Polymer nanocomposites that exhibit stimuli responsive changes in morphology and mechanical behavior are interesting materials for ‘smart’ protective devices or adaptive biomaterials. This dissertation deals with the development of bio-inspired stimuli-responsive mechanically dynamic materials based on the dermis of sea cucumbers. The new materials were based on a low-modulus matrix polymers that were reinforced with a percolating cellulose nanofiber network. Owing to the abundance of surface hydroxyl groups, the cellulose nanofibers display strong interactions between themselves, causing the evenly dispersed percolating nanocomposites to display a high stiffness. The nanofiber-nanofiber interactions were largely switched off by the introduction of a chemical regulator that allows for competitive hydrogen bonding, resulting in a significant decrease in the stiffness of the material. Using a host polymer with a thermal transition in a regime of interest, nanocomposites that demonstrate more than three orders of magnitude modulus changes have been developed.
Tensile storage modulus of PVAc, which has a glass transition of about 42-56 °C (above physiological temperature) increased to about 5 GPa with incorporation of 16.5% v/v tunicate cellulose nanofibers. Exposure to water or artificial cerebrospinal fluid (ACSF) at 37 °C (to simulate physiological conditions) resulted in an uptake of 70-80 %w/w fluid, shift in Tg of the PVAc to about 20 °C and a drop in tensile storage modulus of the materials at 37 °C to about 12 MPa. The enhanced mechanical contrast achieved in these materials (5 GPa to 12 MPa) was a significant advancement over the proof of concept material (800 MPa to 20 MPa) developed earlier by Capadona et al. and the dry state modulus of 5 GPa provided sufficient stiffness for these materials to be used as substrates for cortical electrodes. The high contrast in mechanical behavior, the temperature range (23 °C to 37 °C) and time (1hr) required for switching opens up broad range of applications for these nanocomposites as adaptive biomaterials. Stimuli-responsive mechanically adaptive materials have also been developed using different polymer matrices, filler sources and combination of one or more stimuli and some fundamental insights into the structure-property relationships in these materials have been obtained. The dramatic mechanical morphing of nanocomposites as a result of changing nanoparticle interactions is described in the framework of two mechanical models viz, percolation model and the Halpin Kardos model based on a mean field approach.