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.

Abstract (Swedish)

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

• author: Reichhardt, Nina
• supervisor: Viveka Alfredsson; Tommy Nylander; Beate Klösgen
• opponent: Schmidt, Wolfgang N., Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr, Germany
• organization: Physical Chemistry
• publishing date: 2011
• type: Thesis
• publication status: published
• subject: Physical Chemistry
• keywords: mesoporous silica, SBA-15, intrawall pores, PNIPAAM, lower critical solution temperature, water sorption calorimetry, gas sorption
• pages: 200 pages
• defense location: Center of Chemistry and Chemical Engineering, Lund, Lecture Hall B
• defense date: 2011-11-03 10:30:00
• ISBN: 978-91-7422-281-4
• language: English
• LU publication?: yes
• id: 8b15b736-0e18-4a0d-ac47-7bf11b9fa3f9 (old id 2171895)
• date added to LUP: 2016-04-04 10:32:27
• date last changed: 2018-11-21 20:59:21