A Quantitative Study on Branching Density Dependent Behavior of Polylactide Melt Strength

Area of Science:

• Polymer Chemistry
• Materials Science

Background:

• Polymer melt strength (MS) is crucial for applications but its relationship with molecular structure remains unclear.
• Designing polymers with controlled long-chain branching (LCB) is key to tailoring MS.

Purpose of the Study:

• To prepare designable long-chain branched polylactide (LCB-PLA) in situ.
• To establish a quantitative relationship between LCB structure and MS.
• To provide a feasible route for designing polymers with enhanced melt strength.

Main Methods:

• Synthesis of LCB-PLA using a tailor-made (methyl methacrylate)-co-(glycidyl methacrylate) copolymer (MG).
• Definition and application of a new concept of branching density (φ) to quantify LCB.
• Measurement of zero-shear viscosity (η₀) and melt strength (MS) at varying branching densities.

Main Results:

• A critical branching density (φc = 5.5 mol/10⁴ mol C) was identified.
• Zero-shear viscosity (η₀) showed a slow increase below φc and a sharp increase above it due to chain entanglement.
• Optimized LCB structures led to a >100-fold increase in MS while preserving processability.

Conclusions:

• A quantitative understanding of the relationship between LCB structure and MS in PLA was achieved.
• The identified critical branching density is pivotal for controlling polymer melt strength.
• This work offers a practical strategy for designing high-melt-strength polymers for specialized uses.