Diblock Copolymer Brushes Research Articles

Surface-Active and Stimuli-Responsive Polymer−Si(100) Hybrids from Surface-Initiated Atom Transfer Radical Polymerization for Control of Cell Adhesion

A simple two-step method was developed for the covalent immobilization of atom-transfer radical polymerization (ATRP) initiators on the hydrogen-terminated Si(100) (Si-H) surface. Well-defined functional polymer-Si hybrids, consisting of covalently tethered brushes of poly(ethylene glycol) monomethacrylate (PEGMA) polymer, N-isopropylacrylamide (NIPAAm) polymer, and NIPAAm-PEGMA copolymers and block copolymers on Si-H surfaces, were prepared via surface-initiated ATRP. Kinetics study revealed that the chain growth from the silicon surface was consistent with a "controlled" process. Surface cultures of the cell line 3T3-Swiss albino on the hybrids were evaluated. The PEGMA graft-polymerized silicon [Si-g-P(PEGMA)] surface is very effective in preventing cell attachment and growth. At 37 degrees C [above the lower critical solution temperature (LCST, approximately 32 degrees C) of NIPAAm], the seeded cells adhered, spread, and proliferated on the NIPAAm graft polymerized silicon [Si-g-P(NIPAAm)] surface. Below the LCST, the cells detached from the Si-g-P(NIPAAm) surface spontaneously. Incorporation of PEGMA units into the NIPAAm chains of the Si-g-P(NIPAAm) surface via copolymerization resulted in more rapid cell detachment during the temperature transition. The "active" chain ends on the Si-g-P(PEGMA) and Si-g-P(NIPAAm) hybrids were also used as the macroinitiators for the synthesis of diblock copolymer brushes. Thus, not only are the hybrids potentially useful as stimuli-responsive adhesion modifiers for cells in silicon-based biomedical microdevices but also the active chain ends on the hybrid surfaces offer opportunities for further surface functionalization and molecular design.

Branched fluoropolymer-Si hybrids via surface-initiated ATRP of pentafluorostyrene on hydrogen-terminated Si(100) surfaces.

Linear, branched, and arborescent fluoropolymer-Si hybrids were prepared via surface-initiated atom transfer radical polymerization (ATRP) from the 4-vinylbenzyl chloride (VBC) inimer and ClSO(3)H-modified VBC that were immobilized on hydrogen-terminated Si(100), or Si-H, surfaces. The simple approach of UV-induced coupling of VBC with the Si-H surface provided a stable, Si-C bonded monolayer of "monofunctional" ATRP initiators (the Si-VBC surface). The aromatic rings of the Si-VBC surface were then sulfonated by ClSO(3)H to introduce sulfonyl chloride (-SO(2)Cl) groups and to give rise to a monolayer of "bifunctional" ATRP initiators. Kinetics study indicated that the chain growth of poly(pentafluorostyrene) from the functionalized silicon surfaces was consistent with a "controlled" or "living" process. The chemical composition and functionality of the silicon surface were tailored by the well-defined linear and branched fluoropolymer brushes. Atomic force microscopy images revealed that the surface-initiated ATRP of pentafluorostyrene (PFS) had proceeded uniformly on the Si-VBC surface to give rise to a dense and molecularly flat surface coverage of the linear brushes. The uniformity of surfaces with branched brushes was controlled by varying the feed ratio of the monomer and inimer (VBC in the present case). The living chain ends on the functionalized silicon surfaces were used as the macroinitiators for the synthesis of diblock copolymer brushes, consisting of the PFS and methyl methacrylate polymer blocks.

Polymer brushes––surface immobilized polymers

The synthesis of tethered block copolymer brushes via the use of controlled/‘living’ free radical polymerization techniques presents many significant advantages over traditional free radical polymerization techniques. In our group, we have found that the most versatile controlled/‘living’ free radical polymerization techniques are atom transfer radical polymerization (ATRP) and reversible addition fragmentation transfer (RAFT) polymerization. Both diblock and ABA type triblock copolymer brushes have been synthesized using either ATRP or RAFT. Of particular interest with block copolymer brushes, are their ability to reversibly rearrange upon treatment with selective solvents. This rearrangement of block copolymer brushes can result in the formation of unusual surface morphologies that have been attributed to the formation of either ‘pinned micelles’ or ‘folded’ structures. We have demonstrated that other external stimuli besides block-selective solvents can be used to induce brush reorganization, namely temperature and treatment with supercritical CO 2.

Cover Picture: Macromol. Rapid Commun. 14/2004

Cover: This graphic duplicates the photographs of the water droplets from the manuscript. At the bottom is a simplified representation of the proposed brush rearrangement. This simplified structure is color coded for the two different monomers and corresponds to the more exact chemical structure of the diblock copolymer brush that is given above the pictures.Further details can be found in the Communication by A. M. Granville and W. J. Brittain* on page 1298.

Stimuli‐Responsive Semi‐Fluorinated Polymer Brushes on Porous Silica Substrates

AbstractSummary: A diblock copolymer brush consisting of poly(methyl acrylate)‐block‐poly(pentafluoropropyl acrylate) (Si/SiO2//PMA‐b‐PPFA) was synthesized on a porous silica substrate. The brush was exposed to selective solvents, as well as thermal treatments, to induce a surface rearrangement. The rearrangement resulted in the selective loss or creation of an ultrahydrophobic layer by location of the fluoropolymer segment. This work demonstrates that surface rearrangements observed on flat surfaces can be transferred to porous substrates.Image of a water droplet in contact with an Si/SiO2//PMA‐b‐PPFA ultrahydrophobic polymer brush, synthesized from a porous silica substrate.magnified imageImage of a water droplet in contact with an Si/SiO2//PMA‐b‐PPFA ultrahydrophobic polymer brush, synthesized from a porous silica substrate.

Synthesis and Characterization of Stimuli-Responsive Semifluorinated Polymer Brushes Prepared by Atom Transfer Radical Polymerization

“Grafting from” surface-initiated atom transfer radical polymerization (ATRP) techniques were used to synthesize semifluorinated diblock copolymer brushes from silica substrates. Synthesis of either a polystyrene or poly(methyl acrylate) homopolymer brush was performed first (inner block) followed by polymerization of a semifluorinated monomer (outer block). The semifluorinated outer block consisted either of poly(pentafluorostyrene), poly(heptadecafluorodecyl acrylate), poly(pentafluoropropyl acrylate), or poly(trifluoroethyl acrylate). Analysis of the homopolymer and diblock copolymer brush layers was conducted using ellipsometry, ATR−FTIR, tensiometry, XPS, and AFM. Solvent-induced diblock rearrangement experiments were performed using a selective solvent for the hydrocarbon polymer block to generate a fluorine-deficient surface. With the exception of the systems containing PHFA, all the diblock systems were shown to exhibit water contact angles typical for the hydrocarbon polymer block after solvent treatment. Poor rearrangement for diblock copolymer brushes containing PHFA was observed when compared to all other semifluorinated diblock copolymer systems. The extent of diblock rearrangement was correlated to the calculated values for the solubility parameters of each block.

Synthesis, Characterization, and Properties of Polyelectrolyte Block Copolymer Brushes Prepared by Atom Transfer Radical Polymerization and Their Use in the Synthesis of Metal Nanoparticles

Diblock copolymer polyelectrolyte brushes of either polystyrene (PS) or poly(methyl acrylate) (PMA) and poly(acrylic acid) (PAA) were synthesized using sequential monomer addition from a tethered 2-bromoisobutyrate initiator. Si/SiO2//PS-b-poly(tert-butyl acrylate) (P(t-BA)) and Si/SiO2//PMA-b-P(t-BA) were prepared by the “grafting from” method using atom transfer radical polymerization. The PAA block was prepared by hydrolysis of poly(tert-butyl acrylate) (P(t-BA)) and the polyelectrolyte was formed by subsequent treatment with either aqueous silver acetate or sodium tetrapalladate. Conversion of P(t-BA) to PAA and attachment of either silver or palladium ions to the PAA block was confirmed using ATR-FTIR spectroscopy, water contact angle measurements, and X-ray photoelectron spectroscopy. Solvent switching of the diblock copolymer brush using DMF and anisole was incomplete both before and after treatment with silver acetate, although the degree of switching was larger before treatment. The polyelectrolyte diblock copolymer brushes were used for the synthesis of inorganic nanoparticles by reduction of the treated PAA. These samples were characterized using AFM, XPS, and ATR−FTIR.

Can polymer brushes induce motion of nano-objects?

In searching for new and efficient mechanisms for moving or propelling nano-sizedobjects across a flat surface, we discuss the idea of using the periodicallyswitchable topography of a certain class of polymer substrates. In thispaper, we exploit the peculiar properties of diblock-copolymer and mixedbrushes that can undergo transitions between very distinct topographicalpatterns and a flat state. We investigate if, during the transition process, thereshaping of the surface structure, accompanied by a microphase transition,could deliver a sufficient amount of mechanical work to move objectsadsorbed on the surface. We show this by following the spatial distribution ofan ensemble of silica particles adsorbed on several kinds of poly(methylmethacrylate–b-glycidyl methacrylate) diblock-copolymer brushes, when thesurface topography undergoes many periods of the switching process.

Architecture of multi-component copolymer brushes

In the first part of this article, the method for the preparation of diblock copolymer brushes from free radical homopolymerization of the corresponding diblock macromonomers is reviewed. These diblock copolymer brushes formed a unimolecular structure in dilute solution. Moreover, it was speculated from angular dependence measurements that diblock copolymer brushes with high aspect ratios took a geometrical anisotropic conformation, such as a cylinder. The cylinder brushes were also derived by cross-linking internal domains of the diblock copolymer brush. The second part reviews the synthesis of prototype amphiphilic copolymer brushes by alternating free radical copolymerization of binary macromonomers. To determine the phase-separated cylindrical domains of alternating copolymer brushes, large aggregates of copolymer brushes in water were constructed. The alternating copolymer brushes led to self-assemblies among hydrophobic domains during micelle formation.

Synthesis and solution properties of cylinder brushes derivated by internal domain locking of poly(diblock macromonomer)s

Poly(diblock macromonomer)s (diblock copolymer brush) were synthesized by free-radical homopolymerizations of vinylbenzyl-terminated poly(α-methylstyrene)-block-poly(2-vinylpyridine) (PMS-block-P2VP) diblock macromonomer. Dilute-solution properties of such diblock copolymer brushes were investigated by static (SLS) and dynamic light scatterings (DLS) and small-angle X-ray scattering (SAXS). Effective diffusion coefficient Deff for polymer brushes with various aspect ratios showed the almost constant value in the range of polymer concentration 0–5×10−3g/cm3. These results indicated that polymer brushes formed unimolecule structure even in good solvent such as benzene. It was speculated from angular dependence measurements that polymer brushes with large aspect ratios took geometrical anisotropic conformation such as cylinder. Subsequently, the cylinder brushes were derivated by crosslinking of internal P2VP domains of diblock copolymer brushes with 1,4-diiodobutane. These cylinder brushes also formed unimolecular structure in a dilute solution. However, semi-dilute solution properties of cylinder brush show a clear plateau region, which originates from entanglement effect. This entanglement is caused by the thin thickness of PMS shell.

Synthesis, Characterization, and Properties of Tethered Polystyrene-b-polyacrylate Brushes on Flat Silicate Substrates

Surface-immobilized polystyrene-b-polyacrylate brushes have been synthesized on silicate substrates from chlorine-terminated polystyrene (PS) brushes by atom transfer radical polymerization (ATRP). ATRP of methyl methacrylate (MMA), methyl acrylate (MA), and (N,N-dimethylamino)ethyl methacrylate (DMAEMA) produced tethered diblock copolymer brushes, which were characterized by ellipsometry, FTIR-ATR, tensiometry, and XPS. “Control” polymerizations supported a radical mechanism for the MMA polymerization. The thickness of the polyacrylate terminal block was controlled by free initiator concentration. All three diblock copolymer brushes underwent self-reorganization and exhibited different surface properties in response to different solvent treatments. For PS-b-PMMA and PS-b-PMA brushes, treatment with CH2Cl2 resulted in localization of PMMA and PMA at the air interface; treatment with cyclohexane localized the PS block at the air interface. PS-b-PDMAEMA brushes exhibited a similar behavior using cyclohexane and THF/H2O (v/v = 1:1).

Energy Migration in Diblock Copolymer Micelles and Brushes

Energy Migration in Diblock Copolymer Micelles and Brushes