Surface photovoltage spectra and photoelectrochemical properties of semiconductor-sensitized nanostructured TiO2 electrodes

Three kinds of TiO2 nanostructured thin films and their CdS-sensitized films, consisting of different sizes of TiO2 nanoparticles prepared with different methods, have been investigated. The surface photovoltage spectra (SPS) measurements indicate that the density of surface states on TiO2 is likely dependent upon the details of prepared methods. TiO2 particles prepared from basic sol have more surface states than that prepared from acidic sol. When the TiO2 thin films prepared using the TiO2 sols were sensitized by CdS particles, the SPS responses relative to the surface states on TiO2 from 350 to 800 nm were decreased. The photoelectrochemical properties of nanostructured TiO2 electrodes suggest that the fewer the surface states and the smaller the particle sizes of TiO2, the larger the photocurrent response. For CdS sensitized TiO2 thin film electrode, it is shown that the semiconductor sensitization is an efficient way to decrease the influence of surface states on the charge separation, and can improve the intensity of photocurrent response. (C) 2001 Elsevier Science B.V. All rights reserved.

Nonradiative energy transfer study on the interface of compatibilized linear low density polyethylene/poly(methyl methacrylate) blends

Blends of chromophore-labeled LLDPE and chromophore-labeled PMMA compatibilized by block copolymer of hydrogenated polybutadiene and methyl methacrylate (PHB-b-PMMA) were studied by nonradiative energy transfer (NRET) technique. The ratio of fluorescence intensity of the donor at 336 nm and the acceptor at 408 nm (I-D/I-A) decreased with an increase in block copolymer content. At about 8 wt.-% block copolymer content I-D/I-A reached a minimum value, indicating the interdiffusion of LLDPE chains and PMMA chains in the interface is strongest. The influence of temperature on the interdiffusion of polymer chains in the interface was also examined. Samples quenched in liquid nitrogen from 140 degrees C showed lower energy transfer efficiencies than those annealed from 150 degrees C to room temperature.

Characterization of propylene/ethylene copolymers sequentially polymerized with delta-TiCl3/Et2AlCl catalyst system

Morphological studies of a series of propylene/ethylene sequential polymers have been carried out by permanganic etching and transmission electron microscopy, as an aid to characterization, in conjunction with differential scanning calorimetry. The materials were synthesized using a titanium-based catalyst, with propylene and either ethylene or ethylene/propylene mixture introduced successively, with the aim of examining whether a proportion of block copolymer is obtained. These materials show a complicated phase structure which does not simply reflect polymerization time but varies greatly, especially in regard to the order of introduction of the monomers, and their morphology differs in a number of ways from that of typical commercial materials. Comparison of the materials, as synthesized and after extraction with heptane, suggests that there is a certain amount of material which can compatibilize polypropylene- and ethylene-rich phases, but it was not possible to decide whether it does in fact have block structure.

Miscibility of linear low density polyethylene (LLDPE)-graft-poly(methyl methacrylate) with poly(vinylidene fluoride) (PVF2) and its application as compatibilizer LLDPE/PVF2 blends

Differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were used to study the miscibility of blends of a graft copolymer of poly(methyl methacrylate) on linear low density polyethylene (LLDPE-g-PMMA, G-3) with poly(vinylidene fluoride)(b) (PVF2) and the compatibilization of blends of LLDPE/PVF2. The specific interaction between PMMA side chains and PVF2 in G-3/PVF2 binary blends is weaker than that between the homopolymers PMMA and PVF2. There are two states of PVF2 in the melt of a G-3/PVF2 (60/40, w/w) blend, one as pure PVF2 and the other interacting with PMMA side chains. The miscibility between PMMA side chains and PVF2 affects the crystallization of PVF2. LLDPE-g-PMMA was demonstrated to be a good compatibilizer in LLDPE/PVF2 blends, improving the interfacial adhesion and dispersion in the latter. Diffusion of PMMA side chains into PVF2 in the interfacial region reduces the crystallization rate and lowers the melting point (T-m) and the crystallization temperature (T-c) of PVF2 in the blends.

MOLECULAR-STATE OF GRAFT-COPOLYMERS IN THE INTERFACIAL AREA OF POLYOLEFIN AND POLAR POLYMER

In order to characterize the interface in polymer blends, a new method is suggested, in which the interface is exposed by selectively dissolving in solvent. By means of X-ray photoelectron spectrometry, we studied the molecular state in the interfacial ar

Acid catalyzed synthesis of ordered bifunctionalized mesoporous organosilicas with large pore

Thiol-functionalized mesoporous ethane-silicas with large pore were synthesized by co-condensation of 1,2-bis(trimethoxy-sily)ethane (BTME) with 3-mercaptopropyltrimethoxysilane (MPTMS) using nonionic oligomeric polymer C1H (OCH(2)-CH(2))(10)OH (Brij-76) or poly(alkylene oxide) block copolymer (P123) as surfactant in acidic medium. The results of powder X-ray diffraction (XRD), nitrogen gas adsorption, and transmission electron microscopy (TEM) show that the resultant materials have well-ordered hexagonal mesoscopic structure with uniform pore size distributions. (29)Si MAS NNR, (13)C CP-MAS NMR. FT-IR, and UV Raman spectroscopies confirm the attachment of thiol functionalities in the mesoporous ethane-sificas. The maximum content of the attached thiol group (-SH) in the mesoporous framework is 2.48mmol/g. The ordered mesoporous materials are efficient Hg(2+) adsorbents with almost every -SH site accessible to Hg(2+). In the presence of various kinds of heavy metal ions such as Hg(2+), Cd(2+), Zn(2+), Cu(2+) and Cr(3+), the materials synthesized using poly(alkylene oxide) block cooollxmier (Pluronic 123) g(2+), as surfactant show almost similar affinity to Hg(2+), Cd(2+), and Cr(3+), while the materials synthesized using ofigomeric polymer C(18)H(37)(OCH(2)CH(2))(10)OH (Brij-76) as surfactant exhibit high selectivity to Hg(2+). (C) 2004 Elsevier Inc. All rights reserved.

Effect of binary block-selective solvents on self-assembly of ABA triblock copolymer in dilute solution

We have studied the self-assembly of the ABA triblock copolymer (P4VP-b-PS-b-P4VP) in dilute solution by using binary block-selective solvents, that is, water and methanol. The triblock copolymer was first dissolved in dioxane to form a homogeneous solution. Subsequently, a given volume of selective solvent was added slowly to the solution to induce self-assembly of the copolymer. It was found that the copolymer (P4VP(43)-b-PS366-b-P4VP(43)) tended to form spherical aggregate or bilayer structure when we used methanol or water as the single selective solvent, respectively.

Formation of reversible shell cross-linked micelles from the biodegradable amphiphilic diblock copolymer poly(L-cysteine)-block-poly(L-lactide)

A novel biodegradable diblock copolymer, poly(L-cysteine)-b-Poly(L-lactide) (PLC-b-PLLA), was synthesized by ring-opening polymerization (ROP) of N-carboxyanhydride of beta-benzyloxycarbonyl-L-Cysteine (ZLC-NCA) with amino-terminated Poly(L-lactide) (NH2-PLLA) as a macroinitiator in a convenient way. The diblock copolymer and its precursor were characterized by H-1 NMR, Fourier transform infrared (FT-IR), gel permeation chromatography (GPC), and X-ray photoelectron spectroscopy (XPS) measurements. The length of each block polymer could be tailored by molecular design and the ratios of feeding monomers.

Order-Order Transition of C -> sdG -> sL -> S in ABC Triblock Copolymer Thin Film Induced by Solvent Vapor

The morphology transition of polystyrene-block-poly(butadiene)-block-poly(2-vinylpyridine) (SBV) triblock thin film induced in benzene vapor showing weak selectivity for PS is investigated. The order-order transitions (OOT) in the sequence of core-shell cylinders (C), sphere in 'diblock gyroid' (sdG), sphere in lamella (sL) and sphere (S) are observed. The projection along (111) direction in Gyroid phase (sdG(111)) is found to epitaxially grow from C(001) in the film.

Compositional dependence of the microphase separation transition in styrene-butadiene-styrene triblock copolymer/polystyrene blends

The change in the microphase separation transition (MST) temperature of a styrene-butadiene-styrene (SBS) triblock copolymer induced by the addition of polystyrene (PS) was investigated by small-angle X-ray scattering. It was found that the transition temperature was determined from the molecular weight (M(H)) Of the added PS in relation to that of the corresponding blocks (M(A)) in the copolymer. The MST temperature decreased with added PS if M(H)/M(A) < 1/4, while it increased with added PS when M(H)/M(A) > 1/4 Analysis of the theoretical expression based on the random phase approximation showed exactly the same tendency of change in the transition temperatures as that observed experimentally. The interaction parameter, chi(SB), obtained by nonlinear fitting of the scattering profiles of SBS/PS blends in the disordered state, was found to be a function of temperature and composition. Composition fluctuations were found to exist in SBS/PS blends, increasing with increasing addition of PS but diminishing with increasing molecular weight of the added PS.

Lattice Boltzmann study of hydrodynamic effects in lamellar ordering process of two-dimensional quenched block copolymers

By incorporating self-consistent field theory with lattice Boltzmann method, a model for polymer melts is proposed. Compared with models based on Ginzburg-Landau free energy, our model does not employ phenomenological free energies to describe systems and can consider the chain topological details of polymers. We use this model to study the effects of hydrodynamic interactions on the dynamics of microphase separation for block copolymers. In the early stage of phase separation, an exponential growth predicted by Cahn-Hilliard treatment is found. Simulation results also show that the effect of hydrodynamic interactions can be neglected in the early stage.

Rigidity effect on phase behavior of symmetric ABA triblock copolymers: A Monte Carlo simulation

The phase behavior of symmetric ABA triblock copolymers containing a semiflexible midblock is studied by lattice Monte Carlo simulation. As the midblock evolves from a fully flexible state to a semiflexible state in terms of increase in its persistence length, different phase behaviors are observed while cooling the system from an infinite high temperature to a temperature below T-ODT (order-disorder transition temperature). Within the midblock flexibility range we studied (l(p)/N-c <= 0.105), a lamellar structure is formed at equilibrium state as the situation for fully flexible chains. The fraction of bridge chain is evaluated for the lamellar structures. We find that the increase in midblock rigidity indeed results in the increase in bridge chain fraction within the range from 44.9% to 51.8%.

Effect of the nature of annealing solvent on the morphology of diblock copolymer blend thin films

The morphologies and structures for the thin film of blend systems consisting of two asymmetric polystyrene-block-polybutadiene (SB) diblock copolymers induced by annealing in the vapor of different solvents, namely, cyclohexane, benzene, and heptane, which have different selectivity or preferential affinity for a certain block, were investigated by tapping mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). The results revealed that even a slight preferential affinity of good solvent for one block would strongly alter the morphology of the blend thin film.

Novel pH- and temperature-responsive block copolymers with tunable pH-responsive range

A series of novel pH- and temperature-responsive diblock copolymers composed of poly(N-isopropylacrylamide) (PNIPAM) and poly[(L-glutamic acid)-co-(gamma-benzyl L-glutamate)] [P(GA-co-BLG)] were prepared. The influence of hydrophobic benzyl groups on the phase transition of the copolymers was studied for the first time. With increasing BLG content in P(GA-co-BLG) block, the thermal phase transition of the diblock copolymer became sharper at a designated pH and the critical curve of phase diagram of the diblock copolymer shifted to a higher pH region.

"Sandglass"-shaped self-assembly of coil-rod-coil triblock copolymer containing rigid aniline-pentamer

The amphiphilic PEG1 500-b-EM AP-b-PEG1 500 (EM PAP) triblock copolymer of poly(ethylene glycol) (PEG) and emeraldine aniline-pentamer (EM AP) in its concentrated solution can self-assemble into a special shape like "sandglass", as observed by transmission electron microscopy (TEM), field emission scanning electron microscopy (ESEM) and atomic force microscopy (AFM). This "sandglass"- shaped assembly is composed of several "rods" aggregated in the middle, with every "rod" being about 8 VLrn in length and 300 nm in diameter.