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. 2021 Aug 23;13(16):2835.
doi: 10.3390/polym13162835.

Effect of In-Mold Annealing on the Properties of Asymmetric Poly(l-lactide)/Poly(d-lactide) Blends Incorporated with Nanohydroxyapatite

Martin Boruvka  1 , Cenek Cermak  1 , Lubos Behalek  1 , Pavel Brdlik  1
Affiliations

Effect of In-Mold Annealing on the Properties of Asymmetric Poly(l-lactide)/Poly(d-lactide) Blends Incorporated with Nanohydroxyapatite

Martin Boruvka et al. Polymers (Basel). .

Abstract

The proper choice of a material system for bioresorbable synthetic bone graft substitutes imposes strict requirements for mechanical properties, bioactivity, biocompatibility, and osteoconductivity. This study aims to characterize the effect of in-mold annealing on the properties of nanocomposite systems based on asymmetric poly(l-lactide) (PLLA)/Poly(d-lactide) (PDLA) blends at 5 wt.% PDLA loading, which was incorporated with nano-hydroxyapatite (HA) at various concentrations (1, 5, 10, 15 wt.%). Samples were melt-blended and injection molded into "cold" mold (50 °C) and hot mold (100 °C). The results showed that the tensile modulus, crystallinity, and thermal-resistance were enhanced with increasing content of HA and blending with 5 wt.% of PDLA. In-mold annealing further improved the properties mentioned above by achieving a higher degree of crystallinity. In-mold annealed PLLA/5PDLA/15HA samples showed an increase of crystallinity by ~59%, tensile modulus by ~28%, and VST by ~44% when compared to neat hot molded PLLA. On the other hand, the % elongation values at break as well as tensile strength of the PLLA and asymmetric nanocomposites were lowered with increasing HA content and in-mold annealing. Moreover, in-mold annealing of asymmetric blends and related nanocomposites caused the embrittlement of material systems. Impact toughness, when compared to neat cold molded PLLA, was improved by ~44% with in-mold annealing of PLLA/1HA. Furthermore, fracture morphology revealed fine dispersion and distribution of HA at 1 wt.% concentration. On the other hand, HA at a high concentration of 15 wt.% show agglomerates that worked as stress concentrators during impact loading.

Keywords: crystallization; heat resistance; impact resistance; mechanical properties; nanocomposites; nanohydroxyapatite; poly(d-lactide); poly(l-lactide); stereocomplex.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
DSC curves of the PLLA a PLLA/HA nanocomposites obtained during the first heating.
Figure 2
Figure 2
DSC curves of the asymmetric PLLA/5PDLA a PLLA/5PDLA/HA nanocomposites obtained during the first heating.
Figure 3
Figure 3
Tensile modulus plots of samples from cold and hot molds.
Figure 4
Figure 4
Tensile strength plots of samples from cold and hot molds.
Figure 5
Figure 5
Elongation at break plots of samples from cold and hot molds.
Figure 6
Figure 6
Impact toughness plots of samples from cold and hot molds.
Figure 7
Figure 7
Heat deflection temperature plots of samples from cold and hot molds.
Figure 8
Figure 8
Vicat softening temperature plots of samples from cold and hot molds.
Figure 9
Figure 9
Fracture morphologies of (a) cold and (b) hot mold PLLA.
Figure 10
Figure 10
Fracture morphologies of (a) cold and (b) hot mold PLLA/5PDLA.
Figure 11
Figure 11
Fracture morphologies of (a) cold and (b) hot mold PLLA/1HA.
Figure 12
Figure 12
Fracture morphologies of (a) cold and (b) hot mold PLLA/5PDLA/1HA.
Figure 13
Figure 13
Fracture morphologies of (a) cold and (b) hot mold PLLA/15HA.
Figure 14
Figure 14
Fracture morphologies of (a) cold and (b) hot mold PLLA/5PDLA/15HA.
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