SEC Analysis of Polylactic Acid Using Advanced Polymer Chromatography

Importance of the Topic

Polylactic acid (PLA) is a renewable, biodegradable polymer widely used in packaging, medical devices and agricultural films. Reliable determination of its molecular weight distribution is critical for predicting performance, processability and biodegradation behavior.

Objectives and Study Overview

This work aims to demonstrate rapid method development for size exclusion chromatography (SEC) of PLA using Advanced Polymer Chromatography (APC™) on Waters ACQUITY™ APC XT columns. The goal is to characterize number‐average, weight‐average and Z‐average molecular weights and polydispersity with high throughput.

Methodology and Instrumentation

The PLA sample was dissolved in amylene-stabilized chloroform with varying proportions of unstabilized tetrahydrofuran (THF) to achieve full solubility and minimize column interactions.

• System: Waters ACQUITY APC
• Columns: ACQUITY APC XT 900 Å, 450 Å, 125 Å and 45 Å (4.6 × 150 mm)
• Mobile phase: chloroform/THF (70:30, v/v)
• Flow rate: 0.6 mL/min; Column temp.: 55 °C; Sample temp.: 20 °C
• Sample concentration: 2 mg/mL; Injection volume: 20 µL
• Detector: Refractive index at 55 °C
• Calibration: Narrow polystyrene standards; Data analysis via Empower™ 3 GPC/SEC module

Main Results and Discussion

The optimized solvent blend and BEH-based APC columns delivered complete PLA resolution. Key molecular weight metrics obtained were:

• Mn (number‐average): 63 000 Da
• Mw (weight‐average): 139 000 Da
• Mp (peak molecular weight): 114 000 Da
• Mz (Z‐average): 250 000 Da
• Polydispersity (Mw/Mn): 2.20

The resulting chromatogram and distribution curve confirm a broad molecular weight profile. Exclusion of very high molecular weight calibration points focused the analysis on the region of interest and avoided extrapolation errors.

Benefits and Practical Applications of the Method

The APC approach offers:

• Fast solvent optimization using a multi-solvent valve allowing up to six blends
• Short equilibration times between solvent changes
• High throughput capacity for routine PLA quality control
• Compatibility with advanced detectors for absolute molecular weight and conformational analysis

Future Trends and Potential Applications

Advances in multi-detector SEC (e.g., MALS, viscometry), low-dispersion optics and online micro-viscometers promise improved absolute molecular weight and branching characterization. Integration with ultrahigh-pressure SEC and rapid method screening will further enhance polymer analysis in R&D and QC laboratories.

Conclusion

Advanced Polymer Chromatography on ACQUITY APC XT columns provides a robust, flexible and high-throughput SEC solution for characterizing PLA molecular weight distributions. The method supports efficient development, reproducible results and seamless adaptation to advanced detection for comprehensive polymer profiling.

Reference

1. Rafael Auras; Loong-Tak Lim; Susan E. M. Selke; Hideto Tsuji (eds.). Poly(lactic Acid): Synthesis, Structures, Properties, Processing, and Applications. ISBN 9780470293669.
2. “Using Multi-Detector Advanced Polymer Chromatography (APC) to Better Differentiate Samples by Structure”, Malvern Panalytical Limited, 2018.
3. Trainoff S.; Deng C. “Development of a New Online Micro-Viscometer for APC and UHPLC Applications”, Wyatt Technology Corporation, 2018.