Biocompatibility, antibacterial qualities and drug delivery capabilities are examples of properties which are highly desired for certain materials and products. They are also examples of properties that can be achieved by clever material design, using for example polymer brushes, polyelectrolyte multilayers or hydrogels. One parameter that is important when tailoring the interfacial properties of these materials is the layer conformation, such as cross-linking and degree of hydration.
The polymer conformation at the surface influences the interfacial properties
Polymers and polyelectrolytes of various conformations, such as brushes, multilayers or hydrogels, i.e. cross-linked polymer networks, are used in many applications where there is a need to tailor the interfacial properties to promote a certain interaction with the surrounding environment. Protein adsorption, prevention of bacteria adhesion and drug release, are a few examples of desired interaction scenarios.
Both the polymer brush, multilayer and hydrogel, are more or less hydrated and viscoelastic, depending on the molecular conformation at the interface, and the conformation will have a major impact on the resulting interfacial properties. Properties, which in turn, will influence the interaction with the ambient.
To tailor the materials so that they support the desired interaction with the environment, for example to promote protein adsorption, prevent bacteria adhesion, or release the drug in a stimuli-responsive fashion, it is therefore relevant to characterize the conformational behavior of the polymers at the surface. One tool that can be used to evaluate the layer conformation is the surface sensitive QCM-D technology.
Monitoring water release and crosslinking of thin films
To understand and tailor the interfacial properties of these layers, it is important to characterize and understand the conformational behavior, such as the degree of hydration and transitions from a hydrated to a collapsed or crosslinked state, and vice versa, Figure 1. The swelling and collapse of polymer brushes, and other thin films, can be characterized by QCM-D and other technologies, which will sense the water uptake and water release as changes in mass.
Figure 1. From left to right, this schematic illustration shows how a thick and hydrated film releases water and collapses into a thin layer at the surface.
Example of collapse and cross-linking
As an example of the transition between a hydrated and dehydrated state, let’s look at the swelling and collapse of polymer brushes made of Chitosan . The conformation of the chitosan brush depends both on pH and counter-anion size. At low pH, the brush is water-soluble while at pH above 6.5, it is a collapsed coil and insoluble in water.
The QCM-D sensor is initially coated with the polymer.
Next, the brush layer is exposed to solutions of different pH and counter-anion type, to note the effect on the film thickness.
The results, Figure 2, show that the layer swells at high pH and contracts at low pH.
Figure 21. Thickness of the chitosan layer when exposed to solutions of different pH and counter-anion type.
It is also possible to use anions for crosslinking of the brush . The results, Figure 3, show that when the citrate anions are replacing the acetate anions, ionic cross-links are formed with the ammonium cations on the brush, which results in a collapse of brush layer.
Figure 31. (Top) The thickness of the Chitosan brush layers when exposed to different solution pH and counter-ions. (Bottom) Schematic illustration of the structure of the chitosan brush layer as a function of pH and counter-ion type.
These measurements show how the changes of pH and counter-ion can induce collapse and cross-link the film, and the results give insight into how the polymer brush conformation responds to ambient pH and counter-ion size.
Read more about how to characterize polymer layer swelling, crosslinking and collapse in our application note below.
1. H-S Lee, et al., J. Mater. Chem., 22, 19605, 2012
1. Figures are reproduced from Ref. 1 with permission from The Royal Society of Chemistry
The versatility of polyelectrolyte multilayers, PEMs, is high, which makes them interesting for e.g. biomedical applications. The functionality is largely determined by the layer properties, which needs to be understood to be tailored. Here, we show how PEMs can be characterized with QCM-D.
The ability to take up and release water is central for many materials, such as hydrogels, whose function depend on the ability to hydrate and dehydrate. Hydration and swelling are also central when dealing with hygroscopic materials. QCM-D can be used to characterize such swelling phenomenon.