Long-chain branched polymers to prolong homogeneous stretching and to resist melt breakup Gengxin Liu a, * , Hongwei Ma a, c , Hyojoon Lee b , Hongde Xu a, d , Shiwang Cheng a , Hao Sun a , Taihyun Chang b , Roderic P. Quirk a , Shi-Qing Wang a, * a Department of Polymer Science, University of Akron, Akron, OH 44325-3909, USA b Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea c School of Chemical Engineering, Liaoning Key Lab of Polymer Science and Engineering, Dalian University of Technology, Dalian 116024, China d SINOPEC Beijing Research Institute of Chemical Industry, Yanshan Branch, Beijing 102500, China article info Article history: Received 3 September 2013 Received in revised form 24 September 2013 Accepted 6 October 2013 Available online 14 October 2013 Keywords: Melt stretching Branch polymers Rheology abstract We explored a new synthetic strategy for ultra-high molecular weight long-chain branched (LCB) polymers with equal spacing between adjacent branch points. This method can synthesize LCB poly- styrene (LCB-PS) with total molecular weight of 4.9 million g/mole, 16 branches of 140 kg/mole and polydispersity index of 1.5. The introduction of multiple branch points with long side chains allows the LCB-PS to resist the elastic-driven decohesion. Even after a large step extension of stretching ratio l ¼ 7.4, the specimen would not undergo elastic breakup that occurs in linear PS even at l ¼ 2.7. These LCB-PSs are also extraordinarily more stretchable during startup uniaxial extension, with the maximum engi- neering stress emerging at stretching ratio l max z4 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi M bb =M e p , where M bb is the molecular weight of backbone and M e is the molecular weight between entanglements. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Long-chain branching (LCB) can greatly influence processing and rheological behavior of polymers as demonstrated in previous studies on low-density polyethylene (LDPE) [1e8]. Branched poly- mers show stronger “strain hardening” [7e10] in uniaxial extension and better processability in film blowing [11] than linear polymers. They are also important viscosity modifiers [12e14]. Since the chain architecture in LDPE is rather irregular [15], recent rheological studies focused on model LCB polymers with well-defined branching structures to study the dynamics of branches. These polymers, mostly polystyrenes and polydienes, have defined branch architectures such as hyper-branch [16e19], comb [20e25], H-shape [26e28], and pomepom [29e31] (arm q -backbone-arm q ,a long backbone connecting with another q arms). In this notation, an H-shaped chain is arm 2 -backbone-arm 2 , i.e., q equals 2. Grafting [20] usually cannot control spacing between two branch points thus the branch structure is rather irregular. There are mainly two methods to synthesize H-shape and pomepom polymers (mostly q ¼ 3). The first well-controlled synthetic method [24e27,30] uses bifunctional initiator to make a backbone with two living chain ends, which react with excess SiCl 4 (or MeSiCl 3 , etc. for H-shape) to form Cl 3 Siebackbone-SiCl 3 . Then, linear chains having one living chain end arm-Li are coupled to the Cl 3 Siebackbone-SiCl 3, resulting in a pomepom polymer, arm 3 -Si-backbone-Si-arm 3 . The second widely used method uses 4-(chlorodimethylsilyl) styrene (CDMSS) or its derivatives [22,23,28] to couple linear living chains with CDMSS, resulting in arm q -CDMSS-Li. Subsequently, half of the backbone grows from CDMSS-Li. Finally, a bifunctional coupling agent couples two of such asymmetric stars arm q -(backbone) 0.5 to form a pomepom chain, i.e., arm q -backbone-arm q . Although higher molecular weight of such LCB polymers can be made with these precise methods, no literature reported arms longer than 10 M e . McLeish and co-workers [32] used rheology and neutron scat- tering to study 4 model H-shaped polyisoprenes (arm length 20 to 60 k, backbone length 100 to 200 k, with k indicating kg/mole). The highest molecular weight H-shaped PI had four 60 k arms and one 200 k backbone, with PDI of 1.3. Archer and Juliani [31] carried out step strain tests on one pomepom of arm 3 -backbone-arm 3 with six 21 k arms and 89 k backbone and PDI of 1.2. Gary also used step strain tests to study a series of short-branched comb polymers [33]. Nielsen et al. [29] synthesized one arm 2.5 -backbone-arm 2.5 * Corresponding authors. E-mail addresses: gl15@zips.uakron.edu (G. Liu), swang@uakron.edu (S.-Q. Wang). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.10.007 Polymer 54 (2013) 6608e6616