Structural study of the Even-Odd Effect of polysuccinates with a wide variation of chain lengths, from poly (ethylene succinate) to poly (dodecamethylene succinate)

The variation in chain length, nCH2, in linear polymers with functional groups significantly affects crystallization, leading to the so-called even-odd effect. This effect influences solid-state properties such as crystallization (Tc) and melting (Tm) temperatures, which alternate with increasing nCH2. Generally, even-numbered samples show higher thermal transitions than odd-numbered ones; however, this relationship can shift, as in the so-called odd-even effect. For polyethers and polycarbonate families, when nCH2 increases substantially (e.g., nCH2 = 10), intermolecular interactions weaken, diluting the even-odd effect and causing properties to resemble those of polyethylene. Thus, the properties show a linear increase as nCH2 increases instead of an alternating pattern.

In contrast, for nCH2 as high as 19, such saturation is not reached for polyamides; thus, it seems to depend on the intermolecular interactions of the polymers. Our research group investigates the even-odd effect in polysuccinates within the significant extensive nCH2 range of 2 to 12, novel in the literature. Our preliminary findings reveal that for smaller nCH2 values, even samples exhibit higher Tc and Tm compared to odd samples. As nCH2 increases, the even-odd effect becomes more complex due to the emergence of two melting peaks in odd samples despite a single crystallization peak likely resulting from coincident crystallization. The behaviour of Tm concerning nCH2 can alternate or show linear increases, depending on the Tm, when nCH2 exceeds a certain threshold, indicating a saturation of the even-odd effect. To explore the origins of these melting peaks and the coincident crystallization phenomenon, synchrotron radiation (SR) experiments are crucial. SR provides high-precision, real-time measurements while preserving sample integrity and replicating thermal protocols. Enhancements in signal-to-noise ratio and reduced exposure times make SR vital for understanding coincident crystallization, various melting peaks, particularly in a narrow range of temperatures, and, ultimately, the even-odd effect in this important polymeric family.