|Title:||Breath figures assisted fabrication of polymer films with controllable microstructures for energy harvesting|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Pages:||xxxv, 320 pages : color illustrations|
|Abstract:||The breath figure (BF) technique is a unique one-step strategy of using easily-available and nontoxic water droplet arrays as soft dynamic templates to produce porous films with the size ranging from 200 nm to 20 μm for a wide range of applications in micro/nanofabrication, superhydrophobic/responsive coating, sensing, catalysis, separation, and various biological, optical and electronic devices. It possesses simple, low-cost, time-saving, easily-scalable and non-destructive advantages, and requires no trivial work on the preparation and removal of templates because the condensation, assembly and removal of BFs are all spontaneous. However, considerable challenges and problems still exist in the BF field. Firstly, the reported BFs are usually single-component, which often results in the formation of monotonous porous or spherical structures. And there are still no systematic studies on controllable fabrication and formation mechanism of polymer microstructures via different BFs. Then the optimal experimental conditions for dissimilar materials to obtain ordered pore arrays are usually different, narrow and specific. Currently it is difficult to find a general and scalable fabrication approach for highly ordered porous films with commercial polymeric and non-polymeric materials on different substrates. Thirdly, there are few studies on construction of three-dimensionally (3D) ordered porous film conformably formed on nonplanar substrates (particularly for flexible fabrics with complex surface textures), although uneven and curved surfaces are usually encountered in most real applications. Finally, BFs generated pore arrays (BFAs) and their derived materials have demonstrated promising and valuable applications in various fields, but to our best knowledge, their applications in energy-related fields are significantly less developed. In this thesis, firstly, the BF principle of water was successfully extended to methanol (MeOH) and binary liquids of different water/MeOH ratios for assisted fabrication of various film microstructures under different experimental conditions based on a series of well-defined polystyrene (PS) homopolymers (1600 < number molecular weight (Mn)<19500, and molecular weight distribution (Mn/Mw) < 1.1). Three kinds of solvents with different affinity to water, i.e. completely immiscible carbon disulfide (CS2), partial immiscible chloroform (CHCl3) and highly miscible tetrahydrofuran (THF), were used to prepare solutions with the concentration of 5 mg mL-1 to 640 mg mL-1, respectively. It was found that aqueous BFs led to the fabrication of porous microstructures regardless of molecular weight, solvent type and solution concentration, while MeOH BFs generally resulted in the production of microspherical particles. A surprising phenomenon was that microspherical caps were obtained from the casting solutions with better water affinity by combining use of water and MeOH as binary BFs. The formation of microspherical caps was independent on the molecular weight and solution concentration, while adjusting the methanol ratio of binary BFs, asymmetrical particles ranging from large-portion to small-portion microspherical caps were prepared quickly and directly. Through the comparative and systematic analysis, the whole BF processes starting from the initial spreading of solution film on substrate, the formation and spreading of nonsolvent BFs on solution film, the physical gelation of polymer-rich phase to the final formation of polymer microstructures by complete removal of liquids were thoroughly elucidated based on the different physicochemical properties of nonsolvent, solvent and polymer. Then after unambiguous clarification on formation mechanism behind the BF processes, an additive-assisted strategy was developed for a robust and universal modified BF technique for a range of commercially available polymers which are not good candidates for conventional BF technique, such as PS, PS-b-polybutadiene-b-PS (SBS), poly(ethylene oxide) (PEO), and polycarbonate (PC). Through adjusting the addition of asphalt, all of them were successfully employed to prepare highly ordered BFA films without any cracks in more accessible BF conditions on different substrates with either a planar or nonplanar surface. This asphalt-assisted BF technique was also feasible for incorporating different nanoparticles, such as titanium dioxides nanoparticles (TiO2 NPs), silver nanowires and copper nanowires, into the porous films for tailorable functionality with little change of the regularity of pore arrays. The TiO2 NPs incorporated BFA films were exemplified and demonstrated to harvest luminous energy more efficiently for photocatalytic degradation of pollutants in both air and water.|
Thirdly, the formation of 3D conformal BFAs (3C-BFAs) was systematically investigated on different nonplanar substrates, particular on flexible fabrics. A novel silicon-containing graft copolymer poly(dimethylsiloxane)-graft-polyacrylates (PDMS-g-PAs) that can form non-cracking and ordered BFAs was first explored and demonstrated on a planar substrate by the BF technique in a wide solution concentration. Then based on a special metal fabric, copper/nickel coated conductive polyester fabric, the influence of solution concentration was systematically studied to find an optimal concentration range for the formation of 3C-BFAs. Such modified fabrics may pave the way to a brand-new class of textile composites featured with the porous microstructures of film-forming materials conformably formed on the uneven surface of textile substrates, which possesses customized multifunctional properties of introduced materials, inherent properties (such as breathability and flexibility) and unique texture features of fabrics. Moreover, taking advantages of the silicon-containing characters, PDMS-g-PAs BFA films were used as single-source precursor for in situ formation of honeycomb microstuctured ceramics. Finally, the BFA films on both planar substrates and 3C-BFAs on nonplanar substrates were used as negative micro molds to prepare positive PDMS films with different micro lens arrays (MLAs) and hierarchical MLAs (H-MLAs). They were further used as frictional materials for the assembly of vertical contact-separation mode based triboelectric nanogenerators (TENGs) to convert mechanical energy into electricity, respectively. The influence of surface microstructures on the electric performance of TENGs was systematically investigated by applying different external forces and resistance loads. Based on the PDMS film with an optimal surface microstructure, a portable and wearable TENG device was developed to harvest the mechanical energy generated by human body motions such as finger tapping, hand clapping, and walking. In summary, this study provided a better understanding of the complex mechanism behind the BF approaches for designing and fabricating polymer films with controllable microstructures, and developed a universal additive-assisted BF technique for general fabrication and functionalization of large-area, non-cracking and highly ordered BFA films on either planar or nonplanar substrates with customized polymers and nanoparticles for different practical uses. A brand-new kind of textile composites merging the properties of both porous materials and textiles was first developed and realized by systematically studying the formation of 3C-BFAs on flexible fabric substrates. This benefits the development of novel, flexible and wearable materials with tailorable architectures and functionalities based on traditional textile materials for higher value applications, such as TENGs.
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