Mesomorphic Pathway in Early Stage Crystallization of Polyethylene
Kalman D. Migler, Angela R. Hight Walker, Anthony P. Kotula
The kinetic pathway by which a molten polymer transforms into a multi-length scale semi-crystalline structure upon cooling is an unsolved problem in polymer physics, yet it is critical to the processing, properties and ultimate performance of these materials. Here we utilize Raman spectroscopy to probe the early stage crystallization kinetics of a high-density polyethylene during low undercooling. The importance of Raman as a tool stems from its ability to measure the mass fraction of conformations that are in consecutive trans sequences (locally straight) but are not in an orthorhombic configuration, which we term non-orthorhombic continuous-trans (NOCT). We first demonstrate the existence of the NOCT state by analyzing the Raman spectra of the rotator state of the n-alkane, C21H44. We then apply this concept to early stage crystallization kinetics of polyethylene and find that the Raman peaks indicative of NOCT precede the appearance of the peak which indicates orthorhombic crystallinity. We analyze the spectra within the context of a three-state model to extract the mass fractions of the orthorhombic and the NOCT states as crystallization proceeds. Concomitant birefringence and turbidity measurements indicate that this NOCT state can be understood as a separate mesomorphic phase which emerges from the melt state and precedes crystallization.