Synthesis and characterisation of symmetric 4-armed star copolymers of PS capped with butadiene and PI: importance of the location of the interface M. Moniruzzaman * , Ronald N. Young, J.P.A. Fairclough Department of Chemistry, University of Sheffield, Sheffield S3 7HF, England, UK Received 14 October 2003; received in revised form 9 March 2004; accepted 30 March 2004 Abstract This paper describes the synthesis of three well defined, near monodisperse, heteroarm star block copolymers named as PI 2 (B 13 PS) 2 , PI 2 (B 26 PS) 2 , and PI 2 (B 52 PS) 2 where PI and PS refer to polyisoprene and polystyrene, respectively, and B refers to butadiene. The subscript in B indicates the number of butadiene units. The morphologies determined by transmission electron microscopy of these polymers are discussed. The rheological properties were investigated and the results are compared with those of the related linear AB diblock and the hetero-4 arm star block copolymer PS 2 PI 2 with simple architectures and no butadiene content. The polymer with the lowest number of butadiene units showed the highest T ODT (200 8C) while the polymer with the highest number showed the lowest T ODT value (186 8C). This is believed to be due, in part, to the ability of the PI arms to drag the small section of polybutadiene into their own territory causing the fluctuation of the interface from the connection point, and which also minimises the monomer correlation near the junction point. In addition, since the two PS arms of a star molecule are no longer constrained to occupy adjacent positions on the interface, the star begins to behave like a pair of PI–PS diblocks flexibly connected at their centres. Rheology also reveals the influence of a change of the linking on the linear viscoelastic properties. On increasing the amount of butadiene, a shoulder present in the time – temperature superposition (TTS) master curve of PS 2 PI 2 having simple architecture is reduced in magnitude and almost disappears for the polymer having the highest number of butadiene units. q 2004 Elsevier Ltd. All rights reserved. Keywords: Monodisperse; Heteroarm; Interface 1. Introduction Block copolymers are important commercial materials of great scientific interest both to polymer chemists and physicists because of their model nature. Chemically joined two of more homopolymers result in the formation of block copolymers which, due to their incompatibility caused by different Flory –Huggins interaction parameter ðxÞ; undergo microphase separation from the disordered phase to a variety of ordered structures on the molecular scale of 5– 100 nm [1]. The segregation of these block into an interfacial region can create microdomains causing free energy minimisation [2]. Experimental and theoretical measurements of block copolymers demonstrate the exist- ence of three stable ordered morphologies in both the strong and weak segregation limits: alternating lamellae of A and B (LAM), hexagonally packed cylinders of A in a matrix of B (HEX) or the packing of spheres to give a body centre cubic (bcc) structure [3–5]. The actual microdomain formed is determined by the competition between A and B blocks as to which of them would have to pay the entropic penalty to stretch. A flat interface is created due to competition between an identical volume fraction of each block. However, a curved interface is entropically favoured if two blocks are of dissimilar volume fractions and hence the larger block relaxes on the convex side and the smaller block on the concave side. Non-linear block copolymers, also known as star branched copolymers, consist of three or more identical or different arms extended outwardly from a nucleus generated by a multifunctional linking agent 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.03.100 Polymer 45 (2004) 4121–4131 www.elsevier.com/locate/polymer * Corresponding author. Address: Engineering Systems Department, Royal Military College of Science, Cranfield University, Shrivenham, Swindon, SN6 8LA, UK. Tel./fax: þ44-1793-785682. E-mail address: m.moniruzzaman@cranfield.ac.uk (M. Moniruzzaman).