Mixed Block Copolymer Solutions: Self-Assembly and Interactions

University dissertation from Lund University, Faculty of Science, Department of Chemistry, Division of Physical Chemistry

Abstract: Popular Science SummaryPolymers are giant molecules consisting of a repetition of many small units (monomers) that are linked together by chemical bonds. If we consider a long necklace as a polymer chain, the beads are the monomers and the part of the thread which connects the beads together is the chemical bond. Polymers are important materials in our everyday life and they can be found e.g., in plastics, paints, fabrics, papers, food materials, cosmetics and drugs.When a polymer is composed of more than one type of monomer, it is called a copolymer. Copolymers can have a variety of structures depending on how the different monomers are arranged in the polymer chain. One type of these copolymers is called block copolymers. For instance, if we attach the necklace with the pink beads (block A) to the end of the other necklace with the blue beads (block B), then this is called an A-B block copolymer or generally a ‘’diblock’’ copolymer. One can make A-B-A or A-B-C which are called ‘’triblock copolymers’’ or even block copolymers with more than three blocks. Some of these block copolymers show interesting behavior when they are in contact with water. For instance, they may have a block that hates to be in contact with water (necklace with the red beads) and the other block which loves water (necklace with the blue beads). These types of block copolymers are scientifically called ‘’amphiphilic’’ block copolymers and they are considered polymeric surfactants. To have a favorable condition in water for both blocks, block copolymer chains may arrange themselves in a type of spherical structure that is called a ‘’micelle’’. In this micelle, the blocks which hate water, occupy the core to have less contact with water and the blocks which love water form the corona of the micelle. This spontaneous behavior of the amphiphilic block copolymers in water is called ‘’self-assembly’’ which happens above a specific polymer concentration. Some block copolymers are sensitive to a change in temperature, and hence, temperature can influence their self-assembly. These types of block copolymers are called ‘’thermoresponsive’’ block copolymers. Thus, they self-assemble or form micelles above a specific temperature. Micelles of block copolymers play an important role in treatment of some specific diseases. These micelles, with sizes of about several tens of nanometers, can incorporate drug molecules and act as their carriers into the body. Due to their specific structure and the optimal size, they will not be removed from the blood stream and hence, the circulation time of the drug in the body could be increased. The physical and chemical characteristics of the block copolymer micelle can be tuned in a way that makes it suitable for targeting specific organs or tissues in the body to release the drug. Therefore it is very important to study the physical and chemical properties of the block copolymers and their self-assembly behavior. Meaning, for instance studying how different stimuli such as temperature, addition of acid, salt and other types of chemicals can effect on the self-assembly of these block copolymers.This thesis contains studies on some of the amphiphilic and thermoresponsive diblock and triblock copolymers. Some of these block copolymers have already been used in pharmaceutical applications and some may have the potential to be used. In this thesis it was demonstrated how block copolymer concentration, temperature, addition of salt and a natural body surfactant (bile salt) as well as the length of the loving-water or the hating-water blocks can influence the self-assembly of the micelles and break them up. Various techniques have been applied to study these block copolymer systems. Results demonstrated that some of these block copolymers could be potential candidates in the treatment of specific diseases for instance, hypercholesterolemia.

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