The role of hydrogen bonding in tethered polymer layers

Abstract

A molecular theory to study the properties of end-tethered polymer layers, in which the polymers have the ability to form hydrogen bonds with water, is presented. The approach combines the ideas of the single-chain mean-field theory to treat tethered layers with the approach of Dormidontova (Macromolecules, 2002, 35, 987.) to include hydrogen bonds. The generalization includes the consideration of position-dependent polymer-water and water-water hydrogen bonds. The theory is applied to model poly(ethylene oxide) (PEO), and the predictions are compared with equivalent polymer layers that do not form hydrogen bonds. It is found that increasing the temperature lowers the solubility of the PEO and results in a collapse of the layer at high enough temperatures. The properties of the layer and their temperature dependence are shown to be the result of the coupling between the conformational entropy of the chains, the ability of the polymer to form hydrogen bonds, and the intermolecular interactions. The structural and thermodynamic properties of the PEO layers, such as the lateral pressure-area isotherms and polymer chemical potentials, are studied as a function of temperature and type of tethering surface. The possibility of phase separation of the PEO layer at high enough temperature is predicted due to the reduced solubility induced by breaking of polymer-water hydrogen bonds. A discussion of the advantages and limitations of the theory, together with how to apply the approach to different hydrogen-bonding polymers, is presented.

Figure 1

The average volume fraction of the grafted chains as a function of the distance from the surface for (a) PEO layers, and (b) non-HB polymer layers. The different lines correspond to different temperatures: full line, T = 10°C; dotted line, T = 20°C; dashed line, T = 30°C. For all cases the molecular weight of the polymer chain equals M_w = 5000, which corresponds to N = 114 segments. The surface coverage is σ = 0.1nm⁻².

Figure 2

The average fraction of PEO-water and water-water hydrogen bonds as a function of temperature. The solid line with the circles correspond to the average fraction of PEO-water hydrogen bonds 〈x_p〉, and the dotted line with the square symbols shows the average fraction of water-water hydrogen bonds 〈x_w〉. The molecular weight of the polymer is M_w = 5000, and the surface coverage is σ = 0.1 nm⁻².

Figure 3

The height of the polymer layers as a function of temperature. The dotted line with square symbols corresponds to a PEO layer, while the solid line with circles correspond to a nonHB polymer layer, The molecular weight of the polymer is M_w = 5000, and the surface coverage is σ = 0.1 nm⁻².

Figure 4

The average fraction of PEO-water, 〈x_p〉, and water-water (insert), 〈x_w〉, hydrogen bonds as a function of χ, for a temperature of T = 20°C. The molecular weight of the polymer chain is M_w = 5000, and the surface coverage is σ = 0.1 nm⁻2.

Figure 5

The average fraction of PEO-water hydrogen bonds, 〈x_p〉, and average fraction of water-water hydrogen bonds, 〈x_w〉, as a function of the surface coverage at a fixed temperature T = 20°C. The molecular weight of the polymer is M_w = 5000, and the surface coverage is σ = 0.1 nm⁻2.

Figure 6

Lateral pressure-area isotherms of PEO layers (a), and non-HB polymer layers (b). The solid line corresponds to a temperature of T = 10°C, the dashed line to T = 20°C and the dotted line to T = 30°C. The molecular weight of the polymer is M_w = 5000.

Figure 7

Chemical potential of the polymer as a function of the surface coverage, for (a) PEO layers and (b) non-HB polymer layers. The solid line corresponds to T = 10°C; the dotted line to T = 20°C and the dashed line to T = 30C°. The molecular weight of the polymer is M_w = 5000.

Figure 8

(a) Lateral pressure-area isotherms of PEO layers on attractive surfaces with different attractive strength. (b) Fraction of PEO-water HB as a function of the distance from the surface. The inset shows the volume fractions of PEO layers. Different curves represent different attraction strength ε: solid line: βε = 0; dotted line: βε = −0.5; dashed line: βε = −1.0. The molecular weight of the polymer is M_w = 5000, the surface coverage is σ = 0.1 nm⁻2, and the temperature is T = 20°C.

Figure 9

(a) Lateral pressure-area isotherms, and (b) chemical potential versus the surface coverage of the PEO layer at a temperature of T = 150°C. (c) The volume fraction of the PEO layer with a surface coverage of σ = 0.1nm⁻2 at two different temperatures. The solid line corresponds to T = 150°C, and the dashed line is T = 20°C. The inset shows the fraction of PEO-water hydrogen bonds as a function of distance from the surface. The molecular weight of the polymer is M_w = 5000.