synopsisBound rubber measurement may be very misleading as a measure of elastomer-carbon black interaction because sometimes only part of the apparently bound rubber is truly adsorbed on the carbon black surface. A theory is proposed which utilizes bound rubber measurements, but separates truly adsorbed rubber from other insoluble gel and enables calculation of the adsorbed elastomer lay& thickness, a numerical value of interaction. Measurements of interaction were obtained for many different rubbers, including polybutadienes, styrenebutadienes, EPDMs, and butyl, with several different furnace blacks. An equation for viscosity of a rubber-carbon black composite is proposed, based on the degree of interaction obtained from the theory and the possible varying degree of orientation of the composite as the r@te of shear is changed. This has been applied experimentally both to soluble elastomers and to an elastomer containing a nonrigid gel. The modulus of a vulcanized composite is shown to be related to the effective volume fraction of filler, which is equivalent to the volume fraction of filler plus adsorbed rubber, a t temperatures above the glass transition temperature, regardless of the type of rigid filler. Below the glass transition temperature, the modulus depends only on the filler volume concentration.

Based on the assumption that an equilibrium particle size of dispersed phase will be reached when the breaking-down rate and the coalescence rate are balanced, a theoretical expression was obtained. The theory showed qualitatively that the equilibrium particle size becomes smaller when (1) the stress field is increased, (2) the interfacial tension between matrix and dispersed phase becomes smaller, and (3) the concentration of dispersed phase decreases. Qualitative verification of the theory was obtained by experimental examination of the NR-EPM blend system. In practice, in order to obtain a small particle size in a short time at above 20% volume fraction, the matching of rheological properties of the matrix and the dispersed phase is desirable. On changing from internal mixer to mill, the temperature became one of the most influential factors that control the particle size. Future work, such as the quantitative value of interfacial tension as a function of temperature, and macroscopic breaking energy measurement, etc., is necessary to confirm the theory quantitatively.

Melting point depressions were determined by dilatometric and microscopic measurements for polyacrylonitrile in dimethylformamide and in γ‐butyrolactone. These data give for the glass transition and melting point of the pure polymer 104 and 317°C., respectively. Both the heat and entropy of fusion are low. This suggests that the high melting point of polyacrylonitrile is due to its extended molecular conformation rather than to strong attractions between the molecules in the crystalline state. Average crystallization rates measured dilatometrically indicate that the optimum temperature for crystallization from solutions in the concentration range 17–27% is only 21°C. above the glass transition temperature. On either side of the optimum temperature the rate decreases sharply, and although crystallization then continues for a longer time the amount of crystallinity finally developed is less. For 15–20% solutions the viscosity increases abruptly as the glass transition is approached, and decreases rapidly at about 25°C. below the true melting point. The shear dependence is large at low temperatures but appears to vanish at 25°C. below the melting point. As the temperature is raised a narrow region is observed for each concentration in which the viscosity exhibits an anomalous increase. These temperatures stand in good agreement with the optimum temperature for crystallization as determined dilatometrically. The foregoing observations suggest the presence of molecular aggregates which are held together by regions of small and imperfect crystallites. In addition to the low entropy of fusion, additional evidence for the extended shape of polyacrylonitrile molecules is provided by estimates based on dilute solution measurements. The interaction between nearest‐neighbor nitrile dipoles was investigated in an attempt to correlate large unperturbed dimensions with the molecular structure. This dipolar interaction is repulsive, and its magnitude depends upon the rotational position of the two nitrile groups. By adding a further contribution representing the steric repulsion the total rotational potential is estimated for isotactic and syndiotactic sequences. The dipolar interaction is much larger than the steric repulsion, and the rotation about successive chain bonds must be cooperative to avoid certain high energy conformations. This requirement probably explains the low flexibility and high extension of polyacrylonitrile molecules. The total rotational potential for an isotactic sequence favors the same helical conformation observed in crystalline isotactic polyolefins, while for a syndiotactic sequence some type of helical conformation appears to be preferred rather than the extended planar zigzag.

SynopsisWhen an uncompounded elastomer is processed on a two-roll mill, four different regions of mechanical behavior are observed, depending upon the temperature and the seventy of the nip deformation. This behavior is observed on materials with a wide variety in chemical composition, though the severity varies. The flow a t high temperatures is typical of melt or polymer solution behavior. At lower temperatures unstable flow and elastic solidlike regions are observed. By presuming the elastomer to be an isotropic viscoelastic medium, the stress and velocity fields were computed in the polymer melf region. The unstable regime was found to correspond to a critical value of the ratio of viscoelastic to viscous forces. The mathematical analysis, done in terms of the Green-Rivlin-Noll theory of viscoelastic media, extends earlier studies of deformation in this geometry by Gaskell and Bergen.* In snme instances. the trammarent fluid has changed rolls.

SynopsisElastomer processing operations are discussed and classified as unit operations. The theory of nonlinear viscoelasticity is applied to processing unvulcanized amorphous rubber and the significance of the maximum relaxation time T,,, is emphasized.