Interfacial engineering of composite materials via chemical functionalization for mechanical properties control and additive manufacturing

다국어 초록 (Multilingual Abstract)

The development of composite materials is always closely linked with their manufacturing and processing. Since the origination of additive manufacturing (AM also known as 3D printing) in the 1980s, there has been rapid development with increasing interest in AM technologies for composite due to the advantages of high efficiency, resolution, and customization. However, the composite printed layer by layer has great differences in interfacial behaviors, affecting overall performance. Designing suitable interfaces according to different constituents to control the mechanical properties of composites is a current challenge.
In Chapter 2, MXene (Ti2C) modified by 3-aminopropyl triethoxysilane was grafted onto the carbon fiber (CF) surface in an attempt to improve interfacial properties in continuous CF-reinforced epoxy composites. X-ray photoelectron spectroscopy, scanning electron microscopy, and dynamic contact angle test were employed to characterize the effect of the grafted Ti2C on the interfacial properties. A single fiber fragmentation test together with acoustic emission testing was performed to identify the interface failure mode and also determine the interfacial shear strength (IFSS). The interlaminar shear strength (ILSS) of the laminates was also evaluated with three-point beam testing. It was experimentally observed that Ti2C sheets were uniformly grafted on the fiber surface with covalent bonding. It could provide not only the increase of the CF surface roughness but also an excellent opportunity to create plenty of the polar functional groups thereby leading to a greater surface energy of the CF. The IFSS and ILSS of Ti2C modified CF composites were enhanced by ~78% and ~28% increase, respectively, compared to ones of unsized CF composites. This study shows the great potential of Ti2C as an excellent fiber sizing agent for manufacturing high-performance CF composites.
In Chapter 3, a high-performance, printable lignin-based polylactic acid (PLA) composite was investigated through copolymerizing 2-ethylhexyl acrylate at the interface. It was shown that 10 wt% modified lignin (e-lignin) composites exhibit significantly enhanced toughness from 1.16 to 3.84 MJ/m3 and also impact energy from 2.12 to 6.36 KJ/m2 relative to the pure PLA. The responsible toughening effect was interpreted by the plasticization and the bridging effect of e-Lignin. The low melt viscosity of the dispersed e-Lignin phase causes local thermo-rheological relaxation and promotes the mobility of PLA molecular chains, showing desirable melt viscosity for fused deposition modeling 3D printing. Notable that the adhesion strength between deposited layers during the additive manufacturing was increased due to the high interfacial diffusion of composites, where an approximately 138% improvement of weld energy was achieved in 10 wt% e-lignin composites compared to those of pure PLA. This study shows the great promise to utilize lignin extracted natural materials, particularly in additive manufacturing by encouragingly replacing petroleum-based thermoplastics.
In Chapter 4, a UV-curable lignin-based bio-hydrogel was produced via methacrylation using glycidyl methacrylate, followed by cross-linking with acrylamide. The synthesized lignin hydrogel showed outstanding compressive performance, durability properties, and rheological behaviors, which could be regulated by changing the lignin content. With increasing lignin concentration, the crosslinking density and storage/loss modulus increased, whereas tan δ decreased. Additionally, the hydrogels exhibited excellent swelling ratios (several thousand %), meanwhile, their swelling behavior and dimensional changes varied with the lignin concentration and strongly depended on temperature and pH. Finally, a smart strip-shaped hydrogel was designed to display actuator behavior, showing a fast and reversible swelling-deswelling response between pH 3 and 9. This hydrogel is a promising material for biomedical applications and 4D printing materials.
In chapter 5, the above research topics were re-summarized.

목차 (Table of Contents)

• List of Tables iv
• List of Figures v
• Abstract 1
• Chapter 1. Introduction 4
• 1.1. Development of composite materials 4
• 1.2. Definition, classification, and benefits of composite materials 5
• 1.2.1. Definition of composite materials 5
• 1.2.2. Classification of composite materials 6
• 1.2.3. The benefits of composite materials 9
• 1.3. Interface of composite materials 11
• 1.3.1. Definition of composite materials interface 11
• 1.3.2. Role of interface phase 11
• 1.3.3. Interface mechanisms 12
• 1.3.4. Quantifying and analysis of interfacial adhesion condition 17
• 1.4. 3D printing of polymer matrix composites 18
• 1.4.1. 3D printing technology description 18
• 1.4.2. 3D printing of particle reinforced polymer composites 23
• 1.4.3. 3D printing of fiber reinforced polymer composites 26
• 1.5. Scope and motivation of the dissertation 29
• Chapter 2. Enhancing interfacial properties of carbon fiber reinforced epoxy composites by grafting MXene sheets (Ti2C) 32
• 2.1. Introduction 32
• 2.2. Experiment Section 34
• 2.2.1. Materials 34
• 2.2.2. CF surface grafting process 34
• 2.2.3. Preparation of specimens for IFSS and ILSS test 36
• 2.2.4. Characterization 37
• 2.3. Results and Discussions 39
• 2.3.1. Chemical composition and morphology of CF surface 39
• 2.3.2. The surface energy of CF 42
• 2.3.3. Evaluation of Interfacial properties 42
• 2.3.4. Failure mechanisms in CF composites 45
• 2.4. Conclusions 48
• Chapter 3. Enhancement of 3D printability and mechanical properties of polylactic acid/lignin biocomposites via interface engineering 49
• 3.1. Introduction 49
• 3.2. Experimental Section 51
• 3.2.1. Materials 51
• 3.2.2. Surface modification of lignin 51
• 3.2.3. Preparation of specimens for tensile, Izod impact, and tear tests 52
• 3.2.4. Characterization 53
• 3.3. Results and Discussions 54
• 3.3.1. Characterization of lignin grafting and interfacial effect 54
• 3.3.2. Thermal and mechanical properties of PLA/e-Lignin composites 58
• 3.3.3. Rheological and thermomechanical properties of 3D printed e-Lignin composites 60
• 3.3.4. Toughening mechanisms 63
• 3.4. Conclusions 67
• Chapter 4. Fabrication of dual responsive UV curable lignin-based hydrogels with high compressive performance via interface methacrylation 68
• 4.1. Introduction 68
• 4.2. Experimental Section 70
• 4.2.1. Preparation of Lignin-MA powder and lignin hydrogels 70
• 4.2.2. Mechanical and swelling tests 71
• 4.2.3. Monitoring hydrogel bending behavior 71
• 4.2.4. Characterization 72
• 4.3. Results and Discussions 72
• 4.3.1. Preparation and characterization of Lignin-MA 72
• 4.3.2. Mechanical properties and deformation recoverability of lignin hydrogels 74
• 4.3.3. Viscoelastic properties and crosslinking density of lignin hydrogels 76
• 4.3.4. pH/temperature-dependent swelling behavior 78
• 4.3.5. Responsive properties and actuating performance of bilayer lignin composite hydrogels 80
• 4.4. Conclusions 82
• Chapter 5. Summary and outlook 84
• References 86