Longitudinal growth of polymer crystals from flowing solutions. X. Conditions for continuous growth on the rotor surface

Abstract

AbstractUltrahigh strength polyethylene fibers can be generated by stress‐induced crystallization from a supercooled solution subjected to Couette flow, usually referred to as the “surface‐growth” process. Under appropriate conditions, continuous fiber production can be realized for a period as long as 19 days, whereas under other circumstances a rapid interruption of the growth process is met. The present investigation deals with the origin of fiber fracture during “surface growth.” The limiting values of process variables required to maintain continuous growth have been established. Interruption of the continuous growth can occur in three different ways: (1) formation of a closed fiberloop around the rotor; (2) limited crystal growth rate; (3) rapid crystallization, leading to depletion of the gel on the rotor surface. The gel layer is being formed by adsorption of long molecules on to the rotor surface and subsequent “reptation,” resulting in a dense entanglement network of these molecules. These factors determine the boundaries of the triangularly shaped domain for continuous growth in a graph of the two main variables, namely the takeup speed and the rotor speed. Furthermore, it was noticed that the introduction of a wedge‐shaped groove in the surface of the Couette rotor leads to a substantial reduction of failure. Continuous growth could be established in the temperature range from 103–125°C when p‐xylene was used as a solvent. For p‐xylene solutions at a crystallization temperature of 110°C and using a teflon rotor of 115 mm diameter, a maximum takeup speed and rotor speed were 16 and 180 mm/s, respectively. Basically the restrictions of the process appeared to be due to the limited rate of crystallization and rate of adsorption of polyethylene molecules on the surface of the rotor.