Investigating Chain Dynamics in Highly Crosslinked Polymers using Solid‐State ¹H NMR Spectroscopy

Abstract

Solid state 1H NMR line‐shape analysis and (double quantum) DQ 1H NMR experiments have been used to investigate the segmental and polymer chain dynamics as a function of temperature for a series of thermosetting epoxy resins produced using different diamine curing agents. In these thermosets, chemical crosslinks introduce topological constraints leading to residual stresses during curing. Materials containing a unique ferrocene‐based diamine (FcDA) curing agent were evaluated to address the role of the ferrocene fluxional process on the atomic‐level polymer dynamics. At temperatures above the glass transition temperature (Tg), the DQ 1H NMR experiments provided a measure of the relative effective crosslink and entanglement densities for these materials and revealed significant polymer chain dynamic heterogeneity in the FcDA‐cured thermosets. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 1143–1156

Heterogeneous Polymer Dynamics Explored Using Static 1H NMR Spectra

“…We were unable to reach the low temperature rigid lattice M 2 plateau prior to β-relaxation due to the experimental limitations. The distinct M 2 step has only rarely been observed for thermosets (see Figure 4 in Reference [ 19 ]) consistent with the presence of significant distributions for β-relaxation dynamics. These limitations result in the poorly constrained fitting of the initial β-relaxation process, such that we will concentrate only on the larger α-relaxation (glass transition) region at higher temperatures.…”

“…Only the high-temperature glass transition (α-relaxation) was evaluated, with the E a for the R = 0.0 network not reported because of an insufficient number of high-temperature data points. The E a values are ~2 times smaller than those we reported for epoxy thermosets [ 19 ] and reflect the lower T g values in these thiol-acrylate networks. When decreasing R, the activation energy decreases from 29.2 to 16.7 kJ (even though the T g is increasing), while the pre-exponential term (τ 0 ) increases by a factor of 10 over the same R range ( Table 1 ).…”

“…While the reference curves presented above allowed the distribution in activation energies or correlation times to be assessed, we also wanted to explore the impact distributions have on the Arrhenius analysis that had assumed a single correlation time when analyzing the M 2 temperature behavior. An “effective” correlation time, τ eff , to describe the polymer chain fluctuations near the T g was determined using Equation (A6) (additional discussion is provided in the Appendix A ) [ 6 , 7 , 19 , 20 , 49 ]. Activation energies were then estimated assuming the Arrhenius behavior of τ eff in the high temperature limit.…”

“…Static (non-spinning) NMR line shapes are broadened by homonuclear and heteronuclear dipolar couplings between different nuclei, quadrupolar couplings (for spin I > ½), chemical shielding anisotropy, or chemical shift dispersions, thus providing a handle to probe local structures and dynamics. For example, 7 Li, 23 Na, 31 P, and 133 Cs NMR M 2 results have been reported for glasses in analyses of structures and ion dynamics [ 8 , 9 , 10 , 11 , 12 ], as well as 1 H NMR M 2 studies of dynamics, miscibility, and chain orientations in polymers [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. While static NMR is hampered by low spectral resolution, it does benefit from being sensitive to local motions on the order of the line width and can be employed over very wide temperatures ranges.…”

Synthesis and application of thermal latent initiators of epoxy resins: A review

“…The curing of epoxy resins with apparent curing agents is carried out through addition polymerization or ion polymerization. [15][16][17][18][19][20][21][22][23][24][25][26] A latent initiator mixed with epoxy resin at room temperature can be stored for an extended period of time. When exposed to external stimulation, such as heating and photo-irradiation, the latent initiators initiate the curing reactions of epoxy resins.…”