Volume 15, Issue 4, 01 April 1944
Index of content:
15(1944); http://dx.doi.org/10.1063/1.1707432View Description Hide Description
Natural rubber is the prototype of an important class of materials consisting of long flexible molecules which interact with each other in a particular way. Some of the properties of bulk rubber are strikingly similar to, and may be understood by a consideration of, those of single flexible molecules. For a complete understanding of the behavior of rubberlike materials it is necessary to understand the way in which they are built up from the component flexible molecules. In vulcanized materials, in which plasticity is suppressed, intermolecular bonds link the molecules into a coherent network, very irregular in detail but isotropic and homogeneous on the average. In lightly vulcanized materials these bonds are relatively few, and bring relatively small portions of adjacent molecules into fixed relations to each other. For the most part the interaction of neighboring molecules in the material is that characteristic of liquids. It is the presence of the intermolecular bonds, which link the molecules into a network and thus control its form, which differentiates rubberlike materials from liquids. It is the small number of these bonds, and their weak control of the form of material through the entropy rather than the internal energy, which differentiates rubberlike materials from ordinary solids. On the basis of this picture of the structure of rubber there is derived a form for the stress‐strain curves at moderate extensions which is in good agreement with experiment.
15(1944); http://dx.doi.org/10.1063/1.1707434View Description Hide Description
The nature of hysteresis in products such as pneumatic tires, solid tires, and transmission belts is analyzed and the requirements of a laboratory test for evaluating the relative hysteretic characteristics of natural and synthetic rubber stocks are developed. The significance of various definitions of the ``hysteresis defect'' in rubberlike materials is discussed. A forced resonance vibrator in which rubber samples are deformed in shear at frequencies of 20 to 300 cycles/sec., shear strains of 0.05 to 0.35, and temperatures of −20 to +120°C is described. Experimental results obtained with natural rubber and GR‐S gum and tread stocks are presented. The hysteresis index ωη is found to be nearly independent of dynamic shear strain while the dynamic modulus G decreases moderately with increasing dynamic strain. Neither ωη nor G depends upon the height to diameter ratio of cylindrical samples. These results are at variance with those obtained by previous investigators who, employing compressive vibrations, have reported marked dependences of both modulus and friction upon dynamic strain and the ``shape factor'' of tread type stocks. In agreement with previously reported work, G is found to be independent of frequency and ωη only slightly dependent upon frequency, for treated type stocks. Results are presented for stocks based on Buna S type copolymers with varying monomer ratio and on N‐type Butaprenes, Neoprene, and butyl rubbers.
15(1944); http://dx.doi.org/10.1063/1.1707435View Description Hide Description
Drift tests lasting 8 years have been carried out on rubber blocks in compression at 35°C. Rate of drift, initially high, attains a low constant value in 200 days or less. The initial, rapid drift is termed transient drift; and the slower, constant drift is termed steady drift. Drift varies considerably with the type of accelerator in the compound, Ureka giving the lowest drift of the accelerators tested. A new method has been developed for measuringstress relaxation in tension. The stress is measured to 0.1 percent by the resonance frequency of lateral vibrations of the stretched sample. The vibrations are impressed on the sample by a mechanical oscillator in which the source of vibrational energy is a steel wire which is under adjustable tension and is kept in circular vibration by a pair of air jets. Relaxationmeasurements extending over many months are reported on a soft vulcanized rubber at elongations from 10 to 400 percent at 35° and 70°C. The experimental data can be fitted by a two‐term stress equation of the Tobolsky‐Eyring form representing two slip mechanisms, or transient and steady relaxation. Steady relaxation follows an exponential decay law; and this means that the residual stress goes to zero at infinite time. Other stress relaxation data are reported for rubber tested at 150 percent elongation, at temperatures from 35° to 113°C, in air and in vacuum. Air, as compared with vacuum, produces little effect at 35°; but at 70° and higher, the rate of steady relaxation is greatly increased. Thus oxidation appears to be the major factor in steady relaxation at elevated temperatures. Total transient relaxation is unaccountably increased by vacuum and by heat. A modification of the Tobolsky‐Eyring equation is developed for steady relaxation. For transient relaxation a new theory is developed which leads to the Tobolsky‐Eyring equation, but involves different interpretation of the parameters. In this theory, transient relaxation is attributed to rupture of secondary chemical bonds or crystal forces, followed by longitudinal slippage of chain molecules, with partial or local equalization of tension along the chains. The crystallites then reform, and the rupture and slippage process is repeated. The energy dissipation is attributed primarily to release of local elastic stresses following bond rupture. Transient relaxation is complete when the tension is completely equalized over the total length of each chain between cross links. Neither of the new equations can be distinguished from the Tobolsky‐Eyring equation on the basis of present data.
Elasto‐Viscous and Stress‐Optical Properties of Commercial Polymerized Methyl Methacrylate as a Function of Temperature15(1944); http://dx.doi.org/10.1063/1.1707437View Description Hide Description
The elasto‐viscous and stress‐optical properties of commercial methyl methacrylate polymer have been measured. Between 66°C and 107°C Young's modulus drops from approximately 400,000 pounds per square inch to roughly 200 pounds per square inch and the material behaves like rubber. At 93°C the viscosity is approximately 10 12 poise. This drops to 10 9 at 177°C. While the curve is not strictly linear with 1/T, an activation energy of 30,000 cal./mole can be deduced for this change. Below 93°C three rate constants are necessary to describe the delayed elastic process, but two suffice between 93°C and 135°C. At 149°C one such constant is enough and above this an instantaneous elastic and a viscous flow is sufficient. The change in rate constants with temperature gives rise to elastic activation energies of 9000 to 11,000 cal./mole. Methyl methacrylate is optically negative but has a stress optical sensitivity about that of glass. The stress optical coefficient varies markedly with temperature, showing a sharp maximum at 93.3°C. The stress‐optical coefficient is directly proportional to the average relaxation constant. X‐ray diffraction patterns show four rings corresponding to spacings of 2.19, 3.07, 6.7, and 14.7A. Some slight evidences of crystallinity are shown by diffraction patterns in fibers stretched at 93°C. Fibers stretched at 149°C show a lesser amount of order in agreement with the birefringence studies. The second‐order transition point occurs at 71.1°C.
15(1944); http://dx.doi.org/10.1063/1.1707438View Description Hide Description
An extrusion plastometer for rubberlike materials which measures the shearing stress at a predetermined constant average rate of shear and a given temperature is described. In one filling of the extrusion chamber, requiring about 10 cm 3 of sample, the average rate of shear is set at 3 or 4 different values in the range of 5–100 sec. −1 and corresponding average shearing stresses are read by means of a pressure gauge. A plot of these data thus gives a complete rheological curve from a single determination. Typical curves are given for Hevea smoked sheets, GR‐S, and Butaprene NM and for GR‐S masterbatches and normal tread stocks. The effect of capillary geometry is estimated. The effects of milling and of temperature upon the rheological properties of several rubbers are discussed. Typical data for GR‐S polymers, masterbatches, and tread stocks, together with corresponding data obtained by means of the Mooney shearing disk viscometer and the Firestone constant pressure extrusion plastometer, are presented and their relative significance discussed.
15(1944); http://dx.doi.org/10.1063/1.1707439View Description Hide Description
An instrument has been designed to determine whether a given batch of synthetic rubber tread stock can be forced through a plate die to form a satisfactory tire tread. The instrument takes the form of an extrusion plastometer which uses special dies. The factory tubing properties of a batch can be predicted from the appearance of the sample extruded through this die and from the extrusion rate.
15(1944); http://dx.doi.org/10.1063/1.1707441View Description Hide Description
Vulcanized rubber is composed of a cross‐linked network of chain molecules, segments of which are sufficiently free and mobile in localized regions to form a crystal lattice upon stretching. The crystallites thus formed represent an automatic molecular mechanism for re‐enforcement and are analogous to particles of re‐enforcing pigment which increase the modulus, strength, and tear resistance. These effects depend upon the number of particles present and their size and shape. X‐ray determinations were made of the crystallite sizes in a series of vulcanized gum stocks using the Scherrer method but calculating the diffraction broadening by the formula proposed by Taylor. Evidence was found that the crystallite size distribution was heterogeneous and included small crystallites which broadened the base of the diffraction peaks. It could be shown that a high degree of crystallinity in a compound was associated with small crystallite size. Both of these factors combine to give high modulus stocks. For different cures with a given rubber compounding formula, there is a definite correlation between the amount of combined sulfur and the crystallite size. This indicates that irregularities in the structure caused by the combined sulfur tend to limit the crystallite growth. This interpretation is preferred although the experimental evidence is not decisive as to whether or not lattice distortion contributes to the width of the diffraction spots. Significant conclusions can be drawn from the work in regard to crystallite formation in stretched rubber and effects of crystallites on the physical properties.
15(1944); http://dx.doi.org/10.1063/1.1707442View Description Hide Description
Measurements of stress decay as a function of time made at constant elongation on thin bands of gum and tread type natural rubber (Hevea), Neoprene, Butyl, Buna S, and Butaprene N stocks indicate that both secondary and primary bondrelaxation occur. Practically complete relaxation is observed to take place in the experimental time (about 100 hours) at temperatures at and above 100 degrees C. The manner in which the rate of relaxation depends on temperature and the fact that the rate is independent of elongation and of the presence of carbon black in the vulcanizate indicate that stress decay is caused by a definite chemical reaction which deteriorates the rubberstructure, and oxidative scission is suggested as the mechanism of deterioration of the primary bonds. The stress relaxation data, obtained over a temperature range from −50°C to +150°C, appear to verify modern concepts of the structure of elastomers.Theoretical equations are derived which give very good agreement with the observed relaxation data at high temperatures. The free energy of activation for the oxidative scission is found to be 30.37 kcal. per mole for the Hevea gum stock, and differs from this value by less than ±2.0 kcal. per mole for all other stocks, indicating the same general reaction for all. However, these small differences in free energy of activation correspond to considerable differences in times of decay, which fact might be significant in evaluating the resistance of rubber stocks to deterioration.
Thermal Expansion and Second‐Order Transition Effects in High Polymers: Part I. Experimental Results15(1944); http://dx.doi.org/10.1063/1.1707444View Description Hide Description
Experimental values of second‐order transition temperatures Tm and of cubical expansion coefficients below and above this temperature are presented for several new materials, notably saran. The thermal expansion behavior of two‐component systems of incompatible materials (polystyrene plus polyolefins) has been studied. For relatively coarse dispersions (1000A), Tm is 82°C, independent of composition, while the difference Δβ in cubical expansion coefficient above and below Tm is directly proportional to the volume fraction of polystyrene in the mixture. Unplasticized saran behaves similarly in that Tm is constant while Δβ decreases linearly with increasing crystallinity. For molecular dispersions of incompatible materials both Tm and Δβ are functions of composition. It is shown that for most high polymers Tm increases with increasing intermolecular force constants, while the product of Tm and cubical coefficient of expansion above Tm is roughly constant (0.1 to 0.2).