Inﬂuence of Molecular Structure on the Rheology and Thermorheology of Metallocene Polyethylenes Ibnelwaleed A. Hussein, 1 Tayyab Hameed, 2 Michael C. Williams 3 1 Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia 2 Center for Reﬁning and Petrochemicals, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia 3 Department of Chemical and Materials Engineering, University of Alberta, Edmonton T6G 2G6, Canada Received 23 November 2005; accepted 1 January 2006 DOI 10.1002/app.24353 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The rheology of linear and branched metal- locene polyethylenes (m-PEs) was investigated. The linear metallocenes were prepared by gas-phase polymerization, while the branched PEs were commercial resins. Molecular parameters such as M w , branch type, and molecular weight distribution have inﬂuenced the viscoelastic behavior of both linear and branched PEs, whereas branch content (BC) had little inﬂuence on viscoelastic properties. Plots of log G' versus log G revealed the effect of comonomer type on the viscoelastic behavior of m-PEs. Flow activation energy (E) was found to be sensitive to both M w and BC. Also, E for ethylene-octene copolymers was observed to be always higher than the butene counterparts, which have been caused by the increase in molar volume of the repeating unit. For the effect of BC on E, different trends were ob- served for octene and butene m-LLDPEs. © 2006 Wiley Peri- odicals, Inc. J Appl Polym Sci 102: 1717–1728, 2006 Key words: branch content; comonomer type; molecular weight; thermorheology; activation energy; metallocene polyethylene INTRODUCTION Molecular architecture is known to have a strong in- ﬂuence on the solution, melt, and solid state properties of polyethylenes (PEs). 1–13 Attempt for controlled syn- thesis of PEs has been a very hot activity since the discovery of this material. A good review on the sub- ject is given elsewhere. 1,2 Molecular structure has been documented to affect both the processing and end-use properties. 3–13 Molecular parameters such as M w , mo- lecular weight distribution (MWD), short chain branching (SCB), long chain branching (LCB), distri- bution of branches, and type of branching (comono- mer type) all have been found to affect the rheological and solid state properties. 3– 8,10 –13 Linear low density PE (LLDPE) is a copolymer of ethylene and an -oleﬁn such as butene, hexene, or octene. They were ﬁrst produced by Ziegler–Natta (ZN) heterogeneous cata- lyzes, whose origins can be traced back to the late 1950s. 2 Many studies that previously examined the inﬂuence of molecular architecture used ZN- PEs. 11,13,14 –16 However, these LLDPE resins are known for their compositional heterogeneity. The occurrence of fairly complex comonomer distributions is a basic attribute of Ziegler–Natta LLDPE (ZN-LLDPE) resins. Temperature rising elution fractionation (TREF) of these copolymers revealed that one could obtain two or three regimes within the comonomer distribution (high, medium, and very low density). 16,17 The devel- opment of single site catalysts have made possible the synthesis of PEs with controlled architecture. These polyoleﬁns are catalyzed with what are alternatively called metallocene, or single site catalysts, and have uniform comonomer distribution and narrow MWD. More recent studies appearing on the structure–prop- erty relationships have used these PEs. 3,4,7,9 –12 A very important aspect in these investigations is the isolation of the interaction of molecular parameters controlling a speciﬁc property. This could not be achieved by ZN-LLDPEs. Wood-Adams et al. 3 carried out a detailed investi- gation on the inﬂuence of M w , MWD, and SCB on the linear viscoelastic behavior of m-PEs that had LCB. Linear as well as branched PEs was studied. An in- crease in M w was reported to cause an increase in zero-shear viscosity ( 0 ) and decrease in the shear rate at which shear thinning begins. The  0 (M w ) data was successfully ﬁtted to an expression  = KM w  ; with K = 6.8  10 -15 (Pa s/[g/mol] 3.6 ) and  = 3.6. On the other hand, increasing the MWD distribution was re- ported to broaden the transition zone between New- tonian plateau and the power law zone. Interesting observations were made regarding the inﬂuence of LCB on the viscoelastic behavior of PEs. Increasing Correspondence to: I. A. Hussein (ihussein@kfupm.edu.sa). Journal of Applied Polymer Science, Vol. 102, 1717–1728 (2006) © 2006 Wiley Periodicals, Inc.