Study of polylactide stereocomplex formation with combined Raman spectroscopy and rheology

Significance of the topic

Polylactide (PLA) is a widely used biodegradable polymer with relevance in packaging and biobased materials. However, its slow crystallization and low glass transition temperature limit thermal performance.
Formation of a stereocomplex between PLLA and PDLA enhances thermal stability and increases the melting point from around 165-170 °C to 230-235 °C, enabling PLA to compete with conventional polymers.

Objectives and study overview

This study aimed to investigate the kinetics of PLA stereocomplex (PLA-sc) formation using a novel combined Rheo-Raman approach. The focus was on assessing how shear rate, temperature, and molecular weight influence stereocomplex crystallization.

Methodology

PLA pellets (PLLA and PDLA) with two melt flow indices were ground, sieved, and mixed in equal proportions. The mixture was loaded into a rotational rheometer at 240 °C, subjected to a controlled pre-shear at variable rates, and then cooled to 190, 200, or 210 °C. Small-amplitude oscillatory shear tests measured viscoelastic parameters over time, while in situ Raman spectra monitored stereocomplex formation via shifts in carbonyl stretching bands.

Instrumentation

• Thermo Scientific HAAKE MARS 40 Rotational Rheometer
• Thermo Scientific DXR3 Flex Raman Spectrometer
• Combined in the Thermo Scientific HAAKE MARSXR Rheo-Raman System

Main results and discussion

• Raman spectroscopy distinguished PLA-sc via a carbonyl band shift from 1772 cm⁻¹ (PLLA/PDLA) to 1754 cm⁻¹ (stereocomplex).
• Increased shear rates during pre-shear markedly reduced the induction time for stereocomplex crystallization.
• Viscoelastic data (G', G'', |η*|) correlated with the Raman index, confirming concurrent mechanical and structural transitions.
• Normalization of G' values highlighted the influence of shear history on crystallization kinetics across different molecular weights.

Benefits and practical applications of the method

The combined Rheo-Raman technique provides real-time insights into polymer crystallization, facilitating optimization of processing conditions. The enhanced thermal properties of PLA-sc expand PLA's use in high-temperature applications such as hot beverage cups, disposable tableware, and technical components.

Future trends and applications

• Scale-up of melt compounding processes for industrial stereocomplex production.
• Integration of in situ spectroscopic monitoring in polymer processing lines.
• Development of stereocomplex-based materials for biomedical and packaging sectors.
• Exploration of other enantiomeric polymers and blends using combined rheology-spectroscopy methods.

Conclusion

The study demonstrated that combined Rheo-Raman analysis is a powerful tool to elucidate stereocomplex formation in PLA. Shear rate is a critical parameter governing crystallization kinetics, and the approach offers a framework for optimizing polymer processing to achieve enhanced material performance.

References

1. European Bioplastics Association. Facts and Figures. 2021.
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3. H. Tsuji. Poly(lactic acid) stereocomplexes: A decade of progress. Advanced Drug Delivery Reviews. 2016;107:97-135.
4. H. Tsuji, S. Hyon, and Y. Ikada. Stereocomplex formation between enantiomeric poly(lactic acid)s: Differential scanning calorimetric studies on precipitates from mixed solutions of poly(D-lactic acid) and poly(L-lactic acid). Macromolecules. 1991;24(17):5657-5662.