No abstract

The chain segmental density and diffusional properties of arborescent (highly branched) polystyrenes were investigated by labeling the polymers with pyrene and observing their quenching behavior. A series of arborescent polymers with short (5000 g·mol-1) branches and a linear polymer were investigated. Nitrobenzene and nitrated linear polystyrene were used as quenchers for the pyrene fluorescence. For the nitrobenzene quenching experiments, quencher diffusion coefficients were calculated using the Smoluchowski equation. Comparison of the diffusion coefficients indicates a reduced diffusion rate for small molecules inside arborescent polystyrenes relative to the linear polymer and a decreasing trend with increasing generation number. Quenching experiments with linear nitrated polystyrene led to downward curvature in the Stern−Volmer plots, suggesting that a portion of the chromophores were not accessible to the quencher groups. The fractional Stern−Volmer model was applied, and the quenching rate constants and fraction of accessible chromophores were obtained. The arborescent samples had reduced quenching rate constants, and the fraction of accessible chromophores decreased for higher generation polymers.

The associating coupling process of tertiary-amino-terminated polystyrene (FS) chains with monofunctional and bifunctional dinitrophenol reagents has been studied by means of UV-vis spectroscopy and light-scattering measurements as a function of polymer concentration. The mechanism of coupling was first checked with monofunctional PS chains. The equilibrium constants of the two functional groups of the coupler were found to be slightly different from each other; but with the same average value as for the monofunctional phenol compound. The increase of molecular weight agreed well with predictions from Flory's theory. Much smaller weight average molecular weights were found, however, for the coupling of telechelic PS chains, and the deviations are stronger for the short chains than for the longer ones. The effect is explained by ring formation and three approximations were applied. These are (a) a mean field approximation, (b) the Jacobson-Stockmayer (JS) approach, and (c) the JS approach including excluded volume effect and its influence on Gaussian statistics. Molecular weights were also calculated from the weight fraction of rings and fitted to those obtained by LS where the effective bond length b was chosen to be the adjustable parameter. In the original JS treatment the effective bond length increased with the chain length of the primary chain. In treatment c this could be eliminated by consideration of excluded volume, but absolute values of b are unrealistically low.

SYNOPSISOrganosolv lignin was fractionated on a Sephadex G 75 column with 0.1M aqueous NaOH resulting in 14 fractions. These fractions were acetylated and a high-molecular-weight fraction (F3) was investigated by means of combined static and dynamic light scattering ( LS) in solvents acetone, tetrahydrofuran (THF) , and trifluoroethanol (TFE) . The measurements were found to be reproducible, and recycling of lignin by freeze drying caused slight but unessential changes in solution properties. Depending on the solvent used, weight average molecular weights M , between two and ten million were found. By contrast, M , of the fractions, measured by vapor pressure osmometry (VPO) , was in the range of a few thousands only. Analysis of the angular dependence in static LS by means of a CasassaHoltzer plot and the fractal dimensions showed the presence of chain stiffness, most distinct in TFE. Also, the dynamic light scattering results in TFE are typical for semiflexible chains, while in THF, and to some extent in acetone, the dynamic measurements including viscosity suggest the presence of spherical structures. These findings are being explained by large lignin clusters that consist of stiff subunits.

Dynamic light scattering was used to study the associating coupling process of aminoterminated polystyrene chains with monofunctional and bifunctional dinitrophenol reagents. The tertiary amino end group and the end group which results from association with monofunctional dinitrophenol were found to have no effect on the solution behavior in good solvents. The interaction with the coupling reagent caused a decrease in the mutual diffusion coefficients Dc with increasing polymer concentration. Translational diffusion coefficients Dz°(c) of the associating polymers at each concentration c with a corresponding molecular weight Mw(c) were determined from Dc and its concentration dependence which is expressed by the kD parameter. The molecular weight dependence of the kD parameter was obtained from experimental values of linear polystyrenes in toluene. On the time scale of these experiments, the associates were found to show a diffusion behavior that corresponds to covalently linked chains. Apparently the lifetime of a complex is longer than the measurement time in typical experiments.