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1 Chapter 10 Graphene-based Nanocomposites 10.1 Introduction of composites 10.2 Introduction of graphene-polymer nanocomposites 10.3 Processing of graphene-polymer.

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Presentation on theme: "1 Chapter 10 Graphene-based Nanocomposites 10.1 Introduction of composites 10.2 Introduction of graphene-polymer nanocomposites 10.3 Processing of graphene-polymer."— Presentation transcript:

1 1 Chapter 10 Graphene-based Nanocomposites 10.1 Introduction of composites 10.2 Introduction of graphene-polymer nanocomposites 10.3 Processing of graphene-polymer nanocomposites 10.4 Mechanical properties of graphene-polymer nanocomposites

2 Introduction of composites Definition: A composites is a material having two or more distinct constituents or phases, and have to satisfy the following criteria: 1. Both constituents have to present in reasonable proportions, say greater than 5%; 2. The constituents phases have different properties, and hence the composite properties are noticeably different from the properties of the constituents; 3. A man-made composite is usually produced by intimately mixing and combining the constituents by various means. Nanocomposites: Composites contain two or more nano-sized fillers. (Nano-structural composites)

3 Category of fibers and matrix for composites Fibers: Natural fibers Synthetic organic fibers Aramid fiber Polyethylene fiber Synthetic inorganic fibers Glass fibers Alumina fibers Boron fibers Carbon fibers Si C fibers Whiskers(SiC is available in the form of whiskers, i.e., small single crystals):A few tens of microns in length and less than one micro in diameter. Matrix: Polymer (polymeric composites) Metals (metal matrix composites) Ceramic (ceramic matrix composites) Carbon (carbon/carbon composites)

4 Fillers and matrix for nanocomposites Fillers: Carbon nanotubes Graphene Matrix: Polymer (polymeric composites) Carbon (carbon/carbon composites) The major concern of CNT- or graphene-based nanocomposites is improvement in physical property (such as electrical conductivity) rather than mechanical properties of the nanocomposites.

5 Reinforcement-matrix interface of composites Wettability: Interfacial bonding is due to adhesion between the reinforcement and the matrix and to mechanical keying. Clearly, for adhesion to occur, the matrix must be brought into intimate contact with reinforcements. Wettability defines the extent to which a liquid will spread over a solid surface. Interfacial bonding: Once th ematrix has wet the reinforcement, and is therefore in intimate contact with the reinforcement, bonding will occur. (a) Mechanical bonding A mechanical interlocking or keying of two surfaces can lead to a reasonable bond. (b) Electrostatic bonding Bonding occurs between the matrix and the reinforcement when one surface is positively charges and the other negatively charged. (c) Chemical bonding The bonds formed between chemical groups on the reinforcement surface and compatible groups in the materix. (d) Reaction or interdiffusion bonding The atoms or molecules of the two components of the composite may interdiffuse at the interface.

6 Introduction of graphene-polymer nanocomposites Graphene with the following attracted Challenges: 1.Producing large scale quantities and high quality of isolated pristine graphene sheets. 2. Uniform dispersion of graphene in polymer and bonding of graphene with polymer 1.High specific surface area (2600 m2/g) 2.High modulus of ~1 TPa 3.High fracture strength of ∼ 130 Gpa, 4.High failure strain of 25% 5.High thermal conductivity of 4840−5300 W/mK (estimate using the shift in the Raman G peak) 6.High electron mobility as high as cm2/(V sec).

7 7 To produce isolated pristine graphene sheets, the direct exfoliation in highly polar organic solvents such as dimethylformamide (DMF) and N- methyl-pyrrolidone (NMP) by sonication or in chlorosulfonic acid through simple dissolution, but these methods are not currently suited for polymers since the colloidal suspensions cannot support high graphene concentrations and the stability of the mixture strongly depends on the surface energy of the solvent. [10-1]

8 Processing of graphene-polymer nanocomposites [10-2] Three primary routes for preparing graphene-polymer nanocomposites 1. In situ polymerization[10-1] In situ polymerization intercalates the graphene oxide with a hydrophilic polymer or polar monomers, then polymerizes the intercalated to isolate the single layer sheets. 2. Solution mixing[10-1] Solution casting or melt processing can produce nanocomposites from a variety of polymers in a highly scalable manner. While this is a simple approach, it has not yet been demonstrated to produce single layer graphene sheets without the use of stabilizers or surfactants that may negatively affect the composite, specifically the interface. 3. Melt blending Melt compounding utilizes both high-shear forces and high-temperature melting to blend the filler and matrix material.

9 In situ reduction and polymerization Dispersed GO in N,N-dimethylformamide (DMF) through vortex mixing and mild sonication to form a colloidal suspension of single layer GO sheets; Dissolved PVDF into the DMF/GO solution, then the mixture was casted onto a Telfon substrate to dry overnight at 60 °C under atmospheric. Once the solvent is entirely removed from the polymer matrix, the graphene oxide sheets become immobilized in the rigid polymer matrix (Figure 1a2). The in situ reduction process is then carried out by hot pressing the PVDF/GO film at 200 °C for 2 h, yielding nanocomposites with isolated single layers of reduced-graphene oxide (Figure 1a3).

10 10 These results demonstrate a high level of exfoliation of the GO with a typical sheet thickness less than 1 nm. The color of the film changes during heat treatment from light brown to black as shown in Figure 1b. Color change indicates removal of the oxygen functionalities and the partial restoration of the graphitic structure during thermal treatment. FE-SEM AFM)

11 11 Figure 1e,f shows the fracture surface of virgin PVDF and PVDF with reduced graphene oxide, respectively. The nanocomposites with reduced-graphene oxide become rough compared to the cross section of PVDF. The wrinkled surface indicates the reduced graphene oxide in the polymer as shown in arrow of Figure 1f. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are used to characterize the reaction progress during thermal reduction of the graphene oxide. Figure 2 shows the mass loss of the graphene oxide filler during reduction together with the heat flow and the temperature profile used.

12 12 There is a mass loss below 100 °C that can be attributed to the removal of desorbed water. At 200 °C, there is a dramatic mass loss accompanied by an exothermic peak, which is ascribed to the decomposition of oxygen functional groups present on the GO surface. Samples were prepared by reducing the graphene oxide in the nanocomposite at 200 °C for two hours and thus the temperature profile used in the TGA analysis held the materials at 200 °C to capture the reduction process and determine if longer holding times could benefit the material. There is a sharp exothermic spike around 500 °C, indicating that oxidation of graphene occurs and formation of CO2. Thermal analysis of the material demonstrates that 200°C is sufficient to remove a large portion of the oxygen containing functional groups and shows that higher temperatures are effective in further reducing the materials but not mandatory to achieving filler conductivity.

13 13 Two samples of the filler were produced on copper substrates, one of GO drop cast from DMF at room temperature and another drop cast and reduced at 200 °C for two hours in air. High-resolution C1s scans provide a good assessment of the state of carbon atoms in graphene oxide and reduced graphene oxide because the data can distinguish between different numbers of carbon−oxygen bonds (zero, one, two, or three carbon−oxygen The process is suitable for PAni and epoxy resin.[10-2] (I) (II) (III) (IV) I II I III IV

14 14 The thermal reduction removes many of the hydroxyl groups ( ∼ 3400, 1380, 1052 cm−1) through desorption. Thermal reduction at 200 °C is incapable of removing all carbonyl moieties, thus a significant peak at 1720 cm−1, attributed to C=O stretching, remains after reduction of the graphene oxide. Two closely overlapping peaks are observed at ∼ 1650 and ∼ 1620 cm−1, attributed to COOH and aromatic C=C stretching, respectively. After reduction, the unstable COOH groups are removed from the material while the conjugated network of aromatic groups Increases substantially. The reduced graphene oxide shows no peak at 1650 cm−1 and the increased sp2 hybridization pushes the C=C peak from 1620 to 1580 cm−1.The peak at 1225 cm−1 corresponds to cyclic ether moieties, such as furan or pyran. Graphene oxide has relatively few of these groups and thus a relatively small peak; however, after reduction some of the hydroxyl groups convert into ethers and appear as defects in the graphene structure. COOH C=O stretching C=C stretching C=C stretching cyclic ether

15 Solution mixing[10-1] Solution casting or melt processing can produce nanocomposites from a variety of polymers in a highly scalable manner. While this is a simple approach, it has not yet been demonstrated to produce single layer graphene sheets without the use of stabilizers or surfactants that may negatively affect the composite, specifically the interface.[10-1] Solution mixing is the most straightforward method for preparation of polymer composites. The solvent compatibility of the polymer and the filler is critical in achieving good dispersity. Due to the residual oxygen-containing functional groups, GO can be directly mixed with water soluble polymers, such as poly(vinyl alcohol) (PVA), at various concentrations. For the fillers which are not dissolve in non-polar solvents or limited solubility in both organic and inorganic high-speed shearing combined with ice-cooling has also been applied to mix graphene-based fillers and the polymer matrices. However, in the two approaches mentioned above, re-stacking, aggregation and folding of the graphene based nanosheets are unavoidable during the process, which significantly reduce the specific surface area of the 2D fillers. [10-2] During the subsequent in situ reduction of GO, the polymer matrix prevents the re-aggregation of rGO sheets to retain a homogeneous

16 16 Melt compounding utilizes both high-shear forces and high-temperature melting to blend the filler and matrix material. Hence, it does not require a common solvent for the graphene filler and the polymer matrix. Polylactide (PLA)-exfoliated graphite (EG) composite and PET–rGO graphene composite were successfully prepared by using melt compounding. However, the high shear forces employed in melt compounding can sometimes result in the breakage of the filler materials, such as CNTs and graphenenanosheets.[10-2] PET/graphene nanocomposites were prepared by melt compounding at 285 C using a Brabender mixer. Compounding was performed with an initial screw speed of 50 rpm/min for 4 min; then the screw speed was raised to 100 rpm/min within 1 min and the compounding was conducted at this speed for 5 min. The specimens for microscopy and electrical conductivity measurement were prepared by compression molding at 275 C under a pressure of 15 MPa.[10-10] Photos from network Blend: A polymer material containing two or more constituents within a given phase. [10-9] Melt blending

17 Mechanical properties of graphene-polymer nanocomposites [10-2] Monolayer graphene is one of the strongest materials with a Young’s modulus of 1.0 TPa and a breaking strength of 42 N m-1. By using solution mixing and vacuum filtration, the strong and ductile poly(vinyl alcohol)(PVA)–GO composite paper has been prepared, which shows a Young’s modulus of 4.8 GPa and a tensile yield strength of B110 MPa with 3 wt% of the GO loading. The mechanical properties of graphene-based nanocomposites are apparently lower than the prediction of the pristine graphene. To alleviate or solve this problem, it is necessary to chemically tailor the structure at the filler/matrix interface to facilitate the efficient load transfer. For example, GO filler was covalently bonded to isocyanated-PU matrix via the reaction between the oxygenated groups of GO and the isocyanate groups at the end of PU chains. This chemical bonding has led to the increase in the Young’s modulus and hardness by ~900% and ~327%, Isocyanate is the functional group with the formula R–N=C=O.

18 18 Expandable graphite (EG), known as a graphite intercalation compound, is produced by intercalating sulfuric acid into natural flaked graphite via chemical or electrochemical processes. It can expand up to a hundred times in volume at high temperature. The GO prepared by the oxidation of EG can be exfoliated into graphite oxide nanoplatelets (GONPs) in DMF by ultrasonication directly. This type of GO was specifically called expandable graphite oxide (EGO) Oxygenated groups attached to the GONPs can not only facilitate the dispersion but also provide active sites to form chemical bonding that is an ideal interface between the GONPs and appropriate polymers. Polyurethane (PU) is one of today’s most versatile industrial materials which have been widely used as surface coatings for various substrates. PU is an appropriate polymer that can form the chemical bonding with the GONPs via the reaction between isocyanate groups in the end of PU chains and oxygenated groups on the GONPs., PU materials. Graphite oxide nanoplatelets/PU nanocomposites [10-11]

19 19 [10-7] Stress-strain plots of poly(vinyl alcohol)/RG-O composites as a function of filler loading, showing the pronounced reinforcing effect of RG-O. Tensile strength and elongation at break show opposing trends with increasing volume fraction of RG-O (adapted from Ref. [10-7]). increasing in loading Tensile strength

20 20 The storage modulus of the PU composite containing 4.4 wt% GONPs is about 30% higher than that of the PU at −60 ◦ C. No shift in the transition peak of damping factor (tan δ) associated with the glass transition temperature (Tg) of the soft

21 21

22 22 The properties of polymer nanocomposites depend strongly on how well they are The synthesis of graphene from graphene oxide leaves some epoxide and hydroxyl groups; these greatly facilitate functionalization. Since graphene oxide and CRG are flat sheets, entangled bundles are not an issue. However, restacking of the flat sheets, especially after chemical reduction, can significantly reduce their effectiveness. Restacking can be prevented by either use of surfactants that can stabilize the reduced particle suspensions or blending with polymers prior to the chemical reduction. GO readily exfoliates in water or other protic solvents via hydrogen-bonding Nanocomposites have been created with GO and water-soluble polymers such as poly-(ethylene oxide) (PEO) or poly(vinyl alcohol) (PVA). Using GO after chemical modification with isocyanate or amine, composites have also been produced in aprotic solvents with hydrophobic polymers such as polystyrene (PS), polyurethane (PU), or poly(methyl methacrylate) (PMMA). Example of polar protic solvents R-OH or R-COOH Example of polar aprotic solvents C O∥O∥ CH3 O R H O R H hydrogen- bonding

23 23 Electrical conductivity can be restored via chemical reduction of the graphene oxide. This can also be done in situ in the presence of a polymer. For example added sulfonated polystyrene and then reduced graphene oxide with hydrazine hydrate. Without the sulfonated polystyrene, the reduced sheets rapidly aggregated. However, depending on polymer type and the reducing agent, this in situ reduction technique may result in polymer degradation. Isocyanate: A compound has the functional group with the formula R-N=C=O Amines are organic compounds that contain nitrogen and are basic. Thebasic general form of an amine is shown in the following. Where R represents an alkyl group ( 烷基 ).alkyl group R N HHHH Polystyrene sulfonate Polystyrene

24 24 Graphene composites can be produced via in situ intercalative polymerization of monomers. Successful polymerizations of PVA, PMMA, epoxy, and poly(arylene disulfide) with graphene oxide or silicone foams and PU with TRG have been reported. Especially for poly(arylene disulfide), graphene oxide was used as an oxidation agent which converts thiol salts to disulfide. However, so far monomers have only been polymerized in solvents. The high viscosity of even dilute dispersion of graphene makes bulk-phase polymerization difficult. If functional groups on the chemically modified graphene are reactive with the monomer, grafting of polymer chains onto graphene surfaces can occur. Chain grafting has been demonstrated with the polymerization of poly(2- (dimethylamino)ethylmethacrylate) and PVA and with PU formation.

25 25 Morphologies of graphene-based composites (a)TEM of GONP/DMF, (b) SEM of PU, (c) 、 (d) 、 (e) SEM images of different magnification [10-11]

26 26 (a) 、 (b) TEM of GONP/DMF, (c) SEM of PET nanocomposites [10-9] c Morphologies of graphene-based composites

27 Nano Letters, Tang Chem. Soc. Rev., Huang Polymer Int. Trang Energy Environ. Sci., Pumera Macromolecules, Kim Macromol. Chem. Phys. Du Polymer, Potts Nanotechnology, Du Composite Materials Mel m. Schwartz, Vol. 1, PrinticeHall PTR Polymer Zhang 51 (2010) 1191– Nanotechnology Cai 20 (2009) References


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