Department of Chemical Engineering Faculty Publications

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    Effect of Specific Surface Area on the Rheological Properties of Graphene Nanoplatelet/Poly(Ethylene Oxide) Composites
    (American Institute of Physics (AIP), 2023-05-01) Haridas, Haritha; Kontopoulou, Marianna
    The rheological properties of poly(ethylene oxide) containing graphene nanoplatelets (GNPs) having different specific surface areas (SSAs) are studied using steady shear and small amplitude oscillatory shear experiments. A series of GNPs having SSAs ranging from 175 ± 5 to 430 ± 13 m2/g was prepared using a thermomechanical exfoliation process. The complex viscosity, moduli, and yield stress of the composites increase with SSA, whereas electrical and rheological percolation threshold concentrations decrease, suggesting that higher SSAs promote filler network formation. Modeling of small amplitude oscillatory shear data using a two-phase model confirms that hydrodynamic effects dominate at low concentrations below 8 wt. %, where the particles are noninteracting. At higher concentrations, the response is dominated by filler-phase contributions. We demonstrate that the two-phase model parameters can be used to track the exfoliation of graphite into GNPs. Fitting of rheological percolation curves using Utracki and Lyngaae–Jørgensen models at low concentrations (noninteracting regime) resulted in aspect ratios between 19 and 76. At high concentrations (interacting particles), the aspect ratios determined by the Krieger–Daugherty model ranged between 5 and 24 due to aggregation. The highest aspect ratios (defined as the ratio of major dimension to minor dimension) were associated with GNPs that had the highest SSA of 430 m2/g. Strain sweeps revealed that the critical strain for the onset of nonlinear viscoelasticity scaled with SSA above the percolation threshold. The scaling relationships of the critical strain and storage modulus with volume fraction were used to infer the fractal dimensions of filler networks.
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    A Note on Using Expanded Graphite for Achieving Energy- and Time-Efficient Production of Graphene Nanoplatelets Via Liquid Phase Exfoliation
    (Wiley, 2022-08-03) Awada, Hassan; Kontopoulou, Marianna; Docoslis, Aristides
    Although easily scalable, the production of graphene nanoplatelets (GNP) by the means of liquid-phase exfoliation of graphite flakes (GF) remains an energy- and time-intensive process. In this work, we demonstrate that significant time and energy can be saved in GNP production when employing expanded graphite (EG) in a surfactant-assisted liquid phase exfoliation process. Owing to its increased interlayer distance, the exfoliation of EG can be accomplished in a much shorter time (<30 min) compared to GF (approximately 7 h in the present case). Moreover, the energy required for the EG exfoliation is close to 80-fold lower than that for GF exfoliation. Monitoring of the mean lateral dimension, specific surface area, and graphite flake-to-GNP transition during exfoliation was performed experimentally using several analytical techniques. The EG-derived GNPs are produced much faster and require less energy for exfoliation compared to GF, thus making it a more efficient alternative technique.
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    Accounting for Temperature Effects When Predicting Molecular Weight and Composition Distribution for Gas-Phase Polyethylene Produced Using a Multi-Site Catalyst
    (Wiley, 2022-07-19) Gibson, Lauren A.; Aiello, Jennifer P.; Jiang, Yan; Boller, Timothy; McAuley, Kimberley
    A dynamic model is developed for gas-phase ethylene/1-hexene polymerization with a three-site hafnocene catalyst. The model accurately predicts molecular weight and comonomer composition distributions for fifteen lab-scale copolymerization runs performed at different temperatures. The experimental runs used to fit this model were performed at temperatures between 60 and 85 °C. Gas-phase concentrations were measured every 2.7 minutes throughout each run. Predicted chain-length distributions are discretized to aid model development, keeping the number of ordinary differential equations manageable. Kinetic parameters at the reference temperature of 81 °C and activation energies are estimated. Using parameter subset selection techniques, it is determined that 53 of the 60 model parameters should be estimated using the product characterization and reactor data. An additional data set obtained at 85 °C is used for model validation, confirming the predictive power of the model. The proposed model and its parameter estimates will aid selection of operating conditions to achieve targeted polymer properties.
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    Methacrylate and Styrene Block Copolymer Synthesis by Cu-Mediated Chain Extension of Acrylate Macroinitiator in a Semibatch Reactor
    (Wiley, 2021-12-10) Cooze, Morgan J.; Deacon, Hayden M.; Phe, Katrina; Hutchinson, Robin Arthur
    A process for the well-controlled growth of acrylates by Cu-mediated polymerization has been developed, with macroinitiator synthesized continuously in a copper tubular reactor and subsequently chain-extended in a semibatch reactor without additional copper. Extending this process to methacrylates and styrene, however, has proven difficult due to a significant reduction in reaction rate. This barrier has been overcome by chain-extending the low molecular weight (Mn of 760 g mol–1) poly(methyl acrylate) macroinitiator with methacrylates and styrene using PMDETA (N,N,N′,N″,N″-pentamethyldiethylenetriamine) as ligand. Methyl methacrylate conversions of >80% at 70 °C and diethylene glycol methyl ether methacrylate conversions of >90% at room temperature are achieved in 4 h, with the same room temperature conditions successfully applied for controlled chain extensions with butyl methacrylate. Although styrene conversions are slightly lower (60–70%) over 4 h at 85 °C, the rates achieved are substantially higher than achieved in other studies. Hydrophobic–hydrophilic triblock structures are produced through sequential monomer feeds to the semibatch reactor, keeping total reaction time to less than 5 h. The ability to incorporate methacrylates and/or styrene into structured block polymers greatly extends the range of products that can be efficiently synthesized with low copper levels by this process.
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    Non-Covalent Polymer Surface Modification of Cellulose Nanocrystals Using Block Copolymers
    (Wiley, 2021-12-14) Torres-Rocha, Olga Lidia; Campbell, Sophie; Woodcock, Nicole; Pinaud, Julien; Lacroix-Desmazes, Patrick; Champagne, Pascale; Cunningham, Michael
    Cellulose nanocrystals (CNC) possess desirable mechanical and optical properties that make them a candidate in the development of next generation of polymer-based composites. However, CNC also have a critical issue associated with their use: their hydrophilicity and incompatibility with hydrophobic polymers. CNC surface properties must be modified for them to be successfully implemented by the industrial sector. Grafting (co)polymers chains on the CNC surface can provide compatibility to CNC with hydrophobic matrices and expand their potential range of applications. In this communication we report preliminary results of a simple method to functionalize CNC surface using block copolymers, where a cationic block anchors via complexation to the anionically charged CNC surface and the other block acts as a stabilizing block, providing dispersibility in various solvents. This is a much simpler and less expensive method than current routes based on covalent modification. The block copolymers poly(polyethylene glycol methacrylate)-b-poly(N-butyl-N’-vinyl imidazolium bromide) (PPEGMA-b-PBuVIm) and poly(styrene)-b-PBuVIm (PS-b-PBuVIm) were first synthesized via nitroxide-mediated polymerization and then non-covalently adsorbed on the CNC surface. The functionalization was confirmed via FT-IR and TGA. The dispersion of polymer-modified CNC materials in organic solvents was evaluated via dynamic light scattering. Modified CNC yielded stable dispersions in organic solvents.