TRANSFERENCIAS:

In Chapter 2, we provide insight into the transport of small molecules in chi- tosan/chitin membranes by showing that the water/polymer distribution develops to a well-connected, continuous aqueous domain distributed through-out the membrane with increasing humidity. To the extent that small molecules are located in the aqueous domain, this sample-spanning network is ideal for transport. In fuel cells membranes, where proton transport is desirable, polymer chemistry is manipulated to encourage the development of well-connected aqueous domains. [4,61,291] When

sample-spanning aqueous domains are not present, a percolation threshold appears, resulting in virtually no transport. [217] This same knowledge can be applied to chitosan/chitin membranes for the opposite effect. Manipulation of polymer chemistry leading to isolated and disconnected aqueous domains in the membrane would result in lower oxygen permeability.

In Chapter 3, our goal is to apply the analytical theory to a range of proton exchange membranes and help guide experimental synthesis to determine optimal design of membranes. We recognize that potential performance for proton exchange membranes could far exceed current performance. A key result from this work is that there exists an optimal surface coverage for a given pore size in terms of maximizing proton conductivity.

In Chapter 4 , we believe that for systems where there is a strong correlation to hydration and proton migration, PEG monomers can take the place of water molecules in necessary solvation shell structures and potentially have modestly favorable effects on proton transport. This would imply that in systems whose proton conductivity suffers due to low hydration, PEG can have an enhancement effect on the structural diffusion of protons. Also, as a polymer, given an optimal orientation of the PEG chain, the structural diffusion could show directional preference along the polymer backbone.

In Chapter 5, we show how, given an optimum polymer chain orientation, the PEG polymer chain does have an impact on the diffusivity of H+ _{in an aqueous}

environment. This can impact the design of materials that aim to control charge transport in aqueous domains. This could possibly be achieved by confining the space the polymer is in such that it orients in an elongated fashion, possibly by confinement in a nanpore. Yet, the confinement may have a strong impact on the charge transport and requires further study to validate this approach. This work has provided the molecular-level detail that can aid in material designs that involve PEG. In Chapter 6, we describe our development of a quantum mechanical dataset for training a ReaxFF parameter set for xsPCHD. The impact of this work is in direct relation to performing reactive molecular dynamics of xsPCHD/PEG. The results of these simulations would provide the larger impact for giving atomistic insight into transport processes in these materials and overall membrane design.

The focus of Chapters 4, 5, and 6 is to describe the subtle effects that PEG has on proton transport in trying to understand the enhanced conductivity in xsPCHD/PEG membranes. In Chapter4, we observed the effects of incorporating the PEG oligomer, TEG, into aqueous solution and the differences from proton transport in bulk water. We observed that the free energy of proton transfer between water molecules and the incorporation of TEG chain molecular groups in the hydrogen- bond network were the largest differences. Specifically, we found that the inclusion of methylene as a hydrogen-bond donor was associated with hydrogen-bond networks that where correlated to structural diffusion. This was attributed to the fact that the hydronium ion has an amphiphilic nature, with the lone electron pair attracted to the hydrophobic methylene group. The work in Chapter 5 showed that the orientation

of the PEG chain was the strongest indication for enhanced proton transport and that the free energy of proton transfer between water does not share a correlation to enhanced proton transport. From the observations in Chapter4and5, we believe that the elongated chain, showing preference for proton transport parallel to the polymer backbone, attracts the amphiphilic hydronium ion. The chain also rearranges the hydrogen-bond network to local structures that favor structural diffusion along the axis parallel to the chain backbone. Thus, the mechanistic picture for PEG to enhance proton transport is a coupling of attraction of the hydronium ion to the polymer backbone and the rearrangement of the local hydrogen-bond network surrounding the polymer chain. In the xsPCHD/PEG membranes, the water structures and local environment will certainly be different than in the bulk due to confinement and the inclusion of anion groups. The work in Chapter 6 is the first step to seeing what effect the confinement and anion groups will have on proton transport and how it is coupled to the PEG orientation in the xsPCHD/PEG blend and block copolymer PEMs. From this work, the natural extension would be to revise the analytical model used in Chapter 3to incorporate hydrophilic polymers that can have an effect on proton transport and decouple water and charge transport in PEMs.