Molecular Brushes Research Articles

Computer simulations of the mechanical response of brushes on the surface of cancerous epithelial cells.

We report a model for atomic force microscopy by means of computer simulations of molecular brushes on surfaces of biological interest such as normal and cancerous cervical epithelial cells. Our model predicts that the force needed to produce a given indentation on brushes that can move on the surface of the cell (called “liquid” brushes) is the same as that required for brushes whose ends are fixed on the cell’s surface (called “solid” brushes), as long as the tip of the microscope covers the entire area of the brush. Additionally, we find that cancerous cells are softer than normal ones, in agreement with various experiments. Moreover, soft brushes are found to display larger resistance to compression than stiff ones. This phenomenon is the consequence of the larger equilibrium length of the soft brushes and the cooperative association of solvent molecules trapped within the brushes, which leads to an increase in the osmotic pressure. Our results show that a careful characterization of the brushes on epithelial cells is indispensable when determining the mechanical response of cancerous cells.

Graft Density Dependence of Main Chain Stiffness in Molecular Rod Brushes

The graft density dependence of main chain stiffness in rod brushes composed of a flexible linear polystyrene (PS) main chain and poly(n-hexyl isocyanate) (PHIC) rod side chains has thoroughly been investigated by static light (LS) and small-angle X-ray scattering (SAXS) measurements in tetrahydrofuran (THF) at 25 °C. A series of statistical graft copolymers having different graft densities (σ) and graft chain lengths (PS-g-HIC-Ns-lg, where Ns is the weight-averaged degree of polymerization of HIC and lg is the average distance (spacing) of the main chain between the side chain joints) were prepared by the nearly azeotrope radical copolymerizations of styrene (M1) with PHIC macromonomers (M2) (r1 = 0.84 ± 0.1 and r2 = 1.1 ± 0.3) in n-hexane or the bulk at 60 °C. The compositional heterogeneities of the graft copolymers were carefully characterized by the ratio of the UV to RI signal in the SEC traces and the comparison with those predicted from the theory by Stejskal and Kratochvil [Macromolecules 1987, 20, 2624], implying that they were sufficiently small enough to allow one to study their dimensional characterizations. The graft density dependence of the root-mean-square cross-section radius of gyration (⟨Sc2⟩1/2) and the z-averaged root-mean-square radius of gyration (⟨S2⟩z1/2) of PS-g-HIC-Ns-lg was studied and rationalized as the function of σ and Ns. The σ-dependence of ⟨Sc2⟩1/2 could be quantitatively described by the theory for the semiflexible comb whose main and side chains have different chain stiffness. The weight-averaged degree of polymerization of the main chain (NM) dependence of ⟨S2⟩z1/2 was also quantitatively described by the cylindrical wormlike chain model. It was found that the change in the main chain stiffness (λb–1) resulting from the interactions among the side chains increases in proportion to the scaling law of λb–1 ∝ Ns1σ1.

Chromatographic Analysis of Structural Elements of Molecular Polyimide Brushes with Polymethacrylate Side Chains

The method of thin-layer chromatography under critical conditions was used for the analysis of the functional and structural inhomogeneities of polymethylmethacrylate and poly-tert-butylmethacrylate side chains cleaved from molecular polyimide brushes. Acetic acid–containing mobile phases are suggested, allowing a gradual transition from adsorption to exclusion chromatographic regime to be observed upon variation of the mobile phase composition for both cleaved side chains and “reference” polymethylmethacrylate and poly-tert-butylmethacrylate linear polymers. The presence of a few kinds of cleaved side chains having one terminal carboxylic group and differing in the nature of the other terminal group is revealed.

Methods of controlled radical polymerization for the synthesis of polymer brushes

Methods for the synthesis of polymer (molecular) brushes via various controlled-radical-polymerization approaches are considered, and their properties are analyzed. Special attention is paid to atomtransfer radical polymerization, which is most frequently used for these purposes. The potential of the synthesis of molecular brushes with various chemical structures, including amphiphilic brushes, is demonstrated in many examples.

Well-Defined Polyethylene-Based Random, Block, and Bilayered Molecular Cobrushes

Novel well-defined polyethylene-based random, block, and bilayered molecular cobrushes were synthesized through the macromonomer strategy. Two steps were involved in this approach: (i) synthesis of norbornyl-terminated macromonomers of polyethylene (PE), polycaprolactone (PCL), poly(ethylene oxide) (PEO), and polystyrene (PS), as well as polyethylene-b-polycaprolactone (PE-b-PCL), by esterification of the hydroxyl-terminated precursors (PE, PCL, PEO, PS, and PE-b-PCL) with 5-norbornene-2-carboxylic acid and (ii) ring-opening metathesis (co)polymerization of the resulting macromonomers to afford the PE-based molecular cobrushes. The PE-macromonomers were synthesized by polyhomologation of dimethylsulfoxonium methylide, while the others by anionic polymerization. Proton nuclear magnetic resonance spectroscopy (1H NMR) and high-temperature gel permeation chromatography (HT-GPC) were used to imprint the molecular characteristics of all macromonomers and molecular brushes and differential scanning calorimetry (DSC) for the thermal properties. The bilayered molecular cobrushes of P(PE-b-PCL) adopt a wormlike morphology on silica wafer as visualized by atomic force microscopy (AFM).

Synthesis and phase behavior of a polynorbornene‐based molecular brush with dual “jacketing” effects

ABSTRACTIn this study, a series of well‐defined liquid crystalline molecular brushes with dual “jacketing” effects, polynorbornene‐g‐poly{2,5‐bis[(4‐methoxyhenyl)oxycarbonyl] styrene} (PNb‐g‐PMPCS), were synthesized by the “grafting through” method from ring opening metathesis polymerization of α‐norbornenyl‐terminated PMPCS. The rigid PMPCS side chain was synthesized by Cu(I)‐catalyzed atom transfer radical polymerization initiated by N‐[(2‐bromo‐2‐methylpropanoyl)ethyl]‐cis‐5‐norbornene‐exo‐2,3‐dicarboximide. The chemical structures of the molecular brushes were confirmed by 1H NMR and gel permeation chromatography (GPC), and the thermal properties were studied by thermogravimetric analysis (TGA). GPC results reveal that the molecular brushes have relatively narrow polydispersities. TGA results show that the molecular brushes have excellent thermal stabilities. The PMPCS side chains in all the molecular brushes form the columnar nematic liquid crystalline phase, which is a little different from the behavior of linear PMPCS possibly due to the confinement or other effects of the brush architecture which leads to decreased order. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2116–2123

An investigation of the morphology of Langmuir films based on molecular polyimide brushes containing magnetite nanoparticles

Molecular brushes having a polyimide (PI) backbone with degree of polycondensation n = 33 and polymethylmethacrylate (PMMA) side chains with two different degrees of polymerization (m = 63 and m = 114) were synthesized by the method of controlled atom transfer radical polymerization. Hydrophobic magnetite nanoparticles of a size of 18 ± 2 nm were prepared. Langmuir monolayers on the basis of the polyimide brushes and composite monolayers, containing magnetite nanoparticles with a hydrophobic surface, were formed at the water/air interface. It is found that, in the condensed state of the monolayer at the surface pressure values from 25 to 40 mN/m, the limiting surface area A 0 per side chain of a brush grows with an increase in the length of PMMA side chains of polymer brushes almost by a factor of 2: from A 0 = 744 ± 64 A2 for PI-graft-PMMA-63 to A 0 = 1644 ± 50 A2 for PI-graft-PMMA-114. Increasing the magnetite solution aliquot mixed with the polymer brush solution at the ratios from 1 : 2 and 1 : 1 to 2 : 1 leads to a rise in the limiting surface area values A 0 to 1072 ± 59 A2 for the first two ratios to 2534 ± 79 A2 for the third one. The obtained monolayers were transferred onto mica by the Langmuir-Schaeffer method at different surface pressure values (0.5, 10, and 25 mN/m). With the use of the method of atomic force microscopy, it is shown that a four- to sixfold increase in the mean roughness of a composite film surface due to the inclusion of magnetite nanoparticles into the polymer brush monolayer is typical of all samples.

When dual and multi-responsiveness meet molecular brushes

Molecular brushes are a class of copolymerized polymers by densely attaching the side chains onto the polymer backbone. Due to the unique architecture, the molecular brushes are very informative model systems for the studies of polymer properties, and have applications in many fields, including biomimetics. This Feature Article highlights the recent advances in the molecular brushes which could respond to dual and multi stimuli, such as temperature, pH, light irradiation, ion strength, redox and electric field. We discussed examples that illustrate the designing principles, response manners, and applications of dual and multi-responsive molecular brushes. The challenges and future research trends are also presented.

Two decades of molecular brushes by ATRP

This review presents the development of brush copolymers as macromolecular engineering materials prepared by atom transfer radical polymerization (ATRP) and reported by the Matyjaszewski's group during the last twenty years. The graft structures combine different segments which allowed to precisely design properties of macromolecules to target various applications. The first molecular brushes contained homopolymer side chains, which were extended by other segments yielding core–shell macromolecules with block copolymer side chains. Many variants of brushes, densely vs loosely grafted copolymers, statistical and gradient distribution of grafts, block and statistical graft structures, homografted vs heterografted copolymers, have been designed. The semigrafted copolymers combine linear and brush topologies, whereas block brushes consist of two different grafted segments. These are also known the brushes with much more complex architectures, such as star-like brushes. Examples of various types of brushes, properties and their potential applications are discussed.

Synthesis and characterization of anionic pentablock brush copolymers bearing poly(acrylic acid) side chains on the brush blocks separated by linear poly(butyl methacrylate) blocks

One-pot successive three-step RAFT polymerization of 2-hydroxyethyl methacrylate (HEMA) and butyl methacrylate (BMA) in the presence of difunctional RAFT CTA is reported demonstrating successful synthesis of pentablock copolymers containing hydrophilic blocks of pHEMA separated by hydrophobic blocks of pBMA. Dispersity of pentablock copolymers was lower than that of pHEMA and triblock copolymers, down to 1.11. pHEMA blocks of the pentablock copolymer were decorated by pendent butylthiocarbonothioylthio moieties, which served as multiple RAFT CTA for grafting of acrylic acid (AA). The resulting anionic pentablock brush copolymers consisting of three brush blocks separated by linear blocks were characterized by SEC with triple detection and 1H NMR spectroscopy. Pentablock brush copolymers with short pAA side chains produced aggregates at low pH and showed conformational changes during titration with alkaline solution, while those with medium-length pAA side chains showed ionization behavior typical for anionic molecular brushes. Anionic pentablock brush copolymers are mimicking naturally occurring molecular brushes and deserve attention as potential surface modifiers and lubricants.

Promiscuous Binding of Karyopherinβ1 Modulates FG Nucleoporin Barrier Function and Expedites NTF2 Transport Kinetics

The transport channel of nuclear pore complexes (NPCs) contains a high density of intrinsically disordered proteins that are rich in phenylalanine-glycine (FG)-repeat motifs (FG Nups). The FG Nups interact promiscuously with various nuclear transport receptors (NTRs), such as karyopherins (Kaps), that mediate the trafficking of nucleocytoplasmic cargoes while also generating a selectively permeable barrier against other macromolecules. Although the binding of NTRs to FG Nups increases molecular crowding in the NPC transport channel, it is unclear how this impacts FG Nup barrier function or the movement of other molecules, such as the Ran importer NTF2. Here, we use surface plasmon resonance to evaluate FG Nup conformation, binding equilibria, and interaction kinetics associated with the multivalent binding of NTF2 and karyopherinβ1 (Kapβ1) to Nsp1p molecular brushes. NTF2 and Kapβ1 show different long- and short-lived binding characteristics that emerge from varying degrees of molecular retention and FG repeat binding avidity within the Nsp1p brush. Physiological concentrations of NTF2 produce a collapse of Nsp1p brushes, whereas Kapβ1 binding generates brush extension. However, the presence of prebound Kapβ1 inhibits Nsp1p brush collapse during NTF2 binding, which is dominated by weak, short-lived interactions that derive from steric hindrance and diminished avidity with Nsp1p. This suggests that binding promiscuity confers kinetic advantages to NTF2 by expediting its facilitated diffusion and reinforces the proposal that Kapβ1 contributes to the integral barrier function of the NPC.

Interlayer structure and self-healing in suspensions of brush-stabilized nanoplatelets with smectic order.

We have investigated the rheology of an uncured epoxy fluid containing high aspect ratio (length/thickness ≈ 160) α-zirconium phosphate (ZrP) nanoplatelets with smectic order. The nanoplatelets were exfoliated into monocrystalline sheets with uniform thickness using a monoamine-terminated oligomer. The oligomers were densely grafted to the plate surfaces and behave as a molecular brush. Suspensions containing ∼ 2 vol.% ZrP and above show liquid crystalline order with scattering peaks characteristic of a smectic (layered) mesophase. At much higher loading, ∼ 4 vol.% ZrP, there is a sharp transition in visual appearance, steady shear rheology, and linear and non-linear viscoelasticity that is attributed to the reversible interdigitation of oligomer chains between closely spaced layers. The oligomers are proposed to serve as inter-lamellar bridges that store elastic stresses for intermediate rates of deformation, but are able to relax on longer time scales. Under steady shearing conditions, the smectic suspensions with "overlapped" microstructure show a discontinuous flow curve characteristic of shear banding that is attributed to the dynamic pull-out of oligomer chains from the overlap region. At high shear rates, the limiting viscosity of the concentrated suspensions is on the same order of magnitude as the unfilled suspending fluid. When the rate of deformation is reduced below a critical time scale, the original network strength, and corresponding microstructure, is recovered through a passive self-healing process. The unique combination of concentration-dependent yield stress, low post-yield viscosity, and self-healing is potentially useful for various applications in the liquid state, and desirable for scalable processing of nanocomposite materials for structural applications.

Poly(2-oxazoline) molecular brushes by grafting through of poly(2-oxazoline)methacrylates with aqueous ATRP

Well defined molecular brushes of poly(2-oxazoline)s were synthesized by ATRP of oligo- and poly(2-methyl-, 2-ethyl- and 2-isopropyl-2-oxazoline) macromonomers in aqueous solution.

A novel strategy to synthesize well-defined PS brushes on silica particles by combination of lithium-iodine exchange (LIE) and surface-initiated living anionic polymerization (SI-LAP).

Core-shell hybrid particles, possessing a hard core of silica particles (SiPs) and a soft shell of brushlike polystyrene (PS), were successfully prepared by the combination of lithium-iodine exchange (LIE) and surface-initiated living anionic polymerization (SI-LAP). The molecular weight, graft density and brush thicknesses of the PS brushes were controllable.

Virus-Mimicking Polymer Molecular Brushes Are Potent Antibiotics with Double Selectivity

Evolution of antibiotics-resisting pathogens has become one of the greatest challenges in the battle of bacterial infection. Inspired by the structures of bacteria-invading viruses and antimicrobial peptides, we hypothesize that in addition to a balance of amphiphilicity and electropositivity, the nanoscale architecture is an essential determinant that dictates how membrane-active antibiotics remodel host membranes to achieve desirable activity and selectivity. Here we study the structure-activity relationship of a series of polymer molecular brushes (PMBs) with well-defined nanoscale architectures that mimic spherical and rod-shaped viruses. Our preliminary data based on PMBs with hydrophilic polymer brushes reveal that: (1) amphiphilicity is not a required trait - hydrophilic PMBs can be designed to have potent antibiotic performance as well with no hemolytic side effect; (2) the nanoscale architecture of PMBs defines their double selectivity, not molecular weight per se; (3) PMBs are far more powerful antibiotics than individual linear-chain polymers that make up the PMBs; and (4) nanostructured PMBs induce topological changes of membranes by forming membrane pores that unlikely fit in with any known models of AMP action. These findings challenge existing wisdom and suggest that the spatially-defined, multivalent interactions inherent to PMBs is of great significance for the development of polymer-based antibiotics.

High performance composite polymer electrolytes using polymeric ionic liquid-functionalized graphene molecular brushes

Polymer-functionalized reduced graphene oxide (polymer-FG) combining a polymer ionic liquid and a polymer brush structure is a promising nanofiller for the effective improvement of Li-ion transfer conditions.

Hybrid molecular brushes with chitosan backbone: facile synthesis and surface grafting.

We developed a facile route toward amphiphilic hybrid molecular brushes (HMB) with chitosan backbone and concurrently grafted chains of poly(acrylamide) and polystyrene. The grafting occurs through amino groups of chitosan; no extra modification of chitosan is required. The kinetic and molecular weight characteristics of the primary molecular brush CHI-graft-PAAm are studied. The second step is grafting of PS by emulsion polymerization. The resulting HMB CHI-graft-PAAm-graft-PS form very stable emulsions. We attached the HMB on solid substrates using chitosan backbone by the "grafting to" approach. Thin films of the immobilized HMB of 3-11 nm thickness completely cover the surface. Being amphiphilic by nature, the immobilized HMB reveal the ability to adapt to the medium, which results in shifting of the hydrophobic/hydrophilic balance over a wide range.

Double stimuli-responsive behavior of graft copolymer with polyimide backbone and poly(N,N-dimethylaminoethyl methacrylate) side chains

Comblike block copolymer having a polyimide backbone and poly(N,N-dimethylaminoethyl methacrylate) side chains, i.e., PI-g-PDMAEMA, was prepared through the “grafting from” method using (N,N-dimethylaminoethyl methacrylate) polymerization on a polyimide multicenter macroinitiator by atom transfer radical polymerization (ATRP) mechanism. Its molar mass characteristics were determined in chloroform solution using hydrodynamic methods. PI-g-PDMAEMA is a relatively loose molecular brush with the substitution degree z = 0.44. The behavior of PI-g-PDMAEMA in aqueous solutions at a concentration 0.015 g cm−3 was studied by light scattering within a pH interval from 2.1 to 12.4. The temperature dependences of light scattering intensity and hydrodynamic sizes of dissolved species were obtained. Synthesized copolymer shows clearly temperature and pH responsiveness. PI-g-PDMAEMA is thermo-sensitive at pH ≥ 8.9. The pH growth is accompanied by a decrease in the phase transition temperature and an increase in the width of the phase transition interval and hydrodynamic radii of the micellar aggregates and large loose aggregates.

A New Stationary Phase for Hydrophilic Interaction Chromatography: Polyacrylate-Based Hydrophilic, Monosized-Porous Beads with Zwitterionic Molecular Brushes

Hydrophilic, polyacrylate-based, monosized-porous beads with zwitterionic molecular brushes were synthesized as a new stationary medium for hydrophilic interaction chromatography. Monosized-porous poly(glycerol-1,3-diglycerolate diacrylate-co-glycerol dimethacrylate), poly(GDGDA-co-GDMA), beads 5 μm in size were obtained by a staged-shape template polymerization. As an initiator for surface-initiated atom transfer radical polymerization (SI-ATRP), bromine functionality was obtained on the beads by reacting their hydroxyl groups with 3-(aminopropyl)triethoxysilane and α-bromoisobutyryl bromide, respectively. Zwitterionic molecular brushes on the hydrophilic poly(GDGDA-co-GDMA) beads were generated by SI-ATRP of a sulfobetaine monomer, [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide (MESH). Poly(MESH)-grafted poly(GDGDA-co-GDMA), poly(MESH)g-poly(GDGDAco-GDMA), beads were slurry packed into the microbore columns with 2 mm i.d. and evaluated as stationary medium for the separation of organic acids, nucleosides and peptides using microbore columns in hydrophilic interaction chromatography with the plate numbers up to 30,000 plates m−1.

Advanced photoresist technologies by intricate molecular brush architectures: Diblock brush terpolymer‐based positive‐tone photoresist materials

The reported diblock brush terpolymer displays the fundamental advantages of high macromolecular and regiochemical control in simultaneous top-down/bottom-up construction of advanced positive-tone photoresist materials. By selective partition the substrate adhesion, surface-active migration, and chemically-reactive deblocking functional compositions into the cylindrical polymer brush architectures, large-areas of vertical alignment of the polymers within thin films are realized. The bottom-up premeditated system enables effective top-down lithography to produce near-molecular pixel resolution (2–3 molecules) at low operation dosage.