An eight-bit chipless radio frequency identification tag providing humidity sensing and identification information is proposed. A compact, enhanced-sensitivity resonator based on an interdigital capacitor (IDC) structure is designed for humidity sensing, whereas seven electric-field-coupled inductor capacitor (ELC) resonators are used for identification information. These eight resonators are placed in a two-by-four array arrangement. A step-by-step investigation for the effect of varying the number of elements and array configuration on the resonant frequency and radar cross-section (RCS) magnitude of the IDC resonator is conducted. The RCS value of the resonant peak frequency for the IDC resonator increases as the number of array elements placed nearby increases due to the mutual coupling among the elements, and the increase in the RCS value becomes larger as the number of arrays increases in the vertical direction. Polyvinyl alcohol (PVA) is coated on the IDC-based resonator at a thickness of 0.02 mm. A non-reflective temperature and humidity chamber is fabricated using Styrofoam, and the relative humidity (RH) is varied from 50% to 80% in 10% intervals at 25 °C in order to measure a bistatic RCS of the proposed tag. The humidity sensing performance of the IDC resonator in the proposed tag is measured by the shift in the resonant peak frequency and the RCS value, and is compared with a single ELC resonator. Experiment results show that when RH increased from 50% to 80%, the sensitivities of both the resonant peak frequency and the RCS value of the IDC resonator were better than those of the ELC resonator. The variation in the RCS value is much larger compared to the resonant peak frequency for both IDC and ELC resonators. In addition, the resonant peak frequency and RCS value of the PVA-coated IDC-based resonator change, whereas those of the other seven resonators without a PVA coating do not change.

This study examined the ablation and flammability of hydroxyl-terminated polybutadiene-based polyurethane composites grafted by polyhedral oligomeric silsesquioxane (HTPB-based PU- g-POSS) that were prepared by a one-step PU reaction using the HTPB prepolymer, two types of reactive POSS diol, and isophorone diisocyanate. The ablation property was measured using an oxyacetylene torch, and the ablation rate and relative ablation resistivity were evaluated. The results show that POSS molecules incorporated chemically into the HTPB-based PU matrix make a positive contribution to the ablative resistance. POSS molecules in the HTPB-based PU composites exhibited distinct microphase separation, aggregating into a spherical shape on the nanometer scale. This morphology of the POSS domain in the HTPB-based PU matrix leads to the formation of densely accumulated spherical silica (SiO₂) droplets on the top of the ablated surface of materials under thermally oxidative conditions during ablation, resulting in improved ablation resistance compared with pristine HTPB-based PU. This unique SiO₂-forming ability of POSS molecules can also enhance the flame-retardant behavior of the composites, as illustrated by the limited oxygen index and cone calorimeter measurements.

Reactive energetic plasticizers (REPs) coupled with hydroxy-telechelic poly(glycidyl azide-co-tetrahydrofuran) (PGT)-based energetic polyurethane (PU) binders for use in solid propellants and plastic-bonded explosives (PBXs) were investigated. The generation of gem-dinitro REPs along with a terminal alkyne stemmed from a series of finely designed approaches to not only satisfy common demands as conventional energetic plasticizers, but also to prevent the migration of plasticizers. The miscibility and rheological behavior of a binary mixture of PGT/REP with various REP fractions were quantitatively determined by differential scanning calorimetry (DSC) and rheometer, respectively, highlighting the promising performance of REPs in the formulation process. The kinetics on the distinct reactivity of propargyl vs. 3-butynyl species of REPs towards the azide group of the PGT prepolymer in terms of Cu-free azide-alkyne 1,3-dipolar cycloaddition (1,3-DPCA) was studied by monitoring ¹H nuclear magnetic resonance spectroscopy and analyzing the activation energies (E_a) obtained using DSC. The thermal stability of the finally cured energetic binders with the incorporation of REPs indicated that the thermal stability of the REP/PGT-based PUs was maintained independently of the REP content. The tensile strength and modulus of the PUs increased with an increase in the REP content. In addition, the energetic performance and sensitivity of REP and REP triazole species was predicted.