Water Is a Poor Solvent for Densely Grafted Poly(ethylene oxide) Chains: A Conclusion Drawn from a Self-Consistent Field Theory-Based Analysis of Neutron Reflectivity and Surface Pressure–Area Isotherm Data (original) (raw)
Related papers
Reduced Water Density in a Poly (ethylene oxide) Brush
2012
A model poly(ethylene oxide) (PEO) brush system, prepared by spreading a poly(ethylene oxide)–poly(n-butyl acrylate) (PEO–PnBA) amphiphilic diblock copolymer onto an air–water interface, was investigated under various grafting density conditions by using the X-ray reflectivity (XR) technique. The overall electron density profiles of the PEO–PnBA monolayer in the direction normal to the air–water interface were determined from the XR data. From this analysis, it was found that inside of the PEO brush, the water density is significantly lower than that of bulk water, in particular, in the region close to the PnBA–water interface. Separate XR measurements with a PnBA homopolymer monolayer confirm that the reduced water density within the PEO–PnBA monolayer is not due to unfavorable contacts between the PnBA surface and water. The above result, therefore, lends support to the notion that PEO chains provide a hydrophobic environment for the surrounding water molecules when they exist as polymer brush chains.
Langmuir, 2005
The dilatational rheological properties of monolayers of poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide)-type block copolymers at the air-water interface have been investigated by employing an oscillating ring trough method. The properties of adsorbed monolayers were compared to spread layers over a range of surface concentrations. The studied polymers were PEO26-PPO39-PEO26 (P85), PEO103-PPO40-PEO103 (F88), and PEO99-PPO65-PEO99 (F127). Thus, two of the polymers have similar PPO block size and two of them have similar PEO block size, which allows us to draw conclusions about the relationship between molecular structure and surface dilatational rheology. The dilatational properties of adsorbed monolayers were investigated as a function of time and bulk solution concentration. The time dependence was found to be rather complex, reflecting structural changes in the layer. When the dilatational modulus measured at different concentrations was replotted as a function of surface pressure, one unique master curve was obtained for each polymer. It was found that the dilatational behavior of spread (Langmuir) and adsorbed (Gibbs) monolayers of the same polymer is close to identical up to surface concentrations of ≈0.7 mg/m 2. At higher coverage, the properties are qualitatively alike with respect to dilatational modulus, although some differences are noticeable. Relaxation processes take place mainly within the interfacial layers by a redistribution of polymer segments. Several conformational transitions were shown to occur as the area per molecule decreased. PEO desorbs significantly from the interface at segmental areas below 20 Å 2 , while at higher surface coverage, we propose that segments of PPO are forced to leave the interface to form a mixed sublayer in the aqueous region.
Characterization of the brush regime for grafted polymer layers at the solid-liquid interface
Physical Review Letters, 1991
Using small-angle neutron-scattering techniques, the scaling laws between the thickness of a grafted polymer layer h, its graft density a. , and the molecular weight of the grafted chains M have been determined for polydimethylsiloxane chains end grafted on porous silica. The observed laws h = Mcr in bad solvent and h = Mo' ' in good solvent provide the first experimental evidence of the "brush" regime, where the grafted chains, confined by their neighbors, are stretched normal to the surface.
Macromolecules, 2007
We describe experiments pertaining to the formation of surface-anchored poly(acrylic acid) (PAA) brushes with a gradual variation of the PAA grafting densities on flat surfaces and provide detailed analysis of their properties. The PAA brush gradients are generated by first covering the substrate with a molecular gradient of the polymerization initiator, followed by the "grafting from" polymerization of tert-butyl acrylate (tBA) from these substrate-bound initiator centers, and finally converting the PtBA into PAA. We use spectroscopic ellipsometry to measure the wet thickness of the grafted PAA chains in aqueous solutions at three different pH values (4, 5.8, and 10) and a series of ionic strengths (IS). Our measurements reveal that at low grafting densities, σ, the wet thickness of the PAA brush (H) remains relatively constant, the polymers are in the mushroom regime. Beyond a certain value of σ, the macromolecules enter the brush regime, where H increases with increasing σ. For a given σ, H exhibits a nonmonotonic behavior as a function of the IS. At large IS, the H is small because the charges along PAA are completely screened by the excess of the external salt. As IS decreases, the PAA enters the so-called salt brush (SB) regime, where H increases. At a certain value of IS, H reaches a maximum and then decreases again. The latter is a typical brush behavior in so-called osmotic brush (OB) regime. We provide detailed discussion of the behavior of the grafted PAA chains in the SB and OB regimes.
The Journal of Physical Chemistry B, 2009
The effect of hydrogen-bonding complexation on the interfacial behavior of poly(isoprene)-b-poly(ethylene oxide) (PI-b-PEO) diblock copolymer at the air-water interface has been investigated by Langmuir balance and neutron reflectivity. PI-b-PEO forms Langmuir monolayers with PI as the anchoring block. Introduction of a second diblock, poly(isoprene)-b-poly(acrylic acid) (PI-b-PAA) yields PI-b-PEO/PI-b-PAA mixed layers with interfacial behavior that is pH-dependent. At pH 10.0 and 5.7, the compression (π-A) isotherms exhibit three regions that are characteristic of PEO-type tethered layers, (i) a low-pressure 2-D "pancake" region (region I), (ii) a pseudoplateau where PEO segments desorb and are immerse in the subphase (region II), and (iii) a steep pressure rise region commonly considered as the "brush" regime (region III). At pH 2.5, on the other hand, the π-A isotherm shows only two regions, (I) and (III). This novel behavior is attributed to hydrogen-bonding complexation between the undissociated carboxylic acids and the PEO, forming very compact layers. It appears that desorption of PEO segments is hindered as a consequence of this complexation. Furthermore, no brush-like structure is observed in region III of the isotherml; thus, the steep rise in surface pressure in this case arises primarily from interactions of the anchoring block. The hydrogen-bonded complex of PI-b-PEO/PI-b-PAA monolayers thus shows enhanced surface stability.
2006
Molecular dynamics (MD) simulations were performed to study the structural properties of water molecules confined in functionalized carbon nanotubes (CNTs). Four CNTs, two armchair-type (6, 6), (7, 7) and two zigzag-type (10, 0), (12, 0) CNTs, representing different helicities and different diameters, were chosen and functionalized at their open ends by the hydrophilic -COOH and the hydrophobic -CH 3 groups. The structural properties of water molecules inside the functionalized CNTs, including the orientation distributions of dipole moment and O-H bonds, the length of the single-file water chain, and the average number of hydrogen bonds, were analyzed during a process of simulations. MD simulation results in this work showed that the -CH 3 functional groups exert little special effects on the structural properties of water molecules. It is mainly due to the relatively small size of the -CH 3 group and its hydrophobic nature, which is consistent with hydrophobic CNTs. For CNTs functionalized by -COOH groups, the configurations of -COOH groups, incurvature or excurvature, determine whether water molecules can enter the CNTs. The incurvature or excurvature configurations of -COOH groups are the results of synergy effects of the CNTs' helicity and diameter and control the flow direction of water molecules in CNTs.
Langmuir, 1996
Spread monolayers of poly(styrene)-poly(ethylene oxide) diblock copolymers (PSm-PEOn, m ) 38, n ) 90, 148, 250, and 445) have been studied at the air-water interface by measuring the surface pressurearea (π-A) isotherms at several temperatures. The π-A isotherms exhibit several regions which can be ascribed to different conformations of the polymer chains: a pancake structure at low surface pressures and high areas when the isolated chains are adsorbed by both the PS globule and the PEO segments at the interface; an intermediate structure, quasi-brush, when the PEO segments are solubilized in the subphase; and finally a brush developed at low surface areas when the PEO chains are obliged to stretch away from the interface to avoid overlapping. At surface pressures near 10 mN/m there is a transition between a high-density pancake and the quasi-brush regime. The compression and the subsequent expansion curves superpose at the transition and quasi-brush regions but not at the brush and pancake stages. This points to a high cohesion in the brush structure after compression and to some irreversible entanglement and hydration of the PEO chains when immersed in the subphase. These two local hystereses depend differently on the PEO chain length and temperature. The hysteresis observed at high surface pressures (brush conformation) decreases with the PEO length and temperature, whereas the low surface pressure hysteresis (pancake) increases with PEO chain length, decreases with temperature in the range 283-298 K, and increases in the range 298-315 K. A negative mean transition entropy change was obtained from the temperature dependence of the quasi-SSAL-quasi-brush transition. The results indicate that the extensive properties of the present diblock copolymers at the interface, such as the pancake limiting area and the mean transition entropy, when expressed by PEOmer, are independent of the PEO length.
Macromolecules, 2001
We have investigated the molecular conformations of a lipopolymer with a polyoxazoline headgroup at air/water interfaces as a function of lateral area per molecule with X-ray and neutron reflectometry. The polymer 1,2-dioctadecanyl-sn-glycero-3-poly(2-methyl-2-oxazoline), PMO-(C 18)2, forms stable surface monolayers. Pressure/area isotherms around room temperature show a plateau region, indicative of a phase transition whose origin was examined. For data evaluation, a novel approach was used that acts on explicit quasi-molecular ensemble conformations of the polymer [Politsch et al., preceding paper in this issue]. At lower surface pressure, the polymer density distribution exhibits a maximum near the interface, indicative of attractive interaction between the predominantly hydrophilic polymer chains and the hydrophobic surface. Across the plateau region of the isotherm, a change in the volume density distribution of the alkyl chains was observed which is indicative of a partial immersion of the lipid moieties into the aqueous subphase. In contrast, no major structural change across the phase transition was detected in the polymer volume density profiles which comply with scaling predictions at both sides of the phase transition if deviations due to nonidealities are neglected. We interpret these observations as an alkyl chain ordering induced by the steric interference between the PMO: Immersion of alkyl chains into the subphase relaxes the strain on the hydrophobic anchors which derives from a reduction of the configurational entropy of the PMO chains due to their confinement to the interface.
Interactions of Poly(ethylene oxide) Brushes with Chemically Selective Surfaces
The Journal of Physical Chemistry B, 2000
Poly(ethylene glycol) (PEG) has long been recognized for its unusual ability to resist protein adsorption. This is attributed to the repulsion of proteins by the polymer segments. Despite its successes, there are several reports that PEG does weakly bind proteins. This work tests the hypothesis that the PEG can bind to nonpolar, hydrophobic groups such as the aliphatic side chains of amino acids. To do this we measured the forcedistance profiles between PEG 5000 brushes and self-assembled alkanethiol monolayers with varying amounts of nonpolar methyl-terminal groups. The polymer adhesion to these chemically selective surfaces increased with increasing density of surface methyl groups. The equilibrium thickness of the polymer chains in contact with the alkanethiol monolayer decreased correspondingly. The brush did not adhere to lipid bilayers or to bare mica. The results show that PEG will adsorb to nonpolar, hydrophobic surfaces. These findings may provide a possible explanation for previous direct force measurements of protein-PEG adhesion, and reports of PEG complexation with partially folded proteins.
Molecular Simulation Study of the Structure of High Density Polymer Brushes in Good Solvent
Macromolecules, 2010
Molecular dynamics simulations are presented of coarse-grained, polar, polymer brushes in a good polar solvent at high grafting densities. Chain extension is heavily influenced by temperature, stretching far from the surface at high temperature (350 K) while weak absorption and loop formation is observed at low temperature (300 K). Simulations of isolated free chains of different lengths in solution demonstrate the polymers are in good solvent conditions at both temperatures. Consistent with previous findings, increasing grafting density leads to larger chain extension under all conditions. A saturation limit at about half the bulk polymer density is found for high chain length and grafting density. Even at very high grafting densities a polymer depletion region near the surface is observed at 350 K due to an orthogonal orientation of the chain at the grafting surface. Radial distribution functions reveal that the grafting pattern does not affect the overall brush configuration beyond the first five monomers of each chain as long as the surface is homogeneously covered.