Probing the complex thermo-mechanical properties of a 3D-printed polylactide-hydroxyapatite composite using in situ synchrotron X-ray scattering

Abstract

Polylactide (PLA)-hydroxyapatite (HAp) composite components have attracted extensive attentions for a variety of biomedical applications. This study seeks to explore how the biocompatible PLA matrix and the bioactive HAp fillers respond to thermo-mechanical environment of a PLA-HAp composite manufactured by 3D printing using Fused Filament Fabrication (FFF). The insight is obtained by in situ synchrotron small- and wide- angle X-ray scattering (SAXS/WAXS) techniques. The thermo-mechanical cyclic loading tests (0-20 MPa, 22-56 °C) revealed strain softening (Mullins effect) of PLA-HAp composite at both room and elevated temperatures (<56 °C), which can be attributed primarily to the non-linear deformation of PLA nanometre-scale lamellar structure. In contrast, the strain softening of the PLA amorphous matrix appeared only at elevated temperatures (>50 °C) due to the increased chain mobility. Above this temperature the deformation behaviour of the soft PLA lamella changes drastically. The thermal test (0-110 °C) identified multiple crystallisation mechanisms of the PLA amorphous matrix, including reversible stress-induced large crystal formation at room temperature, reversible coupled stress-temperature-induced PLA crystal formation appearing at around 60 °C, as well as irreversible heating-induced crystallisation above 92 °C. The shape memory test (0-3.75 MPa, 0-70 °C) of the PLA-HAp composite demonstrates a fixing ratio (strain upon unloading/strain before unloading) of 65% and rather a ∼100% recovery ratio, showing an improved shape memory property. These findings provide a new framework for systematic characterisation of the thermo-mechanical response of composites, and open up ways towards improved material design and enhanced functionality for biomedical applications.

Keywords: 3D-printed polylactide-hydroxyapatite composite; Mullins effect; Shape memory effect; Small- and wide-angle X-ray scattering; Thermo-mechanical behaviour.

Graphical abstract

Fig. 1

Experimental setup, 2D SAXS/WAXS patterns, and 1D profiles for data interpretation. (a) Experimental setup incorporating the thermo-mechanical loading rig, imaging detector (“X-ray eye”), WAXS and SAXS detectors. (b) 2D WAXS patterns at initial state, maximum load at each cycle and final state. Examples of isolated bright diffraction spots are highlighted in red dash circles; (c) 2D SAXS patterns at initial state, maximum load at each cycle and final state; (d) 1D line profiles of WAXS patterns at corresponding states in (a); and (e) 1D electron density correlation functions from SAXS patterns for lamella structure analysis.

Fig. 1

Experimental setup, 2D SAXS/WAXS patterns, and 1D profiles for data interpretation. (a) Experimental setup incorporating the thermo-mechanical loading rig, imaging detector (“X-ray eye”), WAXS and SAXS detectors. (b) 2D WAXS patterns at initial state, maximum load at each cycle and final state. Examples of isolated bright diffraction spots are highlighted in red dash circles; (c) 2D SAXS patterns at initial state, maximum load at each cycle and final state; (d) 1D line profiles of WAXS patterns at corresponding states in (a); and (e) 1D electron density correlation functions from SAXS patterns for lamella structure analysis.

Fig. 2

Mullins effect characterisation of PLA-HAp composite. (a) Macroscopic strain evolution obtained by Deben loading rig at different loading cycles; (b) Strain evolution derived from WAXS patterns of the PLA amorphous peaks at different loading cycles; (d) Strain evolution interpreted from WAXS patterns of the HAp crystallite peaks at different loading cycles; (d) Strain evolution calculated from SAXS patterns of the PLA lamella structure at different loading cycles.

Fig. 3

Thermal properties characterisation of PLA-HAp composite. The PLA composite is heated up from 22 °C to 110 °C (in red) and cooled down to 22 °C (in black) at a constant rate of 2 °C/min. The evolution of the PLA amorphous peak centre position is plotted with temperature. The first phase transition temperature range is identified at around 60 °C and cold crystallisation happens at about 90–100 °C and remains during cooling. This is further confirmed by the inset 2D WAXS patterns at selected temperatures. Red dashed circles show the phase transformation from amorphous peaks (22–92 °C) to crystalline peaks (after 110 °C).

Fig. 4

Thermo-mechanical behaviour of PLA-HAp composite. (a) Macroscopic strain evolution at elevated temperatures (22 °C, 50 °C, 52 °C, 54 °C and 56 °C); (b) Strain evolution derived from WAXS patterns of the PLA amorphous peaks at elevated temperatures; (c) Strain evolution interpreted from WAXS patterns of the HAp crystallite peaks at elevated temperatures. (d) Strain evolution calculated from SAXS patterns of the PLA lamella structure at elevated temperature.

Fig. 5

Shape memory effect characterisation of PLA-HAp composite. (a) 3D shape memory cycle contains four stages (I, II, III and IV), representing loading, cooling, unloading and heating; (b) 2D WAXS patterns for stage II cooling from 70 °C to 22 °C and stage IV heating from 22 °C to 70 °C, showing the appearance of strong diffraction from crystal structure (highlighted in red dashed circle).

Fig. 6

Illustration of structure and deformation mechanism of PLA-HAp composite. (a) X-ray tomography image of the cross-section of the PLA-HAp composite produced by FDM 3D-printing. (b) Schematic diagram of the system consisting of HAp filler particle(s), PLA lamella structure and PLA amorphous matrix.