Core-Shell Cylindrical Polymer Brushes with New Properties: A Mini

Chapter 9

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Core-Shell Cylindrical Polymer Brushes with New Properties: A Mini-Review Jun Ling1,* and Axel H. E. Müller2,* 1MOE

Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China 2Institute of Organic Chemistry, Johannes Gutenberg Universität Mainz, 55099 Mainz, Germany *E-mail: [email protected]; [email protected]

We report new properties and applications of polymer and hybrid materials based on the one-dimensional topology of cylindrical polymer brushes (CPBs). We review three examples of core-shell CPBs to illustrate the applications resulting from stretched backbone, dense and stretched inner layer (core) of side chains, and huge number of chain ends of the outer layer (shell).

Introduction The properties of a polymer depend on many aspects, for instance, repeating units, composition and sequence of comonomers, molecular weight averages and distributions. Sometimes chain ends play a role as well. Topology, i.e., the geometry in which repeating units are connected, has a strong impact on the properties of polymers. In this mini-review, we summarize three unique properties derived from the topology of core-shell cylindrical polymer brushes (CPBs). Core-shell CPBs have a molecular backbone and dense side chains consisting of di- or triblock copolymers. Due to the length of the side chains and their steric hindrance, the backbone is stretched and a CPB is a molecule with a onedimensionally rod-like shape containing an inner (core) and an outer (shell) layer (1–3). This unique molecular conformation, which is unseen and unavailable in other materials, develops interesting properties after precise synthesis.

© 2015 American Chemical Society In Controlled Radical Polymerization: Materials; Tsarevsky, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


(1) Side Chains against Backbone Considering the intermolecular interaction energy of 2-12 kJ/mol, a carboncarbon covalent bond is extremely strong with an energy of 348 kJ/mol, about two orders of magnitude higher. It is amazing that a large number of interactions can organize to fight together against and finally “defeat” a single carbon-carbon bond under mild conditions. CPBs provide an ideal architecture for this “battle” since it contains a backbone and a large number of side chains. We reported controlled scissions of carbon-carbon covalent bonds in a CPB backbone caused by interactions between side chains and a substrate surface (4). With precise design, a series of core-shell CPBs with 750 side chains was synthesized (Figure 1) and two solid substrates, i.e. negatively charged mica and neutral silicon, were used. The inner layer of the CPBs was water-soluble poly(oligoethyleneglycol methacrylate) (POEGMA) blocks with DP of 430, which contain a large amount water molecules when cast on a solid substrate. The outer layer varied betwen nothing (b-[O430]750) as a control, poly(2-dimethylamino)ethyl methacrylate) (PDMAEMA) segments with DP of 40 (b-[O430D40]750), and its quaternized analog (b-[O430Dq40]750). As soon as the aqueous CPB solution was transferred, the surface interaction between the cationic outer layer of the CPB side chains and the solid substrate determined how strong the CPB molecules could be immobilized on the surface. Here, the outer layer PDMAEMA units acted as multiple “anchors” to prevent any further movement.

Figure 1. Chemical structures of the homopolymer CPB b-[O430]750 (A), the core-shell CPB b-[O430D40]750 (B) and the core-shell CPB b-[O430Dq40]750 (C) and a schematic illustration of the core-shell CPB (D). [Reproduced with permission from reference (4). Copyright 2013 American Chemical Society.] (see color insert) 128 In Controlled Radical Polymerization: Materials; Tsarevsky, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Upon drying, the inner layer of CPB released water molecules, producing a huge free energy of contraction. This situation might be resolved in two ways, either by stretching along the backbone or perpendicular to it. Since the backbone is already much stretched, the former would lead to a carbon-carbon covalent bond cleavage of the backbone. The latter required to overcome the surface interactions and to make the PDMAEMA units slip on the surface, i.e. pulling the “anchors” from the surface. Thus, a direct competition of the strength of one carbon-carbon covalent bond with a number of intermolecular interactions was established based on the CPB architecture. In fact, scissions of the CPB backbone were observed by means of atomic force microscope (AFM) when surface interaction was strong enough in all cases except those of b-[O430]750 on mica and b-[O430D40]750 on silicon. Moreover, the number of CPB scissions depended exclusively on the interactions between side chains and surface. Stronger interactions resulted in more fragments (Figure 2).

Figure 2. Dependence of scission of CPB backbones on various parameters. ECB and ESI stand for the energies of carbon-carbon bond, and surface inter-actions between side chains and substrate surface, respectively. [Reproduced with permission from reference (4). Copyright 2013 American Chemical Society.] (see color insert)

S. S. Sheiko et al. confined CPBs in a layer of liquid on a water or solid surface. The liquid could vary from organic solvent to bulk CPB melt with low glass transition temperature. Expansion of the liquid pushed the 3D-extended side chains into a denser 2D layer when all side chains intented to expend rather than contract in our cases, and move on surface (5–8). This confinement enhanced steric repulsion between the densely grafted side chains and resulted in scissions 129 In Controlled Radical Polymerization: Materials; Tsarevsky, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

of the backbone (7). Thus, backbones of homo-CPBs, ie. CPBs containing homopolymer side chains, broke in this situation, while they did not show any scission behavior in our cases. In addition, it should be mentioned that slowly increased confinment led to slow scissions and the parts randomly-ruptured from CPBs moved at the same time. It was impossible to distinguish which parts origined from one CPB molecule.


(2) Side Chains Working for the Backbone In a CPB, both backbone and inner layer (core) of side chains are extensively stretched due to steric repulsion. The cross-section of core-shell CPB looks like concentric cylinders wrapping the backbone in the center (Figure 3). It provides an opportunity to mimic the ring-like geometry of pigments in natural light harvesting systems of bacteria and plants also known as “energy cascade” architecture (9).

Figure 3. Illustration of nano-light harvester based on CPB topology and its cross-section. [Reproduced with permission from reference (10). Copyright 2014 Wiley-VCH Weinheim] (see color insert)

Förster resonance energy transfer (FRET) occurs when two chromophores, i.e. energy donor and acceptor, are located close to each other. The increase of donor-acceptor distance dramatically decreases FRET efficiency by 1/R (6). Taking advantage of the stretched inner layer of side chains in CPB, we can control the donor-acceptor distance precisely and thus their FRET efficiency. We reported core-shell CPBs consisting of energy donors in the stretched inner layer of side chains and energy acceptors along backbone acting as a rodlike “nano-light harvesters” (10). In this geometry (Figure 3), the energy donors 130 In Controlled Radical Polymerization: Materials; Tsarevsky, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


(fluorene units) absorbed light acting as antenna. They transferred their excitation energy via FRET to the acceptors (anthracene units) in backbone that emitted light of their characteristic wavelength acting as emitter. FRET directed from the core layer of the CPB into the backbone, a “concentrating effect”, similar to the natural “energy cascade” (10). Whereas the PDMAEMA shell only served the solubilization of the brush, the donor-acceptor distance was easily changeable by either physical or chemical ways. As a physical protocol, the hydrophobic inner layer expanded in organic solvent (THF) and contacted in water, which changed the distances of energy donors and acceptors. An inert PMMA spacer could be introduced between side chains and backbone to separate energy donors and acceptors. Both methods dramatically changed the efficiency of energy transfer.

(3) Crosslinking the Shell Locks Cargo in the Core Compared with micelles, CPBs have several advantages due to their unique topology. CPBs are single molecules constructed by chemical bonds with well-defined geometry and stable sizes, differing from self-assembled micelles suffering dynamic disassembly equilibrium and uncertainty of functional core and shell layers. Moreover, most micelles are spherical, although sometimes different shapes (cylinders, vesicles) may exist. In CPBs, layers from center (backbone) to outside (shell) are clearly distinguished. Core-shell CPBs are comparable to micelles, and core-shell-corona CPBs contain more potential in carrying cargos in various layers. Taking advantages of the one-dimensional structure, our group used CPBs as template and reported hybrid nanorods with iron oxide (11), titania (12), silica (13, 14), and more. A recent example reported the incorporation of rare-earth metal (RE) cations into the core of core-shell CPBs (15). After crosslinking the shell by silica, RE cations were “sealed” in the core and prevented from leaking out (

Core-Shell Cylindrical Polymer Brushes with New Properties: A Mini

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