Free-radical maleation of poly(butylene adipate-co-terephthalate) in supercritical carbon dioxide and its effect on the percolation and deformation mechanism of layered silicate nanocomposites
The utilization of compostable polymers such as poly(butylene adipate-co-terephthalate) (PBAT) in single-use packaging applications can mitigate the water, solvents, and energy required for processing plastic waste. PBAT exhibits exceptional elongational properties, but its moderate water vapor permeability and low tensile modulus limit the applications in which PBAT can be employed. In this work, PBAT nanocomposites were produced at various loadings of organically-modified montmorillonite. The secondary structure of nanoparticles formed within the PBAT matrix was evident in the terminal region of the linear viscoelastic response under oscillatory shear, and a percolation threshold was determined to occur at 3.88% clay by weight. Transmission electron microscopy (TEM), x-ray diffraction (XRD), and the water vapor transmission rate (WVTR) were also utilized to elucidate the clay nanoparticle structure and the effect of increased clay loading on thermal and mechanical and properties was also studied. To improve the level of clay exfoliation, a graft copolymer was developed as a compatiblizer. The maleation of PBAT was demonstrated via a solvent-free free-radical initiated grafting process in a supercritical carbon dioxide medium. The grafted anhydride moieties were modified with benzylamine to aid in FTIR and ¹H NMR spectroscopy, and it was calculated that PBAT-g-MA was produced at a graft level ranging between 1.16 and 1.63%. The graft copolymer was then incorporated in PBAT nanocomposites to examine the compatibilization effects on the development of a nanoparticle network. The addition of 5% PBAT-g-MA resulted in improved clay dispersion and exfoliation evidenced through TEM, XRD, permeation analysis, and tensile testing. Differential scanning calorimetry revealed that the presence of PBAT-g-MA nucleated early onset crystallization but the organized folding of the PBAT chains was hindered by the percolated clay structure resulting in a lower overall percent crystallinity. Thermogravimetric analysis revealed that the compatibilized samples demonstrated onset degradation at lower temperatures and an increased char formation at 600°C.
Missouri University of Science and Technology
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