No abstract

The observation of the morphology of the fibrous molecular assemblages and the Ni±P microtubes was carried out using a Hitachi S-3000N scanning electron microscope and a Hitachi S-800 field-emission scanning electron microscope. Sample characterizations were performed with a Phillips PW-3050 energy-dispersion X-ray analyzer, a Shimadzu ICP-8100 inductively coupled plasma atomic emission spectrometer, a Shimadzu ESCA 3400 X-ray photoelectron spectrometer, a Bio-Rad FTS6000 Fourier-transform IR spectrometer, and a Rigaku RINT2000 powder X-ray diffractometer. Wax-Protected Catalyst Microspheres for Efficient Self-Healing Materials** By Joseph D. Rule, Eric N. Brown, Nancy R. Sottos, Scott R. White, and Jeffrey S. Moore* ReceivedWe have reported a polymeric material that is capable of autonomic crack repair and recovery of structural integrity.[1±5] This self-healing material (Fig. 1a) is a common epoxy which contains solid particles of Grubbs' catalyst and poly(urea-formaldehyde) microcapsules containing liquid dicyclopentadiene (DCPD). When a crack propagates through the epoxy, it also ruptures the microcapsules and releases DCPD into the crack plane. The DCPD then mixes with the Grubbs' catalyst, undergoes ring opening metathesis polymerization (ROMP), and cures to provide structural continuity across the crack plane. This system performs well with a relatively large (2.5 wt.-%) loading of catalyst, but multiple factors have made lower catalyst loadings ineffective. Firstly, the catalyst does not disperse well in the epoxy, so only a few relatively large (~500 lm) catalyst particles are exposed on the crack plane when low catalyst loadings are used. This poor dispersion of catalyst leads to regions on the crack plane where no catalyst is available to cure the DCPD, and healing is incomplete. Secondly, the epoxy's curing agent, diethylenetriamine (DETA), destructively attacks Grubbs' catalyst as the epoxy initially cures, and this destruction reduces the amount of catalyst that is available for the promotion of healing. [2] COMMUNICATIONS

The activation parameters for the ring-opening metathesis polymerization (ROMP) of endo-(1) and exo-dicyclopentadiene (2), endo-1,2-dihydrodicylopentadiene (3), and norbornene (4) in the presence of Grubbs' catalyst were determined using in situ NMR. The exo isomer of DCP was found to be more than an order of magnitude more reactive than the endo isomer. endo-DCP was found to have reactivity similar to its partially saturated counterpart 3, suggesting that the cause of the rate difference between the two isomers of DCP is primarily steric in nature. This interaction is shown to be predominantly entropic and is suspected to originate from an interaction of the penultimate repeat unit and the incoming monomer. Additionally, the alkylidene generated during the polymerization of endo-DCP was found to form an intramolecular complex, but this complex only affects the rate slightly.

The role of the crystal morphology and dissolution kinetics of Grubbs' catalyst on self-healing capability is examined. Self-healing polymers require complete coverage of the crack plane with polymerized healing agent for optimal recovery of mechanical integrity. Lack of catalyst leads to incomplete coverage, partial polymerization, and poor mechanical recovery. Catalyst availability is determined by the competing rates of dissolution of the catalyst and polymerization of the healing agent. First-generation Grubbs' catalyst exists in at least two crystal polymorphs, each with a distinct crystal shape, thermal stability, and dissolution kinetics. The more rapidly dissolving polymorph shows superior healing efficiency when used as the initiator in a self-healing epoxy material based on ring-opening metathesis polymerization of dicyclopentadiene.

Self-healing polymers, based on microencapsulated dicyclopentadiene and Grubbs' catalyst embedded in the polymer matrix, are capable of responding to propagating fatigue cracks by autonomic processes that lead to higher endurance limits and life extension, or even the complete arrest of the crack growth. The amount of fatigue-life extension depends on the relative magnitude of the mechanical kinetics of crack propagation and the chemical kinetics of healing. As the healing kinetics are accelerated, greater fatigue life extension is achieved. The use of wax-protected, recrystallized Grubbs' catalyst leads to a fourfold increase in the rate of polymerization of bulk dicyclopentadiene and extends the fatigue life of a polymer specimen over 30 times longer than a comparable non-healing specimen. The fatigue life of polymers under extremely fast fatigue crack growth can be extended through the incorporation of periodic rest periods, effectively training the self-healing polymeric material to achieve higher endurance limits.