Controlling strain stiffening in elastomeric composites by nanofiber network architecture

In order to reduce the invasiveness of implantable electronic sensor devices, a technology is needed that could create devices that behave soft and flexible as biological tissue but at the same time maintain their electronic functionality and resist to a high values of sheer and tensile stress. The main problem consists in producing a biocompatible material that stiffens at excessive strain and thus could prevent crack formation and delamination in the more fragile thin functional electronic layers. This thesis presents the methodology to fabricate and characterize a smart composite material composed by polymer nanofiber networks incorporated in an elastomeric matrix. Nanofiber networks of the biocompatible polymer Poly(L-lactic acid) (PLLA) are produced by electrospinning. The polymer Polydimethylsiloxane (PDMS) is used as the elastomeric matrix. The objective is to achieve a significant strain-stiffening transition by introducing a nanofiber network with controlled architecture. To this end a methodology is introduced that enables to synthetize polymer nanofibers with controlled curvature. Dynamical mechanical analysis (DMA) and Atomic Force Microscopy (AFM) are employed to characterize the mechanical properties of the composite material as well as of its single components. The results indicate that the mechanical response of the polymer can be largely controlled by the nanonetwork architecture. Stiff response to tensile stress is obtained when the fibers are straight and aligned in the stress direction. Soft and stretchable response results when fibers are curved or oriented parallel to the strain direction. The introduction of the methodology for fabrication as well as the characterization of such nanonetwork composites with controlled architecture will pave the way for further systematic studies leading to composites with tunable strain stiffening transition.