Effect of Short Chain Branching of LDPE on its Miscibility with Linear HDPE Tayyab Hameed, a Ibnelwaleed A. Hussein* Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Fax: 860-4234; E-mail: ihussein@kfupm.edu.sa Received: June 20, 2003; Revised: September 24, 2003; Accepted: September 29, 2003; DOI: 10.1002/mame.200300173 Keywords: blends; branch content; miscibility; polyethylene; rheology Introduction High-density polyethylene (HDPE) has a linear structure with very little or no branching and possesses a high degree of crystallinity, but it is difficult to process. Low-density polyethylene (LDPE) however has a chaotic structure with short and long branches and is easy to process. Blending of polyethylenes (PEs) is a common industrial practice. The principal reasons behind the blending of PEs are property modification and improvement of processability. [1] The miscibility of different PE blends has been a subject of great interest for the past few decades [2–16] due to its direct impact on the processing and final properties of blends. PE blends were reported to be miscible, partially miscible or immiscible depending on molecular parameters such as: molecular weight ( M w ), molecular weight distribution (MWD), branch content (BC) and distribution and com- position distribution (CD). [16–21] Martinez-Salazar and co-workers [18,19] used differential scanning calorimetry (DSC) and correlated thermal proper- ties of HDPE/LDPE systems to the branch content of LDPE. In addition, miscibility was suggested for blend components with BC less than 20 CH 3 /1 000 C. Also, Alamo and co-workers [7,8] used small-angle neutron scat- tering (SANS) and found LDPE/HDPE blends to be com- pletely miscible in the melt in the whole composition range. The LDPEs used in the SANS study had SCB and long chain branches (LCB) in the range 13–16 and 2–3 CH 3 / 1000 C, respectively. However, the MWDs of the LDPEs were broader than those of the HDPEs. On the other hand, Hill and co-workers [2,6] used Transmission Electron Microscopy (TEM) and DSC and reported immiscibility of HDPE and LDPE blends in the melt state. The Summary: The influences of short chain branching (SBC) on the melt miscibility of low-density polyethylene (LDPE) with linear high-density polyethylene (HDPE) were investi- gated by rheological methods. Two LDPE resins with dif- ferent branch contents were blended with the same linear HDPE. Dynamic and steady shear measurements were carri- ed out using a Rheometrics ARES rheometer at 190 8C. The rheology of the low-SCB LDPE (9 CH 3 /1 000 C) blends with HDPE can be predicted by the linear additivity rule. Hence, blends were suggested to be completely miscible at all com- positions. However, blends of the high branch content LDPE (SCB ¼ 19 CH 3 /1 000 C) were completely immiscible. Also, the different viscous and elastic properties of all the immiscible blends were much higher than the corresponding values for the more viscous and elastic blend component. The ratio of interfacial tension to droplet radius was estimated from Scholz et al. model as 1 500 N  m 2 . The level of SCB in LDPE was found to have a strong influence on its miscibility with linear HDPE. Cole-Cole plot for blends of LDPE1 with HDPE. Macromol. Mater. Eng. 2004, 289, 198–203 DOI: 10.1002/mame.200300173 ß 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 198 Full Paper a Current address: Center for Refining & Petrochemicals, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia.