Towards a responsive functional material: Modification of porosity and grafting of poly(N-isopropylacrylamide) in mesoporous silica SBA-15

University dissertation from Department of Chemistry, Lund University

Abstract: Popular Abstract in English Figure 1: Electron microscopy image of mesoporous material seen from above. The left image shows a possible shape of a silica particle (scale bar: 1 μm). The right image shows the mesopores in the particle (grey color) with the silicon dioxide walls (black color) (scale bar: 150 nm). Poly(N-isopropylacrylamide) (PNIPAAM) is a polymer that can provide the active part for a drug delivery vehicle controlling the uptake and release of drugs. Polymers are made of small repeating units forming macromolecules like beads on a necklace, where the small repeating units are connected to form a long chain. Examples of polymers in our daily Popular Scientific Summary in English life are plastics, shampoo and paints. PNIPAAM is a so-called stimuliresponsive polymer. Combining mesoporous silica with PNIPAAM can provide a drug delivery vehicle. The hard mesoporous silica material provides the stability, while PNIPAAM with its soft character provides the active component. Figure 2 illustrates the mechanism of this PNIPAAM-silica composite. The chains of PNIPAAM are attached to the walls of the pores. When the temperature is above 32 °C (Figure 2a) the polymer is collapsed and the pores are open, allowing molecules such as drugs, to enter the pores. When the temperature is decreased below 32 °C (Figure 2b), the PNIPAAM chains become hydrophilic and stretch towards the centres of the pores forming a “jungle” in which the molecules are prevented from moving. Figure 2: Schematic illustration of a mesopore with grafted PNIPAAM: a) above 32 °C allowing drug molecules to be loaded into the pores, and b) below 32 °C showing the drug molecules trapped inside the pore by the PNIPAAM chains. The first part of this work deals with the functionalization of the mesoporous silica material with PNIPAAM. This procedure is carried out in three steps starting with the original mesoporous silica material and resulting in a PNIPAAM-silica composite, in which the PNIPAAM chains are linked to the inner surface of the pores in the mesoporous silica. The materials were characterized using several different techniques. The aim was to understand where the polymer is located inside the mesoporous silica matrix, and to obtain information about the chemical properties of the materials. The second part concerned the mesoporous silica material itself. Different applications require different porous properties. The porosity of a common mesoporous silica material, SBA-15, which consists of two different types of pores, was studied. The primary mesopores, which have a cylindrical shape, are connected by small irregular pores called intrawall SiO2