Modulation of the PLLA Morphology through Racemic Nucleation to Reach Functional Properties Required by 3D Printed Durable Applications.

Doina Dimonie,Sorina Iftimie,Roxana Trusca,Manuela Maria Iftime,Silvia Mathe,Daniela Ionita

doi:10.3390/ma14216650

Abstract

This paper presents an alternative for enhancing the durability of poly (L-lactide) (PLLA) by racemic nucleation following stereo-complexation with a selected poly (D-lactide) (PLDA). The compounds are obtained by melt blending of a PLLA grade, previously designed for 3D printing but with a low heat deflection temperature and impact resistance, with grades of PLDA differing in their molecular weight (Mw), D-lactide content (DS) and concentration. Our method considered how to reveal the racemic nucleation caused by stereo-complexation and its influence on functional properties. The FTIR study we performed showed that, depending on Mw, DS and concentration of the stereo-complexer (PDLA) used, bigger or smaller spectral changes can occur. The stereo-complexation was confirmed by the DSC analysis and, for the selected compound, by the POM, SEM, AFM microscopies, functional property and shapeability as 3D printing filaments. All the obtained results sustain the idea that, if a PLLA with Mw of 4.5 × 104 g·mol−1 is modified with PDLA with a medium Mw of 11.6 × 104 g·mol−1, medium DS of 4% and 1% concentration, a racemic nucleation is possible. It produces a racemic polylactic acid (PDLLA) with improved durability and good shapeability as 3D printing filaments. These results are explicable if the dependence of the intermolecular interactions appears between the PLLA and stereo-complexer PDLA. To enlarge the durable applicability of racemic polylactic acid (PDLLA), future research should identify other parameters controling the PLA stereo-complexing as the intensifying the mobility of the macromolecules, the finding of the optimal recemic cristalization window.

Highlights

Poly(lactic) acid (PLA) is a biodegradable, biocompatible, compostable, semi-crystalline, bio-based thermoplastic aliphatic polyester, which has a relatively low cost, outstanding strength and a high elastic modulus
The stereo-complexation was missing or was very small if the base-PLLA was stereocomplexed with PDLA with a high molecular weight (Mw) (18 × 104 g·mol−1) and medium DS (3.5%), or with PDLA with a high Mw (19 × 104 g·mol−1) and high DS (12%) (Table 3)
The possibility of improving the functional properties of PLA by stereo-complexing of a PLA grade designed for 3D printing was studied

Summary

Introduction

Poly(lactic) acid (PLA) is a biodegradable, biocompatible, compostable, semi-crystalline, bio-based thermoplastic aliphatic polyester, which has a relatively low cost, outstanding strength and a high elastic modulus. PLA is a brittle polymer with a low thermal deformation temperature, poor toughness and low crystallization rates [5,6,7,8,9,10] These disadvantages limit its applicability, for long-life (durable) items, including for the 3D printed grades designed for the automotive industry [11] which need key characteristics such as high temperature melt processability, significant toughness and durability [12]. Durability is defined as the property of polymeric materials to withstand environmental stresses during its life time so that the material’s performance not to be hindered [13] Durability refers to those properties that can fail before the end of the item’s lifetime. The higher the number of nucleation nuclei (particles of nucleant), the more new crystals appear and the smaller their size upon appearance [13]

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