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

Polymorphism is one of the fascinating characteristicsof polymercrystallization. In this work, we report a new crystal modificationof poly(glycolic acid) (PGA), the & beta;-form, tobe distinguished from the already known & alpha;-form PGA crystal reportedin 1968. The & beta;-form with a relatively high amount (X (& beta;) = 43%) is obtained during isothermal crystallizationin a particular temperature domain, i.e., T ( c ) around 60 & DEG;C, after prestretching the amorphoussamples. The orientation of PGA amorphous chains is expected to betterfit the chain conformation in the & beta;-form, i.e., two chains eachwith a 7/2 helical conformation in the unit cell, and thus, the extendedPGA macromolecules feature a lower energy barrier for & beta;-formformation (& UDelta;G (& beta;)) than thatof the & alpha;-form (& UDelta;G (& alpha;)).Moreover, the low crystallization temperature could not provide theextended macromolecules enough energy to overcome the higher & UDelta;G (& alpha;), thus forming predominantly the & beta;-form.The & beta;-form is stable at ambient temperatures, but it is a metastablecrystal modification at elevated temperatures compared to the & alpha;-form.Consequently, an obvious & beta;-& alpha; transition occursin the temperature range of 80-120 & DEG;C via a solid-solidmechanism, while the crystalline orientation degree remains consistentduring the transition. This study enables a better understanding onthe polymorphism and phase transition of PGA, providing a potentialroute to tune its final performance via crystalline structure control.