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

Significance of the Topic

A sustainable and biodegradable polymer, polylactic acid (PLA) offers an eco-friendly alternative to petroleum-based plastics. Its performance and processing depend critically on its molecular weight distribution. Reliable measurement of number average (Mn) and weight average (Mw) molar masses ensures consistent quality in research, industrial production, and quality control.

Objectives and Overview of the Study

This work presents an intralaboratory validation of a GPC-based method for low molecular weight P(D,L)LA in THF. Key aims include comparing conventional calibration (CC) and universal calibration (UC) against absolute methods (1H-NMR), and evaluating method repeatability and robustness under varying experimental conditions.

Methodology

PLA standards (1.5 mg/mL in THF with butylated hydroxytoluene as flow marker) were analyzed by GPC at 25 °C with a 1 mL/min THF mobile phase. A 14-point universal calibration curve was generated using polystyrene standards (580 Da–730 kDa) and Mark–Houwink parameters for PS and PLA. Injection volume was 20 µL, sample concentration 1 mg/mL, data rate 10 Hz.

Instrumentation Used

• Pump: AZURA P6.1L HPG
• Autosampler: AZURA AS 6.1L
• Detector: AZURA RID 2.1L refractive index
• Column thermostat: AZURA CT 2.1
• Pre-column and column: AppliChrom StyDiViBe linear GPC columns
• Software: ClarityChrom 8.2.3 with SEC/GPC extension

Main Results and Discussion

Universal calibration yielded Mn and Mw values within 3 % and 1 % of 1H-NMR references, respectively. Conventional calibration overestimated Mn and Mw by more than 40 %. Repeatability (six injections) showed RSD ≤ 0.43 % (Mn) and ≤ 0.19 % (Mw). Intermediate precision across three days remained below 0.6 % RSD. Robustness tests (±2 °C temperature, ±0.2 mL/min flow rate, 10–30 µL injection, 0.3–3 mg/mL concentration) revealed no significant trends and RSD values under 1 %. Use of a flow marker effectively corrected minor flow fluctuations.

Benefits and Practical Applications

This validated method offers high accuracy, precision, and robustness for low molecular weight PLA characterization. It supports quality assurance in polymer manufacturing, research laboratories, and regulatory compliance. The universal calibration approach and flow marker enhance method reliability across routine analyses.

Future Trends and Potential Uses

Emerging developments may include green solvent systems, integration of multi-angle light scattering or viscometry detectors for absolute molar mass determinations, hyphenation with mass spectrometry for end-group analysis, and automated inline monitoring for process control. Adaptation of universal calibration to diverse biodegradable polymers could broaden application scope.

Conclusion

An intralaboratory validated GPC-THF method using universal calibration and a flow marker reliably determines Mn and Mw of low molecular weight P(D,L)LA. The approach meets ISO 13885-1 criteria for accuracy, repeatability, and robustness, outperforming conventional calibration and providing a practical tool for polymer analysis.

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