This effect can be easily explained since monomers with longer aliphatic chains were introduced to copolyester structure which on the other hand improves material flexibility, as reflected by a decrease of Tg values (from -15 to -21 °C) but on the other, hinders the ability to form crystallites, as reflected by the decrease of Xc and ∆Hm. It is also evident from Figure that the intensity of the crystallization (Tc) and melting (Tm) transition peaks are decreasing and getting broader while changing the number of carbon atoms within α,ω-ALD from 6, 8 to 10, or 12. Moreover, for PDF-DLF and PDDF-DLF copolymers once the glass transition temperature is exceeded during the second heating step, the mobility of macromolecules is gained and further rearrangements take place, as shown by the cold crystallization peak (Tcc). This indicates that the crystallization rate of those materials is slower at the tested time scale and that the crystalline phase in this case is less homogenous and crystallites with variable size distribution are formed. This is also in accordance with the conclusions drawn from XRD data that increasing number of carbon atoms within α,ω-ALD hinders crystallization due to the decreasing chain regularity. Of course, herein again, we also cannot neglect the fact, the differences in thermal transitions of PDF-DLF and PDDF-DLF copolyesters can be also influenced by higher values of averaged molecular weights where Mn and Mw are varying from 5 700 to 20 600 g/mol and from 10 700 to 45 700 g/mol, respectively. As was concluded before, it can also affect the crystallization behaviour of those copolyesters by reducing the polymer chain mobility. However, similar studies To assess the potential application of furan-based copolyesters as environmentally friendly packaging materials for medical sector, we conducted oxygen transmission rate (OTR) measurements on 100µm-thick polymer films that were hot-pressed. Oxygen permeability assessment of packaging materials serves multiple purposes, including evaluating their barrier properties, assessing product shelf life and stability, and complying with regulatory requirements. In this study, our main focus was on measuring the oxygen transmission rate (OTR) to evaluate the suitability of furan-based copolyesters for the sterile barrier system, particularly within the context of medical packaging. While the excellent barrier properties of PEF or PBF materials are known for food packaging sector, we used long aliphatic chains of diols to strike a balance between these properties and the necessary transmission of sterilizing agents, which are essential for medical devices packaging. The emphasis on oxygen transmission rate in medical packaging primarily relates to the efficient ventilation of ethylene oxide after sterilization, ensuring the safety and integrity of packaged medical products. Therefore, understanding and optimizing the oxygen transmission properties of packaging materials are crucial in medical applications.