Polydicyclopentadiene aerogels grafted with PMMA: II. Nanoscopic characterization and origin of macroscopic deformation

Abstract

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 $ 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 noncovalent 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.

Polyurea Aerogels: Synthesis, Material Properties, and Applications

“…Under SEM, these structures appear as if they consist of random assemblies of particles, with a layer of polymer cast over the entire network [ 37 ]. The topography of such an arrangement actually resembles that of polymer-crosslinked oxide aerogels, in which silica primary particles are embedded within a crosslinker-derived polymer that fills the space within the aerogel’s secondary particles [ 39 , 40 ]. Comparisons of particle sizes determined from skeletal density/N 2 sorption data and SAXS have proven useful in understanding the growth mechanisms underlying the formation of polymeric aerogels in general.…”

Scalable, hydrophobic and highly-stretchable poly(isocyanurate–urethane) aerogels

“…Disk-shaped PU samples (5-7 mm in thickness and 9.6-10 mm in diameter) were sandwiched between the incident and transmission bars. The use of an aluminum transmission bar took advantage of the low Young's modulus of aluminum ($1/3 of steel) to reach high signal-to-noise ratios for the weak transmitted signal through aerogels 7,23,24 and attain similar functions to those accessible with hollow transmission steel tubes. 10,14,25 A Cu disk pulse shaper (1.6 mm-thick, 7.4 mm in diameter) was used to reach a dynamic stress equilibrium state and constant strain rates, which is necessary for a valid SHPB experiment.…”

Large, Rapid Swelling of High-cis Polydicyclopentadiene Aerogels Suitable for Solvent-Responsive Actuators

“…The last decade has experienced an unprecedented growth of the types of aerogels available, ranging from inorganic [22][23][24][25] to organic (based on biopolymers [3,14,[26][27][28] and synthetic polymers) and hybrid inorganic-organic aerogels [29][30][31][32][33][34]. The growth of synthetic polymer aerogels in particular has been extremely rapid and now that class includes aerogels based on a wide variety of phenolic resins [35][36][37], polyamides [38][39][40], polyimides [41][42][43][44][45][46][47], polyurethanes [48][49][50][51][52][53], polyureas [54][55][56][57][58] and polymers derived via ring opening metathesis polymerization (ROMP) [42,49,[59][60][61][62][63][64]…”

“…Such crosslinked PDCPD is a rigid polymer with excellent mechanical, chemical and physical properties. PDCPD aerogels combine the unique properties of aerogels with those of PDCPD polymers [59][60][61][62][63][64][65]. Thus, mechanically strong PDCPD-based aerogels can find applications as thermal and acoustic insulators [60], as well as low-density coatings [59,64].…”