Dispersion morphology and correlation to moduli using buckling metrology in clay-biopolymer nanocomposite thin films

Structure-interaction-mechanical property correlation in bionanocomposite thin films is an area of growing interest for research and application areas from barrier to molecular transport to UV blocking layers for polymer solar cells to dielectric properties modification. Here we study flow coated ultrathin to thin films (70-150 nm) of clay bionanocomposites to understand the nanoparticle dispersion and its effect on nanomechanical properties. Binary and ternary thin film systems of polylactide (PLA), polycaprolactone (PCL), and Cloisite 30B (C30B) clay platelets were investigated. While C30B was only partially intercalated by PLA, it was almost completely intercalated by PCL due to strong hydrogen bonding. In addition, the dispersion of C30B improved continuously and linearly with increasing PCL content in homogeneously cast blended PLA:PCL. GIWAXS confirmed that the intercalated clay platelets in PLA and PCL were dominantly oriented parallel to the substrate. The method of strain induced elastic buckling instability for mechanical measurements (SIEBIMM) showed that pure PLA and PCL had in-plane modulus unchanged from bulk values for this range of ultrathin-thin films. In PLA/C30B nanocomposite thin films, the in-plane elastic modulus rapidly increased by up to 26% with 2 wt % C30B, but saturated thereafter up to 10 wt % C30B forming C30B aggregates. On the other hand, the in-plane elastic modulus of PCL/C30B thin films increased linearly by up to 43% with 10 wt % C30B due to the higher interaction driven dispersion, results that were shown to fit well with the Halpin-Tsai model. We conclude that the different strengthening behavior came from different interaction driven dispersion states of C30B in polymer matrices, governed by their molecular structures.