*2.1. Materials*

Trifluoroacetic acid (TFA), trifluoroacetic anhydride (TFAA), microcrystalline cellulose (MCC) from cotton linters (crystallinity ~79%), and chloroform were purchased from Sigma-Aldrich (St. Gallen, Switzerland) and used without additional purification. Cellulose nanofibers (two types: one nominally shorter than the other and labeled as sNF and lNF, respectively) were purchased from Nanografi

(Ankara, Turkey) and used as received. These nanofibers were prepared from wood pulp by using mechanical methods and commercialized as dry powders.

#### *2.2. Fabrication of All-Cellulose Nanocomposites*

The preparation of *all*-cellulose nanocomposites was carried out as follows: first, MCC (450 mg) was dissolved in 30 mL of TFA:TFAA (2:1, v:v) in a 50 mL closed flask and stirred at 50 ◦C until the solution was completely clear (~1 h). Later, cellulose nanofibers (4.5, 22.5, 45, 90 and 135 mg) were mixed with 30 mL chloroform and dispersed by three consecutive 30 s ultrasound cycles using a 3.2 mm diameter tapered microtip at 10% amplitude attached to a VCX 750 ultrasonic processor (Sonics & Materials, Inc., Newtown, CT, USA). Then, both solutions were blended together and cast in glass Petri dishes. The mixture of solvents was completely evaporated after 1 day under an aspirated hood, originating freestanding films. Pure cellulose films were also prepared as a control using the same protocol in order to study the role of the nanofibers as reinforcement of the cellulose matrix. Similarly, to analyze the effect of the solvents on the nanocelluloses, the nanofibers were subjected to the above treatment but without adding MCC. All samples were stored at 44% relative humidity (RH) for 7 days before analysis to ensure the reproducibility of the measurements. Table 1 summarizes the label and the final composition of the different samples.


**Table 1.** Label and final formulation of the samples.
