Chien-Chieh Hu

Tuning nanostructure of graphene oxide/polyelectrolyte LbL assemblies by controlling pH of GO suspension to fabricate transparent and super gas barrier films

A technique of layer-by-layer (LbL) self-assembly was used to prepare transparent multilayered gas barrier films consisting of graphene oxide (GO)/branched poly(ethylenimine) (BPEI) on a poly(ethylene terephthalate) substrate. The effect of the GO suspension pH on the nanostructure and oxygen barrier properties of the GO/BPEI film was investigated. The oxygen barrier properties of the assemblies were shown to be highly dependent on the pH. It was demonstrated that the film assemblies prepared using a GO suspension with a pH of 3.5 exhibited very dense and ordered structures and delivered very low oxygen transmission rates (the lowest was <0.05 cm(3) m(-2) day(-1)). The assemblies were characterized with ultraviolet-visible spectroscopy and ellipsometry to identify the film growth mechanism, and the result indicated a linear growth behavior. To analyze the nanostructure of the films, atomic force microscopy, transmission electronic microscopy, and grazing incidence wide-angle X-ray diffraction were used.

Effect of UV-Ozone Treatment on Poly(dimethylsiloxane) Membranes: Surface Characterization and Gas Separation Performance

A thin SiO(x) selective surface layer was formed on a series of cross-linked poly(dimethylsiloxane) (PDMS) membranes by exposure to ultraviolet light at room temperature in the presence of ozone. The conversion of the cross-linked polysiloxane to SiO(x) was monitored by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX) microanalysis, contact angle analysis, and atomic force microscopy (AFM). The conversion of the cross-linked polysiloxane to SiO(x) increased with UV-ozone exposure time and cross-linking agent content, and the surface possesses highest conversion. The formation of a SiO(x) layer increased surface roughness, but it decreased water contact angle. Gas permeation measurements on the UV-ozone exposure PDMS membranes documented interesting gas separation properties: the O(2) permeability of the cross-linked PDMS membrane before UV-ozone exposure was 777 barrer, and the O(2)/N(2) selectivity was 1.9; after UV-ozone exposure, the permeability decreased to 127 barrer while the selectivity increased to 5.4. The free volume depth profile of the SiO(x) layer was investigated by novel slow positron beam. The results show that free volume size increased with the depth, yet the degree of siloxane conversion to SiO(x) does not affect the amount of free volume.

Preparation and Characterization of Bioplastic-Based Green Renewable Composites from Tapioca with Acetyl Tributyl Citrate as a Plasticizer

Granular tapioca was thermally blended with poly(lactic acid) (PLA). All blends were prepared using a plasti-corder and characterized for tensile properties, thermal properties and morphology. Scanning electron micrographs showed that phase separation occurred, leading to poor tensile properties. Therefore, methylenediphenyl diisocyanate (MDI) was used as an interfacial compatibilizer to improve the mechanical properties of PLA/tapioca blends. The addition of MDI could improve the tensile strength of the blend with 60 wt% tapioca, from 19.8 to 42.6 MPa. In addition, because PLA lacked toughness, acetyl tributyl citrate (ATBC) was added as a plasticizer to improve the ductility of PLA. A significant decrease in the melting point and glass-transition temperature was observed on the basis of differential scanning calorimetry, which indicated that the PLA structure was not dense after ATBC was added. As such, the brittleness was improved, and the elongation at break was extended to several hundred percent. Therefore, mixing ATBC with PLA/tapioca/MDI blends did exhibit the effect of plasticization and biodegradation. The results also revealed that excessive plasticizer would cause the migration of ATBC and decrease the tensile properties.

Correlation between free-volume properties and pervaporative flux of polyurethane-zeolite composites on organic solvent mixtures.

Micron-sized zeolite particles were incorporated into a polyurethane (PU) matrix to prepare ethylbenzene-selective membranes. The resulting composite membranes were used in the pervaporation (PV) of ethylbenzene/styrene (EB/ST) mixtures. The sorption, diffusion, and PV permeation behaviors as a result of zeolite addition were elucidated. Zeolite is less chemically compatible with organic solvents than PU and the PU-zeolite composites, which exhibited suppressed solvent solubilities compared with pristine PU. However, these membranes favor EB transport by diffusion selectivity. The diffusivity and permeation flux increases in parallel with the enlarged radius of the free-volume hole size (R(4) increasing from 3.46 to 3.64 Å using positron annihilation lifetime spectroscopy analysis) by increasing the zeolite content from 0 to 23%. The enlarged free volume at a zeolite loading of 23% promoted pure solvent diffusivities by 10% higher than that of the unfilled film. During the PV operation on the EB/ST mixture, a significant diffusion-coupling was observed, and the permeant diffusion coefficients from the binary mixture exceeded the pure solvent diffusivity. The permeation flux was greatly improved (up to 0.72 kg/m(2)·h) by zeolite addition without any detrimental effect on the separation efficiency.

Interfacially polymerized layers for oxygen enrichment: a method to overcome Robeson's upper-bound limit.

Interfacial polymerization of four aqueous phase monomers, diethylenetriamine (DETA), m-phenylenediamine (mPD), melamine (Mela), and piperazine (PIP), and two organic phase monomers, trimethyl chloride (TMC) and cyanuric chloride (CC), produce a thin-film composite membrane of polymerized polyamide layer capable of O2/N2 separation. To achieve maximum efficiency in gas permeance and O2/N2 permselectivity, the concentrations of monomers, time of interfacial polymerization, number of reactive groups in monomers, and the structure of monomers need to be optimized. By controlling the aqueous/organic monomer ratio between 1.9 and 2.7, we were able to obtain a uniformly interfacial polymerized layer. To achieve a highly cross-linked layer, three reactive groups in both the aqueous and organic phase monomers are required; however, if the monomers were arranged in a planar structure, the likelihood of structural defects also increased. On the contrary, linear polymers are less likely to result in structural defects, and can also produce polymer layers with moderate O2/N2 selectivity. To minimize structural defects while maximizing O2/N2 selectivity, the planar monomer, TMC, containing 3 reactive groups, was reacted with the semirigid monomer, PIP, containing 2 reactive groups to produce a membrane with an adequate gas permeance of 7.72 × 10(-6) cm(3) (STP) s(-1) cm(-2) cm Hg(-1) and a high O2/N2 selectivity of 10.43, allowing us to exceed the upper-bound limit of conventional thin-film composite membranes.

Effect of the surface property of poly(tetrafluoroethylene) support on the mechanism of polyamide active layer formation by interfacial polymerization

The mechanism of polyamide formation by interfacial polymerization is important fundamental knowledge for understanding the properties of the polyamide active layer of thin-film composite (TFC) membranes. In this study, TFC membranes of polyamide using poly(tetrafluoroethylene) (PTFE) as the support were prepared by interfacial polymerization. The effect of the surface property of the PTFE membrane support on the mechanism of formation of the polyamide active layer was investigated. Characterization of polyamide–PTFE composite membranes was performed by attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. Positron annihilation spectroscopy was used to analyze the microstructural variation in the polyamide active layer. The growth of the polyamide film was affected by the surface property of the PTFE support. Positron annihilation lifetime spectroscopy (PALS) results showed that the densest structure was at the interface between the polyamide layer and the PTFE support for the polyamide–hydrophilic PTFE composite membrane system; however, the densest structure was at the top surface of the polyamide active layer for the polyamide–hydrophobic PTFE composite membrane system. The high electronegativity of the –CF2– groups on the PTFE support caused “quenching” and “inhibition” effects, resulting in a dramatic decrease in the o-Ps intensity.

Relationship between polymer structure and gas transport properties in a series of fluorine-containing aromatic polyamide membranes for oxygen enrichment

Abstract In this work, a series of fluorine-containing aromatic polyamide membranes were prepared by direct polycondensation of 4,4′-hexafluoroisopropylidene-dibenzoic acid with various aromatic diamines, and the associated gas transport and sorption properties were investigated. The properties of the prepared aromatic polyamides were examined by elemental analysis, FTIR, and X-ray diffraction. It was found that both gas diffusivity and permeability can be increased by the introduction of bulky substituted groups and arylene ether groups into the polymer backbone. Membranes with oxygen permeability of 3.8 barrers and O2/N2 selectivity of 4.6 were obtained by using the polyamide with 2,5 di-tert-butylbenzene group in the polymer backbone. It was also found that the relationship between the membrane structure and the sorption properties can be well explained by the dual sorption model.

Non-destructive means of probing a composite polyamide membrane for characteristic free volume, void, and chemical composition

Positron annihilation spectroscopy measures free volume in membranes at the sub-nanometer scale (0.1–1 nm). In this study, we used positron annihilation age–momentum correlation (AMOC) spectroscopy coupled to a variable mono-energy slow positron beam; our objectives were to measure not only free volume but also voids that are bigger than 1 nm and to estimate the chemical composition of a composite polyamide membrane. To crosscheck these data, we also used conventional techniques: scanning electron microscopy, transmission electron microscopy, and quantum chemical calculations (QCC). AMOC showed that the free volume diameters and intensities in a polyamide layer were in the range of 0.42–0.68 nm and 9–3%, respectively, and that the void diameters and intensities were 7.2–14.1 nm and 12–18%, respectively, with the size distribution of the voids ranging from 5 to 20 nm. These results were consistent with those from TEM, indicating that the polyamide layer structure consisted of discrete voids distributed throughout a continuously dense phase. QCC validated the S parameter data taken from AMOC, which showed that a highly electronegative environment in the polyamide layer could inhibit the formation of positronium.