Organic and composite aerogels through ring opening metathesis polymerization (ROMP)

Aerogels are open-cell nanoporous materials, unique in terms of low density, low thermal conductivity, low dielectric constants and high acoustic attenuation. Those exceptional properties stem from their complex hierarchical solid framework (agglomerates of porous, fractal secondary nanoparticles), but they also come at a cost: low mechanical strength. This issue has been resolved by crosslinking silica aerogels with organic polymers. The crosslinking polymer has been assumed to form a conformal coating on the surface of the skeletal framework by covalent bridging elementary building blocks. However, assuming is not enough: for correlating nanostructure with bulk material properties, it is important to know the exact location of the polymer on the aerogel backbone. For that investigation, we synthesized a new norbornene derivative of triethoxysilane (Si-NAD) that can be attached to skeletal silica nanoparticles. Those norbornene-modified silica aerogels were crosslinked with polynorbornene by ring opening metathesis polymerization (ROMP). The detailed correlation between nanostructure and mechanical strength was probed with a wide array of characterization methods ranging from molecular to bulk through nano. Subsequently, it was reasoned that since the polymer dominates the exceptional mechanical properties of polymer crosslinked aerogels, purely organic aerogels with the same nanostructure and interparticle connectivity should behave similarly. That was explored and confirmed by: (a) synthesis of a difunctional nadimide monomer (bis-NAD), and preparation of robust polyimide aerogels by ROMP of its norbornene end-caps; and, (b) synthesis of dimensionally stable ROMP-derived polydicyclopentadiene aerogels by grafting the nanostructure with polymethylmethacrylate (PMMA) via free radical chemistry --Abstract, page iv

Assemblies of Nanoparticles as 3D Scaffolds for New Materials Design: from Polymer Crosslinked Aerogels to Polymer Matrix Composites

From a materials perspective, nanotechnology furnishes materials with useful macroscopic properties by manipulating matter in the 1-100 nm size regime. Improvements in performance in terms of strength, modulus and wetability are accomplished by, for example, introducing nanoparticles as fillers in plastics . Two issues that usually interfere with optimal materials performance are agglomeration of the nanoparticles and materials compatibility. Agglomeration cancels the advantage of using nanoparticulate matter as dopant, while lack of materials compatibility introduces a discontinuity at the polymer/dopant interface from where failure may begin. Agglomeration is encountered with surfactants that keep nanoparticles dispersed, while materials compatibility is improved by chemical bonding of the filler with the polymer. Overall the criterion for success is enhancement of the materials properties beyond what is obtained by simple mixing nanoparticles in the matrix. Silica is the most common dopant in use as a filler in plastics. Silica derived through a base-catalyzed sol-gel process consists of interconnected string of nanoparticles dispersed randomly in the 3D space, leaving up to \u3e99% empty mesoporous space between the nanoparticle network. If we consider providing those mesoporous surfaces with functional groups capable of covalent bonding with a polymer formed from monomers introduced in the mesopores, then we can achieve two extreme structures with distinct materials properties: (a) at one end, we may deposit only a thin conformal polymer layer on the nanoparticle network; while, (b) at the other end, we may grow enough polymer to fill the mesopores completely. The first kind of structure emphasizes the materials properties deriving from the porosity, that is lightweight, low thermal conductivity and dielectric constants, and high acoustic impedance. The second kind of structure refers to nanoparticle/matrix polymer composites tackling both issues of dispersion and covalent bonding between matrix and dopant all at once

Luminescent LaF₃:Ce-doped Organically Modified Nanoporous Silica Xerogels

Organically modified silica compounds (ORMOSILs) were synthesized by a sol-gel method from amine-functionalized 3-aminopropyl triethoxylsilane and tetramethylorthosilicate and were doped in situ with LaF3:Ce nanoparticles, which in turn were prepared either in water or in ethanol. Doped ORMOSILs display strong photoluminescence either by UV or X-ray excitation and maintain good transparency up to a loading level of 15.66% w/w. The TEM observations demonstrate that ORMOSILs remain nanoporous with pore diameters in the 5-10 nm range. LaF3:Ce nanoparticles doped into the ORMOSILs are rod-like, 5 nm in diameter and 10-15 nm in length. Compression testing indicates that the nanocomposites have very good strength, without significant lateral dilatation and buckling under quasi-static compression. LaF3:Ce nanoparticle-doped ORMOSILs have potential for applications in radiation detection and solid state lighting

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

Strong Silica Aerogels Crosslinked with Polynorbornene via Ring Opening Metathesis Polymerization (ROMP)

Silica aerogels are porous, self-assembled, 3D networks of silica nanoparticles. In general they are mechanically weak, which limits their applications in spite of their attractive bulk properties (low thermal conductivity, high acoustic impedance etc.). Weak interparticle necks between silica particles responsible for the poor mechanical properties can be reinforced with polymeric tethers to give strong, crosslinked silica aerogels.1 Here we report the crosslinking of silica aerogels by ring opening metathesis polymerization (ROMP) by providing the surface of silica particles with the norbornene functionality using a new nadimide derivative of 3-aminopropyltriethoxysilane (APTES). Norbornene monomer is introduced in the mesopores and a ROMP process is started using 2nd generation Grubbs\u27 catalyst at ambient temperature. The growing polymer engages norbornene moieties bound on the surface of silica forming a conformal coating of polynorbornene on the mesoporous surfaces throughout the entire skeletal framework. The amount of polymer incorporated in the mesoporous structure is controlled by the concentration of the monomer in the mesopores. Despite the increase in bulk density (up to 0.6-0.7 g cm-3), decrease in porosity (down to ~50% v/v), and decrease in surface area (down to ~150 m2 g-1), the materials remain mesoporous. The mechanical properties in terms of strength, modulus and the energy absorption capability relative to the native (non-crosslinked) counterparts is increased dramatically

Monolithic Cellular Graphitic Carbon from Romp-Derived Polydicyclopentadiene Aerogels

Porous carbons and carbon aerogels are useful as electrodes for batteries and fuel cells, catalyst supports and separation/filtration media. Monolithic carbon aerogels are usually obtained by pyrolysis at up to 1100 0C of organic (polymeric) aerogels, mainly based on resorcinol-formaldehyde resin. Other organic aerogels that have been converted to porous carbons include polyacrylonitrile, polyimide, polyurea and polybenzoxazine. Here, we demonstrate the preparation of carbonizable aerogels based on polydicyclopentadiene (pDCPD) synthesized via ring opening metathesis polymerization (ROMP) of the monomer using a second-generation Grubbs\u27 catalyst. Since pDCPD is not substantially crosslinked, the resulting aerogel monoliths are deformed severely relative to the shape of their molds. That issue was resolved by post-gelation grafting of the pDCPD wet-gels with polymethylmethacrylate (PMMA) using AIBN-induced free-radical chemistry. The resulting aerogel monoliths are uniform and robust. Pyrolysis of those aerogels at 800 0C under Ar yields electrically conducting amorphous carbons (yield: 30% w/w), which in turn were graphitized at 2300 0C to yield (93% w/w) highly conducting monolithic cellular (~3 micron diameter) graphitic carbon

Rigid Macroporous Polynorbornene Monoliths by Ring Opening Metathesis Polymerization

Rigid macroporous polymer monoliths are widely used as efficient stationary phases for chromatographic separations, flow-through reactors, catalyst supports etc. An efficient sol-gel bottom-up synthetic approach involves polymerization of soluble monomers into an insoluble polymer that phase-separates into colloidal particles that under the right conditions of molecular structure, concentration, solvent and temperature form gels. Early phase-separation, hence smaller colloidal particles, can be induced by molecular-level cross-linking in the growing polymer akin to that in thermoset resins. Alternatively, for highly soluble linear (thermoplastic) polymers, phase-separation can be controlled with non-solvents. This is demonstrated here with open-cell macroporous polynorbornene monoliths synthesized by ring opening metathesis polymerization (ROMP) of norbornene in toluene using isopropanol (iPrOH) as non-solvent. Wet-gels were solvent-exchanged with liquid CO2 taken out at the end as a supercritical fluid (SCF). Changes in the microstructure as a function of the iPrOH:toluene ratio was probed by SEM, N2 sorption porosimetry. Bulk densities range from 0.40 to 0.68 g cm-3. Porosity ranges from 30 to 60% v/v with average pore diameters in the 1.4-2.3 mm range, but the BET surface area is low (1-3 m2 g-1. The skeletal framework consists of agglomerated particles forming macroglobular structures (1.8 to 4.5 µm in diameter). The monoliths are robust with e.g., ultimate compressive strength under high strain rates (1224 s-1) of samples made with iPrOH:toluene = 7:3 v/v (0.51 g cm-3) equal to 50 MPa at 75% ultimate strain

Correlation of Microstructure and Thermal Conductivity in Nanoporous Solids: The Case of Polyurea Aerogels Synthesized From an Aliphatic Tri-isocyanate and Water

This study correlates microstructure with thermal transport properties in nanoporous solids. The model system is based on polyurea (PUA) aerogels. Those aerogels demonstrate a dramatic change in microstructure with density. Low density aerogels consist of entangled nano-fibers changing into interconnected nanoparticles as the density increases. The nanostructure was probed in terms of both particle size and network interconnectivity with scanning electron microscopy and small angle X-ray scattering. Thermal conductivity values between 0.027 and 0.066 W/mK were obtained with the hot-wire method for PUA samples with densities between 0.04 and 0.53 g/cm3. Both, pressure and temperature dependent experiments were performed for the deconvolution of total thermal conductivity into gaseous, radiative, and transport-through-the-solid- framework contributions. Subsequently, thermal conductivity along the solid framework was considered as a function of microstructure. That leads to a quantitative evaluation of the impact of primary particle characteristics and network interconnectivity on the solid thermal conductivity. © 2013 Elsevier B.V. All rights reserved

Fabrication of Sol-gel Materials with Anisotropic Physical Properties by Photo-cross-linking

A method to vary locally the physical properties of a porous material is presented. A wet gel is first prepared following conventional sol-gel techniques. The pore walls are derivatized by adding to the gelling solution a silane carrying a polymerizable moiety such as trimethoxysilylpropyl. The solvent of the wet gel monolith is then exchanged with a solution of a monomer such as styrene and a photoinitiator such as 2,2′-azobis-isobutyronitrile. Illumination with ultraviolet light initiates polymerization which in turn engages the moiety dangling from the pore surfaces. Supercritically dried monoliths were characterized with techniques such as field-emission scanning electron microscopy (SEM), methylmethacrylate atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy, and Brunauer-Emmett-Teller surface area measurements. These structural characterization techniques showed that the silica nanoparticles making up the backbone of the monoliths were cross-linked by a polymer conformal coating. Mechanical characterization was carried out with nanoindentation and the three-point flexural method and showed that the properties of uniformly photo-cross-linked monoliths could be varied by varying exposure time. So, for example, the monolith density could be varied between about 0.21 and 0.97 g ·cm-3, the porosity between 6 and 87%, and Young\u27s modulus between 9 and about 1800 MPa. Overall, the characterization techniques show that photo-cross-linked monoliths have physical and mechanical properties comparable and often superior to those of monoliths obtained by thermally initiated cross-linking (see, for example, Leventis, N.; Sotiriou-Leventis, C.; Zhang, G.; Rawashdeh, A.-M. M. Nano Lett. 2002, 2, 957-960). More importantly photo-cross-linking allows fabrication of monoliths with anisotropic physical properties. We demonstrate the modulation capabilities of our method by producing transparent and opaque regions within the same monolith and by producing multifunctional two- and three-dimensional patterns