Application of an interpenetrating network model to the necking in the microcrystalline region in four annealed isotactic polypropylene films subjected to uniaxial stretching at room temperature

The microscopic infrared dichroism, mesoscale deformation and macroscopic stress measurements are made on the microcrystalline region in four annealed isotactic polypropylene (iPP) thin films subjected to uniaxial stretching at room temperature. Results reveal that volume dilatation might occur during stretching and the necking causes the anisotropic shrinkage in the thickness and the width directions. The average orientation function fav and the true stress as a function of local draw ratio in the samples showing volume dilatation can be respectively overlapped onto those of the sample undergoing constant volume deformation. The pseudo-affine deformation is applicable for molecular orientation at fav<0.50 and the true stress-strain relationship on the mesoscale can be well described in the same region by the interpenetrating network model previously proposed for necking in the quenched iPP film. This model becomes invalid for deformations above fav=0.50 due to that plastic deformations in the crystalline phase, depending on the annealing time, start to play a major role.