Nonlinear rheological behavior and flow-induced crystallization of low-entangled UHMWPE/HDPE melt blends: Insights from transient shear and extensional flows

The molecular blending achieved by the incorporation of up to 20 wt. % of UHMWPE via melt blending, facilitated by its initially low-entangled nature (dis-UH), is assessed through nonlinear transient shear and extensional rheology. The findings demonstrate that by adding dis-UH, the rheological properties of the blends are enhanced significantly. In a nonlinear shear flow, indicated by the stress ratio (σpeak/σsteady) in comparison to monodisperse and polydisperse solution blended polystyrene, the blends exhibit low deformability. However, increasing the dis-UH fraction leads to greater polymer network stretch, evidenced by a notable increase in maximum strain (γmax) from 2.3 to 3.3. Additionally, the slowdown of polymer dynamics, with increasing dis-UH fraction, is reflected in the long-time stress relaxation rate after shear cessation. Under uniaxial elongational flow, increasing the dis-UH fraction up to 20 wt. % promotes strain hardening (SH) in the vicinity of melting point. The cause of SH is attributed to flow-induced crystallization (FIC), at relatively low supercoolings. Finite extensibility analysis reveals that the onset of SH is triggered at a constant strain of ∼2.63, driven by the stretching of entanglement strands, followed by crystallization. Structural analysis using scanning electron microscopy, wide-angle x-ray diffraction, and small-angle x-ray scattering provides strong evidence of shish-kebab structure formation resulting from FIC. The quantification of long period (Lkebab), orientation degree of the (110) diffraction crystal plane S(110), and kebab structure’s orientation (Skebab) reveals enhanced orientation by increasing both elongational strain and dis-UH content. On comparing Skebab as a function of stress at quench normalized by the entanglement number (Z), we establish the consistency of our findings with previous studies, validating the successful molecular blending of a high fraction of intractable UHMWPE via melt blending, facilitated by its initially low-entangled nature.