Effect of Carbon Nanotubes and Cellulose Nanowiskers on the Phase Separation and Morphology in Thermoplastic Polyurethane Nanocomposites

Carbon nanotubes (CNTs) can be very effective in controlling the hard (HD) and soft (SD) microdomain morphology in thermoplastic polyurethanes (TPU), and their combination with cellulose nanowiskers (CNWs) can further maximize their impact on the morphology and final properties in TPU hybrid composites.

The rheology of the uncured polyol suspensions incorporating either CNTs or CNWs was investigated, and analysis of the results showed that CNTs are more effective in the formation of a filler structure within the liquid suspension. Rheological analyses performed on hybrid suspensions, containing both CNTs and CNWs suggested that the two fillers presenting different size and aspect ratio are able to well interact, resulting in a better distribution and greater potential to form a more continuous and efficient 3D network. TPU nanocomposites were prepared by in-situ polymerization and compression molding, and their morphological features and thermal behavior were investigated. Morphological analyses through scanning electron microscopy (SEM) suggested that incorporation of a small amount of CNWs can have a positive effect on CNT dispersion by limiting the state of aggregation and favoring a more homogeneous distribution. Atomic force microscopy (AFM) indicated that TPU morphology exhibits a multi-scale phase separation characterized by heterogeneous morphologies at the micro- and nano- scales. CNTs appear to preferentially aggregate in the HD regions, favoring the nucleation and growth of smaller and more distributed HD spherulites. Thermal analyses clearly indicated that incorporation of CNTs and CNWs leads to significant changes in the crystallization behavior of TPU nanocomposites and favors multi-scale phase separation.