**1. Introduction**

Orthopaedic procedures are seemingly ever increasing due to both an increase in the aging population and a rise in obesity rates, contributing to the increase in bone fractures. Permanent implants are expected to serve for the whole term of a patient's life, in contrast to temporary implants, which are required for a shorter time period to allow for the healing of broken bones [1]. Current orthopaedic implants of both categories are produced from metallic materials, which display adequate mechanical properties and fatigue and corrosion resistance; however, the stiffness of metallic materials is greater than that of bone, and this can cause stress shielding [2,3]. As a less stiff alternative to metal implants, aliphatic polyesters are a group of biocompatible and bioresorbable polymers used in a wide range of biomedical applications. Some of the most commonly studied polymers include polylactide (PLA), polycaprolactone (PCL) and polyhydroxyalkanoates (PHA) [4–6].

Each of these polymers has advantages and disadvantages for biomedical applications [7]. Poly l-lactide (PLLA) has been used in a range of medical devices [8], including various orthopaedic clinical applications [9], where its favourable mechanical properties when compared to other biopolymers offers a performance advantage. PLLA has a high tensile strength, low ductility and is a semi-crystalline polymer. The crystalline structure can make PLLA mechanically superior when compared to other polyesters in load-bearing applications [4,9–11]. PLLA has glass transition and melting temperatures of 55–80 ◦C

**Citation:** Naseem, R.; Montalbano, G.; German, M.J.; Ferreira, A.M.; Gentile, P.; Dalgarno, K. Influence of PCL and PHBV on PLLA Thermal and Mechanical Properties in Binary and Ternary Polymer Blends. *Molecules* **2022**, *27*, 7633. https:// doi.org/10.3390/molecules27217633

Academic Editors: Marek Brzezi ´nski and Małgorzata Ba´sko

Received: 23 September 2022 Accepted: 4 November 2022 Published: 7 November 2022

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and 170–180 ◦C, respectively. In contrast, polycaprolactone (PCL) is a soft, semi-crystalline polymer with a low melting and glass transition temperatures (55–60 ◦C and −54 ◦C, respectively) [12,13]. Although it possesses high ductility (with a tensile elongation at break of over 700%) and a high impact strength, it has a low tensile strength (~23 MPa) and Young's modulus [3,5,14–16]. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is an aliphatic biodegradable polyester with a melting temperature between 80 and 160 ◦C and a glass transition temperature in the range of −5 to 20 ◦C depending on the HV (hydroxyvalerate) content of the copolymer, which can be adjusted to control the mechanical properties and degradation of the polymer [17]. PHBV has been shown to produce consistent favourable bone tissue adaptation response in addition to the elimination of any undesirable chronic inflammatory responses (up to 12 months after implantation) [18].

Both PLLA and PHBV are hard polymers with poor impact performance [4,5,9,19], which limits their use. The brittle nature of the polymers can be improved through blending with soft ductile polymers [12]. Blending of polymers is a simple ye<sup>t</sup> effective method to obtain new materials with enhanced properties, as the limitations of the dominant component in the blend can be mitigated. Tuning of the physical and mechanical properties of a blend can be achieved with the selection of appropriate materials, adjustment of the blend compositions and appropriate preparation conditions [14]. The blending of PLA/PCL and PLA/PHBV as two co-polymer blends for use in biomedical applications has been previously investigated [6,20–23]. These studies have shown that it is possible to increase the fracture toughness or elongation at break of PLLA by blending with PCL or PHBV, but with reduced modulus or tensile strength [12,24–26]. Ternary blends of the three materials have not been previously assessed; however, the three materials together offer an attractive combination: PLLA providing strength, PCL giving ductility and PHBV enhancing biocompatibility of the material whilst also contributing to the mechanical properties [27].

Ternary blends with polymers which are not established biomaterials have previously been considered in terms of their ability to modify the behaviour of PLLA, PCL and/or PHBV using (i) polypropylene carbonate (PPC) in PLLA/PHBV/PPC blends [28], which showed reduced strength and modulus compared to PLLA alone; (ii) poly(butylene succinate) (PBS) in PLLA/PHBV/PBS blends [29], which showed reduced strength and modulus but increased elongation at break compared to PLLA alone; and (iii) montmorillonite (MMT) nanoclay in PLA/PCL/MMT-nanoclay blends [30], which increased modulus and strength compared to the PLLA/PCL blend, but reduced toughness and impact strength. These studies reinforce the observation from previous studies with PLLA/PCL and PLLA/PHBV blends that a formulation which provides increased ductility without sacrificing modulus and strength has ye<sup>t</sup> to be found.

The aim of this study was, therefore, to characterise binary and ternary blends of PLLA, PCL and PHBV produced through twin screw extrusion for their suitability for use in orthopaedic applications in terms of mechanical performance and processability. Changes in microstructure are assessed via FTIR and DSC, with mechanical properties characterised in tension using modulus, strength, creep rate and viscosity as the key indicators of processability and mechanical behaviour over extended time periods. As orthopaedic devices are commonly applied for extended time periods, the creep behaviour of the blended materials was considered an important property to consider, and one which had not been studied in previous work on blended polyester formulations.

## **2. Materials and Methods**
