Toughening of Poly(lactic acid) and Thermoplastic Cassava Starch Reactive Blends Using Graphene Nanoplatelets

Poly(lactic acid) (PLA) was reactively blended with thermoplastic cassava starch (TPCS) and functionalized with commercial graphene (GRH) nanoplatelets in a twin-screw extruder, and films were produced by cast-film extrusion. Reactive compatibilization between PLA and TPCS phases was reached by introducing maleic anhydride and a peroxide radical during the reactive blending extrusion process. Films with improved elongation at break and toughness for neat PLA and PLA-g-TPCS reactive blends were obtained by an addition of GRH nanoplatelets. Toughness of the PLA-g-TPCS-GRH was improved by ~900% and ~500% when compared to neat PLA and PLA-g-TPCS, respectively. Crack bridging was established as the primary mechanism responsible for the improvement in the mechanical properties of PLA and PLA-g-TPCS in the presence of the nanofiller due to the high aspect ratio of GRH. Scanning electron microscopy images showed a non-uniform distribution of GRH nanoplatelets in the matrix. Transmittance of the reactive blend films decreased due to the TPCS phase. Values obtained for the reactive blends showed ~20% transmittance. PLA-GRH and PLA-g-TPCS-GRH showed a reduction of the oxygen permeability coefficient with respect to PLA of around 35% and 50%, respectively. Thermal properties, molecular structure, surface roughness, XRD pattern, electrical resistivity, and color of the films were also evaluated. Biobased and compostable reactive blend films of PLA-g-TPCS compounded with GRH nanoplatelets could be suitable for food packaging and agricultural applications.


SEM images of graphene nanoplatelets used as a nanofiller
Samples were mounted on aluminum stubs using high vacuum carbon tabs (SPI Supplies, West Chester, PA, US). Samples were examined in a JEOL 7500F (field emission emitter) scanning electron microscope (JEOL Ltd., Tokyo, Japan) at various magnifications at 5 kV.

Tensile test evaluated in cross (CD) and machine (MD) direction
Tensile test method and SEM characterization are described in the Materials and Methods section of the paper.

X-ray diffraction (XRD)
XRD patterns of the cast films were recorded using a Bruker D8 Advance diffractometer (Bruker AXS, Madison, WI, US). Tests were performed under ambient temperature using an X-ray generator voltage of 40 kV and a current of 40 mA. The scan angle was from 5° to 40° with an increment of 0.010°·s -1 . The XRD instrument has a Cu tube with a wavelength of 1.4518 Å, and an anode of Cu with a Kα1 of 1.54060 Å, Kα2 of 1.54440 Å, and Kβ of 1.39224 Å. Three samples of each specimen were tested. DSC of the films for the first heating cycle. Table S1. Tg, Tcc, Tm, Xc from the first heating cycle of DSC

Color of films and opacity
The color of the films produced by twin-screw extrusion-cast-film extrusion (TSE-CF) were measured with the CIE L*a*b* system using a HunterLab LabScan XE spectrophotometer (Hunter Associates Laboratory, Reston, VA, US). The tristimulus values XYZ were measured with the same equipment for determination of the yellowness index (YI).
Three specimens of each sample were measured. The opacity (Op) of the films was evaluated by measuring the absorbance at 550 nm (A550), as described by Bao et al. [1]. The Op was calculated using equation [1], where x is the film thickness (expressed in mm).

A
(1) Table S2 shows the results of the tests.

Electrical Resistivity
Electrical resistivity was evaluated using a FAS2 ™ Femtostat (Gamry Instruments, Warminster, PA, US). Film sample area was 6.3 cm 2 ; three specimens were measured for each type of film. Table S3 shows the results. A piece of copper tape was added to each side of the sample and connected to two terminals of the Potentiostat. Values of impedance, area and thickness of each sample were used to calculate the resistivity of each type of film.