Equibiaxial Extension of Two Polymer Melts: Polystyrene and Low Density Polyethylene

Stress relaxation after rapid extensional strain was measured to obtain the extensional relaxation modulus. The research had the objectives of developing the lubricated squeezing technique for molten polymers, of applying the technique to two different polymers, and of testing the “separability hypothesis.” Equibiaxial extensional flow was generated with a Rheometrics RDS‐LA, using the lubricated squeezing technique. The time dependence of the relaxation modulus was found to be the same in extension as in shear, given by the relaxation modulus of linear viscoelasticity. The strain dependence was markedly different than in shear. Separation of time and strain dependence into a product of two functions is suggested by the experimental results. This is a strong support for the separability hypothesis and simplifies the formulation of rheological constitutive equations. A polymer with linear macromolecules (PS) exhibited stronger strain dependence than a polymer with long chain branched macromolecules (LDPE). The parameters of an integral constitutive equation were determined in rapid strain experiments and the constitutive equation was tested experimentally with stress growth at start‐up of equibiaxial extension.