Prediction of Elastic Strains of Polymer Melts in Shear and Elongation

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Abstract
Transient and steady‐state recoverable strains in shear and uniaxial elongation were measured with rheometers of different type. Results on melts of linear and branched polyethylenes, polypropylene, polystyrene, polyamide 6, and a PIB solution are reported. To represent the linear viscoelastic behavior, the essential basis of the theoretical treatment, discrete relaxation time spectra have been determined from the dynamic moduli in oscillatory shear. Calculations of the primary normal stress and of the shear and elongational viscosities in the non‐Newtonian range are based on a single integral constitutive equation with strain‐dependent memory function. Recoverable strains observed after unloading the samples from steady‐state flow are calculated by means of a quasilinear treatment of the materials, making use of deformation rate dependent effective relaxation strengths. The agreement of predicted and measured recoverable strains over a very wide range of shear and elongation rates strongly supports this new concept, which does not involve an inversion of the integral constitutive equation. In addition, the applicability of two empirical equations that relate the primary normal stress coefficient and the recoverable strain in steady shear flow to the dynamic moduli are checked on the same materials.

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