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

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 (ΔG β) than that of the α-form (ΔG α). Moreover, the low crystallization temperature could not provide the extended macromolecules enough energy to overcome the higher ΔG 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.

Crystal Structure of Poly(glycolic acid) β Form

“…However, as reported in the reference, a new crystal modification (β form) of PGA was discovered in our recent work by stretching the melt-quenched amorphous sample below the glass transition point. The X-ray diffraction pattern of the β form is very different from that of the α form.…”

“…The β form is thermodynamically unstable compared to the α form. In fact, in the present study, the β-to-α phase-transition enthalpy (exothermic) observed in the DSC measurement was about −7.5 J/g for the PGA sample with the degree of crystallinity 0.45 . Since the molecular weight of one monomeric unit is 58 g/mol, the transition enthalpy of the pure crystal phase is estimated as Δ H = H α – H β = −7.5 J/g × 58 g/mol m.u./0.45 = −0.97 kJ/mol m.u.…”

“…Since the glass transition point locates around 40 °C, the thermal motion of the oriented amorphous chain segments might be constrained to some extent, making the relaxation of mechanically tensioned β form difficult to occur in such a low temperature region as 40–70 °C. As discussed, the phase transition from the β to α form of PGA may be a thermodynamical phenomenon. However, the residual strain in the amorphous region may cause superheating of the sample, and so the transition point may be shifted to the higher temperature side.…”

Toward High Strength, Ductility, and Barrier Performance for Poly(glycolic acid)/Poly(butylene adipate-co-terephthalate) Green Films through Reactive Compatibilization and Biaxial Drawing

“…PGA exhibits great competitiveness in green packaging and agricultural films due to its excellent barrier properties. 6 The oxygen permeability of PGA is approximately 100 times lower than that of poly(ethylene terephthalate) (PET). Several studies have enhanced the barrier properties in various polymers such as PET and poly(butylene adipate-co-terephthalate) (PBAT) by incorporating PGA.…”

“…PGA exhibits great competitiveness in green packaging and agricultural films due to its excellent barrier properties . The oxygen permeability of PGA is approximately 100 times lower than that of poly­(ethylene terephthalate) (PET).…”