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Effects of Ratio of Different Side Chain Length on Dispersion of Polycarboxylic Superplasticizer

In the development of high-performance concrete admixtures, polycarboxylate superplasticizers (PCE) are widely used for their excellent water reduction and dispersion stability. The molecular structure—especially the side chain length and ratio—directly determines the working performance.

This study focuses on methyl allyl polyoxyethylene ether (MAPEG) with two side chain lengths (n=18 and n=29) to explore how the mixing ratio affects dispersion, fluidity retention, zeta potential, and molecular weight. The results provide a clear molecular design basis for high-performance polycarboxylate superplasticizers.

What is the Side Chain Length Ratio in PCE?

olycarboxylate superplasticizers have a typical comb‑like structure:
  • Main chain: provides adsorption groups (‑COOH, ‑SO₃H)
  • Side chains: polyoxyethylene ether (PEO) chains that provide steric hindrance
Side-chain length ratio refers to blending short side chains (n=18) and long side chains (n=29) within a single PCE molecule to balance initial fluidity and long‑term slump retention.

Key Roles of Side Chains

  • Short side chains (n=18): improve initial dispersion and fluidity
  • Long side chains (n=29): enhance dispersion stability and slump retention
  • Mixed ratio: balances fluidity and stability for optimal overall performance

Experimental Design

Raw Materials

  • MAPEG macromonomers: n=18 and n=29
  • Monomers: maleic anhydride (MA), sodium allyl sulfonate (SAS)
  • Initiator: ammonium persulfate
  • Cement: P·O 42.5 Portland cement

Test Methods

  • Cement paste fluidity: evaluate dispersion
  • Fluidity retention over 120 min: evaluate stability
  • Zeta potential: analyse adsorption and electrostatic repulsion
  • FTIR: characterize molecular structure
  • Intrinsic viscosity: reflects molecular weight

Optimal Side Chain Ratio: n₁₈:n₂₉ = 1:4

The study tested multiple mixing ratios of MAPEG (n=18) and MAPEG (n=29). The best performance is observed at a 1:4 molar ratio.

Performance at Optimal Ratio

  • Cement paste fluidity: 235 mm (dosage 0.4%, w/c=0.29)
  • 120 min fluidity retention: 87% (205 mm)
  • Balances high initial fluidity and strong stability

Performance Comparison of Different Ratios

  • Only long side chain (0∶1): good stability but low initial fluidity
  • Only short side chain (1∶0): high initial fluidity but fast loss (only 50% retention at 90 min)
  • 1:4 ratio: best combination of fluidity and stability

Why This Ratio Works

1. Steric Hindrance Effect

Long side chains (n=29) provide strong steric hindrance, preventing cement particles from flocculating and maintaining long‑term fluidity.

2. Initial Dispersion

Short side chains (n=18) improve adsorption density and early dispersion, boosting initial flow.

3. Synergistic Effect

The 1:4 ratio maximises the synergy between short and long chains, achieving both high flow and low loss.

Zeta Potential Analysis

Zeta potential reflects the surface charge and dispersion state of cement particles.
  • Zeta potential increases significantly at a dosage 0.3%–0.5%
  • Tends to be stable above 0.5% (saturated adsorption)
  • Mixed side-chain PCE shows a higher absolute zeta potential at low dosage.
This confirms that PCE with mixed side chains adsorbs faster and more effectively.

Molecular Weight & Intrinsic Viscosity

  • The side-chain ratio has a slight influence on the intrinsic viscosity.
  • Molecular weight first increases, then decreases, with a higher long‑chain proportion.
  • The 1:4 formula has a stable molecular weight and good workability.

FTIR Structural Verification

FTIR confirms the successful synthesis:
  • C–H absorption at 2867 cm⁻¹
  • Carbonyl (C=O) of carboxylate at 1721 cm⁻¹, 1583 cm⁻¹
  • Sulfonate group at 1351 cm⁻¹
  • Ether bond (C–O–C) at 1113 cm⁻¹
All characteristic peaks match the designed comb‑like polycarboxylate structure.

Conclusion

  1. The optimal side chain length ratio for MAPEG‑based PCE is n₁₈∶n₂₉ = 1∶4.
  2. This formula achieves an initial fluidity of 235 mm and 87% retention after 120 min.
  3. Mixed short/long side chains balance dispersion and stability better than single‑length chains.
  4. Zeta potential and steric hindrance work together to provide stable cement dispersion.
This ratio is highly valuable for:
  • High‑fluidity ready‑mix concrete
  • Long‑distance transportation of concrete
  • Projects requiring low slump loss
  • High‑efficiency, low‑dosage superplasticiser formulation

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Effects of Ratio of Different Side Chain Length on Dispersion of Polycarboxylic Superplasticizer

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