Text

theses

Nano-additives are important and integral in a vast range of practical applications, especially for enhancing the mechanical properties of composite materials. The development of cost-effective methods to fabricate high-quality nanocomposite materials at high yield is essential. In this work, graphene flakes are delaminated from graphite particles loaded in a high-viscosity liquid polymer solution using a concentric-cylinder shear device. Using poly (methyl methacrylate) (PMMA) as the polymer, with 1%wt graphite loading, shearing for 6 hours leads to the formation of a graphene-reinforced polymer matrix composite (r-PMCs) when subsequently solidified. Here, PMMA dissolved in acetone at four different concentrations, i.e., 0.4, 0.5, 0.6, and 0.7 g/ml, is studied. Among the four different cases, during the exfoliation process, the polymer solution from the 0.7 g/ml concentration exhibits a more pronounced non-Newtonian fluid behavior (i.e., significant shear thinning) and gives the best exfoliation performance. Furthermore, high PMMA/acetone concentration corresponds to high mixture viscosity, which at a high-speed rate (2500 RPM), results in very-high shear stress, increasing the total number of mechanically-exfoliated flakes produced within the PMCs. The liquid-phase UV-Visible spectra analysis shows that 0.225 mg/ml of graphene is created in the G-PMMA during the process at 0.7 g/ml polymer/acetone concentration. The solution is then heat injection molded into a sample at 200°C prior to mechanical testing. The resulting graphene-reinforced PMC exhibits a maximum enhancement of elastic modulus (E) and hardness (H) of about 31% and 28.6% respectively. Additionally, Raman spectroscopy reveals the critical 2D peak to D peak intensity (I2D/IG) ratio to be about 0.92, which correlates to bilayer graphene flakes (n=2). The I2D/IG ratio dramatically changes from 0.45 to 0.92, as the polymer/acetone solution concentration varies from 0.4 to 0.7 g/ml, demonstrating the dependence of the degree of exfoliation on solution viscosity.
The effect of shear exfoliation time (i.e., 2, 6, and 12 hours) on the production of graphene in the liquid phase of the PMMA/acetone solution is evaluated. The Raman spectra show that the number of graphene layers (n ≤ 2) reduces with increasing processing time, with the I2D/IG ratio reaching a maximum of about 1.2 after 12 hours.
Using the same conditions (i.e., PMMA/acetone concentration 0.7 g/ml and processing time 6 hrs), few-layer graphene (G) and hexagonal boron nitride (h-BN) nanoflakes are investigated in terms of their production through exfoliation, along with the properties of the resulting polymer matrix containing them. The process produces both single layer and few-layer 2D nanoflakes, which immediately bond to the surrounding polymer without any external contamination. With pure PMMA as reference, mechanical characterizations, including nanoindentation testing (multi-load testing) and nano-dynamic mechanical analysis (nanoDMA), are conducted on the reinforced polymer matrix nanocomposites. As expected, the H and E of reinforced PMMA increase markedly with the 2D nano additives, in conjunction with heat treatment at 200°C. The results show that there are maximum increases of E and H by 32% and 31%, respectively, for the graphene-reinforced PMMA nanocomposite, whereas the increases are 27.1% and 24.8% for h-BN-reinforced PMMA nanocomposite. The surface morphology and interfacing of the nano-additives with the polymer matrix are investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), showing one to four layers of either graphene or h-BN flakes with excellent interfacing between the matrix and the nanoflakes. Raman spectroscopy is used to assess the number of layers and the dispersion of the 2D flakes in the matrix, via the distributions of peak positions and intensities as a function of wavenumber. The h-BN peak or E2g mode appears at 1367 cm-1 in h-BN-PMMA after exfoliation, with a shift of 2 cm-1 (1365 cm-1 for bulk h-BN). As in the previous study for G-PMMA, the average I2D/IG ratio increases notably to 0.92 at a polymer/acetone concentration 0.7 g/ml. This average value indicates bilayer graphene flakes, with many monolayer graphene flakes (n=1) present also according to TEM imaging.
Finally, microfibers are fabricated by electrospinning the processed PMMA/acetone solution with graphene nanoplatelets for reinforcement. G-PMMA fiber mats with good dispersion of 2D-graphene flakes in the polymer matrix (according to TEM imaging) are produced in the diameter size range from 3 to 6 µm. Nanoindentation testing is performed on the reinforced fiber, as well as the pure polymer fiber for reference. The nanoindentation technique reveals that the mechanical properties of individual graphene-reinforced PMMA microfibers are enhanced (with elastic modulus by 19% and hardness by 17.2%) as compared to those of pure PMMA microfiber produced in the same manner.

ETD_10227

Ph.D.

Includes bibliographical references

doi:10.7282/t3-xd1e-g362

ETD doctoral

The author owns the copyright to this work.