DMI/MEMS Seminar Presented by Prof. Ralph Colby

Title: Nematic Alignment of Chains by Shear Flow and its Role in Accelerating Nucleation and Stabilizing Against Edge Fracture

Abstract: Applying shear flow to some molten polymer liquids imparts a nematic chain alignment that creates form birefringence. Rheo-optical methods are used to quantify this alignment for high-density polyethylene (HDPE) and poly(ether ether ketone) (PEEK). With both polymers, the birefringence starts to be measurable at the same shear rate where shear flow starts to accelerate crystallization kinetics. There is a strong departure from the Cox-Merz rule above this critical shear rate and at much higher shear rates, the shear stress and birefringence both grow as the square root of shear rate, as expected for nematic alignment of chains. Crystallization from the flow-aligned nematic creates a fine-scale morphology that maximizes tie chains between crystals, imparting toughness to these semicrystalline polymers.

Polymer chains stretch in shear flow when the shear rate exceeds the reciprocal of their Rouse time. The high molecular weight tail of the molecular weight distribution of HDPE is fit to an exponential function that allows quantification of an energy conversion factor E that is the volume fraction of chains that get stretched at any shear rate. The measured form birefringence of HDPE tracks perfectly with the shear rate dependence of E in the range 10-6 < E < 10-2. The product of E and specific work is shown to control the magnitude of the crystallization acceleration from shear. When sheared below the equilibrium melting temperature, HDPE forms a network of crystallization precursors that stabilizes the melt against edge fracture.

Host: Michael Rubinstein