Study of polylactide stereocomplex formation with combined Raman spectroscopy and rheology

Importance of the Topic

Polylactide (PLA) is one of the most widely produced biodegradable polymers, yet its slow crystallization rate and low thermal resistance limit applications such as hot beverage cups and disposable cutlery. Stereocomplex formation between enantiomeric PLLA and PDLA significantly elevates the melting point and improves mechanical properties, offering a route to compete with conventional thermoplastics in packaging and engineering applications.

Objectives and Study Overview

This study aimed to elucidate the kinetics of PLA stereocomplex (PLA-sc) formation under melt processing conditions by integrating Raman spectroscopy with rotational rheology. For the first time, the impact of pre-shear rate, temperature, and polymer melt flow index on nucleation and crystallization kinetics was quantified in situ.

Methodology and Instrumentation

Four commercial PLA grades were selected, divided into high‐MFI (30–40 g/10 min) and low‐MFI (14–15 g/10 min) series. PLLA and PDLA pellets were milled, sieved to ~1 mm, mixed in 1:1 ratio, then loaded into a plate‐plate rheometer at 240 °C. After 2 min melt equilibration, samples were cooled to 190, 200 or 210 °C. A defined pre-shear (0–220 s⁻¹ for 15 s) preceded small‐amplitude oscillatory shear tests (100 Pa, 6.28 rad/s) to record G′, G″ and |η*| over time. Concurrent Raman spectra focused on the C=O stretching region to derive a PLA-sc index from the intensity shift between 1754 and 1772 cm⁻¹.

Used Instrumentation

• Thermo Scientific DXR3 Flex Raman Spectrometer
• Thermo Scientific HAAKE MARS 40 Rotational Rheometer

Main Results and Discussion

Raman monitoring revealed the stereocomplex C=O band at 1754 cm⁻¹ versus 1772 cm⁻¹ for homopolymers. Without pre-shear, no PLA-sc formed within 1 800 s, whereas even low shear rates (20 s⁻¹) induced complexation. Increasing shear markedly reduced the induction time and accelerated crystallization, as shown by both rheological G′ growth and Raman index trajectories. Normalized modulus curves confirmed consistent shear-driven kinetics across high and low MFI series, indicating chain alignment dominates over melt viscosity effects.

Benefits and Practical Applications

• Elevated melting temperature (~230 °C) and improved thermo-mechanical properties enable broader PLA use.
• Melt compounding with controlled shear supports scalable industrial production.
• In situ rheo-Raman offers real-time quality control for stereocomplex formation.

Future Trends and Potential Applications

Integration of in situ spectroscopic data with machine learning for predictive process control
Continuous monitoring in extrusion or injection molding lines to ensure consistent PLA-sc content
Design of novel PLA blends and nucleating systems to tailor crystallization kinetics and final properties

Conclusion

The combined rheology–Raman approach provided unprecedented insight into PLA stereocomplex formation under shear and thermal conditions. Mechanical shearing was shown to be a critical driver of nucleation and crystallization, offering a robust strategy to develop high-performance, bio-based polymers for advanced packaging and engineering applications.

Reference

1. European Bioplastics Association, Facts and Figures, 2021.
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3. Tsuji H., Poly(lactic acid) stereocomplexes: A decade of progress, Advanced Drug Delivery Reviews, 107, 97–135 (2016).
4. Tsuji H., Hyon S., Ikada Y., Stereocomplex formation between enantiomeric poly(lactic acid)s: Differential scanning calorimetric studies on precipitates from mixed solutions, Macromolecules, 24(15), 5657–5662 (1991).