Fast switching anisotropic networks obtained by in situ photopolymerization of liquid crystal molecules

Lightly cross-linked anisotropic networks with uniaxial and π/2 twisted orientations were produced by photopolymerization of monotropic mixtures containing liquid crystal mono and diacrylates. In this way the polymer backbone was immobilized and became decoupled from the motion of the mesogenic side groups. The networks showed very good reversibility and even after becoming isotropic, upon cooling, the initial orientation was recovered. In the same way, in the presence of electric fields the mesogenic groups could be reoriented in the direction of the electric field, reverting back to the initial orientation on removal of the field at a rate comparable with those observed in the monomeric state. Combining viscoelastic measurements with the dielectric behaviour of the monomeric liquid crystal and the anisotropic network, a comparison between the internal and bulk rotational viscosities was also made.     

Spectroscopy of alkali atoms and molecules attached to liquid He clusters

Large helium clusters, ranging in size from a few hundred to several thousand atoms, are produced in a nozzle expansion. Combining this source with a pick-up scattering cell in which the clusters can be seeded with chromophores allows us to probe the influence of the helium environment on the atoms and molecules attached to the clusters. Using an alkali as chromophore we recorded laser induced fluorescence spectra of Na atoms and molecules attached to helium clusters. Apart from the spectrum of the Na monomer, we have found spectroscopic bands which can unambiguous be assigned to two bound Na atoms. The first of this bands is due to 1^{1 +} (A) 1¹ _g ⁺ (X) excitations of the covalently bound singlet Na₂ molecule while the second is due to 1³ _g ⁺ 1^{3 +} excitations for the van der Waals bound triplet Na₂ dimer. Both bands have been vibrationally resolved. Furthermore we found very large fluorescence intensities in the region 605–635 nm which are likely due to the excitation of a species containing three Na atoms attached to a helium cluster.     

A particle growth theory for heterogeneous Ziegler polymerization

Samples of “as produced” polypropylene particles at progressively higher yield levels (grams polymer/gram catalyst) were sliced and examined by electron microscopy. In the polymerization of propylene with the TiCl₃–(C₂H₅)₂AlCl catalyst system the catalyst breaks up immediately into basic 100–1000 Å particles. As the yield increases, the catalyst particles gradually disappear and finally become completely dispersed in the polymer particle. These results are compatible with a theory which views the catalyst as a porous crystal containing a single species of active sites uniformly distributed. As polymerization progresses, all sites should eventually initiate a polymer chain whose length should be inversely proportional to the depth of the site below the surface of the particle. Two apparently equivalent statistical models were developed on the basis of this concept. Both models predict a slow increase in the X?_w/X?_n ration (Q) with increasing molecular weight, after an initial rapid increase. The most useful of these models states that Q is equal to the sum of X?_w terms of the simple harmonic series, and that a complete spectrum of x-mers should be present in the product. This agrees satisfactorily with analytically determined values.