Morphology controlled synthesis, characterization, and separation performance of novel polymeric membranes
Creators: name: Nandala, Shiva
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400409
subject: Separation technologies
classificationCode: 401605
subject: Functional materials

description: Membranes have become an integral part of modern daily life today, and their utilization can be found right from domestic applications to large-scale industries. In the twenty-first century, the fast-growing population and industrialization triggered several challenges in the downstream processing of food, biotechnology, and pharmaceutical industries. On the other hand, greenhouse gas emitted from the combustion of fossil fuels causes severe environmental
issues like global warming. In this era, membrane technology has emerged as an efficient separation technique over convectional separation unit operations due to simplicity in operation, small footprint, ease of maintenance, and energy efficiency. Despite many advances, the enhancement of membrane performance still remains an active area of interest in the last few decades. Membranes exhibit a trade-off between permeability and selectivity. In general,
highly permeable membranes are less selective and vice versa. On the other hand, defect control is considered a significant challenge in the large-scale production of membranes without losing their intrinsic properties, where membrane materials, fabrication procedures, and process scheme must be precisely optimized to develop high-performance membranes. This thesis focuses on developing membranes for three major applications (a) synthesis of
highly permeable different molecular weight cut-off (MWCO) membranes for selective separation of biomolecules, (b) fabrication of surface charged porous ultrafiltration membranes for isolation of Lysozyme from the complex protein mixtures, (c) preparation and optimization of dense selective gas permeable membrane for biomethane purification.

In the first step, the physicochemical properties of the membranes were systematically trailed through unique modification approaches to meet the specific needs of said applications. The wet phase inversion and solvent evaporation techniques were used for fabricating asymmetric and dense membranes, respectively. The kinetic parameters such as solvent/non-solvent demixing rates and thermodynamic parameters, including polymer/additive-solvent
interactions, were well controlled to obtain the desired membrane morphology and separation selectivity. UV photo grafting and sulfonation procedures were used for chemical modifications of the membranes.

In the second step, the developed membranes were characterized in terms of molecular weight cut-off, porosity, hydrophilicity, surface roughness, and functional group identification to understand the morphological changes with respect to the change in fabricating conditions. Further, the optimization of membrane modular configurations was studied to achieve high separation efficiencies and low operational costs.

The overall work is divided into five sections. The thesis is structured in 8 chapters, which include Chapter 1 for the introduction and Chapter 8 for the conclusion and future recommendations, while Chapter 2 presents detailed material and methods employed in the fabrication of various morphological membranes and their characterization techniques. Chapters 3 to 7 focus on development of morphologically different membranes for specific applications.

Chapter 3 investigates development of acrylic acid (AA) grafting on polyethersulfone ultrafiltration membranes with fine-tuned MWCO for purification of small therapeutic biomolecules. The membranes were indigenously synthesized by adding a pore former and subsequently grafted with AA monomer under UV-induced photo grafting. As per the size exclusion principle, the resulting membranes were applied to separate urea, cobalamin, and insulin semi-synthetic biomolecules. The separation specificity and improved biocompatibility of these membranes are discussed based on high permeability and retention.

Chapter 4 confers the usefulness of fabricating surface-charged ultrafiltration membranes in protein separation, especially in the food and pharmaceutical manufacturing industries. The study focused on the development of neat and sulfonated polyethersulfone with Polyethylene glycol (PEG) blend membranes for lysozyme separation from chicken egg white. Additionally, the effect of membrane hydrodynamics and physicochemical parameters were examined to
obtain the optimum process conditions for achieving maximum separation of Lysozyme. The research outcomes developed an in-depth understanding of the interrelationship between the PES/sPES with PEG under various membrane formation conditions and subsequent membrane structure and properties, which sheds light on the use of innovative membrane technology to separate complex protein mixtures.

Chapter 5 dedicates to the systematic selection of gas permeation methods and their operational protocols, which have prime importance in the rationalization of developed membranes. This chapter assessed the comprehensive evaluation of transport properties such as diffusivity, solubility, and permeation coefficients using the developed Isochoric apparatus. The design considerations for the economic development of the isochoric apparatus are
presented. Additionally, the pre-screening technique and applicability of downstream pressure rise patterns presented in this chapter provide great flexibility in the identification of nanoscale defects, which could eliminate the possible artifacts during the measurements.

Chapter 6 focuses on establishing the structural correlation, including physical morphology, residual solvent effect, and separation characteristics of membranes. The dense membranes were synthesized in-house using DMF, DMAc, and NMP as solvents by phase inversion technique. The indigenously synthesized membranes were investigated using various spectroscopic, microscopic, thermal, and diffraction techniques. The experimental diffusivity
and solubility of CO2 and CH4 in the prepared membranes were estimated using the Isochoric apparatus (Chapter 5). The atomistic simulations were employed to gain in-depth structural insights; particularly, Grand Canonical Monte Carlo and Molecular Dynamics simulation techniques were employed to understand the sorption behavior and diffusion mechanism, respectively.

Chapter 7 investigates the performance of multilayered composite membranes (MCMs) for the biogas upgrading process. The study also discusses the importance of material selection in fabricating porous support layers to develop MCMs. In addition, the effect of zeolite type and loading percentage on the selective layer performance were examined. Further, the properties of optimized MCM were configured in the membrane model developed in the Aspen HYSYS simulator to perform scale-up studies and the economic feasibility of the membrane-based biomethane enrichment process. The outcome reveals the substantial potential of membrane technology as a biogas upgrading process and offers assistance in developing the appropriate porous support for MCM.

Chapter 8 presents the conclusions from each study with recommendations for future research. The work presented in this thesis demonstrates that the morphology control during fabrication and resulting physicochemical properties are critical in fine-tuning membrane performance. Further, defect control strategies in the membrane fabrication and identification during the membrane performance evaluations are the first steps in the process scale-up. The current work demonstrated the successful implementation of indigenously synthesized membranes in biomolecule purification, protein separation, and biomethane enrichment.