Atomistic simulation of weak polyelectrolytes in aqueous solutions

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Μιντής, Δημήτρης
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The present Ph.D. Thesis has received funding from the European Union’s Horizon 2020 Program (Marie Skłodowska-Curie project titled: “Training in Bio-Inspired Design of Smart Adhesive Materials, BioSmartTrainee”, Grant agreement No. 642861) for carrying out computational work that can aid the rational design and development of new, bio-inspired materials capable of bonding and de-bonding according to the micro-environmental conditions on wet, rough, and fouled surfaces. This project falls under the BioSmartTrainee ETN program (for more information please refer to the following link: and established a network of 10 full partners from academia and industry (BASF, AkzoNobel, and URGO) for developing new links between polymer science, adhesion technology, and biomechanics. In the framework of the BioSmartTrainee ETN program, different material strategies have been suggested for the design of switchable adhesives capable of bonding and de-bonding on demand under wet conditions, including among others: a) polymer brushes, b) micro-patterned surfaces, and c) polyelectrolyte gels (complex coacervates). The objective of the present Ph.D. Thesis was to develop and implement computational algorithms capable of providing quantitative predictions of the microstructure, state of hydration, and dynamics (segmental and terminal) of bulk aqueous solutions of weak polyelectrolytes, and investigate their response to relevant physicochemical parameters (such as pH, total polymer concentration, and chain length) starting from the molecular level. Additional work was carried out for determining the phase boundary of aqueous solutions containing symmetrical (in terms of charge density and molecular length), oppositely charged polyelectrolytes undergoing complex coacervation (that leads to liquid-liquid phase association) by computing the salt-polymer binodal phase diagram. Suitable modelling methodologies at different levels, such as Quantum Mechanics (QM) and Molecular Dynamics (MD), were employed to gain improved insight into the connection between detailed molecular structure and macroscopic properties of aqueous solutions of the following weak polyelectrolytes: • poly(acrylic acid) (PAA) • poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), and • poly(ethylene imine) (PEI) The global and local conformation as well as the dynamics of PAA chains in infinitely dilute solutions were first examined as a function of pH (equivalent to acid, neutral, and basic pH conditions) and chain degree of polymerization N (= 20, 23, 46, 70 and 110). Our predictions were compared to previous experiments and already established scaling theories for flexible polyelectrolytes. The effect of the total polymer concentration on the state of hydration, structure, and dynamics of PDMAEMA aqueous solutions was investigated next. In collaboration with one of our BIOSMART partners (Wageningen University), we also carried out rheological measurements of PDMAEMA solutions as a function of concentration. Aqueous solutions containing PEI of either a linear or a short chain branched architecture were also considered in the present Ph.D. Thesis in order to study the effect of pH and molecular weight on local rigidity, global conformation, and diffusive behaviour of PEI. These systems were simulated in infinitely dilute solution at ambient conditions. A detailed comparison to established theories and previous studies was reported. In the final stages of the Thesis, we computed (using fully atomistic models and free energy calculations) the phase boundary of an aqueous solution of two oppositely and fully charged weak polyelectrolytes, poly(acrylic acid) (PAA) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA). Such a system, under certain conditions (temperature, salt concentration, and molecular length) undergoes a particular type of liquid-liquid phase separation (LLPS) that leads to the formation of two distinct phases co-existing in thermodynamic equilibrium: a dense polymer phase (the coacervate) showing up in the form of liquid droplets and a dilute or polymer-deficient phase (the supernatant). The salt-partitioning in the two phases was also studied, followed by a detailed analysis of the specific interactions between certain pairs of atoms or groups of atoms in the two polyelectrolytes driving complexation and eventually phase separation.
Molecular dynamics, Weak polyelectrolytes