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

Abstract: Mesoporous silica materials, such as SBA-15, have potential as biocompatible carriers for pharmaceutical applications as well as mini-reactors in e.g. catalysis. This potential arises from their well-defined porous structure on the nanometre scale accompanied by a high internal surface area (800 - 1000 m2/g). This work addresses the following issues. i) Surface grafted PNIPAAM was found to be located preferentially in the intrawall pores of mesoporous silica SBA-15. Water sorption could monitor the changes in surface properties and porosity of the different materials, obtained during the modification process. While the isotherm of original SBA-15 exhibits four regimes of water adsorption, SBA-15 with grafted PNIPAAM lacks the second regime, which is characteristic for the presence of intrawall pores. ii) The intrawall porosity of mesoporous silica SBA-15 was removed by addition of NaI without altering the other characteristics of the material. The time of addition is not crucial but influences the efficiency of the method. The method was transferred to other syntheses carried out with Pluronic as the structuredirecting agent, such as of SBA-16, demonstrating the general applicability. Subsequently, PNIPAAM was grafted into SBA-15 without intrawall pores. The mesoporosity of SBA-15 was modified by lowering the synthesis temperature from 55 °C to 5 °C after 1 h of synthesis time. The micelles of the structure-directing agent Pluronic P104 transform from an elongated shape towards a spherical shape. After 1 h of the synthesis the network of SBA-15 is still flexible, which allows adaption too the changes in the shape of the micelles and thus leading to a more corrugated system. iii) Adsorption of γ-CD onto a cationic PNIPAAM co-polymer, which is adsorbed to a silicon surface, in an aqueous environment at 25 °C leads to γ-CD threading onto the polymer chains, comparable to pearls on a necklace. An increase to 40 °C leads to a collapse of the PNIPAAM part and the γ-CD is released. γ-CD was thread onto the PNIPAAM chains grafted onto the pore walls of SBA-15, showing the function of PNIPAAM in SBA-15.