Browsing by Author "Chen, C"

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    The logarithmic relaxation process and the critical temperature of liquids in nano-confined states
    (International Conference on Neutron Scattering, 2017-07-12) Chathoth, SM; Chen, C; Yu, DH
    The logarithmic relaxation process is the slowest of all relaxation processes and is exhibited by only a few molecular liquids and proteins. Bulk salol, which is a glass-forming liquid, is known to exhibit logarithmic decay of intermediate scattering function for the ?-relaxation process. Here, we report the influence of nanoscale confinements on the logarithmic relaxation process and changes in the microscopic glass-transition temperature of salol in the carbon and silica nanopores. The generalized vibrational density-of-states of the confined salol indicates that the interaction of salol with ordered nanoporous carbon is hydrophilic in nature whereas the interaction with silica surfaces is more hydrophobic. The mode-coupling theory critical temperature derived from the QENS data shows that the dynamic transition occurs at much lower temperature in the carbon pores than in silica pores. The results of this study indicate that, under nano-confinements, liquids that display logarithmic ?-relaxation phenomenon undergo a unique glass transition process.
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    Lowering the energetic landscape for negative thermal expansion in 3D-linker metal–organic frameworks
    (ACS Publications, 2023-11-30) Chen, C; Maynard-Casley, HE; Duyker, SG; Barbarao, R; Kepert, CJ; Evans, JD; Macreadie, LK
    Tuning the coefficient of thermal expansion (CTE) of functional materials is paramount for their practical implementation. The multicomponent nature of metal–organic frameworks (MOFs) offers an opportunity to finely adjust negative thermal expansion (NTE) properties by varying the metal ions and linkers used. We describe a new strategy to adjust the NTE by using organic linkers that include additional rotational degrees of freedom. Specifically, we employ cubane-1,4-dicarboxylate and bicyclo[1.1.1]pentane-1,3-dicarboxylate to form the MOFs CUB-5 and 3DL-MOF-1, respectively, where each linker has low torsional energy barriers. The core of these nonconjugated linkers is decoupled from the carboxylate functionalities, which frees the relative movement of these components. This results in enhanced NTE compared to the analogous, conjugated system; VT-PXRD results were used to calculate the CTE for 3DL-MOF-1 (αL = −13.9(2) × 10–6 K–1), and CUB-5 (αL = −14.7(3) × 10–6 K–1), which is greater than the NTE of MOF-5 (αL = −13.1(1) × 10–6 K–1). These results identify a new route to enhanced NTE behaviors in IRMOF materials influenced by low energy molecular torsion of the linker. © American Chemical Society