(479g) Synthesis of Mesoporous TiO2 Using Amphiphilic Diblock Copolymer (PMMA-b-PAA) as a Self-Assembling Agent as a Template by RAFT Polymerization

A mesoporous material is a material containing pores with diameters between 2 and 50 nm. They have huge surface areas, providing a vast number of sites where sorption processes can occur. These materials have numerous applications in catalysis, separation and many other fields. The synthesis of these materials is of considerable interest and is constantly being developed to introduce different properties. One of the current scientific challenges is the creation of mesoporous crystalline metal oxide. Currently, there are many synthetic methods to produce high quality metal oxides, including sol-gel, hydrothermal, microemulsions, electrospinning, chemical vapor deposition (CVD), thermal decomposition, templating, and self-assembly techniques. The synthesis of nanostructured materials using the template method has become extremely popular during the last decade. One important strategy of nanostructure synthesis is self-assembly using soft chemistry which is inexpensive and easy to scale-up. The possible technological applications of block copolymers is dependent mainly on these materials' ability to self assemble into periodic morphologies. Moreover, block copolymers self-assemble on the nanoscale, and thus offer intriguing possibilities as templates for preparation of mesoporous bulk materials and mesoporous thin films. When a block copolymer is placed in a solvent that preferentially dissolves one block, micelles are formed in solution. These micelles are spherical structures in which the inner core, or insoluble block, is shielded from the solvent by an outer shell formed by the soluble block. In this study, PMMA-b-PAA, an amphiphilic diblock copolymer, was synthesized via living/radical polymerization, reversible addition fragmentation chain-transfer polymerization (RAFT), to use a self-assembling agent to synthesize of mesoporous TiO2. Using RAFT technique, at different time, diblock copolymer with different chain length of PAA were obtained; PMMA-b-PAA1, PMMA-b-PAA2, and PMMA-b-PAA3 after 1, 2, and 3 hours, respectively. GPC was used to investigate the livingness polymerization and results show the linear increase of the molecular weight with conversion and reaction time, and narrow molecular weight distribution. To investigate the morphology of block copolymer of PMMA-b-PAA at different times, SEM and TEM were employed. SEM and TEM images show different self-assemblies of diblock copolymer as the chain length of PAA is increasing. PMMA-b-PAA1 (T1), PMMA-b-PAA2 (T2), and PMMA-b-PAA3 (T3) were used as templates to synthesize of mesoporous TiO2. Ti (VI) is known to be able to form acetate complexes. In the complex, the acetate can potentially coordinate with the metal as a chelating, bridging bidentate or monodentate. In the case of amphiphilic diblock copolymer of PMMA-b-PAA, based on the need of carboxylic group of PAA, isopropanol was chosen as a solvent. In order to investigate the polycondensation reaction process of the synthesis of TiO2, in situ FTIR was utilized. In a nonaqueous solvent, if the metal alkoxide mixes with polycarboxylic acid, the carboxylic group will quickly coordinate the metal ions and form a metal-carboxylate complex along the polymer chain. Formation of the Ti-acetate complex, and poly condensation products along with consuming of carboxylic acid group of polyacrylic acid and titanium alkoxide, were observed. The crystal structure of the TiO2 particles synthesized at 65 °C and calcined at 600oC was examined by XRD. In the wide-angle powder XRD pattern, five crystal peaks at high 2 values of 25.28o, 37.80o, 48o, 53.89o, and 62.7o appear, which is consistent with the anatase phase of titanium dioxide materials. The isotherm plots show that the TiO2 materials exhibit a type IV isotherm with a hysteresis loop. The calculated pore volumes and surface areas shows that for materials synthesized using T1, the pore volume was 0.085cm3/g and a maximum pore volume 0.14cm3/g was reached for T2 and became 0.37cm3/g for material synthesized using T3 as templates. The pore size distributions of the mesoporous materials were investigated using BJH. The pore size distributions are bimodal for T1 and T2; the average pore diameter for the first peak in the pore size distribution was ca. 21-23 nm, which according to the IUPAC definition, still is in the mesoporous range. However, the second peaks at ca. 300 nm, are in the macroporous range. The materials synthesized using T3 shows a multimodal distribution; the first peak in the pore size distribution was ca. 21-23 nm while the second and third peaks are in macroporous range. The difference pore size distribution can be attributed to the morphology of the used templates. The BET surface area calculated from the N2 adsorption reveals that the specific surface area of the synthesized materials varied significantly with the chain length of PAA in diblock copolymer, such as 15 m2 /g for T1 which increase to 27 m2 /g and 105 m2/g for T2 and T3, respectively. The significant increasing in surface area is attributed to the morphological change. The size of the synthesized mesoporous TiO2 was investigated by dynamic light scattering (DLS) measurement. The samples were dispersed in THF using sonication. As the chains of PAA are increasing, while other reaction conditions remained unchanged, the particle size of mesoporous TiO2 was changed from 88, 173, and 350 nm. The effects of the chain length of PAA in PMMA-b-PAA on the microstructures and morphology of the TiO2 nanomaterials were characterized by SEM and TEM analysis. SEM was used to study the morphology of mesoporous TiO2 after calcination at 600oC. Samples with various morphologies were obtained when the chain length of PAA in PMMA-b-PAA is changed. As observed in these micrographs, the short PAA in diblock copolymer template (T1) produced the agglomerated spherical TiO2 particles. With increasing the chain length of PAA (T2), the morphology of TiO2 has been changed from spherical to rod/cylinder with diameter and length ranging from 40-300 nm and 200-4000 nm, respectively. It can be attributed to the morphology of block copolymer template, cross-linked pearl necklace. Further increasing the PAA length (T3), the TiO2 particles assembles into hollow core, having the wall thickness ~ 110 nm. TEM image of mesoporous TiO2 calcined at 600 °C for 2 hours the short chain length of PAA (T1) show both the crystalline and amorphous phases can be observed. Selected area electron diffraction patterns recorded on mesoporous TiO2 show the wall of these materials are comprised of nano crystalline oxides that characteristic diffuse electron diffraction rings, which is consistent with the XRD measurement. Upon increasing PAA chains resulting changing the morphology of T2 template, the rod/cylinder mesostructure TiO2 with short-range order can be observed. The TEM image indicates that the morphology of synthesized TiO2 after increasing the chain length of PAA in diblock copolymer template (T3) was changed to hollow core/mesoporous shell. However, because of the thickness of the shell wall ca. ~110 nm and the agglomeration of TiO2 particles, electron beams can't pass the shell which results the dark image of core. The mosoporosity of the shell can be observed from TEM images.