Polydicyclopentadiene Aerogels Grafted with PMMA: I. Molecular and Interparticle Crosslinking
Mohite, Dhairyashil P.
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Polydicyclopentadiene (pDCPD) is a polymer of emerging technological significance from separations to armor. It is a paradigm of ring opening metathesis polymerization (ROMP) and some of its remarkable properties (e. g., strength) have been attributed to crosslinking of the pendant cyclopentenes. pDCPD should be an ideal candidate for strong nanoporous solids (aerogels), however, excessive swelling of the wet-gels precursors in toluene (up to 200% v/v), followed by de-swelling and severe deformation in acetone, renders the resulting aerogels unusable. Based on spectroscopic evidence (IR, solid state C-13 NMR and several liquid H-1 NMR controls), only 4-5% of the pendant cyclopentene double bonds of pDCPD are engaged in crosslinking, via Wagener-type olefin coupling. Deformation was rectified via free radical polymerization of methylmethacrylate (MMA) in the pores of pDCPD wet-gels. The uptake of PMMA was varied in the 13-28% w/w range by varying the concentration of MMA. Evidence (e. g., differential scanning calorimetry) though suggests that PMMA remains a linear polymer, hence the pDCPD/PMMA network resist deformation, not because of molecular-level crosslinking, but due to a synergism related to the nano-topology of the two components (see next paper of this issue). With cylindrical monoliths available, the nature of the interparticle chemical bonding in pDCPD/PMMA aerogels was probed top-down with thermal conductivity and compression testing, using linear-polynorbornene (pNB) aerogels as a control system. The latter, with no pendant cyclopentenes, has no chance for interpolymer chain crosslinking. The solid thermal conduction and stiffness of pDCPD/PMMA and pNB aerogels scale similarly, pointing to a common mechanism for interparticle bonding. That was assigned to cross-metathesis, effectively extending the polymer chains of one nanoparticle into another, and was reflected on very high polydispersities (8-13).
"This project was supported by the Army Research Office under Award no. W911NF-10-1-0476. Initial funding was provided by the National Science Foundation under Agreement no. CHE-0809562. We also thank Dr Wei Wycoff of the University of Missouri Columbia for her help with the solid state 13C NMR."