Modeling of PLLA near glass transition temperatures

Abstract

The treatment for Coronary Heart Diseases -caused by the deposition of plaque on arterial walls -which has been the deployment of a metal stent in the blocked artery, poses serious complications in the form of accumulation of blood clots due to the permanent and rigid nature of metals. Meanwhile, Bioresorbable Vascular Scaffolds (BVSs) -made up of poly l-lactide (PLLA) -are transient implants offering support to the artery for 6-9 months and leaving no trace behind once dissolved after 2 years. PLLA, however, has lower strength compared to metals and thus requires a higher thickness to make it useful for transplants. Thermal annealing, which can alter the microstructure of PLLA to make it thinner and stronger, is a good solution for this problem. In order to implement thermal annealing, experimentation and simulations are required to study the behaviour of the microstructure during the process. In general, its behaviour below glass transition temperature and at short time-scale is glassy, while above glass transition temperature and at long time scale is rubbery. With constitutive models available for other polymers, such as polyethylene terephthalate (PET), it is intended to develop a constitutive model for PLLA accordingly. In this regard, the Buckley model is implemented. The model allows studying the behaviour in PLLA at the temperatures of interest during thermal annealing. Numerical results will be presented and compared for neat PLLA samples for different strain rates and temperatures.