The role of differential scanning calorimetry and X‐ray diffraction in identifying crystal continuity in highly oriented samples of die‐drawn polypropylene

X‐ray diffraction and differential scanning calorimetry studies of die‐drawn polypropylene have indicated the presence of a crystalline structure which does not identify with any of the usual crystallographic forms and has a melting point slightly below that of the monoclinic a form. The melting of this structure is stress sensitive and shows the superheating effect expected from the melting of an extended chain structure, whose chain ends are physically constrained. It is postulated that this structure bridges the gaps between adjacent monoclinic crystal blocks and provides the framework necessary to maintain mechanical stability at temperatures well above that of the glass transition. Moreover, together with taut tie molecules, this structure is considered to be the main source of the high material stiffness.