Poly(urethane-norbornene) Aerogels via Ring Opening Metathesis Polymerization of Dendritic Urethane-Norbornene Monomers: Structure-Property Relationships as a Function of an Aliphatic Versus an Aromatic Core and the Number of Peripheral Norbornene Moieties

Abstract

We report the synthesis and characterization of synthetic polymer aerogels based on dendritic-type urethane-norbornene monomers. The core of those monomers is based either on an aromatic/rigid (TIPM/Desmodur RE), or an aliphatic/flexible (Desmodur N3300) triisocyanate. The terminal norbornene groups (three at the tip of each of the three branches) were polymerized via ROMP using the inexpensive 1st generation Grubbs catalyst. The polymerization/gelation conditions were optimized by varying the amount of the catalyst. The resulting wet-gels were dried either from pentane under ambient pressure at 50 °C, or from t-butanol via freeze-drying, or by using supercritical fluid (SCF) CO2. Monomers were characterized with high resolution mass spectrometry (HRMS), 1H- and solid-state 13C-NMR. Aerogels were characterized with ATR-FTIR and solid-state 13C-NMR. The porous network was probed with N2-sorption and SEM. The thermal stability of monomers and aerogels was studied with TGA, which also provides evidence for the number of norbornene groups that reacted via ROMP. At low densities (<0.1 g cm−3) all aerogels were highly porous (porosity > 90%), mostly macroporous materials; aerogels based on the aliphatic/flexible core were fragile, whereas aerogels containing the aromatic/rigid core were plastic, and at even lower densities (0.03 g cm−3) foamy. At higher densities (0.2–0.7 g cm−3) all materials were stiff, strong, and hard. At low monomer concentrations all aerogels consisted of discrete primary particles that formed spherical secondary aggregates. At higher monomer concentrations the structure consisted of fused particles with the size of the previous secondary aggregates, due to the low solubility of the developing polymer, which phase-separated and formed a primary particle network. Same-size fused aggregates were observed for both aliphatic and aromatic triisocyanate-derived aerogels, leading to the conclusion that it is not the aliphatic or aromatic core that determines phase separation, but rather the solubility of the polymeric backbone (polynorbornene) that is in both cases the same. The material properties were compared to those of analogous aerogels bearing only one norbornene moiety at the tip of each branch deriving from the same cores.

Synthetic Polymer Aerogels in Particulate Form

“…The BET surface areas were higher when hexane was used; the highest value was obtained in propylene carbonate/hexane (481 m 2 g –1 ; Table 1). In all samples a 15–38% of the total surface area was assigned to micropores (Table 1), which in turn was attributed to the rigid structure of the aromatic triisocyanate—in accord with previous observations in similar materials [81,82,92]. Average pore diameters in the range of 1.7–300 nm were all found between 10 nm and 30 nm for all powders, and followed the same trend with the values obtained for the BET surface areas.…”

“…In that regard, although there is a vast literature for monolithic aerogels derived from synthetic polymers, including aerogels based on phenolic resins (e.g., resorcinol-formaldehyde [19,55,56,57,58,59,60,61], polybenzoxazines [62,63,64,65,66], polyureas [67,68,69], polyimides [70,71,72,73,74], polyamides (Kevlar TM -like) [74,75,76], polyurethanes [77,78,79,80,81,82,83], as well as aerogels derived via Ring Opening Metathesis Polymerization (ROMP), including polynorbornene and polydicyclopentadiene [81,84,85,86,87], only a few examples of synthetic polymer aerogel particles are known, and they have become the subject of this mini-review. Those polymeric materials include mostly polyurea aerogels, and a few examples of polyimide and phenolic resin aerogels.…”

Polyurea-crosslinked biopolymer aerogel beads

“…In the spectrum of X-Ca-alginate aerogels, a shoulder at 1733 cm À1 can be assigned to the urethane carbonyl group. 6,11,12,61,62 No peak is shown at 2266 cm À1 , assigned to the vibration of the isocyanate groups, 9 showing that all isocyanate groups of Desmodur N3300 have reacted. The 13 C CPMAS spectra of X-Ca-alginate aerogel beads (Fig.…”

“…Aerogels were invented in the early 1930's by Kistler 1,2 and are dened as solid colloidal or polymeric networks of nanoparticles expanded throughout their entire volume by a gas. 3,4 Early emphasis was based on silica and other inorganic aerogels, 1,2,5 while organic aerogels, based on organic polymers [6][7][8][9][10][11][12] or biopolymers, [13][14][15][16] and hybrid organic-inorganic aerogels [17][18][19][20][21] were developed later.…”

Millimeter-Size Spherical Polyurea Aerogel Beads with Narrow Size Distribution

“…[2]. Several types of aerogels have been reported, including inorganic [5,6,7,8,9], organic (based on biopolymers [10,11,12,13] or synthetic polymers [14,15,16,17]), and hybrid inorganic/organic [18,19,20,21,22].…”