Self-healing kinetics and the stereoisomers of dicyclopentadiene

Abstract

While original epoxy resin-based self-healing systems used the commercially available endo-isomer of dicyclopentadiene (DCPD), the exo-stereoisomer is known to have much faster olefin metathesis reaction rates with first-generation Grubbs' catalyst. Here, we measure the energy to failure of healed specimens as a function of healing time and compare the kinetics of damage repair for endo- and exo-DCPD, and mixtures of the two isomers. Using catalyst loading levels previously reported to be effective for endo-DCPD, exo-DCPD was found to heal approximately 20 times faster than the endo-isomer, but with a lower healing efficiency. The fracture toughness of the repaired specimens decreased when the exo content of the blends was greater than 40% and, for the pure exo-DCPD, when the catalyst loadings were below 1%. Possible causes of the reduced healing efficiencies of the exo-DCPD healing agent are discussed.

Figure 1

Chemical structures of (a) endo- and (b) exo-DCPD.

Figure 2

Progress of healing efficiency for endo- and exo-DCPD healing agents tested with self-activated protocols. An overall catalyst loading of 0.25 wt% was delivered via embedded wax microspheres (5 wt%) loaded with 5 wt% first-generation lyophilized Grubbs' catalyst.

Figure 3

Representative ESEM images of TDCB epoxy matrix fracture planes for endo- and exo-DCPD healing agents. Images were taken on self-activated samples that were healed and subsequently fractured. Overall catalyst loading was 0.25 wt%, delivered by embedded wax microspheres (5 wt%) containing 5 wt% first-generation lyophilized Grubbs' catalyst: (a) endo-DCPD (scale bar, 200 μm), (b) exo-DCPD (scale bar, 500 μm) and (c) exo-DCPD (scale bar, 200 μm).

Figure 4

Effect of overall catalyst loading on energy required to fail samples healed with self-activated protocols. Catalyst was delivered via embedded wax microspheres (2.5, 10 or 20 wt%) loaded with 10 wt% first-generation lyophilized Grubbs' catalyst.

Figure 5

Effect of exo/endo-DCPD blends on healing efficiency for in situ healed samples containing an overall catalyst loading of 0.25 wt% delivered via embedded wax microspheres (5 wt%) containing 5 wt% first-generation lyophilized Grubbs' catalyst.

Figure 6

Load–displacement curves of virgin and in situ healed TDCB samples of (a) endo- and (b) exo-DCPD. Samples contained a catalyst loading of 0.25 wt% first-generation lyophilized Grubbs' catalyst delivered via embedded microcapsules (5 wt%) containing 5 wt% catalyst. Healing was carried out in a cold room (0–4°C) for 24 h.