Self-assembly of a liquid crystal ABA triblock copolymer in a nematic liquid crystal solvent Mohammad Tariqul Islam a , Tahseen Kamal a , Taegyu Shin b , Baekseok Seong b , Soo-Young Park a, * a Department of Polymer Science, Kyungpook National University, #1370 Sangyuk-dong, Buk-gu, Daegu 702-701, Republic of Korea b Korea Atomic Energy Research Institute, Neutron Science Division, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic of Korea article info Article history: Received 18 January 2014 Received in revised form 10 May 2014 Accepted 4 June 2014 Available online 11 June 2014 Keywords: Liquid crystal triblock copolymer Nematic liquid crystal solvent Self assembly abstract An ABA type triblock copolymer, consisting of liquid crystalline polymer (LCP, poly(4-cyanobiphenyl-4- oxyundecylacrylate)) ‘A’ end blocks and a deuterated polystyrene (dPS) ‘B’ mid block (LCPedPSeLCP) was successfully synthesized by atom transfer radical polymerization (ATRP). The number average mo- lecular weight (M n ) of LCPedPSeLCP was LCP (7.1 K)edPS (19.4 K)eLCP (7.1 K) with a polydispersity index (PDI) of 1.41. LCPedPSeLCP was self-assembled in a nematic liquid crystal solvent of 4-pentyl-4 0 - cyanobiphenyl (5CB) into spherical micelles with a LCP corona and a dPS core, in which dPS was folded to produce a V-shape structure. Micellar structures of LCPedPSeLCP in 5CB were examined by small angle neutron scattering at various block copolymer concentrations and temperatures using a curve fitting method. The critical micelle concentration was 0.25 wt% and the self-assembled micelles dissociated into unimers at 33  C, which is lower than the nematic to isotropic transition temperature (T ni ) of 5CB (36  C). The entropic penalty imposed on dPS by the ordered nematic state of the 5CB solvent caused phase separation of the flexible dPS block to form micelles, which vanished above the T ni of the 5CB solvent. Magnetic field-induced global orientation of 5CB revealed the structure of the dPS core of the micelle to be prolate (an elongated sphere) oriented with its long axis along the direction of the applied magnetic field. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Small-molecule nematic liquid crystals (NLCs) are particularly interesting solvents for polymers because they can undergo a first- order transition between ordered (nematic) and disordered (isotropic) phases. Furthermore, their Flory interaction parameters (c) change discontinuously at the nematic to isotropic transition temperature (T ni ) [1]. NLCs tend to orient cooperatively in a preferred direction, which gives to useful properties, such as, high birefringence, excellent dielectric anisotropy, and orientation elasticity, which are forbidden by symmetry in isotropic liquids. The coupling of order with fluidity makes NLCs intriguing materials because their orientationedependent properties can be influenced using readily accessible external fields. Block copolymers (BCPs) composed of liquid crystal polymer (LCP) and isotropic polymer blocks provide an attractive basis for the development of new functional materials due to the unique functionalities of LCs, which can be introduced via LCP blocks into the self-assembled structures of BCPs [2,3]. In a dilute selective solvent for one block, BCPs self-assemble into micelles, comprised of a dense insoluble core and a less-dense soluble shell [4]. To control the self-assembled micellar structures of BCPs in a solution, the balance between interfacial energy, repulsion between soluble blocks in the shell, and the entropy penalty of insoluble blocks packed in the core must be manipulated [5]. For a BCP with a given chemical structure, it has been demonstrated that adjusting BCP composition and solution conditions could effectively change self- assembled micelle morphology [6e8]. One way of controlling solvent quality is by mixing two selective solvents. Our group studied the micelle structures of poly(styrene-b-vinyl4pridine) (PS-b-P4VP) in toluene/ethanol mixtures, which covered PS- selective, neutral, and P4VP-selective solvents, by varying the mixing ratio [9e12]. Changing the solvent temperature provides another way of controlling solvent selectivity, because c is also a function of temperature. An increase in solvent temperature will decrease c monotonically for common organic solvents in an upper * Corresponding author. Tel.: þ82 53 950 5630; fax: þ82 53 950 6623. E-mail address: psy@knu.ac.kr (S.-Y. Park). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer http://dx.doi.org/10.1016/j.polymer.2014.06.009 0032-3861/© 2014 Elsevier Ltd. All rights reserved. Polymer 55 (2014) 3995e4002