Intramolecular excimer formation and energy migration in head-to-head polystyrene

Head-to-head polystyrene (HH-PS), in which the phenyl rings are separated by either two or four carbon atoms, displays predominantly fluorescence from monomeric units. No excimer emission is observable even when the polymer is dissolved in increasingly thermodynamically poorer solvents. This is strongly suggestive that excimer formation in normal head-to-tail polystyrene (HT-PS) occurs primarily between nearest-neighbouring chromophores. Intramolecular energy migration is shown to be negligible in both fluid and solid solutions of HH-PS. This is in direct contrast to the case with HT-PS in fluid solutions in which an energy migration coefficient of 7.5 × 10^?5 cm²s^?1 and an energy migration frequency of 4.1 × 10¹¹s^?1 were determined.     

Tacticity dependence of excimer formation in polystyrene solutions

Summary This work deals with the excimer formation in dilute solution for a series of polystyrenes of various tacticity prepared by epimerization of an initial isotactic sample. The main result is that the relative excimer yield remains equal to that obtained for iPS if the percentage of isotactic dyads is higher than 65%.The range of energy migration could be roughly approximated to 6 chromophores.     

Fluorescence excitation spectroscopy of polystyrene near the critical concentration c*

The critical concentration c* was measured using both fluorescence excitation spectroscopy and light scattering for four narrow molecular weight distribution samples of polystyrene (MW = 225,000–1,500,000) in bicyclo[4.4.0]decane (decahydronaphthalene, decalin) at 20 and 30°C. A discontinuity in the corrected intensity of a band in the excitation spectrum (assigned to the dimmer complex) as a function of concentration allowed determination of the critical concentration, c*, from the excitation spectra. The c* resulting from fluorescence and light scattering were identical within experimental error and were comparable to c* values calculated from literature methods. Fluorescence excitation spectroscopy should be a convenient method for determining c* for aromatic polymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 360–364, 2007     

Intramolecular excimer formation in copolymers of methylmethacrylate and indene

A set of fifteen samples of indene (IN) with methylmethacrylate (MMA) copolymers was synthesized by free radical polymerization in bulk. Reactivity ratios, mean sequence lengths and distribution‐functions of monomeric units indicate the formation of quasirandom copolymers, with a larger tendency to blockiness for MMA units. UV and NMR results are consistent with these observations. No typical signals of strongly interacting aromatic moieties can be identified either in the UV short‐wavelength spectra region, or in the ¹H NMR aromatic proton region. Photophysical behavior is analysed. © 2000 Society of Chemical Industry     

The concentration dependence of diffusion in semi-dilute polymer solutions

Dynamic light-scattering data for polystyrene fractions in a good solvent (benzene) and a theta solvent (cyclopentane, 20.4°C) are presented. They encompass a broad concentration interval in the semi-dilute range. These data are compared with those for the systems poly(ethylene oxide)/water and hydroxyethylcellulose/water. The entanglement network in these systems is reflected in part by the nature of the concentration dependence of the mutual diffusion coefficient and in part by the presence of a slow relaxation, information on which may be derived from the non-exponential autocorrelation function.     

Self-diffusion in concentrated polystyrene solutions measured by fluorescence recovery after photobleaching

A polystyrene polymer of narrow molecular weight distribution was carboxylated, then reduced, and finally esterified with NBD-aminohexanoic acid [6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-aminohexanoic acid]. The self-diffusion of the NBD-labelled polystyrene polymer in concentrated solutions of the unlabelled polystyrene polymer was measured by the method of fluorescence recovery after photobleaching over a concentration range from 0.017 g/ml to 0.41 g/ml at room temperature. In the semi-dilute region, the concentration dependence of diffusion coefficient was found to be in agreement with the predictions of scaling concepts.     

Effect of sequence distribution of PES/PEES random,block, and alternative copolymers on excimer formation in solution

Three copolymers of poly(ether sulfone) and poly(ether ether sulfone) with the same composition but different sequence distribution were synthesized by three kinds of methods. Their molecular aggregation in dichloromethane was studied by fluorescence spectrophotometer and electron microscope. The experimental results revealed that the formation of intermolecular excimers in alternative copolymer (A50) dichloromethane solution were observed at a A50 concentration about 1.6 × 10^?2 g/mL by the fluorescence analysis, but the formation of intermolecular excimers in dichloromethane were not found for random copolymer (R50) and block copolymer (B50). The electron micrograph of three copolymer films, heat‐treated at 200°C for 7 days, presented a diffraction micrograph, which suggest that three copolymer molecular aggregation is changed from a randomly coiled amorphous phase to an ordered one, and the order structure of alternative copolymer (A50) was the most distinct in three copolymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Morphological studies of elongated PMMA by excimer formation

The technique of excimer fluorescence has been employed to study the orientational structure of poly (methyl methacrylate) (PMMA). The PMMA films used contain a small amount of naphthalene‐labeled PMMA, and were uniaxially stretched to various elongation at 80°C. The results show that the fluorescence intensity ratio of excimer to monomer, I_E/I_M , increases with the elongation ratio. The increase of excimer forming sites in the system after drawing is attributed to the orientation of both the chain segments and side groups along the drawing direction. The orientation leads to highly dense packing of the PMMA molecules. The conclusion drawn from excimer fluorescence is in agreement with those from other techniques. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1869–1872, 2000     

Cyclization dynamics of polymers: 9. The effect of polymer concentration on the slowest internal relaxation mode of a labelled polystyrene chain

A combination of steady-state and fluorescence decay techniques permits one to measure the dynamics of end-to-end cyclization of a polymer chain substituted at both ends with pyrene groups. In the limit of low concentration, the rate constant for cyclization, k_cy, can be identified with the slowest relaxation rate $\text{τ}{}_1{}^{\mathrm{?1}}$ of a Rouse—Zimm chain. Experiments are reported which allow k_cy to be examined for two chain lengths of polystyrene substituted on both ends with pyrene groups. These chains have $\text{M}\text{?}{}_{\mathrm{n}}\text{= 9200}$ and 25 000 ($\text{M}\text{?}{}_{\mathrm{w}}\text{M}\text{?}{}_{\mathrm{n}}\text{? 1.15}$). Added unlabelled polystyrene polymer [PS] causes $\text{k}\text{?}{}_{\mathrm{<mtext>cy</mtext>}}$ to decrease in cyclohexane just above the θ-temperature, whereas in toluene, a good solvent, k_cy is largely unaffected, even at [PS] concentrations of 50 wt%. These results are explained in terms of frictional effects—hydrodynamic screening—dominating in the poor solvent, whereas other factors tend to have offsetting effects in the good solvent.     

Viscosity of polystyrene solutions expanded with carbon dioxide

The viscosity of solutions of polystyrene (PS) in decahydronaphthalene (DHN) was measured in the presence of carbon dioxide (CO₂) with a moving‐piston viscometer. The effects of the CO₂ pressure (0–21 MPa), polymer concentration (1–15 wt %), temperature (306–423 K), and polymer molecular weight (126 and 412 kDa) on the viscosity were investigated. In the absence of CO₂, the increase in the viscosity with increasing polymer concentration was approximately exponential in concentration and was well described by the Martin equation. All the data fell on a single line when the relative viscosity was plotted against cM^0.50 (where c is the concentration of the polymer in solution and M is the molecular weight of the polymer). The viscosity of the polymer solution decreased with increasing CO₂ pressure under otherwise constant conditions. For a given CO₂ pressure, the viscosity reduction was greatest when the relative viscosity was high in the absence of CO₂, that is, for high‐molecular‐weight polymer, high polymer concentrations, and low temperatures. Reductions in the relative viscosity of almost 2 orders of magnitude were observed in some cases. The viscosity of solutions of PS in DHN also was measured in the presence of sulfur hexafluoride (SF₆). At a given pressure, SF₆ was about as effective as CO₂ in reducing the solution viscosity. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 540–549, 2006     

Magnetic birefringence of polystyrene sulphonate: Molecular weight and concentration dependence

The specific Cotton-Mouton constant of water solutions of PSS-Na⁺ of different molecular weight have been measured at concentrations below 2% in the absence and presence of added salt. A bare persistence length q=12 Å is found from the high ionic-strength molecular weight and concentration independent value. The molecular weight and concentration dependence at low ionic-strength is discussed in terms of the electrostatic contribution to the persistence length and the structure of semidilute polyelectrolyte solutions.     

Effect of the parent solution concentration on the flocculation performance of PAAm flocculants and the relation between the optimal parent solution concentration and critical concentrations

Flocculation performance of three kinds of polyacrylamide (PAAm), linear‐PAAm, Al(OH)₃–PAAm hybrid, and star–PAAm, in kaolin suspensions have been investigated by Spectrophotometer. It was found that the flocculation performance of the polymer flocculant is enhanced at the beginning and then impaired with increasing parent solution concentration (C_p) and an optimal parent solution concentration (C_op) exists, which is directly proportional to both critical concentrations of C* and C_s of the polymer in the dilute aqueous solution, and can be roughly expressed by an empirical formula, C_op = 3.1 × 10^?3 + 643.1C_s. The findings suggests that flocculation performance of a given polymer is dependent on both of the interchain association and the chain contact of the polymer chains in the parent solution and in the kaolin suspension, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1585–1592, 2007     

Polarization of excimer emission in solution

Polarization of fluorescent emission of excimer structures for polystyrene and poly (α-methyl styrene) has been examined in cyclohexane solution. No depolarization was observed resulting in the conclusion that energy migration does not take place under the experimental conditions studied.     

Viscometric study of extremely dilute macromolecular solutions: Critical concentration c and the intrinsic viscosity of the polystyrene through scaling laws. The values of the Huggins constant

A crossover is observed in the variation of the reduced viscosity, η_sp/c, as a function of the concentration, c, for polystyrene solutions in cyclohexane and dioxane. This crossover is attributed to the critical concentration c at which the macromolecular coils start to contact. The critical concentration c is related to the molecular weight and the intrinsic viscosity of the polystyrene through scaling laws. The values of the Huggins constant k' determined in the concentration regions below c (extremely dilute solutions) confirm the Peterson-Fixman theory.     

Tuning the particle size and charge density of the crosslinked polystyrene particles

We report the synthesis of charged spherical colloidal particles of poly [styrene‐(co‐2‐propene sulfonic acid)] crosslinked with divinylbenzene by emulsion polymerization. The effects of concentration of both the emulsifier and initiator on the polymerization, particle size, and charge density are studied. The particle size is found to be dependent on both the emulsifier and initiator concentration and their power dependencies are different. Below critical micelle concentration (CMC), the particle size varies significantly within a small range of emulsifier concentration. In contrast, particle size decrease is not very pronounced at the heterogeneous (micellar) particle nucleation regime where the emulsifier concentration is well above of the CMC. The power dependencies of the number of particles on surfactant concentration are explained in the light of conversion–time profile of the polymerization. The surface charge density of the colloidal particles also varies with both the emulsifier and initiator concentration. Both the particle size and charge density show an inverse relation with the molecular weight of the polymer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The influence of polymer concentration on the internal motion — intramolecular pyrene excimer formation — of a low molecular weight probe in solution

Fluorescence decay measurements were used to study the kinetics of end-to-end cyclization of the molecule Py(CH₂)₅Si(Me₂)OSi(Me₂)(CH₂)₅Py (1), where Py=1 ? pyrenyl. Experiments were carried out in toluene at 22°C and in cyclohexane at 35°C, in the absence and in the presence of added polystyrene (PS). Addition of even small amounts (0.01 g/ml) of PS caused the cyclization rate of probe molecule 1 to decrease substantially in cyclohexane, whereas in toluene, a much milder retardation of cyclization rate with increasing PS was observed. These effects are interpreted in terms of a coupling between the chain dyanmics and that of the probe molecule 1, mediated by the solvent. These hydrodynamic screening interactions depend sensitively upon whether the polymer is in a good or a poor solvent.     

Photo-oxidation of polystyrene in dichloromethane solutions

Photo-oxidation of polystyrene in dichloromethane solution irradiated with light absorbed by the polymer was investigated. The quantum yields of the formation of acetophenone-type carbonyl compounds as well as α, β-enone and dicarbonyl products were calculated based on the analysis of u.v. absorption spectra of irradiated solutions. The relationship between the concentration of these products was found.     

Radical copolymerization of sulphur dioxide and styrene: 4. Intramolecular excimer formation in poly(styrene sulphone)s

Intramolecular excimer formation in poly(styrene sulphone)s with various compositions has been investigated based on the characteristic monomer sequence distributions which were determined experimentally. The fluorescence spectra of the poly(styrene sulphone)s show two emission bands at 285 nm and 330 nm as for polystyrene, corresponding to the monomer and the excimer bands, respectively. The ratio of the excimer to the monomer emission intensities (I_e/I_m) is linearly correlated with the mole fraction of styrene. This observation is a consequence of the characteristic sequence distributions in poly(styrene sulphone)s. containing a very small fraction of SMS units (S = SO₂; M = styrene). The efficiency of excimer formation in poly(styrene sulphone)s with regular styrene sequences, e.g. dyad sequence only, was calculated from the sequence distribution and empirical I_e/I_m. It is concluded that the excimer formation in poly(styrene sulphone)s is very efficient. Energy migration along the copolymer chain was also demonstrated by measuring the degree of polarization as a function of copolymer composition. The efficient excimer formation and the depolarization in poly(styrene sulphone)s suggest that the SO₂ unit in poly(styrene sulphone)s does not act as a barrier or trap to energy migration unlike the methyl methacrylate unit.     

超细聚苯乙烯(PS)树脂的制备工艺研究

采用O/W型苯乙烯微乳液体系,研究了乳化剂OP-10、助乳化剂DBS、交联剂DVB等对聚苯乙烯(PS)粒径分布的影响,基于正交试验,找出了量佳微乳液体系配方。采用激光粒度仪与透射电镜(TEM)对纳米PS粒子的粒径进行了测量研究,结果表明:交联PS粒子单分散性好,大小较均匀。