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03: Solution, gel and membrane structure
of rigid amorphous polymers and biomolecules
Rigid amorphous polymers for fuel cells
Recent advances in the synthesis of substituted polyparaphenylenes (PPPs)
and polybenzimidazoles (PBIs) have lead to the development of high molecular
weight amorphous rod-like polymers that are soluble in common laboratory solvents,
such as tetrahydrofuran and N,N-dimethylacetamide, and
can be readily processed into films and fibers from solution. PPPs and PBIs
exhibit exceptional properties. At present, these polymers are preferred for
applications in the military and civilian sectors that require a high level
of heat resistance, corrosion resistance and high modulus. Acid doped PBI has
emerged as a promising candidate for low-cost and high performance fuel cell
material. Polymer electrolyte membranes (PEMs) based on PBI can operate reliably
at temperatures above 120oC without humidification, thus offering many advantages
such as fast electrode kinetics, high tolerance to fuel impurities such as
carbon monoxide, and greatly simplified system design. Although it can be inferred
from prior studies by Benicewicz and co-workers that a sol-gel transition leads
to a variation in the morphology of the sol-gel membrane (based on different
chemical structures) for polybenzimidazoles, knowledge of the chain conformation
of this polymer in solution could have an impact on these variations in morphology.
In addition, the morphology of the sol-gel membrane may also influence the
proton conductivity and mechanical properties of these membranes for fuel cell
applications.
In collaboration with Benicewicz (PEMEAS GmbH and DOE-BES under DE-FG02-05ER46258)
we have recently discovered a random coil to wormlike chain and a wormlike
chain to extended wormlike chain conformational transition in the polymer chain
that can be induced by varying salt concentration or polymer concentration.
Furthermore, the presence of optical activity obtained from circular dichroism
measurements was linked to the wormlike chain to extended wormlike chain conformational
transition, which occurred at higher polymer concentrations. Birefringence
measurements indicated the possibility of relaxation caused by the degree of
chain flexibility and by deformation in the polymer chains due to the influence
of laser intensity.
Our current efforts are to study the solution behavior by static light scattering (Brookhaven Instruments Bi-MwA), synchrotron x-ray scattering (BNL and APS) and neutron scattering (NIST Center for Neutron Research) to understand the formation of associating structures at
the high polymer concentrations used to fabricate PEMs for fuel cells. These
studies can provide a basis for elucidating the role of polymer conformation
and fabrication conditions on proton conductivity and mechanical properties
of PEMs for fuel cells.
Biomolecules for sensing and medical imaging (Rensselaer NSEC/Morehouse/Spelman Collaboration under NSF DMR-01177792)
Many biological functions depend on the assembly of molecules into gels, networks, complexes or crystals. For example, carbohydrates play a critical role in cellular processes and are one of the most important classes of biomolecules. Some of these biomolecules have been utilized for the synthesis of nanoparticles such as iron hydro-oxides. These metal complexes of carbohydrates have been used to develop techniques for grafting other polymers onto starch and cellulose. These complexes also offer unique approaches for synthesizing nanoparticles for use in imaging, sensing and delivery agents because of their excellent ability to chelate an array of transition metal ions. The success of nanoparticles prepared using biomacromolecules requires the study of fundamentals such as characterization of the metal ion-biomacromolecules interactions, ligand/complex formation mechanisms, structure and morphology of the metal ions and the biomacromolecule. Currently, the proposed mechanism for metal-ion complexes involves the formation of metal-carbohydrate complex which dissociates to yield reduced metal ion and opening of the ring in the carbohydrate.
In collaboration with Bhatia (Morehouse College) and Ravi (Spelman College), Abdulahad (current graduate student in Akpalu's group) has undertaken studies to establish the physico-chemical properties of complexes of iron (II and III) ions with glucose, cellobiose, and cellulose. His is using Mossbauer spectroscopy to study the structural and magnetic properties of these complexes. A key result is that spectra of these complexes are typical quadrupole doublets and the parameters correspond to respective oxidation states of the metal ion in octahedral geometry. It is observed that the method of preparation plays a crucial role in the properties of the complexes. This work will be presented by Abdulahad at the 2006 Fall ACS National Meeting. To understand the detailed molecular mechanism underlying the structure and function of macromolecules involves characterizing structure on sizes that range from a few nanometers to several microns.
At Rensselaer, we are using static light scattering (Brookhaven Instruments Bi-MwA), synchrotron x-ray scattering (BNL and APS) and neutron scattering (NIST Center for Neutron Research) to characterize the size, shape, conformation and aggregation behavior of these complexes as a function solution conditions (e.g., pH, salt, biomolecule concentration).
Related Publications
- Shogbon C, Brousseau J-L, Zhang H, Benicewicz BC, Akpalu YA*, “Determination of the molecular parameters and studies of the chain conformation of polybenzimidazole in DMAc/LiCl”, Macromolecules 2006, 39: 9409-9418.
Related Press Releases
BI-MwA praised by Rensselaer Polytechnic Institute
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