Scattering and Physical Aging in Intrinsically Microporous Polymers
Open Access
- Author:
- Mcdermott, Amanda G
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 05, 2012
- Committee Members:
- James Patrick Runt, Dissertation Advisor/Co-Advisor
Coray M Colina, Committee Member
Ralph H Colby, Committee Member
Scott Thomas Milner, Committee Member - Keywords:
- scattering
SAXS
WAXS
physical aging
PIMs
polymers of intrinsic microporosity
microporous polymers
free volume - Abstract:
- Polymers of intrinsic microporosity (PIMs) form glassy, rigid membranes featuring a large concentration of pores smaller than 1 nm, a large internal surface area, and high gas permeability and selectivity. Porosity in these materials—closely related to free volume—arises from an unusual chain structure combining rigid segments with sites of contortion. Linear PIMs can be easily solution-cast into films whose interconnected networks of micropores can be exploited for applications such as gas separation and storage. Like other glasses, though, PIMs are subject to physical aging: a slow increase in density over time. This is accompanied by a decrease in permeability that reduces their performance as gas separation membranes. Several characterization methods are routinely employed to measure the structural properties of microporous materials, but none are as widely available and as easily applied to film samples with varied sample histories as small- and wide-angle X-ray scattering (SAXS and WAXS). Although it is possible to derive useful information such as surface areas and pore sizes from the scattering patterns of many porous materials, scattering from PIMs includes some unusual features whose interpretation is not readily apparent. In this work, a robust interpretation of PIM SAXS and WAXS features is developed with support from molecular dynamics simulations. The sensitivity of these patterns to time, temperature and film thickness is shown to be qualitatively consistent with physical aging, demonstrating that high-free-volume, porous polymeric glasses present a unique opportunity to study structural changes during physical aging using scattering methods. Models for extracting quantitative information about changes in the sizes and volume fraction of pores are also explored. Although quantitative interpretation of scattering patterns remains challenging, results of the aging study suggest that there may be two distinct mechanisms of aging in PIMs. Several tangential investigations are also presented, including (1) a study of small-molecule porous materials using scattering experiments; (2) a preliminary study of alcohol adsorption in PIMs using small-angle neutron scattering (SANS) and quartz crystal microbalance (QCM) techniques; and (3) a study of the low-q power-law scattering observed in a wide variety of amorphous, homogeneous polymers, including PIMs.