Mohamad Al-Sheikhly

Ultra-high molecular weight polyethylene (UHMWPE) is commonly used as a lining material in total joint arthroplasty; however, irradiation used for sterilization and crosslinking generates free radicals that increase the material's susceptibility to oxidative aging. This study explores the efficacy of nitroxides in scavenging free radicals in UHMWPE and their impact on crosslink density. The irradiation and crosslinking of UHMWPE polymer specimens were performed using a ⁶⁰Co γ-ray source and a 7 MeV electron accelerator at the University of Maryland. The scavenging of alkyl radicals and the degree of crosslinking in UHMWPE were assessed using Electron Paramagnetic Resonance (EPR) spectroscopy and gel swelling in decalin solvent, as measured by a Thermomechanical Analyzer (TMA). The nitroxide radicals TEMPO and TEMPOL demonstrated high efficiency in scavenging alkyl radicals. It was noted that without oxygen and antioxidants, most alkyl radicals undergo bimolecular reactions, resulting in crosslinks. When nitroxides are present, residual radicals combine with nitroxides or form long-lasting allyl radicals. The addition of nitroxide antioxidants, which efficiently scavenge free radicals, can prevent crosslinking if they diffuse into the polymer before irradiation, scavenging residual radicals within a crosslinked polymer. While it is possible to create highly crosslinked UHMWPE with nitroxides by adjusting factors such as dose, dose rate, irradiation temperature, and the duration of nitroxide doping, it was found that at low gamma dose rates, nitroxides nearly entirely inhibited crosslinking. When added before or after irradiation, nitroxides effectively scavenge residual free radicals while maintaining crosslink density. However, specific scenarios may inhibit crosslinking in the presence of nitroxide.

Carbon capture remains a crucial and growing area of research, driven by market demand for innovative and scalable solutions in CO2 abatement and related industrial reuse. Primary and secondary amines are essential for CO2 binding. This research investigates the radiation-induced graft polymerization of allylamine onto Fluorinated Ethylene Propylene (FEP) and Ethylene tetrafluoroethylene (ETFE) films, with FEP exhibiting a higher Degree of Grafting (DoG) and enhanced CO2 capture capacity. The stability of free radicals in FEP may explain its superior performance. Analyses revealed minimal surface modifications and low nitrogen incorporation in the films. Because optimizing RIGP parameters is complex, a generative machine-learning framework is proposed to optimize the grafting process, supported by experimental data from other monomers. A dataset involving the grafting of 4-vinylpyridine onto FEP was selected due to data availability for model training. The ML framework is designed to be a customizable generative model in which the grafting process can be modeled by the algorithm of choice and optimized through iterative cycles of Gaussian Processes and Bayesian Optimization. This approach provided flexible learning and demonstrated strong predictive ability in identifying the optimal conditions for achieving a higher degree of grafting while minimizing experimental costs and resource use.

Herein, we describe the design, synthesis, and analysis of anhydrous fuel cell membranes that can operate at temperatures above 100 °C, in view of enhanced performance and stability. Traditional polymer electrolyte membrane fuel cells (PEMFCs) do not operate efficiently above 100 °C because water is used as a proton-conductive medium through the Grotthuss hopping mechanism. By substitution of water with heterocyclic amine monomers and use of ionizing radiation to graft them onto fluoropolymer films, proton-conductive network solid-state polymer electrolyte membranes (PEMs) were developed. PEMs were synthesized using indirect radiation grafting of the following heterocyclic amine monomers: 4-vinylpyridine and 5-vinylpyrimidine onto fluorocarbon substrates. The resulting PEMs have proton conductivities greater than 10^–2 S/cm above 100 °C and perform independent of humidity conditions. These PEMs also demonstrate a positive correlation of increased proton conductivity with increasing temperatures above 100 °C. The chemical properties and structures of the grafted monomers affect the proton-conductive mechanism and performance of the PEMs. The data generated through this research will further the development of anhydrous PEMs through radiation grafting to achieve higher proton conductivity, enhanced performance, and stability.

Polymeric materials have become important components in prefilled syringes, drug delivery systems, and advanced medical devices. Background/Objectives: Nitrogen dioxide gas is used for the terminal sterilization of drug delivery systems. For the implementation of sterilization methods, compatibility with materials must be demonstrated such that the materials maintain product requirements and specifications after sterilization and at the time of use (i.e., product shelf life). Methods: Commonly used polymers were selected based on their chemical structures to provide insight into the nature of reactions that occur at the temperature and NO₂ concentration levels used in the sterilization process. After exposure to the NO₂ process, materials were evaluated for chemical, mechanical, and biocompatibility properties. Results: In this paper, we demonstrated the compatibility of polymers comprising carbonyl, unsaturated ester, and ketone groups which have been used in medical devices sterilized with NO₂. No significant chemical or physical changes were observed upon the treatment of Amorphous Polyester, Polysulfone (PSU), Polycarbonate (PC), PolyEtherEtherKetone (PEEK), PolyArylEtherKetone (PAEK), and Polypropylene (PP) with NO₂ at a sterilization temperature of 20 °C. At this relatively low sterilization temperature, the reactions of NO₂ with the polymer do not typically occur because the activation energies of these reactions require much higher temperatures. Conclusions: Not all materials will be compatible with NO₂ sterilization, and even with the established data, many devices will need to have their polymers evaluated for compatibility before moving to NO₂ sterilization. These results will provide guidance to device designers selecting materials for new drug delivery devices and to regulators that review the safety and efficacy of these devices.

Poly(N-vinyl-2-pyrrolidone), or PVP, nanogels loaded with gadolinium nitrate (Gd(NO₃)₃·6H₂O) were synthesized by ionizing irradiation, aiming for potential applications in magnetic resonance imaging (MRI). A comprehensive characterization of PVP and Gd aqueous solutions with different VP-monomer-to-Gd ratios was conducted before and after irradiation. The results indicate a complexation between PVP and Gd ions before irradiation. The size of the nanogels exhibited a strong dependence on several factors, including PVP molecular weight, concentration, temperature, and the precise timing of Gd introduction relative to the irradiation process. A quantification study was conducted to investigate the impact of molecular weight, the VP/Gd ratio, and Gd addition before or after the irradiation process on the concentration of free Gd ions. These findings offer valuable insights into optimizing the synthesis of Gd-loaded PVP nanogels for potential applications, highlighting the critical factors that influence their size and stability.

The use of ionizing radiation offers a boundless range of applications for polymer scientists, from inducing crosslinking and/or degradation to grafting a wide variety of monomers onto polymeric chains. This review in particular aims to introduce the field of ionizing radiation as it relates to the degradation and recycling of cellulose and its derivatives. The review discusses the main mechanisms of the radiolytic sessions of the cellulose molecules in the presence and absence of water. During the radiolysis of cellulose, in the absence of water, the primary and secondary electrons from the electron beam, and the photoelectric, Compton effect electrons from gamma radiolysis attack the glycosidic bonds (C-O-C) on the backbone of the cellulose chains. This radiation-induced session results in the formation of alkoxyl radicals and C-centered radicals. In the presence of water, the radiolytically produced hydroxyl radicals (^●OH) will abstract hydrogen atoms, leading to the formation of C-centered radicals, which undergo various reactions leading to the backbone session of the cellulose. Based on the structures of the radiolytically produced free radicals in presence and absence of water, covalent grafting of vinyl monomers on the cellulose backbone is inconceivable.

Nanohydrogel particles of polyethylene glycol (PEG), gelatin (GEL), and PEG–GEL mixtures (MIXs) were synthesized with a high electron beam and ⁶⁰Co gamma-ray radiation. The relatively novel technique of Asymmetrical Flow Field Flow Fractionation (AF4 or AFFFF) coupled to a Multi-Angle Laser Light Scattering (MALLS) detector was mainly used to determine the hydrodynamic diameter (D_h) of the radiation-synthesized PEG, GEL, and PEG–GEL nanohydrogel particles. Our approach to achieving nanohydrogel particles is to enhance the intracrosslinking reactions and decrease the intercrosslinking reactions of the C-centered radicals of the PEG and GEL. The intracrosslinking reactions of these free radicals were enhanced via irradiation at temperatures of 77–80 °C and using a high dose rate and pulsed irradiation. The shorter average distance between the C-centered free radicals on the backbone of the thermally collapsed PEG and GEL chain, due to the destruction of hydrogen bonds, enhances the intracrosslinking reactions. It was observed that increasing the dose and dose rate decreased the D_h. DLS results lined up with AF4 measurements. This study provides researchers with a clean method to produce GEL–PEG hydrogels without the use of toxic reagents. Particle size can be tuned with dose, dose rate, and temperature as demonstrated in this work. This is ideal for medical applications as the use of ionizing radiation eliminates toxicity concerns and provides simultaneous sterilization of the material.