Effect of nanotubes on shape memory effect in liquid-crystalline epoxy-based elastomers

In this work, shape memory nanocomposites were prepared and characterized. The polymer matrix consisted in an epoxy-based liquid crystalline elastomer (LCE). Multi-walled carbon nanotubes (MWCNT) and graphite nanoplatelets (GNP) were selected as fillers. The influence of different contents of nanofillers on mechanical, thermal and shape memory properties was evaluated. In order to disperse and homogeneously distribute the nanofillers within the polymer matrix an in-depth evaluation on the optimal conditions to synthesize the materials was carried out. These conditions had a substantial influence on the final distribution of the nanofillers within the epoxy-based matrix, which was analyzed from a macroscopic and microscopic morphological point of view. The best results were obtained through a chemical surface modification of the nanoparticles. The chemical modification of MWCNTs consisted in grafting the selected epoxy monomers on the surface. The obtained adducts were characterized in terms of chemical, thermal and morphological features. Concerning GNP, a similar protocol based on surface modification was carried out. In this case, a preliminary oxidation process was performed in order to promote the exfoliation of graphene sheets, in form of graphene oxide (GO), and to favour their dispersion within the polymer matrix. Different degrees of oxidation were attempted. GO nanoparticles were successively modified with epoxy monomers. Also in this case chemical, morphological, structural and thermal characterization was carried out. Surface modified carbonaceous nanoparticles were then dispersed in varying amounts in the organic matrix. The obtained nanocomposite systems were characterized in their chemical-physical and morphological properties. The adopted compatibilization strategies used for both MWCNTs and GNP were found to be extremely effective to get homogeneous and to enable a dramatic enhancement of the actuation extent at low nanofiller content. Moreover, the stress threshold required to trigger the reversible thermomechanical actuation was significantly decreased. The effect of nanoparticles on thermomechanical properties of the materials was correlated to the microstructure and the phase behavior of the host system. Results demonstrated that the incorporation of carbon nanofillers amplified the soft-elastic response of the liquid crystalline phase to external stimuli. Tunable thermomechanical properties of these systems make them suitable for a variety of potential advanced applications ranging to robotics, sensing and actuation, and artificial muscles.