*2.3. Characterization*

## 2.3.1. Film Thickness

Film thickness was measured with a digital micrometer series S00014, having ±0.001 mm accuracy, from Mitutoyo Corporation (Kawasaki, Japan) at three random positions. The post-processed samples had a thickness of typically 55 ± 4 μm.

#### 2.3.2. Scanning Electron Microscopy

A S-4800 microscope from Hitachi (Tokyo, Japan) was used to observe by scanning electron microscopy (SEM) the morphology of the electrospun PHB fibers and the film cross-sections and surfaces. Cross-sections of the samples were prepared by cryo-fracture of the electrospun PHB films using liquid nitrogen. Then, they were fixed to beveled holders using conductive double-sided adhesive tape, sputtered with a mixture of gold-palladium under vacuum, and observed using an accelerating voltage of 5 kV. Image J Launcher v 1.41 software was used to determine the average fiber diameter and standard deviation by measuring the diameter of at least 100 fibers.

#### 2.3.3. Transmission Electronic Microscopy

The morphology and distribution of the PdNPs was studied using a JEOL 1010 from JEOL USA, Inc. (Peabody, MA, USA) by TEM using an accelerating voltage of 80 kV.

#### 2.3.4. Differential Scanning Calorimetry

Thermal properties of the neat electrospun PHB fibers and films and of the multilayer systems were evaluated by differential scanning calorimetry (DSC) using a Perkin-Elmer DSC 8000 (Waltham, MA, USA) thermal analysis system under nitrogen atmosphere. The measurement was carried out on ~3 mg of each sample using a two-step program from 0 to 200 ◦C followed by a subsequent cooling down to −50 ◦C, both at a heating rate of 10 ◦C/min. The DSC equipment was previously calibrated with indium as a standard and the slope of the thermograms was corrected by subtracting similar scans of an empty pan. Tests were done, at least, in triplicate.
