Photo-oxidation of polymers—II: The sensitized decomposition of hydroperoxides as the main path for initiation of the photo-oxidation of polystyrene irradiated at 253.7 nm

Decrease of the intensity of excimer fluorescence during the photo-oxidation of polystyrene indicates that energy transfer occurs from excited phenyl chromophores to oxidation products. Among these, hydroperoxide groups are prominent because, after acting as energy acceptors, they decompose efficiently into free radicals which initiate the photo-oxidation. This effect is clearly demonstrated using cumene hydroperoxide as a model compound for polystyrene hydroperoxides. It is concluded that the sensitized decomposition of hydroperoxide groups provides a general explanation for the characteristic kinetic behaviour of polymers in photo-oxidation as opposed to autoxidation.     

Local compositional fluctuations in PPO/HIPSand PPO/SBS blends

Solid-state NMR relaxation has been used to explore the distribution of components in poly(phenylene oxide) (PPO) high impact polystyrene (HIPS) and PPO/poly(styrene-b-butadiene-b-styrene) (SBS) blends. The nuclear relaxation of PPO in the former system is single exponential for all compositions, but the relaxation of PS in the blend is simple exponential only when the PPO content is low but is otherwise nonexponential. The nuclear magnetization decay curves were analyzed in terms of statistical compositional fluctuation at the scale of spin diffusion distances of several nm. Distribution functions for nuclear relaxation and for blend composition have been derived. Extraction of low molecular weight occluded PS from HIPS resulted in blends having reduced homogeneity. Addition of low molecular weight PS enhanced homogeneity in both the PPO/HIPS and PPO/SBS blends. © 1994 John Wiley & Sons, Inc.     

Gas permeation behavior of PS/PPO blends

Miscible blends of polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) were prepared by a solution casting method to examine the permeation characteristics for oxygen and nitrogen. Gas permeation characteristics of PS/PPO membranes prepared using two different methods, slowly cooled and quenched, after being treated at various annealing temperatures were examined with respect to the controlled free volume. A variety of compositions of PS/PPO membranes provided varying permeability coefficients and ideal separation factors. This indicates that the present miscible blends gave rise to a decrease in the free volume by the physical interaction between two polymer components. For the slowly cooled PS/PPO membranes, the permeability coefficient decreased but the ideal separation factor increased with decreasing annealing temperatures due to the fast macromolecular relaxation or the efficient packing effect of macromolecular chains. On the other hand, the permeability coefficient of the quenched PS/PPO membranes increased with increasing annealing temperatures, as the quenching from higher annealing temperatures led to the existence of more free volume.     

LIGHT-INDUCED QUENCHING OF PHOTOSYSTEM II FLUORESCENCE AT 77 K

Abstract— Light-induced quenching of the low temperature fluorescence emission from photosystem II (PS II) at 695 nm ( F ₆₉₅) has been observed in chloroplasts and whole leaves of spinach. Photosystem I (PS I) fluorescence emission at 735 nm ( F ₇₃₅) is quenched to a lesser degree but this quenching is thought to originate from PS II and is manifest in a reduced amount of excitation energy available for spillover to PS I. Differential quenching of these two fluorescence emissions leads to an increase in the F ₇₃₅/ F ₆₈₅ ratio on exposure to light at 77 K. Rewarming the sample from -196°C discharges the thermoluminescence Z-band and much of the original unquenched fluorescence is recovered. The relationship between the thermoluminescence Z-band and the quenching of the low temperature fluorescence emission ( F ₆₉₅) is discussed with respect to the formation of reduced pheophytin in the PS II reaction center at 77 K.     

THE ROLE OF CYCLIC ELECTRON FLOW AROUND PHOTOSYSTEM I and EXCITATION ENERGY DISTRIBUTION BETWEEN THE PHOTOSYSTEMS UPON ACCLIMATION TO HIGH IONIC STRESS IN Dunaliella salina

Abstract— The distribution of excitation energy between the two photosystems in the halophylic alga Dunaliella salina has been analyzed under ionic stress. In the transition from state 1 to state 2, it was found that a, the absorption cross-section of photosystem (PS) I increased from 42 to 49% until an equal distribution between PS I and PS II was obtained in state 2. Acclimation of the algae to different salt concentrations did not change the fractions of light absorbed in PS II and PS I, but slowed down the transition time from state 1 to state 2. A large increase in ΔpH induced fluorescence quenching was observed which was abolished by the uncoupler nigericin. Photoacoustic quantum yield spectra of energy storage indicated a larger energy storage at 700 nm induced upon stress. The additional ΔpH quenching of fluorescence and the additional quantum yield of energy storage at 700 nm, in the stressed algae, are consistent with the operation of a cyclic, energy-storing pathway in PS I which is uncoupler sensitive.     

Calix[4]arene-based, Hg2+ -induced intramolecular fluorescence resonance energy transfer chemosensor

A novel calix[4]arene-based chemosensor 1 based on Hg2+-induced fluorescence resonance energy transfer (FRET) was synthesized, and its sensing behavior toward metal ions was investigated by UV/vis and fluorescence spectroscopies. Addition of Hg2+ to a CH3CN solution of 1 gave a significantly enhanced fluorescence at approximately 575 nm via energy transfer (FRET-ON) from the pyrenyl excimer to a ring-opened rhodamine moiety. In contrast, addition of Al3+ induced a distinct increase of pyrenyl excimer emission ( approximately 475 nm), while no obvious FRET-ON phenomenon was observed. Different binding behaviors of 1 toward Hg2+ and Al3+ were also proposed for the interesting observation.     

Photochemical degradation of blends of polystyrene and poly(2,6 -dimethyl-1,4-phenylene oxide)

Blends of polystyrene and poly(2,6-dimethyl-1,4-phenylene oxide) that cover the entire compositional range have been subjected to the action of singlet oxygen from microwave discharge, dye-sensitized reaction, and photochemical oxidation. With the applied analytical technique, which consisted of infrared (IR) analysis, including ATR technique and a spectroscopic method combined with chemical analysis for hydroperoxide groups, it was not possible to detect any effect of the singlet oxygen treatment. For that reason singlet oxygen does not appear to be important to the initiation of the photooxidation of these blends. In connection with photochemical oxidation the interaction observed between the two components probably involves energy transfer from PS to PPO. This interaction results in the enhancement of reactions in PPO that lead to greater carbonyl group formation and crosslinking. Simultaneously, the probability of chain scission in the PS is lowered with increased PPO content, found by determining the changes in the molecular weights.     

Fluorescence quenching of pyrene monomer and excimer by CH3I

The fluorescence quenching rate constants of pyrene monomer and excimer by CH₃I were obtained at several temperatures in methylcyclohexane. Both quenching processes are kinetically controlled, allowing insight on the mechanism of quenching. The rate constants have both temperature-independent and temperature-dependent components. The temperature-independent component for both monomer and excimer fluorescence is consistent with quenching due to enhanced intersystem crossing to a lower energy triplet state. The monomer temperature-dependent component comes from the enhancement of the intersystem crossing to a higher energy triplet state. The thermally activated excimer quenching is associated with the excimer dissociation step to give a pyrene in a second triplet state plus a ground state pyrene molecule.     

阴离子表面活性剂存在下光系统II中酪氨酸残基的性能研究

通过荧光光谱和富立叶变换红外(FT-IR)光谱研究了阴离子型表面活性剂－十 二烷基硫酸钠（SDS）与光系统II（PSII）的相互作用。结果表明,PSII表现出酪 氨酸荧光的特性。在PSII蛋白质内部,存在着232 nm处的组分与酪氨酸之间以及这 两种氨基酸列基与叶绿素a之间的能量传递。SDS的存在会使这些能量传递以及 PSII中蛋白的骨架结构和酪氨酸残基的结构发生改变,而变化方式又明显受SDS在 溶液中聚集状态的影响。低于其临界胶束浓度(cmc)时,SDS会促进蛋白质中232 nm外的组分与酪氨酸之间的能量传递,并且使酪氨酸残基外于极性更小的环境; 而大于cmc时,SDS却产生相反的效应。但不同浓度的SDS均会抑制酪氨酸残基至叶 绿素a的能量传递。     

Reactively and physically compatibilized immiscible polymer blends: stability of the copolymer at the interface

This paper reports on the interfacial behaviour of block and graft copolymers used as compatibilizers in immiscible polymer blends. A limited residence time of the copolymer at the interface has been shown in both reactive blending and blend compatibilization by preformed copolymers. Polystyrene (PS)/polyamide6 (PA6), polyphenylene oxide (PPO)/PA6 and polymethylmethacrylate (PMMA)/PA6 blends have been reactively compatibilized by a styrene-maleic anhydride copolymer SMA. The extent of miscibility of SMA with PS, PPO and PMMA is a key criterion for the stability of the graft copolymer at the interface. For the first 10 to 15 minutes of mixing, the in situ formed copolymer is able to decrease the particle size of the dispersed phase and to prevent it from coalescencing. However, upon increasing mixing time, the copolymer leaves the interface which results in phase coalescence. In PS/LDPE blends compatibilized by preformed PS/hydrogenated polybutadiene (hPB) block copolymers, a tapered diblock stabilizes efficiently a co-continuous two-phase morphology, in contrast to a triblock copolymer that was unable to prevent phase coarsening during annealing at 180°C for 150 minutes.     

Photo-oxidation of polymers—VII: A re-investigation of the photo-oxidation of polystyrene based on a model compound study

Fluorescence quenching experiments indicate that energy transfer occurs from cumene excited at 254 nm to cumene hydroperoxide. Quantum yields show that the sensitized decomposition of the hydroperoxide occurs quantitatively and that 2-phenylpropanol-2 is the main photoproduct. In the presence of oxygen, this process plays a dominant role in the initiation of the photo-oxidation. When benzophenone is excited to the first triplet state by irradiation at 365 nm in the presence of cumene hydroperoxide, phosphorescence quenching experiments and laser flash photolysis suggest that an exciplex is formed. This exciplex dissociates into cumylperoxy and ketyl radicals in such a way that 80% of the excited ketone molecules are transformed into the corresponding pinacol. In the presence of oxygen, benzophenone primarily initiates the photo-oxidation of cumene by hydrogen abstraction but, as cumene hydroperoxide is formed, formation and reaction of the exciplex become progressively more and more important. The photochemical behaviour a fluorenone is quite different from that of benzophenone. The sensitized decomposition of cumene hydroperoxide occurs in the presence of that ketone. Surprisingly, fluorenone also initiates the photo-oxidation of cumene; the mechanism of that reaction is discussed. The whole set of results provides a sound basis for the interpretation of the photo-oxidation of polystyrene in various conditions.     

Quenching of exciplex and charge-transfer complex fluorescence of poly(N-vinylcarbazole) – benzanthrone system

When benzanthrone (Bt), a weak electron acceptor, is doped into poly(N-vinylcarbazole) (PVCz) solution or film, an excited carbazole chromophore (D*) interacts with Bt to form a new exciplex state, which gives a broad fluorescence band (λ_max = 440 nm) in solution and a new state, which gives broad fluorescence (λ_max = 550 nm) in the film. In order to elucidate the origin of these new states, we have studied the results of experiments for absorption, concentration dependence of the excimer and exciplex fluorescence quenching, both in solution and in the film, and electric field-induced fluorescence quenching in the film. Taking into account that (i) the new state formation in the PVCz film containing small amounts of Bt enhances the photocurrent in the absorption region, where the photon energy is insufficient to excite polymer molecules directly into the conduction state, (ii) the 550 nm fluorescence of the PVCz - Bt system in film is only partly quenched by electric field, (iii) the appearance of structureless tail in the fluorescence excitation spectrum, the charge transfer interaction model of the PVCz - Bt system in film is proposed.     

PPO/SEBS-g-MAH共混体系的形态结构与冲击性能

从亚微相态和冲击性能出发 ,对比了采用熔融挤出法制备的PPO/SEBS和PPO/SEBS g MAH两种共混物 .结果表明 ,在本文所研究的弹性体用量范围内 ,PPO/SEBS为“海 岛”型结构 ,而PPO/SEBS g MAH呈现网状结构 ;PPO/SEBS体系无脆韧转变现象 ,PPO/SEBS g MAH体系则在弹性体用量为 10 %～ 15 %时出现明显的脆韧转变 ,缺口冲击强度达到 95 0J/m ,这种超韧现象源于其网状结构的形成 .文中进一步用DSC和毛细管流变仪对共混体系的热性能和流变性能进行了测试 ,探讨了PPO/SEBS g MAH共混物网状结构的形成原因     

Chain orientation in polystyrene/poly(2,6-dimethyl-1,4-phenylene oxide) blends

The differential orientation of polymer chains has been measured in polystyrene (PS)/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) compatible blends. Density measurements are reported as a function of binary blend composition at 23°C. Drawing was performed by solid-state coextrusion. PS/PPO blend compositions of 90/10 and 75/25 were drawn within sandwiches of polyethylene at 145°C and isotactic polypropylene at 155°C, i.e. at ca. 25°C above the glass transition temperatures of the two blends. The change in Fourier-transform infrared dichroisms on drawing these blends was measured at 906 and 1190 cm^?1, corresponding to predominantly PS and PPO, respectively. The orientation of PS and PPO was observed as a function of draw ratio λ in the range 1–5; orientations increased with λ for both PS and PPO in both blends but to different degrees. Both polymers decreased in orientation with increasing PPO content. Annealing with fixed ends showed that the PPO chains disorient more slowly than those of PS. All binary systems were found to be amorphous and compatible.     

Surface segregation in blends of miscible amorphous polymers

The isothermal luminescence decay kinetics in near-surface nanolayers of plasma-activated bulk samples of amorphous polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and their miscible blends with weight ratios PS/PPO of 75/25 and 50/50 has been studied at 77 K. The intensities of isothermal luminescence (I) of homopolymer and blend surfaces have been compared. It has been found that the ratio between the luminescence intensities for PS and PPO (I(PS)/I(PPO)) may be as high as 50, while the luminescence intensities for the PS–PPO blends are close to I(PPO). The results obtained indicate that the PPO concentration in the surface layers of the blends is higher than that in the bulk.     

Syntheses and some fluorescence properties of poly(2-methacrylamido-3-arylpropionic acid)

2-Methacrylamido-3-arylpropionic acid, where aryl is 1-naphthyl (M1NpPA), 2-naphthyl (M2NpPA), or 1-pyrenyl (M1PyPA), was synthesized. Free radical polymerization gave polymers which were soluble in water at pH > 6. In aqueous solution, poly(M1NpPA) and poly(M2NpPA) exhibited strong excimer fluorescence as a dominant component, whereas poly(M1PvPA) showed weak monomer fluorescence as a main component. Excitation spectra and fluorescence quenching by Tl⁺ ions suggested that naphthyl groups in poly(M1NpPA) and poly(M2NpPA) interacted so as to form excimer sites, by which migrating excited singlets were trapped, leading to the strong excimer emission. It was suggested, however, that pyrenyl groups in poly(M1PyPA) were sterically so constrained that they could not attain the full-overlap excimer conformation but could only interact to form self-quenching sites, to which energy migration occurred, leading to weak fluorescence. © 1993 John Wiley & Sons, Inc.     

荧光光谱法研究PS分子链在不同溶剂中的荧光特性

利用荧光光谱法研究了溶剂和浓度对聚苯乙烯（PS）荧光特性的影响. 研究表明,PS在四氢呋喃溶液中的荧光发射波长为345 nm,其荧光强度达到最大时的饱和浓度为0.1 mg/mL;PS在甲苯溶液中的荧光发射波长为310 nm,其荧光强度达到最大时的饱和浓度为0.075 mg/mL. 造成这种现象的原因在于:四氢呋喃不具荧光性,不与PS分子链中的苯环发生相互作用,而甲苯溶剂自身具有苯环结构,PS和甲苯生色团荧光频率接近,同时PS分子链上苯环与甲苯上的苯环的间距非常小,相邻苯环上的π电子可以互相重叠,形成T型构造的苯环二聚体,使体系的荧光发射峰蓝移至310 nm. 另外,随着PS在溶液中浓度的增大,开始呈线性增加,当浓度大于饱和浓度C*后,随后其荧光强度呈现非线性下降,这主要是PS在溶剂中形成了二聚体.     

聚甲基苯乙基硅烷的荧光光谱

本文研究了聚甲基苯乙基硅烷(PMPES)的荧光光谱。从浓度效应和猝灭规律证明了PMPES溶液中既有分子间激基缔合物又有分子内的激基缔合物。低温荧光光谱说明在基态时有部分生色团呈激基缔合物构象。     

Energy Transfer between Flavin and Cytochrome c

Energy transfer between photoexcited flavin and cytochrome c has been investigated in order to estimate intermolecular forces between flavin and cytochrome c. The quenching of the fluorescence of flavin by cytochrome c excited at 372 nm was found to be much greater than that excited at 465 nm. This dependence of the quenching on the exciting wavelength is considered to be due to the “prerelaxational” fast energy transfer. From the analysis of the quenching of the fluorescence of FMN and lumiflavin by cytochrome c excited at 465 nm, it was concluded that 1) the quenching is mainly controlled by resonance energy transfer, and 2) the heterogeneous dispersion state of molecules due to electrostatic forces makes the critical transfer distance, R ₀, of the resonance process longer than the real distance. For the quenching of the fluorescence of flavodoxin by cytochrome c, it was found that complex formation is a dominant process and is controlled to a great extent by electrostatic forces. Furthermore, fluorescence decay curves were measured by a single-photon counting method in order to estimate the dynamic processes of flavin fluorescence. The results also showed that the resonance process exists in the energy transfer between flavin and cytochrome c.     

光谱法研究牡荆素与蛋白质相互作用及共存金属离子的影响

利用荧光光谱法和紫外-可见光谱法研究了牡荆素(VT)与牛血清白蛋白(BSA)之间的相互作用.VT对BSA的荧光猝灭为动态猝灭过程,测定了不同温度下的猝灭常数;根据F(o)rster非辐射能量转移理论,计算出VT在BSA中的结合位置与色氨酸残基间的距离为2.675.nm;通过热力学参数推断出VT与BSA之间主要靠疏水作用...