Influence of Chain Stiffness on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

We extend previous systematic investigations of a family of model coarse-grained polymer network-forming materials by focusing on the influence of varying molecular rigidity on basic conformational properties (i.e., radius of gyration Rg and persistence length lp), segmental dynamics (structural relaxation time τα), and mechanical properties (shear G and bulk B moduli) of this broad class of materials. We find that increasing molecular rigidity increases the radius of gyration Rg and persistence length lp of the network chains, as observed before in polymer solutions. The increase in molecular rigidity leads to a significant slowing down in the segmental dynamics and a strong increase in the characteristic temperatures (i.e., onset temperature TA, glass transition temperature Tg, and Vogel temperature T0) and fragility of glass formation. We also find that the structural relaxation time τα, along with G and B, does not exhibit the near-universal scaling with reduced temperature T/Tg as found previously for fully flexible cross-linked networks having variable cross-link density and cohesive energy, but a fixed chain bending stiffness. Both of these moduli become progressively smaller in magnitude as the network chains become stiffer over the T range investigated. Moreover, τα and the moduli (G, B) all exhibit strong correlative relationships with the Debye–Waller parameter ⟨u2⟩, which is correspondingly utilized to define a local measure of material "stiffness". Color maps based on this stiffness measure indicate that both the average value and variance of the local stiffness fluctuations decrease with an increasing chain stiffness at the same reduced temperature, T/Tg. Our simulation observations provide novel physical insights into how varying chain stiffness influences the glass formation of cross-linked networks, which should be helpful in the design of cross-linked thermoset materials.