PSI - Issue 15

Raasti Naseem et al. / Procedia Structural Integrity 15 (2019) 55–59 Naseem et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction PLLA is a brittle material with considerable degradation time (over 2~3 years), To address the shortcomings of neat PLLA, alternative approaches to manufacturing adjustments were used to enhance mechanical properties of the polymer. One such method is to blend it with elastomers, typically polymeric materials with rubber-like properties. One of the most studied polymers to co-polymerize PLLA with is polycaprolactone (PCL). PCL is a hydrophobic, semi-crystalline polymer, whose crystallinity decreases with increasing molecular weight. Along with good solubility, PCL has a low melting point along with high blend-compatibility. PCL attracted much interest due to its good processability, cost efficiency and high toughness, which is related to its low glass-transition and melting temperatures. The use of PLLA and PCL are both approved by the FDA. Both polymers can be modified to obtain a final material with desirable properties. Incorporation of PCL into PLLA enhances its fracture toughness (Todo, 2007) and reduces fragility, whilst the incorporation of PLLA into PCL helps to increase the otherwise high degradation rate of PCL (Lebourg et al., 2008). Matta et al. (2014) characterized a biodegradable PLA/PCL blend, demonstrating an increase in its percentage elongation and impact toughness, but a decrease in strength and modulus, in comparison to PLA. Different weight ratios of PLA/ PCL were investigated, with the 80/20% of PLA/PCL blend showing the highest strength and elongation. Chavalitpanya and Phattanarudee (2013) focused on assessing the use of a block copolymers (poly(ethylene glycol) and poly(propylene glycol)) as a potential compatibilizer for PLA/PCL blends, with the intention to improve the phase miscibility in the polymer blend. It was found that the PCL phase was more miscible within the PLA matrix. As the concentration of poly (ethylene glycol)-poly (propylene glycol) (PEG-PPG) increased, there was an increase in strain at break; however, this was accompanied by a concomitant decrease in modulus and maximum strength. As with most polymers, the mechanical and physical properties for either individual or blended ones are dependent on molecular weight, crystallinity and manufacturing processes the samples undertake. Each sample is unique and its property and performance cannot be derived from results in the literature. The aim of this work is to assess a novel co-polymer developed for potential cardiovascular applications to address the limitations of previous cardiovascular stents. Ideally, the material should have sufficient intrinsic toughness for handling and deployment, an adequate radial strength, an ideal degradation rate which matches that of healing-tissue regeneration, and be susceptible to surface functionalisation to aid with the elimination of thrombosis (Zhang, 2017). In this paper, focus was given to characterise the mechanical properties of a novel PLLA/PCL copolymer in virgin state as well as over degradation. 2. Methodology The material is a PLA/PCL-PEG co-polymer. The poly(L-lactide) was the same grade as used for most of the commercial bioresorbable stents (Corbion PURAC® PL 38). The PLA-(PCL-PEG) is a custom copolymer with lower molecular weight (~150-200 kDa), amorphous structure and a glass transition temperature of 20 ° C. The weight ratio between PLA and PCL-PEG was 60 to 40. The novelty of this material was the addition of phosphate glass particles into the mix, aiming to toughen the material. They are less than 2 µm in size and loaded to 10 wt%. The material was supplied in the form of tube. The inner and outer diameter of the tube was 2.7 mm and 3 mm, respectively, with a wall thickness of approximately 200 µm. The sectioned tube samples were subjected to accelerated degradation conditions, through incubation in phosphate buffered saline (PBS) at 50 ° C. The samples were assessed every 2 days over a 14-day period. A 10 µm-radius spherical tip was used for nano-indentation experimentation. A loading rate of 0.5 mN/s and an unloading rate of 5 mN/s were chosen for the testing. Offset between indents was maintained at 40 µm. The elastic modulus of the materials was determined using the Oliver-Pharr theory, based on the slope of the unloading curve (top portion; 20%). Both continuous and singular indentations were carried out to assess the Young’s modulus as well as the stress-strain response of the material.