Abstract
It is shown that at unprecedented (high) temperatures an extremely slow long lasting shear treatment, which requires almost an hour for its unwinding, can produce a highly oriented structure in the melt of a crystallizing polypropylene. It seems that a convection process, which is characteristic for shear flow, furthers intimate contacts between parts of passing by macromolecular coils. In this respect an inspection of the broad spectrum of crystallization speeds of quiescent melts of a variety of polymers is instructive. The conception, which is defended in the present paper, is that thread-like nuclei are the result of the straightening of assemblages of already loosely connected macromolecules. At the high temperature applied the melt must immediately be quenched after the cessation of the flow in order to preserve the obtained thread-like nuclei. Apparently, only the number of successful encounters is relevant and not the frequency of their occurrence. For a lower frequency more time is required. It is evident that even mild touches can cause permanent adhesions, if the mutual local conformations just fit. One is reminded of the click of a slot. A new apparatus for unlimited shearing is described. This apparatus uses polymer samples, which are ring-shaped, being cut from molded plates of reasonable thickness. These rings possess an unusually large radius of 18 cm, in order to keep the curvature of the flow lines low. Rectilinear unlimited shearing is approached in this way.
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