SCANNING ELECTRON-MICROSCOPY AND DIFFERENTIAL SCANNING CALORIMETRY STUDY OF THE TRANSITION FRONT IN UNIAXIALLY STRETCHED ISOTACTIC POLYPROPYLENE

This work discusses the potential of calorimetric (DSC) measurements t o study the deformation process of semicrystalline polymer samples. It has been pointed out that parameters such as lamellar fragmentation a nd the parameter of intrachain melting cooperativity suffered signific ant changes, showing that the stretching process strongly modifies the structure of the isotatic polypropylene (iPP) sample. Furthermore, th e observed changes are restricted to a small region between the isotro pic and fibrillar regions which is called the transition front. By obs erving the evolution of the parameter of intrachain melting cooperativ ity in comparison with lamellar thickness, it has been possible to cha racterize, at least in a qualitative way, the origin and further evolu tion of the double-fold molecules; totally extended polymer chain conn ecting two adjacent crystallites in the fibrillar structure-''tie mole cules.'' Thus, the procedure described here allows, in a simple way, t he observation of these molecules, being a possible alternative to TEM methods. Finally, scanning electron microscopy helped in the interpre tation of the calorimetric results, revealing that the intermediate va lues of the different measurements made in this work must be understoo d as the macroscopic average of the inhomogeneous mixture of the two d ifferent structures coexisting in the transition front, i.e., newly fo rmed fibrillar structures and the remains of the original spherulitic structure. (C) 1996 John Wiley & Sons, Inc.