Self-Nucleation of Polymers with Flow: The Case of Bimodal Polyethylene

A small amount of high molecular weight molecules can have a dramatic influence on the flow-induced crystallization kinetics and morphology of polymers. In this paper, it is shown that with the addition of 7 wt % of high molecular weight molecules a melt of low molecular weight polyethylene can be tailored for process-induced self-nucleation. To ensure mixing between molecules of high and low molecular weight, we make use of a special synthesis route where both fractions are generated simultaneously by two catalysts immobilized on the same support. Under the influence of flow, the high molecular weight molecules make crystallization possible at temperatures as high as 137 degrees C, where the matrix alone cannot crystallize. Moreover, flow can lead to fibrillar precursors of crystallization at temperatures as high as 142 degrees C, i.e., in the vicinity of the equilibrium melting point. On cooling below 139 degrees C, these precursors crystallize and transform into shishes while the growth of kebabs is still suppressed because of the high temperature. With exact lattice matching and an excellent state of dispersion, shishes generated at this high temperature are ideal as a substrate for heterogeneous crystallization (self-nucleation). In fact, with the processing conditions explored here, they can shift the onset of crystallization, 124 degrees C in quiescent conditions, up to temperatures as high as 132 degrees C during cooling at 5 degrees C/min. The effect of shear at 142 degrees C on the nonisothermal crystallization of the material is explored varying systematically shear rate and shear time. These novel findings on self-nucleation also demonstrate that the crystallization temperature of polymers is a processing parameter that depends on the whole thermomechanical history and not only on the cooling rate. In particular, for the bimodal polyethylene investigated in this work, after shear at 142 degrees C, the crystallization temperature and other crystallization parameters are governed essentially by the macroscopic strain.