Functionalized High Aspect Ratio Cellulose Nanocrystal Filled Composites for Gas and Liquid Separations

Abstract

Separating mixtures into their components is a ubiquitous feature of industry, and these separations are necessary for every facet of life down to the simple functions of breathing clean air and drinking potable water. These chemical separations account for a large portion of the total energy use both in the United States and globally. Polymer membrane based separations are desirable when applicable due to their lower energy requirements relative to thermal methods such as distillation. This has led to increases in membrane usage to reduce energy costs; however, membrane separations are not without limitations relating to the membrane material and application requirements. Herein I will address membrane separation technologies, their limitations, and the impact of incorporation of high aspect ratio cellulose nanocrystals (CNCs) on the performance of the resulting polymer composite membranes for desalination and gas phase separations. Lack of available drinking water is an increasing problem across the world with much of the world living in water scarce regions. Desalination using reverse osmosis (RO) membranes is one of the most effective methods of producing clean drinking water. Aromatic polyamide based thin film composite membranes (TFCs) are the most commonly used for commercial desalination and have been since the late 1970s. These TFCs suffer from drawbacks including irreversible performance reduction from fully drying the membrane before use and susceptibility to biological fouling. One technique to mitigate issues with TFCs is to utilize the desirable properties of nanoparticles through their incorporation in the TFC selective layer to create thin film nanocomposite membranes (TFNs). CNCs were selected for this work due to their high aspect ratio, potential for surface modification, attractive mechanical properties, sustainable feedstock, and low toxicity. Membranes containing as received CNCs, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanocrystals (TOCNs), tertiary amine functionalized cellulose nanocrystals (aCNCs), or zwitterionic functionalized cellulose nanocrystals (zCNCs) were synthesized to investigate the effects of nanoparticle functionality and loading level on the brackish water desalination, drying behavior, and fouling resistance of polyamide based TFNs. Loading level was investigating using TOCN containing TFNs which exhibited an increase in water flux and sodium salt rejection up to a maximum when the m-phenylenediamine monomer to TOCN ratio was 20:1 followed by a decrease in both water flux and salt rejection with more TOCN added to the membrane. At the optimal loading level there was a 25% increase in the water flux and 0.2% increase in salt rejection relative to the unloaded control for the membranes kept hydrated and a 146% increase in water flux and 1.6% increase in salt rejection relative to the unloaded control for membranes that were dried. These increases yielded equivalent water flux and salt rejection for the membranes kept hydrated and those dried prior to use at the optimal loading level. The changes in desalination performance are attributed to the introduction of a new water transport pathway at the interface of the TOCN nanoparticle and the polymer matrix and a structural reinforcement effect preventing the collapse of pores present in the polymer during the drying process. The optimal loading level from the previous investigation was used for all work with the other CNC functionalities. The TFNs containing CNCs yielded a 10% increase in water flux and no change in salt rejection relative to the unloaded control while those containing aCNCs and zCNCs yielded no change in water flux and a 0.6% and 0.3% decrease in salt rejection respectively. These differences in behavior relative to the TOCN loaded TFNs are attributed to the transport pathway and structural reinforcement effects being subject to the interaction between the polymer and functionality of the nanoparticle as well as the size and shape of the functional group leading to the differences for each CNC functionality. There were no changes in the foulant resistance for any of the membranes when exposed to water containing bovine serum albumin and sodium alginate as probe foulants. This is attributed to the synthesis procedure in which the nanoparticles are added to the membrane in the denser aqueous phase of the interfacial polymerization. The CNCs will not diffuse well through the polymer as it begins to form, so they would be likely to be concentrated deeper in the membrane while fouling is a surface sensitive behavior, so if the nanoparticles aren't near the surface they will not affect that behavior. Gas separations are of interest for investigation into the effects of high aspect ratio nanoparticles in composite membranes as it allows for investigating more fundamental information through control of the membrane morphology and mixture composition. The range of molecule sizes in the separation is much smaller for gas separations compared to desalination with kinetic diameter differences on the order of 0.1-1 Å compared to 4.5Å. Additionally, with the lower pressure requirements for gas transport relative to reverse osmosis, simple membrane geometries can be investigated using dense films rather than TFNs. In this investigation, dense film composite membranes were made consisting 0, 0.07, 0.7, 3.6, 7.2% or 15% CNCs by volume in a thermoplastic polyurethane (TPU) matrix. The addition of TPU showed increased structural strength in the film with loading modulus increasing from 10 MPa for the unloaded TPU to 58 MPa for 3.6% CNC loaded TPU and 105 MPa for 7.2% CNC loaded TPU. The gases tested during this investigation are CO2, He, Ar, O2, and N2. As the CNC loading level increased, the gas permeability for each gas decreased. For the gases other than CO2, there 0.07, 0.7, and 3.6% CNC films all had the same permeability with all, but Ar, 47 ± 3 % less than the unloaded film permeability. The 15% CNC permeabilities were all 44 ± 1 % less than that of the 0.07, 0.7, and 3.6% CNC films. For CO2, the permeability decreased with each addition of CNC. None of these decreases are described by simple space filling by an impermeable particle. This indicates that the structural reinforcement providing strength to the membrane may be limiting some of the chain mobility inhibiting the diffusion of gases through the membrane which is seen in the diffusion coefficient of CO2 which decreases with increasing CNC loading.