Ehsan Ghobadi

Influence of different heating regimes on the shape-recovery behavior of poly(L-lactide) in simulated thermomechanical tests

Aim Multifunctional polymer-based biomaterials, which combine degradability with a shape-memory capability and in this way enable the design of actively moving implants such as self-anchoring implants or controlled release systems, have been recently introduced. Of particular interest are approved degradable polymers such as poly(L-lactide) (PLLA), which can be easily functionalized with a shape-memory effect. In the case of semicrystalline PLLA, the glass transition can be utilized as shape-memory switching domain. Methods In this work we applied a fully atomistic molecular dynamics simulation to study the shape-memory behavior of PLLA. A heating-deformation-cooling programming procedure was applied to atomistic PLLA packing models followed by a recovery module under stress-free conditions allowing the shape recovery. The recovery was simulated by heating the samples from Tlow = 250 K to Thigh = 500 K with different heating rates β of 125, 40 and 4 K·ns−1. Results We could demonstrate that the obtained strain recovery rate (Rr) was strongly influenced by the applied simulation time and heating rate, whereby Rr values in the range from 46% to 63% were achieved. On its own the application of a heating rate of 4 K·ns−1 enabled us to determine a characteristic switching temperature of Tsw = 473 K for the modeled samples. Conclusions We anticipate that the atomistic modeling approach presented should be capable of enabling further study of Tsw with respect to the molecular structure of the investigated SMP and therefore could be applied in the context of design and development of new shape-memory (bio)materials.

Simulation of volumetric swelling of degradable poly[(rac-lactide)-co-glycolide] based polyesterurethanes containing different urethane-linkers

Aim The hydrolytic degradation behavior of degradable aliphatic polyester-based polymers is strongly influenced by the uptake or transport of water into the polymer matrix and also the hydrolysis rate of ester bonds. Methods We examined the volumetric swelling behavior of poly[(rac-lactide)-co-glycolide] (PLGA) and PLGA-based poly-urethanes (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). Results 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. Conclusions It can be anticipated that the incorporation of urethane linkers might be a helpful tool to adjust the degradation behavior of polyesters.

Electrodeposition and mechanical and corrosion resistance properties of tertiary Ni-W/Al2O3/CNT nanocomposite coatings

In this paper, the effects of the average size of nanometric particles (ASNP) from submicron scale (less than 1 µm) to nanometric scale (less than 10 nm) have been studied on the properties of nanocomposite layers which usually have not been studied or rarely studied. These properties consist of corrosion current density after long time immersion, roughness of obtained layer and distribution of nanometric particles. All of the other effective factors for fabrication of nanocomposite coatings have been fixed for better studying the effect of the ASNP. It has been seen that decreasing the ASNP will lead to lower corrosion current densities however in some cases pitting phenomena has been observed. The roughness illustrated a minimum level while the distribution of nanometric particles will be more uniform by decreasing the ASNP.