The mechanisms of latex particle formation and growth in the emulsion polymerization of styrene using the surfactant sodium dodecyl sulfate

A new method is presented that provides experimental information which is qualitatively and quantitatively sensitive to assumptions made as to the mechanisms of free radical entry and of latex particle formation in emulsion polymerization systems. The method consists of (1) obtaining (by electron microscopy) the full particle-size distributions (PSDs) at several different times soon after the cessation of latex particle nucleation, (2) using these PSDs to determine the volume dependences of the various rate coefficients governing particle growth by fitting the data to the appropriate evolution equations, and (3) employing these empirical rate coefficients to find that time dependence of the nucleation rate which fits the early-time PSD (again using the evolution equations). This method is quite sensitive to mechanistic assumptions: for example, one is able to determine whether or not the nucleation rate is an increasing or decreasing function of time. The technique is applied to a styrene nucleation system employing sodium dodecyl sulfate as surfactant at well above the critical micelle conventration. The data cannot be fitted even qualitatively by a simple one-step nucleation mechanis, whether it involes micellar entry or homogeneous nucleation. It is found, on the other hand, that the results can be accurately fitted by assuming that coagulation events between primary colloidal particles, perhaps formed by homogeneous nucleation, dominate both the nucleation process and the entry of free radicals into mature latex particles. In addition, the data indicate that the rate of free radical entry into the latex particles decreases with increasing particle size, at least for particles of unswollen radius less than ca. 40 nm.     

Muon level crossing resonance in aluminium

Cross relaxation between implanted positive muons and²⁷Al nuclei in Al metal, lightly doped with Cu to impede the muon diffusion, shows weak resonances at 2.3 and 4.0 mT. Assignment of these to polarization transfervia the 1/2 3/2 and 3/2 5/2 transitions of the (J=5/2) spins leads to a quadrupole coupling constante ²qQ/h=1.8(1) MHz,i.e. an electric field gradient on Al nuclei immediately adjacent to the muon siteq=0.048(3) a.u., with a small departure from axial symmetry.     

Structure of scillaglaucoside. 56. Section on cardiac glycosides

The structure of scilliglaucoside, a cardiac active squill glycoside, is revised. The assignment of the new structural formula 1 is made on the basis of NMR. spectral data.     

Transverse-and zero-field μSR investigation of magnetism and superconductivity in (Y1−xPr x )Ba2Cu3O7

Zero-field muon-spin-rotation (μSR) measurements on (Y₁? _x Pr _x )Ba₂Cu₃O₇ [x=1.0, 0.8, 0.6, and 0.54] show evidence for antiferromagnetic ordering of the Cu moments within the Cu?O planes, with Néel temperatures 285, 220, 35. 30 and 20 K, respectively. Forx=1.0 the local muon magnetic field is ≈16 mT, but decreases to ≈12 mT at 17K, due to additional magnetic ordering. The zero-field data, in conjunction with transport data, allow construction of a complete phase diagram for this system. Transverse-field (1 kOe) μSR data forx=0.2 (T _c =75 K) show that the muon depolarization is determined primarily by the Cu nuclear moments forT>T _c , and by the vortex state forT _c . Fitting the superconducting-state data to a BCS model yields an extrapolated zero-temperature magnetic penetration depth of 2170 Å.     

Muon bonding sites in rare earth orthoferrites

A muon site search has been performed for the RFeO₃ series based on a calculation of dipole fields and assumptions that a μ-0 bond is formed at identical sites in each sample. The site previously identified by Holzschuh et al. /1/ has been verified and additional sites located which together explain all the observed μSR frequencies in the orthoferrites. Effects due to muon motion and covalent contributions to the internal fields are briefly discussed.     

Muon Knight shift and zero-field relaxation in (U,Th)Be13

We report μ⁺ zero-field relaxation and Knight shift studies of the heavy-fermion superconductors U_1−xTh_xBe₁₃, x=0 and 0.033. The Knight shift in UBe₁₃ shows a strong decrease as the temperature is reduced in the superconducting state, unlike U_0.967Th_0.033Be₁₃ in which the shift remains at about the normal state value. If the superconducting state in UBe₁₃ has odd-parity, the decrease of K_μ below T_c suggests that the order parameter is pinned to the lattice. Either spin-orbit scattering or the existence of two distinct superconducting states with different spin susceptibilities in the (U,Th)Be₁₃ system would explain the differences observed in the Th-doped and pure UBe₁₃ materials. The latter hypothesis would exclude conventional BCS superconductivity. No evidence for magnetic order is seen in the zero-field relaxation rate for either material down to 0.3 K.