Polydicyclopentadiene Aerogels Grafted With PMMA: II. Nanoscopic Characterization and Origin of Macroscopic Deformation
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 should be an ideal candidate for strong nanoporous solids (aerogels), however, excessive swelling of pDCPD wet-gels in toluene (up to 200% v/v), followed by de-swelling and severe deformation in acetone, renders the resulting aerogels unusable. With only 4-5% of the pendant cyclopentene double bonds of pDCPD engaged in crosslinking (see previous paper of this issue), introducing additional crosslinking with polymethylmethacrylate (PMMA) was deemed appropriate. Thus, even with an uptake of PMMA as low as 13% w/w, the resulting aerogels kept the shape and dimensions of their molds. Evidence though suggests (e.g., DSC) that PMMA remains a linear polymer, hence pDCPD/PMMA networks resist deformation, not because of molecular-level crosslinking, but due to a synergism related to the nanotopology of the two components. SEM and N-2 sorption on dry aerogels show that macroscopic deformation of wet-gels is accompanied by coalescence of nanoparticles. Small angle X-ray scattering (SAXS) shows that both deformed (pDCPD) and non-deformed (pDCPD/PMMA) aerogels consist of same-size primary particles (8-9 nm radius) that form non-mass-fractal secondary particles (21-27 nm radius). On the other hand, rheology shows that the pDCPD gel network is formed by mass fractal aggregates (D-f similar to 2.4). Putting this information together, it is concluded that the pDCPD network is formed by aggregates of secondary particles. It is suggested that particles coalescence is driven by non-covalent interactions that squeeze deformable secondary particles of one fractal assembly inside the empty space of another. As supported by skeletal density considerations, PMMA fills the space between primary particles; thus, secondary particles become rigid and can no longer squeeze past one another into the empty space of their higher fractal aggregates.