2.3. Characterization and Processing Techniques

NMR analysis: ¹H NMR spectra were measured using a Bruker AMX 400 (Bruker Nederland, Leiderdorp, the Netherlands). Around 10–20 mg of polymer was dissolved in ~0.7 mL CDCl₃. Spectra were referenced to the residual signal of CDCl₃ (7.26 ppm). MestReNova software (version: 14.1.1-24571, released 2019-12-02) was used to process and analyze the NMR data.

DSC analysis: Differential scanning calorimetry (DSC) analysis was performed using a DSC 3+ STARe system from Mettler Toledo (Greifensee, Switzerland). A perforated aluminum 40 µL crucible containing polymer sample (5–7 mg) was heated under nitrogen flow in two cycles from 25 °C to 300 °C with a rate of 10 K/min. The second heating cycle of each sample was used for the determination of its glass transition temperature.

TGA analysis: The TGA/DSC 3+ STARe system from Mettler Toledo was used for TGA analysis. A perforated aluminum 100 µL crucible containing a polymer sample (~20 mg) was heated under nitrogen flow at a rate of 5 K/min.

SEC analysis: Size exclusion chromatography (SEC) was performed on an Agilent HPLC system (1260 Infinity II, Agilent Santa Clara, CA, USA) with two PLgel 5 µm MIXED-C (300 × 7.5 mm) columns and a 1260 Infinity II Refractive Index detector. DCM was used as the mobile phase, with a flow of 1 mL/min at T = 35 °C. Polystyrene standards (Mn = 550 g/mol to 6,025,000 g/mol, PS-H Easy Vial from Sigma Aldrich) were used for calibration. The polymer was dissolved (5 mg/mL) in DCM and filtered (40 µm), and of this, 50 µL was injected. Agilent GPC/SEC software for OpenLAB CDS (GPC/SEC Software: Build 1.4.0.84, Data Analysis: Build 2.205.0.1344) was used to interpret and integrate spectra and calculate the M_n and mass average molar mass (M_w) of the main peak.

Polymer film: Before processing, chunks of polymer were dried overnight in a vacuum oven (70–80 °C). To create films through compression molding, pressure was applied using a preheated hot press: Carver Auto Four/30 (4533.2NE0000). A temperature of 180 °C was used for the polymer containing the lowest amount of 2,3-BDO. This temperature was increased based on the increasing 2,3-BDO content of the polymer, up to a maximum temperature of 210 °C. The polymer (~1.5 g) was melted in a circular template (Ø 10 cm), and subsequently, the pressure was increased to 10 tons. This resulted in films with a thickness in the range of 0.1 to 0.2 mm. A similar procedure was applied for PETG at 190 °C. The average thickness of the polymer film was measured on a ferrous surface using a Voltcraft SDM 115 layer thickness tester.

Water and oxygen transmission: A Totalperm (Permtech s.r.l., Pieve Fosciana, Italy) instrument was used to measure the water vapor and oxygen transmission rate. A calibration of the system for oxygen transmission was carried out with a standard PET film provided by Permtech (Italy), according to the ASTM F1927-14 standard [44], and for water vapor according to the ASTM E96/E96M-15 standard [45].

Injection molding: Tensile bars were produced by injection molding ca. 2 g of the dried polymer. For the impact bars, ~5 g of dried material was used. For this, a Thermo Scientific (Waltham, MA, USA) HAAKE Minijet II apparatus was used with a pressure of 960 bar for 6 s (subsequently 6 s at post-pressure). There was some deviation in applied pressure for a few tensile bars: more information on the specific samples can be found in the supporting information. The mold and cylinder temperatures were 40 °C and 250 °C, respectively, for the synthesized copolymers, 40 °C and 260 °C for PETG, and 60 °C and 300 °C for PET.

Tensile test: The tensile bars had a width of 4 mm, a thickness of 1.95 mm, and a parallel length of 20 mm. To determine tensile properties, an Instron 5565 machine with a load cell (10 kN) and an Instron (Instron, Norwood, MA, USA) strain gauge extensometer (2630-106, 25 mm) at a test speed of 5 mm/s were used. The extensometer was removed at an extension of ~15 mm, and the measurement was continued after. At least three specimens for each polymer were tested. More information on individual sample results can be found in the supporting information (Tables S2–S6 and Graphs S1–S5).

Impact test: The impact bars (five per polymer, 10 mm width, 4 mm thickness, 80 mm length) were notched, and then after at least 48 h of rest, the bars were tested using a Zwicker impact tester equipped with a 5 kpcm hammer. The Charpy edgewise impact with a single-notched specimen impact was calculated as described in the international standard ISO 179 [46]. Thus, the area of break is calculated by multiplying the specimen’s thickness (4 mm) with the remaining width (8 mm). The energy absorbed during the breaking of the test specimen is divided by this area to give the Charpy impact strength of notched specimens (αcN).