Experimental investigation of flow induced molecular weight fractionation phenomenon for two linear HDPE polymer melts having same Mn and Mw but different Mz and Mz+1 average molecular weights

Abstract

In this work, flow induced molecular weight fractionation for two linear HDPE polymers having the same polydispersity index (Mw/Mn) but different Mz and Mz+1 molecular weight averages, has been experimentally investigated. It has been revealed that first, onset as well as intensity of slip–stick instability and flow induced molecular weight fractionation are in direct relationship, i.e., low molecular weight polymer chains start to fractionate from the main polymer melt stream at the onset of slip–stick flow regime initiating formation of low viscosity layer at the die wall, which leads to its consequent unwanted accumulation at the die lips–die drool phenomenon. Second, due to enhanced melt elasticity (by the presence of long linear chains of linear HDPE polymer), the flow induced molecular weight fractionation is more effective which results in narrow molecular weight distribution of die drool sample containing small amount of long chains. Third, buildup ratio sensitivity to apparent shear rate is the highest in the slip–stick regime and its value increases with increased content of very long chains. Finally, the flow induced molecular weight fractionation takes place only in a very thin layer near the die wall (i.e., within less than 8% of the channel radius for the studied processing conditions and HDPE polymer samples).