Flow as an effective promotor of nucleation in polymer melts: A quantitative evaluation

An apparatus is presented which enables the application of defined portions of mechanical work to the polymer sample in its state of undercooled melt. For the purpose mainly intermittent shear creep is used. Results are presented for an industrial grade of polypropylene. A three-dimensional picture is presented, in which the resulting numbers of nuclei (per unit volume) are plotted against two responsible parameters: crystallization temperature and mechanical work. With decreasing temperature and with increasing mechanical work the number of nuclei increases by many decades. At sufficiently high mechanical loads a transition to thread-like precursors ("shishs") has been observed previously. Oriented structures (kind of "shish-kebabs") are formed in this way. The periods of shearing applied have always been extremely short compared with the time until crystallization becomes observable. In this way an accumulation of various processes could be avoided. The description of shear induced crystallization, as previously given, is modified in the light of the present results.