Haksoo Han

Water sorption and diffusion behaviours in thin films of photosensitive polyimides

Abstract Several photosensitive polyimide (PSPI) precursors were synthesised by the acid/base complexations of conventional poly(amic acid) precursors with photochemically cross-linkable 2-(dimethylamino)ethyl methacrylate. PSPIs in films were prepared from the photosensitive precursors by thermal imidisation, whereas the corresponding PIs in films were prepared from the conventional poly(amic acid)s: rod-like poly(p-phenylene pyromellitimide) (PMDA-PDA), rigid (poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), semi-flexible poly(4,4′-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA), semi-flexible poly(4,4′-oxydiphenylene pyromellit-imide) (PMDA-ODA), and flexible poly(4,4′-oxydiphenylene benzophenonetetracarboximide) (BTDA-ODA). Water sorption and diffusion behaviours in the PSPIs were gravimetrically measured at 25°C in 100% relative humidity and compared with those of the corresponding PIs. For the PSPIs as well as the PIs, the water sorption and diffusion behaviours were nearly Fickian, regardless of the backbone chemistry. However, those were strongly dependent upon the polyimide backbone chemistry and precursor origin. In addition, those in polyimides were affected by the bulky photosenistive groups, even though they were temporarily linked to the precursor polymers and then debonded from the backbones and ultimately outgassed during the thermal imidisation process. For PMDA-PDA, BPDA-PDA and BPDA-ODA, the PSPIs absorbed water more quickly than the corresponding PIs, whereas for PMDA-ODA and BTDA-ODA, the PSPIs absorbed water less quickly than the corresponding PIs. In contrast to the water diffusion, all the PSPIs absorbed slightly more or a great deal more water than the corresponding PIs, depending on the backbone chemistry. All the measured water sorption and diffusion behaviours in the PSPIs were understood by considering changes in the morphological structure (namely, chain order and orientation), voids, and residues possibly induced by the bulky photosensitive groups, in addition to the Tg as well as the chemical affinity to water.

Special susceptive aqueous ammonia chemi-sensor: extended applications of novel UV-curable polyurethane-clay nanohybrid.

In this contribution, chemical sensor for the detection of aqueous ammonia has been fabricated using UV-curable polyurethane acrylate (PU) and nanohybrids (NH-1, NH-3 and NH-5). PU has been prepared by reacting polycaprolactone triol (PCLT) and isophorone diisocyanate (IPDI) while the nanohybrids, NH-1, NH-3, and NH-5 have been synthesized by solution blending method using PU with 1, 3, and 5 wt% loading levels of C-20B. PU and their nanohybrids showed higher sensitivity investigated by I-V technique using aqueous ammonia as a target chemical. All the nanohybrids showed higher sensitivity as compared to neat PU. The sensitivity increased with increase in clay content and the nanohybrid containing 5 wt% of clay showed the highest sensitivity (8.5254 μA cm(-2) mM(-1)) with the limit of detection (LOD) of 0.0175 ± 0.001 μM, being 7.8 times higher than pure PU. The calibration plot for all the sensors was linear over the large range of 0.05 μM to 0.05 M. The response time of the fabricated sensor was <10.0 s. Therefore, one can fabricate efficient aqueous ammonia sensor by utilization of nanohybrid as an efficient electron mediator.

Synthesis and characterization of new polyimides containing calix[4]arenes in the polymer backbone

Two diaminocalix[4]arene monomers were synthesized from p-tert-butylcalix[4]arene through a 4-step reaction sequence. New copoly(amic acid)s containing calix[4]arene moieties on the polymer backbone were successfully synthesized in N-methyl-2-pyrrolidone by polycondensations of 4,4′-oxydiphthalic anhydride (ODPA) with the diaminocalix[4]arene monomers using 4,4′-oxydiphenylene diamine (ODA) as a comonomer. These copoly(amic acid)s were soluble in aprotic polar solvents, so that they can be processed in various ways. The copoly(amic acid) precursors were thermally converted to the corresponding copolyimides in films. The copolyimide films are amorphous, but insoluble in common solvents. They are thermally stable up to 366°C. The copolyimides exhibit relatively high TECs, low Tgs, low refractive index, low dielectric constant, low optical anisotropy, low dielectric anisotropy, and low water uptake, compared to those of conventional ODPA-ODA polyimide. These property characteristics were interpreted in regard to bulky, cone-like calix[4]arene moieties and their effects on the chain conformation and morphological structure. The processability and property characteristics support that both of the copolyimides containing calix[4]arene moieties are potential candidate materials suitable for membranes, antioxidant additives, chemical sensor devices, and microelectronic devices.

Shape effects of cuprous oxide particles on stability in water and photocatalytic water splitting

Cuprous oxide (Cu2O) has received much attention as a photocatalyst due to its direct band gap structure, small band gap energy, non-toxicity, and abundance. However, Cu2O usually suffers from poor stability because the oxidation state of copper is easily changed. In this work, Cu2O particles of three different shapes were prepared with distinct surface structures: cubes with (100) facets, octahedra with (111) facets, and rhombic dodecahedra with (110) facets. Their shape stability was estimated in deionized water with or without light irradiation. The Cu2O(100) facets were selectively deformed under dark conditions, as expected from density functional theory calculations. The rhombic dodecahedra showed the most violent degradation under light irradiation, with many large thorns appearing on the surface. When water splitting was attempted using the shaped Cu2O particles, the rhombic dodecahedra produced the most hydrogen, whereas the cubes produced none. Oxygen was not measured because the holes generated upon light absorption were used to oxidize the Cu2O surface to CuO. A conformal TiIrOx overlayer was successfully formed on the rhombic dodecahedral Cu2O particles, and the coated particles presented overall water splitting producing both hydrogen and oxygen. They also showed significantly improved stability over repeated water splitting reactions relative to bare Cu2O particles or TiOx-coated Cu2O particles.

The effect of curing history on the residual stress behavior of polyimide thin films

The effect of curing history on the residual stress behaviors in semiflexible structure poly(4,4′-oxydiphenylene pyromellitimide) (PMDA–ODA) and rigid structure poly(p-phenylene biphenyltetracarboximide) (BPDA–PDA) polyimide was investigated. Depending upon the curing history and different structures of polyimide, the residual stress behaviors and the morphology of polyimide thin films were detected in situ by using a wafer bending technique and wide angle X-ray diffraction (WAXD), respectively. For the rigid structure BPDA–PDA polyimide, the residual stress and the slope decreased from 11.7 MPa and 0.058 MPa/°C to 4.2 MPa and 0.007 MPa/°C as the curing temperature increased, and the annealing process is done. However, for the semiflexible structure PMDA–ODA, the change of the residual stress and the slope was relatively not significant. In addition, it was found that the cured polyimide prepared at a higher temperature with a multistep curing process showed a higher order of chain in-plain orientation and packing order than does the polyimide film prepared at a lower temperature with a one-step curing process. These residual stress behaviors of polyimide thin films show good agreement with WAXD results, such as polyimide chain order, orientation, and intermolecular packing order, due to curing history. Specifically, it shows that the effect of curing history on residual stress as well as morphological change was significant in rigid BPDA–PDA polyimide but, not in semiflexible PMDA–ODA polyimide. Therefore, it suggests that the morphological structure depends upon curing history, and the polyimide backbone structure might be one of important factors to lead the low residual stress in polyimide thin films.

Dependence of water sorption in polyimides on the internal linkage in the diamine component

The dependence of the water sorption behavior of polyimide thin films upon different diamine fragments was gravimetrically investigated. It was found that the diffusion coefficient and water uptake were strongly dependent upon the internal linkage in the diamine fragment, and varied in the range of 1.6×10−10 13.5×10−10 cm2/s and 1.52 ∼ 5.63 wt.%, respectively. The water sorption behavior of the polyimide films decreased with increasing bulkiness of the internal linkages in the diamine fragments: BPDA-PDA<BPDA-Bz<BPDA-ODA<BPDA-DDS<BPDA-FDA. Specifically, BPDA-DDS and BPDA-FDA polyimide films with bulky internal linkages in the diamine fragments showed poor chain orientation and low intermolecular packing order, leading to relatively high water sorption (i.e. fast diffusion and high water uptake).

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.

Water sorption behaviour of polyimide thin films with various internal linkages in the dianhydride component

Abstract Polyimide films have been prepared from reaction of 1,4-phenylenediamine (PDA) with 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 4,4′-oxydiphthalic dianhydride (ODPA), diphenylsulfone tetracarboxylic dianhydride (DSDA) or (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The water sorption behaviour of thin films of the polyimides was strongly dependent on the chemical structure of the internal linkage and the morphological structure of the films. The in-plane orientation and molecular chain order decreased with increasing bulkiness of the internal linkage, leading to relatively low diffusion coefficient in DSDA-PDA and 6FDA-PDA films. The water diffusion coefficient varied in the range of 1.7×10 −10 to 10.5×10 −10 cm 2 /s, and was in the increasing order: BPDA-PDA

Stress behaviors and thermal properties of polyimide thin films depending on the different curing process

The effect of high boiling point solvent on the residual stress behaviors of semiflexible structure poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA) and pseudo-rodlike poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) polyimide was investigated. As a solvent, a mixed solution of 20 wt % cyclohexyl-2-pyrrolidone (CHP; bp = 307 °C) and 80 wt % n-methyl-2-pyrrolidone (NMP; bp = 202 °C) was used. The effects of solvent system and imidizing history on the morphological structure, as well as residual stress, were significantly high in the BPDA-PDA having high chain rigidity, but relatively low in the semiflexible PMDA-ODA with low chain rigidity. In addition, rapidly cured films prepared from PAA (NMP/CHP) showed higher residual stress and a lower degree of molecular anisotropy than slowly cured film imidized from PAA (NMP). This was induced by high chain mobility in polyimide thin films prepared from PAA (NMP/CHP) during the thermal cure process. Therefore, molecular anisotropy, depending on the solvent system and imidizing history, might be one of the important factors leading to low residual stress in polyimide thin films.

A thermally and mechanically stable eco-friendly nanocomposite for chemical sensor applications

Ethanol chemical sensors have been developed by proficient exploitation of polypropylene carbonate (PPC) and PPC/cloisite 20B (clay) nanocomposite (NC) for the detection and quantification of ethanol in the environment. NC was synthesized by the addition of polypropylene carbonate (PPC) into 5 wt% of cloisite 20B. The physicochemical structure was characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy. The thermal and mechanical properties of PPC and NC were investigated by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and nanoindentation analyzer, respectively. NC displayed high thermal and mechanical properties. TGA results revealed that the thermal decomposition temperature (Td50%) of PPC increased significantly, being 43 °C higher than that of pure PPC, while DSC measurements indicated that NC increased the glass transition temperature from 21 to 32 °C. Accordingly, NC showed a high elastic modulus and hardness as compared to PPC. By applying to ethanol sensing, both PPC and NC performed as the best ethanol chemi-sensors in terms of sensitivity. NC showed 3.24 times higher sensitivity (0.8231 μA cm−2 mM−1) as compared to pure PCC (0.2543 μA cm−2 mM−1).