Simulation of Volumetric Swelling of Degradable Poly[(Rac-Lactide)-Co-Glycolide] Based Polyesterurethanes Containing Different Urethane-Linkers

Abstract

The hydrolytic degradation behavior of degradable aliphatic polyester-based polymers is strongly influenced by the up-take or transport of water into the polymer matrix and also the hydrolysis rate of ester bonds. We examined the volumetric swelling behavior of poly[(rac-lactide)-co-glycolide] (PLGA) and PLGA-based polyurethanes (PLGA-PU) with water contents of 0 wt%, 2 wt% and 7 wt% water at 310 K using a molecular modeling approach. Polymer systems with a number average molecular weight of Mn = 10,126 g∙mol(-1) were constructed from PLGA with a lactide content of 67 mol%, whereby PLGA-PU systems were composed of five PLGA segments with Mn = 2052 g∙mol(-1), which were connected via urethane linkers originated from 2,2,4-trimethyl hexamethylene-1,6-diisocyanate (TMDI), hexamethyl-1,6-diisocyanate (HDI), or L-lysine-1,6-diisocyanate (LDI). The calculated densities of the dry PLGA-PU systems were found to be lower than for pure PLGA. The obtained volumetric swelling of the PLGA-PU was depending on the type of urethane linker, whereby all swollen PLGA-PUs contained larger free volume distribution compared to pure PLGA. The mean square displacement curves for dry PLGA and PLGA-PUs showed that urethane linker units reduce the mobility of the polymer chains, while an increase in backbone atoms mobility was found, when water was added to these systems. Consequently, an increased water uptake of PLGA-PU matrices combined with a higher mobility of the chain segments should result in an accelerated hydrolytic chain scission rate in comparison to PLGA. It can be anticipated that the incorporation of urethane linkers might be a helpful tool to adjust the degradation behavior of polyesters.