Morphology of polylactide stereocomplexes in blends of poly[(L-lactide)-co-(ε-caprolactone) and -poly(D-lactic acid) with varying molar masses

Abstract text (incl. figure legends and references)

Introduction

Poly[(L-lactide)-co -(ε-caprolactone)] (PLC) and poly(D-lactic acid) (PDLA) are biodegradable polyesters that can be produced from renewable resources and are now widely used in standard, biomedical and pharmaceutical applications. Improving the mechanical properties and heat resistance of materials based on poly(ε-caprolactone) (PCL) and poly(lactic acid) (PLA)) is a major concern when they are used in biomedical and consumer products [1][2]. One of the most effective methods to improve the mechanical properties and thermal stability of PLA-based materials is the formation of stereocomplexes between poly(L-lactide) (PLLA) and poly(D-lactide). This stereocomplex (SC) formation is defined as stereocomplex crystallization between PLLA and PDLA and contributes significantly to the control of polymer properties [3].

Objectives

The aim of this study is to identify and visualize the different morphologies, especially crystallinity and stereocomplexation, in the blends of PLC and PDLA of different compositions prepared by solution casting, in order to understand the SC formation and the associated new properties of the polymer.

Materials and methods

The obtained polymer blends were morphologically studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Supporting analytical methods such as wide angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC), were used to detect crystallization, while polymer characterization by GPC and NMR was employed to relate the morphology to polymer. Tensile tests were performed to determine the mechanical properties.

Results

Visualization of the morphology allowed identification of the amorphous and (stereo-) crystallizing regions. Together with the additional characterizations, maximal overall crystallization and, specifically, stereocrystallization was quantifiably shown. In polymer blends with too short lactide segment lengths, no SC could be detected. From a number average oligolactide sequence length l_LA ≥ 4.2, the formation of SC was observed, which further increased with increasing lactide sequence and lattice content. However, the crystallites show a decrease in SC size, which can be attributed to phase dilution. Mixtures with a high PDLA mol mass yielded smaller SC crystallites than mixtures with lower mol mass. Limited SC formation exists due to steric hindrance of long molecular chains and partial crystallization of the homopolymer.

Conclusion

The totality of the applied analytical methods indicates that the adjustment of the morphology of the polymer blends at the nanoscale is necessary to achieve the desired macroscopic properties of the blends.

References

[1] A. T. Neffe et al., Nanomaterials 2021, 11, 1472.

[2] V. Izraylit et al., Eur. Polym. J. 2020, 137, 109916

[3] X. Su et al., Polymers 2020, 12, 2515.