Rheology of polyisobutylene. III. Elastic recovery, non‐Newtonian flow, and molecular weight distribution

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Abstract
Moments of the molecular weight distribution have been measured in the equilibrium ultracentrifuge under ideal conditions for two rough fractions of polyisobutylene (Mw 1.6 × 104 and 1.31 × 105). Flow and elastic recovery measurements were made on these materials and on a series of blends; the non‐Newtonian flow at small shear stresses was characterized by the reciprocal of the Ferry “internal shear modulus,” Gi, and the magnitude of the elastic recovery by the steady‐state elastic compliance, Je. This was found to be about two to three times the magnitude of 1/Gi. Measured values of Mz+1 for the fractions appeared unreasonably high compared to values obtained by assuming a Lansing‐Kraemer distribution. With either set of values of Mz+1 for the fractions, it was found that Je was proportional to the square of (MzMz+1/Mw) for these materials, in disagreement with the results of molecular theories, which predict proportionality with the first power of this function.