MARTINI-based simulation method for step-growth polymerization and its analysis by size exclusion characterization:a case study of cross-linked polyurethane

\u3cp\u3eSimulation studies of step-growth polymerization, e.g., polymerization of polyurethane systems, hold great promise due to having complete control over the reaction conditions and being able to perform an in-depth analysis of network structures. In this work, we developed a (completely automated) simulation method based on a coarse-grained (CG) methodology, i.e., the MARTINI model, to study the cross-linking reaction of a diol, a tri-isocyanate molecule and one-hydroxyl functional molecule to form a polyurethane network without and with dangling chains. This method is capable of simulating the cross-linking reactions not only up to very high conversions, but also under rather complicated reaction conditions, i.e., a non-stoichiometric ratio of the reactants, solvent evaporation and multi-step addition of the reactants. We introduced a novel network analysis, similar to size-exclusion chromatography based on graph theory, to study the growth of the network during the polymerization process. By combining the reaction simulations with these analysis methods, a set of correlations between the reaction conditions, reaction mechanisms and final network structure and properties is revealed. For instance, a two-step addition of materials for the reaction, i.e., first the dangling chain to the tri-isocyanate and then the diol, leads to the highest integrated network structure. We observed that different reaction conditions lead to different glass transition temperatures (Tg) of the network due to the distinct differences in the final network structures obtained. For example, by addition of dangling chains to the network, the Tg decreases as compared to the network without dangling chains, as also is commonly observed experimentally.\u3c/p\u3

Effect of Network Topology on the Protein Adsorption Behavior of Hydrophilic Polymeric Coatings

We prepared polyurethane (PU) network coatings with various cross-linking densities that were based on polypropylene glycol (PPG) and polytetramethylene glycol (PTMG) macrodiols with different lengths and containing similar amounts of hydrophilic methoxy polyethylene glycol (mPEG) dangling chains. Then, we investigated the effect of the network cross-linking density on the coating-water interface and protein adsorption through coarse-grained (CG) molecular dynamics (MD) simulations and experimental studies on molecular and macroscopic scales. Our CG MD simulations reveal that although a higher cross-linking density provides more connecting sites for hydrophilic dangling chains in the PU network, it diminishes the orientation of the hydrophilic dangling chains toward the water interface. Besides, our experimental results confirm that tighter networks with a similar total mPEG content display lower hydrophilicity (larger advancing water contact angle), a lower amount of mPEG migration to the interface (lower surface roughness measured by atomic force microscope), and higher human serum albumin and human fibrinogen adsorption, in agreement with CG MD simulation results