SynopsisSteady shear viscosities, dynamic viscosities and moduli, and the corresponding activation energies for flow were examined for a branched polyethylene, a linear polyethylene, and three of their blends a t 150' and 190°C. The polyethylenes were chosen to have closely matched molecular weights and distributions. An R-17 Weissenberg rheogoniometer and an Instron capillary rheometer were used. At lower stress, the branched polymer had a higher viscosity than the linear one, possibly because of the contribution of long branches to entanglements. At high stress, this contribution is reduced and the inherently smaller coil dimensions likely become responsible for the lower viscosity of the branched polymer. The activation energy for the branched polymer is high and decreases with stress, in contrast to the low and almost-constant value for the linear polymer. The effects here of pressure on compression are considered. The entanglements of long branches may also decrease with increasing temperature. With decreasing stress, the activation energy for branched polymer tends to become constant, corresponding to an absence of pressure effects and an equilibrium entanglement of long branches for a given temperature range. The linear relationship between activation energy and blend composition problably means that any compressional effects, like free volume, are additive and that long-branch entanglements rearrange with added linear molecules. The linearity may be the result, in part, of a broad distribution for the lengths of long branches.
A detailed study has been made of the intrinsic viscosity‐molecular weight relationship for a polybutadiene prepared at 50°C. and one prepared at 5°C. Original fractions were at least twice refractionated from very dilute solutions in an attempt to obtain fairly sharp distributions. Fox‐Flory K constants are observed to decrease with increasing molecular weight in each case, and the intrinsic viscosity‐molecular weight plots show a curvature. This is interpreted to be a result of branching. A method of plotting the data is proposed which simultaneously yields the K for unbranched material and an estimate of the degree of branching. It is estimated that, in polybutadiene prepared at 50°C., one out of 6500 monomer units is involved in a branch, and that in 5°C. polybutadiene, one out of 10,000.
An intrinsic viscosity‐molecular weight study has been made of several polybutadienes and synthetic polyisoprenes. The Fox‐Flory treatment of such data was applied in each case. This serves for the determination of a constant characteristic only of the polymer and related to the unperturbed end‐to‐end distance of the polymer chain. A comparison of the dependence of this constant on polymer structure with that predicted theoretically shows good agreement. A further comparison of the values of this constant for the synthetics with those for natural polyisoprenes leads to the conclusion that the diene polymers, natural and synthetic, do not differ markedly in hindrances to free rotation.
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