Competing Effects of Cohesive Energy and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

Abstract

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 (ε) 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 T g and fragility of glass formation, while the fragility decreases with an increase in ε. These competing effects of ε 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 ε. More interestingly, the macroscopic mechanical properties are found to be strongly correlated with the Debye–Waller parameter ⟨u 2⟩, a measure of material “stiffness” at a molecular level. In particular, the distribution of local molecular stiffness, 1/⟨u 2⟩, exhibits a nearly universal Gaussian distribution at a fixed reduced temperature T/T g. 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.

Heterogeneous Local Dynamics in Mussel-Inspired Elastomers

“…Furthermore, reduced fragility with increasing network topology (Figure b) nicely correlates with the reduced liquid heat capacity, Δ c p at T g (Figure ). We should mention here that the origin of the inverse proportionality of T g with fragility (increasing the cross-linking density causes an increase of T g and a simultaneous reduction in fragility) can be discussed in view of recent molecular dynamics simulations of segmental dynamics in cross-linked polymers. , The simulations revealed that increasing cross-linking density (by the presence of charged functional groups of highly polar cross-linking agents) can increase T g and at the same time reduce the fragility as observed experimentally. The origin of the effect could be traced to competing effects involving the cohesive interaction energy.…”

“…The origin of the effect could be traced to competing effects involving the cohesive interaction energy. Increasing the cross-link density can increase the cohesive interaction strength when the cross-linking agent is highly polar, which can lead to a decreasing fragility that counterbalances the normal fragility increase from the geometrical constraints of cross-linking. , …”

“…Increasing the cross-link density can increase the cohesive interaction strength when the crosslinking agent is highly polar, which can lead to a decreasing fragility that counterbalances the normal fragility increase from the geometrical constraints of cross-linking. 39,40 3.2. Effects of Pressure.…”

Influence of Functionalization on the Crystallinity and Basic Thermodynamic Properties of Polyethylene

“…We confirm in Figure 12 that B is highly correlated with ⟨u 2 ⟩, in qualitative agreement with both linear 103 and highly branched thermoset polymers. 66,104 Previous simulation observations on linear and thermoset polymers indicated roughly universal curves for B versus ⟨u 2 ⟩ for fixed values of cross-linking density 66 or fixed values of the cohesive interaction parameter 104 governing the strength of the polymer−polymer attractive interaction. However, when the data are examined in greater detail, it was found that the variation of these molecular parameters gives rise to a family of curves having nearly the same shape, but which are clearly distinct.…”

Molecular Dynamics Investigation of the Pressure Dependence of Glass Formation in a Charged Polymer Melt

“…As with all polymer materials, properties related to glass formation play a central role in the design and characterization of materials suitable for their intended applications. − While there have been a significant number of studies of glass formation in uncharged polymer materials, relatively little is known about glass formation in charged polymer materials and the oligomeric analog of these materials, ionic liquids. Correspondingly, fundamental studies of glass formation in charged, and “analog” uncharged polymer materials as a reference point, would clearly be of interest in the development of this class of materials.…”