Crystallization and dissolution of flow-induced precursors

We make use of a specially synthesized linear high density polyethylene with a bimodal molecular weight distribution (MWD) to demonstrate that it is possible to produce a suspension of extended-chain (shish) crystals only. Such a suspension can be generated at high temperatures, above but close to the equilibrium melting temperature of the unconstrained extended-chain crystals (Tm0=141.2¿°C) and requires stretch of the longest chains of the MWD. After the application of a shear flow of 120¿¿s-1 for 1 s at 142¿°C, x-ray scattering suggests the presence of a large number of metastable needlelike precursors with limited or no crystallinity . Precursors that are too small dissolve on a timescale that correlates perfectly with the reptation time of the longest polymer molecules. Whereas, precursors that exceed a critical size crystallize forming extended-chain shishes