Nanostructured Composites with Layered Morphology: Structure and Properties

Using different microscopic techniques, we investigate the morphology and the micro-deformation processes in two entirely different classes of polymer based composites: natural biocomposites and synthetic polymer composites. The emphasis has been put on the comparison of the micromechanical properties of those composite materials. In the natural layered composites exemplified by human cortical bone, analogous to the synthetic glassy polymers, craze-like deformation zones were formed. A strong dependence of deformation mechanisms (such as transition from formation of single crazes to multiple crazing behaviour) on the layer dimension was observed in the layered composites made up of different amorphous polymers.     

Determination of the space-charge field amplitude in polymeric photorefractive polymers

The space-charge field built in a polymeric photorefractive polymer was calculated by a simple method based on the oriented gas model. When anisotropic chromophores in a photorefractive polymer were exposed to an external field, they oriented preferentially to exhibit a birefringence. Then, under illumination of two coherent beams and an external field, they reoriented to form a photorefractive grating. During the formation of the grating, the chromophores were reoriented by the space-charge field as well as by the external applied field. The birefringence induced in the material by an external electric field was determined by measuring the transmittance of the sample which is placed between crossed polarizers, where birefringence depicts the orientation of the chromophores. By measuring the diffraction efficiency with a modified degenerate four-wave mixing setup, the index amplitude of the grating was determined. Finally, the space-charge field was determined by comparing the diffraction efficiency with the birefringence with respect to the applied electric field. In our study, the space-charge field was about 20% of the external applied field, which coincided with previous results obtained from our laboratory.     

Infrared photorefractive composites based on polyvinylcarbazole and carbon nanotubes

Photorefractive materials based on unplasticized polymers that have a high glass-transition temperature and the frozen random orientation of chromophores were prepared by layer casting. Under these conditions, only the third-order susceptibility has a nonzero value, increasing with an increase in the conjugation-chain length and reaching considerable values in the case of nanosized molecules, such as single-wall carbon nanotubes (SWNTs). In SWNT-containing polyvinylcarbazole, the photoelectric sensitivity and photorefractive characteristics were measured at 1064 nm. Using electric-field induced second-harmonic generation, the third-order susceptibility was estimated. In a composite containing 0.26 wt % SWNT, the diffraction grating is displaced by 5° relative to the interference pattern, the result that is presumably due to the close mobility of unlike charge carriers. Therefore, the beam-coupling gain coefficient Γ and the net gain Γ-α have low values, which are 53 and 42 cm^?1, respectively, at 115 V/μm.     

Enhanced performance of photorefractive poly(N‐vinyl carbazole) composites

We present the enhanced photorefractive performance of high molecular weight poly(N‐vinyl carbazole) (PVCz)‐based composites. Higher diffraction efficiency with faster speed of grating build‐up was obtained by optimizing the composition of the PVCz composites. At relatively low applied electric field of E = 45 V μm^?1, diffraction efficiency of 26% for p‐polarized probe beam and corresponding that of 5.1% for s‐polarized probe beam were measured with faster grating build‐up speed of 48.3 s^?1 for the composite with 2,4,7‐trinitrofluorenone (TNF) as a sensitizer. Fastest speed of grating build‐up of 100 s^?1 and large optical gain up to 110 cm^?1 were measured at E = 80 V μm^?1 for the composite with fullerene derivative of PCBM as a sensitizer. These improved performances are due to a large orientational enhancement effect with faster response speed in addition to Pockels effect for the samples with appropriate glass transition temperature. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Photorefractive polymer composites based on nanosized nonlinear optical chromophores

Photorefractive (PR) polymer composites based on polymers with a high glass transition temperature in which the random distribution of a photosensitizer and a nonlinear optical chromophore as dopants is “frozen” were designed. In the case of the random distribution of chromophores, only the third-order electric susceptibility has a nonzero value. Therefore, nanosized structures having high third-order polarizability due to an extended conjugated-bond system (or cooperative electronic excitation) were used as nonlinear chromophores. Good PR characteristics are displayed by polymeric composites containing nanosized structures, such as cyanine dye J aggregates, supramolecular assemblies of ruthenium(II) complexes, and single-wall carbon nanotubes. The use of extended nanosized chromophores as simultaneous spectral sensitizers allowed polymer composites with PR sensitivity in the near IR region at 1064 and 1550 nm to be designed.     

PHOTOINDUCED SURFACE RELIEF GRATINGS ON AZOCELLULOSE FILMS

Surface relief gratings (SRG) on films of azobenzene–modified cellulose (azocellulose) with ultrahigh molecular weight were holographically recorded in a single step. Surface modulation depth of the SRGs increased with the degree of substitution (DS) of the azobenzene chromophores in the cellulose as well as with the grating spacing. Large surface modulations over 500 nm on all samples were obtained with a grating spacing of 4 μm. A linear increase of photoinduced birefringence with the azobenzene chromophore concentration in these photodynamic polymers was also observed. The maximum birefringence of 0.11 was achieved for an azocellulose polymer with a DS of 0.99.

     

Photorefractive effect of ferroelectric liquid crystals

This paper reviews our recent work on the photorefractive effect of ferroelectric liquid crystals (FLCs). The photorefractive effect is defined as the optical modulation of the refractive index of a medium as a result of a variety of processes. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon enables the creation of different types of photonic applications. FLCs exhibit fast electric field response, and the orientation of the molecular axis of FLCs changes its direction according to the change in direction of the spontaneous polarization (Ps). When two laser beams interfere in a photoconductive FLC, an orientational grating is formed. The mechanism of the formation of the grating is based on the response of the Ps to the photoinduced internal electric field. The time of formation of the refractive index grating is significantly shorter in FLC materials.     

Novel water-soluble photosensitizer based on starch and containing porphyrin

Novel water-soluble polymeric photosensitizers (SPO) based on starch and containing porphyrin chromophores were synthesized and studied. The polymers were soluble in water and in dimethyl sulfoxide. Photophysical studies and solubilization of molecular probes proved the formation of hydrophobic, rigid microdomains in an aqueous solution of SPO; they were created due to the clustering of porphyrin chromophores attached to the polymer chain. SPO polymers absorbed light from the UV-visible spectral region. The polymers could sensitize photochemical reactions mediated by electron transfer, energy transfer or both, from the singlet-excited state of porphyrin chromophores to the molecules of organic compounds solubilized in the hydrophobic microdomains or residing in the water phase.     

若干新型有机光电功能材料的分子设计、合成与性能 研究

介绍本研究组得到国家自然科学基金资助的四个方面工作的进展。研究了金属有机化合物的结构与非线性光学性质的关系,总结了从分子几何构型着手,根据不同用途,对金属有机非线性光学材料进行分子设计的经验规律;提出了利用“组合式共轭桥”进行有机非线性光学发色团分子设计的新思路,所合成的几个有机化合物既有很大的光学非线性,又有紫移的最大吸收峰;通过化学键将有机发色团分子张到各种高分子的侧链上,合成和表征了潜在的电光高分子和光折变高分子材料;采用无机-有机夹层复合的思路对兼有导电性和强磁性的分子材料进行了探索,将一些有机小分子和导电高分子分别插入了层状无机物MPS3的层间,得到了8个新的分子磁体,而另一夹层化合物则表现了较高的电导率。     

The influence of photosensitizers on the photorefractivity in poly[methyl-3-(9-carbazolyl)propylsiloxane]-based composites

We investigated the photosensitizers effect on the photorefractive (PR) properties in five poly[methyl-3-(9-carbazolyl)propylsiloxane] (PSX-Cz)-based PR composites which were doped with various photosensitizers having each different electron affinity, such as 2,4,5,7-tetranitro-9H-fluorine-9yilden malonitrile (TeNFM), 2,4,7-trinitro-9-fluorenone (TNF), 9-dicyanomethylene-2,4,7-trinitro-fluorenone (TNFM), tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinnodimethane (TCNQ). At 632.8 nm, photo-charge generation efficiencies, photoconductivities, space charge field, four wave mixing diffraction efficiencies, and PR grating buildup times were measured as a function of external electric field. The photo-charge generation, which is dependent on the light absorption, was achieved through the charge transfer (CT) complexes between the PSX-Cz and each of the photosensitizers. The photon energy of the CT transition decreased with increasing electron affinity of the photosensitizer. In composites doped with TeNFM, TNF, and TNFM, the space charge field (E_sc) increased as the photo-charge generation efficiency increased; the grating buildup in these composites is rate-limited by the photo-charge generation speed. In sample doped with TCNE, and TCNQ, the hole mobility was reduced due to the larger amount of photosensitizer anion traps produced by photoreduction of the photosensitizer. Then, the grating buildup speed became hole mobility limited, and smaller buildup rates were observed. The magnitude of space charge field was sustained as the charge and trap density increased. In all composites, the refractive index modulation is increased with the magnitude of space charge field.     

Synthesis and characterization of polyurethane anionomers with bisazoaromatic chromophores and carboxylate groups

A new diol with a bisazoaromatic pendant was prepared to obtain photosensible polymers suitable for dyed aqueous systems. A polyurethane bearing bisazoaromatic chromophores, based on a poly(tetramethylene oxide) diol (average molecular weight = 2000), 2,4‐tolylene diisocyanate, and the aforementioned azo diol, was synthesized and characterized. Bichromophoric polyurethane anionomers, prepared by a two‐step substitution of urethane hydrogen atoms with sodium carboxylate groups, were studied. The influence of the concentration of carboxylate groups (30–158 mequiv of ionic groups/100 g of polymer) on some polymer properties and photoisomerism in polymer solutions and thin films was examined. In particular, the polymer structure and its morphology dictated the proximity of anchored bisazo chromophores and the capability of intermolecular forces between dyes producing hydrogen aggregates in solutions and thin films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5463–5470, 2004     

Novel polymers containing carbazole chromophores for nonlinear optics: Photorefractive application

Oligomers and polymers containing carbazole chromophores in the main chain and/or side chain have been designed and synthesized as a model compound of monolithic photorefractive materials. Nonlinear optical properties of these compounds and image processing application have been studied.     

Atomistic molecular modeling of the effect of chromophore concentration on the electro-optic coefficient in nonlinear optical polymers

We employ fully atomistic molecular modeling to investigate the concentration dependence of the electro-optic coefficient of two guest-host polymer composites. Using classical molecular dynamics, we record the time-evolution of the guest-host system under the application of an external electric field. Through analysis of the orientation of the nonlinear optical chromophores in the guest-host composite with respect to the direction of the external electric field, we calculate the orientational parameter N < cos(3)theta >, with N being the number density of chromophores in the composite. This parameter is directly proportional to the electro-optic coefficient. We find agreement between the concentration dependence of the electro-optic coefficient calculated through our simulation and that from experimental data and also from Monte Carlo models.     

Efficient Nonlinear Optical Properties of Dyes Confined in Interlayer Nanospaces of Clay Minerals

Nonlinear optical (NLO) responses from organic dyes can be maximized when the dyes are aligned in appropriate manners in bulk materials. The use of restricted nanospaces provided by interlayer spacing of inorganic layered materials is a promising strategy for imposing suitable molecular alignments for NLO materials on dyes. The hybrid materials thus obtained exhibit salient NLO responses owing to the improved molecular orientation. In some cases, extension of the π‐electron system as a consequence of improved molecular planarity, obtained by the intercalation of a dye into the 2‐dimensional interlayer space of an inorganic layered material, is also observed as a factor that enhances NLO responses of chromophores at the molecular level. This review focuses on recent progress in the strategies for controlling the molecular orientation of NLO‐phores by employing clay minerals, which are one of the typical inorganic layered materials. In addition, development of a means for fabricating composites that satisfy the properties of an optical material, such as a sufficient size and thickness, a flat surface, and low light‐scattering characteristics is required to utilize the superior NLO properties observed for clay/dye hybrid materials for practical applications. A novel means for obtaining such a hybrid material is also outlined.     

Photoelectric, nonlinear optical, and photorefractive properties of polyvinylcarbazole composites with graphene

Polyvinylcarbazole (PVK) composites containing graphene in an amount of somewhat less than 0.15 wt % exhibit third-order dielectric susceptibility due to the presence of graphene, as well as photoelectric and photorefractive sensitivity at 1064 nm. The photorefractive (PR) effect is known to occur in a polymer composite that possesses both photoelectric sensitivity and nonlinear optical properties. The photoelectric, nonlinear optical, and PR properties of PVK composites with graphene have been considered in this paper.     

Carbazole main-chain polymers for multifunctional materials: Synthesis and characterization

A series of α,ω-bis(3-formylcarbazolyl)alkane monomers have been prepared starting from carbazole and α,ω-dibromoalkane in two steps. Main-chain polymers with carbazole substituted with one acceptor group as the nonlinear optical (NLO) chromophores and photoconductive moieties were synthesized by the Knoevenagel polycondensation of α,ω-bis(3-formylcarbazolyl)alkane and 1,6-bis(cyanoacetoxy)hexane using 4-(N,N-dimethyl)pyridine (DMAP) as a base. This Knoevenagel polycondensation involved two stages: polycondensation was first carried out in tetrahydrofuran solution, and then polycondensation was allowed to continue in the solid-state after removal of solvent. All of these main-chain polymers characterized by spectroscopic methods and elemental analyses are soluble in common low boiling organic solvents, such as chloroform. These polymers were found to be amorphous with glass transition temperatures in the range of 94–117°C by differential scanning calorimetry. This two-stage polycondensation gave polymers with weight-average molecular weight in the range of 25,000–72,000 g/mol by gel permeation chromatography. The study of the nonlinear optical and photorefractive properties of these polymers is in progress. © 1996 John Wiley & Sons, Inc.     

Bandgap engineering of indenofluorene‐based conjugated copolymers with pendant donor‐π‐acceptor chromophores for photovoltaic applications

Three narrow‐band‐gap conjugated copolymers based on indenofluorene and triphenylamine with pendant donor‐π‐acceptor chromophores were successfully synthesized by post‐functionalization approach. All the polymers have good solubility in common solvents and excellent thermal stability. The photophysical properties, energy levels and band gaps of the polymers were well manipulated by introducing different acceptor groups onto the end of their conjugated side chains. By using different acceptor groups, the band gaps of the polymers were narrowed from 1.86 to 1.53 eV by lowering their lowest unoccupied molecular orbital levels, whereas their relatively deep highest occupied molecular orbital levels of approximately ?5.35 eV were maintained. Bulk‐heterojunction solar cells with these polymers as electron donors and (6,6)‐phenyl‐C₇₁‐butyric acid methyl ester as acceptor showed power conversion efficiencies as high as 3.1% and high open circuit voltages more than 0.88 eV. The relationships between the performance and film morphology, energy levels, charge mobilities were discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and characterization of tin telluride inorganic/organic composite materials with nanoscale periodicity through solution-phase self-assembly: a new class of composite materials based on Zintl cluster self-oligomerization

In this work, we demonstrate the synthesis of semiconducting tin telluride inorganic/organic composite materials with nanoscale periodicity prepared using solution phase self-assembly. Oligomerization of anionic SnTe ₄ ^4? clusters by halogen-mediated tellurium elimination in the presence of surfactant leads to the formation of a meosotructured composite. The composites initially forms as a mixture of mesophases, usually some combination of a layered phase and a phase based on cylindrical building blocks. Post synthetic treatment leads to a solid-state structural change which converts the composites to a single mesophase architecture with a hexagonal honeycomb (p6mm) morphology on the nanometer length scale. A by product of this reaction, however, is bulk tellurium. Changes in the electronic structure of the materials during synthesis and solid-state restructuring are probed using electron spin resonance (ESR) spectroscopy.     

功能化的无机复合体系

通过化学物理相互作用将不同组分进行复合可以形成各种各样的复合体系。如果体系中的组分均为无机物或以无机物为主则相应的复合体系称为无机复合体系。通过对组成、结构及形貌等进行设计与调控,可以赋予无机复合体系独特的性质和功能。常见的无机复合体系主要包括主-客体复合物、配位聚合物以及各种纳米复合体系等。这些无机复合体系的功能化对新材料及新能源的开发和利用具有重要意义。本文综述了各种新型无机复合体系的最新研究进展,总结了本课题组在无机复合体系及其功能化的设计与开发方面取得的最新结果,并对功能化的无机复合体系作为新型材料的应用进行了展望。