Crystallization Kinetics in an Immiscible Polyolefin Blend

Motivated by the problem of brittle mechanical behavior in recycled blends of high density polyethylene (HDPE) and isotactic polypropylene (iPP), we employ optical microscopy, rheo-Raman, and differential scanning calorimetry (DSC) to measure the composition dependence of their crystallization kinetics. Raman spectra are analyzed via multivariate curve resolution with alternating least-squares (MCR-ALS) to provide component crystallization values. We find that iPP crystallization behavior varies strongly with blend composition. Optical microscopy shows that three crystallization kinetic regimes correspond to three underlying two-phase morphologies: HDPE droplets in iPP, the inverse, and cocontinuous structures. In the HDPE droplet regime, iPP crystallization temperature decreases sharply with increasing HDPE composition. For cocontinuous morphologies, iPP crystallization is delayed, but the onset temperature changes little with the exact blend composition. In the iPP droplet regime, the two components crystallize nearly concurrently. Rheological measurements are consistent with these observations. DSC indicates that the enthalpy of crystallization of the blends is less than the weighted values of the individual components, providing a possible clue for the decreased iPP crystallization temperatures.

Automated summary

Motivated by the issue of brittle mechanical behavior in recycled blends of HDPE and iPP, this study investigates the composition dependence of their crystallization kinetics using optical microscopy, rheo-Raman, and DSC. The results show that the crystallization behavior of iPP varies with blend composition, with three distinct two-phase morphologies observed. The crystallization temperature of iPP decreases sharply with increasing HDPE composition in the HDPE droplet regime, while it is delayed but shows little change in the onset temperature in cocontinuous morphologies. In the iPP droplet regime, the two components crystallize nearly concurrently. Rheological measurements support these findings. DSC analysis suggests that the enthalpy of crystallization of the blends is less than the weighted values of the individual components.