Competing Effects of Cohesive Energy and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers
XR Zheng and YF Guo and JF Douglas and WJ Xia, MACROMOLECULES, 55, 9990-10004 (2022).
To develop structure-property relationships for cross-linked thermosetting polymers, it is crucial to better understand key factors that control their segmental dynamics and macroscopic properties. Here, we employ a coarse-grained (CG) polymer model to systematically explore the combined effect of varying the cohesive energy (epsilon) and cross- link density (c) on the segmental relaxation time and mechanical properties for a model cross-linked glass-forming thermoset material. We find that increasing c increases both the glass transition temperature Tg and fragility of glass formation, while the fragility decreases with an increase in epsilon. These competing effects of epsilon and c on fragility are practically important since fragility determines the overall temperature width of the glass formation over which the non- Arrhenius temperature dependence is observed. Our simulation results show that the basic mechanical properties (i.e., bulk and shear moduli) of cross-linked thermosets are mainly influenced by epsilon. More interestingly, the macroscopic mechanical properties are found to be strongly correlated with the Debye-Waller parameter (u2), a measure of material "stiffness" at a molecular level. In particular, the distribution of local molecular stiffness, 1/(u2), exhibits a nearly universal Gaussian distribution at a fixed reduced temperature T/Tg. Our work reveals the key and competitive roles of cohesive energy and cross- link density in controlling the segmental dynamics, large scale, and local mechanical properties of cross-linked thermosets, providing an understanding that should be useful in the molecular design of these materials.
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