Determining molecular weights for P(D,L)LA in ethyl acetate – an internal validation

Importance of the Topic

Biodegradable and bio-based plastics such as polylactic acid play a crucial role in sustainable material development. Accurate characterization of molecular weight distribution parameters Mn and Mw is essential for predicting mechanical and degradation behavior of PLA in industrial and research settings. Gel permeation chromatography (GPC) remains the primary tool for these determinations.

Aims and Study Overview

This work presents an intralaboratory validation of a GPC method using universal calibration to quantify low molecular weight P(D,L)LA in ethyl acetate. The method performance was benchmarked against conventional calibration and absolute techniques, and key parameters such as repeatability and robustness were assessed.

Methodology and Instrumentation

Sample Preparation

• PMMA calibration standards (800 Da to 2 200 kDa) dissolved in ethyl acetate with butylated hydroxytoluene flow marker at 1.5 mg/ml
• P(D,L)LA samples (approx. 20 mg) dissolved overnight to 3 mg/ml then diluted to 1 mg/ml

Chromatographic Conditions

• Solvent: ethyl acetate at 1 ml/min
• Column temperature: 25 °C
• Injection volume: 20 µl
• Refractive index detection with flow correction via BHT marker

Instrument Configuration

• Pump AZURA P6.1L HPG
• Autosampler AZURA AS 6.1L
• Detector AZURA RID 2.1L
• Thermostat AZURA CT 2.1
• Pre-column and analytical column AppliChrom ABOA StyDiViBe series
• Data acquisition with ClarityChrom software

Main Results and Discussion

• Universal calibration yielded Mn within 8% and Mw within 4% of values from absolute methods, while conventional calibration deviated by over 100%
• Repeatability tests (six injections) showed RSD of 0.44% for Mn and 0.13% for Mw; intermediate precision over three days remained below 0.5% RSD
• Robustness evaluations demonstrated stable Mn and Mw under ±2 °C temperature shifts, ±0.2 ml/min flow rate changes, injection volumes from 10 to 30 µl, and sample concentrations from 0.1 to 3 mg/ml
• Use of a flow marker effectively compensates for minor flow fluctuations, ensuring consistent retention volumes

Benefits and Practical Applications

The validated GPC method in ethyl acetate provides a green alternative to traditional solvents, offering precise and reproducible molecular weight distributions of low molecular weight PLA. This approach supports quality control in polymer manufacturing, research on biodegradable materials, and regulatory compliance.

Future Trends and Opportunities

Opportunities include extending universal calibration protocols to other sustainable polymers, integrating detectors such as multi angle light scattering for absolute measurements, and developing standardized calibration kits for routine industrial use.

Conclusion

An intralaboratory validation confirms that GPC with universal calibration in ethyl acetate is a robust, accurate, and environmentally friendly method for characterizing low molecular weight P(D,L)LA. The protocol meets ISO 13885-1 criteria and is suitable for widespread analytical applications.

Reference

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