Conformations of star-branched macromolecules

The dimensions of star-branched macromolecules in dilute solutions were investigated using scaling concepts. The quality of the solvent, the rigidity of star branches, their number and degree of polymerization are taken into account. The relationship between the approaches based on scaling approximations and mean-field approximations is shown. The results obtained are compared to the data of Monte-Carlo simulations and other theoretical results.     

Micro- and macroconformation of macromolecules

Summary Configurative as well as conformational variations with respect to the polymer structure yield comparable effects in the chemical shift of ¹³C-NMR signals. With the example of the head- to head diad within the isomeric polypropylenes, it is demonstrated with the aid of corresponding low molecular weight model compounds that the conformational informations in the ¹³C-NMR spectrum can be used for the determination of the rotational isomers. For the meso- head- to head diad the temperature dependence of the methyl group coalescence signals can be interpreted with the aid of the slow exchange spectra of the low molecular model compounds. The consequences of the microconformations on the macroconformation of the macromolecules are discussed.Dedicated to Prof. C.I. Simionescu on the occasion of his 60th birthday     

Drag reduction effectiveness of macromolecules

A working hypothesis has been developed to account for observed drag reduction properties of dilute polymer solutions. Drag reduction effectiveness of polymer solutes is attributed to their ability to form a deformable network structure which inhibits the formation of microvortices in the solvent and retards their ability to migrate through the fluid, coalesce, and result in fully developed turbulence centres. The size of microvortex precursors is tentatively set in the range of 100 Å, and it is assumed that the damping (drag reduction) effect of macromolecules is due to strong association between solvent molecules and polymer chains, immobilizing many of these active precursors. The hypothesis indicates that drag reduction effectiveness of polymers should depend strongly on polymer/solvent interactions in addition to the recognized variables of molecular weight, concentration and geometry of the flow-system. The hypothesis accounts for a number of published anomalous observations and leads to new predictions of drag reduction variations with polymer molecular weight distribution, and temperature. These and related predictions are the focal points of new experimental research studies of the drag reduction phenomenon.     

Micro- and macroconformation of macromolecules

Summary From the temperature dependence of ¹³C-NMR chemical shift measurements under fast exchange conditions, using the methyl carbon atom as indicator, it was concluded that for erythrodiisotactic head-to-head tail-to-tail polypropylenes the temperature dependence of the conformers around the methine-methine bond may be analyzed quantitatively. In the present paper the ¹³C-NMR analysis has been expanded to inverted polypropylenes of arbitrary ditacticity. Methyl, methylene as well as methine resonances have been taken into account for assignements. Conformational influences on the ¹³C-NMR spectrum are discussed as well as the relevance of low molecular weight models for diads.Herrn Dr. Rudolf Krüger zu seinem 80. Geburtstag gewidmet     

Micro- and macroconformation of macromolecules

Summary Thermoelastic data of eis- trans- 1,4- polybutadiene networks swollen in n- octane are presented. On the basis of the theory of the Gaussian chain the analysis of the energy part of the retractive force and of the temperature dependence of the unperturbed dimensions has been carried out as the function of the cis- trans- ratio of the subunits within the macromolecular chains. Temperature invariant macroconformation has been verified by quasi- isoenergetic chains consisting on 40 % trans and 60 % cis- units within statistically arranged cis- trans- polybutadiene networks.Herrn Prof. Dr. G. V. Schulz zu seinem 75. Geburtstag gewidmetProf. Dr. H. A. SCHNEIDER we are obliged for fruitfull discussions. Thanks to Mr. A. HASENHINDEL for technical assistance and Mr. BANDARA for X-ray diagrams. Financial support of DEUTSCHE FORSCHUNGSGEMEINSCHAFT is gratefully acknowledged.     

Cross-linking of macromolecules in coal

Cross-linking in bituminous coals has been studied by internal friction methods. A low temperature damping peak occurring at 130 K for a frequency of 1 Hz has been used to deduce the character of cross-linking in coal. The molecular units responsible for the internal friction are found to be methylene chains connecting ring structures of the macromolecular material. The number varies progressively from a Braunkohle with 67wt% carbon to anthracites of > 90 wt% carbon. The number goes through a maximum at ≈80 wt%C, showing that the cross-link density reaches a maximum at that carbon level. It falls of markedly at higher carbon concentrations and remains at a low level > 85 wt%C. These findings correlate well with models of coal structure.     

Micro- and macroconformation of macromolecules

Summary The first results are reported about the conformational analysis of an amorphous polymer in the glassy state by means of proton enhanced variable-temperature magic angle spinning ¹³C NMR experiments. In amorphous threodiisotactic poly(1,2-dimethyltetramethylene) at 220 K different conformational diads could be resolved and assigned. From the spectrum of the glassy polymer the shift increment associated with a conformational change of main chain bonds from anti to gauche is determined. In addition the populations of the conformations of CH-CH bonds and of CH-CH-CH₂ bond pairs of poly(1,2-dimethyltetramethylene)s are determined in the glassy state and energy differences between these conformations are estimated.Herrn Prof. Dr. O. Kratky zu seinem 80. Geburtstag gewidmet     

Stretching of self-interacting wormlike macromolecules

Structural and elastic properties of non-charged polymers of stiffness ranging from flexible to rigid chains are computed by Monte Carlo simulations. A discrete wormlike chain (WLC) model with self-interacting units is applied to chains of intermediate lengths of interest in the AFM measurements. Variations of the persistence length and mean chain dimensions with bending stiffness are presented. The chain-end distribution functions, the Helmholtz elastic energy and the force-extension profiles of chains of variable stiffness are computed in an isometric ensemble. Occurrence of a plateau on the force-extension curves at intermediate chain stiffness is noted. Qualitative differences are found between force profiles from simulations and from the standard (ideal) WLC model. The differences can be ascribed to an inherent dissimilarity between isometric and isotensional ensembles used and, at small extensions, to the excluded-volume effects. The single-chain functions from simulations were employed to investigate the influence of bending stiffness on elasticity of networks of semiflexible chains by the three-chain model. A stark reduction of degree of elongation of a network with rising stiffness is found. Stress-strain relations show a highly non-linear behavior with the marked strain-stiffening effect.     

DNA compaction by nonbinding macromolecules

Compaction of DNA by nonbinding macromolecules such as uncharged flexible polymer chains and negatively charged globular proteins is thought to have various applications in biophysics, for example in the formation of a nucleoid structure in bacteria. A simple experimental model that has been very well studied is the classic DNA ??-condensation induced by polymers and salt. In recent years, compaction of DNA by nonbinding macromolecules has been reconsidered under conditions that are closer to the biophysical applications, in various respect. This work is reviewed here. Topics that are considered are: DNA compaction by nonbinding globular proteins, the influence of DNA binding proteins and DNA topology on ??-condensation, and finally, the impact of confinement on DNA ??-condensation.     

Defining topologigical equivalences in macromolecules

We describe a completely automated and objective method fordefining topological equivalents in macromolecules. The methodis based on well established techniques for identifying topologicallyand topographically equivalent atoms in small molecules andhas been used in structural alignment of proteins and RNA molecules,and to extract fragments of molecules (protein secondary structuresand RNA and DNA double helices) from structural databases consistentwith some specified template structure.     

Micro- and macroconformation of macromolecules

Summary A semiempirical modification of FERRY's extension of the ROUSE spectrum in the entanglement region is proposed, with a smooth transition in the relaxation behaviour. Consequently the zero shear viscosity-molecular weight relationship is modelled. With the molecular weight between the entanglements as the only parameter measurements on cis- 1, 4-polybutadiene could be reproduced accurately, comparing favourably with the recent multiparameter theory by SHEN.A simple linear weighting scheme for the average friction coefficient of chemical and configurational copolymers, e. g. cis-trans-polybutadienes, is proposed.Herrn Prof. Dr. Helmut Holzer zu seinem 60. Geburtstag gewidmet     

Elaborate synthesis of biological macromolecules

Egged on: Elaborate syntheses of biological macromolecules consisting of more than two different components is developing. Kajihara and co-workers have used a bio-resource to develop a new strategy for the semisynthesis of glycoproteins.     

Micro- and macroconformation of macromolecules

Summary Cyclotetracosane exhibits complex phase behaviour in the solid state as well as in the melt as evidenced by complementary methods, DSC, BRILLOUIN scattering, dielectric constant and PE-MAS 13c-NMR. Three solid phases have been verified, one of them — within the range from 299.5 to 312 K — exhibits extremely high molecular mobility. Between the start of melting, 312 K, and the first order transition at 321 K a mesomorphic type region develops. Two additional transitions have been detected in the melt. That occuring at 361 K (DSC), 356 K (BRILLOUIN data) resembles the T_u as observed in n-alkanes.Generous financial support of DEUTSCHE FORSCHUNGSGEMEINSCHAFT, particularly within the SFB 130, is gratefuly acknowledged. Prof. Dr. H. G. Unruh we thank cordially for fruitful discussions.     

The θ-behaviour of heterogeneous macromolecules

A polymer chain consisting of N_r segments with a repulsive interaction (binary cluster integral β_r) and N_a ? N_r segments with a stronger, attractive and pairwise saturable interaction (β_a), which is at the averaged θ-point N²_rβ_r + N²_aβ_a = 0 deviations from the predictions of the two parameter theory: $\text{α}{}^2{}_{\mathrm{<mtext>R</mtext>}}\text{? 1 ～ δz}{}_{\mathrm{<mtext>r</mtext>}}\text{< 0}$ and A₂ ～ δz_r > 0 with $\text{δz}{}_{\mathrm{<mtext>r</mtext>}}\text{～ β}{}_{\mathrm{<mtext>r</mtext>}}\text{(}\text{N}{}_{\mathrm{<mtext>a</mtext>}}\text{N}{}_{\mathrm{<mtext>r</mtext>}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. It is shown that the deviations from the universal behaviour are due to the existence of an intermediate length scale $\text{～}\text{N}{}_{\mathrm{<mtext>a</mtext>}}\text{N}{}_{\mathrm{<mtext>r</mtext>}}$.     

Viscosity and density measurements of macromolecules

A micro-differential capillary pressure system has been established to measure the density and viscosity of macromolecular solutions as well as regular chemical solutions. Easy handling, short operational time, accuracy, low cost, high sensitivity and adaptability for on-line control of the instrument are demonstrated by the measurement on poly(ethylene glycol)/water, poly(acrylic acid)/water, sugar/water, glycerol/water, and poly(methyl methacrylate)/tetrahydrofuran systems.     

Modelling ‘melting’ in macromolecules

The melting behaviour of polymers was modelled using polyethylene parameters. The model was based on the idea of melting followed by recrystallization and remelting, which can lead to multiple melting peaks. Experimental data is qualitatively fit by the model for changes in heating rate and original lamellar thickness. In addition one can use the model to predict less well documented changes in other parameters. For example predicted changes in the themograms with changes in thickness distribution follow the kind of behaviour that one would anticipate. Theoretical melting curves often show exothermic types of responses, however these exotherms are effectively eliminated from the observed thermogram when the theoretical curve is convoluted with an instrumental broadening function.     

Three-dimensional conjugated macromolecules as light-emitting materials

In this feature article, we present a short review on the recent development in the construction of structurally perfect red and blue light-emitting materials and highlight such research in our group. We focus mainly on the three-dimensional conjugated structures such as dendrimers, hyperbranched polymers, star polymers, spirobifluorene-based polymers, and spiro-bridged ladder-type oligomers and polymers. Results indicated three-dimensional conjugated structures can efficiently decrease the aggregation of materials, resulting in the improving of the light-emitting efficiency.