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Polymer Brushes: Efficient Synthesis and Applications Chun Feng and Xiaoyu Huang*
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Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China CONSPECTUS: Polymer brushes are special macromolecular structures with polymer chains densely tethered to another polymer chain (one-dimensional, 1D) or the surface of a planar (two-dimensional, 2D), spherical or cylindrical (threedimensional, 3D) solid via a stable covalent or noncovalent bond linkage. In comparison with the corresponding linear counterpart with similar molecular composition, one-dimension polymer brushes have some fascinating properties including wormlike conformation, compact molecular dimension, and notable chain end effects due to their compact and confined densely grafted structure. The introduction of polymer chains onto the surface of planar and spherical or cylindrical matrix will not only significantly change the surface-related properties of the matrix but also endows the obtained hybrid polymer brushes with new functionalities. Thus, polymer brushes are of great interest in the fields of polymer and material science due to their broad applications, such as catalysis, nanolithography, biomineralization, drug delivery, medical diagnosis, optoelectronics, and so on. Although a variety of 1D, 2D, and 3D polymer brushes have been prepared with the advent of living/controlled polymerization, the development of more efficient and facile synthetic protocols that permit access to polymer brushes with precisely controlled composition, structure, and functionality still represents a key contemporary challenge. In this Account, we summarize our recent efforts on the development of efficient methods to prepare 1D, 2D, and 3D polymer brushes and exploration of their potential applications in drug delivery, antifouling coating, catalysis, and lithium-ion batteries and also highlight related achievements by other groups. First, we briefly introduce the precedent examples of efficient synthesis of polymer brushes with different structures and functionalities by the combination of monomer design with living/controlled polymerization. Given the excellent tolerance and use of the same catalytic system without any mutual interference of ATRP and Cu-catalyzed alkyne−azide cyclization (CuAAC) click reaction, a versatile and efficient platform for precise synthesis of complex asymmetric (Janus-type) 1D polymer brushes was developed on the basis of the “trifunctional monomer” strategy without polymeric functionality transformation. Subsequently, a noncovalent strategy based on crystallization-driven self assembly to prepare well-defined polymer brushes with precise control over their composition and dimensions is described. Notably, the crystallization-driven self assembly can be treated as a living/controlled polymerization of “polymeric monomer” with a special building segment for crystallization, which allows for preparing linear polymer brushes with length as high as tens of micrometers. Moreover, the properties and related applications of polymer brushes as interesting building blocks for constructing hierarchical nanostructures, efficient drug deliver carriers, antifouling films, and lithium-ion batteries are addressed by some typical examples. These advancements in this field will provide a new avenue for obtaining fascinating polymer-brushbased functional materials.
1. INTRODUCTION
result in some fascinating properties, such as wormlike conformation, compact molecular dimension, and notable chain end effects. 2D and 3D polymer brushes are a class of hybrids with polymer chains densely attached onto the surface of various organic or inorganic matrices.2 The polymer chains will provide the matrix with exquisite properties such as corrosion protection, colloid stability, adhesive behavior, stimuli-responsiveness, lubrication and friction properties, and so forth. With great progress in polymer, organic, and supramolecular chemistry, various living/controlled polymerization methods,
With the advent of living/controlled polymerization, a great variety of unprecedented polymers and polymer/inorganic hybrids with intricate structures and functionalities have been prepared so as to explore new functional materials for meeting increasing and diverse requirements from industry and academia.1,2 In recent years, much attention has been paid to one-dimensional (1D), 2D, and 3D polymer brushes, where polymer chains are tethered onto linear polymers, planar surfaces, and spherical particles, respectively.1,2 1D polymer brushes are also called graft copolymers, macromolecular brushes, or molecular bottle-brushes, consisting of a linear backbone densely grafted with polymeric side chains.1 The confined and compact structure of 1D polymer brushes can © XXXX American Chemical Society
Received: June 26, 2018
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DOI: 10.1021/acs.accounts.8b00307 Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. Synthesis of 1D polymer brushes with a carboxyl backbone.
Figure 2. Synthesis of 1D polymer brushes by “trifunctional monomer” strategy.
controllable preparation. However, the emergence of living/ controlled polymerization and development of organic reactions enhances our capacity to circumvent this problem. Up to now, a large variety of 1D polymer brushes have been prepared with the combination of grafting-from, grafting-onto, and/or grafting-through strategies and living/controlled polymerization. 1 Poly(2-hydroxyethyl methacrylate) (PHEMA) was usually used as a backbone for 1D polymer brushes of which hydroxyls were utilized to introduce halogencontaining atom transfer radical polymerization (ATRP) initiating groups for the growth of side chains.3 Although it is a convenient method, the backbone can not be further functionalized because hydroxyls have been consumed. In 2006, our group developed a convenient method to prepare amphiphilic 1D polymer brushes with retaining ester groups (potential carboxyls) of the polyacrylate backbone.4 ATRP initiating groups were installed onto the α-carbon of ester groups of poly(methoxymethyl acrylate) (PMOMA) backbone by treatment with lithium diisopropylamide (LDA) and α-bromopropionyl bromide (Figure 1). The macroinitiator could be employed for growing poly(butyl methacrylate) (PBMA) side chains via the combination of grafting-from strategy and ATRP. Because the side chains were connected with the backbone by C−C bonds, instead of ester groups, the preserved ester groups can be easily hydrolyzed to carboxyls. This merit will not only make the backbone of formed polymer brushes hydrophilic but also allows for further functionalization. This strategy was also extended to synthesize heterografted amphiphilic centipede-like and starlike polymer brushes.5,6 However, the grafting density can not be achieved very high because a postpolymerization functionalization
organic reactions, and supramolecular strategies have been employed in the preparation of 1D, 2D, and 3D polymer brushes with excellent control over the composition, architecture, and length of tethered polymer chains and backbone of 1D polymer brushes.1,2 However, one has to admit the fact that the efficient and controllable preparation of polymer brushes is still a key issue and remains a challenge to some extent. In addition, we still need more information on the influence of structure (composition, architecture, and length) of tethered polymer chains and matrix of polymer chains attached on the properties of polymer brushes to optimize the performance of materials made from polymer brushes. We also need to prepare new polymer brushes and investigate their properties so as to explore new functional polymer brushes. In this Account, we focus on the contributions from our own group in the synthesis of polymer brushes and exploration of their potential applications. We begin by describing a “trifunctional monomer” strategy for the synthesis of 1D polymer brushes and then briefly discuss the methods developed for preparing 2D and 3D polymer brushes. In the last section, we highlight some applications of 1D, 2D, and 3D polymer brushes in the fabrication of nanostructures, drug delivery, antifouling film, and cathode material of lithium-ion batteries.
2. SYNTHESIS OF POLYMER BRUSHES 2.1. Synthesis of 1D Polymer Brushes
As is well-known, the confined and compact architecture of 1D polymer brushes consisting of dense side chains grafted onto a backbone via covalent or noncovalent interaction is considered to be the major troublesome hindrance for their efficient and B
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Figure 3. Construction of asymmetric molecular double brushes in a one-shot system via the combination of ATRP and CuAAC click reaction.
(ethylene glycol) (PA-g- PDMAEA/PEG), polyacrylate-gpoly(pentafluorophenyl methacrylate)/poly(ethylene glycol) (PA-g-PPFMA/PEG), and polyacrylate-g-polystyrene/poly(ethylene glycol) (PA-g-PS/PEG) in a one-shot fashion by concurrent grafting-from via ATRP and grafting-onto via CuAAC. This approach is a versatile and efficient platform for precise synthesis of complex asymmetric (Janus-type) 1D polymer brushes with high compatibility of ATRP and CuAAC with functionalities, good tunability on the chain length for ATRP and grafting density for CuAAC, and relative minimum number of reaction steps without polymeric functionality transformation. Ruthenium-mediated ring-open metathesis polymerization (ROMP) of exo-norbornene-based monomer has attractive characteristics including livingness, stability, and steric and functional group tolerance, which enables synthesizing Janusand block-type polymer brushes with ultrahigh molecular weights.16−18 Cheng et al. synthesized Janus-type polymer brushes by ROMP of exo-norbornene-functionalized macromonomer with a PEG and PLA side chain.16 Grubbs et al. reported the preparation of block-type polymer brushes by sequential ROMP of exo-norbornene- functionalized macromonomers with different side chains.17,18 Photonic crystals with precisely tunable wavelengths of reflected light can be fabricated by self-assembly of these polymer brushes with ultrahigh molecular weights.
strategy with relatively harsh conditions was employed for the introduction of initiating groups. In 2010, we reported a more efficient approach to prepare 1D polymer brushes with excellent control over the grafting density from 0 to 100% without polymeric functional group transformation (Figure 2).7 The key of the approach is the synthesis of a trifunctional monomer of tert-butyl 2-((2bromopropanoyloxy)methyl) acrylate with a polymerizable double bond, an ATRP initiating group, and a potential carboxyl moiety. Macroinitiators with tunable initiating group densities were then obtained by copolymerization with tertbutyl acrylate, which can be used for polymerization of monomers suitable for ATRP to construct a variety of 1D polymer brushes via grafting-from strategy.8,9 This method allows for better control over both grafting density and chain length of side chains. For further expanding the scope of the applicability of the “trifunctional monomer” strategy, an approach by the combination of grafting-onto strategy and atom transfer nitroxide radical coupling reaction (ATNRC) was developed. This method allowed for preparing 1D polymer brushes, polymer chains of which, not able to be prepared by ATRP directly, e.g., poly(propylene oxide) can be connected to the macroinitiator.10 In addition, we also prepared a new trifunctional monomer with a polymerizable double bond, a ring-opening polymerization (ROP) initiating group, and a potential carboxyl moiety (Figure 2).11 The obtained backbone was capable of initiating ROP of ε-caprolactone and lactide, respectively, for preparing 1D polymer brushes with biocompatible and degradable side chains and poly(acrylic acid) backbone.11,12 Moreover, we also prepared a trifunctional monomer bearing a polymerizable double bond, an ATRP initiating group, and hydroxyl functionality (Figure 2).13 This monomer can be employed to prepare polymer brushes with pendent hydroxyls capable of further functionalization in each repeat unit of the backbone without any postpolymerization modification.13,14 Given the excellent tolerance and usage of same catalytic system without any mutual interference of ATRP and Cucatalyzed alkyne−azide cyclization (CuAAC) click reaction, we first prepared a polymeric backbone from a trifunctional monomer consisting of a polymerizable double bond, an ATRP initiating group, and an alkynyl by RAFT polymerization (Figure 3).15 This backbone was then used for synthesizing a series of asymmetric (Janus-type) 1D polymer brushes, such as polyacrylate-g-poly((2-(dimethylamino)ether acrylate)/poly-
2.2. Synthesis of 2D Polymer Brushes
The grafting of polymeric chains onto the surface of 2D matrices is one feasible method to offer some merits of polymer brushes, e.g., antifouling property and stimuliresponsiveness, and synergetic merits such as processability and mechanical strength for improving the performance of 2D matrices and expanding the scope of potential applications.2 We recently developed a facile strategy to introduce polymer chains onto 2D matrices, e.g., graphene oxide (GO), by the combination of grafting-onto strategy and ATNRC reaction.19 Radical scavenger species of 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO) were first bonded to GO. Br-terminated poly(N-isopropylacrylamide) (PNIPAM) chains with controlled chain lengths obtained by single-electron transfer-living radical polymerization (SET-LRP) were then coupled with TEMPO of GO by ATNRC reaction. To offer more reactive groups for installation of polymeric chains onto the surface of GO, GO was first treated with tris(hydroxymethyl) aminomethane followed by reaction with 2-bromo-2-methylpropionC
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Figure 4. Synthesis of GiF- and GeF-based polymer brushes by photochemically mediated ATRP.23 Reprinted from ref 23. Copyright 2016, American Chemical Society.
Figure 5. Schematic illustration of (A) gold nanoparticles coated with PEG-b-PS25 and (B) preparation of ultrastable and biofunctionalizable lanthanide nanoparticles.26 Reprinted from ref 25 and 26. Copyright 2014 and 2017, American Chemical Society.
yl bromide.20 The Br content was ∼10-times that of functionalized GO sheets reported before,21 which meant that more polymer chains can be introduced. Overall, this method offers intriguing possibilities for preparing GO-based polymer brushes due to the robustness of ATRP, SET-LRP, and ATNRC. Fluorinated graphite (GiF) and fluorinated graphene (GeF) are special derivatives of graphite and graphene, respectively, which show attractive electrochemical and electronic properties. In comparison with graphite, graphene, and GO, which can be easily functionalized by covalent or noncovalent methods, surface functionalization of GiF and GeF is a great challenge due to the lack of aromatic systems for noncovalent interaction and inert C−F bonds on the surface. Inspired by our work on direct modification of poly(vinyl chloride) (PVC) by using inactivated C−Cl groups as initiating sites for graft polymerization,22 we found that C−F bonds of GiF and GeF can also be activated by photoredox catalyst of Ir(ppy)3 under blue light to serve as initiating sites for ATRP of methacrylatebased monomers (Figure 4).23 Given the pervasive nature of photopolymerization and broad application of GiF and GeF, this strategy is a promising tool for fabrication of functional materials from GiF- and GeF-based polymer brushes.
common method of attaching water-soluble homopolymers onto the surface of inorganic nanoparticles for offering them water solubility and colloidal stability, we developed an approach to improve the colloidal stability of inorganic nanoparticles by introducing an extra hydrophobic layer on the surface of inorganic nanoparticles. A series of PEG-b-PS amphiphilic diblock copolymers with a thiol group at the end of PS block were anchored onto the surface of gold nanoparticles (AuNP) to give PEG-b-PS-coated gold nanoparticles (AuNP@PEG-b-PS).25 In comparison with the AuNP covered with only PEG chains, the presence of hydrophobic PS layer not only significantly improved the stability of AuNP@ PEG-b-PS against electrolyte-induced aggregation and competitive displacement of dithiothreitol but also endowed AuNP@PEG-b-PS with excellent reusability in the reduction reaction of 4-NP into 4-AP without loss of catalytic activity (Figure 5A). It was found that the grafting density decreased from 4.2 ± 0.1 to 1.8 ± 0.1 (chains/nm2) as the average number of repeat units of PS increased from 12 to 65. Furthermore, both colloidal stability and catalytic activity increased with the decrease in PS chain length, resulting from a more compact coverage of polymer chains and a thinner PS layer, respectively. We also developed an inner layer cross-linking strategy to improve colloidal stability of lanthanide-containing nanoparticles, which offers great potential in biological applications due to their unique magnetic and upconversion fluorescent
2.3. Synthesis of 3D Polymer Brushes
For inorganic nanoparticles to be endowed with colloidal stability and functionality, surface modification with polymer is one efficient and practical strategy.24 Different from the D
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Figure 6. Construction of 1D noncovalent polymer brushes via “living” crystallization-driven self assembly.
properties (Figure 5B).26 PEG-b-PPFMA with pendent phosphonic acid groups were grafted onto NaGdF4:Yb,Er nanoparticles. Subsequently, nanoparticles covered by PEG-bPPFMA/phosphonic acid were treated with NH2-PEG-NH2 to form a cross-linker layer by virtue of high reactivity of amino groups with pentafluorophenyl esters. The outermost PEG brushes along with inner cross-linked PEG layer provided the nanoparticles with not only long-term colloidal stability in PBS buffer in the temperature range from 25 to 50 °C but also antifouling property against proteins, e.g., BSA. More importantly, the densely grafted and partially cross-linking PEG brushes also offered stability against the freeze-drying process for lyophilization. The freeze-dried nanoparticles can redisperse into aqueous media after stirring without any aggregation. In addition, the high reactivity of pentafluorophenyl ester to the amino group allows easy surface functionalization.
and side chains of polymer brushes, respectively. The length of covalent polymer brushes is normally less than 100 nm,16−18 whereas this method affords a facile strategy to prepare polymer brushes with length in the range of approximately 50 nm to 1.2 μm. In the self-seeding process, the regions of OPV domains with the highest crystalline order survive, and the rest of the OPV domains with lower crystallinity dissolve. Upon cooling, the surviving crystallites would act as nuclei to initiate growth by deposition of the dissolved polymer, leading to the elongation of polymer brushes.27 Therefore, the length increased with the annealing temperature; theoretically, the length of 1D polymer brushes can be precisely tuned by annealing temperature. This method was also applicable for OPV-b-PNIPAM with different chain lengths of PNIPAM and OPV-b-PDEAEMA to prepare 1D polymer brushes with different lengths. More importantly, we found that the ends of polymer brushes with an OPV backbone remain active toward further growth via crystallization or face-to-face packing of OPV motifs upon the addition of OPV-based copolymers into the solution of short seeds of polymer brushes with face-to-face packing OPV backbone.28 Significantly, the mean lengths of OPV backbone of polymer brushes were consistent with the predicted lengths as a function of the unimer-to-seed ratio. This phenomenon indicates that this process of seeded growth resembles “living” polymerization. Thus, the length of OPV backbone of polymer brushes can be precisely tuned by the unimer-to-seed ratio. Satisfyingly, the seeded growth strategy was also successfully employed to prepare A-B-A-type multiblock polymer brushes by adding OPV5-b-PNIPAM49 into the solution containing seeded polymer brushes of OPV5-b-PDEAEMA60 (Figure 6). Moreover, the strategy of “living” CDSA can also be used to prepare 1D and 2D polymer brushes with precise control over the dimensions and composition by Manners and co-workers.29
2.4. Synthesis of Polymer Brushes by a Noncovalent Strategy
Because the incorporation of noncovalent motifs into polymers will not only bring about extra functionalities of the motif but also endow the polymers with reversibility and stimuliresponsiveness, noncovalent interactions including hydrogen bonding, host−guest modulation, and π−π interaction have been widely utilized to connect side chains with backbone for preparing 1D polymer brushes. By taking advantage of the crystalline property of oligo(p-phenylenevinylene) (OPV) segment, we are able to prepare a series of 1D polymer brushes (fiberlike micelles) containing face-to-face packing OPV core and PNIPAM coronas with average lengths of approximately 40 nm to 1.2 μm via temperature-induced selfseeding approach (Figure 6).27 This kind of fiberlike micelles can be treated as 1D polymer brushes given the morphological similarities between 1D polymer brushes and fiberlike micelles, where the core of face-to-face packing OPV segments and corona of PNIPAM chains are considered to be the backbone E
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Figure 7. (A) Schematic illustration of the formation of vesicular and tubular nanostructures. TEM images of formed vesicle (B) and tube (C) from AuNP@ PS-b-PEG.33 Reprinted from ref 33. Copyright 2012, American Chemical Society.
3. APPLICATIONS OF POLYMER BRUSHES
sizes of 3−5 nm from 1D amphiphilic polymer brushes in solution.32 3D polymer brushes were employed as building blocks for well-defined nanostructures as well. Nie et al. prepared a series of spherical AuNP-based 2D polymer brushes (AuNP@PS-bPEG) by introducing linear PEG-b-PS with different chain lengths to the surface of AuNPs with different sizes (Figure 7A).33 A THF solution of AuNP@PS-b-PEG was first cast on a glass substrate to form a thin film. Subsequently, the rehydration of film was performed in water, a poor solvent for PS, with sonication or heating. They found that tubular or vesicular nanostructures, depending on the size of AuNPs and molecular weight of PEG-b-PS, can be obtained with precise control over interparticle distance of AuNPs (Figure 7B and C). The same group also attached PEG-b-PS onto the surface of gold nanorods (AuNRs) to afford AuNR@PS-b-PEG. These AuNR@PS-b-PEG 3D polymer brushes can form raftlike disks, giant vesicles, and spherical micelles by a microfluidic platform, where hydrodynamic flow was employed to control the kinetic aspects of the self-assembly process and the morphology and size of formed nanostructures.34
3.1. Building Blocks for Constructing Nanostructures
Amphiphilic copolymers, especially linear block copolymers, are widely used as building block for preparing a large variety of nanostructures with diverse morphologies including sphere, vesicle, and wormlike micelles.2 Similar to linear amphiphilic block copolymers, 1D amphiphilic polymer brushes normally self-assemble to form nanostructures with diverse morphologies. However, amphiphilic polymer brushes are able to demonstrate some special self-assembly behaviors due to their densely grafted structures compared to their linear analogues.1 For example, we found that PNIPAM-b-[poly(ethyl acrylate)-g-PDEAEMA] (PNIPAM-b-(PEA-g-PDEAEMA)) double hydrophilic copolymer was able to form unimolecular micelles consisting of a single PNIPAM core and PEA-g-PDEAEMA outer corona above 32 °C in an acidic environment (pH 2.0). 30 It is speculated that dense PDEAEMA side chains blocked the intermolecular chain entanglements upon the collapse of PNIPAM chains. Moreover, 1D polymer brushes were also employed to fabricate fibular and tubular structures by taking advantage of the wormlike configuration of polymer brushes. Muller et al. utilized amphiphilic 1D polymer brushes as templates to prepare organo-silica hybrid nanowires.31 Because both backbone and side chains of polymer brushes were prepared by ATRP, the length and diameter of hybrid nanowires could be tuned by changing the degree of polymerization of backbone and side chains.31 By using a similar strategy, Rzayev et al. reported the preparation of tubular structures with lengths of 36−87 nm, thicknesses of 5−7 nm, and pore
3.2. Application in Drug Delivery Carrier
Nanoparticles with appropriate size and surface properties can target tumors in the human body through the enhanced permeability and retention (EPR) effect by the virtue of abnormalities of tumor tissues. Polymeric nanoparticles are considered to be one of the most attractive drug delivery carriers. Given the biodegradability and biocompatibility of PLA, we used spherical micelles with sizes of 70−110 nm formed by self assembly of PEG-b-(PAA-g-PLA) 1D polymer brushes as drug delivery carriers of doxorubicin (DOX).12 The F
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Figure 8. (A) Schematic illustration of the structure of GO-PEG/PTX and relative cell viability of cells after treatment with different concentrations of PTX and GO-PEG/PTX for 72 h, (B) A-549, and (C) MCF-7 with different times at a constant PTX concentration of 40 nM.35 Reprinted from ref 35. Copyright 2014, American Chemical Society.
Figure 9. (A) Structure of PA-g-PFMA/PEG Janus-type asymmetric polymer brushes. (B) Schematic illustration of the mechanism for antifouling property of the film of PA-g-PFMA/PEG. Images of HaCaT cells grown on bare ITO (C) and the film of PA-g-PFMA/PEG (D).39 Reprinted from ref 39. Copyright 2017, American Chemical Society.
covalent method to load drugs.36 PTX was still used as a model drug, which was attached onto the chain end of 6-armed PEG by amidation reaction between terminated amino groups of 6armed PEG and carboxyls of modified PTX. Subsequently, PTX-terminated 6-armed PEG chains were introduced onto the surface of GO via the amidation reaction between the remaining amino groups of 6-armed PEG and carboxyls of GO to afford GO-PEG-PTX. Although GO-PEG-PTX and free PTX showed similar cytotoxicity against A549 and MCF-7 cells with a PTX concentration below 1 nM, GO-PEG-PTX showed an obviously higher cytotoxic effect on both cells than free PTX under similar conditions while the PTX concentration ranged over 5 to 160 nM. These results demonstrated that GO grafted with 6-armed PEG could be a promising drug delivery carrier for enhancing bioavailability of drugs by noncovalent or covalent strategies.
in vitro cytotoxicity of micelles on SMMC-7721 and SH-SY5Y cells showed that cell viability can be retained as high as 96% for the concentration up to 20 μM. GO has obtained increasing attention in the area of drug delivery due to its excellent biocompatibility and low toxicity. We developed an efficient method to introduce a high amount of PEG onto the surface of GO with sizes of 50−200 nm, where 6-armed PEG chains, instead of linear PEG chains, were covalently attached onto the surface of GO (Figure 8A).35 These GO-based polymer brushes (GO-PEG) showed longterm stability in PBS buffer and low cytotoxicity against A549 lung cancer and MCF-7 breast cancer cells. Paclitaxel (PTX) was loaded onto GO-PEG via π−π stacking and hydrophobic interactions to afford a nanocomplex of GO-PEG/PTX. GOPEG/PTX showed a much higher cytotoxicity on tumor cells than free PTX under the same conditions (Figure 8B and C), probably due to the enhanced PTX cellular uptake for GOPEG/PTX. To efficiently deliver the drugs not able to be loaded via π−π stacking and hydrophobic interactions, we also developed a
3.3. Application in Antifouling Coating
The prevention of nonspecific bimolecular and microorganism attachment on the surface is one of most important challenges G
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Figure 10. (A) Schematic illustration of the energy storage mechanism of G-g-PTMA/RGO electrode. Electrochemical performance of G-gPTMA/RGO cathode at (B) various current rates and (C) 0.04 A g−1 over 250 cycles.41 Reprinted from ref 41. Copyright 2016, American Chemical Society.
(Figure 9C and D). Furthermore, more than 45% of proteins and 70% of cells preattached can be released from the surface upon washing by PBS buffer. We speculated that the enhanced antifouling property might be attributed to a synergistic effect, where PEG chains endow the surface with a nonfouling characteristic and PFMA side chains offer fouling-release functionality (Figure 9B). The structure of polymer brushes was also found to affect the antifouling behaviors, where a longer backbone and PEG side chains would result in better resistance to the protein adsorption and cell adhesion.
in many applications ranging from biomedical devices to ship hulls.37 The spin-casting of a polymer film onto the surface of a matrix is considered to be an easy, convenient, and scalable strategy. Polymer film formed by spin-casting of amphiphilic polymers can mitigate biofouling effectively because compositional heterogeneities would discourage thermodynamically favorable interactions between the foulant and surface, leading to limited adsorption events. We infer that heterografted polymer brushes with hydrophilic and hydrophobic side chains randomly distributed along the backbone might be a better choice for surface coating due to their densely branched structures in comparison with linear counterparts.38 To test our assumption, we synthesized a series of asymmetric polymer brushes containing hetero-side-chain of hydrophobic PS and hydrophilic PEG and subsequently prepared uniform thin films by spin-casting. A low amount of proteins can be adsorbed onto the films, and very few HaCaT cells were able to grow on the polymer brush thin film as well. Moreover, it is interesting to note that the film fabricated from the polymer brushes with shorter PS side chains and higher PEG content led to a better antifouling behavior. Fluorinated polymers have a lower surface energy than hydrophobic PS, which has been proven to be a better building block to improve the antifouling property. Thus, we synthesized a series of Janus-type asymmetric polymer brushes of PA-g- PFMA/PEG with both PEG and poly(2,2,2trifluoroethyl methacrylate) (PFMA) side chains densely distributed on the same repeat unit along the polymeric backbone by the facile one-shot strategy (Figure 9A).15,39 The antifouling behaviors of films formed via spin-casting of PA-gPFMA/PEG brushes were examined by measuring the protein (BSA) absorption and cell (HaCaT cells) adhesion. The films of all Janus-type asymmetric polymer brushes demonstrated significantly higher antifouling property with much less protein adsorption and cell adhesion than those of the bare surface
3.4. Application in Lithium Ion Batteries
Graphene and reduced graphene oxide (RGO) have attracted increasing attention as next-generation energy storage materials for lithium ion batteries (LIBs) due to their high electronic conductivity, high specific surface area, and good mechanical and chemical stability. The formation of polymer brushes on the surface of graphene and RGO is an effective route to prevent RGO from restacking and endow graphene and RGO with new functionality by polymer itself or synergistic effect of the composite.40 Although the combination of radical polymers with graphene sheets was found to improve the electric conductivity of composite with preserving the high-rate charge storage capability of radical polymers,40 the capacities of composite electrodes was limited to a low level. On the basis of the strategy to prepare GO-based 2D polymer brushes,20,21 we introduced poly(2,2,6,6-tetramethylpiperidin-4-yl methacrylate) (PTMPM) chains onto the surface of RGO by surface-initiated ATRP followed by the oxidation of PTMPM to afford graphene-g-poly(2,2,6,6tetramethylpiperidin-1-oxyl-4-yl methacrylate) (G-g-PTMA) (Figure 10A).41 Subsequently, a composite cathode material (G-g-PTMA/RGO) containing G-g-PTMA and RGO was fabricated through a conventional dispersing-depositing process. The covalent connection between RGO and PTMA not only improved electron and ion transportation but also provided excellent dispersibility of RGO sheets without H
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restacking during the electrode fabrication process. Thus, a high specific capacity (466 mAh g−1) of G-g-PTMA/RGO, good cycle performance to the cathode with fast one-electrode redox reaction of PTMA, and enhanced surface Faradaic reaction of RGO were achieved (Figure 10B and C).
Chun Feng is an associate professor at SIOC. His current research interest focuses on the preparation of hierarchical nanostructures by the self assembly of crystalline-coil copolymers. Xiaoyu Huang is a professor at SIOC. His current research interest focuses on the synthesis, self assembly, and application of functional polymers.
4. CONCLUSIONS In this Account, we have summarized recent progress of preparation, properties, and applications of 1D, 2D, and 3D polymer brushes. A “trifunctional monomer” strategy was developed, which allows efficient synthesis of a large variety of 1D polymer brushes with controlled grafting density and lengths of backbone and side chain by the combination of diverse synthetic strategies, living/controlled polymerization methods, and efficient coupling reactions. In addition, a noncovalent route to fabricate 1D polymer brushes was also described by “living” crystallization-driven self assembly. Although recent works showed that this approach might just be applicable to certain copolymers with a crystallizable segment and several copolymers exhibited “living” characteristics over the self assembly,27−29 this method has still attracted increasing interest because it gives access to polymer brushes with a large dimension (μm scale), which is extremely important for the preparation of large-dimension functional materials. Although there are obvious advances in the preparation of 2D and 3D polymer brushes, efficient introduction of polymer chains with diverse functionalities onto the surface of different inorganic spherical and planar surfaces remains a challenge because of the chemical “inertness” of the surface. 1D polymer brushes can be an interesting building block or template in the fabrication of well-defined nanostructures given the fact that composition and length of backbone and side chain, and grafting density and distribution of side chain can be properly tuned as desired. It is anticipated that the selfassembly of 1D polymer brushes, instead of linear copolymers, could be a versatile approach for the preparation of hierarchical nano- or microstructures for diverse applications.16,42 As a unique class of functional materials, 2D and 3D polymer brushes combine the functionalities of matrix of inorganic nanostructures, such as magnetic, optical, electronic, and mechanical properties, and the properties of tethered polymers, including stimulus- responsiveness, superhydrophobicity, antifouling, and so forth; thus, they show great potential in an array of applications by taking advantage of their structurederived and synergetic properties. It is a safe bet that the field of polymer brushes will continue to be an exciting area, and we hope that this Account could be a trigger for more fascinating ideas in this field among the readership.
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ACKNOWLEDGMENTS We are thankful for financial support from the National Basic Research Program of China (2015CB931900), National Key R&D Program of China (2016YFA0202900), National Natural Science Foundation of China (21474127, 21504102, 21632009, 51773222 and 51873229), Strategic Priority Research Program of CAS (XDB20000000), Youth Innovation Promotion Association of CAS (2016233), and Shanghai Scientific and Technological Innovation Project (16JC1402500, 16520710300, 17DZ1205402, 18JC1410600, 18JC1415500 and 18520711900).
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[email protected]. ORCID
Chun Feng: 0000-0003-1034-9831 Xiaoyu Huang: 0000-0002-9781-972X Notes
The authors declare no competing financial interest. I
DOI: 10.1021/acs.accounts.8b00307 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.accounts.8b00307 Acc. Chem. Res. XXXX, XXX, XXX−XXX
