Tuning the Johari-Goldstein β-Relaxation and Its Separation from α-Relaxation of Poly(n-alkyl methacrylate)s by Small Molecule-bridged Hydrogen Bonds

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.