In Woo Cheong

Effect of process variables on molecular weight and mechanical properties of water-based polyurethane dispersion

Abstract Water-based polyurethane dispersions (PUD) were prepared by polyaddition reaction using poly (hexamethylene adipate) glycol (polyol 66), isophorone diisocyanate (IPDI), dimethylol propionic acid (DMPA), and 1,6-hexane diamine (HDA) as chain extender. Various formulations were designed to investigate the effects of process variables such as molar ratio of NCO to OH (NCO/OH), DMPA content, and neutralization degree on molecular weight and mechanical properties. Universal testing machine (UTM) was used for the mechanical properties of emulsion cast film of the PUD. The molecular weights were measured by gel permeation chromatography (GPC). Evolution of the weight average molecular weight ( M w ) and mechanical properties were significantly affected by the neutralization degree during the chain extension. It was found that the M w increased when the NCO/OH molar ratio increased, subsequently mechanical properties such as tensile strength and 100% modulus improved dramatically. However, the elongation decreased as the NCO/OH increased.

Chain extension study of aqueous polyurethane dispersions

Abstract In the several decades, aqueous polyurethane dispersion has been investigated by many researchers. However, relatively little systematic work has been reported in detail on chain extension process. This work describes the reaction of chain extension step (chain extended reaction) as the variation of residual NCO group and subsequent weight average molecular weight of the polyurethane during the chain extension step. Polyurethane dispersion was prepared by neutralization emulsification method. The dispersions with prepared different average particle sizes were obtained by varying the degree of neutralization and each of them chain extended subsequently by water-soluble chain extender; 1,6-hexanediamine. Time-dependent change of free and residual NCO group was measured by FT-IR spectroscopy during the chain extension process. As the total surface area of polyurethane particle decreased, the amount of residual NCO group and consequently required amount chain extender for optimum chain extension decreased. According to the GPC and FT-IR data, optimum amount of 1,6-hexanediamine was determined by the location of the NCO group and contact area between the residual NCO groups and water molecules. Additionally, it was found that excess amount of chain extender had an unfavorable influence on adhesive strength.

Enhanced thermoelectric properties of PEDOT:PSS nanofilms by a chemical dedoping process

We report that a simple chemical dedoping treatment of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanofilms enhances the thermoelectric properties of the polymer nanofilms. The dedoping process was done by over-coating a mixture of dimethyl sulfoxide (DMSO) and hydrazine (HZ), a strong chemical reducing agent, onto the PEDOT:PSS nanofilms. This additional step led to the removal of excess PSS chains and the formation of neutral states of PEDOT chains, resulting in an improvement in the Seebeck coefficient, from 30 μV K−1 to 142 μV K−1, and a decrease in the electrical conductivity from 726 S cm−1 to 2 S cm−1. By controlling the concentration of HZ, we obtained an optimized power factor of 112 μW m−1 K−2 at 0.0175 wt% of HZ in DMSO at room temperature. The corresponding electrical conductivity and Seebeck coefficient under optimized conditions were 578 S cm−1 and 67 μV K−1, respectively. We expect that this simple dedoping process can be applied to general thermoelectric nanofilms based on chemically doped polymers in order to enhance the power factor.

Transparent and flexible organic semiconductor nanofilms with enhanced thermoelectric efficiency

Sequential doping and dedoping increased the conductivity and optimized the oxidation level of transparent and flexible poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS) films, resulting in an improvement in the thermoelectric figure of merit ZT. The electrical conductivity (σ) increased from 970 to 1260 S cm−1 and the power factor from 66.5 to 70.7 μW mK−2 at the optimum concentration of the chemical dopant p-toluenesulfonic acid monohydrate (TSA). Then, the doped PEDOT:PSS films were treated with hydrazine/DMSO solutions with different hydrazine concentrations to precisely control the oxidation level. During the hydrazine/DMSO treatment (dedoping), σ of the films continuously decreased from 1647 to 783 S cm−1 due to a decrease in the carrier concentration, whereas the Seebeck coefficient (S) steeply increased from 28 to 49.3 μV K−1 at the optimum oxidation level. A power factor of 318.4 μW mK−2 (σ = 1310 S cm−1, S = 49.3 μV K−1), the highest among all existing thermoelectric nanofilms, was achieved while maintaining polymer film flexibility and transparency (88.3% of optical transmittance). In addition, the thermal conductivity (κ) of the PEDOT:PSS films decreased from 0.38 to 0.30 W mK−1 upon removal of PSS. At the lowest κ value, a high ZT value of 0.31 was achieved at room temperature.

Microencapsulation of fragrant oil via in situ polymerization: effects of pH and melamine-formaldehyde molar ratio.

Microcapsules containing fragrant oil (Foral oil) were synthesized via the in situ polymerization method using melamine-formaldehyde (M-F) as a wall material. The encapsulation efficiency and other physical properties were analysed with varying formaldehyde/melamine (F/M) mole ratio and pH of emulsion medium. The pH of the reaction medium was varied from 5.0-6.0 and the F/M molar ratio, 2.3 ~ 5.5. Microcapsules containing fragrant oil were synthesized successfully and their particle sizes ranged from 12-15 #181;m. Encapsulation efficiency of fragrant oil varied from 67-81%. It was found that both pH and F/M molar ratio have an effect on the separation of M-F prepolymer, consequently the morphology of the surface of the microcapsule was changed as well as encapsulation efficiency. The encapsulation mechanism, focusing on the liquid-liquid phase separation of methylolmelamines and formation of M-F precursor particle, was described to explain the surface morphology and encapsulation efficiency.

Preparation of uniform microspheres using a simple fluidic device and their crystallization into close-packed lattices.

A versatile technique for producing monodisperse microspheres from both hydrophobic and hydrophilic polymers using a simple fluidic device fabricated with a poly(vinyl chloride) (PVC) tube, a syringe needle, and a glass capillary tube is described. The technique is successfully applied to a variety of different materials, including poly(e-caprolactone) (PCL) as an example of a hydrophobic polymer, ethyl-2cyanoacrylate (ECA) as an example of organic monomer, and gelatin as an example of a hydrophilic, natural polymer. From the calculated capillary number (Ca) andWeber number (We), the system is confirmed to work in the dripping regime. Precise control over particle size can be achieved by varying the polymer concentration and/or the flow rate for the continuous phase. An increase in flow rate for the continuous phase or a decrease in polymer concentration results in the reduction of particle size. The production of raspberry-like microspheres with a mixture of PCL and ECA is also demonstrated. In addition, we have developed a tapping method based on solvent evaporation on a concave glass for crystallizing these microspheres into close-packed lattices. Microspheres with uniform diameters are of great importance in many applications, including, among others, cosmetics, printing, coating, drug delivery, tissue engineering, and photonics. The best established method for producing such spheres relies on the formation of stable oil-in-water (O/ W) or water-in-oil (W/O) emulsions. Two approaches are commonly employed to make these emulsions: emulsification under amechanical or shear force and emulsification involving uniform breakup of a stream of the discontinuous phase as

Swelling and drug release behavior of tablets coated with aqueous hydroxypropyl methylcellulose phthalate (HPMCP) nanoparticles

Organic solvent-based enteric coating technology using hydroxypropyl methyl cellulose phthalate (HPMCP) has been developed for many years due to low water solubility of HPMCP. In this work, aqueous HPMCP nanoparticles (HPMCP-NPs) were prepared by neutralization emulsification method using HPMCP powder and ammonium hydroxide (NH(4)OH) in the absence of any organic solvent and emulsifier. Tablets for enteric use were coated with HPMCP-NP dispersions having different degree of neutralization that was manipulated by ion-exchange process. Disintegration and dissolution behavior of coated tablets were investigated using UV-visible spectrophotometer based on USP method (pH 1.2 and at 37 degrees C) and simulated intestinal fluid (pH 6.8 and at 37 degrees C for 60 min), respectively. The ion-exchange process, which was directly achieved by the protonation of dissociated carboxylic acid group of the aqueous HPMCP-NPs, was introduced as a useful way to control the release rate of drug and hydrophobic nature of HPMCP coating layer with a view for pharmaceutical application. The drug release and swelling were increased with increase in conductivity of aqueous HPMCP-NPs. On the other hand, particle size and polydispersity were decreased with increase in degree of neutralization.

Seeded emulsion polymerization of methyl methacrylate using aqueous polyurethane dispersion: effect of hard segment on grafting efficiency

Abstract Seeded emulsion polymerization of methyl methacrylate (MMA) using aqueous polyurethane dispersion as seed particle was undertaken to investigate the effect of hard segment on the grafting efficiency. The hard segment consists of isophorone diisocyanate (IPDI) and dimethylol propionic acid (DMPA), where the portion of hard segment varied from 19.6 to 35.4%. The effects of IPDI and DMPA contents on the grafting efficiency were confirmed by both FT-IR and thin layer chromatograph equipped with flame ionization detector (TLC/FID). The grafting efficiency increased with increasing hard segment content, i.e. IPDI and DMPA. From the quantitative results of FT-IR spectra and TLC/FID analysis, it was found that the relative area of NH peak (1550 cm −1 ) varied significantly against the contents of IPDI. These results suggested that NH bonds in the hard segment play an effective role of grafting reaction with MMA.

Fabrication of polymeric Janus particles by droplet microfluidics

Janus particles (JPs), with their fascinating property of asymmetry, have received considerable attention in recent years in the fields of colloidal and particulate chemistry. The particles offer a range of exciting potential applications as they possess two distinctive parts with different chemistry, colors, polarities, and/or surfaces. Currently, a number of methodologies are available for the synthesis of JPs. This review presents a short description of polymeric JPs synthesized by droplet microfluidics.

Preparation and characterization of MRI-active gadolinium nanocomposite particles for neutron capture therapy

We demonstrate the synthesis and characteristics of MRI-active Gd2O3 core/SiO2 shell/poly(2-methacryloyloxyethyl phosphorylcholine) corona composite nanoparticles (Gd2O3@SiO2@PMPC NPs). The prepared NPs have a number of attractive features in cancer diagnosis and neutron capture therapy (NCT): biocompatibility, colloidal stability, low cytotoxicity, nucleus affinity, passive targeting, etc. Monodisperse and highly crystalline Gd2O3 NPs were prepared using a polyol protocol to control the average particle size and surface properties. The Gd2O3 NPs were then functionalized with SiO2 and a biomimetic layer of PMPC, to achieve reduced toxicity and enhanced nucleus affinity, for use as an MRI-active Gd-NCT agent. The size of the NPs was tailored to be from 50 to 100 nm for passive accumulation in tumor tissue through loosened capillary vessels. The morphologies and structures of Gd2O3, Gd2O3@SiO2–Br, and Gd2O3@SiO2@PMPC NPs were studied by FT-IR, XRD, HR-TEM, and TGA. In vitro cytotoxicity was investigated with three kinds of normal and cancer cells, and in vitro and in vivo MRI analyses were performed to confirm the contrast ability, accumulation, and sustentation of NPs in tumor tissues.