Ahmed A. Abdala

Functionalized graphene sheets for polymer nanocomposites

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, highly stiff fibres in a polymer matrix, high-performance lightweight composites could be developed and tailored to individual applications. Today we stand at a similar threshold in the realm of polymer nanocomposites with the promise of strong, durable, multifunctional materials with low nanofiller content. However, the cost of nanoparticles, their availability and the challenges that remain to achieve good dispersion pose significant obstacles to these goals. Here, we report the creation of polymer nanocomposites with functionalized graphene sheets, which overcome these obstacles and provide superb polymer-particle interactions. An unprecedented shift in glass transition temperature of over 40 degrees C is obtained for poly(acrylonitrile) at 1 wt% functionalized graphene sheet, and with only 0.05 wt% functionalized graphene sheet in poly(methyl methacrylate) there is an improvement of nearly 30 degrees C. Modulus, ultimate strength and thermal stability follow a similar trend, with values for functionalized graphene sheet- poly(methyl methacrylate) rivaling those for single-walled carbon nanotube-poly(methyl methacrylate) composites.

Recent advances in chemical modifications of graphene

Graphene has attracted the interest of chemists, physicists, and materials scientists due to its extraordinary structural, mechanical, and electronic properties. While pristine graphene is desirable for applications that require a high electrical conductivity, many other applications require modified or functionalized forms of graphene, such as graphene oxide, reduced graphene, or other functionalized forms. Structurally modifying graphene through chemical functionalization reveals the numerous possibilities for tuning its structure; several chemical and physical functionalization methods have been explored to improve the stabilization and modification of graphene. In this review, we report recent progress towards the chemical modification of graphene, including both covalent and noncovalent methods, for use in various applications.

Solution rheology of hydrophobically modified associative polymers: Effects of backbone composition and hydrophobe concentration

We investigate the effects of polymer molecular structure on the solution rheology of a hydrophobically modified associative polymer comprised of macromonomers with alkyl hydrophobes attached to a poly(ethyl acrylate-co-methacrylic acid) backbone. In particular, the effect of polymer backbone composition with variable proportions of methacrylic acid (MAA) and ethyl acrylate (EA) are examined. We find that the concentration of the MAA monomer has a large impact on polymer viscoelasticity. Polymers with low MAA content have smaller hydrodynamic size that result in lower viscosities and dynamic elastic moduli compared to polymers with high MAA content. Moreover, the balance between the polymer hydrodynamic size, the chain flexibility, and the aggregation of the EA blocks yield maxima in these material functions with respect to the MAA concentration. The scaling of shear viscosity, high frequency elastic modulus, and creep compliance with polymer concentration exhibits power-law behavior with different exponents. In all cases, three power-law regimes, regardless of the MAA content, are observed that can be attributed to the presence of different modes of hydrophobic interaction. However, the transitions shift to lower concentrations as the MAA content increases. With regards to the effects of the macromonomer side-chain concentration, we observe a substantial increase in viscosity at intermediate macromonomer content (1 mol %), possibly due to an increase in the number of intermolecular junctions as the number of hydrophobes per chain increases. This is in contrast to (i) low macromonomer concentration (0.3 mol %) behavior that reveals low viscosity due to weak hydrophobic associations, and (ii) high macromonomer concentration (1.9 mol %) behavior that favors more intramolecular association resulting in lower viscoelastic properties compared to intermediate macromonomer concentrations.We investigate the effects of polymer molecular structure on the solution rheology of a hydrophobically modified associative polymer comprised of macromonomers with alkyl hydrophobes attached to a poly(ethyl acrylate-co-methacrylic acid) backbone. In particular, the effect of polymer backbone composition with variable proportions of methacrylic acid (MAA) and ethyl acrylate (EA) are examined. We find that the concentration of the MAA monomer has a large impact on polymer viscoelasticity. Polymers with low MAA content have smaller hydrodynamic size that result in lower viscosities and dynamic elastic moduli compared to polymers with high MAA content. Moreover, the balance between the polymer hydrodynamic size, the chain flexibility, and the aggregation of the EA blocks yield maxima in these material functions with respect to the MAA concentration. The scaling of shear viscosity, high frequency elastic modulus, and creep compliance with polymer concentration exhibits power-law behavior with different expone...

Solution rheology of hydrophobically modified associative polymers: Solvent quality and hydrophobic interactions

The rheological behavior of a model hydrophobically modified alkali soluble emulsion (HASE) polymer, comprised of a random copolymer backbone of methacrylic acid and ethylacrylate and pendant hydrophobic macromonomers, is examined in cosolvents of water and propylene glycol (PG) of different proportions. We find the solvent solubility parameter to have a direct impact on both the steady and dynamic behavior of the polymer solutions. In particular, scaling of the relative viscosity (ηrel) and the elastic modulus (G′) at a fixed frequency with the solvent solubility parameter (δs) reveals the presence of two distinct regimes with different dependences on δs. In “water-rich” solvents, both ηrel and G′ show a strong dependence on δs in contrast to “PG-rich” solvents, in which there is slight or no dependence on δs. The concentration dependences of both ηrel and G′ are also found to be different in water-rich solvents from that in PG-rich solvents. In water-rich solvents, ηrel and G′ reveal power-law dependenc...

Effect of solvent on the uncatalyzed synthesis of aminosilane-functionalized graphene

Uncatalyzed functionalization of thermally reduced graphene (TRG) with 3-aminopropyltriethoxy silane (APTS) is reported and the effect of the solvent on selective functionalization is discussed. The chemical, morphological and thermal properties of the functionalized TRG (f-TRG) have been studied using FTIR, XPS, EELS, Raman spectroscopy, TEM, and TGA. Our results indicate that the use of organic solvent during the silylation reaction not only increases grafting yield from 7 to 8 atomic% of Si attachment but also directs APTS groups to the edges of TRG as revealed using energy filtered TEM elemental mapping. A reaction mechanism based on attachment of the silane groups on the TRG surface through residual, surface bound phenolic and carbonyl groups is proposed and discussed. The present approach provides an economical route for mass production of APTS-f-TRG and sheds light on the role of organic solvents in silane functionalization of graphene.

Tracer microrheology study of a hydrophobically modified comblike associative polymer.

The viscoelastic properties of associative polymers are important not only for their use as rheology modifiers but also to understand their complex structure in aqueous media. In this study, the dynamics of comblike hydrophobically modified alkali swellable associative (HASE) polymers are probed using diffusing wave spectroscopy (DWS) based tracer microrheology. DWS-based tracer microrheology accurately probes the dynamics of HASE polymers, and the extracted microrheological moduli versus frequency profile obtained from this technique closely matches that obtained from rotational rheometry measurements. Quantitatively, however, the moduli extracted from DWS-based tracer microrheology measurements are slightly higher than those obtained using rotational rheometry. The creep compliance, elastic modulus, and relaxation time concentration scaling behavior exhibits a power-law dependence. The length scale associated with the elastic to glassy behavior change is obtained from the time-dependent diffusion coefficient. The Zimm-Rouse type scaling is recovered at high frequencies but shows a concentration effect switching from Zimm to more Rouse-like behavior at higher concentrations.

In situ formed graphene/ZnO nanostructured composites for low temperature hydrogen sulfide removal from natural gas

Nanostructured composites of graphene and highly dispersed sub-20 nm sized ZnO nanoparticles (TRGZ) were prepared via a novel method combining freeze-drying and thermal annealing. A direct synthesis implies thermal reduction of graphite oxide and in situ ZnO formation which acted as mutual spacers preventing restacking of the graphene layers and eventual aggregation of the nanoparticles at moderate temperatures. A series of compositions with different weight ratios of ZnO nanoparticles were prepared and used as a reactive sorbent in low temperature hydrogen sulfide (H2S) removal from natural gas. The composite sorbent having a ZnO mass ratio of 45.1 wt% showed a significantly greater H2S adsorption capacity (3.46 mmol g−1) than that of pure ZnO (1.06 mmol g−1), indicating that hybridization of ZnO with grpahene significantly improved the H2S removal ability. Such enhancement was mainly attributed to the higher surface area, greater pore volume and unique morphology at the nanoscale in the graphene–ZnO hybrid.

Manipulation of hydrophobic interactions in associative polymers using cyclodextrin and enzyme

We examine a new approach to reversibly modulate hydrophobic interactions in associative polymers using cyclodextrins (CD) and enzymes that cause scission of the α-1, 4 linkages in cyclodextrins. The associative polymers have a comb-like structure with pendant hydrophobic groups randomly attached to the polymer backbone. The intermolecular interaction between hydrophobic groups forms a transient network resulting in thickening of solutions containing the polymer. The CDs, doughnut-shaped cyclic polysaccharides, encapsulate the hydrophobes within their hydrophobic cavity and eliminate hydrophobic interactions. This results in several orders of magnitude reduction in solution viscosity and other viscoelastic properties. Subsequent degradation of the CDs by enzymes restores the hydrophobic interactions and the original rheological properties. A rheokinetic model is developed to describe the kinetics of the enzymatic reactions. The model accounts for equilibrium between the CD bound to the hydrophobes and free CD in solution and assumes the enzyme hydrolyzes only the free CD in the solution, which causes the release of the bound CDs in order to maintain equilibrium. The reaction is assumed to follow Michaelis Menten kinetics and the kinetic parameters are determined by tracking the changes in the viscoelastic properties of the polymer solution during the enzymatic scission of CD. The effects of enzyme concentration, polymer concentration and temperature on the rate of recovery of the original rheological properties are experimentally determined, and used to validate the trends of the rheokinetic model.

Applications of Graphene in Catalysis

The extraordinary and unique physical, chemical, and mechanical properties of graphene have led to the development of graphene-based materials for a wide range of applications in different fields. Amongst, the use of graphene-based materials in the field of catalysis has attracted the interests of researchers in the last few years. Due to its extremely high surface area and adsorption capacities, graphene is expected to function as an excellent catalyst support material. Moreover, an ability to tune its structure using desired functionalities have added significant versatility for such materials in metal free catalyst systems. The interest is due to the activity and stability of graphene based catalysts through tailoring its structures/morphologies, catalytic performance, and design for synthesis, catalytic mechanisms. This editorial note summarizes the versatile applications of graphene-based catalysts in organic synthesis as a carbocatalyst, metal free catalysis, in photocatalysis, and as a catalyst support and provides an outlook on future trends and perspectives for graphene applications in sustainable catalysis.

Water Absorption and Stress Relaxation Behavior of PP/Date Palm Fiber Composite Materials

In this study, the effects of the fiber loading, the coupling agent content and the amount of the absorbed humidity on the stress relaxation behavior of the PP/date palm fibers composite material were characterized, using a dynamic mechanical analyzer (DMA). The obtained results have indicated that the rate of stress relaxation decreases as the date palm fiber content increases. It is therefore suggested that the presence of the natural fibers in composite materials do not help polymer molecules to recover their original configurations, generally present in the polymer matrix before any mechanical loading. The use of the coupling agent was found to also reduce such rate, due to the improvement of the quality of adhesion between the reinforcing phase and the polymer molecules. By increasing the amount of the absorbed humidity, a higher relaxation rate was obtained. This is an expected result since the presence of water molecules in composite materials encourages the relative movement (sliding) of polymer molecules, reducing therefore their recovering time and these can regain their initial configuration at a higher rate.