Molecular Simulations of Nanoporous Glassy Materials

Open Access
Author:
Abbott, Lauren Jeanine
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 12, 2013
Committee Members:
  • Coray M Colina, Dissertation Advisor
  • Ralph H Colby, Committee Member
  • James Patrick Runt, Committee Member
  • Scott Thomas Milner, Committee Member
Keywords:
  • molecular simulations
  • polymers
  • microporous materials
  • intrinsic microporosity
  • gas adsorption
  • polymatic
Abstract:
Microporous materials have pores smaller than 2 nanometers in size, which show enhanced adsorption of small molecules (e.g., methane, carbon dioxide, nitrogen) due to interactions with multiple pore walls. As such, these materials are of interest for gas storage and separations applications, including natural gas storage for on-board vehicular storage or the capture of carbon dioxide generated from burning fossil fuels. The performance of microporous materials is closely tied to nanoscale properties, such as pore sizes, accessible surface area, and total pore volume, but the current limitations in characterization techniques prohibit a clear understanding of their structures at this level. Molecular simulations can provide necessary insight into molecular-level structure and phenomena in this regard, complementary to experiments. Unfortunately, however, the generation of accurate molecular models of amorphous microporous materials is a non-trivial problem due to their complex structures. This dissertation presents the development of computational methodologies for accurately simulating amorphous, microporous materials. In particular, a general algorithm for constructing models of amorphous polymers by a simulated polymerization approach is discussed. This algorithm, termed Polymatic, has been released as open-source code for the benefit of the community. The presented structure generation techniques are validated for a wide range of linear and network polymers through comparison of structural, thermal, and adsorptive properties with available experimental data. Moreover, utilizing these methodologies, the structure-property relationships for amorphous microporous materials are studied extensively. For instance, the porosity of network polymers is shown to increase significantly with larger degrees of crosslinking, but decrease when the crosslinking is performed at higher densities. The molecular-level detail is also exploited to provide interpretation and analysis of experimental data, including peak assignment of X-ray scattering patterns to specific structural elements. Lastly, a computational screening of an array of intrinsically microporous materials is utilized to reveal important design principles for increasing porosity in these types of materials, namely rigidity, bulky end groups, and three-dimensionality, allowing for a directed approach to designing new materials.