Doctoral student Zhang Yuwenya and colleagues, under the joint supervision of Researcher Li Liangbin, Researcher Zhang Hongjun, and Postdoctoral Fellow Chen Jungen, focused on the microscopic structural evolution mechanisms of semicrystalline polymer materials. Using a self-developed positron annihilation lifetime spectroscopy technique, the team achieved, for the first time, in situ real-time measurement of the free volume in the amorphous regions of oriented polyethylene terephthalate (PET) films during tensile deformation. They further proposed the innovative concept of “free volume modulus,” providing a new approach for establishing a quantitative relationship between microscopic structure and macroscopic mechanical properties. The related research findings have been published in Macromolecules.
In this study, the research team precisely regulated the initial structure of the samples through annealing treatments at different temperatures, and systematically revealed the evolution of free volume at different stages of deformation. The experiments showed that, in the linear region of tensile deformation, the response mechanism of free volume is closely related to the initial structure: samples annealed at lower temperatures, with less free volume, mainly exhibited enlargement of existing holes, whereas samples annealed at higher temperatures, with more free volume, mainly exhibited the generation of new holes. Despite these different pathways, the team identified a common pattern: regardless of the initial structure, the relative increase in the relative free volume fraction of the amorphous regions showed a highly consistent linear relationship with stress, with slopes stabilizing at approximately 1.3/GPa in the elastic region and 4.0/GPa in the plastic region.
Based on this significant finding, the team proposed the new concept of free volume modulus (Kf) to quantitatively describe the relationship between changes in stress-field intensity and changes in free-volume hole volume. In PET, the work measured Kf values of approximately 0.77 GPa in the elastic region and 0.25 GPa in the plastic region. The study also found that the modulus of the amorphous regions (Eam) is negatively correlated with hole number density, while showing little correlation with hole size, indicating that the key factor determining stiffness is the number of highly mobile chain segments surrounding the holes.
This work establishes, for the first time in PET, a quantitative linear relationship between stress and free volume fraction, and proposes the new parameter of free volume modulus. It deepens the understanding of the deformation mechanisms of semicrystalline polymers and provides crucial experimental evidence and theoretical insight for the future development of quantitative constitutive models that take microscopic structural evolution into account.
This work was supported by the Chinese Academy of Sciences (JZHKYPT-2021-04) and the National Natural Science Foundation of China (52303048, 12275270).
Zhang Y, Liu L, Kang W,et al.Quantitative Correlation between Mechanical Behaviors and Free Volume of Oriented Polyethylene Terephthalate: an In-Situ PALS Study. Macromolecules 2026,59,6, 3789–3799.
Paper Link: https://pubs.acs.org/doi/10.1021/acs.macromol.5c03108.
