Evaluation of tungsten hexachloride as a ROMP active catalyst precursor for self-healing polymers.

摘要

Self-healing polymers and composites have been demonstrated via a microencapsulated healing agent and catalytic trigger within the matrix material. The microcapsules are ruptured by a propagating crack and healing agent is released into the crack plane by capillary action. Contact with the catalyst initiates a chemical reaction and the resulting solid polymer bonds the crack faces together. Initial work using the microcapsule/catalytic trigger concept focused on the ring-opening-metathesis-polymerization (ROMP) of endo-dicyclopentadiene with Grubbs' catalyst. Monotonic fracture recovery in excess of 90% and extended fatigue life have been reported.; A novel self-healing system based on the ROMP active catalyst tungsten (VI) chloride (WCl6) shows great promise for addressing the limitations of Grubbs' catalyst, specifically the high cost and low thermal stability. The healing chemistry utilizes WCl6 in solid phase and exo-dicyclopentadiene (exo-DCPD) monomer. The exo-DCPD monomer is the same molecule as used in previously self-healing materials, but is a different isomer and exhibits much higher ROMP reactivity and kinetics. The effect of two different initiators, phenylacetylene and 1-decyne, was explored, as well as an alternative monomer, 5-ethylidene-2-norbornene (ENB).; Three catalyst forms (as-received, recrystallized, and wax-protected) were investigated to address the known sensitivity of WCl6 to moisture and amine curing agents. Fracture experiments were performed to evaluate the healing efficiency of each catalyst type. A tapered double cantilever beam (TDCB) specimen geometry was used to evaluate the virgin and healed fracture toughness of the material. Fracture recovery of ca. 90% was achieved in both the recrystallized and as-received WCl6 cases at 12 wt% catalyst loading. The wax-protected form of the catalyst gives healing efficiencies of ca. 50% at only 4 wt% overall catalyst loading. In situ samples achieved healing efficiencies of up to 30% for recrystallized catalyst loadings of 10 wt% with 15 wt% microcapsules. In fatigue, the benefits of the self-healing system becomes more evident. An in situ sample containing 10 wt% recrystallized catalyst and 15 wt% microcapsules showed no crack growth after 2.5 million cycles, and in fact exhibited a regression of the initial precrack.; A drawback to current catalysts used in self-healing materials is the relatively low temperatures at which they deactivate. An advantage of WCl 6 is that the high melting temperature (Tm=275°C) allows for the use of the catalyst in higher temperature applications. Typical polymer and fiber reinforced plastic processing temperatures are 121 and 177°C. The ability of WCl6 catalyst to withstand these processing temperatures was probed. Several thermal analysis techniques were used to evaluate the activity of WCl6 after high temperature exposure. At both 121 and 177°C in an inert environment, the WCl6 retained sufficient activity for use as a ROMP precursor for self-healing applications. Mechanical evaluation through fracture experiments showed healing efficiencies as high as 92% if the processing and testing environment was carefully controlled.