Synthesis, characterization and modeling of novel polymers and polymer-templated mesoporous materials

This research focused on the development of novel mesoporous silicate molecular sieves using amine-functionalized dendrimers and helical polymers as templates. The pseudo-spherical dendrimer templates were 4th and 5th generation poly(propylene)imines, while the amphiphilic polymers that formed ordered helical secondary structures were meta-poly(phenyleneethynylene) (m-PPE) oligomers. Since the exact template structure is important to the design of a given pore structure, molecular dynamics modeling studies were undertaken to predict the preferred structure of the helical polymer in solution. The 4th and 5th generation poly(propylene)imine were used as templating agents for the synthesis of mesoporous silicates and titanosilicates via sol-gel techniques that employed tetraethylorthosilicate (TEOS) as the silica source, and HCl as the hydrolyzing agent. The physical properties of these mesoporous materials were investigated by TGA, SEM, EDX, PXD, and nitrogen adsorption techniques. Results from these analyses showed that the catalytically active transition metal (Ti) is distributed evenly throughout the mesoporous silicates, which have interconnected spherical pores and high surface areas of about 650 m2 g-1.; The helical templates investigated were meta-poly(phenyleneethynylene) (m-PPE) oligomers. Molecular dynamics (MD) and replica exchange MD modeling techniques were used to determine which functional groups would cause the oligomer to rapidly form the desired helical structure needed for silicate templating. These simulation methods employed the parallelized GROMACS MD simulation code and OPLS force field. Simulation results showed that the helix structure is the preferred minimum energy conformation of a single oligomer in water, and that Lennard-Jones interactions are the dominant forces for the stabilization of the helix. The butoxy and methoxy m-PPE oligomers were prepared via palladium-catalyzed coupling; however, they exhibited low solubility in all solvents except DMSO. Thus, the butoxy oligomer, which was more soluble in DMSO, was used to template mesoporous materials using modified sol-gel techniques, with DMSO as a cosolvent. Results from nitrogen adsoption experiments showed that moderate increases in surface area and pore diameter were observed due to the low levels of the oligomer incorporated into the material. In summary, amine-functionalized polymers are good templates for mesoporous silicates; however, appreciable solubility of the polymers is essential to obtain high surface area materials.