Non-covalent interaction induces controlled reinforcement of thermoplastic elastomer composites homologously incorporated with hydrophobized cellulose nanocrystals

Abstract

Surface hydrophobization of cellulose nanocrystal (CNC) is developed through Pickering emulsion method and esterification-grafting, consequentially enhancing its thermal properties and hydrophobicity, and tensile modulus in thermoplastic elastomer composites. n-Tetradecenylsuccinic anhydride (TDSA) provides a high level of compatibility owing to the structural homology by non-covalent interactions between CNC-graft-TDSA and poly(styrene)-block-poly(isoprene)-block-poly(styrene) (SIS). Theoretical calculations supports the molecular-level insight on how the nanofiller and SIS interacts. Density functional theory calculations and the energy decomposition analysis reveal the stabilization free energy of −2.59 kcal mol−1 gained through hydrophobic and OH–π interactions, which is assigned as the key component for the superior mechanical properties. At the filler contents of 0.6–2.5 vol %, the moduli of the TPE nanocomposites gradually strengthen while retaining thermoplasticity, without sacrificing the ultimate tensile characteristics of SIS, although it is below the calculated φc of 5.9 vol % (critical percolation threshold) for filler-filler interactions. Origin of the controlled boost in filler-matrix binding is rationalized by pronounced hydrophobic interactions in isoprene-rich rubbery phase (86 wt % in SIS). Interestingly, at 5.1 vol % near to filler-filler interactions, even though reinforcements in storage/initial modulus, and toughness increase by up to 14/4.0-, and 1.2-fold, the tensile stress at break only decreases by 27 %.