Longitudinal growth of polymer crystals from flowing solutions III. Polyethylene crystals in Couette flow

Abstract

A detailed description is presented of several phenomena observed during the longitudinal growth of fibrillar polyethylene crystals from xylene solutions subjected to Couette flow. The rate at which the extended chain seed crystals grow longitudinally in this flow field could be established by measuring the increment in length of the macrofiber in a given period of time and also by finding the take-up speed under steady-state conditions which is equal to the growth rate. Continuous macrofibers of polyethylene with lengths of several hundred meters could be produced by this technique. The growth rate of the seed crystals anchored at a distance of approximately i mm from the surface of the rotating inner cylinder was found to increase linearly with the rotor speed, and these growth rate data agreed remarkably well with those values obtained in Poiseuille flow at corresponding local velocity gradients. Scanning electron micrographs reveal that these macrofibers, having diameter in the micron range, are composed of bundles of elementary fibrils of the Shish-Kebab type. It is believed that these Shish-Kebab backbones grow simultaneously by the action of the local flow field. The diameters of the macrofibers increase with the rotor speed as a result of branching of the backbone tips. The growth of seed crystals pushed against the rotor surface is considerably faster than the growth of seeds, around which the polymer solution flows freely. This fast growth is promoted by the roughness of the rotor surface and by the presence of methyl groups on the glass rotor surface introduced by silanization. These experimental observations suggest that the active element in this fast “surface-growth” is a layer of adsorbed polyethylene molecules on the rotor surface. By this surface-growth technique, continuous fibrillar crystallization appeared to be possible at a temperature of even 120.5 °C, which is well above the thermodynamic equilibrium temperature in a dilute p-xylene solution, and the longitudinal growth rate of the polyethylene macrofiber at this high temperature was still 6 cm/min.