Aerogels as diverse nanomaterials

Aerogels are 3-D light-weight nanoporous materials pursued for their low thermal conductivity, low dielectric constant and high acoustic attenuation. Those exceptional macroscopic properties of aerogels are dependent on the chemical nature of nanoparticles, complex hierarchical solid skeletal framework and porosity. Also, the free space can become host for functional guests such as pharmaceuticals. In chapter I, we investigated randomly mesoporous bio-compatible polymer-crosslinked dysprosia aerogels as drug delivery vehicles and demonstrated storage and release of drugs under physiological conditions. Comparative study with ordered and randomly mesoporous silica showed high drug uptake and slower release rate for random nanostructures (silica or dysprosia) relative to ordered silica. Drug release data from dysprosia aerogels showed that drug is stored successively in three hierarchical pore sites on the skeletal framework. In chapter II, we developed flexible polyurethane-acrylate aerogels from star monomer containing urethane linkage and terminal acrylate bonds by free-radical polymerization. Lower density samples were flexible, while higher density samples were mechanically strong. Those results were dependent on the particle size and interparticle connectivity of skeletal framework, pointing to a nanoscopic origin for their flexibility, rather than to a molecular one. Further, the acrylate bonds were converted to norbornene moieties and the gelation process was brought down to room temperature by using ring opening metathesis polymerization (ROMP). In chapter III, we developed polydicyclopentadiene (pDCPD) based aerogels using two different Grubbs catalysts (GC-I and GC-II) with different catalytic activity towards ROMP. The different behavior of pDCPD aerogels was traced to a different polymer configuration at molecular level. --Abstract, page v

Flexible to Rigid Nanoporous Polyurethane-Acrylate (PUAC) Type Materials for Structural and Thermal Insulation Applications

Novel urethane-acrylate (UAC) Star monomers and polyurethane-acrylate (PUAC) aerogel polymers derived therefrom are described herein, along with other novel, related monomers and polymers. Also described herein are processes for preparing the UAC Star monomers, the PUAC aerogel polymers, and the other related monomers and polymers. The PUAC and related polymers herein are useful in various applications including in structural and thermal insulation

Flexible Aerogels From Hyperbranched Polyurethanes: Probing the Role of Molecular Rigidity with Poly(Urethane Acrylates) vs. Poly(Urethane Norbornenes)

Flexible and foldable aerogels have commercial value for applications in thermal insulation. This study investigates the molecular connection of macroscopic flexibility using polymeric aerogels based on star-shaped polyurethane-acrylate versus urethane-norbornene monomers. the core of those monomers is based either on a rigid/aromatic, or a flexible/aliphatic triisocyanate. Terminal acrylates or norbornenes at the tips of the star branches were polymerized with free radical chemistry, or with ROMP, respectively. at the molecular level, aerogels were characterized with FTIR and solid-state 13C NMR. the porous network was probed with N2-sorption and Hg-intrusion porosimetry, SEM and SAXS. the interparticle connectivity was assessed in a top-down fashion with thermal conductivity measurements and compression testing. All aerogels of this study consist of aggregates of nanoparticles, whose size depends on the aliphatic/aromatic content of the monomer, the rigidity/flexibility of the polymeric backbone, and generally varies with density. at higher densities (0.3-0.7 g cm-3), all materials were stiff, strong, and tough. Aerogels based on urethane-acrylates built around a rigid/aromatic core exhibited a rapid decrease of their elastic modulus with density (slopes of the log-log plots \u3e5.0), and at about 0.14 g cm-3, they were foldable. Data support that molecular properties of the monomer affect macroscopic flexibility indirectly, not so through the particle size, but rather through the growth mechanism and consequently through the interparticle contact area. Thus, flexible aerogels of this study showed no indication for polymer accumulation onto the primary nanostructure (particle sizes via N2-sorption and SAXS were comparable), and their interparticle contact area was comparatively lower. Because for flexibility purposes, interparticle contact area is related to interparticle bonding, it is speculated that if the latter is controlled properly (e.g., through adjustment of the monomer functional group density) it might lead to superelasticity and shape-memory effects

Nanoporous Flexible Polyurethane-Acrylate Aerogels

Aerogels are porous, low-density 3D assemblies of nanoparticles with large surface-to-volume ratios. Flexible aerogels are particularly attractive materials for thermal insulation. Herein, we report flexible aerogels synthesized via polyurethane-acrylate (PUAC) chemistry. A star-shaped monomer was prepared from tris(4-isocyanatophenyl)methane (TIPM) and 2-hydroxyethyl acrylate (HEA) in anhydrous acetone using dibutyltin dilaurate (DBTDL) as catalyst. Crosslinking was initiated with 2,2\u27-azobisisobutyronitrile (AIBN) in one pot. Wet-gels were dried with supercritical fluid CO2 to monolithic PUAC aerogels, which were characterized at the molecular (solids 13C NMR), nanoscopic (SEM, SAXS) and macroscopic level (compression and 3-point bending). Lower density PUAC aerogels (0.14 g cm-3) consist of large primary particles (88 nm in diameter) and are macroporous and flexible. Higher density samples (0.66 g cm-3) consist of smaller particles (18 nm in diameter), they are rigid and mechanically strong

Polydicyclopentadiene Aerogels via ROMP: Nanostructure Control with First and Second Generation Grubbs Catalysts

Polydicyclopentadiene (pDCPD)is a polymer synthesized via ROMP from readily available dicyclopentadiene (DCPD), an inexpensive byproduct of petroleum refinery, and is emerging as an attractive material for diverse applications from separation media to body armor. Here, we developed pDCPD-based aerogels using first and second generation Grubbs Catalysts (GC-I and GC-II) known for their different catalytic activity and tolerance towards wide range of functional groups. pDCPD wet-gels with GC-II show excessive swelling in toluene (up to 200% v/v) followed by de-swelling and uneven shrinkage in acetone, resulting in severely deformed aerogels. However, wet-gels from GC-I retain their shape throughout processing. Microscopically, pDCPD aerogels derived from GC-I and GC-II catalysts show different morphologies: fibrous versus particulate, respectively. High concentration pDCPD aerogels obtained from GC-I are mechanically strong, undergo compression without buckling, making them suitable material for ballistic protection

Mechanically Strong Acrylonitrile-based Aerogels Via Free Radical Polymerization and their Conversion to Porous Carbons

We became interested in polyacrylonitrile (PAN) aerogels by considering together the fact that PAN is the major industrial source of carbon fiber, and the practical applications of porous carbons. Although free-radical solution polymerization of acrylonitrile may afford gels, those linear-polymer gels collapse upon drying and they cannot be converted to aerogels. Using 1,6-hexanediol diacrylate (HDDA) or ethylene glycol dimethacrylate (EGDMA) as crosslinkers induces early phase-separation of “live” particles that react with one another forming the robust 3D network of mechanically strong aerogels. SEM shows, and N2 sorption confirms that acrylic aerogels are mainly macroporous materials, and therefore they can be dried under ambient pressure. Pyrolysis of acrylic aerogels under Ar yields porous carbon aerogels with electrical conductivities in the range of 0.6-3.5 mho cm-1. The higher conductivities of samples made with EGDMA was attributed to their higher surface areas (180-200 m2 g-1) compared to those made with HDDA (40-60 m2 g-1)

Polydicyclopentadiene Aerogels From First- vs. Second-generation Grubbs’ Catalysts: A Molecular vs. 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

Flexible Aerogels from Hyperbranched Polyurethanes: Probing the Role of Molecular Rigidity with Poly(Urethane Acrylates) Versus Poly(Urethane Norbornenes)

Flexible and foldable aerogels have commercial value for applications in thermal insulation. This study investigates the molecular connection of macroscopic flexibility using polymeric aerogels based on star-shaped polyurethane-acrylate versus urethane-norbornene monomers. The core of those monomers is based either on a rigid/aromatic, or a flexible/aliphatic triisocyanate. Terminal acrylates or norbornenes at the tips of the star branches were polymerized with free radical chemistry, or with ROMP, respectively. At the molecular level, aerogels were characterized with FTIR and solid-state ¹³C NMR. The porous network was probed with N₂-sorption and Hg-intrusion porosimetry, SEM and SAXS. The interparticle connectivity was assessed in a top-down fashion with thermal conductivity measurements and compression testing. All aerogels of this study consist of aggregates of nanoparticles, whose size depends on the aliphatic/aromatic content of the monomer, the rigidity/flexibility of the polymeric backbone, and generally varies with density. At higher densities (0.3–0.7 g cm^–3), all materials were stiff, strong, and tough. Aerogels based on urethane-acrylates built around a rigid/aromatic core exhibited a rapid decrease of their elastic modulus with density (slopes of the log–log plots >5.0), and at about 0.14 g cm^–3, they were foldable. Data support that molecular properties of the monomer affect macroscopic flexibility indirectly, not so through the particle size, but rather through the growth mechanism and consequently through the interparticle contact area. Thus, flexible aerogels of this study showed no indication for polymer accumulation onto the primary nanostructure (particle sizes via N₂-sorption and SAXS were comparable), and their interparticle contact area was comparatively lower. Because for flexibility purposes, interparticle contact area is related to interparticle bonding, it is speculated that if the latter is controlled properly (e.g., through adjustment of the monomer functional group density) it might lead to superelasticity and shape-memory effects