Biopolymer-Aerogele und -Schäume: Chemie, Eigenschaften und Anwendungen

Heat Confinement Aerogel Enables Supramagnetothermal Effect for Triggering Nitric Oxide Generation

Reducing heat dissipation plays an indispensable role in boosting the magnetothermal effect but has received scant attention. Herein, a magnetothermal aerogel (MA) combining an efficient magnetothermal convertor for heat generation and a highly porous aerogel for reducing heat dissipation is developed. Such a heat confinement MA shows a large thermal resistance and high infrared absorption that can effectively confine the heat by regulating interior thermal conduction and radiation, exhibiting a supramagnetothermal effect. In addition, a waterproof beeswax coated MA achieves negligible heat loss and a supramagnetothermal effect even in high-thermal-diffusion aqueous media. As a proof of concept, a synthesized heat-triggered nitric oxide (NO) precursor is integrated into an MA, and the rapid NO generation (∼22 μM/min) resulting in an antibacterial effect further verifies the supramagnetothermal effect of the MA. This work provides an efficient strategy to promote the magnetothermal effect and offers inspiration for building a heat-triggering system.

Macroscopic Ultralight Aerogel Monoliths of Imine-based Covalent Organic Frameworks

The use of covalent organic frameworks (COFs) in practical applications demands shaping them into macroscopic objects, which remains challenging. Herein, we report a simple three-step method to produce COF aerogels, based on sol-gel transition, solvent-exchange, and supercritical CO₂ drying, in which 2D imine-based COF sheets link together to form hierarchical porous structures. The resultant COF aerogel monoliths have extremely low densities (ca. 0.02 g cm^-3 ), high porosity (total porosity values of ca. 99 %), and mechanically behave as elastic materials under a moderate strain (<25-35 %) but become plastic under greater strain. Moreover, these COF aerogels maintain the micro- and meso-porosity of their constituent COFs, and show excellent absorption capacity (e.g. toluene uptake: 32 g g^-1 ), with high removal efficiency (ca. 99 %). The same three-step method can be used to create functional composites of these COF aerogels with nanomaterials.

Silica-silk fibroin hybrid (bio)aerogels: two-step versus one-step hybridization

In this study, silk fibroin as a highly promising naturally occurring biopolymer extracted from silkworm cocoon is applied to mechanically reinforce silica aerogels. To this aim, two different approaches for the incorporation of silk fibroin into the silica network are compared: (1) a one-step acid catalyzed and (2) a two-step acid-base catalyzed sol-gel reaction. The total organosilane concentration, as well as the SF to silane mass fractions, regulated the hybridization process to proceed either through a one-step or two-step sol-gel reaction. In both processes, for an efficient chemical mixing the silk fibroin components with the silane phase, a silane coupling agent, 5-(trimethoxysilyl) pentanoic acid (TMSPA), comprising carboxylic acid groups and a pentyl hydrocarbon chain has been used. For a low organosilane content (3.4 mmol) along with a high SF to silane mass ratio (15-30%), the gelation of the silane and silk fibroin phases took place in a one-pot/one-step process in the presence of an acid catalyst in an entirely aqueous system. In the two-step synthesis approach, which was applied for high initial silane contents (17 mmol), and low SF to silane mass ratios (1-4%), first, the gelation of the silk fibroin phase was triggered by addition of an acid catalyst followed by a more pronounced condensation of the silane catalyzed by the addition of the base. Both synthesis approaches led to materials with promising mechanical properties-being 1) the one-step process resulting in gels with much better compressibility (up to 70% of strain), low density (0.17-0.22 g cm^-3) and three orders of magnitude improvement in the Young's modulus (13.5 MPa) compared to that of the pristine silica aerogel but with rather high shrinkage (30-40%). The two-step process in principle could result in the hybrid aerogel with interesting bulk density (0.17-0.28 g cm^-3) with lower shrinkage (10%), but the resultant aerogel was stiff and fragile. Also, both approaches led to a significant reduction in the time required to prepare strong hybrid aerogels compared to conventional hybrid polymer-silica aerogels with the utilization of an entirely aqueous synthesis approach for a wide range of applications.

Self-Assembled Dipeptide Aerogels with Tunable Wettability

Constructing supramolecular materials with tunable properties and functions is a great challenge due to the complex competition between multiple assembly pathways. Herein, we report that dipeptides can self-assemble into aerogels with entirely different surface wettability through precisely controlling the assembly pathways. Charged groups or aromatic residues are selectively exposed on the surface of their nanoscale building blocks which results either in a superhydrophilic or highly hydrophobic surface. With this special property, single component dipeptide aerogels can play diverse roles in medical care applications. This study suggests great promise in the synthesis of supramolecular materials with different targeted functions from the same molecular unit.

Host-Guest Chemistry Within Cellulose Nanocrystal Gel Receptors

Cellulose nanocrystals (CNCs) spontaneously assemble into gels when mixed with a polyionic organic or inorganic salt. Here, we have used this ion-induced gelation strategy to create functional CNC gels with a rigid tetracationic macrocycle, cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺ ). Addition of [CBPQT]Cl₄ to CNCs causes gelation and embeds an active host inside the material. The fabricated CNC gels can reversibly absorb guest molecules from solution then undergo molecular recognition processes that create colorful host-guest complexes. These materials have been implemented in gel chromatography (for guest exchange and separation), and as elements to encode 2- and 3-dimensional patterns. We anticipate that this concept might be extended to design a set of responsive and selective gel-like materials functioning as, for instance, water-pollutant scavengers, substrates for chiral separations, or molecular flasks.

Fabrication and Application of Carboxymethyl Cellulose-Carbon Nanotube Aerogels

In this study, composite aerogels with excellent mechanical properties were prepared by using carboxymethyl cellulose (CMC) as raw materials, with carboxylic carbon nanotubes (CNTs) as reinforcement. By controlling the mass fraction of CNTs, composite aerogels with different CNTs were prepared, and the surface morphology, specific surface area, compressive modulus, density and adsorption capacities towards different oils were studied. Compared to the pure CMC aerogel, the specific surface areas of CMC/CNTs were decreased because of the agglomeration of CNTs. However, the densities of composite aerogels were lower than pure CMC aerogel. This is because the CNTs were first dispersed in water and then added to CMC solution. The results indicated that it was easy for the low CMC initial concentration to be converted to low density aerogel. The compressive modulus was increased from 0.3 MPa of pure CMC aerogel to 0.5 MPa of 5 wt % CMC/CNTs aerogel. Meanwhile, the prepared aerogels showed promising properties as the adsorption materials. Because of the high viscosity, liquid possesses strong adhesion to the pore wall, the adsorption capacity of the CMC aerogel to the liquid increases as the viscosity of the liquid increases.

Novel multifunctional polymethylsilsesquioxane-silk fibroin aerogel hybrids for environmental and thermal insulation applications

The development of aerogels with improved mechanical properties, to expand their utility in high-performance applications, is still a big challenge. Besides fossil-fuel based polymers that have been extensively utilized as platforms to enhance the mechanical strength of silsesquioxane and silica-based aerogels, using green biopolymers from various sustainable renewable resources are currently drawing significant attention. In this work, we process silk fibroin (SF) proteins, extracted from silkworm cocoons, with organically substituted alkoxysilanes in an entirely aqueous based solution via a successive sol-gel approach, and show for the first time that it is possible to produce homogeneous interpenetrated (IPN) polymethylsilsesquioxane (PMSQ)-SF hybrid aerogel monoliths with significantly improved mechanical properties. Emphasis is given to an improvement of the molecular interaction of the two components (SF biopolymer and PMSQ) using a silane coupling agent and to the design of pore structure. We succeeded in developing a novel class of compressible, light-weight, and hierarchically organized meso-macroporous PMSQ-SF IPN hybrid aerogels by carefully controlling the sol-gel parameters at a molecular level. Typically, these aerogels have a compressive strength (δ max) of up to 14 MPa, together with high flexibility in both compression and bending, compressibility up to 80% strain with very low bulk density (ρ b) of 0.08-0.23 g cm-3. By considering these promising properties, the superhydrophobic/oleophilic PMSQ-SF aerogel hybrids exhibited a high competency for selective absorption of a variety of organic pollutants (absorption capacities ∼500-2600 g g-1 %) from water and acted as a high-performance filter for continuous water/oil separation. Moreover, they have demonstrated impressive thermal insulation performance (λ = 0.032-0.044 W m-1 K-1) with excellent fire retardancy and self-extinguishing capabilities. Therefore, the PMSQ-SF aerogel hybrids would be a new class of open porous material and are expected to further extend the practical applications of this class of porous compounds.