Raymond Chien-Chao Tsiang

Infrared studies of thermal oxidative degradation of polystyrene-block polybutadiene-block-polystyrene thermoplastic elastomers

Thermal oxidative degradation of a polystyrene-block-polybutadiene-block-polystyrene thermoplastic elastomer (SBS rubber) has been conducted in an in-situ infrared cell. By monitoring the disappearance of trans-1,4 and vinyl-1,2 double bonds and the appearance of the hydroxyl and the carboxyl/ carboxylate groups in the FTIR spectra, the temperature, air, antioxidant, and molecular microstructure dependence of the polymer degradation was studied. The experimental results indicate that the 1,4-polybutadiene portion of the SBS polymer is easier to degrade than the 1,2-polybutadiene portion and the hydroxyl group appears concomitantly with the disappearance of the polybutadiene. Based on data from the temperature-programmed desorption (TPD) of H2O, it is concluded that the formation of hydroxyl group makes the polymer hydrophilic and promotes the H2O adsorption on it. The amount of H2O adsorption varies with the temperature and the process appears reversible. At low temperatures, the moisture adsorbed onto a degraded polymer sample amounts to approximately 7–10% of total hydroxyls. 1 wt% of Irganox 1076 (antioxidant) would effectively thwart the thermal oxidation process even at severe conditions as high as 225 °C for 12 h. No degradation occurs in an air-free environment.

Using heavy ethers as structure modifiers in the synthesis of SBS block copolymers in cyclohexane

Heavy ethers—diethoxyethane (DIOXO) and triethylorthoacetate (TEOA)—were evaluated and compared with monoethers such as tetrahydrofuran (THF) and diethylether (DEE) as structure modifiers during the synthesis of linear styrene-butadiene block copolymers of polyA-block-polyB-block-polyA type (SBS). A smaller amount of a heavy ether than a monoether was needed to achieve the same targeted content of the 1,2-polybutadiene (vinyl) microstructure. The vinyl content increased from 10 to 71% with increase in the amount of TEOA from 10 ppm to 1 wt %, while the trans-1,4 and cis-1,4 units decreased. Similarly, increasing the amount of DIOXO from 10 ppm to 1 wt % increased the vinyl content from 17 to 89%. TEOA, 300 ppm, or DIOXO, 50 ppm, were suggested for making an SBS copolymer with a targeted 40% vinyl content. The addition of heavy ethers as structure modifiers also increased the rates of polymerization for both styrene and butadiene. Among all ethers, DIOXO enhanced the rate of butadiene polymerization the most, whereas TEOA caused the highest rate of styrene polymerization. Heavy ethers accelerated the rates of polymerization more than did monoethers. Furthermore, for an SBS polymer synthesized via a sequential method, the addition of heavy ethers enhanced the crossover efficiency, resulting in a narrower molecular weight distribution with a lower polydispersity. For an SBS polymer made via a coupling method, the coupling efficiency decreased and varied with the type of the coupling agent used.

PEDOT:PSS/Graphene Nanocomposite Hole-Injection Layer in Polymer Light-Emitting Diodes

We report on effects of doping graphene in poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate), PEDOT:PSS, as a PEDOT:PSS/graphene nanocomposite hole injection layer on the performance enhancement of polymer light-emitting diodes (PLEDs). Graphene oxides were first synthesized and then mixed in the PEDOT:PSS solution with specifically various amounts. Graphenes were reduced in the PEDOT:PSS matrix through thermal reduction. PLED devices with hole-injection nanocomposite layer containing particular doping concentration were fabricated, and the influence of doping concentration on device performance was examined by systematically characterizations of various device properties. Through the graphene doping, the resistance in the hole-injection layer and the turn-on voltage could be effectively reduced that benefited the injection and transport of holes and resulted in a higher overall efficiency. The conductivity of the hole-injection layer was monotonically increased with the increase of doping concentration, performance indices from various aspects, however, did not show the same dependence because faster injected holes might alter not only the balance of holes and electrons but also their combination locations in the light-emitting layer. Results show that optimal doping concentration was the case with 0.03 wt% of graphene oxide.

Epoxidation of partially hydrogenated styrene-butadiene block copolymers using peracetic acid in a cyclohexane/water heterogeneous system

The preparation of partially saturated lightly functionalized styrene-butadiene block copolymers of polyA-block-polyB-block-polyA type (SBS) is described. The work involves epoxidizing partially hydrogenated SBS block copolymers using peracetic acid in a cyclohexane/water heterogeneous system. Five partially hydrogenated model polymers containing low levels of unsaturated aliphatic double bonds were used to study the epoxidation reaction and kinetics. The existence of the epoxide functional group on the product polymer was evidenced by IR and 1 H-NMR spectra and the epoxide concentration was determined by direct titration. The partially hydrogenated SBS copolymers were more difficult to epoxidize than the unhydrogenated ones. The temperature dependence of the epoxidation rate was

Rheological, extractive and thermal studies of the room temperature vulcanized polydimethylsiloxane

The rheological behavior of the room temperature vulcanized polydimethylsiloxane (precursor) with tetraethylorthosilicate (crosslinker) was studied. Data collected from a constant stress rheometer showed that, during the vulcanization, the phase angle increased until reaching the gel point where the elasticity predominated over the viscosity causing a drastic decrease. At a higher ratio of crosslinker to precursor, the vulcanization accelerated and the gel point was reached sooner, thus resulting in a shorter gel time. However, data collected from our extraction experiments indicated that the degree of completion for vulcanization was actually lowered because of an increased amount of sol species, thus necessitating a longer cycle time for a complete cure. The validity of this observation was corroborated by comparing the data collected from the extraction method against those collected from two independent thermal analytical methods, namely the heat capacity method and the cold crystallization method. The degree of completion for vulcanization at each reaction instant determined using these three methods were in excellent agreement with each other. In addition, the presence of residue sol species in the vulcanized products caused a slight decrease in the thermal stability.

Synthesis and characterization of ZnO/ZnMgO multiple quantum wells by molecular beam epitaxy

The growth of single and multiple (three) ZnO/ZnMgO quantum well samples on sapphire substrates, through a two-step temperature variation growth of ZnO buffer layers by molecular beam epitaxy (MBE), were investigated. For single quantum well (QW) growth, the thicker first ZnMgO barrier layer about 220 nm on the high-temperature growth ZnO (HT-ZnO) buffer layer, accumulated larger compressive stress, to achieve higher quality ZnO/ZnMgO QW growth. In the temperature-dependent photoluminescence (PL) results, the obvious S-shape variation of emission peak positions presented the stronger exciton confinement ability of QW in the higher magnesium concentrations of ZnMgO barrier layer growth. Compared to the control sample, the quantum confinement resulted in blueshift PL peaks of QW samples at low temperature. The multiple quantum well (MQWs) structure increased the exciton confinement ability to enhance the light emission efficiency of the sample. The three ZnO/ZnMgO MQWs structures were found clearly by high-resolution transmission electron microscopy.

Preparation and characterization of a star-shaped polystyrene-b-poly(ethylene-co-propylene) block copolymer as a viscosity index improver of lubricant

A saturated star-shaped polystyrene-b-poly(ethylene-co-propylene) block copolymer, (SEP)star, was synthesized for use as a viscosity index improver in lubricants. Polystyrene-b-polyisoprene arms were first made anionically, followed by a linking reaction at the optimum temperature of 60°C with divinylbenzene. The resulting star-shaped (SI)star was hydrogenated to eliminate the double bonds on the polyisoprene segment, thus forming the star-shaped (SEP)star. The number of arms on each molecule increased with an increase in the mol ratio of divinylbenzene to n-butyllithium. Increasing the arm length adversely affected the linking efficiency but caused a slight increase in the degree of branching. The Tg of the poly(ethylene-co-propylene) block was 13°C higher than that of the original polyisoprene block. Compared with (SI)star, (SEP)star has a thermal decomposition temperature 50°C higher but independent of the arm length or the degree of branching. Viscosity measurements for (SEP)star revealed that intrinsic viscosity depends only on the arm length, but not on the degree of branching. Adding 1 wt % of (SEP)star markedly increased the viscosity index of a HN base oil. With a fixed arm length, a (SEP)star having a higher degree of branching increased the viscosity index more than that having a lower degree of branching. On the other hand, the viscosity index increased with an increase in the arm length when the degree of branching was fixed. The addition of 1wt % of (SEP)star increased the vioscosity index up to a number between 111 and 166, with the exact number depending upon its arm length and degree of branching.

New organo‐soluble aromatic polyimides based on 3,3′,5,5′‐tetrabromo‐2,2‐bis[4‐(3,4‐dicarboxyphenoxy)phenyl]propane dianhydride and aromatic diamines

New aromatic tetracarboxylic dianhydride, having isopropylidene and bro- mo-substituted arylene ether structure 3,39,5,59-tetrabromo-2,2-bis(4-(3,4-dicarboxy- phenoxy)phenyl)propane dianhydride, was synthesized by the reaction of 4-nitro- phthalonitrile with 3,39,5,59-tetrabromobisphenol A, followed by alkaline hydrolysis of the intermediate bis(ether dinitrile) and subsequent dehydration of the resulting bis- (ether diacid). The novel aromatic polyetherimides having inherent viscosities up to 1.04 dL g 21 were obtained by either a one-step or a conventional two-step polymeriza- tion process starting from the bis(ether anhydride) and various aromatic diamines. All the polyimides showed typical amorphous diffraction patterns. Most of the polyimides were readily soluble in common organic solvents such as N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), pyridine, and even in less polar solvents like chloroform and tetrahydrofuran (THF). These aromatic polyimides had glass transition temperatures in the range of 256 -303°C, depending on the nature of the diamine moiety. Thermogravimetric analysis (TGA) showed that all polymers were stable, with 10% weight loss recorded above 470°C in nitrogen.

Effects of tetrahydrofuran as a structure modifier in preparation of SBS thermoplastic block copolymers in cyclohexane

Linear styrene-butadiene block copolymers of polyA-block-polyB-block-polyA type (SBS) were synthesized in the presence of varying amounts of THF functioning as the polar structure modifier. The efficiency of this modifier was studied by analyzing the microstructure of synthesized polymers using 13C-NMR, and the effect of THF on polymerization kinetics was determined by progressive buildup of the molecular weight measured by GPC. Polymerization at 4000 ppm THF concentration resulted in the highest styrene polymerization rate while a 1 wt % concentration gave the highest butadiene polymerization rate. The vinyl content increased from 20 to 64% with an increase in the amount of THF from 200 ppm to 1 wt % while the content of trans-1,4 and cis-1,4 units decreased. For SBS polymer synthesized via a sequential process, the use of THF as the structure modifier enhanced the crossover efficiency that would otherwise result in a skewed molecular weight distribution with a higher polydispersity. For SBS polymer made via a coupling process, the coupling efficiency decreased when the amount of THF exceeded a certain limit. The temperature dependence of the coupling efficiency was also investigated.

Indium droplet formation in InGaN thin films with single and double heterojunctions prepared by MOCVD.

Indium gallium nitride (InGaN) samples with single heterojunction (SH) and double heterojunction (DH) were prepared using metal-organic chemical vapor deposition. SH has a layer of InGaN thin film (thicknesses, 25, 50, 100, and 200 nm) grown on an uGaN film (thickness, 2 μm). The DH samples are distinguished by DH uGaN film (thickness, 120 nm) grown on the InGaN layer. Reciprocal space mapping measurements reveal that the DH samples are fully strained with different thicknesses, whereas the strain in the SH samples are significantly relaxed with the increasing thickness of the InGaN film. Scanning electron microscopy results show that the surface roughness of the sample increases when the sample is relaxed. High-resolution transmission electron microscopy images of the structure of indium droplets in the DH sample indicate that the thickness of the InGaN layer decreases with the density of indium droplets. The formation of these droplets is attributed to the insufficient kinetic energy of indium atom to react with the elements of group V, resulting to aggregation. The gallium atoms in the GaN thin film will not be uniformly replaced by indium atoms; the InGaN thin film has an uneven distribution of indium atoms and the quality of the epitaxial layer is degraded.