Investigation of the effects of polycaprolactone molecular weight and graphene content on crystallinity, mechanical properties and shape memory behavior of polyurethane/graphene nanocomposites.

In the following work, different shape memory polyurethanes (SMPUs) were synthesized using polycaprolactone (PCL) with various molecular weights, hexamethylene diisocyanate (HDI), and 1,4-butanediol (BDO). Afterward, polyurethane (PU)-based nanocomposites were prepared with different graphene nanosheets contents via solution casting method. Hydrogen nuclear magnetic resonance (1H-NMR) was used to confirm the chemical structure of PCLs and calculate their actual molecular weights. The chemical structure and hydrogen bonding content of PUs and their nanocomposites were investigated by Fourier-transform infrared spectroscopy (FTIR). According to the results, the hydrogen bonding contents of nanocomposites were reduced by graphene nanosheets inhibition from the formation of hydrogen bonds between polyurethane chains. Thermal properties and crystalline morphology of samples were studied using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results indicated that the transition temperature and crystallinity of samples were changed by variation of the molecular weight of the PCL component and of the concentration of the graphene nanosheets. Graphene nanosheets dispersion in polyurethane matrix was investigated using the field emission scanning electron microscope (FE-SEM). The mechanical and shape memory properties of different PUs and their nanocomposites were determined at both 75 °C and room temperature. It can be deduced from the results that the modulus of the samples increased due to the rigidity of nanosheets. Furthermore, the restricted mobility of PCL chains, due to the presence of nanosheets, led to higher shape fixity ratio. Moreover, the nanosheets prevented the stress transfer on the hard segments which increased the shape recovery ratio.