AmericanChemicalSociety.com

The objective of this work is to study the chemical and physical properties of zwitterionic polymers. We have been tackling this problem from three angles – (a) studying phase behaviors in a polymer gel where three transition points are identified; (b) investigating the packing behaviors of polymer brushes where the transition from hydrophilic to hydrophobic is identified; (c) exploring the effect of the chain length between two charged groups on hydration where a maximum hydration is identified.  

Phase Transitions in a Polymer Gel: In this work, we performed molecular simulation studies of the swelling of a zwitterionic gel in water in order to provide a fundamental understanding of water interactions with a zwitterionic polymer network. The polymer studied was polycarboxybetaine methacrylate (pCBMA) and its derivative. For its derivative, a hydroxyl group attached to the carbon atom between positively and negatively charged groups was introduced in each CBMA repeat unit in order to study the effect of hydrogen bonding on the hydration of the polymer network. A series of all-atom gel models were built using this modified pCBMA repeat unit. These models contained different numbers of water molecules, ranging from 19% to 90% of the total weight of the gel. Molecular dynamics simulations were performed to study the properties of the gel. Results show that three transition states of the gel can be observed as water context increases. At 36% water content, a kink in the diffusion coefficient of water was found, indicating the hydration of the polymer gel starts to be saturated, representing the first transition. The equilibrium water content of the gel was determined to be 60% from the stability analysis of physical crosslinkers. The further increase in water content shows the second transition state where the associations of zwitterionic side chains break. Finally, we observed an apparent change in the bonded energy of the modified CBMA gel when the water content goes beyond 80%. This water content is believed to be the possible maximum water content for this gel, representing the third phase transition. These results are comparable to those from previously published experimental studies about similar zwitterionic gels. Although our discussion largely focuses on the zwitterionic gel in this work, the approach can also be applied to help identify the critical transition states of other gels from simulations. This work is highlighted in the Nugget and the TOC figure. A paper has been prepared and is under review.

Molecular Packing of Zwitterionic Polymer Brushes: Using molecular dynamics (MD) simulations, we investigated the surface properties of pCBMA polymer brushes as a function of their chain distances (or packing densities). Each chain carries ten units. The chain distance is defined as the distance between the mass centers of the adjacent backbones. The entire surface was composed of 144 chains (12×12). After minimization and 1.0-ns molecular dynamics in an explicit water solvent, we first investigated the contact angles of two surfaces with different chain packing densities (or chain distances) using a cubic water box with 1.5 × 4.0 ×4.0 nm³ as a probe. The MD results show that the contact angle of the surfaces depends on the chain distance. As the chain distance decreased from 1.4 nm to 1.0 nm, the water contact angle changed from nearly zero to a significant contact angle. This increase in contact angle indicates that the polymer surface turns to be more hydrophobic as surface packing densities increase although the polymer component is identical. We are now investigating the effect of the chain distance on the structural and dynamic properties of the hydration layer, including the hydrogen bond structure, the spatial distribution and orientation of water molecules, and the residence time and reorientation of water molecules. The work will be extended to the surface of nanoparticles. This work will be highlighted in the next report.

Chain-length dependence of Hydration of Zwitterionic Compounds: CBMA molecules contain an acidic carboxylate group covalently linked to a basic quaternary amine group, producing zwitterionic molecules. In this work, quantum chemical calculations and molecular dynamics simulations were performed to investigate the conformation and solute-solvent interactions of CB molecules with one to five methylene spacer groups between the charged quaternary amine and carboxylate groups in water and a sodium chloride solution of physiological ionic strength. As the number of methylene spacer groups in the CB molecules increases, the number of possible molecular conformations also increases.  The conformational potential energy was calculated to determine the most stable conformation for each CB molecule studied.  The optimized CB structure and the resulting force field parameters for each conformation were then used to calculate the solvation free energy.  The calculated solvation free energy was used to evaluate hydration interactions. Results are compared with those from experiments. Results show that the chemical structure of CB (or the number of methylene spacer groups in CB) can significantly impact its molecular conformation.  These conformational differences lead to differences in the CB solute salvation capacity.  A paper has been prepared and is under review.