Molecular architecture requirements for polymer-grafted lignin superplasticizers

Abstract

Superplasticizers are a class of anionic polymer dispersants used to inhibit aggregation in hydraulic cement, lowering the yield stress of cement pastes to improve workability and reduce water requirements. The plant-derived biopolymer lignin is commonly used as a low-cost/low-performance plasticizer, but attempts to improve its effects on cement rheology through copolymerization with synthetic monomers have not led to significant improvements. Here we demonstrate that kraft lignin can form the basis for high-performance superplasticizers in hydraulic cement, but the molecular architecture must be based on a lignin core with a synthetic-polymer corona that can be produced via controlled radical polymerization. Using slump tests of ordinary Portland cement pastes, we show that polyacrylamide-grafted lignin prepared via reversible addition-fragmentation chain transfer polymerization can reduce the yield stress of cement paste to similar levels as a leading commercial polycarboxylate ether superplasticizer at concentrations ten-fold lower, although the lignin material produced via controlled radical polymerization does not appear to reduce the dynamic viscosity of cement paste as effectively as the polycarboxylate superplasticizer, despite having a similar affinity for the individual mineral components of ordinary Portland cement. In contrast, polyacrylamide copolymerized with a methacrylated kraft lignin via conventional free radical polymerization having a similar overall composition did not reduce the yield stress or the viscosity of cement pastes. While further work is required to elucidate the mechanism of this effect, these results indicate that controlling the architecture of polymer-grafted lignin can significantly enhance its performance as a superplasticizer for cement.

Rheological Properties and Flow Behaviour of Cement-Based Materials Modified by Carbon Nanotubes and Plasticising Admixtures

“…The changes in molecular structure of Pl provide the improvement of their plasticising efficiency. Nowadays, there are a significant number of research works on the synthesis of Pl with new molecular architecture, which contributes to the changes in rheological and technological properties of cement-based materials [43][44][45][46][47].…”

Lignin‐based polymers via graft copolymerization

“…Superplasticizers are a class of high‐performance anionic polymers that is designed to modify rheological properties of hydraulic cement . Gupta et al demonstrated high‐performance dispersants obtained through a grafted architecture using RAFT polymerization of polyacrylamide from a lignin macroinitiator [Scheme (b)] . Significant reductions in yield stress were observed in ordinary portland cement when it used with the prepared lignin‐graft‐polyacylamide.…”

“…(a) Graft‐from RAFT polymerization of acrylic acid and acrylamide . (b) Graft‐from RAFT polymerization of acrylamide . (c) Graft‐from RAFT polymerization of soybean oil‐derived monomers .…”

“…Among various integration methods between lignin and synthetic polymers, graft copolymerization has the potential to result in the most well‐defined products because the precise control of copolymerization is possible through modern polymerization techniques, including atom transfer radical polymerization (ATRP), reversible addition‐fragmentation chain‐transfer (RAFT) polymerization, and click reactions. A few of the reported special properties of lignin‐based graft copolymers are enhanced elasticity, superplasticizer capability, UV absorbent capabilities, surfactant capabilities, and gene delivery properties . In addition to previous achievements of lignin‐based polymers, consistent developments of new lignin graft copolymerization methods are needed to serve the rapidly growing interest and needs in sustainable biomass‐based functional materials.…”

Synthesis of block cationic polyacrylamide precursors using an aqueous RAFT dispersion polymerization

“…28 Gra, star, or branch copolymers are also possible with RAFT polymerization. [29][30][31][32][33][34][35][36][37][38][39] Shan et al 40 used a multi-arm star RAFT chain transfer agent (CTA) to synthesize star PAM copolymers. A similar approach was used for preparation of hyperbranched copolymers with AM and acrylonitrile.…”