Stress-Induced Crystallization of the Metastable β-Form of Poly((R)-3-hydroxybutyrate-co-4-hydroxybutyrate)

The stress-induced crystallization of the metastable β-form of poly((R)-3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) during the stress loading and unloading process was systematically investigated by in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) measurement. During deformation, the major content of the β-form is found to be transformed from the fully extended tie chain, while a minor part from the deformed α-form. During retraction, the β-form finally transforms into a highly oriented α-form. The stress-induced crystallization rate of the β-form is not affected by temperature, implying the constant nucleation barrier of the β-form at different temperatures. The critical stress of β-form formation and melting is determined to be about 7.8 and 1.5 MPa, respectively. The necessary conditions for the formation of the β-form are summarized as follows: (1) the tie chains between lamellae need to be fully extended, and (2) the applied stress needs to exceed the critical stress of formation of the β-form. The unveiled β-form formation mechanism can directly contribute to our understanding of the origin of enhanced macroscopic performances of biodegradable polyhydroxyalkanoates (PHAs) through incorporating comonomer.