Wang Xuan,Xiao Kesong,Liu Xiangnong,Wu Hao,Gao Cai
The phenomenological models based on fictive temperature and nonlinear Adam–Gibbs formulation have been widely used to describe the enthalpy relaxation kinetics. Although the parameters in Adam–Gibbs formulation are physically explicable, the reasonabilities of the parameters obtained from curve-fitting procedures have not been systematically discussed yet. In this work, differential scanning calorimetry was employed to measure the heat capacities of maltitol in both the amorphous and the crystalline states, and the data were used to generate the model parameters. Results showed that the configurational ground state plays an important role in extracting the model parameters. Selecting the crystal rather than the glass as the ground state seems to be effective to improve the reasonabilities of the fitted parameters. It suggested that even if the fit quality seems sound, the physical reasonability of the best-fit parameters still needs to be treated with caution.
High-Tg polymers generally exhibit high dynamic fragility (m). Here, we propose to design striking polymeric materials with low m but high Tg by introducing ionic coordination. The enthalpy relaxation of poly(vinylpyrrolidone)/metal chloride complexes was investigated to elucidate the influences of the coordination strength on Tg and m. Findings reveal that Tg increases with the addition of metallic ions, while m decreases for weakly coordinated complexes (Li+) and moderate systems (Zn2+ and Mg2+). Meanwhile, m increases continuously for strongly coordinated systems (La3+). Further experiments revealed that m is well correlated to heat capacity jump ΔCp and enthalpy hysteresis ΔHR as predicted by Adam–Gibbs theory. Ionic coordinated polymers with low m but high Tg can be achieved only when ΔHR changes more than Tg·ΔCp, suggesting that the enthalpy change plays a leading role in the activation free energy of segmental cooperative rearrangement. These results not only add new evidence of the thermodynamic link to dynamic fragility but also help clarify the mechanism of the independent tuning of Tg and m.
Xuan Ge,Pingsheng Lai,Caijuan Shi,Xiaowei Xu,Jian Wang,Tao Du,Morten M. Smedskjær,Dongliang Yang,Fan Yang,Wenquan Lu,Jingyu Qin,Jianguo Li,Qiaodan Hu
Annealed titanate glasses are important candidate materials for high-performance electronic components, but their structural response to temperature-induced properties upon thermal annealing remains elusive. Here, using high-energy synchrotron X-ray scattering, high-temperature Raman spectroscopy , and empirical potential structure refinement simulation, we track the structural evolution in situ of a barium dititanate (BaTi 2 O 5 ) glass upon annealing around and below the glass transition temperature . We find that the network structure is intrinsically floppy, as the network units ([TiO m ] polyhedra) themselves and their topological packing can be easily modulated by annealing temperature. The floppy nature of the glassy titanate network challenges the currently well-cognized Rigid-Unit Mode model explaining temperature-driven structural reorganization of prototypical network glasses (e.g., silicate/borate glasses). As temperature increases, the floppy network of BaTi 2 O 5 glass undergoes a non-monotonic, two-stage network rearrangement. After cooling back to room temperature, the distortion of network units is found recoverable whereas the change of network connectivity in the intermedium-range order is irreversible. Simulation ensembles further show the irreversible connectivity change can be ascribed to the local development of crystal-like cationic motifs, which in turn induce excess enthalpy relaxation . Our findings provide an atomic-scale perspective on temperature-dependent structural and enthalpy relaxation of a model titanate glass upon annealing, which are crucial for understanding thermal-related changes in the properties of titanate glasses.
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