A β<i>-</i>Form Crystal Modification of Poly(glycolic acid): Formation, Stabilization, and β–α Transition
Deyu Niu, Hai Wang, Ye Ma, Pengwu Xu, Jiaxuan Li, Weijun Yang, Tianxi Liu, Kohji Tashiro, P. J. Lemstra, Piming Ma
Abstract
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 ( X β = 43%) is obtained during isothermal crystallization in a particular temperature domain, i.e., T c 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 (Δ G β ) than that of the α-form (Δ G α ). Moreover, the low crystallization temperature could not provide the extended macromolecules enough energy to overcome the higher Δ G α, 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.