1 2 Microphase separation in diblock copolymers with amphiphilic block: 3 Local chemical structure can dictate global morphology 4 Alexei R. Khokhlov a,b , Pavel G. Khalatur b,c, * 5 a Physics Department, Moscow State University, Moscow 119899, Russia 6 b Department of Polymer Science, University of Ulm, Ulm D-89069, Germany 7 c Institute of Organoelement Compounds, Russian Academy of Science, Moscow 119991, Russia 8 9 11 article info 12 Article history: 13 Received 17 April 2008 14 In final form 17 June 2008 15 Available online xxxx 16 17 abstract 18 Using dissipative particle dynamics, we study microphase separation in the melt of amphiphilic/nonpolar 19 diblock copolymers in the strong segregation regime. We show that the phase diagram for the copoly- 20 mers with an amphiphilic block can be significantly different from that known for the conventional 21 diblocks. In the limit of significant amphiphilicity (surface activity), the resulting morphology corre- 22 sponds to thin channels and slits of amphiphilic units penetrating through the matrix of a majority non- 23 polar component. The physical reason behind this is connected with the surface activity of amphiphilic 24 monomers, which forces them to be located in the regions of maximum concentration gradient. 25 Ó 2008 Published by Elsevier B.V. 26 27 1. Introduction 28 Diblock copolymers consist of two linear blocks, poly-A and 29 poly-B, connected by a covalent chemical bond. If A- and B-blocks 30 are immiscible with each other, then in a melt of AB diblock 31 copolymers a microphase separation should take place, i.e., forma- 32 tion of alternating A-rich and B-rich microdomains. The theory of 33 microphase separation in block copolymers has been developed 34 in detail [1–7]. In particular, it was shown that for conformational- 35 ly symmetric diblock copolymers (‘symmetric’ means here the case 36 when both blocks have the same volume per monomer unit, Kuhn 37 segment length, etc.) the phase diagram for the melt has the form 38 shown in Fig. 1 [7]. Here, v is the Flory–Huggins parameter describ- 39 ing the repulsion between the components A and B, N is the num- 40 ber of monomers in the diblock macromolecules, and f is the 41 fraction of the units of type A. 42 In Fig. 1 one can see the regions of stability of ‘classical’ micro- 43 domain phases: A-spheres with close-packed symmetry (region 44 CPS, f < 1/2), A-spheres arranged in a body-centered cubic lattice 45 in B-matrix (region C, f < 1/2), A-cylinders arranged in a hexagonal 46 lattice in B-matrix (region H, f < 1/2), alternating A- and B-lamellae 47 (region L), B-cylinders hexagonally packed in A-matrix (region H, 48 f > 1/2), B-spheres arranged in a body-centered cubic lattice in A- 49 matrix (region C, f > 1/2), close-packed B-spheres in A-matrix (re- 50 gion CPS, f > 1/2), as well as the region of stability of gyroid phase 51 (region G), which was discovered more recently [8,9]. These or- 52 dered structures are key to the material properties which make di- 53 block copolymers of great technological importance. 54 The phase diagram shown in Fig. 1 is valid in the limit of infinite 55 chain length for structureless (‘bead-like’) monomers A and B 56 which repel each other. It is commonly believed that the micro- 57 phase-separated morphology is dictated only by the global features 58 of copolymers: their architecture and chemical composition, chain 59 length, interaction between segments, etc. In the present study we 60 will show that even the accounting for such rough structural fea- 61 tures of monomer units as their amphiphilicity can break the 62 universality of Fig. 1 and lead to emerging of new motifs for micro- 63 phase separation in the melt of diblock copolymers. 64 The simplest model of amphiphilic polymer chain was intro- 65 duced by us in the papers [10,11]. In this model, each amphiphilic 66 monomer is represented as a ‘dumbbell’ or ‘dipole’ (see, Fig. 2a). 67 The two beads (A and B) in the dumbbell are strongly repelling 68 each other, so that the amphiphilic unit prefers to be at the A/B 69 boundary, rather than in A- or B-bulk, i.e., this unit possesses a sig- 70 nificant surface activity and behaves as a surfactant, an interface 71 modifier. In contrast to conventional block copolymers, surfactants 72 are generally very strongly segregated, i.e., the interface separating 73 the domains formed by the polar groups from those built of the 74 hydrophobic segments is very sharp. It was shown that these fea- 75 tures can lead to a completely different self-organization of glob- 76 ules made from amphiphilic chains, or amphiphilic/nonpolar 77 copolymers [10,11]. Here we will show that the microphase sepa- 78 ration in the melt of diblocks shown in Fig. 2c (B-block linked to a 79 block of A–B dumbbells) significantly differs from that in the melt 80 of usual diblocks shown in Fig. 2b. To this end, we will use com- 81 puter simulations. 0009-2614/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.cplett.2008.06.054 * Corresponding author. Address: Department of Polymer Science, University of Ulm, Ulm D-89069, Germany. Fax: +49 731 50 31399. E-mail address: khalatur@germany.ru (P.G. Khalatur). Chemical Physics Letters xxx (2008) xxx–xxx Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett CPLETT 26207 No. of Pages 6, Model 5G 26 June 2008 Disk Used ARTICLE IN PRESS Please cite this article in press as: A.R. Khokhlov, P.G. Khalatur, Chem. Phys. Lett. (2008), doi:10.1016/j.cplett.2008.06.054