Determination of the distribution of straight-chain segment lengths in crystalline polyethylene from the Raman LAM-1 band

It is shown how the shape of the longitudinal acoustic vibration observed in the low-frequency region of the Raman spectra of crystalline polymers can be used to obtain a quantitative distribution of lengths of straight-chain segments associated with polymer lamellas. The procedure is demonstrated for a “solid-state” extrudate of polyethylene and for a bulk-crystallized specimen of the same polymer. Equations relating the shape of the LAM-1 band to the shape of the distribution curve are given. The low intensity observed for the LAM-3 mode relative to LAM-1 is explained quantitatively without recourse to end effects. LAM-5 has been observed for bulk-crystallized polyethylene. For the extrudate we find the distribution of lengths of straight-chain segments to have a tail on the long-length side which is not present for the bulk-crystallized sample. The Raman technique is shown to provide new morphological data unattainable at present by other methods.     

The accuracy of the pseudo-fermion method

The pseudo-fermion method for dynamical fermions is studied using Monte Carlo techniques. Various approximations are tested by calculating the mass gap of the Schwinger model and by comparing it with results obtained with an exact algorithm. The mass found is consistent with that obtained from the exact algorithm, implying that the systematic errors are small. In addition, the mass is almost independent of the number of pseudo-sweeps used, implying that the statistical error inherent in the Monte Carlo of the pseudo-fermion field does not have a strong effect on the observable. Numerical evidence is presented to show that the algorithm gives the correct answer for even a single pseudo-sweep in the limit that the Metropolis hit size goes to zero.     

Effect of temperature on carbon-black agglomeration in hydrocarbon liquid with adsorbed dispersant

Suspensions of carbon black in oil, stabilized with adsorbed polyisobutylene succinimide (PIBSI) dispersant, are commonly used as model systems for investigating the soot-handling characteristics of motor oils. The structure of the carbon-black agglomerates changes dramatically with temperature; this results in a concomitant change in the suspension rheology. Linear and nonlinear rheological experiments indicate a large increase of the interparticle attractions as the temperature is raised. To elucidate the origin of this behavior, we investigate the effect of temperature on the stabilizing effect of the dispersant. Measurements of adsorption isotherms of the dispersant on carbon black indicate that there is little variation of the binding energy with temperature. Intrinsic viscosity measurements of PIBSI dispersants in solution clearly exhibit an inverse dependence of the dispersant chain dimension with temperature. These results suggest that the temperature-dependent changes in the chain conformation of the PIBSI dispersant are primarily responsible for the changes in the dispersion rheology, and we propose a simple model to account for these data.