2018-11-07
10:15 at HCP F 43.4

A coil-stretch transition and configurational microphase separation in elongational flow of an entangled polyethylene liquid

Brian Edwards

Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, United States

Net-shape manufacturing of plastic and rubber products is typically performed via processing of a molten polymeric fluid under an imposed flow field. Until recently, the general state of knowledge has maintained that the orientation and deformation of the constituent chain-like molecules under elongational flow is homogeneous and essentially obey Gaussian statistics; however, recent experimentation and simulation have called this notion into question. Virtual experimentation of atomistic entangled polyethylene melts undergoing planar elongational flow reveals an amazingly detailed depiction of individual macromolecular dynamics and the resulting effects on bistable configurational states. A clear coil-stretch transition was evident, in much the same form as first envisioned by de Gennes for dilute solutions of high polymers, resulting in an associated hysteresis in the configurational flow profile over the range of strain rates predicted by theory. Simulations conducted at steady state revealed bimodal distribution functions in which equilibrium configurational states were simultaneously populated by relatively coiled and stretched molecules, which could transition from one conformational mode to the other over a relatively long timescale at critical values of strain rate. The implication of such behavior points to a double-well conformational free energy potential with an activation barrier between the two configurational minima. The configurational hysteresis can also lead to a similar phenomenon with respect to the rheological properties of the fluid such as the extensional viscosity. Furthermore, visualization of the liquid in the bistable regime revealed that the stretched and coiled states not only coexist, but also that the constituent macromolecules become segregated via an inhomogeneous microphase separation into large domains of relatively coiled molecules surrounded by thinner sheets of highly stretched chains. Such startling behavior represents a remarkable departure from the theoretical expectation, and as such, presents a difficult challenge for theoreticians to assimilate into the prevailing rheological worldview.