Homopolymer Brushes Research Articles

Synthesis and characterization of well-defined hydrophilic block copolymer brushes by aqueous ATRP

Homopolymer brushes of poly(N,N-dimethylacrylamide) (PDMA), poly(methoxyethylacrylamide) (PMEA) and poly(N-isopropylacrylamide)(PNIPAM) grown on atom transfer radical polymerization (ATRP) initiator functionalized latex particles were used as macroinitiators for the synthesis of PDMA-b-PNIPAM/PMEA, PMEA-b-PDMA/PNIPAM and PNIPAM-b-PDMA block copolymer brushes by surface initiated aqueous ATRP. The grafted homopolymer and block copolymer brushes were analyzed for molecular weight, molecular weight distribution, chain grafting density, composition and hydrodynamic thickness (HT) using gel permeation chromatography–multi-angle laser light scattering, 1H NMR, particle size analysis and atomic force microscopy (AFM) techniques. The measured graft molecular weight increased following the second ATRP reaction in all cases, indicating the second block had been added. Chain growth depended on the nature of the monomer used for block copolymerization and its concentration. Unimodal distribution of polymer chains in GPC with non-overlap of molar mass–elution volume curves implied an efficient block copolymerization. This was supported by the increase in HT measured by particle size analysis, equilibrium thickness observed by AFM and the composition of the block copolymer layer by 1H NMR analysis, both in situ and on cleaved chains in solution. 1H NMR analysis of the grafted latex and cleaved polymers from the surface demonstrated that accurate determination of the copolymer composition by this method is possible without detaching polymer chains from surface. Block copolymer brushes obey the same power law dependence of HT on molecular weight as homopolymer brushes in good solvent conditions. The NIPAM-containing block copolymer brushes were sensitive to changes in the environment as shown by a decrease in HT with increase in the temperature of the medium.

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.

Attractive Bridging Interactions in Dense Polymer Brushes in Good Solvent Measured by Atomic Force Microscopy

Using an atomic force microscope (AFM), we have investigated the interaction forces exerted by latex particles bearing densely grafted polymer brushes consisting of poly(N,N-dimethylacrylamide) (PDMA), poly(methoxyethylacrylamide) (PMEA), poly(N-isopropylacrylamide) (PNIPAM), and PMEA-b-PNIPAM in aqueous media (good solvent). The brushes were prepared by controlled surface-initiated atom transfer radical polymerization, and the hydrodynamic thicknesses were measured by dynamic light scattering. The molecular weight (Mn), grafting density (sigma), and polydispersity (PDI) of the brushes were determined by gel permeation chromatography and multiangle laser light scattering after cleaving the polymer from the latex surface by hydrolysis. Force profiles of PDMA (0.017 nm(-2) < or = sigma < or = 0.17 nm-2) and PMEA (sigma = 0.054 nm-2) brushes were purely repulsive upon compression, with forces increasing with Mn and a, as expected, due to excluded volume interactions. At a sufficiently low grafting density (sigma = 0.012 nm-2), PDMA exhibited a long-range exponentially increasing attractive force followed by repulsion upon further compression. The long-range attractive force is believed to be due to bridging between the free chain ends and the AFM tip. The PNIPAM brush exhibited a bridging force at a grafting density of 0.037 nm(-2), a value lower than the sigma needed to induce bridging in the PDMA brush. Bridging was therefore found to depend on grafting density as well as on the nature of the monomer. The grafting densities of these polymers were larger than those typically associated with bridging. Bridging interactions were used to confirm the presence of PNIPAM in a block copolymer PMEA-b-PNIPAMA brush given that the original PMEA homopolymer brush produced a purely repulsive force. The attractive force was first detected in the block copolymer brush at a separation that increased with the length of the PNIPAM block.

Plasma protein adsorption to surfaces grafted with dense homopolymer and copolymer brushes containing poly(N-isopropylacrylamide)

Growing polymer chains from surface initiators in principle allows much more dense polymer surface layers to be created than can be produced by grafting of whole (self-excluding) chains. We have utilized aqueous atom transfer radical polymerization to graft a series of cleavable hydrophilic poly(N-isopropylacrylamide) (PNIPAM) homopolymers and block copolymers of substituted acrylamides from polystyrene latex to give brushes of controlled MW and surface density. Average chain separations much less than their free solution radii of gyration have been achieved. Exposure to radiolabeled single proteins or to whole plasma and subsequent analysis by SDS-PAGE shows that PNIPAM brushes decrease protein adsorption relative to the latex surface or other substituted polyacrylamides. The PNIPAM brushes exhibit a second-order phase transition around 30°C as reflected by a decrease in the hydrodynamic thickness of the brush at higher temperatures. Total plasma protein adsorption is increased at 40°C compared to 20°C but there is significant differential adsorption behavior among the proteins detected by gel-electrophoresis analysis.

Controlled Grafting of Well-Defined Polymers on Hydrogen-Terminated Silicon Substrates by Surface-Initiated Atom Transfer Radical Polymerization

Controlled grafting of well-defined polymer brushes on the hydrogen-terminated Si(100) substrates (the Si−H substrate) was carried out via the surface-initiated atom transfer radical polymerization (ATRP). Surface initiators were immobilized on the Si−H substrates in three consecutive steps: (i) coupling of an ω-unsaturated alkyl ester to the Si−H surface under UV irradiation, (ii) reduction of the ester groups by LiAlH4, and (iii) esterification of the surface-tethered hydroxyl groups with 2-bromoisobutyrate bromide. Homopolymer brushes of methyl methacrylate (MMA), (2-dimethylamino)ethyl methacrylate (DMAEMA), and poly(ethylene glycol) monomethacrylate (PEGMA) were prepared by ATRP from the α-bromoester functionalized silicon surface. The chemical composition and topography of the graft-functionalized silicon surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. Kinetic studies revealed a linear increase in polymer film thickness with rea...

A switchable polymer layer: Chain folding in end-charged polymer brushes

We use a self-consistent field approximation to model the configurations of end-charged homopolymer and block copolymer brushes in response to an external electric field due to charges on the grafting surface. By varying the charge density on the grafting surface, we can cause the chains either to extend outward, greatly increasing the brush height, or to loop back to the grafting surface. We show that such a copolymer brush can present one block at the exposed surface in the extended state and present the other block in the retracted state. This occurs for both a solvated brush and a dry brush. We also compare these results to those of a modified Alexander–de Gennes model for the end-charged homopolymer brush.

Amphiphilic Polymer Brushes Grown from the Silicon Surface by Atom Transfer Radical Polymerization

Homopolymer and block amphiphilic polymer brushes on silicon wafers have been successfully prepared by combining the self-assembled monolayer of initiator and atom transfer radical polymerization (ATRP). Two methods for self-assembly of initiator monolayer were compared: One was to treat the silicon surface with 6-((2-bromo-2-methyl)propionyloxy)hexyltrichlorosilane solution (solution method), and the other was to treat it with the vapor of 3-aminopropyl)triethoxysilane, followed by amidization with 2-bromoisobutyric acid (vapor method). While the solution method produced particles due to cross-linked products of the chlorosilane on silicon surface, the vapor method gave a uniform surface. The homopolymer brushes of poly(methyl methacrylate) (PMMA) and poly(acrylamide) (PAAM) were prepared by ATRP on the modified silicon wafer by the initiator. The amphiphilic block copolymer brushes consisting of PAAM and PMMA blocks were obtained by using the homopolymer brush as the macroinitiator for the second ATRP ...