Processing–property interactions in poly(vinylidene fluoride). I. An analysis of melt stress history in an extensional flow geometry

A numerical and experimental analysis of the melt flow history of poly(vinylidene fluoride), PVF 2 , in an extensional flow, film‐processing geometry is presented. Numerical computations were carried out for both Newtonian and power law flow in an impinging channels die using a finite‐element technique. Computations demonstrate that a strong extensional flow exists in the region from the stream impingement point to a distance about 0.75 D downstream where D is the channel height at the impingement point. Measurements of the stress fields using the technique of flow birefringence showed that in consequence of the changing flow kinematics from the stream impingement region to the downstream converging channels region, both the isochromatic and isoclinic patterns exhibit a pronounced axial positional dependence. Excellent qualitative and quantitative agreement was found between measured and calculated shear stress fields. However, large differences were exhibited between calculated and measured normal stress differences along the symmetry axis of flow. Results to be presented in a companion paper (Part II) demonstrate that the strong extensional flow which occurs in this geometry is sufficient to produce oriented, row‐nucleated structures in the extruded PVF 2 films.