A β-Form Crystal Modification of Poly(glycolic acid): Formation, Stabilization, and β–α Transition

Polymorphism is one of the fascinating characteristics of polymer crystallization. In this work, we report a new crystal modification of poly(glycolic acid) (PGA), the β-form, to be distinguished from the already known α-form PGA crystal reported in 1968. The β-form with a relatively high amount (Xβ = 43%) is obtained during isothermal crystallization in a particular temperature domain, i.e., Tc around 60 °C, after prestretching the amorphous samples. The orientation of PGA amorphous chains is expected to better fit the chain conformation in the β-form, i.e., two chains each with a 7/2 helical conformation in the unit cell, and thus, the extended PGA macromolecules feature a lower energy barrier for β-form formation (ΔGβ) than that of the α-form (ΔGα). Moreover, the low crystallization temperature could not provide the extended macromolecules enough energy to overcome the higher ΔGα, thus forming predominantly the β-form. The β-form is stable at ambient temperatures, but it is a metastable crystal modification at elevated temperatures compared to the α-form. Consequently, an obvious β–α transition occurs in the temperature range of 80–120 °C via a solid–solid mechanism, while the crystalline orientation degree remains consistent during the transition. This study enables a better understanding on the polymorphism and phase transition of PGA, providing a potential route to tune its final performance via crystalline structure control.