This study concerns the effect of thermal aging on mechanical properties of polyurethane. Polyurethane samples were exposed at 85° and 120°C under inert atmosphere. Mechanical tests were carried out on these samples the aging period. Tensile tests were performed to see the effect of aging on elastic modulus (E), stress (σ_r), and strain (ϵ_r) at break. It was shown that there are two distinct periods. Due to aging, E and σ_r increase in the first period, then they decrease in the second period. ϵ_r decreases first and then increases. Fatigue tests were performed on unaged and aged samples. It was shown that the fatigue behavior of polyurethane (PU) is improved the same way during the first stage of aging. In the second step, the number of cycles to failure increases due to aging. The results show that aging has an important effect on mechanical properties of PU. The strain at break decreases during the first step of aging due to post-cross-linking and then increases due to chain scission in the network. Based on these results, the effect of cross-linking and chain scission on the mechanical properties of PU was discussed.

In this study some new concepts regarding certain aspects related to shape memory polymers are presented. A blend of polylactic acid (PLA) (80%) and polybutylene succinate (PBS) (20%) was prepared first by extrusion, then by injection molding to obtain the samples. Tensile, stress-relaxation and recovery tests were performed on these samples at 70 °C. The results indicated that the blend can only regain 24% of its initial shape. It was shown that, this partial shape memory effect could be improved by successive cycles of shape memory tests. After a fourth cycle, the blend is able to regain 82% of its shape. These original results indicated that a polymer without (or with partial) shape memory effect may be transformed into a shape memory polymer without any chemical modification. In this work, we have also shown the relationship between shape memory and property memory effect. Mono and multi-frequency DMA (dynamic mechanical analyzer) tests on virgin and 100% recovered samples of polyurethane (PU) revealed that the polymer at the end of the shape memory tests regains 100% of its initial form without regaining some of its physical properties like glass transition temperature, tensile modulus, heat expansion coefficient and free volume fraction. Shape memory (with and without stress-relaxation) tests were performed on the samples in order to show the role of residual stresses during recovery tests. On the basis of the results we have tried to show the origin of the driving force responsible for shape memory effect.

Les polymères qui se caractérisent par un effet mémoire de forme sont capables de mémoriser leur forme macroscopique permanente, d’être déformés puis fixés dans une forme temporaire, ensuite sous l’effet d’un stimulus externe, ils peuvent récupérer leurs formes initiales. Dans ce travail nous nous sommes intéressés au mécanisme de l’effet mémoire de forme ainsi que les propriétés du polymère au cours d’un cycle de mémoire de forme, afin d’établir une relation entre l’effet mémoire de forme et l’effet mémoire de propriétés. La deuxième partie de ce travail a été consacré à un nouveau concept qui est l’effet mémoire de forme partiel, où un polymère ne peut récupérer qu’une partie de sa forme initiale. Il a été montré que cette capacité partielle peut être améliorée et augmentée à travers des essais multi-cycles de mémoire de forme sous stimulation thermique, pour devenir une capacité totale de récupération de la forme initiale. Enfin, nous avons étudié la force motrice "Driving force" responsable de l’effet mémoire de forme en établissant une relation entre le taux de recouvrance et les contraintes résiduelles introduites dans le polymère au cours de sa déformation. Nous avons suivi l’évolution de la contrainte pendant un essai de recouvrance à longueur fixe pour la relier avec la déformation pendant un essai de recouvrance libre, ensuite nous avons comparé ce résultat avec une méthode directe originale que nous avons mise en place, en utilisant un échantillon témoin. Nous avons ainsi pu suivre l’évolution de la force motrice et la déformation relative à la recouvrance due à une stimulation thermique.