Polymer Physics and Modeling of Polycarboxylate-based Superplasticizers

Faculty Development Competitive Research Grants 2020-2022

Polycarboxylate-based Superplasticizers (hereafter abbreviated as PCE SPs, where SP is for superplasticizer and PCE is for polycarboxylate ester or polycarboxylate ether) are comb-shaped copolymers (Fig. 1A & 1B), possessing a linear anionic backbone (typically a polyacrylate or polymethacrylate) and charge-neutral side chains (typically made of polyethylene oxides). PCEs can offered far better suspension stability relative to simple polyelectrolytes species, such as polyacrylate acid (PAA) and polymethacrylate acid (PMAA), particularly in a high ionic strength environment (Fig. 1C) where electrostatic screening strongly penalizes electrostatic stabilization (Kirby 2004). Nowadays, PCE SPs are widely used in high performance and self-consolidating concretes, the two main innovations in concrete technology in recent decades, and the introduction of PCE SPs has been regarded as the most important innovation in the technology of cementitious materials (Scrivener 2008). However, applications of PCEs, as effective polymeric dispersants, are not limited to cement-based materials, but also include many other particulate or colloidal suspensions, such as silica (Whitby 2003), BaTiO3 suspensions (Kirby 2004), TiO2 nanoparticles (Koch 2016), carbon nanotubes (Liebscher 2017), graphene oxide (Lu 2017), to name a few. Generally, despite the important and widespread applications of PCE SPs, there are still debates on very basic understandings of (1) PCE adsorption on like-charged surfaces, (2) contributions of non-adsorbed PCEs to inter-particle dispersion forces, and (3) adsorption conformations of PCEs close to surface saturation. Those three aspects, as summarized in Fig. 2, are interrelated, and some recent experimental studies have challenged the tradition view of the adsorption-dispersion mechanism (i.e., single layer adsorption with backbone being responsible to adsorption and non-adsorbing side-chains providing steric exclusion) of PCEs. As the current design principles of PCEs are heavily relied on the traditional view of the adsorption-dispersion mechanism (Marchon 2019), a clearly understanding of the aforementioned three aspects is in urgent need in order to advance PCE SPs’ technology. The proposed research is to develop a computational toolbox for simulation and modeling of the structural and adsorption properties of PCE SPs in high-salt aqueous solution and at liquid/solid interfaces. The toolbox to be developed consists of three categories of tools, namely, (1) existing analytical theories by Flatt and coworkers (Flatt 2009, Aïtcin & Flatt 2016, Marchon 2019) and by the project PI and coworkers (Wang 2018), (2) the Scheutjens-Fleer self-consistent field (SF-SCF) theory as has been applied by Feuz and coworkers (Feuz 2005, Feuz 2006, Feuz 2008) to poly(L-lysine)-graft-poly(ethylene glycol) (PLL- g-PEG), a similar but different comb-shaped polyelectrolytes, and (3) stochastic simulations (Monte Carlo (MC) and Brownian Dynamics (BD)) based on coarse-grained polyelectrolyte-surface models. Our selection of basic tools in the planned computational toolbox is based on an extensive literature survey, taking into consideration whether the method is suitable to model industrially relevant PCEs with our computational resources, whether they can be complementary to each other. Our primary goal is to make such computational models accessible to ourselves and to experimentalists and R&D researchers interested in the structure-property relationships of PCEs. At present researchers working on PCE SPs have background in different scientific disciplines, e.g. chemistry, materials science and engineering, and civil engineering, and it is our hope that by developing tools/platforms that are easily accessible, all researchers may benefit from the power and the promise of computational approaches for enhancing our understanding of and our ability to control complex physical phenomena of polyelectrolytes at interfaces. Our second goal is to enhance our understandings of (A) PCE adsorption on like-charged surfaces, (B) contributions of non-adsorbed PCEs to inter-particle dispersion forces, and (C) adsorption conformations of PCEs close to surface saturation by designing model systems and performing extensive in silico experiments using the “computational toolbox” developed in this project, especially the stochastic simulation methods. We would also like to examine possible mechanisms and the range of parameters that may lead to some peculiar observations and confronting views reported in the literature.