H. P. Grossmann

On Present Theories of the Condensed Polymer State

Abstract An improvement of the bundle model gives the possibility to discuss quantitatively the solid state transition and the final melting of PE and calculate the transition data as well as the cooperativity. A similar quantitative treatment is carried out to explain the kink-block transitions in clay organic complexes.

Some thermodynamic quantities of n-alkanes as a function of chain length

SummaryResults from high pressure dilatometry onn-alkanes and linear polyethylene and literature data yield a linear relation between specific volume, entropy and enthalpy of fusion, the reciprocal melting temperature and 1/n, wheren denotes the number of C-atoms per molecule. Extrapolating towards infiniten one always obtains polyethylene data.The differences in the properties ofn-alkanes and polyethylene can be ascribed to the influence of the chain ends. Assuming entropy and enthalpy contributions from these chain ends as made probable by a molecular model one can quantitatively explain then-dependence of the above mentioned quantities including their pressure dependence.ZusammenfassungAus druckdilatometrischen Messungen ann-Alkanen und an linearem Polyäthylen und aus Literaturergebnissen erhält man lineare Abhängigkeiten des spezifischen Volumens, der Umwandlungsentropie, der Umwandlungsenthalpie und der reziproken Umwandlungstemperatur von 1/n, wobein die Anzahl der C-Atome pro Molekül ist. Für den Grenzfalln→∞ führt die Extrapolation stets zu den Werten des Polyäthylens.Die Unterschiede zwischen den Eigenschaften dern-Alkane und des Polyäthylens lassen sich auf den Einfluß der Kettenenden zurückführen. Nimmt man für diese Kettenenden Entropie- und Enthalpiebeiträge an, wie sie durch ein molekulares Modell nahegelegt werden, so kann dien-Abhängigkeit der oben genannten Größen, einschließlich ihrer Druckabhängigkeit, quantitativ erklärt werden.

Medium Access Concept for VANETs Based on Clustering

Vehicle-to-vehicle communication is a promising technology for enhanced road safety as it provides the possibility to get a detailed knowledge about the traffic situation a long way ahead. Vehicles transmit information about nearby dangers like obstacles on the road as well as warning messages over long distances to all vehicles in a certain area. Therefore the dissemination of the data has to be done in broadcast transmission. This transmission concept causes severe problems regarding the organization of the medium access. The often used RTS/CTS mechanism is no longer feasible, which is especially problematic in road scenarios, where the hidden station problem is a major cause of data loss due to collisions. In addition the regarded data makes high demands on the provided quality of service. Especially reliability and delay are crucial for the efficiency of the warning system. In this paper we introduce a concept for a medium access control which minimizes the effects of the hidden station problem by introducing a clustering scheme. Therefore clusterheads among the vehicles are selected, which are then responsible for assigning bandwidth to the other vehicles in their clusters. This paper presents an algorithm for selecting the clusterheads along with a concept for assigning bandwidth to the nodes.

Atomistic calculations of kink defects in the polyethylene crystal by means of semiempirical potentials

SummaryA method to calculate defects in polymer crystals is introduced and applied to the polyethylene crystal. It is tested for the ideal crystal and is then used to investigate defects (kinks,Reneker defects, kink blocks). Two stable kink positions have been determined at a lattice point. The defect energy of the kinks is about 8.5 kcal/mole. TheReneker defect has been found to have a remarkably higher defect energy of about 13.5 kcal/mole in the crystal. It has been verified that block arrangements of kinks decrease the defect energy per kink. The amount of decrease is about 4 kcal/mole per kink for planar kink blocks and is about 2 kcal/mole per kink for linear kink blocks.ZusammenfassungDiese Arbeit gibt am Beispiel des Polyäthylens eine Methode an, mit der Defekte in Polymerkristallen berechnet werden können. Das Verfahren wurde am Idealkristall überprüft und dann zur Berechnung von Kinken im Kristall verwendet. Es ergaben sich zwei stabile Kinklagen an jedem Gitterplatz. Die Defektenergie für die Kinken beträgt ca. 8,5 kcal/mol. DieReneker-Kinke (Renekerdefekt) im Kristall führt mit 13,5 kcal/mol zu einer wesentlich höheren Defektenergie. Blockanordnungen von Kinken erniedrigen die Defektenergie beträchtlich. Die Energieabsenkung pro Kinke beträgt für einen flächenhaften Block ungefähr 4 kcal/mol, für lineare Blöcke etwa 2 kcal/mol.

Cross-Layer Design for Packet Routing in Vehicular Ad Hoc Networks

In this paper we present a cross-layer design between packet routing and medium access control (MAC) for a vehicular ad hoc network (VANET) in order to guarantee special quality of service (QoS) requirements. As precondition we consider vehicles equipped with driving-assistant-systems that exchange environmental object information through the VANET. The object information is collected by onboard sensors and gets obsolete after some time because of the fast changing surroundings. Therefore it should be distributed to other vehicles with a lower delay than the data gets outdated. This introduces a special low delay QoS requirement the VANET has to provide to the application. As a solution we propose a cross-layered combination of MAC and routing mechanisms. In detail we use advantages of a cluster-based forwarding mechanism and adapt them to the MAC. This helps to decrease collisions and to control packet forwarding. On the other hand the MAC protocol provides cluster and neighbor node information to the routing algorithm. We show that this QoS mechanism satisfies the required packet delay for a desired information propagation area. Finally VANET simulations of certain traffic scenarios validate the proposed cross-layer protocol and show the performance of this mechanism.

IR-spectroscopic investigations of cycloalkanes

SummaryA homologous series of cycloalkanes from (CH2)14 up to (CH2)96 as model systems for tight folding in CH2-chain molecules was investigated by infrared spectroscopy. In comparison with the corresponding spectra of the nalkanes we found two additional new absorption bands at 700 cm−1 and 1442 cm−1 which can be assigned to a characteristic fold vibration. The already known fold band in the wagging region at about 1344 cm−1 could also be found in all cycloalkane spectra. A numerical decomposition of the bands enables the calibration of the fold concentration. The Davydov splitting of the (CH2)-rocking and (CH2)-bending vibration in the melt crystallized form of the molecules (CH2)72 and (CH2)96 show the occurence of a orthorhombic subcell of the larger rings whereas the smaller rings and the larger ones in the solution crystallized form only crystallize in a monoclinic form.

Measurement of the orientational elastic constants and the twist viscosity of nematic side chain polymers

Abstract The temperature dependence of the three elastic constants k ii (i = 1, 2, 3) and the twist viscosity γ1 of two nematic side chain polyacrylates and one comparable low molecular weight compound have been measured by means of the Freedericksz effect. The change from the low to the high molecular liquid crystal causes a change of the ratio k 33/k 11 from greater to less than unity, but the order of magnitude of the elastic constants remains the same. In contrast, the twist viscosity of the polymeric liquid crystal is about 1000 times greater in magnitude than that of a comparable low molecular weight liquid crystal. The activation energy for the viscosity of the polymer differs by a factor 3–4 from that of the low molecular weight liquid crystal. The elastic constants as well as the twist viscosity show a quadratic dependence on the order parameter S over a wide temperature range.

The high temperature transition of polyethylene into a Condis phase, described on the basis of the cluster-entropy hypothesis (CEH)

On the basis of atomistic calculations a rotator model of the high temperature Condis phase of polyethylene is proposed. Its cooperative statistical treatment (using CEH for the lateral packing of chains) explains the first order transition from the all-trans crystal as well as the transition dataTt,ΔSt,ΔVt. We found that the normal melting (i. e. at pressures below the triple point) is also governed by the onset of the Condis phase.

Meander model of amorphous polymers

Assuming bundles (of shortrange ordered macromolecules, folding back and forth statistically), their equilibrium superstructure and diameter are described on the basis of cluster-entropy-hypothesis (CEH). As primary blocks in the bulk polymer and in thin films coupled meander cubes are most probable, which are linked via their cube diagonals serving as axis of statistical rotation and aggregate to coarse grains. Magnetic birefringence, SANS and elctronmicroscopy are used as further methods to determine the cube side length. Applying the same concept to myosin-, collagen-, and elastin-aggregates, these can be interpreted as equilibrium meander fibrils, additionally stabilized by specific interactions and by the length of the molecules.

Evidence of a liquid-liquid-transition in n-Tetracosane the behavior of the specific volume

SummaryDilatometric measurements of n-Tetracosane from 330 K up to 420 K are reported. These experiments show evidence for a small kink in the specific volume — temperature curve. The correct statistical interpretation of the measured data yield a discret transition temperature above the melting point at about 378 K in close agreement with the results from other experimental methods.