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https://hdl.handle.net/2142/21032
Description
Title
Bulk synthesis of two-dimensional polymers
Author(s)
Son, Sehwan
Issue Date
1994
Doctoral Committee Chair(s)
Stupp, Samuel I.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Polymer
Engineering, Materials Science
Language
eng
Abstract
"The main objective of this research involves the bulk synthesis of two-dimensional (2D) polymers. Two different approaches, the chiral recognition approach and the hairpin approach, have been used for the ""bulk preparation of the two-dimensional polymers""."
Firstly, the chiral recognition approach is to confine two-different reactive groups in well-separated planes of layered monomeric units and a liquid crystalline chiral precursor was designed that contains two reactive groups (a chiral nitrile and an acrylate) to meet this condition. If there is only one polymerizable group in the monomer, polymerization of the layered monomers would yield a comb-shaped 1-D polymer. Since the monomer is intentionally designed to have ability to self-organize into layered structures, preorganization of monomers through external force is not required, and simple heat treatment converts monomers to 2D polymers in a flask. This represents the first example of such a reaction reported in the literature (Science 259, 59, 1993). The resulting 2D polymer has monodisperse thickness (50.2 angstroms) and molecular weight in the order of millions of daltons. TEM experiments revealed 2D polymers stack to form lamellas phases analogous to smectic phases. More interestingly, this 2D polymer is 90% by weight soluble in chloroform and has a sharp isotropization transition at 230$\sp\circ$C. This is believed to be the consequence of a flexible backbone in these 2D polymer molecules and the presence of holes (defects) created by the monomers that do not participate in the polymerization reaction and are later extracted either by solvent or thermal energy. A comparison between the 2D polymer synthesized and its 1D analogue reveals higher thermal and temporal stability in films containing the molecular sheets. Apart from the two-dimensional polymer synthesis, a derivative of the two-dimensional polymer precursor gave rise to enormous second order nonlinear optical signal larger than that of KTP.
Secondly, the hairpin approach includes designing and synthesizing precursors with various head groups to induce molecular folding. Precursors contain two diacetylene groups to prevent one-dimensional polymer formation. Bulk polymerization of liquid crystalline hairpin shaped monomer was found to maintain smectic ordering with high thermal stability up to 346$\sp\circ$C. Surface functionalization of two-dimensional polymers has been also achieved using hydroxy terminated or metal containing precursors. During these investigation, we found that one of the precursor formed highly ordered two-dimensional polymer through covalent-hydrogen bonding. Moreover, this polymer revealed usually high stability to intensive laser beams and third order nonlinear optical susceptibility. Since the conjugated backbone is embedded inside the polymer, the conjugated structure may be inert to moisture or oxygen attack.
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