Transient flow-induced crystallization of a polyethylene melt

Abstract

Extensional, flow-induced crystallization (FIC) of a high-density polyethylene (HDPE) melt has been studied using a four-roll mill flow cell. Simultaneous measurement of the birefringence and scattering dichroism are used to quantify the crystallization process during and following transient flow deformation in planar extensional flow. Suspension of the HDPE phase as a droplet in a linear low-density polyethylene carrier phase prevents die blockage on crystallization and allows measurement of the flow kinematics. Initial crystallization rates following a transient flow deformation show a stress-strain dependence. Crystallization induction times during flow correlate with the extension rate during the transient flow deformation. Measurement of the HDPE melt steady and oscillatory flow rheological properties, along with measurements of time constants following step-shear and extensional strains, allow determination of the viscoelastic properties which enhance FIC. Parameters obtained from these experiments are used in a phenomenological model for FIC which allows qualitative and semiquantitative analysis of the data trends, particularly the relaxation behavior of the birefringence during flow cessation/crystallization.

Comparison of the structure-property relationships for PTT and PET fibers spun at various take-up speeds

“…In this study, although we could not find the birefringence plateau in the take-up speed range below 4000 m/min, we experienced a similar phenomenon, that is a decrease of birefringence at the take-up speed range above 4000 m/min for PTT fibers and above 5000 m/min for PET fibers, respectively. With increasing take-up speed, the oriented amorphous phases seem to act as nucleation sites for crystallization [25,26] and folded-chain crystals may grow laterally using such nucleation sites. At very high take-up speeds, it is inferred that many small crystallites were formed under stress and the elongational flow of melt spinning and that these rigid crystallites bear most of the stress in the spinning line.…”

A Thermodynamically Inspired Method for Quantifying Phase Transitions in Polymeric Liquids with Application to Flow-Induced Crystallization of a Polyethylene Melt

“…Experimentally, capturing crystal nucleation events via methods such as microscopy, , X-ray, and neutron scattering , as well as scanning electron microscopy and atomic force microscopy , is very challenging for several reasons, all of which are fundamentally related to the short length and time scales associated with this phenomenon. − Several theories of FIC have been developed over the past 30 years ( e.g. , see refs , − ), but their predictive power is limited by the lack of fundamental understanding of the relevant physical mechanisms and dynamics, which experiment alone cannot provide. During the past decade, atomistic simulation has become a useful tool to model and predict FIC in flowing polymer solutions and melts. ,− These simulations have led to new insight into FIC, but they have been limited by the inordinate amount of computational resources necessary to initiate and track flow-enhanced nucleation (FEN) events to eventual FIC over the long length and time scales associated with the crystallization kinetics, which are often several orders of magnitude larger than those associated with FEN.…”

A numerical treatment of crystallization in tube flow

“…The impact of both flow on crystallization and crystallization on flow is important, especially when we consider industrial processes, where the polymer is subjected to high stresses and strains. In the last decade, many of the theoretical ideas have converged on flow‐induced crystallization (FIC), connecting flow variables to crystallinity as detailed by Bushman and McHugh [10, 11], Eder and Janeschitz‐Kriegl [7], Zheng and Kennedy [12], Tanner and Qi [13], and more recently by Peters et al [14]. Despite the various ideas and significant amounts of research effort, there is still not a universally accepted theory as to the mechanisms that cause FIC and its full effect on rheology.…”