surface by different behaviors toward the water. Superhydrophilic surfaces, where the water contact angle is less than 5°, can be cleaned by the spreading of water across the surface or a stream of water, like rainfall. [6] In contrast, on superhydrophobic surfaces, water forms spherical droplets that roll across the surface, carrying away contamination particles. [7] This phenomenon is known as the "lotus effect," [8] whose origin is the surface energy and the roughness of the corresponding surface. [9] In comparison, there are more advantages in choosing superhydrophobicity over superhydrophilicity as self-cleaning materials due to the reduction of bacterial and contamination adhesion, [10] and their waterproof and antimist characteristics. [11] To achieve a superhydrophobic surface with a water contact angle of more than 150°, a low surface energy material is necessary, as well as a high roughness of micro-or nanostructured surface architecture. Therefore, hydrophobic polymers, alkyl silanes, or semifluorinated silanes are candidates for the generation of superhydrophobic surfaces. [12] Methods for constructing nano-or microstructure surfaces are important for realizing superhydrophobic surfaces. However, these methods are limited to particular substrates (e.g., silicon wafer or aluminum foil) or by the complexity of processing (e.g., lithography and anodization), which restrict its broader applications. Moreover, surface superhydrophobicity could be damaged by the oily contamination from organic compounds or bacterial biofilm. [13] Thus, a functionality which can maintain the surface superhydrophobicity is of pivotal importance to keep the durability of self-cleaning materials. One of the most convenient and effective ways to achieve that is through compositing photocatalysts.Artificial photocatalytic surfaces have become increasingly popular over the recent decades since their discovery by Fujishima and Honda in 1972. [14] Among photocatalytic materials, TiO 2 has attracted much attention in scientific research as well as commercial applications. TiO 2 is well-known as an efficient photocatalyst in air and water purification, water splitting, and the disinfection of surfaces. [15] It has been demonstrated that including TiO 2 in the composition of a surface coating is a good approach to fabricating self-cleaning materials. [16] Self-cleaning materials, which are inspired and derived from natural phenomena, have gained significant scientific and commercial interest in the past decades as they are energy-and labor-saving and environmentally friendly. Several technologies are developed to obtain self-cleaning materials. The combination of superhydrophobic and photocatalytic properties enables the efficient removal of solid particles and organic contaminations, which could reduce or damage the superhydrophobicity. However, the fragility of the nanoscale roughness of the superhydrophobic surface limits its practical application. Here, a hierarchical structure approach combining micro-and nanoscale architecture...
Core–shell nanoparticles consisting of silica as core and surface-grafted poly(dimethylsiloxane) (PDMS) as shell with different diameters were prepared and used as heterogeneous nucleation agents to obtain CO2-blown poly(methyl methacrylate) (PMMA) nanocomposite foams. PDMS was selected as the shell material as it possesses a low surface energy and high CO2-philicity. The successful synthesis of core–shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The cell size and cell density of the PMMA micro- and nanocellular materials were determined by scanning electron microscopy. The cell nucleation efficiency using core–shell nanoparticles was significantly enhanced when compared to that of unmodified silica. The highest nucleation efficiency observed had a value of ∼0.5 for nanoparticles with a core diameter of 80 nm. The particle size dependence of cell nucleation efficiency is discussed taking into account line tension effects. Complete engulfment by the polymer matrix of particles with a core diameter below 40 nm at the cell wall interface was observed corresponding to line tension values of approximately 0.42 nN. This line tension significantly increases the energy barrier of heterogeneous nucleation and thus reduces the nucleation efficiency. The increase of the CO2 saturation pressure to 300 bar prior to batch foaming resulted in an increased line tension length. We observed a decrease of the heterogeneous nucleation efficiency for foaming after saturation with CO2 at 300 bar, which we attribute to homogenous nucleation becoming more favorable at the expense of heterogeneous nucleation in this case. Overall, it is shown that the contribution of line tension to the free energy barrier of heterogeneous foam cell nucleation must be considered to understand foaming of viscoelastic materials. This finding emphasizes the need for new strategies including the use of designer nucleating particles to enhance the foam cell nucleation efficiency.
We describe a successful strategy to substantially enhance cell nucleation efficiency in polymer foams by using designer nanoparticles as nucleating agents. Bare and poly(dimethylsilane) (PDMS)-grafted raspberry-like silica nanoparticles with diameters ranging from ∼80 nm to ∼200 nm were synthesized and utilized as highly efficient cell nucleators in CO2-blown nanocellular polymethyl methacrylate (PMMA) foams. The successful synthesis of core–shell nanoparticles was confirmed by Fourier transform infrared spectroscopy, thermogravimetric analysis, Brunauer–Emmett–Teller measurements, and transmission electron microscopy. The cell size and cell density of the obtained PMMA micro- and nanocellular foams were determined by scanning electron microscopy. The results show that increased surface roughness enhances the nucleation efficiency of the designer silica particles. This effect is ascribed to a decreased nucleation free energy for foam cell nucleation in the nanocavities at the melt–nucleator interface. For PDMS grafted raspberry-like silica nanoparticles with diameters of 155 and 200 nm, multiple cell nucleation events were observed. These hybrid particles had nucleation efficiencies of 3.7 and 6.2, respectively. The surprising increase in nucleation efficiency to above unity is ascribed to the significant increase in CO2 absorption and capillary condensation in the corresponding PMMA during saturation. This increase results in the presence of large amounts of the physical blowing agent close to energetically favorable nucleation points. Additionally, it is shown that as a consequence of cell coalescence, the increased number of foam cells is rapidly reduced during the first seconds of foaming. Hence, the design of highly efficient nucleating particles, as well as careful selection of foam matrix materials, seems to be of pivotal importance for obtaining polymer cellular materials with cell dimensions at the nanoscale. These findings contribute to the fabrication of polymer foams with high thermal insulation capacity and have relevance in general to the area of cellular materials.
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