@article{GhobadiHeuchelKratzetal.2014, author = {Ghobadi, Ehsan and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas}, title = {Atomistic simulation of the shape-memory effect in dry and water swollen Poly[(rac-lactide)-co-glycolide] and copolyester urethanes thereof}, series = {Macromolecular chemistry and physics}, volume = {215}, journal = {Macromolecular chemistry and physics}, number = {1}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1022-1352}, doi = {10.1002/macp.201300507}, pages = {65 -- 75}, year = {2014}, abstract = {An atomistic molecular dynamics simulation approach is applied to model the influence of urethane linker units as well as the addition of water molecules on the simulated shape-memory properties of poly[(rac-lactide)-co-glycolide] (PLGA) and PLGA-based copolyester urethanes comprising different urethane linkers. The shape-memory performance of these amorphous packing models is explored in a simulated heating-deformation-cooling-heating procedure. Depending on the type of incorporated urethane linker, the mechanical properties of the dry copolyester urethanes are found to be significantly improved compared with PLGA, which can be attributed to the number of intermolecular hydrogen bonds between the urethane units. Good shape-memory properties are observed for all the modeled systems. In the dry state, the shape fixation is found to be improved by implementation of urethane units. After swelling of the copolymer models with water, which results in a reduction of their glass transition temperatures, the relaxation kinetics during unloading and shape recovery are found to be substantially accelerated.}, language = {en} } @article{GhobadiHeuchelKratzetal.2013, author = {Ghobadi, Ehsan and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas}, title = {Simulating the shape-Memory behavior of amorphous switching domains of Poly(L-lactide) by molecular dynamics}, series = {Macromolecular chemistry and physics}, volume = {214}, journal = {Macromolecular chemistry and physics}, number = {11}, publisher = {Wiley-VCH}, address = {Weinheim}, issn = {1022-1352}, doi = {10.1002/macp.201200450}, pages = {1273 -- 1283}, year = {2013}, abstract = {The thermally induced shape-memory effect of polymers is typically characterized by cyclic uniaxial thermomechanical tests. Here, a molecular-dynamics (MD) simulation approach of such a cyclic uniaxial thermomechanical test is presented for amorphous switching domains of poly(L-lactide) (PLLA). Uniaxial deformation of the constructed PLLA models is simulated with a Parinello-Rahman scheme, as well as a pragmatic geometrical approach. We are able to describe two subsequent test cycles using the presented simulation approach. The obtained simulated shape-memory properties in both test cycles are similar and independent of the applied deformation protocols. The simulated PLLA shows high shape fixity ratios (Rf 94\%), but only a moderate shape recovery ratio is obtained (Rr 30\%). Finally, the structural changes during the simulated test are characterized by analysis of the changes in the dihedral angle distributions.}, language = {en} } @article{GhobadiHeuchelKratzetal.2012, author = {Ghobadi, Ehsan and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas}, title = {Simulation of volumetric swelling of degradable poly[(rac-lactide)-co-glycolide] based polyesterurethanes containing different urethane-linkers}, series = {Journal of applied biomaterials \& functional materials}, volume = {10}, journal = {Journal of applied biomaterials \& functional materials}, number = {3}, publisher = {Wichtig}, address = {Milano}, issn = {2280-8000}, doi = {10.5301/JABFM.2012.10432}, pages = {293 -- 301}, year = {2012}, abstract = {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 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 M-n = 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 M-n = 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.}, language = {en} } @article{GhobadiHeuchelKratzetal.2012, author = {Ghobadi, Ehsan and Heuchel, Matthias and Kratz, Karl and Lendlein, Andreas}, title = {Influence of different heating regimes on the shape-recovery behavior of poly(L-lactide) in simulated thermomechanical tests}, series = {Journal of applied biomaterials \& functional materials}, volume = {10}, journal = {Journal of applied biomaterials \& functional materials}, number = {3}, publisher = {Wichtig}, address = {Milano}, issn = {2280-8000}, doi = {10.5301/JABFM.2012.10440}, pages = {259 -- 264}, year = {2012}, abstract = {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 T-low = 250 K to T-high = 500 K with different heating rates beta of 125, 40 and 4 K.ns(-1). Results: We could demonstrate that the obtained strain recovery rate (R-r) was strongly influenced by the applied simulation time and heating rate, whereby R-r 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 T-sw = 473 K for the modeled samples. Conclusions: We anticipate that the atomistic modeling approach presented should be capable of enabling further study of T-sw 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.}, language = {en} } @article{AliofkhazraeiRouhaghdamGhobadietal.2009, author = {Aliofkhazraei, Mahmood and Rouhaghdam, Alireza Sabour and Ghobadi, Ehsan and Mohsenian, Ehsan}, title = {Study of shape and distribution of TiO2 nanorods produced by atmospheric pressure plasma}, issn = {1612-8850}, doi = {10.1002/ppap.200930504}, year = {2009}, abstract = {A new method in plasma surface engineering called pulsed plasma electrolytic deposition (PPED) was applied to copper substrates. Corrosion properties of the produced layers were evaluated via potentiodynamic polarization. Results indicated relations among treatment time as well as peak applied voltage, frequency, and duty cycle on average density of TiO2 nanorods significantly. Investigation of the corrosion characteristics showed that the magnitude of corrosion resistance strongly followed from the experimental parameters of applied treatment as well as the average density of TiO2 nanorods. The average density of TiO2 nanorods measurements was done via figure analysis with minimum possible errors.}, language = {en} } @article{MofidiAliofkhazraeiRouhaghdametal.2009, author = {Mofidi, Seyed Haman Hedayat and Aliofkhazraei, Mahmood and Rouhaghdam, Alireza Sabour and Ghobadi, Ehsan and Mohsenian, Ehsan}, title = {Improvement of surface characteristics by electroplating hard chromium coating post treated by nanocrystalline plasma electrolytic carbonitriding}, issn = {1612-8850}, doi = {10.1002/ppap.200930702}, year = {2009}, abstract = {Nanocrystalline carbonitrides were performed by pulsed plasma electrolytic carbonitriding on hard chromium coating deposited on AISI 1035 substrate by electroplating. The electroplated samples were connected cathodically to a high-current pulsed power supply and biased to a negative voltage. The treatment times were 30, 60 and 60 min. A thick compound layer was formed on the surface of Cr coating with microhardness of about 1200 HV0.15. The nanostructure of the treated layers depends strongly on the applied voltage. The wear resistance of the treated layers depended on process parameters. Overall mechanical properties of treated samples show strong relation to morphology and distribution of complex carbonitride nanocrystallites.}, language = {en} }