Acta Polymerica最新文献

The research field of “Macromolecular chemistry” has changed considerably during the last 10 years. There has been unexpected break throughs in the area of bulk polymers despite the fact that research in this area could have been considered to be complete. The metallocene catalysts open up the synthesis of new polymer combinations with completely new structures. Their potential has not yet been exhausted. The trend toward building polymeric structures with small dimensions and to nanotechnology is decisive. Many workgroups within germany and abroad have been working on nano-particles and layers and with reactions at interfaces with respect to their technological and biochemical application. Evidently this trend has been promoted though the foundation of competence centers of nanotechnology. The following report for the year 1998 gives some information on these developments without claiming to be complete.

The hyperbranched polyester based on 4,4-bis-(4′-hydroxyphenyl)pentanoic acid was synthesized with different DP's. A complete ¹H and ¹³C NMR assignment of the oligomers was carried out. Six different structural units were quantified by NMR analysis. The conversion dependence of the content of these structural units and of the degree of branching DB follows the theoretically derived equations. Furthermore, six different diads were quantified by ¹³C NMR spectroscopy. The conversion dependence of the diads was calculated and again the measured data agree well with the theory. The good fit between theoretical and measured data proves that the polycondensation of the 4,4-bis-(4′-hydroxyphenyl)pentanoic acid follows a random growth of the different units with equal reaction rates of all reaction steps. Finally, spin-lattice relaxation times (T_1C) and nuclear Overhauser enhancements (NOE) were determined and discussed.

Symmetric diblock copolymers in the lamellar phase are studied by NMR. ²H NMR spectra obtained in macroscopically oriented monodomains show an absolute frequency shift of a few ppm, which depends of the orientation of the sample with respect to the magnetic field. This is interpreted as due to the anisotropy of the diamagnetic susceptibility, related to the lamellar structure itself rather than to an orientational order of polymer chain segments. The spectra obtained in both 2D ¹³C and ¹H rotor-synchronized MAS experiments reflect the same effect. Specifically, we show that ¹H rotor-synchronized MAS experiments allow us to discriminate unambiguously the effect of diamagnetic susceptibility anisotropy and the effect of molecular orientation.

Poly(vinyl alcohol) (PVA) and two ethylene-vinyl alcohol copolymers were esterified with undecenoyl chloride and a mixture of undecenoyl and benzoyl chlorides to test how the double bond content and aromatic or aliphatic spacers can affect the further crosslinking reaction. In all cases crosslinking was only possible by using a peroxide initiator to prevent simultaneous thermal degradation. Isoconversional kinetic analysis was applied to dynamic DSC data. This study reveals the complexity of the crosslinking process because the activation energy depends on the conversion degree. In the polymers which contain most double bonds, diffusion is the rate limiting step at high conversion. As a general trend, E_a values increase when the number of double bonds decreases. However, when there are many benzoate moieties, E_a values are lower probably due to the more expanded conformation that the random coil must adopted.

AFM images of isothermally crystallized samples of syndiotactic polypropylene and poly(propene-co-octene) show that the lamellar crystallites are composed of blocks. Upon heating the granular structure vanishes. Observations support the view recently established by SAXS experiments that polymer crystallization is a two-step process, with the block formation near the growth front as a first and their fusion into a lamella as a second step. It is proposed that the blocks have the minimum size necessary to remain intrinsically stable. This could explain the observed invariability of the crystal thickness with changing co-unit content.

The nucleophilic substitution reactions on poly(vinyl chloride) (PVC) with sodium benzenethiolate, sodium thiobenzoate and sodium 2-mercaptobenzothiazolate have been carried out in solution of cyclohexanone. The aim was to study from a comparative point of view the influence of nucleophile characteristics, in particular bulkiness, on the reaction mechanisms through the microstructural evolution with degree of substitution of the same PVC sample with the three kinds of nucleophile in the same experimental conditions. The quantitative microstructural analysis with conversion has been followed by ¹³C NMR spectroscopy. Owing to steric hindrance of 2-mercaptobenzenethiolate the stereoselective nature of the reaction is enhanced making the reaction through the mr triads of mmr inoperative in practice unlike the other nucleophiles. This conclusion was confirmed on the basis of the FTIR results.

Different syntheses of ordered condensation polymers from nonsymmetric monomers are reviewed. For several classes of polymers, the approach exploiting reactivity differences between the functional groups of the nonsymmetric monomers (ordered polymers from in situ generated intermediates) is compared with that where the relevant intermediates are used as symmetrical preformed and/or isolated precursors for the ordered polymers.

A polymer (TPD-PPV), incorporating both efficient light-emitting and hole transporting moieties has been synthesized. This polymer also possesses excellent film-forming property, good thermal stability, and high electrochemical reversibility and stability. The HOMO-LUMO energy levels were determined by cyclic voltammetry and UV-Vis measurement. The diode with the structure of ITO/CuPc/TPD-PPV/Al showed high rectification ratio (10⁸) and low turn-on voltage (4.2 V). A bright green-yellow light-emission was observed in day-light under forward bias.

A side-chain liquid crystalline polyacrylate containing a salicylaldimine group has been synthesized and complexed with copper(II) ions to generate metallomesogenic crosslinking of polymeric chains. The mesomorphic properties of the parent and complexed polymers have been studied by optical microscopy, DSC and X-ray diffraction. In addition, these materials have been investigated in depth by dielectric spectroscopy. Dielectric measurements of the parent polymer reveal the presence of two sub-glass relaxations (β and γ). The α relaxation associated with the T_g is masked by a conductive process, but at higher temperatures a δ relaxation is observed. These relaxation processes have been characterized and their activation energies evaluated. The influence of the mesogenic order and metal crosslinking on some relaxation processes has also been determined.