Choonkeun Lee

Water‐sorption behavior of P‐phenylene diamine–based polyimide thin films

Four different p-PDA–based polyimide thin films were prepared from their respective poly(amic acid)s through thermal imidization at 400°C: poly(p-phenylene pyromellitimide) (PMDA-PDA); poly(p-phenylene biphenyltetra carboximide) (BPDA-PDA); poly(p-phenylene 3,3′,4,4′-oxydiphthalimide) (ODPA-PDA); and poly(p-phenylene 4,4′-hexafluoroisopropylidene diphthalimide) (6FDA-PDA). Water-sorption behaviors of polyimide films were gravimetrically investigated at 25°C and 22–100% relative humidity by using the modified electromicrobalance (Thin Film Diffusion Analyzer). The diffusion coefficients of water for the polyimides varies in the range of 1.6 to 10.5 × 10−10 cm2/s, and are in the increasing order: BPDA-PDA < PMDA-PDA ∼ ODPA-PDA < 6FDA-PDA. The water uptakes of polyimides vary from 1.46 to 5.80 wt %, and are in the increasing order: BPDA-PDA < ODPA-PDA < 6FDA-PDA < PMDA-PDA. The water-sorption behaviors for the p-PDA–based polyimides are closely related to the morphological structure; specifically, the diffusion coefficients in p-PDA–based polyimide thin films are closely related to the in-plane orientation and mean intermolecular distance, whereas the water uptakes are affected by the packing order.

Application of L‐band differential SAR interferometry to subsidence rate estimation in reclaimed coastal land

The subsidence rate in a reclaimed coastal land has been estimated by using JERS‐1 L‐band SAR two‐pass differential interferometry. Measurement of subsidence rate during reclamation is difficult to obtain as a two‐dimensional subsidence map based on sparsely distributed field measurements is required. Owing to the severe temporal decorrelation induced by frequent soil loading and intense deformation gradients, the L‐band was found to be effective for observing subsidence in reclaimed land. We evaluated the accuracy of the estimated subsidence rate using field measurements obtained by a magnetic probe extensometer from 42 ground stations. A two‐dimensional subsidence map was generated from seven qualified pairs. The correlation coefficient R between the two‐dimensional radar measurements and the in situ data was 0.87 with a rms error of 1.42 cm. Two interferometric pairs obtained from an adjoining JERS‐1 path were also used to verify the results. Independent estimations from the two different JERS‐1 paths correlated each other with correlation coefficients R of 0.97 and 0.80. The main sources of the error were the reference DEM errors and additional phase noises calculated from phase fluctuation at stable points. The error from these two sources was ±1.27 cm. The estimated maximum subsidence was about 60 cm over 352 days. The results demonstrate that L‐band differential SAR interferometry is a useful tool in geological engineering applications.

Water sorption behaviors of the BPDA‐based polyimide films depending upon the structural isomers of diamine

For the biphenyltetracarboxylic dianhydride (BPDA)-based polyimide thin films, the water sorption behaviors were gravimetrically investigated by using a thin film diffusion analyzer. The water sorption behaviors of the polyimide thin films are quite different and strongly dependent upon the sort of polyimide. The diffusion coefficients of the polyimide thin films vary in the range of 1.6 × 10−10 to 12.4 × 10−10cm2/s and the water uptakes vary from 1.52 to 5.25 wt %. Both the diffusion coefficient and water uptake of the polyimide thin films are in the increasing order: BPDA-pPDA < BPDA-p,p′ODA < BPDA-p,m′ODA < BPDA-mPDA ∼ BPDA-p,p′DDS < BPDA-m,m′DDS. Specifically, the polyimide films with para-oriented linkages in backbone structure showed relatively lower diffusion coefficient and water uptake than the corresponding polyimide films with meta-oriented linkages because of the well-developed crystalline structure and good intermolecular chain ordering. In addition, the polyimide thin films having higher chain order showed relatively lower diffusion coefficient and water uptake. The crystallinity and intermolecular chain ordering in the morphological structure are critical parameters in controlling the water sorption behaviors of the polyimide thin films.

Spaceborne radar interferometry for coastal DEM construction

Topographic features in coastal regions including tidal flats change more significantly than landmass, and are characterized by extremely low slopes. High precision DEMs are required to monitor dynamic changes in coastal topography. It is difficult to obtain coherent interferometric SAR pairs especially over tidal flats mainly because of variation of tidal conditions. Here we focus on i) coherence of multi-pass ERS SAR interferometric pairs and ii) DEM construction from ERS- ENVISAT pairs. Coherences of multi-pass ERS interferograms were good enough to construct DEM under favorable tidal conditions. Coherence in sand dominant area was generally higher than that in muddy surface. The coarse grained coastal areas are favorable for multi-pass interferometry. Utilization of ERS-ENVISAT interferometric pairs is taken a growing interest. We carried out investigation using a cross-interferometric pair with a normal baseline of about 1.3 km, a 30 minutes temporal separation and the height sensitivity of about 6 meters. Preliminary results of ERS-ENVISAT interferometry were not successful due to baseline and unfavorable scattering conditions.