This work discusses the potential of calorimetric (DSC) measurements to study the deformation process of semicrystalline polymer samples. It has been pointed out that parameters such as lamellar fragmentation and the parameter of intrachain melting cooperativity suffered significant changes, showing that the stretching process strongly modifies the structure of the isotatic polypropylene (iPP) sample. Furthermore, the observed changes are restricted to a small region between the isotropic and fibrillar regions which is called the transition front. By observing the evolution of the parameter of intrachain melting cooperativity in comparison with lamellar thickness, it has been possible to characterize, at least in a qualitative way, the origin and further evolution of the double-fold molecules; totally extended polymer chain connecting two adjacent crystallites in the fibrillar structure-"tie molecules." Thus, the procedure described here allows, in a simple way, the observation of these molecules, being a possible alternative to TEM methods. Finally, scanning electron microscopy helped in the interpretation of the calorimetric results, revealing that the intermediate values of the different measurements made in this work must be understood as the macroscopic average of the inhomogeneous mixture of the two different structures coexisting in the transition front, i.e., newly formed fibrillar structures and the remains of the original spherulitic structure.