Polydicyclopentadiene aerogels from first- versus second-generation Grubbs’ catalysts: a molecular versus a nanoscopic perspective
Polydicyclopentadiene (pDCPD) aerogels obtained via ring-opening metathesis polymerization using the first-generation Grubbs’ catalyst (GC-I) are well-shaped monoliths; those obtained from the second-generation Grubbs’ catalyst (GC-II) were severely deformed. At lower densities, materials from either catalyst consisted of entangled nanofibers, turning into random aggregates of nanoparticles as density increased. Nanoscopically, all those microstructures consisted of similar mass-fractal aggregates of secondary particles (by rheology), which in turn are closely packed assemblies of primary particles (by SAXS). Soluble oligomers along gelation were observed only with GC-II (by 1H NMR); nevertheless, all monomers were eventually incorporated in the skeletal framework of both materials (gravimetrically). The extent of cross-linking by olefin addition (via solid-state 13C NMR) was in the same range with both catalysts (19–25 % of pendant cyclopentenes). The only significant difference in the two kinds of aerogels was in the cis versus trans configuration of the polymeric backbone (by IR and solid-state 13C NMR). Deformation of GC-II-derived aerogels has been rectified by filling the empty space among primary particles (about 36 % v/v) with a hard polymer. Those aerogels have the same mechanical properties with those derived from GC-I, meaning that deformation is due to rearrangement at a level below the load-bearing macroporous network. Thus, a self-consistent model for deformation calls for primary particles of mostly trans pDCPD (via GC-I) being more rigid and more difficult to squeeze; thus, higher mass-fractal aggregates of secondary particles do not penetrate into the empty space of one another. Conversely, primary particles of more malleable cis/trans pDCPD (via GC-II) are squeezable, allowing higher aggregates to partially penetrate into one another. This model may also be related to frequently noted drying shrinkage of wet-gels even after converting the pore-filling solvent into a supercritical fluid, whereas all surface tension forces should have been eliminated.
KeywordsPolydicyclopentadiene Aerogels Grubbs’ catalyst Mechanism Deformation
This study was funded by the Army Research Office (W911NF-14-1-0369) and BASF Polyurethanes GmbH (0039509).
Conflict of interest
The authors declare that they have no conflict of interest.
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