*3.2. TGA Results of Pectin and the Pectin-Chitosan Composites*

The TGA results reveal that pectin decomposes at 236 ◦C, which is in agreement with the results reported by Cao [23], where the main thermal event was attributed to scission of the saccharide rings. Pal and Kaityar reported the synthesis of a lactic acid oligomer-grafted chitosan that undergoes decomposition at a lower onset temperature (ca. 200 ◦C) relative to that of pristine chitosan (ca. 200 ◦C) [24]. By comparison, the pectin–chitosan composite (PC11 S DMSO) prepared under sonication-assisted synthesis in DMSO reveals a higher decomposition temperature in Figure 2, as compared with the composites prepared in water (PC11 W and PC51 W). The greater decomposition temperature of the composites formed under sonication in DMSO (PC51 S DMSO, PC11 S DMSO, and PC15 S DMSO) provides support of the different bonding that results from water- versus DMSO-based composite syntheses. Materials prepared in water are anticipated to favor the formation of PECs due to the higher dielectric constant of aqueous media. Composites synthesized in DMSO are more likely to form CBFs due to amide bond formation between pectin and chitosan biopolymers. The FTIR results in Figure 3a for DMSO-based preparations provide confirmation of secondary amine features (two bands ca. 2900 cm−1) that provide support for amide bond formation [25]. By contrast, pectin and chitosan composites prepared in water reveal a prominent thermal event at 220–230 ◦C that indicates the formation of PECs by electrostatic interactions. The TGA results for the PEC and CBF materials are in agreement with independent results from the preparation of related composites [12,26,27].

**Figure 2.** Thermal gravimetric analysis (TGA) results of pectin and pectin–chitosan composites.

**Figure 3.** (**a**) Infrared (IR) spectra of pectin, chitosan, and their composites prepared in DMSO under assisted sonication, and (**b**) IR spectra of pectin, chitosan, and composites prepared in water and DMSO solvents.
