Irreversible deformation of macromolecular networks

Based upon a thermodynamical approach, the generalized Onsager type of relaxation of van der Waals networks is presented. By linearly and identically coupling the set hidden variables to the network, the memory function of the system can be related to the equilibrium strain-energy function. The relaxation behavior of real networks on stretching can quantitatively be described by means of a distribution of relaxation times known from small strain experiments. Some new and interesting conclusions are discussed as to how the macroscopically non-linear visco-elastic response might be interpreted.     

Fundamental aspects of rubber-elasticity in real networks

Based on the van der Waals-equation of state a characterization of the elastic properties of molecular networks in the total range of strain is presented by the use of Mooney-Rivlin plots. This discussion elucidates the importance of finite chain extensibility as well as global interactions for a quantitative interpretation of the deformation properties of molecular networks. An interpretation of the so-called Mooney-Rivlin coefficients in terms of the van der Waals-parameters delivers at least a novel, yet very consistent understanding of simple extension of swollen networks.     

Kinetic Study of Metals Sorption on a Resin Modified with Tetrakis(carboxyphenyl)Porphyrin

 The sorption of metal ions (e.g. Cd²⁺, Cu²⁺, Ni²⁺ and Pb²⁺) from aqueous solution on the anion exchange resin Amberlite IRA-904 modified with tetrakis(4-carboxyphenyl)porphyrin (TCPP) was studied in batch equilibrium experiments. The influence of operating variables such as initial pH and contact time between solution and the resin on the equilibrium parameters was measured. The selectivity order of investigated metal ions was evaluated as follows: Pb²⁺ > Ni²⁺ > Cu²⁺ > Cd²⁺. The matrix cations, such as Mg²⁺ and Ca²⁺, exhibit very low affinity for TCPP-modified resin. The rate uptake of the sorption procedure is predominantly controlled by film diffusion. The best retention kinetic was observed for lead where half the saturation of the TCPP-modified sorbent was achieved in less than 5 min. Author for correspondence. E-mail: kryspyrz@chem.uw.edu.pl Received November 20, 2002; accepted January 26, 2003 Published online May 5, 2003     

Kristallisieren und Schmelzen von verschiedenen Hochpolymeren nach röntgenographischen Messungen

Zusammenfassung Es wird an Hand von röntgenographischen Messungen an verschiedenen Hochpolymeren gezeigt, daß es durch die spezielle Bauart des Makromoleküls bedingte, anisotrope Kristallstrukturen gibt, die allgemein beim Kristallisieren abgeschreckter Proben eingenommen werden. Dabei ist vorausgesetzt, daß durch das Abschrecken die Kristallisation der betreffenden Hochpolymeren ganz oder auch nur teilweise unterdrückt werden kann. Es werden die Ursachen für die stufenweise Kristallisation der angegebenen Polymeren beim Tempern diskutiert. Die Zunahme der Langperiode während des Strukturwechsels Pseudostruktur Kristallstruktur wird dem selektiven Aufschmelzen der kleinsten Teilchen bei gleichzeitigem Wachsen der stabilen Kristalle zugeschrieben. Dies führt zu der Auffassung, daß das Schmelzen der Hochpolymeren in einem Temperaturintervall von der mittleren Kristallitgröße und von der Kristallitgrößenverteilung abhängt. Beide Größen können i. a. durch unterschiedliche thermische Vorbehandlung der Proben verändert werden, wie es am Beispiel des Polyurethans und des Terephthalsäureglykolesters gezeigt wird. Das Schmelzverhalten verschieden verzweigter Polyäthylene, des Teflons und eines Copolymerisates dieses Polymeren mit Hexafluorpropylen kann durch eine Theorie vonFlory qualitativ richtig wiedergegeben werden. Hierbei werden die mittlere Kristallitgröße und die Kristallitgrößenverteilung durch den Einbau nicht kristallisationsfähiger Atomgruppen in das Makromolekül entscheidend bestimmt. Der Schmelzvorgang selbst wird als eine Umwandlung 1. Ordnung angesehen. Copolymere werden als spezielles Zweikomponentensystem aufgefaßt. Diese schmelzen prinzipiell in einem Temperaturintervall.Ich danke der Deutschen Forschungsgemeinschaft für die Bereitstellung eines Teils der Geriäte und sachlichen Mittel.     

Computer simulations of nematic droplets

Abstract

Nematic droplets are intimately connected with disclinations, because in nematic droplets, point and line-shaped defects, as well as surface defects, are not generated at random, but inevitably by topological constraints. Thus, droplets provide a good means for investigating nematic defects. There is a growing interest in both topics due to the applications in polymer dispersed liquid crystal devices [1–3], but also in classical display modelling, where nematic defects are to be avoided. Various types of droplets are investigated theoretically with the aid of a previously developed numerical algorithm [4,5], which is based on a dynamic equation for the alignment tensor a _μv. The rotational diffusion, the influence of an orienting external field, and the Frank elasticity (in the one-coefficient approximation) are taken into account, but flow processes are neglected. For the application to nematic droplets, a new type of boundary conditions had to be used, which I have called ‘true planar anchoring’. I simulate the relaxation of the director field of nematic droplets from the isotropic state and vice versa for various types of anchoring and cavity shapes. Contrast pictures, as if viewed under crossed Nicols, are computed and compared to experiment. The results obtained elucidate the nature of the surface disclinations of strength one (boojums). In particular, it is found that their occurrence can be understood as a consequence of the planar anchoring, without any further assumptions. Moreover, a phase transition-like transformation of the director configuration is predicted which is temperature controlled and occurs, as the blue phases do, close to the nematic-isotropic transition temperature T_c.