A Hidden Relaxation Process in Poly(2-vinylpyridine) Homopolymers, Copolymers, and Nanocomposites

Abstract

In broadband dielectric spectroscopy experiments, we find that introducing an air gap between the top electrode and the polymer sample reduces DC conductivity substantially, allowing the study of low-frequency relaxations, whose signal would otherwise be hidden by the DC conductivity signal. An extra process slower than the α-relaxation process is observed in poly­(2-vinylpyridine) (P2VP), consistent with some earlier reports. This “slower process” was studied in two heterogeneous systems to examine the interaction between the 2VP and other species to elucidate the mechanism behind the slower process signal. In a random copolymer of styrene and 2VP, the relaxation strength of the slower process relative to the α-process increases substantially at low 2VP mole fractions. Additionally, in a composite of P2VP and octa­(aminophenyl) silsesquioxane (OAPS), the presence of OAPS increases both the strength and timescale of the slower process, leaving the α-relaxation process relatively unchanged. This suggests that the slower process could be caused by cooperative polymer relaxation coupled to the transport of heterogeneous components and impurities. Further studies are needed to probe the molecular-level mechanism of this slower process and its effect on interfacial properties.

Intermediate Polymer Relaxation Explains the Anomalous Rheology of Nanocomposites with Ultrasmall Attractive POSS Nanoparticles

“…The time scale of this process agrees with that of the recently observed slower Arrhenius process (SAP), 38 which appears in polymer thin films under confinement, but the slower process appears to follow Vogel−Fulcher−Tammann (VFT) temperature dependence rather than Arrhenius. 37 Thus, the physical interpretation of the results presented in this work is that entanglement relaxations occur at similar time scales independently of OAPS loading; however, they do not dominate the stress response of the material until frequencies lower than τ e , at which point the slower process relaxation is complete. This slower relaxation would have an associated length scale that is larger than the polymer segment length or the OAPS molecular diameter.…”

“…This loading-dependent change in τ α corresponds well to the significant change in T g reported by DSC but is much weaker than the other trends observed. 37 In other words, the change in T g with increasing ϕ OAPS can only be responsible for 1/5 of the change in time scale observed in the rheological measurements. The terminal flow time τ t of the entangled polymers is essentially independent of ϕ OAPS (n = 0.2 ± 1.5 to 0.6 ± 0.5; data shown in Figure S4).…”

“…The time scales of terminal flow τ t for the low M n nanocomposites are shown in Figure a, and the time scale of entanglement onset τ e is shown in Figure b for the high M n samples. Both are plotted against the slower relaxation time and the α relaxation time of P2VP/OAPS with M n = 15 kDa, adapted from our former work . Data for the 101 kDa nanocomposites is adapted from Cheng et al These critical time scales (τ t and τ e ) are also plotted in Figure c, normalized to those of the neat polymer.…”

“…Both are plotted against the slower relaxation time and the α relaxation time of P2VP/OAPS with M n = 15 kDa, adapted from our former work. 37 Data for the 101 kDa nanocomposites is adapted from Cheng et al 18 These critical time scales (τ t and τ e ) are also plotted in Figure 6c, normalized to those of the neat polymer. The terminal flow time, entanglement onset time, slower process time, and α relaxation time are fitted to log 10 τ ∝ n • ϕ OAPS , where n is a temperaturedependent exponent and indicates how sensitive τ is to ϕ OAPS .…”

“…Increasing the OAPS volume fraction (ϕ OAPS ) in P2VP from ϕ OAPS = 0.00 to ϕ OAPS = 0.35 significantly increased the time scale of the slower process by a factor of 50, compared to a factor of 2 increase in the α process time. 37 Such a drastic increase in the slower process relaxation time with the loading of the OAPS implies that this process is an additional crucial component of rheological properties in P2VP/OAPS nanocomposites, manifesting as a response to both mechanical perturbations (rheology) and electrical perturbations (DRS). In this study, the rheological response of a model P2VP/ OAPS composite system is studied and rationalized based on the principles of gelation at high OAPS loadings and the emergence of the slower process as the dominant relaxation process at time scales between the α process and disentanglement.…”