Flow-induced crystallinity in polyethylene as probed by time-resolved Raman spectroscopy and optical microscopy

The crystallization of polymer melts under flow is critically relevant to the polymer processing industry. Optical, thermal, and mechanical properties of semi-crystalline polymer products depend on the final crystallinity of the material, which depends on the structure and orientation of the initial molecular-scale crystal nuclei formed in flow. Here, we present a combination of optical microscopy and Raman scattering techniques to analyze structure formation in high density polyethylene during steady shear at low degrees of undercooling. When low shear rates ($\sim$ 1~s$^{-1})$ are imposed after a temperature quench to less than 10~$^{\circ}$C undercooling, fibrous structures aligned parallel to the flow direction appear over a period of minutes. Raman spectra indicate that chain straightening (consecutive \textit{trans} chain conformation) precedes the growth of the crystalline phase, and increased shear increases the rate of growth of the consecutive \textit{trans} and crystalline fractions. Complimentary Raman scattering experiments are performed on $n$-alkanes to compare conformational similarities in the molecular structure during the crystallization process.