Flow-induced crystallization regimes and rheology of isotactic polypropylene

We investigated the effect of flow in combination with molecular architecture on crystallization for linear and branched polypropylenes by means of dynamical mechanical spectroscopy. Compared to the linear polymer, the branched one exhibits much slower relaxation after deformation, causing higher levels of molecular orientation and molecular stretch for the same flow conditions. Different regimes of flow-induced crystallization are observed as a function of an increased level of molecular orientation and stretch. These regime changes cause the structure development in the material to vary from 3-dimensional spherulitical growth to nearly 1-dimensional fiber-like growth. Intermediate behavior is observed when the flow is strong (for the linear polymer) or weak enough (for the branched polymer), or in time when the effect of flow is exhausting. It is important to note that dynamical mechanical measurements can be used to probe different aspects of the (flow induced) crystallization process, such as a relative level of crystallinity and timescales of the evolution, but do not give absolute values. A correlation is found with the level of crystallinity obtained from real-time WAXS measurements and the kinetics of linear viscoelastic properties, but this correlation is not unique and depends on the pre-shear conditions applied.