as observed previously by others, has been found in this study to hold only if low sliding speeds and newly prepared surfaces are used. At high sliding speed, the nature of the surface is irreversibly changed such that subsequent sliding at low speed reveals a two‐ to threefold increase in the friction coefficient.In addition to the effect of high speed sliding, there are reversible changes in the friction of Teflon on Teflon dependent in part on temperature. As the temperature of Teflon is increased from below room temperature, a sharp and pronounced increase in the coefficient of friction is observed in the vicinity of 20°C. The existence of a phase transition in Teflon at this temperature indicates a close correlation between the frictional and the structural properties of this material.

ISSN:0021-8979    

DOI:10.1063/1.1722156

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

Glass temperatures (Tg) of pure and plasticized elastomers have been obtained from dilatometric measurements and from the temperature dependence of angular response to torsional stress. The depression ofTgby a diluent, determined by either method, is a convenient measure of its plasticizing ability. Some compounds which are normally crystalline aboveTgof the pure polymer, but nevertheless compatible with the polymer, depressed the glass temperature. The polymer‐diluent compatibility at low temperatures results either from supercooling, the depression of the melting point of diluent by polymer, or possibly a combination of both phenomena.Tgdecreased as the diluent loading increased to a more or less well defined limit, beyond which further dilution had negligible effect. For diester type plasticizers both &Dgr;Tgmaxand the limiting diluent concentration were linear functions of compatibility, as defined by the extent to which standard polymer vulcanizates were swollen by plasticizer. The viscosities and apparent activation energies for flow (Ev) of hydrocarbon oils correlated with their low temperature plasticizing efficiencies. Pour points and glass temperatures may also be significant properties of the diluents. It is concluded that lowEvand high solvency for polymer are desirable properties for low temperature plasticizers.

ISSN:0021-8979    

DOI:10.1063/1.1722157

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

The proton magnetic resonance absorption has been observed in several linear polyamides from 77°K to 400°K. The absorption line narrows gradually with increasing temperature up to about 280°K. A marked decrease in line width with increasing temperature occurs over a span of 20 or 30 degrees within the range of 280 to 400°K, and is presumed to result from the onset of rotation of chain segments. In a series of polyamides possessing an even number of carbon atoms between polar groups, this transition occurs at a lower temperature as the hydrocarbon portions are made longer (polydecamethylene octadecanediamide compared to polyhexamethylene adipamide). In the copolymer polyhexamethylene adipamide‐sebacamide, the transition region is at a lower temperature than in the pure adipamide or pure sebacamide. The x‐ray diffraction studies on polyamide fibers show that the chain packing becomes more symmetrical over these same temperatures. The resonance results are interpreted in terms of simple models.

ISSN:0021-8979    

DOI:10.1063/1.1722158

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

One‐dimensional compression measurements have been made on samples of natural rubber gum stock. The experimental stress‐strain curve and the theoretical relationf=G(L−1/L2) agreed quite well over the entire range (0%–32%) studied. No hysteresis loop was evident.The friction at the faces of the sample was eliminated by a new method of lubrication. The sample essentially floated between two thin films of a liquid which flowed under pressure between the sample and compressing plates allowing uniform lateral expansion of the sample.

ISSN:0021-8979    

DOI:10.1063/1.1722160

出版商:AIP    

年代:1955

数据来源: AIP

摘要:

An instrument has been constructed to allow simultaneous measurements of stress and birefringence in polymer samples to be made as a function of time and temperature. The stress is measured by attaching one end of the sample to a cantilever beam near the fulcrum; the small deflection of the beam is indicated by means of a differential transformer, which in turn is attached to an automatic recorder.The birefringence is measured with a Senarmont compensator (quarter‐wave plate method). Measurements can be made either manually or with a continuously rotating analyzer. This latter method permits the automatic recording of the birefringence and is also adaptable to the study of rapid birefringence changes. The sample is housed in a copper pipe surrounded by a thermostatically controlled liquid bath.Some data are presented showing the use of this apparatus in measuring birefringence and stress changes during stress relaxation experiments (sample held at constant elongation), birefringence changes during creep under constant load, and stress‐optical coefficients.

ISSN:0021-8979    

DOI:10.1063/1.1722161

出版商:AIP    

年代:1955

数据来源: AIP