Xin Shu

pH-induced conformational changes of comb-like polycarboxylate investigated by experiment and simulation

Four types of polycarboxylate ether (PCE) polymers with consistent backbone length and side-chain (poly(ethylene glycol) monomethyl ether methacrylate MAA-M2000) length but different ratios of acrylic acid (AA) to methacrylic acid (MAA) were synthesized. The ratio of the methyl group would directly affect the backbone stiffness and hydrophobicity of the PCE. Laser light scattering (LLS) and molecular dynamic (MD) simulations were used to investigate the conformations of PCE at various pH values. It was found that the conformation of PCE remained more extended with the decrease of ratio of the backbone methyl group, which resulted in a low exposed extent of carboxylic groups on the backbone at the same ionization degree. The complexation of the carboxylic oxygen atoms of PCE with Ca2+ indicated that the differences of the complexing capacity resulted from the differences in the spatial distribution of carboxylic groups, which depended on the solution conformation of PCE molecules.

Effects of Sequence Structure of Polycarboxylate Superplasticizers on the Dispersion Behavior of Cement Paste

Different AA-OEGMA copolymers with random and block distributions were synthesized using free radical polymerization and reversible addition-fragmentation chain transfer polymerization, respectively. Studies on the dispersion ability, adsorption isotherm, adsorption conformation, and zeta potential revealed that the random and block architecture behaved differently. Sequence structure of polycarboxylate polymers (PCPs) had a significant influence on its performance. Both monomer ratio and sequence structure had influences on the dispersion of cement paste. Compared with random PCPs, PCPs with block distribution adsorbed faster on cement particle surfaces because of the higher density of exposed carboxylic groups. For random PCPs, the adsorption was a thermodynamic spontaneous process and driven by entropy, while it was driven by Gibbs free energy for block PCPs. Besides, the hydrodynamic radius of random PCPs in solution was larger than the block PCPs. However, the adsorbed layer thickness of random PCPs was close to that of block PCPs. Furthermore, the zeta potential illustrated that the PCPs with block distribution may adopt a more extended conformation compared with random PCPs. All these findings found from the differences between random PCPs and block PCPs will help the researchers to explore high-performance PCPs. GRAPHICAL ABSTRACT

Effect of Hydrophobic Units of Polycarboxylate Superplasticizer on the Flow Behavior of Cement Paste

ABSTRACT For the tuning of conformation of polycarboxylate (PCE) superplasticizers, hydrophobic groups of different stiffness were incorporated, including styrene (St), methyl methacrylate (MMA), ethyl acrylate (EA), and n-butyl acrylate (n-BA) units. The effect of these hydrophobic groups on the dispersing performance, adsorption process and, rheology of cement paste were investigated. Investigation on the solution conformation and adsorption layer thickness indicated the action mechanism of these groups. High backbone stiffness resulted in a lower extent of conformation condensation from pure aqueous solution to pore solution, and therefore more carboxylic groups could be accessible for adsorption. However, the conformation change after adsorption might also be limited and the size of single molecule after adsorption should be small. Hydrophobic groups always resulted in a coiled PCE conformation in salt solution, which indicated a lower adsorption affinity and thinner adsorption layer for these PCE molecules. GRAPHICAL ABSTRACT

Effect of counterions on comb-like polycarboxylate conformation in aqueous solutions

ABSTRACT Laser light scattering (LLS) and conductivity experiments were performed to investigate the effect of counterions on the conformation of polycarboxylate comb-like copolymers (PCEs) in aqueous solutions. The addition of monovalent ions (i.e., Na+ and K+) to dilute polycarboxylate comb-like copolymer solutions induced a slight shrinking of the molecular chains because of the screening of electrostatic intramolecular repulsion. Varying complexation phenomena, such as the formation of intramolecular complexation at certain Ca2+ concentrations and a transition between intermolecular and intramolecular complexations at different Ca2+ concentrations, were closely associated with Ca2+ concentration. Therefore, Ca2+ exerted a more complex influence on the conformation of PCEs with side chains containing polyethylene oxide (PEO) with different grafting densities. In addition, various combination types of Ca2+ with carboxylic groups were confirmed by theoretical simulation. GRAPHICAL ABSTRACT

A novel and controllable route for preparing high solid-content and low-viscosity poly(acrylamide-co-acrylic acid) aqueous latex dispersions

High solid-content and low-viscosity poly(acrylamide-co-acrylic acid) aqueous latex dispersions have been successfully synthesized by copolymerization of acrylamide (AM) and acrylic acid (AA) in an aqueous solution of ammonium sulfate (AS) and lithium sulfate (LS) based on a distinctly novel strategy, the so called Swollen-Diffusion-In situ redox Polymerization (SDIP), which involves swelling followed by diffusion and redox initialized polymerization inside the seed particle, avoiding the high viscosity progress resulting from homogeneous nucleation in the continuous phase. Compared to the widely used one stage synthetic protocol, this process affords much more effective control over the viscosity of the dispersion and molecular weight of the resultant polymer by simply changing the concentration of inorganic salts and addition rate of the oxidant. The synthesized aqueous latex dispersions have been characterized using Fourier-transform infrared (FT-IR) spectroscopy, H nuclear magnetic resonance (1HNMR) spectroscopy, and optical microscopy. The mechanism governing the formation of the latex dispersion is also extensively discussed.

Adsorption of organic molecules on mineral surfaces studied by first-principle calculations: A review

First-principle calculations, especially by the density functional theory (DFT) methods, are becoming a power technique to study molecular structure and properties of organic/inorganic interfaces. This review introduces some recent examples on the study of adsorption models of organic molecules or oligomers on mineral surfaces and interfacial properties obtained from first-principles calculations. The aim of this contribution is to inspire scientists to benefit from first-principle calculations and to apply the similar strategies when studying and tailoring interfacial properties at the atomistic scale, especially for those interested in the design and development of new molecules and new products.

Synthesis of SiO2-PCE Core-Shell Nanoparticles and its Modification Effects on Cement Hydration

NanoSiO2 was widely used to modify the property of cementitious materials, however, for nanoparticles used in cement-based materials, key problem is the effective dispersion. The surface modification technology can be introduced to promote dispersion of nanoparticles in aqueous system, especially in cement pore solution, which possess high concentration of ions. In this study, at first, NanoSiO2-polycarboxylate superplasticizer (SiO2-PCE) core-shell nanoparticle was synthesized from silanized polycarboxylate superplasticizer and colloidal nanoSiO2 by the “grafting to” method, then SiO2-PCE was testified by UV-Vis, FTIR, and TGA. Additionally, stability of SiO2-PCE and its effect on cement hydration were investigated. Results shows: SiO2-PCE possess higher stability in saturated calcium hydroxide solution compared to nanoSiO2, and heat development of cement hydration can be regulated by shell structure of SiO2-PCE. The research implied a new approach for nanoSiO2 to optimize cement-based composites.

Effect of Molecular Weight of Polycarboxylate Superplasticizer on Its Dispersion, Adsorption, and Hydration of a Cementitious System

AbstractThe aim of this study was to link the molecular weight (MW) of polycarboxylate superplasticizer (PCE) with the performance of cementitious system for the purpose of developing new products; PCE synthesized by copolymerization of methacrylic acid and methyl poly(ethylene oxide) methacrylate was characterized by proton nuclear magnetic resonance (H1-NMR) and gel permeation chromatography (GPC). The polycarboylate (PC) was separated into four fractions with different MWs using an ultrafiltration method. The interaction behaviors between PC fractions and cement particles were investigated by measuring the adsorption, zeta potential, and hydration. Results showed that PC fractions with medium MW showed a good dispersion and dispersion retention owing to the adsorption amount continuously increasing with time and delayed the longest cement hydration time. The highest MW of PC exhibited poor dispersion and dispersion retention, having the greatest adsorption amount on the cement surface and poor adsorpti...

Effect of side chains in block polycarboxylate superplasticizers on early-age properties of cement paste

Even though numerous research projects have been carried out on the subject concerning interactions between sequence structure of polycarboxylate superplasticizers (PC) and cement, many questions remain unsolved, such as the influence of PC on early-age microstructural development and PC adsorption behavior. The study first successfully synthesized well-defined block PC with varying side chains length and side chains density using reversible addition–fragmentation chain transfer polymerization. The influence of side chains in block PC on the early-age properties of cement paste was systematically studied by various characterization methods such as paste flow, adsorption properties, calorimetric measurements and amount of hydration products. The results have illustrated that PC with longer side chains and lower side chains density shows higher adsorption amount, thereby a better workability for a given mass ratio of adsorption groups to side chains. Furthermore, the adsorption amount decreases with the increasing side chains length for a fixed length of main chain as well as side chains density. However, there is better initial paste flow and higher paste flow retention capability owing to longer side chains. It is worth noting that PC with longer side chains promotes the cement hydration process thus increasing the hydration products owing to larger surface coverage. All these findings close the gap between side chains in block PC and early-age properties, providing suggestions for the design of PC.

Binding of calcium cations with three different types of oxygen-based functional groups of superplasticizers studied by atomistic simulations

AbstractThis work investigated interactions between calcium cations (Ca2+) and three common types of oxygen-based functional groups of concrete superplasticizers using density functional theory (DFT) calculations and all-atom molecular dynamics (MD) simulations. The three common types of oxygen-based functional groups were modeled as three hypothetical, low-molecular-weight organic molecules, each containing a methyl-terminated oxyethylene dimer and an adsorbing head of two oxygen-based functional groups, and are referred to as carboxylate, sulfonate, and phosphate groups, respectively, following the usual terminology in the field of concrete admixtures. Our DFT results show that the binding strength of the three groups with calcium cations follows (from high to low) phosphate>carboxylate>sulfonate, and both the electrophilic attack and the chemical reactivity of the three groups contribute significantly to the binding strength. The MD simulation results indicate that the adsorption of the three small molecules on the calcite (1 0 4) surface in aqueous solution shares a similar pattern in the sense that just two oxygen atoms of two adjacent anchor groups adsorb on the calcium atoms on the top layer of the crystal. The adsorption strength among the three types of functional groups follows the same order as the binding strength obtained from DFT calculations; both results corroborate a similar rule-of-thumb established by experiments. Furthermore, interactions of the three types of groups with water molecules suggest that strong hydrogen-bonding interactions exist in those systems. Graphical abstractBinding of calcium cations with three different types of oxygen-based functional groups of superplasticizersᅟ