Effect of In-Mold Annealing on the Properties of Asymmetric Poly(<span style="font-variant: small-caps">l</span>-lactide)/Poly(<span style="font-variant: small-caps">d</span>-lactide) Blends Incorporated with Nanohydroxyapatite

• Main Authors: Martin Boruvka, Cenek Cermak, Lubos Behalek, Pavel Brdlik
• Format: Article
• Language: English
• Published: MDPI AG 2021-08-01
• Series: Polymers
• Subjects: poly(<span style="font-variant: small-caps">l</span>-lactide); poly(<span style="font-variant: small-caps">d</span>-lactide); stereocomplex; nanohydroxyapatite; nanocomposites; mechanical properties
• Online Access: https://www.mdpi.com/2073-4360/13/16/2835

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(<span style="font-variant: small-caps;">l</span>-lactide) (PLLA)/Poly(<span style="font-variant: small-caps;">d</span>-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.