Atomistic simulations of the structure of highly crosslinked sulfonated poly(styrene-co-divinylbenzene) ion exchange resins

Abstract

The microscopic structure of highly crosslinked sulfonated poly(styrene-codivinylbenze) resins have been modeled by generating atomistic microstructures using stochastic-like algorithms that are , subsequently, relaxed using molecular dynamics.Two different generation algorithms have been tested. Relaxation of the microstructures generated by the first algorithm, which is based on a homogeneous construction of the resin, leads to a significant overestimation of the experimental density as well as to an unsatisfactory description of the porosity. In contrast, the generation approach that combines algorithms for the heterogeneous growing and branching of the chains enables the formation of crosslinks with different topologies. In particular, the intrinsic heterogeneity observed in these resins is well reproduced when topological loops, which are defined by two or more crosslinks closing a cycle, are present in their microscopic description. Thus, the apparent density, porosity and pore volume estimated using microstructures with these topological loops, called super-crosslinks, are in very good agreement with experimental measures. Although the backbone dihedral angle distribution of the generated and relaxed models is not influenced by the topology, the number and type of crosslinks affect the medium-and long-range atomic disposition of the backbone atoms and the distribution of the sulfonic groups. Analysis of the distribution of the local density indicates that super-crosslinks are responsible of the heterogeneous homogenization observed during the MD relaxation. Finally, - stacking interactions between aromatic phenyl groups have been analyzed, those in which the two rings adopt a T-shaped disposition being considerably more abundant than those with the rings co-facially oriented, independently of the resin topology.3

Influencing Molecular Dynamics Simulations of Ion-Exchange Membranes by Considering Comonomer Propagation

“…The GAFF was used in all MD simulations to predict properties of the polymers . This force field has been successfully applied to study sulfonated poly­(styrene- co -divinylbenzene) , and cross-linked polyamide membranes. , …”

1-Butanol absorption in poly(styrene-divinylbenzene) ion exchange resins for catalysis

“…Atomic charges for sulfonated P(S-DVB) resin were taken from our previous work. 20 Atomic charges for BuOH were taken from a data base 25 and scaled (c = 0.71) to reflect that the polarization in condensed media is smaller than that in the gas-phase. 26,27 Before MD trajectories, the initial structures of the polymer network and absorbed BuOH were equilibrated using the following strategy.…”

“…The construction of the crosslinked network was reported in previous work. 20 The polymer matrix presented a complex topology characterized by a very heterogeneous distribution of both the crosslinks and the length of polymer segments as well as by the existence of internal loops (i.e. closed polymer chains) of different sizes.…”

“…apparent density, porosity and pore volume) in the dry state. 20 BuOH molecules were randomly introduced at sterically accessible positions of the equilibrated dry resin. Seven resin/ solvent systems, which differ in the amount of BuOH (10%, 15%, 20%, 26%, 30%, 40% and 50% w/w), were considered for the simulations described in this work.…”

“…those with 10%, 15%, 20%, 26%, 30%, 40% and 50% w/w BuOH). The average volume of the dry resin is 759.55 nm3 (see ref 20)…”