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Research on the fabrication of convex structure composite film from waste paper and polyvinyl alcohol for application in triboelectric nanogenerators

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  • Chapter 1: OVERVIEW (21)
    • 1.1 Introduction to materials (21)
      • 1.1.1 Introduce the paper industry (21)
      • 1.1.2 Introduce paper and paper-making methods (21)
      • 1.1.3 Introduce pulp in paper-making (23)
      • 1.1.4 Chemical composition of paper (23)
        • 1.1.4.1 Limestone (CaCO 3 ) (24)
        • 1.1.4.2 Kaolin (Al 2 SO 3 , talc MgO.SiO 3 .nH 2 O) (24)
        • 1.1.4.3 Others oxide (24)
      • 1.1.5 Waste paper pulp (24)
      • 1.1.6 Polyvinyl Alcohol (26)
    • 1.2 Overview of the cellulose (27)
      • 1.2.1 Introduce cellulose and its properties (30)
      • 1.2.2 Hemicellulose (31)
      • 1.2.3 Lignin (31)
    • 1.3 Triboelectric Nanogenerator – TENG and applications (32)
      • 1.3.1 Four fundamental principle modes of Triboelectric Nanogenerators (32)
      • 1.3.2 Typical structure of TENG (34)
        • 1.3.2.1 Flat structure (34)
        • 1.3.2.2 Arch-shaped structure (34)
      • 1.3.3 Applications of TENG (35)
    • 1.4 High-order porous structure polymeric materials (36)
      • 1.4.1 Introduction porous polymer materials (36)
      • 1.4.2 Porous structure making methods (37)
        • 1.4.2.1 Conventional phase separation method (37)
        • 1.4.2.2 Breath Figures method (38)
        • 1.4.2.3 The emulsion method (38)
        • 1.4.2.4 Improved Phase Separation method (IPS) (39)
    • 1.5 Introduction convex structure films (40)
    • 1.6 Biocomposite materials and films (42)
  • Chapter 2: EXPERIMENTAL (43)
    • 2.1 Materials and chemicals (43)
    • 2.2 Equipment and laboratory instruments (44)
    • 2.3 Experimental process (44)
      • 2.3.1 Waste paper treatment processing (44)
      • 2.3.2 Concave Polystyrene thin film making process (46)
      • 2.3.3 Convex composite film making process (47)
      • 2.3.4 TENG device fabrication process (48)
    • 2.4 Characterization (50)
      • 2.4.1 Optical microscope (OM) (50)
      • 2.4.2 Scanning electron microscope (SEM) (50)
      • 2.4.3 Fourier transform infrared spectroscopy (FTIR) (50)
      • 2.4.5 Oscilloscope (51)
      • 2.4.6 Tensile properties (51)
      • 2.4.7 Thermal analysis (53)
      • 2.4.8 The moisture content of cellulose pulp (54)
      • 2.4.9 Water absorption (54)
      • 2.4.10 Water contact angle (55)
  • Chapter 3: RESULTS AND DISCUSSION (56)
    • 3.1 Characterization of cellulose pulp treated from waste paper (56)
      • 3.1.1 Morphology structure of cellulose pulp (56)
      • 3.1.2 Evaluation of functional group composition in cellulose pulp by FTIR spectroscopy (56)
      • 3.1.3 Moisture content of cellulose pulp (58)
    • 3.2 Characterization of the concave Polystyrene thin film (59)
    • 3.3 Characterization of the composite films from cellulose and PVA (60)
      • 3.3.1 Flat composite films (60)
      • 3.3.2 Convex composite films (61)
    • 3.4 Water absorption of the convex composite film (66)
    • 3.5 Thermal analysis results of composite film and PVA film (67)
    • 3.6 Water contact angle of composite films (69)
    • 3.7 Tensile properties of composite films (70)
    • 3.8 Electrical measurement results of composite films (71)
      • 3.8.1 Output voltage of convex composite films (71)
      • 3.8.2 Compare the voltage difference between two types of electrode (76)
      • 3.8.3 Compare the voltage difference between convex and flat composite films (77)
      • 3.8.4 Capacitor charging speed of TENG (78)
      • 3.8.5 Durability of TENG after 10,000 cycles (79)
      • 3.8.6 Applicability of TENG (80)
  • from 1-5 Hz (0)

Nội dung

OVERVIEW

Introduction to materials

Paper is an indispensable product in the social activities of any country Although the means of informatics in information and storage have developed strongly, the word has always been an irreplaceable product in educational activities such as printing, newspapers, learning, painting, etc As the national economy develops, the increasing social demand will increase the demand for paper-based packaging, and the need for other types of paper will increase

Paper materials seem indispensable to us humans in modern society Most of the uses of paper are for note-taking and printing Of course, it is not the only use, but many other uses have been invented We can see that paper is applied in the following fields: Education, culture-art, business-commerce, transportation packaging, The main raw materials to create paper are wood, bamboo, and agricultural waste such as straw, bagasse, and other plants Paper is produced from cellulose, so the cellulose content is very high because, in the forming process, the lignin content has been removed almost completely In the paper, there is still some residual hemicellulose and lignin; cellulose still accounts for the largest percentage

Figure 1.1 Paper factory in the paper industry

1.1.2 Introduce paper and paper-making methods

Paper is a product of cellulose fibers in the form of sheets in which fibers and fiber components are linked together to create three-dimensional spatial networks since ancient times people have been able to make paper from papyrus children and then stack them on top of each other illustrating the paper's layered structure

In the paper-forming process, the fibers are brought into contact at the pressing and drying stage to form bonds and thereby provide the mechanical and mechanical properties of the paper tape The mechanical strength of the paper tape will depend on the contact area and the strength of the bonds formed The fundamental determining factor of cohesion is the free surface area of the fibrous muscle or the part capable of binding This parameter is related to pulp and paper production technology

The heterogeneity of the paper tape is more pronounced in the cases where the base is oriented in the direction of the double-sided paper rolls and has a less flat forming in addition to the manufacturing process properties of the fibers, especially the fiber size, which affects the homogeneity of the structure

Figure 1.2 Describe the paper production and recycling process

Once the paper tape is formed this structural array not only has mechanical strength but also has to drop some other properties such as coverage, porosity, smoothness, and water repellency These requirements will be tailored to the application of each paper product The first step is to choose the raw material, that is, to choose the properties for the fiber, but in the pulp production process, it is a phenomenon that some properties of the product can be affected when we try to separate ways to improve some other properties Therefore, it is necessary to adjust the shaping process or use additives to find the most suitable response [13]

1.1.3 Introduce pulp in paper-making

Paper is a source of fibrous material used to make paper Pulp is usually of vegetable origin, but special paper can also be made from animal fibers, inorganic fibers, or synthetic fibers The pulp is chemically treated to produce non-paper-like products, collectively known as soluble pulp, these pulps are used to produce cellulose derivatives such as cellulose, cellulose acetate, cellulose nitrate, cellulose nitrate Soluble pulp can be produced from cellulose sulfite or modified sulfate pulp, the issue that needs to be closely examined and is the cleanliness and uniformity of the pulp after bleaching

Figure 1.3 Types of pulp commonly used to make paper

There is also a form of pulp prepared like cotton, which is used in special fields that are completely different from paper such as products with high adsorbent properties

The chemical properties of the gender are determined by the pulp production process, the type and content of non-fibrous additives Chemical properties are important for some types of paper such as photo paper with color and photocopier insulation with food packaging tamper-proof paper as in m for silver steel wrapping fireproof paper re-opening the paper depending on the application that has different tests

In addition to pulp (cellulose), there are additives in the paper to increase whiteness, smoothness, smoothness, reflectivity, etc These additives are called fillers Fillers are white, fine, water-insoluble substances, soluble in some alkalis, added to the pulp suspension to increase some important features of paper such as whiteness, and smoothness, reduce paper deformation, and increase the cost of paper Commonly used fillers: limestone CaCO3, kaolin Al2SO3, talc MgO.SiO3.nH2O, TiO2, [13]

Known as: Calcium carbonate, with a molecular weight of 100,087 g/mol, usually appears as a white pulp, is insoluble in water, and melts at 825ºC There are two main roles in paper production as fillers and surface whiteners: Fillers in paper are gloss and luminance enhancers, and higher opacity and smoothness will increase printability

1.1.4.2 Kaolin (Al 2 SO 3 , talc MgO.SiO 3 nH 2 O)

Theoretical composition: Al2O3 39.48%, SiO2 46.6%, H2O 13.92% Density 2.57-2.61 g/cm 3 , hardness 1-2.5, grain diameter 0.2-12 μm Kaolin helps paper products have a smoother surface, increases tightness, reduces optical transparency, and increases ink absorption at the best level

Titanium dioxide pulp (TiO2): Has the highest whiteness and best turbidity but the cost is high Usually used for papers that use less filler but need high whiteness and opacity

Talc pulp (Magnesium Silicate 3MgO.4SiO2.H2O) is a kind of filler that gives paper softness However, this substance is harmful to human health, so when producing paper, people often use it in very small amounts [13]

Natural fibers like flax, hemp, jute, wood, and others are frequently used in polymer matrix composites because of their low cost, high specific strength, lower emissions of pollutants, and biodegradability, among other benefits which makes them a better choice for reinforcement than synthetic fibers Cellulosic fiber-based polymer composites were initially designed to replace glass fiber-reinforced composites Among all regular cellulosic filaments, lignocellulosic strands are the most generally utilized, as built-up material because of their minimal expense, and effectively accessible and these strands have various fascinating mechanical and actual properties [14]

Paper is an example of a valuable material that can be recycled Manuel (2002) found that it is an eco-friendly material due to the considerable content of recyclable waste paper The disposable paper that is abundantly available around the world is composed primarily of short, natural cellulose fibers and has been used in many local raw materials Waste paper comes from many different sources such as newspapers, offices, printing paper, and boxes Each has a different yarn quality and mixing all these different quality papers will reduce the purity of the highest quality yarn The quality of waste determines the final quality of recycled paper [15]

Pulp and paper are made from unrefined substances containing short-length cellulose strands, which come from lignocellulosic biomass or reused paper In 2013, the world's paper and pulp industries produced approximately 400 million tons of paper and board for use by consumers in a variety of nations As waste is being produced at a

Overview of the cellulose

Cellulose was discovered in 1838 by French chemist Anselme Payen who isolated it from plants and determined its chemical formula [19],[20] Cellulose was used to produce the first successful thermoplastic polymer, celluloid, by the Hyatt Manufacturing Company in 1870 Production of rayon ("artificial silk") from cellulose began in the 1890s, and cellophane was introduced invented in 1912 Hermann Staudinger determined the polymeric structure of cellulose in 1920 This compound was first chemically synthesized (without the use of any biologically derived enzymes) in

Figure 1.5 Introduction of cellulose origin

Cellulose is an organic compound with the formula (C6H10O5) n, which is a polysaccharide consisting of a linear chain of several hundred to many thousands of D- glucose units linked to β(1→4) [19][22] Cellulose is an important structural component of the primary cell walls of green plants, many forms of algae and oomycetes Cellulose is the most abundant organic polymer on Earth [23] The cellulose content of cotton fiber is 90%, that of wood is 40–50% and that of dried hemp is about 57%

Figure 1.6 Structure of simple cellulose

Cellulose is used mainly for the production of paperboard and paper Smaller quantities are converted into a variety of derivative products such as cellophane and rayon The conversion of cellulose from energy crops into biofuels such as cellulosic ethanol is being developed as a renewable fuel source Cellulose for industrial use is mainly obtained from wood pulp and cotton [23]

Cellulose is a long-chain polysaccharide composed of glucose units It is the main structural component of the cell walls of plants and is the most abundant organic compound on Earth Cellulose is an essential material for various industries, including paper, textiles, pharmaceuticals, and food

The history of the structure of cellulose is long and complicated One of the previously favored models for natural cellulose was a microfiber with a crystalline core surrounded by a fringe micelle [24] Cellulose is a complex carbohydrate and the main structural component of the cell walls of plants It is composed of long chains of glucose molecules linked together through chemical bonds Cellulose is indigestible to humans and many other animals because they lacks the enzymes needed to break down its strong bonds

However, cellulose is an essential dietary fiber that provides bulk to the diet, aiding digestion and promoting bowel regularity It is also used in various industries, such as paper and textile manufacturing, as well as in the production of biofuels

Figure 1.7 Structure of a cellulose long chain

Cellulose is tasteless, odorless, hydrophilic with a contact angle of 20–30 degrees

[25], insoluble in water and most organic solvents, antagonistic, and biodegradable It has been shown to melt at 467 °C in pulse tests performed by Dauenhauer et al (2016)

[26] It can be chemically broken down into glucose units by treating it with concentrated mineral acids at high temperatures [27]

Cellulose is derived from D-glucose units, condensed through β(1→4)- glycosidic bonds This binding motif contrasts with the α(1→4)-glycosidic binding motif found in starch and glycogen Cellulose is a linear polymer Unlike starch, no twisting or branching occurs, and the molecule has an elongated and rather rigid rod- shaped structure supported by the equatorial structure of the glucose moiety Multiple hydroxyl groups on glucose from one chain form hydrogen bonds with oxygen atoms on the same chain or on neighboring chains, holding these chains firmly next to each other and forming high-tensile microfibrils This creates tensile strength in the cell wall, where the cellulose microfibrils are bonded together into a polysaccharide matrix The high tensile strength of tree trunks and wood also arises from the arrangement of cellulose fibers tightly distributed in the lignin matrix The mechanical role of cellulose fibers in the wood matrix responsible for its strong structural strength can somewhat be compared to that of reinforcing bars in concrete, with lignin here playing the role of the hard cement paste acting as the "glue" in between the cellulose fibers The mechanical properties of cellulose in the primary plant cell wall are correlated with plant cell growth and expansion [28]

1.2.1 Introduce cellulose and its properties

Cellulose is a white, odorless, tasteless substance Cellulose is insoluble in water even when heated and in common organic solvents Soluble in some strong inorganic acid solutions such as HCl, HNO3, some salt solutions: ZnCl2, PbCl2, Is the main component that makes up the plant cell membrane that helps plant tissues have mechanical strength and elasticity Cellulose is abundant in cotton (95-99 %), jute, hemp, bamboo, wood, (cellulose contains 40-45 % in wood) [13]

Cellulose is a complex and stable compound, insoluble in water and in many other organic solvents, dilute alkaline solutions are also ineffective, only decomposed by heating to react with concentrated acids or alkalis

Reacting with inorganic or organic acids will produce esters (esterification reactions) For example, by heating cellulose in a concentrated sulfuric acid solution, concentrated nitric acid yields cellulose nitrate

Cellulose is made up of chains of ꞵ-D-Glucose linked together by 1,4-Glucoside bonds, which are unstable to acids Under the action of acids, cellulose will be hydrolyzed to produce products with poor mechanical strength but insoluble When cellulose is completely hydrolyzed, the final product is glucose

When immersed in a polar liquid such as water, dimethylforamide, dimethylsulfoxyde, tetrahydrofuran, pyridine, and cellulose fibers will swell The hydroxyl groups in the cellulose chain of the expanding filament can still be used for other chemical reactions, but the polar solvent molecules will be trapped in the cellulose structure In a non-polar environment, such as benzene, toluene, gasoline, etc., the hydroxyl groups return to the inside of the cellulose chain structure These non-polar solvent molecules have the ability to gradually replace the polar solvent molecule trapped inside the cellulose fiber bundle, changing the environment from polar to less

15 polar As a result, it creates and maintains a non-polar environment within the expanding cellulose fibers [13]

Hemicellulose is a carbohydrate located adjacent to the cellulose wall Insoluble in water but soluble in hot or cold dilute alkali Is a branched chain polymer with low polymerization from 70-200 monomers, short chains of about 500-3000 sugar units, mainly in the form of amorphous, a few straight chains, can crystallize, creating adhesion between cellulose and lignin Hemicellulose is divided into 2 types:

- Homopolymer: Consists of a base unit that easily separates during heating

- Copolymers: Consists of many types of basic units such as xylan and mannose

The hemicellulose composition depends on the species, growth rate, plant type, and location in the cell [13]

Hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in most terrestrial plant cell walls Cellulose is crystalline, strong, and can be hydrolyzed Hemicellulose is branched, shorter in length than cellulose, and also tends to crystallize They can be hydrolyzed by dilute acids or bases as well as by numerous hemicellulase enzymes [29],[30]

Lignin: In nature, lignin is a substance found mainly in the intermediate layer between cell walls, responsible for the adhesion of fiber walls The composition of lignin consists of 62-65% C, about 5-6% H, and many free methoxyl (-OCH3) and hydroxyl (-OH) groups Lignin is synthesized from hydroxyl phenyl propane, and has a complex structure, the bonds being mainly C-C bonds and ether bonds In plants, lignin acts as a special adhesive, binding cells together, increasing the stiffness of cellulose fibers,

16 increasing the mechanical strength of cells, stabilizing cell size, and repelling insects, water repellants, and external microorganisms such as molds and fungi [13]

Triboelectric Nanogenerator – TENG and applications

1.3.1 Four fundamental principle modes of Triboelectric Nanogenerators

The first mode, shown in Fig 1.11a (Zhu et al., 2012; Wang et al., 2012), is the vertical contact separation mode Two dielectric films face each other, and electrodes are attached to the top and bottom of the stack Physical contact between the two dielectric films creates oppositely charged surfaces A potential drip is created when the two surfaces are separated by a small gap due to an external force If the two electrodes are electrically connected by a load, free electrons in one electrode will flow to the other electrode and create an opposite potential to balance the electrostatic field Once the gap is closed, the potential generated by the triboelectric charge disappears, and the electrons flow back (Niu et al., 2013)

The lateral sliding mode structure has the same initial state as the vertical contact separation When two dielectric films are in contact, relative sliding parallel to the surfaces also generates triboelectric charges on the two surfaces, as shown in Figure 11b (Wang et al., 2013; Zhu et al., 2013a) This introduces lateral polarization along the sliding direction, causing electrons to flow at the top and bottom electrodes to fully balance the field generated by the triboelectric charges By periodically expanding and closing, an AC output is generated This is the TENG gliding mode Sliding can be planar motion, cylinder rotation, or disk rotation (Jing et al 2014; Lin et al 2013)

Both of the above modes have two electrodes connected together through a load This TENG can move freely and be used for mobile shells In some cases, an object part of a TENG cannot be electrically connected to a load because it is a moving object, such as a person walking on the ground A single-electrode TENG is introduced to harvest energy in this situation, where the electrode at the lower part of the TENG is grounded,

17 as shown in Figure 11c If the size of the TENG is finite, the approach or distance of the upper object from the lower object will change the local electric field distribution, so electron exchange will occur between the lower electrode and the ground to maintain the electrode potential change This energy harvesting strategy can adopt contact- separation mode and sliding mode (Yang et al 2013; Zhang et al 2013; Niu et al 2014a)

Figure 1.11 Principle modes of Triboelectric Nanogenerators[9]

The separated triboelectric layer pattern is shown in Figure 1.11d (Wang et al., 2014a) A moving object is naturally charged due to its interaction with air or other objects (such as Our usually charged shoes are placed on the ground The charge will remain on the surface for hours, during which no contact or friction is required because the charge density reaches a maximum value If we place a pair of symmetrical electrodes below a dielectric layer, and the size of the electrodes and the distance between the two are of the same order of magnitude as the size of the moving object, the approach and/or distance of the object from the electrodes will create an asymmetric charge distribution in the medium, causing electrons to flow between the two electrodes to balance the local potential distribution Electrons between the paired electrodes Oscillations generate electricity The moving object does not have to directly touch the upper dielectric layer of the electrodesso that, in rotation mode, free rotation is possible without direct mechanical contact; the wear of the surfaces can be drastically reduced This is a good approach for extending the durability of the TENGs Such a design has

18 shown the potential for harvesting energy from a freely moving object without an electric connection

This is the most common structure of TENG It can be divided into two groups: 2D and 3D Professor Wang's group presented a flat-shaped TENG in simple 3D, where two different friction layers are separated by a spacer, as shown in Figure a This structure is suitable for vertical operations based on contact-separation mode and has higher output as shown in Figure b,c To save space, the Peking University team proposed a large area TENG with distributed buffers, as shown in Figure 12d With a simple manufacturing method, this device has a very low cost of $2/m 2 , showing great commercial potential in the market This large area TENG can be used as a smart home mat to collect energy from human walking [31] [32]

The important element of 3D flat-shape design is the spacer; it should be reliable and stable for multi-cycle operations to separate the friction layers; without the buffer layer, the generator can not work

Figure 1.12 Fabrication of flat TENG and the outputs a) 2D structure TENG b) Output voltage of 2D TENG c) Output current of 2D TENG d) Structure of 3D TENG e) Output voltage of 3D TENG f) Output current of 3D TENG [33]

The arch structure is one of TENG's most developed structures to achieve full contact and separation when pressed and released to improve charge transfer efficiency The arch-shaped TENG from Professor Wang's group improved the performance significantly, as shown in Figure It has very high power, around 3.56 mW/cm 2 and 128

19 mW/cm 3 , with an open circuit voltage (VOC) of 230 V and a short circuit current (ISC) of 0.13 mA, respectively [34]

In general, pairs of arches, inverted shapes, or stacked structures are among the most used designs of TENG, as they can improve power transmission efficiency and achieve high output

Figure 1.13 Arch-shaped structure TENG and its output [35]

Thanks to the advantages of TENG devices, such as lightweight, simple construction, low cost and wide selection of materials and structures, TENG has been widely applied as a micro-power source for self-sufficient systems powered by capturing biomechanical or ambient energy, such as human walking, heart rate, machine vibration, and wind energy Because of its outstanding performance at low frequencies, biomechanical energy harvesting using TENGs is of great importance and has been explored at an early stage of TENG development with a smart backpack [36][37], smart, self-powered meter and acoustic energy receiver [38][39][40] Shape-adaptive devices use elastic and deformable materials so they can be applied to the human body more naturally and effectively

A tubular, silicon TENG has been developed to capture electrical energy from human walking This device can be installed under shoes or on clothing to power electronic wearables such as watches and heart rate monitors The highly stretchable TENG consists of conductive liquid and soft rubber, with conductive liquids enclosed

20 in soft rubber The device can withstand tension up to 300% without obvious deterioration, even after a strain cycle of almost 55K

Optimal distortion with maximum electrical power exists due to the conflicting effects of contact area and electrode resistance As strain increases, the increased contact area results in more tribunate charge and higher output, while the increased length and reduced cross-sectional area of the stretched liquid electrode produce higher resistance and low output than This stretchy TENG can easily conform to uneven surfaces and is proven as a shoe insole and bracelet to capture energy from human walking and manual manipulation The stretchability of TENG is further improved by using an ionic conductor as the electrode [41]

TENG has a sandwich-like structure with an ion hydrogel layer enclosed between two elastic membranes The ionic hydrogel consists of polyacrylamide and lithium chloride; the elastomers used are PDMS and very high bond (VHB) Both PDMS and VHB-STENG have high transparency, stretchability, and mechanical strength of STENG can be adjusted by adjusting the combination of material components The maximum power density of 35 mWm −2 was achieved by using nylon to isolate contact with the PDMS-STENG and it has been shown to power an electronic watch under hand touch operation The sandwich structure significantly reduces the water loss of the ionic hydrogel and achieves long-term stability of the electrical outputs even after storage at 30% relative humidity at 30°C for one month Furthermore, the drag of the elongated hydrogel has a much smaller increment; thus, the performance degradation in the stretched state is less of a concern [42].

High-order porous structure polymeric materials

The honeycomb porous polymer material consists of hexagonal pores arranged in a honeycomb shape formed by evaporating the solvent and condensing water vapor in air humidity The formation of highly ordered porous structures has been studied by different methods since 1994, leading to many applications in areas such as mold making, membrane filters, and energy-absorbing materials

Phase separation is another solution-based technique used to create a highly porous structure For polymer solutions, the phase separation technique is based on solvent crystallization, the different processing steps are illustrated in Figure 15 The solution is prepared in a container and then placed in a freezer The decrease in temperature causes the solvent to crystallize and freeze, creating a solid-liquid phase separation This solidified mixture is transferred to a freeze dryer, where the solvent is removed by vacuum The result is a highly porous foam-like structure This technique is not limited to polymer solutions

Figure 1.15 Conventional phase separation mechanism [44]

Regarding the porosity of the structure, the pore size and shape themselves can largely be controlled Still, the pores are not entirely interconnected and often create long zigzag paths, which causes inconvenience, and in beneficial for several biomedical applications such as signal or nutrient release, flow in bio-composite structures [44]

Phase separation occurs based on a temperature change (TIPS method), the addition of a certain amount of non-solvent substance to the polymer solution (NIPS method), or vapor phase separation (VIPS) According to the crystallization temperature of the solvent used, it can be divided into the solid-liquid phase and the liquid-liquid phase Solid-liquid phase separation, the crystallization temperature of the solvent used is higher than the liquid-liquid phase separation temperature [44]

The NIPS method is a three-component system consisting of polymers, solvents, and nonsolvents The process begins by mixing polymers and solvents to form a homogeneous solution The polymer solution forms a thin film on a flat substrate by casting The sample is placed in a cryogenic bath containing nonsolvent to initiate phase separation and precipitation in the polymer solution

VIPS method: This is an essential technique for synthesizing porous structural films Unlike NIPS, the nonsolvent of VIPS is vapor and exists in a volatile solution The main advantage of membranes prepared by the VIPS method is the simple controllability of the shape

TIPS method: similar to the NIPS method, a homogenous polymer solution is made from a solvent (high boiling point liquid, low molecular weight) called high- temperature diluent [45]

The formation of breath figures (BF) was first studied in 1893 by Aitken, and in

1911 by Lord Rayleigh Depending on the wetting properties of the surface, condensed liquid can form either a uniform, dark film (black BF) or a collection of small, light- scattering, and white droplets (grey BF) The BF pattern is formed when breathing on a cold surface and is named after the method of creation

In 1994, FranÅois et al discovered the formation of an ordered polymeric porous film when a star-shaped drop of polystyrene (PS)/carbon disulfide (CS2) solution was exposed to a stream of moist air The micrometer-sized pores exhibiting a very regular hexagonal arrangement after removal of the upper layer of these membranes are known as honeycomb structures Therefore, the ordered holes in the polymer film prepared by such a method are named breath-shaped arrays (BFA) Adjustable shapes and sizes of condensate droplets are useful in controlling samples This method is cost-effective and widely applicable in creating porous membranes [46]

An emulsion is two heterogeneous solvents mixed together in one phase as droplets dispersed in the other phase Furthermore, emulsions are mainly made up of three parts: continuous phase or outer phase, dispersed phase or internal phase (aqueous solution), and surfactant For example, an emulsion can be developed by dispersing water droplets in the oil phase, known as a water-in-oil (W/O) emulsion, or by dispersing oil-in-water droplets, called an emulsion oil in water (O/W) The emulsion method is a versatile technique for preparing highly porous organic polymers with droplets that have an average size of a few àm and a reasonably wide size distribution

Two mutually insoluble liquids are stirred to disperse one liquid in another in the form of drops An emulsion comprises three parts: a continuous phase - outside, a dispersed phase - inside, and a surfactant Emulsifying molecules coat the surface of the dispersed droplets (reducing interfacial tension) to create repulsion between the droplets and cause them to remain suspended in the dispersion medium For synthesizing porous polymers, it is possible to polymerize monomers in a continuous phase - used to lock the emulsion structure and to remove the dispersed phase - to form a porous polymer with a size of micrometers [47]

1.4.2.4 Improved Phase Separation method (IPS)

The BF method is a popular way to make honeycomb polymeric films, but it requires specific polymers and equipment for high humidity and precise humidity control, limiting its use for large-scale applications Phase separation is an alternative technique that uses precipitation, solvent-free vapor, or solvent evaporation to create porous films, but it still faces challenges in producing structures with high porosity and controllable pore sizes

In the methods for creating porous films mentioned above, it is clear that there are both advantages and disadvantages However, IPS's advanced phase separation method can overcome most of the disadvantages of the above methods IPS can be synthesized on a large scale without requiring moisture or surfactants and does not need polymers It must have a special construction and be able to use industrial coating equipment With these advantages, IPS can scale up production and commercialize with low processing costs while still achieving the desired high-order porous film properties

In the past few years, a new method, the "Improved Phase Separation Method" (IPS), has been developed to modulate highly ordered honeycomb light from many polymeric materials [48].The improved phase separation method combines NIPS and

BF methods, offering advantages like easy industrial expansion, air-formed honeycomb structure with no surfactants, and controllable membrane morphology by solvent ratios

Improved phase separation can lead to increased process efficiency, time-saving, and improved product quality This is especially essential for large-scale industrial processes where time savings can lead to increased productivity and economic growth Additionally, better stage separation can lead to the production of higher-quality final products at a lower cost and on a large surface Improving the phase separation process can be challenging and improvements may not always be successful, leading to potential defects

Factors that affect the structure of a polymer include its concentration, solvent/nonsolvent ratio, and environmental humidity The concentration of polymer determines the thickness and thinness of the film when coating The formation of the pore structure and size is determined by the solvent/nonsolvent ratio Depending on the type of polymer, select the corresponding ratio pair The IPS method is a combination of two traditional methods of phase separation and BF, so the decisive humidity is also a good factor in determining the formation of the film's structure The higher the humidity, the larger the pore size, and the foam increases

Figure 1.16 Honeycomb filmmaking with IPS method [50]

Introduction convex structure films

The concept of convex structural membranes is no longer strange to scientific researchers in recent years Thanks to the published results, scientists have contributed to promoting the research and application of the properties that convex membranes bring Compared with flat-structured films, convex films have many advantages beyond expectations such as contact area, mechanical strength, optical transmittance, and electrical properties

The external structure depends on the needs of the user, it can have the structure of the parent material or it can be changed depending on the technique and production conditions For example, with the same fusing method, we can change the shape and structure of the periphery just by changing some factors like temperature, time, pressure, etc is an important factors in evaluating the success of a study

Figure 1.17 SEM image of convex PDMS boundary region a) Surface SEM image and b) Cross-sectional SEM image [8]

To obtain a film with a convex structure, we can proceed in two ways:

Direct fabrication: Create convex films directly by methods such as photolithography, 3D printing, thermal evaporation, sputtering, For these methods, the accuracy is relatively high and the products are homogeneous On the downside, requires a high cost for equipment, requires experience for workers, high risk of occurrence

Indirect fabrication: Through the use of a mold with a concave structure, then proceed to place the polymer material on the mold surface, spread it evenly, vacuum, and finally separate the film from the mold by using Use suitable solvents or can be directly dissected to obtain a film with a micro-convex structure In terms of advantages, the method is simple, easy to implement, and significantly saves production costs

Figure 1.18 Principle of convex membrane fabrication a) Direct manufacturing, b) Indirect manufacturing [51][52]

Convex structural polymer membranes have become an important part of our daily lives and are widely explored from both a fundamental point of view to an application point of view Polymer materials have controllable morphology, surface adhesion and are easy to process on large areas Convex structures are increasingly being used in technological applications such as transistors, lithium-ion batteries, supercapacitors, pressure sensors, artificial skin, solar cells, organic laser devices, and more mechanics and many other fields [53].

Biocomposite materials and films

A biocomposite is a material composed of two or more distinct constituent materials (one being naturally derived) which are combined to yield a new material with improved performance over individual constituent materials [54]

Biocomposite is a composite material composed of a matrix (resin) and natural fiber reinforcement (Fazeli et al., 2018) Polymers produced from sustainable and non- renewable resources constitute the matrix phase The matrix not only protects the fibers from environmental degradation and mechanical damage but also serves to hold them together and transfer stress Biofibers are also major components of biocomposite materials, which are produced from biological entities such as crop fibers (cotton, flax, or hemp), recycled wood, waste paper, agricultural-based by-products, or regenerated cellulose fibers (viscose/rayon) Animals and plants naturally produce high-strength biocomposite materials composed of fibrous biopolymers Cellulose is a prominent example because it consists of whisker-like microfibrils that are synthesized and deposited in a specific way to exhibit considerable strength Biocomposites can also be used in a wide range of products due to their similar mechanical properties [55]

EXPERIMENTAL

Materials and chemicals

Table 2.1 Materials and Chemicals Used

No Name Specifications, properties Origin

Volume 96 pages/book, 60 g/m 2 , usable volume 5 kg

White solid, small granules, density d=2.13 g/cm 3 , purity 96 %, vial 500 g

They are commonly used as detergents, titration alkaline solution

Manufactured in China, purchased at Hoa Nam Chemical

Solution form, concentration 30%, density d=1.11 g/cm 3 , vial 500ml

Manufactured in China, purchased at Hoa Nam Chemical

White granular solid, purity 99%, vial 500g

Manufactured in India, purchased at Hoa Nam Chemical

Single-distilled water, usage volume 30 liters

Materials Technology Laboratory, Ho Chi Minh City University of Technology and Education

Transparent granules, M ,000 g/mol, d40 kg/m 3 Ningbo,China

Molecular Weight: 119.378 grams/mol Boiling temperature: 61℃

Molecular Weight: 32.04 grams/mol Boiling temperature: 64.5℃

Equipment and laboratory instruments

Table 2.2 Equipment and Laboratory Instruments

No Name Specifications Model, Origin

1 Magnetic stirrer Maximum 1200 rpm, heating up to 200ºC IKA C-MAG HS 7, India

2 Overhead Stirrer From 60 to 2000 rpm IKA RW 20 Digital, India

3 Oven UN30 Up to +300ºC Memmert, Germany

4 Spin coating machine From 500 to 2000 rpm Vietnam

5 Analytical balance Maximum 220g/0.00001g Labex balance, HC-JF2204,

6 Becher 250 ml, 100 ml, 50 ml Onelab, China

7 Pipette 10 ml / 0.05 ml Isolab, China

8 Alcohol thermometer Maximum 100ºC/1ºC China

Experimental process

Using chemical treatment of materials to help remove certain types of composition, structure, properties, etc., present in the material From there, offer processes, solutions, and improvements to suit the most desired materials and products The most optimal method for processing this fiber is the hydrothermal method The hydrothermal method is known to be environmentally friendly because when it is used, there is no need for a catalyst, low corrosion, high energy efficiency, etc [56] The hydrothermal treatment process can increase the surface area if treated by chemical methods combined with hydrothermal will help shorten the processing time [57] The use of the hydrothermal method combined with the chemical treatment method helps to remove part of the lignin, wax, and oil from the outer surface of fiber cells [58]

Figure 2.1 Scheme of waste paper treatment processing

The process of making cellulose pulp from waste paper is carried out according to the steps below:

Explain the experimental procedure for processing waste paper to obtain cellulose:

Step 1: Weigh the paper, wash and soak it in hot water for 30 minutes to remove surface dirt and to soften and drain the paper.;

Step 2: Proceed to bring the paper soaked in hot water to be treated with 10 wt% NaOH solution for 4 hours at 70 ºC, to remove the hemicellulose in the paper Until a green paper mixture is obtained Then filter and wash with clean water several times to remove excess solution;

Step 3: Treat the paper with 5 wt% H2O2 solution for 2 hours at 80 ºC to bleach, remove filler and part of lignin, then filter and wash with distilled water to pH 7, obtain a white opaque paste sample;

Step 4: Dry the sample at 60 ºC for 24 hours to remove water in the sample, dry the sample;

Step 5: Dry the sample at 100 ºC for 1 hour to completely dry the sample, prepare to grind the sample with the ball mill to make cellulose pulp;

Step 7: Use a sieve of size 1mm to floor the pulp sample, and obtain a fine pulp of size 1mm

2.3.2 Concave Polystyrene thin film making process

Figure 2.2 Scheme of making concave PS thin film

Explain the fabrication process of PS plastic film with concave structure by advanced phase separation method (IPS):

Step 1: Weigh the PS plastic and use the pipette to get the amount of chloroform solvent needed to make 10 wt% PS resin solution Stir the PS plastic and solvent mixture for about 4 hours at room temperature until completely dissolved;

Step 2: Prepare flat copper pieces and clean the surface thoroughly, then attach the copper base to the spin coating machine, and set the spinning speed accordingly Next, drops of PS plastic solution are applied and then spin-coating into the thin film;

Step 3: Use the pipette to get the amount of soluble solvent (chloroform) and insoluble solvent (methanol) in the ratio of 90/10 and stir the mixture for 4 hours at room temperature;

Step 4: Pour the solvent mixture into a glass beaker, then proceed to dip the PS thin film in 3 seconds, get the copper base out of the cup, and let it dry naturally for 10 minutes to obtain a concave structure PS plastic film on copper base

2.3.3 Convex composite film making process

Figure 2.3 Scheme of making convex composite films

Explain the experimental process of making convex composite films

Step 1: Weigh the fine pulp according to the weight of the composite film to be poured; Step 2: Weigh the amount of PVA needed to mix PVA solution with distilled water;

Step 3: Mix PVA weighed with distilled water according to pre-calculated mass percent, stir from heat, and heat at 80 ºC for 4 hours until PVA is completely dissolved in water;

Step 4: Mix the pulp and PVA solution according to the pre-calculated volume together and spread it evenly on the mold surface;

Step 5: After drying at 60 ºC for more than 30 hours, the mixture dried, obtaining cellulose/PVA convex composite film

Table 2.3 Table of symbols for composite film sample names

Sample names Cellulose pulp PVA polymer solution Symbol Composite film with the ratio of

75 % of the solid- weight mass

25 % of the solid- weight mass

Composite film with the ratio of

70 % of the solid- weight mass

30 % of the solid- weight mass

The composite film with the ratio of 65/35-Cellulose/PVA

65 % of the solid- weight mass

35 % of the solid- weight mass

Figure 2.4 Scheme of making the TENG device

Explain the TENG-making process:

Step 1: Prepare two 30 mm x 30 mm pieces of mica plastic, clean the surface

Step 2: Stick the aluminum tape so that it covers the entire surface of the mica, use gloves to avoid direct contact with the aluminum surface, leading to damage to the surface

Step 3: Place the convex PDMS film and the convex composite film on the aluminum surface, which can be fixed with double-sided tape

Step 4: Use aluminum tape to glue the copper wire to the side of the aluminum base to connect to the measuring device

TENG operates in vertical contact separation mode based on a combination of friction and electrostatic induction effects In the initial state, on the surface of the composite film, PDMS, and aluminum, there is no movement in charge and the difference in potential between the two composite film and PDMS electrodes When two films have direct contact with each other, causing friction, opposite electrostatic charges will be formed on the surface of the two polymer films A negative charge will be formed on the PDMS film because PDMS material has many electrons in the

33 outermost layer, which can attract electrons from other materials to create a negatively charged material As for the positive charges that will be formed on the composite film, in contrast to PDMS, the composite film material has few electrons in the outermost layer, which is likely to be attracted by other materials, so the composite film becomes an integrated material Positive electricity This contact process when on the surface of two dielectric layers in contact with each other and increases the contact surface between the dielectric layers Separation occurs when no more force is applied, the two surfaces are far apart, and there is a potential difference The contact separation process continues to repeat until saturation is reached after several contact separation cycles

Figure 2.5 Contact-separation mechanism of TENg device

Characterization

The shape, structure, and uniformity of the film surface structure were observed by optical electron microscopy Samples were observed at 50X and 100x magnification due to the microscopic size of the sample structure Thus, we know the structure and morphology of the sample before taking SEM

Figure 2.6 Computer system and optical microscope in

The composite film surface analysis method is measured by scanning electron microscope SEM, the instrument code is TM4000, Japan with different magnifications The prepared sample must be a clean and dry sample, cut with a specialized knife according to the specified size of the SEM machine The applied voltage was set up at

2.4.3 Fourier transform infrared spectroscopy (FTIR)

The infrared spectroscopy method based on the principle of selective absorption of infrared radiation of a molecule measures the natural vibrational frequency of the bonds in the molecule by the frequency of the incident radiation and causes the variation their dipole moments When absorbing infrared radiation, the vibrational and rotational motions of the molecules are excited with different frequencies and emit a spectrum of infrared radiation absorption The different spectral peaks present in the infrared spectrum correspond to the characteristic and bonding functional groups present in the chemical compound [59]

The FTIR analysis method was measured with an IR spectrophotometer, the Thermo NICOLET 6700, USA The sample to be prepared shall be a clean and dry

35 sample, chopped sufficiently for measurement Measurement parameters are wavenumber range from 400 – 4000 cm -1 , at a spectral resolution of 4 cm -1

An oscilloscope is a laboratory instrument commonly used to display and analyze the waveform of electronic signals In effect, the device draws a graph of the instantaneous signal voltage as a function of time

Figure 2.7 Oscilloscope at HCM City University of Technology

Tensile strength, vertical strain, and elastic modulus for each material were measured at room temperature with a Tension testing machine at the Consumer Goods laboratory, at the Metrology Standards Engineering Center Quality 3, according to ASTM D882-02

Figure 2.8 Tension testing machine Consumer Goods Lab, QUATEST 3

Tensile samples are machined and shaped with dimensions according to ASTM D882-02 standards as shown in the figure and table below:

Figure 2.9 Prepare tensile test specimens according to ASTM D882-02

Table 2.4 Tensile gauge sample preparation parameters

Measure the width (WO) and thickness (T) at many points on the waist area of the sample, and then take the average value Next, start-up and set the device parameters according to the measuring standards and the sample size (length x width x thickness), pulling speed of 12.5 mm/min Place the sample in the two clamps and measure the sample Record data: Tensile force at the time of yield or break, module/slope, yield tensile stress, tensile stress at break, and data processing

Tensile strength is the maximum tensile effect of the specimen during tensile measurement When the extreme occurrence occurs at the stress point (the first point on the power-strain curve at which there is an increase in strain without an increase in strain without an increase in stress), then Tensile strength is recorded at the yield point When extreme application occurs at the point of observation, the tensile strength is recorded as elongation

Tensile strength, 𝜎𝑚 (MPa) calculated according to the following formula:

A: Cross-section at waist of sample A = W × T (mm 2 )

𝜎𝑏: Deformation (%) l: Sample length after deformation (mm) l0: Initial length (mm)

The modulus of elasticity is the strain ratio between the tensile stress at break and the strain at break

The modulus of elasticity (MPa) is calculated using the formula E = 𝜎 𝑚

Thermal gravimetric analysis (TGA) is a group of techniques that study the change in properties of materials with temperature when a sample is heated under specified conditions

Figure 2.10 Thermal analyzer at the Ho Chi Minh City University of Natural Sciences

The thermal properties of all experimental product samples were analyzed using a STA Pt 1600 analyzer (Linseis, Germany) at the Ho Chi Minh City University of Natural Sciences The samples were dried before performing the tests Then, 10-15 mg of the sample was used for analysis in an inert argon atmosphere and heated from room temperature to 700 ºC at a heating rate of 10ºC/min

2.4.8 The moisture content of cellulose pulp

The moisture content of cellulose pulp can be tested and calculated according to ASTM D1348-94 (Standard Test Method for Moisture in Cellulose) These test methods include the determination of moisture in cellulose using a two-stage oven-drying process and a Karl Fischer procedure

Sample preparation: Weigh five cellulose pulp samples with a weight of 1 g (weighing accuracy 0.00001) then put the samples in the drying oven for 24 hours at 50ºC, then take the samples out and put them into the desiccator for 30 minutes and reweigh the mass

The moisture content of cellulose pulp is calculated according to the formula:

M: Moisture content (%) m0: Initial weight (g) m1: Sample weight after drying (g)

Water absorption tests are performed according to ASTM D570–98 with custom sample and water volume The 20 mm x 20 mm samples were pre-dried in a vacuum oven at 50 ºC for 24 hours and then cooled to room temperature in a desiccator before weighing their initial dry weight These samples were immersed in 80 ml of distilled water at room temperature for 24 hours The test specimens are removed from the water and excess water is gently removed by wiping with a clean cotton cloth Immediately, the final sample mass is recorded with an analytical balance Three samples were tested for all three materials and the mean represents the water absorption property The water absorption property can be calculated according to the following equation:

Contact angle measurement is a qualitative way to evaluate whether the surface has a hydrophobic or hydrophilic characteristic It is based on the observation of the intermolecular interactions between the surface and a small drop of water when the drop meets the surface

Sessile drop measurements are most often done with water If the water contact angle is lower than 90 degrees the surface is said to be hydrophilic and if the contact angle is higher than 90 the surface is hydrophobic Hydrophilicity and hydrophobicity are important in many applications

The typical drop size for contact angle measurements is 1 to 10 μl However, in recent years, the interest in picoliter droplets has increased due to the need to measure small micropatterned areas or otherwise small objects

Sample preparation: Prepare a sample of 5 × 5 (mm) size and stick it on a flat surface Number of samples: Four samples - one convex complex sample and three flat complex samples with different proportions Measuring equipment: Contact angle meter (SEO, Korea) at the Ho Chi Minh City University of Technology

Figure 2.11 Computer system and contact angle meter, SEO, Korea

RESULTS AND DISCUSSION

Characterization of cellulose pulp treated from waste paper

3.1.1 Morphology structure of cellulose pulp

These are SEM images of pulp samples treated with 10 wt% NaOH solution for

4 hours at 70 ºC and 5 wt% H2O2 solution for 2 hours at 80 ºC Observing the SEM image, we can see that the cellulose pulp sample has unequal sizes, and tends to clump together, the average size is 42.47 àm

Figure 3.1 SEM image of cellulose pulp after treatment a) Magnification x500; b) Magnification x1000

3.1.2 Evaluation of functional group composition in cellulose pulp by FTIR spectroscopy

FTIR spectroscopy results of waste paper and treated cellulose pulp from waste paper

Figure 3.2 FTIR spectra of cellulose pulp and waste paper

Figure 3.2 shows that the FTIR spectroscopy results of the cellulose from the waste paper sample show the appearance of characteristic peaks for the bond vibrations The absorption peaks in the range 3000-3700 cm -1 are typical for the stretching vibrations in the O-H groups of cellulose, hemicellulose, and lignin; the peak range between 2800 cm -1 and 2900 cm -1 corresponds to the symmetric and asymmetric stretching vibrations of C-H bond of the cellulose, the peak at about 1643 cm -1 corresponds to bending vibration of O-H group in adsorbed water molecules [60] Although the FTIR measurement sample is dried, the water adsorbed in the cellulose molecules is difficult to remove completely, due to cellulose and water interactions Meanwhile, the peaks that appeared between 1427 cm -1 and 1376 cm -1 in the FTIR spectrum are the bending vibrations of the CH2 group, and the bending vibrations of the C-H and C-O groups in the polysaccharide, respectively According to Mincheva et al, the absorption peak between 1161 cm -1 and 1063 cm -1 corresponds to the C-O relaxation vibration and the C-H transverse displacement vibration in the pyranose ring framework The peak ranges from 900 cm -1 to 896 cm -1 in the spectrum corresponding to the glycoside bonds (corresponding to the strain fluctuations of C-H in polysaccharides, representing the glucoside bonds between glucose units in cellulose) [61]–[63]

After treating waste paper with NaOH and H2O2 solutions, the sample has a small change in composition and properties, we see that the absorbance peak for cellulose groups such as OH, C-H, and C-O-C groups increases due to the substance The fillers and additives in the paper were removed and the moisture content of the sample was also increased Cellulose pulp from waste paper can be used to make composite films if the moisture content is acceptable

Table 3.1 The number of characteristic wavelengths of functional groups present in natural fibers [64]

3337 Stretching O – H groups on cellulose molecules

1620 OH of water Water absorption of cellulose

1526 C = C in the aromatic ring Lignin

1420 Asymmetric C – H2 bending vibration Cellulose, lignin

1153 - 1056 C – O – C stretch Typical for pure cellulose

Figure 3.3 FTIR spectra of two cellulose pulp samples treated with different concentrations of NaOH solution

Treating waste paper samples with the concentration of NaOH solution reduced from 20 wt% to 10 wt% The FTIR results of both Ce1 (20 wt% NaOH concentration) and Ce2 (10 wt% NaOH concentration) cellulose pulp samples showed that the same cellulose pulp results can be achieved with a 10 wt% NaOH solution, which helps reduce both chemical usage and environmental pollution

3.1.3 Moisture content of cellulose pulp

After testing the pulp sample after treatment with ASTM D1348-94 standard and calculated the moisture content in the pulp is 2.51 ± 0.26 % We can conclude that the input material of the composite film has a slight hygroscopic ability, which can cause the composite film to become water absorbent, reducing the strength of the film when in contact with water, solution, or when operating in a high humidity environment, however low hygroscopicity may not greatly affect the durability of the composite film This is nonetheless a common hygroscopic property of cellulose [65]

Figure 3.4 Moisture content chart of cellulose pulp sample after treatment

Characterization of the concave Polystyrene thin film

We can see in Figure 3.5 that the concave structure of PS plastic film is evenly coated on the copper substrate with very high uniformity, the hole size is measured and calculated using the ImageJ application

Figure 3.5 SEM image of concave PS film surface with magnifications a) x1000k b) x3000

Figure 3.6 Size distribution of concave PS film

Figure 3.6 shows that a concave PS plastic film has been successfully fabricated to be used as a mold for a convex composite film, the concave film has achieved a relatively uniform size of pores, and the average diameter is 2.08 ± 0.14 àm The PS concave will decide the quality of the convex structure die The height of the convex structure will depend on the holes of the PS die, if the holes of the PS are uniform in size, the deeper the concave dimensions, the higher the height of the convex structure.

Characterization of the composite films from cellulose and PVA

Composite film samples with Cellulose/PVA ratios of 75/25, 70/30, and 65/35, respectively, using 12 wt% PVA solution gave better film forming results than samples with a ratio of 90/10, 80/ 20, 85/15 (Appendix 4) Use the manual pressing method by applying thick tape and pressing at room temperature by hand force, then fixed and air dry in the oven

Figure 3.7 Actual photos of composite films of the ratios a) 75/25, b) 70/30, c) 65/35

Figure 3.7 illustrates that a composite film of cellulose and PVA can be easily molded to a thickness of about 0.500 àm while maintaining good bonding between the matrix and reinforcement With the method of manual pressing and heat drying to create the film, PVA resin is quite evenly distributed in the film However, because the cellulose used is a cellulose pulp from waste paper, the size is still quite large, causing foam in the film to appear The gas causes the surface structure of the film to not achieve a high level of flatness

Figure 3.8 SEM images of the flat composite surface with different ratios a) 75/25 b) 70/30 c) 65/35

Through the SEM image of the composite film sample, we can see that the PVA resin solution can do a good job as a binder of cellulose pulp together, but there are still gaps on the film surface With the ratio with the most PVA solution, the ratio 65/35, we can see that the composite film has a lot of PVA to bind to make the film more seamless than the sample with the least PVA solution, which is the ratio of 75/25

The fabricated convex structure composite films are 30 mm x 30 mm in size, with the average thickness of different ratios are 0.42 ± 0.03 mm (65/35), 0.44 ± 0.02 mm (70/30), and 0.47 ± 0.02 mm (75/25), these thicknesses of convex structure composite film are suitable for making TENG devices Surface with convex structure is fabricated by the indirect method using concave structure PS plastic mold, vacuum to fill concave positions to form a composite convex film

Figure 3.9 Actual and SEM images of composite convex film sample a) 75/25 b) 70/30 c) 65/35 d) SEM x2000 of 75/25 e) SEM x2000 of 70/30 f) SEM x2000 of 65/35

Table 3.2 Average thickness of composite films

Figure 3.10 SEM images (a-c) and convex structure size distribution histograms (d-f) of composite films at different ratios a and d) 75/25, b and e) 70/30, c and f) 65/35

Figures 3.9, and 3.10 show that the composite film with a convex structure has been successfully fabricated thanks to the indirect method of manually pressing the composite with a concave structure PS thin film mold The convex structure of the composite film has a slightly rounded half-spherical shape and depends heavily on the concave shape and size of the PS plastic film; we can see a fairly uniform size distribution over the entire convex film surface, with the film proportional to 70/30 has an average diameter of the smallest convex structure of the three scales of 2.23 ± 0.12 àm d) e) f)

Figure 3.11 Cross-sectional SEM images of convex composite film at 70/30 ratio with different magnification a) x500, and b) x1000

Figure 3.11 shows the combination of cellulose pulp from waste paper and PVA polymer binder In addition, the cross-sectional SEM image also shows that the convex structure of the film appears quite uniform on the entire surface, but the surface of the convex composite film still has some roughness due to the manual heat-pressing-drying method using light and uneven force

Figure 3.12 FTIR spectra of composite film samples with different ratios

Figure 3.12 shows that the large bands observed between 3550 and

3200 cm − 1 are linked to the stretching O-H from the intermolecular and intramolecular hydrogen bonds The vibrational band observed between 2840 and 3000 cm − 1 refers to the stretching C-H from alkyl groups and the peaks between 1750-1735 cm − 1 are due to the stretching C=O and C-O from the acetate group remaining from PVA [66] the increase in intensity occurs in the amorphous samples, compared to the original samples, the absorption band at the peak 1734 cm −1 , ascribed to the C=O bending vibration in cellulose/PVA films Absorbance at 1661 cm -1 corresponds to an acetyl C=O group, which could be explained on the basis of intra/inter molecular hydrogen bonding with the adjacent OH group In addition, the corresponding oscillatory binding of C=O can be seen at the 1259 cm −1 peak [18] The sharp band 1090 cm -1 corresponds to the C-O stretching of acetyl groups present on the PVA backbone The peak at 840 cm -1 refers to the stretching of the C-C bond [67] The FTIR spectroscopy result shows that the composite film samples have a combination of cellulose and PVA bonded together to form a thin film

Water absorption of the convex composite film

PVA-based materials have a disadvantage in certain applications due to the high water absorption and solubility caused by their hydroxyl groups (-OH) The mechanical strength of both pure PVA and nanocomposites decreases as a result of water absorption The introduction of nano-fillers affects water absorption differently, depending on various factors The presence of gaps in the nanocomposite films increases the diffusion of water, affecting water absorption The microstructure and interface between matrix and reinforcement are significant factors that influence the barrier properties of nanocomposites Basically, both PVA and cellulose are polar and hydrophilic [68]

Figure 3.13 Water absorption of composite films

After testing and calculating the water absorption of composite convex film at different ratios according to ASTM D570 standard, we obtained the highest average water absorption of 30.62 ± 2.90 % of the convex film ratio 65/35 and then 25.17 ± 2.04

% of the 75/25 ratio convex film, gave the lowest result belonging to convex film ratio 70/30 with an average water absorption of 15.58 ± 2.76 % This was possible due to the hydrogen bonding of the water molecules to the free hydroxyl groups present in the cellulosic cell wall of fibrous materials and the diffusion of water molecules into the microfibrils interfaces From a chemical view, this result could be explained by the differing chemical compositions of PVA compared to cellulose Cellulose is a macromolecule with a compact chemical structure, hence its hydroxyl groups are less accessible compared to PVA Therefore it would be expected to show lower water uptake compared to PVA-containing composites The poor absorption resistance of the cellulosic materials is mainly due to the presence of polar groups, which attract water

51 molecules through hydrogen bonding This phenomenon leads to a moisture build-up in the fiber cell wall (fiber swelling) and also in the microfibrils interfaces Based on this result, we can conclude that the 70/30 ratio convex composite films can be used in a humid environment because its water absorption is at an acceptable level, so after a period of use in an environment with moderate humidity, without losing its structure or the film being destroyed [18].

Thermal analysis results of composite film and PVA film

Figure 3.14 Thermogravimetric analysis of composite and PVA film samples a) TG analysis curve; b) DTG analysis

The TG and DTG curves of the composite and PVA films are shown in Figures

3.14a, and 3.14b The degradation of PVA film and cellulose/PVA composite films was divided into three main weight loss regions The initial weight loss of the PVA film was 8.117 % from 50 ºC to 190 ºC, which is attributed to the loss of weakly bound moisture and absorbed water [69][70][71] The initial mass loss of the composite film was 7.465

% from 50 ºC to 150 ºC with a 70/30 ratio composite film

The primary degradation of the pure PVA film occurred in the second region from 245 ºC to 379 ºC, which was a 54.941 % weight loss This stage involves the separation of monomers and cleavage of bonds in the backbone of the polymer [72] According to Figure 3.14b, the pure PVA films started to degrade earlier than other films, clearly demonstrating that cellulose limits the mobility of the polymer chains

[45] The decomposition of the composite film reduced by 62.289 % in mass at the temperature range of 220 ºC to 404 ºC, we can conclude that the composite film has a lower weight loss rate than pure PVA films (as a result of enhanced cross-linking between PVA and cellulose affecting the mobility of the polymer chain As a result, the composite films clearly show better thermal stability than pure PVA film The third weight loss phase of the pure PVA film starts at about 392 ºC to 519 ºC, during which the film continues to lose 23.733 % by mass, predominating for a small amount of hydrocarbons, that is, separation and segregation destroy carbon materials [73] Pure PVA films have less residue than cellulose/PVA composite films due to the lower carbon material

The temperature range of 550 ºC to 600 ºC can be considered as the complete decomposition of combustible materials and the formation of coal dust Therefore, it can be concluded that the pulp decomposes in different thermal regions depending on the presence of components such as cellulose, hemicellulose, and lignin in the pulp Since the composite film is mainly composed of cellulose, more heat stable than PVA film and, when decomposed at high temperature, will leave more residue than pure PVA film

Water contact angle of composite films

A surface is considered hydrophobic or hydrophilic based on the contact angle between the droplet and the solid surface When the contact angle is less than 90º, we have a hydrophilic surface, greater than 90º, we have a hydrophobic surface When the contact angle is greater than 150º, the surface becomes superhydrophobic [74]

Figure 3.15 Water contact angle of surfaces a) Flat film 65/35 b) Flat film 70/30 c) Flat film 75/25 d) Convex composite film

Contact angle and the wetting behavior of solid particles are influenced by many physical and chemical factors such as surface roughness and heterogeneity as well as particle shape and size [75] The figure shows that the composite films at different ratios with flat surfaces have hydrophobic surface properties The contact angle results have small differences between the ratios of 101.45 º , 102.48 º , and 104.81 º with the sample 75/25, 65/35, and 70/30 ratios

The convex structure composite film with the ratio of 70/30 has a contact angle of 112.37 º , about 10 º bigger than the flat film samples, showing that the convex- structured film is more hydrophobic than the flat films and may have self-cleaning ability and anti-dust properties Hydrophobic substances are usually non-polar substances [76], but in this research, the composite film samples have both flat and convex structures made up of polar materials such as cellulose and PVA, which generally absorb water very quickly ( water contact angle < 90º) [74], but thanks to the bonding forces between the components such as hydrogen bonding, Van der Waals forces and the micro-convex structure, the water droplet is trapped on the surface of the material

Tensile properties of composite films

Figure 3.16 Stress-strain diagram of composite film samples

Figure 3.17 Tensile strength and tensile modulus plots of film samples

Figures 3.16 and 3.17 show that the film samples in all three ratios have relatively poor mechanical properties because they are made from recycled cellulose materials from waste paper and use a small amount of adhesive polymer, the young modulus of the films are shown in Fig 3.28 The 75/25 film is the highest with 247.28 MPa, followed by a 70/30 film with 147.04 MPa, and the lowest is a 65/35 film with a tensile modulus of 72.70 MPa

The 75/25 film displays minimal deformation when pulled due to the lowest amount of PVA present, resulting in reduced flexibility The 65/35 ratio composite films have the highest amount of PVA with the best tensile strain of the three ratios (strain >

3 %) but with the lowest tensile strength (2.38 MPa), because it does not require too much force to deform the film, and the 65/35 ratio film, is unstable with higher force Increasing the PVA content increased the elongation of the samples Specifically, the elongation increases from 1.4 % to 3.1 % (Figure 3.16) The reason can be explained that PVA and cellulose both contain hydroxyl groups (-OH) capable of forming hydrogen bonds As the PVA content is increased, more H-bonds can be formed between the PVA and cellulose chains, increasing the plasticity of the mixture Therefore, the sample elongation will increase linearly

The 70/30 ratio composite film gave the best tensile strength properties of the three samples with the tensile strength of 3.87 MPa and Young’s modulus of 147.04 MPa, Figure 3.17 shows that the 70/30 ratio film requires more force than the 75/25, and 65/35 composite films to deform and be destroyed, but in terms of strain, the 65/35 ratio films have the best elongation of three ratios It can be concluded that the composite film with a ratio of 70/30 has acceptable tensile properties among the investigated samples Concluding that the mechanical properties of different ratios composite films will be different.

Electrical measurement results of composite films

3.8.1 Output voltage of convex composite films

The film sample used for electrical measurement is 3 cm x 3cm and has a thickness of about 0.50 mm, taped up the plastic base covered with an aluminium tape of the same size; the counter electrode of the TENG is a PDMS film attached to the plastic base covered with aluminum tape.The TENG devices with composite convex films operate with the contact- separation mechanism Conduct electrical measurements with frequencies 1 Hz, 2 Hz, 3 Hz,

4 Hz, and 5 Hz at room temperature (± 27 ºC) and in a closed chamber

The output voltage results of 75/25 convex composite film:

Figure 3.18 Output voltage of 75/25 convex composite film indifferent frequency from 1-5 Hz

The positive-negative dielectric layer creates an electrode that uses a convex cellulose/PVA composite film - PDMS film At 1 Hz frequency, the output voltage reaches

132 V The output voltage increases to 148 V, 160 V, 184 V, and 196 V at 2 Hz, 3 Hz, 4 Hz, and 5 Hz, respectively The slight voltage change at higher operating frequencies is the main advantage of TENG when collecting energy from low frequency and irregular motions

The output voltage results of 75/25 vacuumed convex composite film:

Figure 3.19 Output voltage of 75/25 vacuumed convex composite film indifferent frequency from 1-5 Hz

The output voltage of the electrode created by the positive-negative dielectric layer is a convex Cellulose/PVA composite film - PDMS film at 1 Hz frequency reaches

104 V and reaches 148 V, 180 V, 196 V, and 200 V at 2 Hz, 3 Hz, 4 Hz, and 5 Hz The above results showed that the vacuumed convex composite can generate higher voltage signals than the normal convex composite because It has a uniform convex structure over a larger area thanks to the vacuum process

The output voltage results of 65/35 vacuumed convex composite film:

Figure 3.20 Output voltage of 65/35 vacuumed convex composite film in different frequency from 1-5 Hz

The output voltage of the electrode created by the positive-negative dielectric layer is a 65/35 ratio convex composite film - PDMS film at 1Hz frequency reaches 120V and reaches 148 V, 180 V, 196 V, and 216 V at 2 Hz, 3 Hz, 4 Hz, and 5 Hz

The output voltage results of 70/30 vacuumed convex composite film:

Figure 3.21 Output voltage of 70/30 vacuumed convex composite film in different frequency from 1-5 Hz

The output voltage of the electrode created by the positive-negative dielectric layer is a convex Cellulose/PVA composite film – PDMS film at 1Hz frequency reaches

138 V and reaches 168 V, 188 V, 212 V, and 220 V at 2 Hz, 3 Hz, 4 Hz, and 5 Hz

Figure 3.22 Analysis of voltage signal of 70/30 vacuumed convex composite film

Figures 3.18-3.22 demonstrate that the output voltage of the TENG device remains consistent at 200-220 V for composite convex films with various film ratios (75/25, 70/30, and 65/35), indicating that the convex structure composite film can generate voltage without being too dependent on the composite ratio Due to its high output voltage, the 70/30 ratio convex composite film will be chosen to measure other electrical parameters such as capacitor charging speed and durability after 10000 cycles

3.8.2 Compare the voltage difference between two types of electrode

The TENG devices comprise the same components of a 70/30 ratio convex composite, only replacing the convex PDMS electrode and the aluminum electrode to conduct comparative surveys

Table 3.3 Output voltage comparison of electrode changes

Convex film with the aluminum electrode

Convex film with the convex PDMS electrode Frequency

(Hz) Output voltage (V) Output voltage (V)

The table indicates that using two convex films (convex composite-convex PDMS) as electrodes in a TENG device generates a much higher output voltage (approximately 61 times at 5 Hz frequency) than using a convex composite film and aluminum electrode Specifically, the voltage signal at 5 Hz input frequency of TENG with aluminum electrode is 3.6 V, whereas that of PDMS convex electrode TENG is

220 V When paired with a convex PDMS electrode, the convex composite film is suitable for use as a TENG device

3.8.3 Compare the voltage difference between convex and flat composite films

The TENG devices comprise the same components of the convex PDMS film electrode, only replacing a 70/30 ratio flat composite film electrode and the 70/30 ratio convex composite electrode to conduct comparative surveys

Figure 3.23 Output voltage of TENG devices with a) flat composite films, and b) convex composite film Table 3.4 Output voltage comparison of composite film structure changes

Flat composite film with the convex PDMS electrode

Convex composite film with the convex PDMS electrode

(Hz) Output voltage (V) Output voltage (V)

The results indicate that using a convex composite film in the TENG device results in an output voltage signal 3.7 times greater (220 V and 59.2V at 5 Hz) compared to a flat composite film To achieve higher output volts, a convex structure is recommended

3.8.4 Capacitor charging speed of TENG

The TENG device consists of a convex composite film and a convex PDMS film connected in a circuit, as shown in the figure to measure capacitor charging time

Figure 3.24 Circuit diagram of measuring capacitor

Figure 3.25 Capacitor charging time with different operating frequency of TENG

Figure 3.25 shows that the time it takes to fully charge a 10 àF capacitor varies based on the frequency of the TENG device operating Charging times at different frequencies are: 200 seconds at 1 Hz, 198 seconds at 2 Hz, 190 seconds at 3 Hz, 187 seconds at 4 Hz, and 184 seconds at 5 Hz

Figure 3.26 Capacitor charging time with different capacitance at the same frequency

We can assess the power generation capability of TENG in power supply by analyzing the charging time of capacitors with varying capacitance values at the same operating frequency of the device Figure 3.26 shows that the TENG system can rapidly charge a 0.22 uF capacitor (in 18 seconds), but larger capacitance capacitors such as 10 àF, 33 àF, and 44 àF take longer to reach full charge with times of 184 seconds, 196 seconds, and 200 seconds, respectively

3.8.5 Durability of TENG after 10,000 cycles

Operating the TENG for over 10,000 continuous cycles (at 5 Hz for more than 2,000 seconds) gives a positive result in terms of durability The device can achieve a maximum output voltage of 150V and both electrodes remain intact with no damage The power generation efficiency of the convex composite TENG decreases by 31.32 % after more than 10,000 cycles of continuous operation, indicating a potential for energy collection from low-frequency motion over a long period of use

Figure 3.27 Output voltage of TENG after 10,000 continuous cycles

After investigating the electrical properties, we made a model of 65 green LEDs directly connected to the electrodes of the TENG device Using a cyclic oscillator with temperature and humidity control, measured at 5 Hz frequency, we found that TENG provides direct lighting power for 65 LEDs

Figure 3.28 Model of LED lighted by TENG

• The morphology, size, chemical composition, and moisture content of cellulose from waste paper are affected by chemical treatment During treatment from pretreatment, alkali treatment, and H2O2 treatment, components such as fillers, dirt, surfactants, additives, hemicellulose, and part of lignin in the pulp were removed which were identified by FTIR spectroscopy, size average of cellulose pulp reached 42.47 àm, relatively uniform when observed from SEM images, the sample of cellulose pulp has a moisture content of 2.51 ± 0.26 %

• The composite film has been successfully fabricated from waste paper cellulose and polyvinyl alcohol by heat pressing-drying method with the following manufacturing parameters: waste paper treatment process includes: hot water at 100 ºC for 30 minutes,

10 wt% NaOH solution for 4 hours, 5 wt% H2O2 solution for 2 hours; pressing process consists of treated cellulose pulp and 12 wt% PVA solution mixed together in the ratio of 75/25, 70/30, 65/35 wt%, manual pressed and dried at 50 ºC for 30 hours The average thickness of 70/30 ratio convex composite film is 0.44 ± 0.02 mm which is suitable for making TENG The mechanical properties of composite films are slightly different between composite film ratios, the convex composite films have mechanical properties in terms of tensile strength 2.38-3.87 MPa, tensile modulus 72.70-247.30 MPa, water absorption of the film is 15.58 ± 2.76 %, the water contact angle is 112.37º, the convex structure of composite film has a uniform distribution and has a size of 2.23 ± 0.12 àm

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