Size exclusion chromatography of polylactic acid in three different solvents

Importance of the Topic

Polylactic acid (PLA) represents one of the most promising sustainable plastics in the modern polymer industry. Produced from bio-based feedstocks via bacterial synthesis, PLA offers biodegradability and a lower environmental impact relative to conventional non-degradable polymers. Critical performance attributes such as melting point, mechanical strength and elasticity are dictated by molecular weight distributions and polydispersity. Gel permeation chromatography (GPC) remains the reference method for accurate characterization of these parameters.

Objectives and Study Overview

This study evaluates the performance of a KNAUER HPLC system equipped with GPC columns for size-exclusion analysis of PLA. Three solvents (tetrahydrofuran, chloroform and ethyl acetate) are compared using two calibration strategies based on polystyrene (PS) and polymethyl methacrylate (PMMA) standards. A 20 kDa PLA standard and two real world samples (3D-printing filaments and a disposable PLA bottle) are analyzed to assess method reliability and identify sample impurities.

Methodology and Instrumentation

Sample preparation involved overnight dissolution of calibration standards and PLA samples in each solvent with butylated hydroxytoluene as a flow marker. Molar mass calibrations employed PS (580 Da–6.57 MDa) and PMMA (800 Da–2.2 MDa) kits, fitted with fifth-degree polynomials. Universal calibration for PLA used literature Mark–Houwink parameters for each solvent. The KNAUER AZURA HPLC system operated at 25 °C with 1 mL/min flow, 20 µL injection volume, refractive index and diode-array detection at 245 nm and 280 nm.

Main Results and Discussion

PS and PMMA calibrations in chloroform and THF provided PLA molecular weights within ±20 % of NMR-determined values when universal calibration was applied. In ethyl acetate, PS calibration significantly underestimated PLA molar masses by up to 150 % due to interactions with the stationary phase, whereas PMMA calibration remained accurate (<±11 % deviation). Analysis of filament samples revealed one pure PLA filament matching calibration signals, while the second filament exhibited a shifted UV response at 280 nm, suggesting blend or impurity presence. Molecular weight and polydispersity of all real samples fell in the range of 65 kDa–80 kDa with PD values around 1.7–1.8.

Benefits and Practical Applications of the Method

• Rapid, reliable determination of PLA molecular weight and polydispersity
• Ability to detect sample impurities or polymer blends through dual UV and RI detection
• Use of ethyl acetate as a greener alternative solvent for low-molecular-weight PLA analysis
• Compatibility with standard GPC columns and HPLC instrumentation

Future Trends and Potential Applications

Advances in green chromatography will focus on optimizing environmentally friendly solvents such as ethyl acetate and developing polymer-specific calibration strategies. Integration of multi-detector arrays and advanced data analysis will further improve characterization of complex biopolymer blends. Expanded use of fiber-reinforced PLA composites calls for tailored GPC methods to monitor molecular weight changes during recycling and processing.

Conclusion

The presented method demonstrates that a KNAUER HPLC system with GPC columns can accurately characterize PLA in THF and chloroform using PS or PMMA calibrations and can employ ethyl acetate with PMMA calibration as a green alternative. Dual detection enables assessment of polymer purity. This setup offers laboratories a robust, user-friendly approach for routine PLA quality control.

Used Instrumentation

• AZURA P 6.1L HPG pump
• AZURA AS 6.1L autosampler
• AZURA RID 2.1L refractive index detector
• AZURA DAD 2.1L diode-array detector
• AZURA CT 2.1 column thermostat
• AppliChrom StyDiViBe GPC columns (THF, CHCl₃, EtOAc)
• ClarityChrom 8.2.3 software with SEC/GPC extension

References

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