(355d) Gas Transport of Paraffins and Olefins in Liquid Crystal Membranes

Gas
Transport of Paraffins and Olefins in Liquid Crystal Membranes

Abstract

A solubility induced phase separation (SIPS) method was used to form 8CB and MBBA liquid crystal domains in a
polysulfone matrix.  The membranes were characterized for their temperature dependent phase
behavior and structural characteristics using DSC, POM, and FE-SEM.  Paraffin and olefin separations were investigated at different temperatures and different
liquid crystal phases using a mixed gas permeation apparatus.  Propane and propylene were
chosen to minimize size effects of diffusion and sorption.

            We
studied the effects of permeation and selectivity on intermolecular ordering of
the small molecular liquid crystal.  The
liquid crystal phase plays an important role for diffusion and solubility on
the solute.  The crystalline state
exhibited the lowest permeability with an increase in both propylene transport
and propylene selectivity over propane as the crystalline domain sizes
increased.  Transport through the liquid
crystals at their mesophase resulted in both an increase in permeability and
selectivity.  The contributing factor to
this phenomenon was concluded to be solubility
selectivity of propylene over propane due to enhanced intermolecular
interactions between the ordered domains and the more polar propylene solute.  A further increase in temperature resulted in
a two fold magnitude
increase in permeability while maintaining a selectivity of 4.

            A
cyanobiphenyl liquid crystal attached to a butadiene backbone with a flexible
alkane spacer was synthesized for the investigation of
its gas transport properties.  Room
temperature transport studies showed a helium selectivity of 60 over methane.  This was primarily due to size selective
diffusion across the membrane with very little solubility or sorption.  The effect of propane and propylene transport
on pressure and phase dependence was tested. 
Transport for temperatures under the LCPs glass transition was dominated by diffusivity.  A transition to a smectic liquid crystal
phase resulted in an increase in permeability and selectivity.  The contributing factor was the increase of
propylene solubility compared to that of propane.  A feed pressure increase resulted in an
increase in permeability selectivity, which was also due to an increase in
solubility selectivity.  Mixed gas
transport experiments correlated well with single gas transport.

            A
series of methacrylate liquid crystal polymers with nitro azobenzene and cyano biphenyl mesogens were synthesized and characterized.  These liquid crystal monomers were
copolymerized with 2-ethylhexyl acrylate (2-EHA) using a free radical
polymerization method.  Liquid crystal
phases were present for LC content of over 50 mol% for
the cyanobiphenyl and 60 mol% for the nitro azobenzene.  Mesophase stability increased with increasing
LC content.  A method to form acrylate
networks for the fabrication of stable membranes was developed and partially
characterized.  We will test its
transport properties in the future.