Structural behavior of competitive temperature and pH-responsive tethered polymer layers

Abstract

Herein, we develop a molecular theory to examine a class of pH and temperature-responsive tethered polymer layers. The response of pH depends on intramolecular charge repulsion of weakly acidic monomers and the response of temperature depends on hydrogen bonding between polymer monomers and water molecules akin to the behavior of water-soluble polymers such as PEG (poly-ethylene glycol) or NIPAAm (n-isopropylacrylamide). We investigate the changes in structural behavior that result for various end-tethered copolymers: pH/T responsive monomers alone, in alternating sequence with hydrophobic monomers, and as 50/50 diblocks with hydrophobic monomers. We find that the sequence and location of hydrophobic units play a critical role in the thermodynamic stability and structural behavior of these responsive polymer layers. Additionally, the polymers exhibit tunable collapse when varying the surface coverage, location and sequence of hydrophobic units as a function of temperature and pH. As far as we know, our results present the first molecularly detailed theory for end-tethered polymers that are both pH and temperature-responsive via hydrogen bonding. We propose that this work holds predictive power for the guided design of future biomaterials.

Figure 1

Average polymer volume fractions shown as a function of distance from the surface for purely responsive polymers (A₅₀). Temperature response (a) is shown at pH 1 and pH response (b) is shown at 27°C. For both cases the polymers are 50 segments in length. The surface coverage is σ 0.1 chains/nm² and the salt concentration is 0.1 M.

Figure 2

Average polymer volume fractions shown as a function of distance from the surface for polymers with an alternating sequence of responsive and neutral, hydrophobic monomers (AB)₂₅. Temperature response (a) is shown at pH 1 and pH response (b) is shown at 27°C. For both cases the polymers are 50 segments in length. The surface coverage is σ 0.1 chains/nm² and the salt concentration is 0.1 M.

Figure 3

Thermodynamic properties of (AB)₂₅ system. Chemical potential of polymers shown as a function of surface coverage for alternating A-B polymers for pH 5 at 27°C (3a). The osmotic pressure associated with pH 5 is shown (3b). The chemical potential is also shown for wider pH range (3c). The polymers are 50 segments in length. The surface coverage is σ 0.1 chains/nm² and the salt concentration is 0.1 M.

Figure 4

3D plot of average height of polymer layer as a function of both pH and temperature (4a). In 4b, maxima were extracted from the chemical potential profiles for the (AB)₂₅ system that exhibited loop behavior. They are plotted here to show predictive potential of theory in determining the zone where the system will collapse for a given pH and surface coverage. Results are shown for room temperature (T = 27°C).

Figure 4

3D plot of average height of polymer layer as a function of both pH and temperature (4a). In 4b, maxima were extracted from the chemical potential profiles for the (AB)₂₅ system that exhibited loop behavior. They are plotted here to show predictive potential of theory in determining the zone where the system will collapse for a given pH and surface coverage. Results are shown for room temperature (T = 27°C).

Figure 5

Average polymer volume fractions are shown as a function of distance from the surface for diblock cases with the hydrophobic block closer to the surface (a,c; B₂₅A₂₅)or away from the surface (b,d; A₂₅B₂₅). Figs. 5c and 5d are the average polymer volume fractions of A and B blocks separately at σ 0.1 nm^-2. Volume fractions were splined through the points.

Figure 6

Chemical potential shown as a function of surface coverage for alternating A-B polymers for pH 5. With increasing temperature at constant pH, the p-w hydrogen bonds decrease favouring brush collapse.

Figure 7

Average degree of dissociation, f_d, as a function of pH for different salt concentrations and different sequences (a, b, c).

Figure 8

The pH inside the tethered layer for a given bulk pH is shown for corresponding sequences (a, b, c) and different salt concentrations.