When Flow Creates Order: Nematic Alignment and Form Birefringence in Shear for High-Density Polyethylene Melts

Making use of the exponential character of the high molecular weight tail of high-density polyethylene allows the quantification of the fraction of chains that get stretched in shear flow, which tracks with measured birefringence and Raman intensity. At high enough shear rates, apparent viscosity and birefringence show the shear rate dependence of chains that exhibit nematic alignment. In the nematic regime, a strong increase in birefringence and simultaneous strong decrease in primary normal stress difference mean there is a failure of the linear stress-optical rule, suggesting that form birefringence results from nematic alignment, which was confirmed using optical techniques.