| dc.description.abstract | Bicontinuous functional porous polymers are desirable in terms of pore accessibility, particularly for applications in templating, chromatographic separation, and catalysis. Bicontinuous morphologies can be obtained by block copolymer self-assembly only over a very narrow compositional window, which makes it synthetically challenging. On the other hand, polymerization induced phase separation (PIPS) methodology for generating bicontinuous porous structures, which was discovered several decades ago, is an easy strategy, but it does not permit precise control over the pore size, especially at smaller sizes. To achieve multiple objectives in a single step, Seo and Hillmyer ingeniously combined the concepts of PIPS and BCP self-assembly to generate crosslinked polymer matrices, wherein they introduced a covalent linkage between the pore-forming segment and the matrix, allowing microphase separation to occur at the nanoscale dimensions; thus generating nanoporous polymers. The bicontinuous pores were obtained because of kinetic trapping of the microphase-separated domains via in situ crosslinking.
The main objective of my thesis is to develop an alternate strategy using a polymerizable porogen, wherein a polymerizable Styryl unit is linked to the pore-forming PEG segment via a thermally labile linker, namely a urethane. Copolymerization of the polymerizable porogen (PolyPo) with a crosslinker, divinyl benzene (DVB), leads to the formation of a microphase separated crosslinked matrix; one of the defining ideas of the study is the exploitation of the thermal reversibility of the urethane linkage between the pore-forming segment and the matrix, which not only disconnects the porogen to generate the porous matrix but also leaves behind amine groups (upon reaction with water) that lines walls of the pore. Careful examination of the in situ microphase separation process during the copolymerization of the PolyPo, revealed that slowing down the polymerization by using controlled radical polymerization is essential to prevent premature crosslinking and allow effective microphase separation. The pore volume and surface area in these systems could be easily controlled by varying the ratio of porogen to the crosslinker, whereas the average pore size depended only on the length of the PEG porogenic segment. This concept was extended to the distyryl systems where both ends of the porogen PEG segment were linked to polymerizable groups; it was shown that the pore size was dependent only on the size of the porogenic segment and was unaffected by the presence of two polymerizable units. Further, it was shown that carrying out the copolymerization of the PolyPo in the presence of free PEG porogen could be an effective strategy for controlling the average pore dimension, despite broadening of the size distribution; likewise, copolymerization of two PolyPos carrying different PEG segments proved to be an alternate approach for fine-tuning the average pore size.
Finally, we showed that the pore-generating PEG segment could also be designed as a counter-ion to a suitable polymerizable unit; in this case PEG-trimethylammonium, 4-vinylbenzoate was designed as an ionic PolyPo. Here, a simple MeOH-HCl wash at 70°C was adequate for near-complete removal of the porogen PEG segment. The pore size, pore volume, and surface area of these counter ion-based systems varied much like their covalent counterparts. Most importantly, the porous crosslinked matrix that was generated using this approach carried carboxylic acid groups, unlike the earlier urethane-based strategy that left behind amine groups. In summary, we demonstrated novel single-step strategies for the preparation of crosslinked bicontinuous porous polymers carrying tailorable functional groups on the pore walls, with excellent control over the pore size and surface area. | en_US |
