We have carried out a systematic study of the time-dependent rheology and morphology of poly(dimethylsiloxane) (PDMS) ionomers with tailored number of monomers between ions and number of ions per chain as a function of the cation. Small-angle X-ray scattering (SAXS) and scanning transmission electron microscopy (STEM) were used to examine the structure of the samples. The flow behavior of nonequilibrated freshly precipitated ionomers varies from flowing liquids to weak networks. Low mol % zinc and cobalt ionomers that flow do not show aggregates in STEM images whereas gallium, barium, and high mol % zinc ionomers that precipitate as gels show a diverse range of aggregates. The equilibrium state of all ionomers is reached very slowly at room temperature or more rapidly at high temperatures and is a physically cross-linked network irrespective of the cation. Very different morphologies were observed for different cations. Low mol % (∼1 mol %) barium ionomers have rod-shaped and spherical ionic aggregates; gallium ionomers have inhomogeneously distributed highly polydisperse spherical aggregates; and low mol % zinc and cobalt ionomers have no aggregates whereas high mol % (∼7.4 mol %) zinc ionomers have spherical and rodlike aggregates. The nature of the cation has little influence on the elastic modulus of the equilibrated samples.

Two important issues in the area of stress relaxation in elastomers are addressed experimentally in this paper. The first deals with the dependence of the terminal relaxation time on the number of entanglements that a chain tethered to a network makes with the network, and the second deals with the validity of the empirical Chasset-Thirion equation. By carefully end-linking PDMS chains to form networks with few defects besides the controlled amount of long pendent chains, our results show a much weaker exponential dependence of the terminal relaxation time on number of entanglements per tethered chain than predicted by the Doi-Kuzuu theory for a fixed entanglement network but closer to the exponential dependence predicted by lattice calculations with a harmonic potential. The ChassetThirion equation is found to hold only in an intermediate time regime of the relaxation spectrum.

We describe a synthesis scheme for the preparation of "model" poly(dimethylsiloxane) ionomers with tailored number of monomers between ions and number of ions per chain. Melts of low ion concentration (0.3-1.3 mol %) model zinc and cobalt ionomers, and their unneutralized COOH precursors are found to precipitate as polymers that flow and exhibit a zero-shear viscosity but equilibrate to physical gels. The gel time follows an Arrhenius relationship and is used to predict and verify physical gel formation at room temperature over a time scale from several months to years. The gel time depends chiefly on the ion concentration (calculated here as the average number of ions per monomer units (×100)) and functionality (defined as the average number of ions per chain); for ionomers with comparable overall molar mass, the ion concentration and functionality act counter to each other. Unlike traditional hydrocarbon ionomers, gel formation in these polysiloxane ionomers is favored by high temperature, and the gel moduli are comparable to moduli of end-linked PDMS networks. We propose this is due to transformation of intramolecular interactions to intermolecular interactions at high temperatures. Sodium ionomers form critical gels immediately after precipitation which we attribute to the more ionic nature of sodium relative to the transition-metal ionomers.

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Synthetic aperture radar (SAR) is a well-known imaging technique and most commonly used up to the microwave frequency spectrum (below 30 GHz) which provides spatial resolution in the sub-m range. To enhance the resolution, higher frequency spectra such as millimeter-wave (mmWave) and terahertz (THz) regions are being investigated. The mmWave and THz spectral ranges extend the SAR applications to nondestructive testing (NDT), material characterization, and sub-mm resolution imaging. However, the higher frequency spectrum suffers from higher path loss and potentially higher atmospheric absorption that limits the propagation distance. Nevertheless, the mmWave/THz spectrum is suitable for short-range applications such as indoor room profiling. From theoretical analysis, it can be summarized that the higher frequency spectrum provides better resolution but a comparative study on the impact on the image quality of the frequency spectrum ranging from GHz to THz has not been presented. Besides, as of the hardware complexity of the THz devices, the optimum range of the spectrum is always under investigation. The optimum range is defined where no strong improvements in the image quality are achievable with further increases in the frequency spectrum. Therefore, this paper presents an overview of electronics-based imaging using the SAR technique for the frequency spectrum ranging from GHz to THz with the focus on NDT and high-resolution imaging. Seven frequency bands: 5-10 GHz, 68-92 GHz, 75-110 GHz, 0.122-0.168 THz, 0.22-0.33 THz, 0.325-0.5 THz, and 0.85-1.1 THz are selected for a comparative analysis. The results are presented for 2D and 3D imaging using the backprojection algorithm. Additionally, state-of-the-art imaging based on SAR technique with electronics transceiver modules has only been demonstrated up to the sub-0.75 THz, whereas in this paper the spectrum up to 1.1 THz has been addressed.INDEX TERMS GHz and THz comparison, high-resolution imaging, non-destructive testing, synthetic aperture radar, terahertz imaging, radar imaging.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

In radar remote sensing, the Terahertz (THz) spectrum is presently being investigated worldwide with focus on short-range indoor and outdoor applications. The spectrum broadens the unmanned aerial vehicle (UAV) based synthetic aperture radar (SAR) applications to indoor room profiling with submm resolution and material characterization as many materials have unique fingerprints at this spectrum. SAR technique requires precise localization information of the mobile radar sensor, which in conventional SAR is achieved using an existing localization infrastructure, such as a global positioning system (GPS) and inertial measurement unit (IMU). For the indoor THz SAR, the GPS does not provide coverage in indoor complex environments, and also the state-of-art compact IMU does not provide the required sub-mm accuracy. These limitations can be overcome by utilizing an indoor localization system. Therefore, this paper presents an indoor THz simultaneous localization and mapping (SLAM) system. The system comprises of passive tags based radio frequency identification (RFID) localization system and SAR that provides the UAV localization and mapping of the in-room objects. Another challenge for the UAV based indoor THz SAR that is addressed in this paper is motion compensation (MOCO). At the THz, MOCO requires special consideration due to very small trajectory deviation is in the range of carrier wavelength. Therefore, to study the effects of the sub-mm translational errors, a testbed has been set up, and measurement results are presented in this paper along with the 3D electromagnetic simulation results for a carrier frequency of 275 GHz and bandwidth of 50 GHz. Further, to compensate these errors, the sub-mm localization system is used and the results are presented to validate the proposed solution for indoor THz SAR MOCO.

2 H nuclear magnetic resonance (NMR) and small angle neutron scattering (SANS) data are reported for deuterated guest chains of polydiethysiloxane (PDES) in end-linked PDES networks as a function of the molecular weight of guest chains relative to that of the network elastic chains. We exploit the ability of PDES networks to form a strain-induced mesophase to demonstrate the tendency of longer guest chains to phase separate or partition selectively to the amorphous phase and the tendency of smaller guest chains to remain distributed between the mesophase and the amorphous phase. The segmental orientation of the guest chains measured via 2 H NMR peak splitting can be interpreted in terms of an enthalpic orientational coupling of the chain segments in the amorphous state. SANS results show that the radius of gyration of guest chains in the unstretched networks and in the networks stretched below the mesomorphic transition remains essentially unchanged from that in the melt state. The scattering intensity from SANS patterns for networks with guest chains of any size has a peak in the direction parallel to the stretch direction that reflects the domain spacing of the lamellae in the mesophase.

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