47038-AC7
Self-Assembly of Block Copolymer Bilayer Membranes at Oil-Water Interfaces
This project involves studies of molecular assembly at liquid/liquid interfaces. Two types of assemblies have been investigated during the previous grant period. The first of these is made by adsorption of copolymer molecules at the interface between two immiscible liquids, and the second involves assemblies that form between miscible liquids containing different solute molecules. Different types of questions are addressed in each type of experiment, as described in more detail below:
1) Effect of sequence distribution on copolymer interfacial activity.
Oil/water interfaces are ideally suited for studying the effects of sequence distribution on the interfacial activity of copolymer molecules. In our experiments the oil phase is chloroform, and the model copolymers contain methacrylic acid and methyl methacrylate units. Three different types of copolymers are considered: statistically random copolymers where the different repeating units are distributed randomly along the backbones of the polymer molecules, block copolymers where the different repeating units exist as distinct chemical blocks, and gradient copolymers where the average copolymer composition varies smoothly from one end of the molecule to the other. Gradient copolymers represent a relatively new class of polymeric materials that has emerged as a result in advances in controlled radical polymerization techniques. The polymers used in our experiments were produced as part of a collaboration with Prof. John Torkelson's group here at Northwestern. The copolymers are dissolved in a pendant chloroform droplet that is embedded in water, and the interfacial tension between the two liquid phases is obtained from an automated shape analysis of the droplet profile. Interfacial segregation of the copolymer molecules is driven by the relative insolubility of the methacrylic acid groups in the chloroform phase, and by the preference for these groups to be in contact with the aqueous phase. Interfacial segregation is limited by the formation of micelles in the bulk chloroform phase (for block or gradient copolymers) or for phase separation into a polymer-rich phase (for the random copolymers). We have shown that the interfacial activity is maximized for gradient copolymers, a result that we attribute to the reduced tendency for these copolymers to form micelles.
In addition to studies of the equilibrium adsorption of the copolymers to the oil/water interface, we have also investigated the mechanical properties of interfacial layer formed by this adsorption process. This information is obtained by shrinking the chloroform droplet and measuring the interfacial tension over times that are too short for any desorption of the copolymer molecules to be observed. These experiments allow us to obtain the area modulus of the interfacial layer, an important mechanical characteristic of the adsorbed layer from which structural information can be inferred.
2) Structure-property relationships of polymeric membranes formed a the interface between miscible liquids.
The drop shape experiments described above are an example of the determination of the elastic properties of interfacial membranes that form under well-controlled conditions. We have recently used a similar protocol to investigate the properties of membranes that form at the interface between two aqueous liquids with different compositions. This work is being done in collaboration with the group of Prof. Sam Stupp, a faculty colleague at Northwestern. The experimental geometry is similar to the geometry used to study adsorption to the chloroform/water interface, but in this case the chloroform droplet is replaced with an aqueous solution of a high molecular weight polyelectrolyte. The aqueous matrix phase contains a low concentration of peptide amphiphiles that form a fibrous network when brought into contact with the polyelectrolyte solution. This network interacts with the polyelectrolyte molecules to form a two-dimensional elastic membrane at the interface between the two liquids. This membrane thickens with time, and the factors controlling the elastic modulus, biaxial tensile strength and water permeability of these membrane structures are being determined.
