Constructing tesnegrity-inspired microstructures in a polymer nanocomposite with cellulosic nanomaterials
Orr, Matthew Philip
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The objective of this research is to develop and investigate prescribed microstructures based on the idea of tensegrity in a semicrystalline thermoplastic polymer matrix with cellulose nanomaterials (CNs). Two CNs, cellulose nanocrystals (CNCs) and nanofibrillated cellulose (CNFs) were used as nanofiller in two polymer matrices, polyethylene-co-vinyl alcohol with either 44 mol.% or 48 mol.% ethylene comonomer content (44EVOH and 48EVOH). Three different processing methods were used to investigate the level of CNC dispersion in EVOH: melt mixing, solution casting, and a multi-step protocol involving first solution casting followed by melt mixing CNCs with EVOH. The level of CNC dispersion in the nanocomposites was initially characterized with polarized optical microscopy below and above the melting temperature of EVOH. The nanocomposites’ thermomechanical, thermal, mechanical, and structural properties were also investigated as a function of CNC loading. The results suggested a multi-step protocol increased the level of CNC dispersion in EVOH the most compared to only melt or solution processing strategies. Next, prescribed microstructures were developed using a sequential biaxial stretching technique. The structures of the stretched samples were characterized with x-ray diffraction and thermomechanical properties were also investigated of the stretched films as a function of CNC loading. The results indicated storage modulus values increased in the direction of applied strain for a 2.5 wt.% CNC/44EVOH nanocomposite when it was uniaxially stretched versus the unstretched composite sample and these higher storage modulus values were retained and more uniform in-plane when the composite was biaxially stretched. Overall, the results indicated nanocomposites with anisotropic CNs designed in specified spatial arrangements in a thermoplastic matrix could increase thermomechanical properties of the polymer, and these polymer-particle arrangements behaved like tensegrity-inspired microstructures. The work presented herein contributes to the overall understanding of polymer processing- structure- property relationships using processing strategies commonly employed in commercial applications to incorporate and draw polymer and composite films.