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

Chain dynamics and crystalline network structure of poly[R-3-hydroxybutyrate-co-4-hydroxybutyrate] as revealed by solid-state NMR

The chain dynamics and crystalline network structure of poly[R-3-hydroxybutyrate-co-4-hydroxybutyrate] (P(3HB-co-4HB)) were systematically investigated by the combination of various solid-state NMR techniques. High-resolution 13C cross-polarization (CP) and direct-polarization (DP) MAS with selective recycle delay times were first used to check the presence or absence of the 4HB unit in the crystalline domain. The results show that the 4HB unit is excluded from the crystalline domain. Afterward, 1H MAS Nuclear Overhauser Effect Spectroscopy (NOESY) with different mixing times was used, which shows that no micro-phase separation exists in the amorphous domain. 1H magic-sandwich-echo (MSE)-FID at elevated temperature shows the absence of motions on a timescale of 100 μs and below in the crystalline domain, as evidenced by the invariant second moment M2 of the proton line shape. Finally, the crystalline based network density was characterized directly by magic and polarization echo (MAPE)-double quantum (DQ) NMR, which shows a significant decreasing tendency after 80 °C. Such a decreasing crystalline network density, together with the reduced relaxation time, results in the significant decrement of the maximum stretch ratio and modulus in the high-temperature region.

Heterogeneous Polymer Dynamics Explored Using Static 1H NMR Spectra

NMR spectroscopy continues to provide important molecular level details of dynamics in different polymer materials, ranging from rubbers to highly crosslinked composites. It has been argued that thermoset polymers containing dynamic and chemical heterogeneities can be fully cured at temperatures well below the final glass transition temperature (T_g). In this paper, we described the use of static solid-state ¹H NMR spectroscopy to measure the activation of different chain dynamics as a function of temperature. Near T_g, increasing polymer segmental chain fluctuations lead to dynamic averaging of the local homonuclear proton-proton (¹H-¹H) dipolar couplings, as reflected in the reduction of the NMR line shape second moment (M₂) when motions are faster than the magnitude of the dipolar coupling. In general, for polymer systems, distributions in the dynamic correlation times are commonly expected. To help identify the limitations and pitfalls of M₂ analyses, the impact of activation energy or, equivalently, correlation time distributions, on the analysis of ¹H NMR M₂ temperature variations is explored. It is shown by using normalized reference curves that the distributions in dynamic activation energies can be measured from the M₂ temperature behavior. An example of the M₂ analysis for a series of thermosetting polymers with systematically varied dynamic heterogeneity is presented and discussed.