Understanding the process-structure-property relationship in biodegradable polymer nanocomposite films
Sullivan, Erin M.
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The focus of this study was to explore process-structure-property relationships in biodegradable polymer nanocomposite films in order to eliminate the commonly used trial and error approach to materials design and to enable manufacturing of composites with tailored properties for targeted applications. The nanofiller type and concentration, manufacturing method and compounding technique, as well as processing conditions were systematically altered in order to study the process-structure-property relationships. Polylactic acid (PLA) was used as the polymer and exfoliated graphite nanoplatelets (GNP), carbon nanotubes (CNT), and cellulose nanocrystals (CNC) were used as reinforcement. The nanocomposite films were fabricated using three different methods: 1) melt compounding and melt fiber spinning followed by compression molding, 2) solution mixing and solvent casting, and 3) solution mixing and electrospinning followed by compression molding. Furthermore, the physical properties of the polymer, namely the crystallization characteristics were altered by using two different cooling rates during compression molding. The electrical response of the composite films was examined using impedance spectroscopy and it was shown that by altering the physical properties of the insulating polymer matrix, increasing degree of crystallinity, the percolation threshold of the GNP/PLA films is significantly reduced. Additionally, design of experiments was used to examine the influence of nanofiller type (CNT versus GNP), nanofiller content, and processing conditions (cooling rate during compression molding) on the elastic modulus of the composite films and it was concluded that the cooling rate is the primary factor influencing the elastic modulus of both melt compounded CNT/PLA and GNP/PLA films. Furthermore, the effect of nanofiller geometry and compounding method was examined and it was shown that the high nanofiller aspect ratio in the CNT/PLA films led to decreased percolation threshold compared to the GNP/PLA films. The melt compounded GNP/PLA films displayed a lower percolation threshold than the solution cast GNP/PLA films most likely due to the more homogeneous distribution and dispersion of GNP in the solution cast films. Fully biodegradable and biorenewable nanocomposite films were fabricated and examined through the incorporation of CNC in PLA. Through the addition of CNC, the degree of crystallinity of the matrix was significantly increased. Focusing the design space through investigation of process-structure-property relationships in PLA nanocomposites, can help facilitate nanocomposites with tailored properties for targeted applications.