摘要:

In the general theory of the behavior of simple fluids with fading memory in slow deformations, the constitutive equation of those fluids calledsecond‐order fluidsgives to the equation of Newtonian fluids a correction for viscoelastic effects that is complete to within terms of order greater than two in the time scale. The hydrodynamical behavior of incompressible second‐order fluids is determined by three material constants: &eegr;0, &bgr;, &ggr;, besides the density. We show here that not only the viscosity &eegr;0, but also the constant &ggr; which governs certain normal stress effects, is determined by the shear‐relaxation modulus of classical infinitesimal viscoelasticity. This result enables us to show that in a slow Couette flow the viscoelastic contribution to the difference in normal thrusts on the inner and outer cylindrical walls has a sign opposite to that of the inertial contribution. We also suggest several practicable methods for measuring &bgr; and &ggr;.

ISSN:0021-8979    

DOI:10.1063/1.1713068

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The spin‐lattice nuclear magnetic resonance relaxation has been measured in a series of poly(normal &agr;‐olefins) in which the side chains ranged in length from methyl to hexadecyl. The studies were carried out as a function of temperature, from −160° to 150°C, and as a function of the radiofrequency, specifically at 10, 20, 30, and 50 Mc/sec. The dependence of the NMR relaxation upon temperature and frequency shows that increase in the length of branches contributes to increased segmental mobility. Effects of thermal history are examined. The rate of side‐chain crystallization of polydodecene can be measured from the time dependence of the spin‐lattice relaxation.

ISSN:0021-8979    

DOI:10.1063/1.1713014

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

Narrow, well‐defined molecular weight fractions of polyisobutylene were irradiated inn‐hexadecane as 3% concentrations by a 10‐kc/sec sonic oscillator. The irradiated polymers were recovered by precipitation and evaporation; the molecular weight distributions were determined by column chromatographic fractionation. Narrow fractions for study had viscosity molecular weights of 15 100, 40 000, 74 400, and 137 000. The changes in distribution are compared with those from a broad distribution polymer with a molecular weight of 40 000. The limiting degree of polymerization below which degradation will not take place is at least as low as 200, and probably 100, for polyisobutylene in solution, considerably lower than the 1000–2000 reported for several polymers.

ISSN:0021-8979    

DOI:10.1063/1.1713059

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

In the development of molecular theories of liquid and polymer structure, great importance is attached to stress and time dependences of the apparent viscosity of materials and their solutions. Some of these dependences can be accounted for by the heating of the fluid which is a consequence of the flow. In this report, solutions for the time and space distributions of the temperature in an incompressible fluid flowing steadily in a capillary with isothermal walls are presented. A comparison of the computed temperature changes with observed changes in apparent viscosity of some lubricating oils suggests that the temperature effect is significant. The possible usefulness of the solutions for understanding the origin of turbulence in flow and the heating of a cylindrical rod loaded in torsion is also discussed.

ISSN:0021-8979    

DOI:10.1063/1.1713066

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

It has been stated in a brief report that a gum rubber Hevea stock exhibited a linear stress—strain relationship in two‐dimensional shear up to 300% shear. The experimental procedures and results are reported here in detail. Detailed results are reported also of a series of dynamic stress—strain measurements with the St. Joe Flexometer, briefly referred to in 1940. These dynamic shear—shear stress curves are linear within a given stress range. Whenever the greatest previous stress is exceeded, a new linear curve is obtained, of lower modulus.Nonreversible crystallinity has been generally neglected in theories of rubber hysteresis and in experimental efforts to obtain truly reversible stress—strain curves.

ISSN:0021-8979    

DOI:10.1063/1.1713076

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The tensile stress‐at‐break &sgr;b(based on the initial cross‐sectional area) and the corresponding ultimate extension ratio &lgr;bof unfilled vulcanizates of silicone, hydrofluorocarbon (Viton B), butyl (both sulfur‐cured and resin‐cured), and natural rubber were determined at many strain rates and temperatures; the latter ranged from slightly above the glass transition temperatureTg, up to a temperature somewhat below that at which chemical degradation affected the results. For each vulcanizate except natural rubber, data obtained over an extended temperature range superposed to give a time‐ and temperature‐independent failure envelope on a plot of log(&sgr;b273/T) vs log(&lgr;b−1), whereTis the test temperature in °K; for natural rubber, data obtained between 90° and 120°C superposed, but those at lower temperatures did not because of strain‐induced crystallization. For each vulcanizate, data at elevated temperatures gave, or tended toward, a line of unit slope on a plot of log (&lgr;b&sgr;b273/T) vs log(&lgr;b−1), where &lgr;b&sgr;bis the breaking stress based on the cross‐sectional area at the moment of rupture. The position of each line corresponded to the equilibrium modulusEederived from stress—strain curves. Failure envelopes previously obtained for two styrene—butadiene vulcanizates, which had different crosslink densities, superposed to give a master failure envelope on a plot of log(&lgr;b&sgr;b273/T) vs logEe(&lgr;b−1). On this type of plot, failure envelopes for all the vulcanizates except silicone and natural rubber were found to be essentially identical. At a given value of &lgr;b&sgr;b, silicone had a smaller &lgr;band natural rubber a somewhat larger &lgr;bthan the vulcanizates of the three other rubbery polymers.

ISSN:0021-8979    

DOI:10.1063/1.1713094

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The tensile strength and ultimate elongation properties of any given amorphous elastomer can be described by a characteristic failure curve. It is shown in this paper that the failure curve can be predicted from a knowledge of the creep curve of the elastomer together with the data from a Mooney‐Rivlin plot. The theory relating the ultimate properties to the viscoelastic properties of the elastomer is based upon the idea of a propagating crack, the rate of propagation being limited by viscoelastic mechanisms. Data for the failure curves and creep response for EPR and SBR elastomers are presented and shown to support the theory. Literature data for butyl rubber are also shown to confirm the theory.

ISSN:0021-8979    

DOI:10.1063/1.1713095

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The scattering of light from crystalline polyolefins results principally from a correlation in crystal orientation described by an empirical Debye‐Bueche correlation functionf(r)=exp(−r/a) for crystals separated by distancer. A one‐dimensional theory is developed in which neighboring crystals may differ in orientation by a small angle ±&dgr;. A random‐walk theory results in the exponential correlation function where the correlation distance is given bya=Kd/&dgr;2for crystals separated by distanced. Kis a numerical factor which is ½ for crystals confined to a plane and ⅔ for crystals permitted to orient about a cone of apex angle &dgr; with respect to the preceding crystal. The extension to two dimensions is proposed by means of a computer simulation of the lattice, and preliminary results of such an analysis are presented.

ISSN:0021-8979    

DOI:10.1063/1.1713096

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

The birefringence of polyolefins undergoes a change from an initial negative value at low elongation to a positive value at higher elongations. The extent of the negative contribution depends upon time after quenching the sample and upon temperature. The changes result from a change in the mode of crystal orientation. A model involving spherulite deformation and reorientation of crystals within the spherulite is proposed. Changes with time and temperature are ascribed to changes in a parameter &ggr; describing the mobility of crystals within the spherulites. The model is also used to account for variation in the dynamic birefringence with straining frequency.

ISSN:0021-8979    

DOI:10.1063/1.1713097

出版商:AIP    

年代:1964

数据来源: AIP

摘要:

To test theories of the twisting or correlated orientation of crystallites along the radial arms of sperulites, we measured the radial growth rates (G) and the extinction spacings (s) of a series of molecular weight fractions of linear polyethylene. At equivalent degrees of supercooling the radial growth rate is faster for the lower molecular weight fractions. Comparison with earlier measurements shows that the critical free energy for the formation of a growth nucleus is essentially the same for a single crystal growing from dilute solution as for a spherulite crystallizing from the melt. The data are not consistent with the hypothesis thatsis directly proportional to the parameter &dgr;=D/G, but indicate an additional factor that has the temperature dependence of a nucleation process.

ISSN:0021-8979    

DOI:10.1063/1.1713098

出版商:AIP    

年代:1964

数据来源: AIP