*2.4. Cast Film Preparation*

The starch-glycerol and nanocomposite films were prepared by solution casting according to the method of Alves et al. [33]. Precursor solutions of the nanocomposite films with the total mass *m*total had a water content *c*aq of 97 wt %. Five different nanoparticle concentrations 0 ≤ *c*NP ≤ 9 wt % were added as well as glycerol as plasticizer with a concentration *c*gly of 30 wt %, both relative to the weighed portion of starch *m*starch. The weighed portions of chemicals were calculated according to Equation (1), considering the mass of water added *m*H2O, corrected for the water content of the other components.

$$m\_{\text{total}} = \left(1 + c\_{\text{gly}} + c\_{\text{NP}}\right) m\_{\text{starch}} + c\_{\text{aq}} m\_{\text{H}\_2\text{O}} \tag{1}$$

Starch was added to preheated water with a temperature of 70 ◦C in a water bath and stirred for 1 h to allow plastification. Glycerol and nanoparticles were then added and stirring was continued for 20 min to allow plasticization and mixing of the composite. A dry film thickness of 50 μm was targeted. Before casting in polystyrene Petri dishes (Greiner Bio-One, distributed by VWR, Ismaning, Germany), the filmogenic solutions were homogenized in an ultrasonic bath for 3 min. Excess water was evaporated from the dishes overnight in a climatic chamber (ICH 110, Memmert, Schwabach, Germany) at 40 ◦C and a relative humidity (r.h.) of 47%. The films were then peeled off from the Petri dish and turned upside down for double-sided drying overnight.

#### *2.5. Instrument Measurements*

If applicable, all measurements were performed at least in triplicate and are presented with the 95% confidence interval of the mean. The uncertainty of quantities depending on multiple variables is given by the propagation of error. For the determination of the oxygen permeability, the mean value of two measurements is given with the minimum and the maximum value.

#### 2.5.1. Particle Size and Viscosity

The hydrodynamic apparent particle size of the nanoparticle dispersions was measured by dynamic light scattering (DLS) using a Zetasizer Nano ZSP (Malvern Instruments, Worcestershire, UK). Aliquots were filtered with syringe filters with a hydrophilic PES membrane and a pore size of 1 μm (Chromafil PES, Macherey-Nagel, Düren, Germany). The harmonic intensity averaged particle diameter (*z*-average) and the polydispersity index (PdI) from the cumulants analysis were obtained for 0.025 wt % nanoparticle dispersions after equilibration for 30 min at 25 ◦C.

The volume-weighed particle size of residual microparticles was measured using a HELOS/KR laser diffraction particle size analyzer with a QUIXEL wet dispersion system (Sympatec, Clausthal-Zellerfeld, Germany) at 23 ◦C and an optical concentration of 10%. The balanced mean size *x*1*,*<sup>3</sup> and the *span* were evaluated according to Equations (2) and (3) as

$$\mathbf{x}\_{1,3} = \int\_{x\_{\text{min}}}^{x\_{\text{max}}} \mathbf{x} q\_3(\mathbf{x}) d\mathbf{x} \text{ and } \tag{2}$$

$$spam = \frac{\mathbf{x}\_{\\$0} - \mathbf{x}\_{\\$0}}{\mathbf{x}\_{\\$0}}.\tag{3}$$

Here, *x*min and *x*max are the smallest and the largest particle size, *x* is the class midpoint and *q*3(*x*) is the volume-weighted particle size distribution. *x*10, *x*<sup>50</sup> and *x*<sup>90</sup> are the particle sizes corresponding to 10%, 50% and 90% of the cumulative undersize distribution, respectively.

The viscosity of the concentrated CNC dispersion was determined using a Physica MCR 501 rheometer (Anton Paar, Graz, Austria) at 25 ◦C with a cone-plate geometry. Shear flow curves were measured in the range from 0.01 to 1000 s<sup>−</sup>1.
