Synthesis of Functional Porous Materials from Block Copolymer-Based Multicomponent Blends

Abstract

Since the pioneering work of the Mobil researchers in 1992, ordered mesoporous materials have been extensively studied in the past decades because of their outstanding properties including high specific surface area, large pore volume, tuneable pore size and pore structures. Thus, ordered mesoporous materials have attracted considerable attention as prospective candidates for a wide range of applications such as adsorption, separation, drug delivery, catalysis, and energy storage and conversion. Self-assembly of block copolymers (BCP) holds great promise for synthesis of functional mesoporous materials with tuneable architectures on the nanometer scale. Despite the considerable progress in BCP directed co-assembly, major challenges still remain in (i) selective incorporation of active species (e.g., metal catalysts) in a spatially organized manner, (ii) direct synthesis of hierarchically meso-macroporous materials with well-defined pore networks and (iii) control over macroscopic particle morphologies (i.e., uniform and regular geometry such as spherical shape). Previous approaches mostly rely on the use of complicated, laborious multi-processes or the integration of multiple templates/techniques, which often require specific chemistries and thus are only applicable to a few materials (Chapter 1). The primary objective of the present study (Ph.D. thesis) is development of facile, straightforward synthetic approaches that overcome the aforementioned challenges by using BCP directed self-assembly of multicomponent blends. To this end, we combine inorganic sol-gel chemistry, synthetic polymer chemistry, and phase behaviour of multicomponent blends. In chapter 2, a straightforward synthesis of Sn/carbon/silica (Sn-CS) composites with unique morphologies is described. The method uses the selective interaction among the precursors and block copolymer; hydrophilic silica and carbon precursors are selectively mixed with polyethylene block, whereas hydrophobic Sn precursors are incorporated into polystyrene block exclusively. We successfully synthesize the Sn nanowires or Sn nanoparticles that are embedded in CS frameworks. In chapter 3, we report a simple method for the synthesis of inorganic oxide materials with multiscale porosity. The inorganic precursor co-assembles with PEO-b-PS to form ordered mesostructures by microphase separation. At the same time, in-situ polymerized organic precursors (resol) segregate out, forming organic-rich 3D-macrodomains (macrophase separation). After calcination, hierarchical meso/macroporous oxides, such as SiO2 and TiO2, with three dimensionally interconnected pore-networks are obtained. Nanoscale X-ray computed tomography clearly visualizes the continuously interconnected macrostructures. Finally, we develop generalized synthesis of monolithic macro-mesoporous and spherical mesoporous materials. We simultaneously combined the micro- and macrophase separation in a predictive and systematic manner and used them as rational tools for synthesizing materials with intended nano- and macrostructures. Macrophase separated homopolymer domains act as an in-situ generated sacrificial template that provides macropores or molds the macrostructures into spherical morphology. The size of both macropores and spherical particles is highly uniform, and is readily tunable in a length regime of 500 nm ~ 10 µm and 200 nm ~5 µm, respectively. This thesis is of particular importance because it (i) expands the base of BCP self-assembly to complex porous structures; (ii) shows that interplay of micro-and macro-phase separation can be fully utilized for the design of useful materials with intended nano- and macrostructures; and therefore (iii) provides an insight into strategies for the researchers in materials science and polymer science.