Article pubs.acs.org/Macromolecules
Sub-10 nm Nano-Organization in AB2- and AB3‑Type Miktoarm Star Copolymers Consisting of Maltoheptaose and Polycaprolactone Takuya Isono,† Issei Otsuka,‡ Yohei Kondo,† Sami Halila,‡ Sébastien Fort,‡ Cyrille Rochas,‡ Toshifumi Satoh,§ Redouane Borsali,*,‡ and Toyoji Kakuchi*,§ †
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-8628, Japan Centre de Recherches sur les Macromolécules Végétales (CERMAV, UPR-CNRS 5301), affiliated with the Université Joseph Fourier (UJF) and member of the Institute de Chimie Moléculaire de Grenoble (ICMG, FR-CNRS 2607), BP53, 38041 Grenoble Cedex 9, France § Division of Biotechnology and Macromolecular Chemistry, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan ‡
S Supporting Information *
ABSTRACT: The AB2- and AB3-type miktoarm star copolymers consisting of maltoheptaose (MH, as A block) and poly(ε-caprolactone) (PCL, as B block), namely MH-b(PCL)2 and MH-b-(PCL)3, were synthesized, and their nano-organization was characterized. The syntheses of MHb-(PCL)2 and MH-b-(PCL)3 were carried out through two reaction steps: (1) preparation of linear and three-branched PCLs bearing an azido group on the chain center (N3-(PCL)2 and N3-(PCL)3) by the diphenyl phosphate-catalyzed ringopening polymerization of ε-caprolactone (ε-CL) using azidofunctionalized di- and triols (N3-(OH)2 and N3-(OH)3) as the initiators and (2) the copper-catalyzed azide−alkyne cycloaddition of N3-(PCL)2 and N3-(PCL)3 with the ethynyl-functionalized MH. The miktoarm star copolymers having Mn for the PCL block of ca. 5000 (MH-b-(PCL2.5k)2 and MH-b-(PCL1.7k)3) and 10 000 (MH-b-(PCL5k)2 and MH-b-(PCL3.3k)3) were obtained with a very narrow polydispersity index of less than 1.05. Bulk samples of the four types of miktoarm star copolymers exhibited body-centered cubic phases, as determined by small-angle X-ray diffraction experiments. The domain-spacing were determined to be 9.8 nm for MH-b-(PCL2.5k)2, 8.8 nm for MH-b-(PCL1.7k)3, 10.5 nm for MH-b-(PCL5k)2, and 9.8 nm for MH-b-(PCL3.3k)3, which were smaller than those of the corresponding linear diblock copolymers.
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INTRODUCTION One of the most interesting properties of block copolymers (BCPs) is the self-assembly property of producing a variety of ordered periodic structures, such as lamellar, hexagonally closepacked cylinder, gyroid, and body-centered cubic phases, with a 10−100 nm length scale.1−3 Therefore, there are many efforts to control and characterize the BCP self-assembly from the viewpoint of applications for optical and electronic devices,4 and particular interest has focused on downsizing the microphase-separated structures for use in the next-generation lithography.5−7 Since the domain-spacing of a microphaseseparated structure (d) is a function of the degree of polymerization (N), reducing N of BCP is the rational choice to decrease the self-assembled feature size.8 It is important to note that to promote phase separation in a small BCP molar mass, the Flory−Huggins interaction parameter (χ) should be high enough to reach a critical value, χN > 10.5, for symmetric diblock copolymers.8 The BCPs consisting of saccharidic and synthetic polymer blocks, i.e., saccharide-based hybrid BCPs, are some of the promising candidates for the downsizing of d because they have high χ values due to the strong segregation © 2013 American Chemical Society
between the saccharidic and synthetic polymer blocks originating from their hydrophilic/hydrophobic imbalance. Saccharide-based hybrid BCPs were produced using several different strategies, and the end-to-end coupling of the saccharidic and synthetic polymer chain ends is one of the simple synthetic strategies.9 Although reductive amination between the reducing end of the saccharides and amino endfunctionalized polymers was utilized as the end-to-end coupling strategy, inefficient reaction conditions were required, such as a large excess of saccharide (5−100 equiv) and long reaction time (7−8 days).10,11 In contrast, the click reaction between ethynyl end-functionalized saccharides and azido end-functionalized polymers provided the end-to-end coupling strategy to afford the desired hybrid BCPs under mild experimental conditions without tedious purifications.12−15 More importantly, the click approach enabled us to synthesize a wide variety of hybrid BCPs in combination with clickable polymers prepared using Received: December 28, 2012 Revised: January 29, 2013 Published: February 6, 2013 1461
dx.doi.org/10.1021/ma3026578 | Macromolecules 2013, 46, 1461−1469
Macromolecules
Article
Scheme 1. Synthetic Pathways for Miktoarm-Type Hybrid BCPs Consisting of Maltoheptaose and Poly(ε-caprolactone), MH-b(PCL)2 and MH-b-(PCL)3
control their morphological behaviors. We now report the synthesis of an ABn miktoarm-type saccharide-based hybrid BCPs (n = 2, 3) consisting of maltoheptaose (MH, as A block) and poly(ε-caprolactone) (PCL, as B block) by utilizing the click reaction as the end-to-end coupling strategy, as shown in Scheme 1. The linear and 3-branched PCLs bearing an azido group on the chain center (N3-(PCL)2 and N3-(PCL)3) are respectively synthesized by the living ring-opening polymerization of ε-caprolactone (ε-CL) using azido-functionalized diand triols of N3-(OH)2 and N3-(OH)3 as the initiators. The miktoarm star copolymers, maltoheptaose-block-[poly(ε-caprolactone)]2 (MH-b-(PCL)2) and maltoheptaose-block-[poly(εcaprolactone)]3 (MH-b-(PCL)3), are synthesized by the CuAAC of N3-(PCL)2 and N3-(PCL)3 with N-maltoheptaosyl-3-acetamido-1-propyne (MH−CCH), respectively. In addition, the morphologies of MH-b-(PCL)2 and MH-b(PCL)3 in both the bulk and thin film states were studied using time-resolved small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM), respectively, together with the linear diblock copolymer of MH-b-PCL.
living polymerization methods. For example, we utilized the copper-catalyzed azide−alkyne cycloaddition (CuAAC) to synthesize a series of hybrid BCPs, such as maltoheptaoseblock-polystyrene16 and maltoheptaose-block-poly(p-trimethylsilylstyrene),17 which formed microphase-separated structures with a roughly 10 nm domain-spacing in a thin film and in the bulk. In addition, we achieved the 10 nm scale morphological control in maltoheptaose-block-poly(ε-caprolactone) (MH-bPCL), which was driven from the changes in the saccharide volume fraction due to thermal caramelization.18 Thus, there is a significant interest in the design and precise synthesis of macromolecular architectures based on saccharide-based hybrid BCPs capable of forming specific morphological structures. Some pioneering works on the morphological studies for BCPs with nonlinear structures, such as macrocyclic BCPs,19 dendritic-linear BCPs,20,21 (AB)n-type star BCPs,22,23 and miktoarm star copolymers,24,25 have shown that the nonlinear structures strongly affected their phase behaviors. In particular, miktoarm star copolymers, defined as the star-shaped polymers consisting of more than two chemically different polymer chains, are a relatively new class of macromolecular architectures, which have drawn considerable attention because they show unique and unusual morphologies due to their characteristic branched architectures.24,25 For example, the A2Btype miktoarm star copolymers of (polystyrene)2(polyisoprene) exhibited interesting morphological features either by moving the borders of the classical phase boundary of linear block copolymers or by forming new morphological structures,26 strongly indicating that the BCP self-assembled structures were controlled by tuning the polymer chain architectures. Hence, saccharide-based hybrid BCPs with miktoarm star shapes should become a powerful tool to
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EXPERIMENTAL SECTION
Materials. 6-Azido-1-O-tosylhexane (N3-OTs),27 (2,2,5-trimethyl1,3-dioxan-5-yl)methanol,28 pentaerythritol orthoacetate,29 and MH− CCH14 were prepared by following the reported methods. MH-bPCL5k and MH-b-PCL10k used for the AFM measurements were the same samples that were described in our previous report.18 Sodium azide and 60% sodium hydride were purchased from Tokyo Chemical Industry Co., Ltd. (TCI), and used as received. ε-Caprolactone (ε-CL) was purchased from TCI and purified by distillation from CaH2 under reduced pressure. Diphenyl phosphate (DPP) was purchased from TCI and dried under high vacuum before use. N,N,N′,N″,N″Pentamethyldiethylenetriamine (PMDETA) and copper(I) bromide 1462
dx.doi.org/10.1021/ma3026578 | Macromolecules 2013, 46, 1461−1469
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were purchased from Sigma-Aldrich Chemicals Co. and used as received. Dimethyl sulfoxide (DMSO), dry dimethylformamide (DMF, >99.5%; water content, 99.5%; water content, 99.5%; water content,
