UCLA

Plastic scintillators, known for their fast response, scalability, and mechanical durability, are promising candidates for γ-ray detection. However, their moderate light yield and inability to absorb the full gamma energy due to their all-organic composition have hindered their ability to achieve spectroscopic identification of gamma sources. This dissertation strives to address these limitations by developing plastic scintillators with enhanced light yield performance and spectroscopic capabilities through optimized formulations.To improve light yield, the first approach involves modifying the polymer matrix with a cross-linkable fluorene derivative, 9,9-bis(4-vinylbenzyl)-9H-fluorene (SF), copolymerized within the matrix to enhance Förster resonance energy transfer (FRET). Additionally, non-curable 9,9-dimethyl-9H-fluorene (MF) and 9,9’-spirobifluorene (SBF) were introduced to bridge the energy transfer gap between the polymer matrix and the primary dye. This approach achieved a light yield of 11,600 photons/MeV with a scintillation decay time of 2.3 ns in a plastic scintillator containing 5 wt% SF and 20 wt% MF. The second approach utilizes triplet exciton harvesting compounds. A triplet harvesting matrix incorporating 1,3-di(9H-carbazol-9-yl)benzene (mCP) and MF as triplet hosts facilitated Dexter energy transfer to bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrPic), a blue phosphorescent dye , resulting in a gamma light yield of 14,800 photons/MeV for a plastic scintillator containing 20 wt% MF, 10 wt% mCP, and 2 wt% FIrPic.

To enable spectroscopic identification, hafnium oxide nanoparticles were incorporated into the optimized matrices, producing nanocomposite scintillators. Fluorene-based polymer matrix containing MF (up to 20 wt%) and hafnium oxide nanoparticles (up to 70 wt%) maintained high transmittance, achieved a light yield of ~5000 photons/MeV, and produced a prominent gamma photopeak with an energy resolution of 8.8% at 662 keV. Besides, nanocomposite scintillators composed of the triplet harvesting matrix with 10 wt% mCP, 20 wt% MF, and 2 wt% FIrPic, and loaded with 20-35 wt% hafnium oxide nanoparticles, exhibited superior gamma light yields of 8,800-10,800 photons/MeV. These samples retained prominent gamma photopeaks with energy resolutions of 6.4-9.7% at 662 keV, demonstrating the effectiveness of triplet exciton utilization and high-Z nanoparticle integration. These advancements underscore the potential of modified organic matrices and high-Z nanoparticle integration to revolutionize plastic scintillator technology for critical applications in nuclear nonproliferation, medical imaging, and high-energy physics.

Dielectric elastomers based on commercial acrylic dielectric elastomers (VHB adhesive films) have been widely investigated for soft actuators due to their large electrically driven actuation strain and high work density. However, the VHB films require prestretching to overcome electromechanical instability, which adds fabrication complexity. In addition, their high viscoelasticity leads to a low response speed. Interpenetrated polymer networks (IPNs) were developed to lock the prestrain in VHB films, resulting in free-standing films that are capable of large-strain actuation. In this work, a prestrain-locked high-performance dielectric elastomer thin film (VHB-IPN-P) by introducing 1,6-hexanediol diacrylate to create an IPN in the VHB network and a plasticizer to enhance the actuation speed is reported. VHB-IPN-P based actuators exhibit stable actuation at 60% strain up to 10 Hz and reach a peak energy density of 102 J/kg. In addition, a hybrid process has also been developed for the fabrication of multilayer stacks of VHB-IPN-P with strong inter-layer bonding and structural integrity. Four-layer stacks fabricated preserve the strain and energy density of single-layer VHB-IPN-P films but with linearly scaled force and work output.