Grafting Density Of Side Chains Research Articles

Cylindrical Molecular Brushes of Poly(2-oxazoline)s from 2-Isopropenyl-2-oxazoline

We report on the synthesis and characterization of cylindrical molecular brushes based on poly(2-oxazoline)s (POx). The dual-functional monomer, 2-isopropenyl-2-oxazoline (IPOx), was first converted to a poly(2-isopropenyl-2-oxazoline), backbone by free radical (PIPOxR) or living anionic polymerization (PIPOxA). Quantitative reaction with methyl triflate yields a macroinitiator salt (PIPOxOTfR/A) for the preparation of molecular brushes via the grafting from approach by living cationic polymerization of 2-oxazolines (2-methyl-, 2-ethyl-, and 2-isopropyl-2-oxazoline). Characterization of the resulting molecular brushes by NMR and FTIR spectroscopy indicates a very high side chain grafting density and quantitative reactions. Visualization of adsorbed molecular brushes by AFM corroborates this assumption. Furthermore, the lower critical solution temperatures of the POx molecular brushes were determined. The transition temperatures were found to be very defined, reversible, and with no noticeable hysteresis.

Adsorption of Molecular Brushes with Polyelectrolyte Backbones onto Oppositely Charged Surfaces: A Self-Consistent Field Theory

The two-gradient version of the Scheutjens-Fleer self-consistent field (SF-SCF) theory is employed to model the interaction between a molecular bottle brush with a polyelectrolyte backbone and neutral hydrophilic side chains and an oppositely charged surface. Our system mimics graft-copolymers with a cationic main chain (among which poly( L-lysine)- graft-poly(ethylene glycol) (PLL- g-PEG) or poly( L-lysine)- graft-polyoxazoline are well-known examples) interacting with negatively charged (metal oxide) solid surfaces. We aim to analyze the copolymer-surface interaction patterns as a function of the molecular architecture parameters. Two regimes are investigated: First, we compute the effective interaction potential versus the distance from the surface for individual bottle brush macromolecules. Here, depending on the grafting ratio and the degree of polymerization of the side chains, the interplay of electrostatic attractions of the main chain to the surface and the steric repulsion of the grafts results in different patterns in the interaction potential and, therefore, in qualitatively different adsorption behavior. In particular, we demonstrate that, at high side chain grafting density and short grafts, the molecular brushes are strongly adsorbed electrostatically onto negatively charged substrates, whereas, in the opposite case of low grafting ratio and high molecular weight of grafts, the steric repulsion of the side chains from the surface dominates the polymer-surface interaction. At intermediate grafting ratios, the adsorption/depletion scenario depends essentially on the ratio between the electrostatic screening length and the thickness of the molecular bottle brush. We further have analyzed the equilibrium adsorbed amount as a function of the macromolecular architecture. Our results are based on a detailed account of attractive and repulsive (including intermolecular in-plane) interactions, and suggest a nonmonotonic dependence of the adsorbed amount on the grafting ratio, in good agreement with the experimental studies for PLL- g-PEG adsorption onto Nb2O5 surfaces. The results of the theoretical modeling are discussed in the context of the optimization of the PLL-g-PEG molecular design parameters in order to create protein-resistant surfaces.

Poly-2-methyl-2-oxazoline: A Peptide-like Polymer for Protein-Repellent Surfaces

Surface coatings with so-called protein-repellent or nonfouling polymers have become indispensable for the development of modern therapeutic and diagnostic medical devices such as biosensors, drug-delivery capsules, and biomedical implants. Nowadays, poly(ethylene glycol) (PEG) is routinely used for these purposes. However, there is increasing evidence that PEG has limited long-term stability, particularly in vivo. Here we investigate poly(2-methyl-2-oxazoline) (PMOXA) as a potential alternative polymer. We designed comb copolymers consisting of a polycationic poly(l-lysine) backbone and PMOXA side chains by analogy to precisely studied and highly protein-repellent PEG-based systems. Using optical waveguide lightmode spectroscopy, we quantitatively compare the in situ self-assembly of the comb copolymers on negatively charged surfaces and the exposure of the formed monolayers to full human serum. We find that the PMOXA-based coatings with an optimal side-chain grafting density eliminate protein adsorption to a level of <2 ng/cm2; that is, they quantitatively equal the protein-repellent properties of the best PEG-based coatings.

Surface Induced Tilt Propagation in Thin Films of Semifluorinated Liquid Crystalline Side Chain Block Copolymers

The supramolecular self-assembly within a liquid crystalline block copolymer thin film with mesogenic semifluorinated alkyl side groups was studied using both grazing incidence X-ray diffraction and small angle scattering. Previous studies employing near-edge X-ray absorption fine structure spectroscopy indicated that mesogens at the film surface are tilted relative to the surface-normal. This tilt may arise from the interplay between the low surface energy of perfluoromethyl groups and the entropic degrees of freedom of each side chain (constrained by neighboring mesogens in the smectic layer and the alkyl linker attachment to the polymer backbone). Here, we show that the mesogen tilt at the surface propagates over several smectic layers into the film interior. We propose that the high grafting density of side chains in combination with a head-to-head arrangement of side chain free ends plays a significant role in transmitting mesogen tilt from the surface to the adjacent smectic layers within the film interior. In addition to scattering from individual mesogens, we observed characteristic scattering patterns in the plane of incidence produced by the horizontal smectic layers. The amorphous polystyrene block formed vertical domains interspersed in the liquid crystalline matrix, resulting in out-of-plane Bragg rods.

Topology and nanomechanics of polyethylene networks

We show in this paper that three-dimensional networks in bulk polyethylene (mean molecular weight:350 000 g mol−1, polydispersity of 2.62) with a defined grafting density of side-chains of 9.6% can berevealed by spin-coating the gel-like swollen polymer onto a flat surface. The polymercontained side-chains with one (33%), two (20%), three (13%) and more (34%) carbonatoms. The polymer structure after spin-coating consists of bundles, dropletsat interconnects and globules. The combined tapping mode and contact modeatomic force microscopy (AFM) is able to distinguish topological differences ininterconnects, such as entangled junctions from simple crossings of polymer bundles.A solvent treatment of the polymer network on the sample surface reveals that some of thedroplets are in fact superpositions of more than one droplet, indicating again that athree-dimensional network is collapsed to two dimensions. We have observed that polymerbundles do branch near such multiple droplets.Finally, the recorded lateral forces while scanning across a polymer bundle in contact modemight be useful to model the mechanical behaviour of polymer bundles. However, without asolid theoretical model it is difficult to translate the experimental data into meaningfulmaterials properties.

Synthesis and Single Molecule Force Spectroscopy of Graft Copolymers of Poly(2-hydroxyethyl methacrylate-g-ethylene glycol)

In this study, an end-functionalized graft copolymer, thiol-terminated poly(2-hydroxyethyl methacrylate-g-ethylene glycol) or SH-poly(HEMA-g-EG), was synthesized by the atom transfer radical polymerization (ATRP) method and then characterized by 1H nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), and light scattering (LS). The polymer was covalently end-grafted to an Au substrate at low enough grafting densities to produce well-separated individual polymer “mushrooms” and single molecule force spectroscopy (SMFS) was then employed to measure the single polymer extensional elastic properties in phosphate buffered saline solution (PBS, ionic strength (IS) = 0.15 M, pH = 7.4) and deionized H2O (IS ∼ 6 × 10-5 M, pH = 5). In both solvents, the polymer behaved as an extensible freely jointed chain with a statistical segment length, a = 0.52 ± 0.09 nm, and a segment elasticity, ksegment = 10.5 ± 3.3 N/m, in PBS and a = 1.03 ± 0.01 nm and ksegment = 4.2 ± 0.5 N/m in H2O. Comparison to the known single molecule elasticity behavior for the PEG homopolymer suggests a smaller number and/or smaller magnitude of the noncovalent inter- and intramolecular interactions along the HEMA backbone and that even at this low PEG side chain grafting density, the side chains are still quite effective in causing local expansion of the PHEMA backbone by overcoming hydrophobic collapsing forces relative to the PHEMA homopolymer. Since the PEG chains are still well-solvated in PBS, one explanation for the small variation in nanomechanical properties with solvent is that in PBS the salt ions compete for and weaken the inter- and intramolecular H-bonding that does exist along the PHEMA backbone, causing a local contraction and an increased segmental stiffness due to the reduced noncovalent nature of the segments.