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

Prof. Ralph Colby, Penn State College of Earth and Mineral Sciences

Abstract: Rheo-optical methods allow quantification of birefringence related to chain alignment in flow. We first apply such methods to melts of aromatic backbone polymers: poly(ether ether ketone) (PEEK), poly(phenylene sulfide) and polysulfones. Each polymer shows strong birefringence that grows above a critical shear rate, consistent with a coil-stretch transition, and adopts full nematic alignment at roughly an order of magnitude higher shear rate. To our surprise, we also see very similar behavior with much more flexible polymer melts, such as isotactic polypropylene (iPP) and high density polyethylene (HDPE). For the semicrystalline polymers, in particular PEEK, iPP and HDPE, the shear rate at which the strong birefringence is first observed coincides with the minimum shear rate needed for shear flow to accelerate nucleation (flow-induced crystallization) when shearing between the nominal melting temperature and the equilibrium melting temperature, where a network of crystallization precursors is formed. For iPP, we have shown that critical shear rate is the reciprocal of the Rouse time of the longest chains. For HDPE, strong edge fracture is observed when shearing at 150 oC, which is above the equilibrium melting temperature (141 oC) and hence, too high a temperature for the nematic alignment to create the precursor network and edge fracture occurs at modest shear rates. In contrast, the nematic alignment creates the precursor network when shearing the same HDPE at 135 oC and the precursor network apparently stabilizes the melt against edge fracture, even though normal stresses are larger at the lower temperature.