Atomic Layer Deposition onto Thermoplastic Polymeric Nanofibrous Aerogel Templates for Tailored Surface Properties

Poly(vinyl alcohol-co-ethylene) (EVOH) nanofibrous aerogel (NFA) templates were fabricated through vacuum freeze-drying from EVOH nanofibrous suspensions. Aluminum oxide (Al₂O₃) layers were deposited onto highly porous templates to form organic-inorganic hybrid aerogels by the atomic layer deposition (ALD) technique. Chemical and physical measurements showed that mechanical properties were improved through ALD. In addition, the surface chemistry of ALD modified aerogels showed a fascinating cyclic change based on the number of ALD deposition cycles. A transition from hydrophilicity to hydrophobicity was observed after a few cycles of ALD coating; however, additional deposition cycles changed the wettability characteristics back to hydrophilicity. This hydrophilic-hydrophobic-hydrophilic variation is shown to be governed by a combination of geometrical and chemical surface properties. Furthermore, the deposited Al₂O₃ could substantially improve aerogels strength and reduce permanent deformation after cyclic compression. The Young's modulus of aerogels increased from 5.54 to 33.27 kPa, and the maximum stress at 80% strain went up from 31.13 to 176.11 kPa, after 100 cycles of trimethyl-aluminum (TMA)/water ALD. Thermogravimetric analysis (TGA) results confirm that ALD can effectively improve the heat resistance characteristics of polymeric aerogel. The onset temperature and the residual mass increased with increasing numbers of ALD cycles. During pyrolysis, the nanofiber cores were decomposed, and the brittle pure Al₂O₃ self-supporting nanotube aerogels with the continuous hollow nanotubular network were formed. A coating of continuous thickness Al₂O₃ layer on individual nanofiber was achieved after 100 ALD cycles. In additional to mechanical strength and physical property changes, the ALD modified aerogel also shows a superhydrophobic and oleophilic surface chemistry, which could potentially be used to remove oils/organic solvents from water. The resultant aerogels exhibit excellent absorption capacity (31-73 g/g) for various liquids, and the material could be reused after distillation or squeezing. A successful scale-up of such materials could provide some insights into the design and development of thermoplastic polymeric NFAs with substantial industrial applications.

Strong, Ultralight Nanofoams with Extreme Recovery and Dissipation by Manipulation of Internal Adhesive Contacts

Advances in three-dimensional nanofabrication techniques have enabled the development of lightweight solids, such as hollow nanolattices, having record values of specific stiffness and strength, albeit at low production throughput. At the length scales of the structural elements of these solids-which are often tens of nanometers or smaller-forces required for elastic deformation can be comparable to adhesive forces, rendering the possibility to tailor bulk mechanical properties based on the relative balance of these forces. Herein, we study this interplay via the mechanics of ultralight ceramic-coated carbon nanotube (CNT) structures. We show that ceramic-CNT foams surpass other architected nanomaterials in density-normalized strength and that, when the structures are designed to minimize internal adhesive interactions between CNTs, more than 97% of the strain after compression beyond densification is recovered. Via experiments and modeling, we study the dependence of the recovery and dissipation on the coating thickness, demonstrate that internal adhesive contacts impede recovery, and identify design guidelines for ultralight materials to have maximum recovery. The combination of high recovery and dissipation in ceramic-CNT foams may be useful in structural damping and shock absorption, and the general principles could be broadly applied to both architected and stochastic nanofoams.

Robust monolithic polymer(resorcinol-formaldehyde) reinforced alumina aerogel composites with mutually interpenetrating networks

Monolithic polymer(resorcinol-formaldehyde) reinforced alumina (RF/Al₂O₃) aerogel composites were prepared using a sol–gel method and supercritical fluid CO₂ drying. The formation mechanism, chemical compositions, pore structures, morphologies, thermal and mechanical performances of RF/Al₂O₃ aerogel composites with different RF/Al molar ratios were investigated. The results show that the two networks of organic resorcinol-formaldehyde and inorganic alumina are completely independent of one another. The as-synthesized RF/Al₂O₃ aerogels consist of spherical organic carbon particles and fibrous alumina, which possess low bulk density (0.077–0.112 g cm⁻³), low shrinkage (1.55–2.76%), low thermal conductivity (0.024–0.028 W m⁻¹ K⁻¹), and high specific surface area (453.26–722.75 m² g⁻¹). Especially, the sample prepared with molar ratio RF/Al = 1 shows the best network structure with the higher compressive strength (1.83 MPa) and Young's modulus (122.57 MPa). The resulting robust RF/Al₂O₃ aerogel composites could be potentially used as thermal insulators, catalysts and adsorbents.

Monolithic polymer(resorcinol-formaldehyde) reinforced alumina (RF/Al₂O₃) aerogel composites were prepared using a sol–gel method and supercritical fluid CO₂ drying.

Modifying the morphology and properties of aligned CNT foams through secondary CNT growth

In this work, we report for the first time, growth of secondary carbon nanotubes (CNTs) throughout a three-dimensional assembly of CNTs. The assembly of nanotubes was in the form of aligned CNT/carbon (ACNT/C) foams. These low-density CNT foams were conformally coated with an alumina buffer layer using atomic layer deposition. Chemical vapor deposition was further used to grow new CNTs. The CNT foam's extremely high porosity allowed for growth of secondary CNTs inside the bulk of the foams. Due to the heavy growth of new nanotubes, density of the foams increased more than 2.5 times. Secondary nanotubes had the same graphitic quality as the primary CNTs. Microscopy and chemical analysis revealed that the thickness of the buffer layer affected the diameter, nucleation density as well as growth uniformity across the thickness of the foams. The effects of secondary nanotubes on the compressive mechanical properties of the foams was also investigated.