Miscibility of hexene-LLDPE and LDPE blends: influence of branch content and composition distribution Ibnelwaleed A. Hussein a, * , Tayyab Hameed a , Basel F. Abu Sharkh a , Khaled Mezghani b a Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia b Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 2 February 2003; received in revised form 7 May 2003; accepted 28 May 2003 Abstract The influences of branch content (BC) and composition distribution (CD) of hexene linear low-density polyethylene (LLDPE) on its miscibility with low-density polyethylene (LDPE) were investigated. Ziegler – Natta (ZN) and metallocene-LLDPE (m-LLDPE) were used to study the melt miscibility using rheological tools. Dynamic, steady shear and transient measurements were carried out in an ARES rheometer at 190 8C. The miscibility was revealed by the dependence of their h o ; h 0 ðvÞ; G 0 ; and N 1 ð _ gÞ on blend composition. The CD of LLDPE has influenced its miscibility with LDPE. The ZN–LLDPE blend with LDPE was found to be more miscible than an m-LLDPE of the same M w and similar BC. On the other hand, a high-BC m-LLDPE (32.2 CH 3 /1000 C) was found to be more miscible with LDPE than a low-BC m-LLDPE (14.4 CH 3 /1000 C) of the same M w and MWD. The high-BC m-LLDPE blends with LDPE were partially miscible and immiscibility is likely to develop in LDPE-rich blends. Agreement was observed between the measured rheology and theoretical predictions of Einstein, Scholz et al., Palierne, and Bousmina emulsion models. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Polyethylene blend miscibility; Branch content; Composition distribution 1. Introduction Low-density polyethylene (LDPE) is usually blended with linear low-density polyethylene (LLDPE) to improve the processing of LLDPE. Their blends are widely used in film applications. So far, the blend miscibility of LLDPE and LDPE has received far less attention than blends of high-density polyethylene (HDPE) with LLDPE and LDPE [1–7]. Utracki and Schlund [8] found that a blend of LLDPE/ LDPE was immiscible; however, other blends of LLDPE/ LDPE were reported to be partially miscible [7,9–13]. Molecular parameters such as: molecular weight ðM w Þ and molecular weight distribution (MWD) were reported to have strong influence on miscibility of PE/PE blends [1–5, 11–15]. In addition, the details of molecular structure like branch content (BC) and composition distribution (CD) are important given that ‘pure’ Ziegler – Natta (ZN) LLDPEs can phase separate [16–18]. This raises questions about the use of ZN – LLDPE in most of the previous blend miscibility studies. It should be noted that ZN catalysis produces both linear and branched chains with more branches on smaller molecules [4,5,13,16 – 19]. The interactions of molecular parameters such as heterogeneity of structure and branch distribution make it very difficult to isolate the effects of BC and CD. The use of metallocene-LLDPE (m-LLDPE) is expected to isolate these interactions due to its intermole- cular chain homogeneity [16,17]. Also, m-LLDPE has almost uniform distribution of branches [18]. The effect of BC on the miscibility of blends of branched (LLDPE and LDPE) and HDPE was the subject of many investigations [2–5,11,15,20–26]. The previous work has mainly used transmission electron microscopy (TEM) and small angle neutron scattering (SANS). Although, there exists a basic disagreement over the issue of miscibility of HDPE with LLDPE or LDPE, it was agreed that BC of branched component was the major molecular factor that controls miscibility of HDPE/LLDPE and HDPE/LDPE blends [2–5,22–26]. Hill and coworkers [2,20 – 22] used DSC and TEM and reported that 60 branches/1000 C are required for HDPE/LLDPE blends to phase separate. While 0032-3861/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0032-3861(03)00437-3 Polymer 44 (2003) 4665–4672 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 966-3-860-2235; fax: þ966-3-860-4234. E-mail address: ihussein@kfupm.edu.sa (I.A. Hussein).