Interplay of Segmental and Normal Mode Dynamics in Polymer Networks Undergoing Chemical Cross-Linking. Epoxy/Amine-Terminated Linear and Star PPO Formulations

Abstract

An investigation was conducted of segmental and normal mode dynamics during cross-linking of reactive systems where one of the components exhibits, in addition to the transverse dipole moment (μ⊥) component that gives rise to the segmental α process, a persistent cumulative dipole moment (μ∥) along the chain contour that can be relaxed via the normal mode process. The systems studied were composed of an amine-terminated linear or three-arm star poly(propylene oxide), which contains both μ⊥ and μ∥, and a bifunctional epoxy prepolymer. The kinetics of network formation were evaluated by Fourier transform near-infrared spectroscopy (NIR), and the dynamics were investigated by broad-band dielectric relaxation spectroscopy (DRS) and dynamic mechanical spectroscopy (DMS). The dynamics of networks containing linear and star chains were similar but not identical. The average relaxation time for segmental (τS) and normal mode (τN) increases in the course of network formation, but the distance between τS and τN varies little and the T g-scaled fragility remains unchanged. The spectra become thermodielectrically complex following the onset of reactions and broaden in the course of cure. Segmental and normal mode relaxations overlap increasingly during cure but, interestingly, retain their identities. There is a decrease in the dielectric relaxation strength for the segmental process (ΔεS) and a simultaneous (unexpected) increase in the dielectric relaxation strength for the normal mode process (ΔεN). Before gelation, the DMS response was characterized by segmental and terminal relaxation zone. The gel point was observed at a conversion above that predicted by the gelation theory, and an explanation was put forward.

Real time dielectric monitoring of glass transition in n-vinyl pyrrolidone polymerization

“…The evolution of e 00 (x) in VP polymerization differs from the results reported by Mijovic et al [13]. The differences are no doubt due to differences in step and radical polymerization kinetics.…”

“…Studies by Shephard and Senturia [1], Kranbuehl et al [2,3], Johari [4][5][6], Rolla [7] Williams [8][9][10], Mijovic [11][12][13] and Friedrich [14] were mostly devoted to epoxy systems and their modifications. These works established the dipolar aspects of molecular dynamics, a-and b-relaxation kinetics, and the contribution of different relaxation processes to the dielectric function.…”

Effect of Molecular Architecture on Dynamics of Multigraft Copolymers: Combs, Centipedes, and Barbwires

“…A similar finding has been recently reported in polymer networks where one reactive component was a type A chain. 58 The effect of molecular architecture on the dynamics was quantified in terms of ∆τ N , the difference between the normal mode relaxation time for each MG copolymer, τ N(PIPS) , and its PI precursor, τ N(PI) . Thus, ∆τ N ) τ N(PIPS) -τ N(PI) .…”

Dielectric relaxation in polymer–clay nanocomposites

“…Having in mind that the shift of T g in the PCNCs has been related with retardation (increase of T g ) or acceleration (decrease of T g ) of the segmental molecular mobility of the polymers in the presence of the clay platelets, it is of great importance to investigate comparatively the molecular dynamics of the intercalated Jeffamines. At this point it is worthy mentioning that poly(propylene oxide) amines chains possess a dipole moment along the chain contour, apart the dipole moment perpendicular to the chain axis, and, consequently, they exhibit the dielectric normal mode (nm relaxation) due to the fluctuation of the end-to-end vector in addition to the segmental mode (a relaxation) due to local, segmental fluctuations (Mijovic et al, 2003). thus, dielectric techniques can provide information on the long length scale chain dynamics monitoring not only the segmental mobility (related to the glass transition) but also the global chain translational and rotational motions (normal mode process).…”