Zixing Zhen,Xuan Ge,Zhencai Li,Morten M. Smedskjaer,Wenquan Lu,Fan Yang,Jianguo Li,Qiaodan Hu
Primary ( α ) structural relaxation is known to influence the nucleation behavior in glasses and it has recently been proposed to account for the so-called classical nucleation theory “breakdown” in oxide glasses around the glass transition temperature . However, it is unclear whether secondary ( β -) relaxation, predominant at lower temperatures, also plays a role in promoting crystallization. Here, we study the correlation between crystallization kinetics and β -relaxation of three types of oxide glasses with distinct chemical bonds by subjecting the glasses to low-temperature annealing. We find that the activation energy of crystallization ( E a ) exhibits an exponential decay with increasing annealing duration for all studied glasses. Such a decrease in E a is found to be associated with the structural rearrangements in the glasses, especially showing a negative correlation with the enhanced atomistic interplays around modifying cations. Our findings thus point to a universal linkage between β -relaxation and nucleation kinetics in oxide network glasses.
Polyimide (PI), as an ideal interlayer dielectric material, is widely used in traditional communication fields. However, with the rapid development of 5G communication, conventional polyimide exhibits disadvantages such as signal delay and missing and is in urgent need of modification. Based on the β-relaxation theory, we designed and prepared three types of triphenylenediamine-containing monomers with different side group sizes (A-AmM, A-AmP, and A-AmBP) and polymerized them with benzene tetracarboxylic dianhydride (PMDA) to obtain three types of polyimides (MPI, PPI, BPPI), to investigate the effect of the relaxation behavior on the dielectric constant (D k ) of the polyimide. It was also polymerized with 4,4′-(hexafluoroisopropyl)diphthalic anhydride (6FDA) to three fluorinated Polyimides (MFPI, PFPI, BFPPI) to investigate the effect of polar groups on D k and the dielectric loss (D f ). In addition, we also calculate the free volume of polyimide molecules under different side groups by materials studio simulation to further verify the effect of side groups on the D k . The results show that, firstly, with the increase of side group volume, polyimide still maintains excellent thermal stability, and its dielectric constant gradually decreases and mechanical strength gradually increases. Secondly, the dielectric constant of FPIs is significantly lower than that of the PIs due to the low polar group, and the thermal and mechanical properties are almost unchanged. In addition, benefiting from the combined effects of β-relaxation and trifluoromethyl groups, BPFPI exhibits a low D k (2.64@10 GHz) and D f (0.0092@10 GHz), excellent thermal properties (glass transition temperature (T g ) = 282.3 °C, 5 % decomposition temperature (T d5% ) = 537 °C) and excellent mechanical properties (tensile modulus = 1.71 GPa, tensile strength = 134.7 MPa).
Introducing small molecule-bridged hydrogen bonds (HBs) between polymer chains has been reported to effectively reduce the interchain cooperativity despite of strengthening the intermolecular interaction. Here, a systematic investigation on tuning the Johari-Goldstein β ( β JG ) relaxation by adding various low-molecular-weight phenols in poly( n -alkyl methacrylate)s is carried out to further clarify the anomalous dynamics. Given these small molecules capable of coupling the motion with pendent groups of host polymers due to forming at least two HBs per molecule, poly( n -alkyl methacrylate) mixtures exhibit rich dynamic changes in the β JG -properties and α, β JG separations. An increased loading of phenols with a small size and strong inter-HB strength (Δ υ i ) clearly benefits for significant retardation and suppression of the β JG -relaxation, narrows the α, β JG separation and converges the β JG -peak with the α -peak, which demonstrates the alleviation of inter-chain topological constraints. However, small molecules with a relatively big size and weak Δ υ i are found to amplify the magnitude of the α, β JG separation of poly(butyl methacrylate), even though experimental results of changes in α -dispersion and dynamic fragility confirm a reduction of the coupling factor n in all of these hybrids. The counterintuitive phenomenon suggests that the crossover time t c in the Coupling Model is no longer a universal quantity if the inter-chain interaction of polymers is strengthened by HBs. These compelling findings shed vital insights into the HB-induced anomalous dynamics, and provide essential guidance for tailoring the β JG behavior and designing glassy polymeric materials.
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