Stress Relaxation in Epoxy Thermosets via a Ferrocene-Based Amine Curing Agent

Abstract

Physical stress relaxation in rubbery, thermoset polymers is limited by cross-links, which impede segmental motion and restrict relaxation to network defects, such as chain ends. In parallel, the cure shrinkage associated with thermoset polymerizations leads to the development of internal residual stress that cannot be effectively relaxed. Recent strategies have reduced or eliminated such cure stress in thermoset polymers largely by exploiting chemical relaxation processes, wherein temporary cross-links or otherwise transient bonds are incorporated into the polymer network. Here, we explore an alternative approach, wherein physical relaxation is enhanced by the incorporation of organometallic sandwich moieties into the backbone of the polymer network. A standard epoxy resin is cured with a diamine derivative of ferrocene and compared to conventional diamine curing agents. The ferrocene-based thermoset is clearly distinguished from the conventional materials by reduced cure stress with increasing cure temperature as well as unique stress relaxation behavior above its glass transition in the fully cured state. The relaxation experiments exhibit features characteristic of a physical relaxation process. Furthermore, the cure stress is observed to vanish precipitously upon deliberate introduction of network defects through an increasing imbalance of epoxy and amine functional groups. We postulate that these beneficial properties arise from fluxional motion of the cyclopentadienyl ligands on the polymer backbone.

Phosphinated Poly(aryl ether)s with Acetic/Phenyl Methacrylic/Vinylbenzyl Ether Moieties for High-Tg and Low-Dielectric Thermosets

“…Therefore, a material with a low dielectric value will exhibit a high signal propagating speed and a low signal propagating loss. Generally, epoxy resins are cured by reacting the epoxy groups with active hydrogen-containing compounds such as phenol novolac, diamine, or thiol. − The reaction of active hydrogens with epoxy leads to secondary alcohol linkages, which are highly polar and increase the dielectric constant of epoxy thermosets. Therefore, compared with other low-dielectric thermosets, epoxy thermosets show a moderate-to-high dielectric constant.…”

Manufacturing and Characterization of Hybrid Composites with Basalt and Flax Fabrics and a Partially Bio-based Epoxy Resin

“…Although the crosslinked structure is not lost, some segments can move, vibrate, and so on, leading to this rubbery behaviour. The internal structural stresses due to the fully crosslinked epoxy resin, is relaxed at moderate-to-high temperatures, thus indicating a transition from a rigid state to a rubbery state, which is representative for the glass transition temperature (Tg) this movement is made between the crosslinking points in the glass-to-rubber transition, and this is related to the tan δ peak [64]. As one can see, a clear decrease in G' takes place during the -relaxation or Tg.…”

Molecular Ball Joints: Mechanochemical Perturbation of Bullvalene Hardy-Cope Rearrangements in Polymer Networks

“…Physical systems take advantage of structural adaptations to external forces (i.e., bond rotations, non-covalent interactions); specific examples include soft materials containing "expandable", 28,29,30 mechanically-interlocked, [31][32][33][34][35] hydrogen-bonded, [36][37][38][39] and/or metal-ligated elements. [40][41][42][43][44][45][46] Adaptations in these systems are generally reversible with low activation barriers. On the other hand, systems bearing chemical mechanophores use high energy barrier chemical transformations to adapt to applied mechanical force (e.g., cyclo-reversion) [20][21][22][23][24]27 ; such processes are irreversible, however.…”