Acta Polymerica最新文献

On the basis of an extended series of monodisperse oligomers of the dialkoxy-substituted phenyleneethenylenes 1a–i the Eqs. (3) and (4) were conceived in order to determine the limiting values of the energies E_i and the wavelengths λ_i of the UV/vis absorption. The convergence of the E_i, and λ_i values with a growing number n of repeating units permits a precise prediction of the E_i,∞ and λ_i,∞ values of the corresponding polymer 1j as well as a statement about the overall effect of conjugation ΔE_i and the effective conjugation length ECL. A great variety of different conjugated oligomers 2–14 can be evaluated by the same algorithm.

The occurrence of banded spherulites in blends of poly(ε-caprolactone) with poly(styrene-ran-acrylonitrile) is studied by means of optical-, scanning electron-, environmental scanning electron-, and atomic force-microscopy as well as smallangle X-ray scattering. The measurements reveal that lamellae are confined in fibrils that bend periodically from the center of spherulites in a radial direction. The fibrils show also a regular spacing in the lateral direction. Single bent lamellae can be observed by atomic force microscopy after permanganic etching. The enrichment of amorphous material near to the growth front of the spherulites can be directly observed by polarized light microscopy after temperature jump experiments. The excluded amorphous material on the surface of the growing spherulites has the shape of droplets. This might be the main reason for the initiation of the bending process because the result is a non-uniform stress distribution acting on the growing lamellae at the interface between spherulites and the surrounding melt. The amorphous material of the blends can be found in three areas as observed by small-angle X-ray scattering experiments and microscopical methods: (i) between the lamellae, (ii) excluded from the interlamellar region but within the spherulite, and (iii) for high contents of non-crystallizable material, it can be excluded from the spherulite. The banding periodicity of spherulites as a function of the crystallization temperature can be described in terms of two models, based either (i) on the inherent twisting of lamellae perpendicular to the axis of a screw dislocation or (ii) on the dependence of the diffusion of amorphous material away from the growth front of lamellae and the temperature dependence of the rate of crystallization. Both models fail at very low supercoolings near to temperatures where the banding disappears completely.

Oxidative addition to a metal in a low valent state produces catalysts that allow reporter or functional groups to be introduced to a polymer chain end. Substituted allyltrifluoroacetates were added to Ni(0)(COD)₂. The resulting π-allyl-Ni trifluoroacetates are dimeric in the solid state as well as in solution. The crystal structure of bis[(η³-2-phenylallyl)(trifluoroacetato)-nickel(II)], bis[(η³-2-trimethylsilylallyl)(trifluoroacetato)nickel(II)], are given and correlated with the reactivity as polymerization catalysts. Butadiene, styrene, and norbornene are used as monomers. The substituent of the allyl group is found quantitatively to be the end group of each macromolecule. Thus it is demonstrated that no chain transfer occurs.

Syntheses of aromatic polyketones in the form of particles were performed using a very small monomer concentration (0.05 mol/1) by the precipitation polycondensation method. Some of these particles present highly ordered elongated needle-like structures (whiskers) particularly for the all-para polyketone samples. The use of isophthaloyl instead of terephthaloyl for the same low monomer concentration produces, in addition to the globular particles, bundles of ribbon-like structures. The average size of the needle-like particles is 1−5 μm in width and 150−250 μm in length. The factors controlling the morphology texture and size of the whiskers and ribbons are shown to be the polymer structure and the reaction conditions. Formation mechanisms are discussed.

This paper reports the polycondensation of benzyl 2-amino-2-deoxy-α-D-glucopyranoside hydrochloride (1) with carbon dioxide using the triphenylphosphine/carbon tetrachloride/DBU system as a condensing agent to give poly(urethane) (2). The polycondensation of 1 with CO₂ was carried out in DMF solvent in the presence of the condensing agent under CO₂ atmosphere. From the NMR and IR analyses of the product, the polymerization proceeded regioselectively to form 2. The molecular weight was estimated as 2000–3000 by GPC measurement after acetylation of 2, corresponding to DP = 5–8.

The polypropylene growth on metallocene/MAO catalysts supported on a silica carrier has been examined by scanning electron microscopy. The kinetic induction period in early polymerization stages can be explained by the formation of a discrete polymer layer only on the outer surface of the catalyst particle. Differences in the fragmentation process at later stages are pointed out, depending on the supporting procedure of the catalyst system.

The structure formation of growing spherulites by branching lamellae is simulated by a computer model. In addition to the process of linear growth of lamellae, three different mechanisms are implemented in the simulation program: the fork-like branching of lamellae at their growth front (“front branching”), the generation of secondary lamellae at the sides of already existing lamellae (“side branching”) and the crystallization of lamellae parallel to existing lamellae (“clustering”). The simulation is performed stepwise and the branching takes place in dependence on branching probability and angle, leading to a multiplication of the lamellae and to disc-like superstructures. Moreover, the side branching as well as the clustering mechanisms result in a gradient of “density” (crystallized entities per area unit) inside the spherulites.

The structure of the crystalline hard domains of polyurethane elastomers based on 1,5-naphthalene diisocyanate (NDI) has been investigated by X-ray methods. The specimens were pressed films prepared from NDI and 1,4-butanediol (chain extender), with poly(oxytetramethylene) as the soft segments. Small angle X-ray data show that the hard domains are lamellae oriented preferentially with their planes parallel to the film surface. The corresponding wide angle data show that the hard domains are crystalline, and that the degree of crystallinity can be more than 60%. The hard segment chains are relatively extended and are oriented perpendicular to the film surface: they probably form a staggered array in which they are linked in sheets by hydrogen bonds. Molecular models suggest that the hydrogen bonds are shorter, and hence stronger, than those formed in the hard domains of equivalent elastomers based on diphenylmethane diisocyanate (MDI) or dicyclohexylmethane diisocyanate (HMDI). In addition, the stacking of the naphthylene units is unencumbered by the presence of the bridge methylene groups in the latter systems, and these structural features probably account for the higher melting temperature of the hard domains.