Characterisation of Poly(Lactic Acid) Self-reinforcement and its Influence on Material Morphology and Crystallinity

Description

Poly(Lactic Acid) (PLA) is vastly studied as a biodegradable and biocompatible
alternative to synthetic commodity polymers, due to its positive life cycle assessment and comparable material properties [1]. Nonetheless, in order to broaden PLA´s range of applications, its thermal and mechanical properties have to be improved, like thermal stability and heat resistance [1], as well as melt and impact strength [2]. This could be achieved by obtaining desirable morphologies and crystallinities via nucleation [1].
The current study aims to observe the self-reinforcement of Poly(Lactic Acid) by
incorporating low concentrations of a PLA stereocomplex (PLA-SC) into PLA of
different optical purities (i.e., D-isomer content) under industrial processing conditions. PLA-SC was produced by melt-blending Poly(L-Lactic Acid) (PLLA) and Poly(D-Lactic Acid) (PDLA) in a 1:1 ratio, followed by a two-step post-annealing. Different threshold temperatures (200 ºC, 210 ºC and 220 ºC) were selected for the introduction of PLA-SC in the matrices (PLLA and Poly(D,L-Lactic Acid) (PDLLA)), in order to investigate the impact of melt flow-induced precursor structures on homopolymer self-nucleation.
Incorporation and the resulting properties were investigated by combining thermal Differential Scanning Calorimetry (DSC) and Stepwise Isothermal Segregation Technique (SIST) DSC, microscopic (optical and Atomic Force Microscopy (AFM), Raman spectroscopic, and mechanical analysis.
The initial DSC measurements of PDLLA/PLA-SC pellets with 1, 3 and 5 wt.% PLA-SC content revealed that the homopolymers were partially detached from the PLA-SC at processing temperatures above 200 ºC, when the internal hydrogen-bonding (-CH₃⋯O=C) between the two enantiomers within the SC crystallites could be weakened [1]. This partial melting of the PLA-SC was also supported by Raman Spectroscopy. Additional SIST DSC analysis indicated the self-nucleation effect of PLA-SC on the PDLLA matrix, which was observed for the PLLA matrix as well via POM, where the pellets were melted at 200 °C on a heating stage, then isothermally crystallised at 120 °C for 5 minutes. An increase in the percentage of SC-PLA resulted in a notable rise in the number of nuclei and an initial formation of bigger spherulitic structures. The thin films so-obtained were further analysed using AFM, which revealed two distinct morphologies, namely spherulites and shish-kebabs, the latter being known as flow-induced oriented semicrystalline conformations [3]. Static mechanical testing was implemented in order to determine intensive tensile properties of the materials (e.g., Young´s modulus and tensile strength) and to investigate a correlation between these and morphology and crystallinity, respectively. The samples were prepared analogous to the aforementioned ones, including a post-annealing step at 120 ºC to allow melt crystallisation of the homopolymers within the matrices.

Literature: [1] D. Dimonie, Materials 2021, 14, 6650. [2] S. Rathi, Polymer 2021, 52, 4184. [3] R. H. Somani, Polymer 2005, 46, 8587.

Period	11 Mar 2025
Event title	ANAKON 2025
Event type	Conference
Location	Leipzig, GermanyShow on map