bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stereospecific polymerization of perfluoroolefins
Authors: Sianesi Dario , Caporiccio Gerardo 
Published: April 2003

AbstractPreliminary results of research work in the field of stereospecific polymerization of fluorine‐containing olefins are reported. It is shown for the first time that ZIEGLER‐NATTA type catalysts may be used for polymerizing fully fluorinated olefins. By employing catalysts based on titanium tetraalcoholates and alkylaluminum derivatives, in halogenated solvents, slow conversion to crystalline polymers was obtained from tetrafluoroethylene and hexafluoropropylene. To the new crystalline polyhexafluoropropylene, whose main properties are described, an isotactic structure can probably be assigned. Some features of the stereospecific polymerization of perfluoroolefins are reported and briefly discussed.
Frequency shifts in the infrared spectrum of human deoxyribonucleic acid
Authors: Zerbi Giuseppe , Bianchi Paolo , Polli Elio E. 
Published: April 2003

AbstractThe infrared spectra of DNA from calf thymus, human normal and human leukaemic leucocytes was recorded in solution of H2O and D2O. Spectra have been also obtained after the samples in D2O have been subjected to the following treatments: heating, acid and enzymatic hydrolysis. The spectrum of apurinic acid has been determined. A detailed study is carried out in the frequency region from 1750 to 1550 cm−1 with improved resolution. The experimental difficulties for recording the spectrum in this spectral range are discussed. In disagreement with previous works it was found that the pair of strong bands at 1645 and 1680 cm−1 in the spectrum of the starting material shifts to 1660 and 1690 cm−1 respectively along with changes in intensity after treatment. Assignments of the bands in the same frequency range are tentatively proposed and the frequency shifts are explained in terms of the intermolecular interaction between purine and pyrimidine units. An infrared criterion for the existence of the double‐stranded helix is postulated as follows: the pair of infrared bands at 1645 and 1680 cm−1 characterizes a regular double‐stranded helix; a strong band at 1660 cm−1 and a weaker one at 1690 cm−1 indicate a deformed or denaturated DNA.
Cyclic Polymers by Kinetically Controlled Step‐Growth Polymerization
Authors: Kricheldorf Hans R. , Schwarz Gert 
Published: April 2003

AbstractThe theory of step‐growth polymerizations including the cascade theory is discussed in the light of new results focussing on the role of cyclization reactions. The identification of cyclic oligomers and polymers in reaction products of step‐growth polymerizations has been eased considerably by means of MALDI‐TOF mass spectrometry. Experimental examples concern syntheses of polyesters, polycarbonates, polyamides, polyimides, poly(ether sulfone)s, poly(ether ketone)s and polyurethanes. It was found in all cases that the percentage and molecular weight of the cycles increases when the reaction conditions favor high molecular weights. In the absence of side reactions all reaction products will be cycles when conversion approaches 100%. Cyclization may even take place in the nematic phase but even‐numbered cycles are favored over odd‐numbered ones due to electronic interactions between mesogens aligned in parallel. In contrast to Flory's cascade theory, cyclization also plays a decisive role in polycondensations of abn‐type monomers, and at 100% conversion all hyperbranched polymers have a cyclic core. Furthermore, it is demonstrated that in a2+b3 polycondensations intensive cyclization in the early stages of the process has the consequence that either no gelation occurs or the resulting networks consist of cyclic and bicyclic oligomers as building blocks. Finally, a comparison between cyclization of synthetic polymers and biopolymers is discussed.
Schematic representation of a network structure mainly consisting of cyclic oligomers and multicyclic building blocks as derived from “a2” + “b3” polycondensation.magnified imageSchematic representation of a network structure mainly consisting of cyclic oligomers and multicyclic building blocks as derived from “a2” + “b3” polycondensation.
Transition in swollen polymer networks induced by intramolecular condensation
Authors: Dušek K. , Patterson D. 
Published: April 2003

AbstractIt is predicted that the net repulsion between the segments of a polymer network and a poor solvent can cause a phase transition marked by a sudden change in the degree of swelling. This is analogous to the “coil–globule” transition recently predicted by Ptitsyn to occur for a macromolecule in solution. The critical conditions for the transition. as well as phase diagrams, are calculated for the gel in free swelling and under uniaxial tension, which facilitates the transition. The transition depends on the gel being formed of chains crosslinked while greatly swollen by a diluent and also having a high degree of crosslinking. It is concluded that it would be difficult to attain the conditions necessary for the transition in the free‐swelling case, but that it should be possible for gel under tension.
Role of diffusion in gel permeation chromatography
Authors: Hermans J. J. 
Published: April 2003

AbstractA theory is developed that describes the diffusion of solute into the gel particles during a gel permeation chromatographic experiment. The particles are treated as homogeneous spheres of radius a, into which diffusion takes place with diffusion coefficient Ds. The concentration in the mobile phase at any level at any time is supposed to be uniform throughout the cross‐section of the column. It is shown that in the usual columns the effect of diffusion in the mobile phase is unimportant. A determinative quantity in the process is the parameter a2/Dst, where t is the time. For large values of a2/Dst an explicit expression for concentration versus time in the mobile phase at the end of the column is derived [eq. (26) and Fig. 1]. It shows a relatively long tail at large efflux volumes V, where the concentration varies at V−3/2. For arbitrary values of a2/Dst the first three moments of the concentration versus time curve are calculated [eqs. (33)–(37)]. Pronounced skewness of the curve is found unless a2/Dst is small.
Second virial coefficient of polymers in good solvents: The case of polyelectrolytes
Authors: Alexandrowicz Z. 
Published: April 2003

AbstractA stepwise segment‐by‐segment cluster development of the interaction energy of two polymer coils at given distance leads to a closed expression, describing the second virial coefficient A2 in terms of certain intersegmental contact probabilities. An approximate expression for A2(α,z) in good solvents is then derived by using the uniform segment cloud model; the result being equivalent to that obtained by Flory. This expression is combined with the author's theory for α2(z) to give A2(z), and then with a subsequent adaptation of that theory to polyelectrolytes, to derive the dependence of A2 on ionic strength. The α2(z) and A2(z) equations are both compared with recently reported experimental data for NaPSS.
Temporary changes in electrical properties of polymer dielectrics due to ionizing radiation
Authors: Adamec V. 
Published: April 2003

AbstractMeasurements of conductivity, permittivity, and dissipation factor on polystyrene, low‐density polyethylene, poly(ethylene terephthalate), and polytetrafluoroethylene under irradiation with x‐rays at exposure rates from 0.004 to 400 r/sec. are presented. The radiation‐induced anomalous conductivity as well as the induced dielectric loss are interpreted by Maxwell‐Wagner polarization due to radiation imbalance in surface layers of the specimen. The nature of radiation‐induced steady‐state conductivity is also discussed.
Morphology of polypropylene crystals. III. Lamellar crystals of thermally decomposed polypropylene
Authors: Kojima Morio 
Published: April 2003

AbstractMorphology and crystal structure of solution‐grown and melt‐grown crystals of thermally decomposed polypropylene have been studied by electron microscopy and x‐ray diffraction. By crystallization from dilute α‐chloronaphthalene or carbitol solutions well‐defined lamellar crystals about 100–150 A. in thickness are obtained from fractions (number‐average molecular weights 1600–2100) of thermally decomposed crystalline polypropylene. The structure is monoclinic as crystallized from very dilute α‐chloronaphthalene or carbitol solutions (0.02–0.005 wt.‐%). However crystals of the triclinic as well as of the monoclinic forms are precipitated from carbitol solutions of higher concentrations (0.05‐1 wt.‐%). On the other hand, the separated triclinic form has been obtained from the melt. Crystals of both modifications have similar morphology. In addition, the chain molecules cannot be expected to fold, within the thickness of lamellae in crystals of either modification prepared from the low molecular weight fractions used in this study.
Small‐angle x‐ray diffraction studies of plastically deformed polyethylene. II. Influence of draw temperature, draw ratio, annealing temperature, and time
Authors: Peterlin A. , Corneliussen R. 
Published: April 2003

AbstractThe angular dependence of scattering intensity of drawn polyethylene (PE) was investigated with a small‐angle Kratky camera. At constant drawing temperature the intensity drops drastically with increasing draw ratio; however, the position and the half‐width of the first maximum remain nearly unchanged. The drop in intensity can be explained only by a reduction of effective electron density difference between amorphous and crystalline components. The latter contains more vacancies, and the former contains more and better packed tie molecules. This increases the average density of the amorphous layer and decreases that of the crystalline component. As the temperature of the drawing increases, the draw ratio attainable at the applied draw rate drops and the intensity of scattering and the long period rapidly increase. In addition, a second‐order maximum appears, indicating a better order of lamellar stacking, in good agreement with electron microscopy. The first annealing effect is an extremely rapid increase in scattering intensity and long period. The subsequent increase is rather slow and proportional to the logarithm of annealing time. The long period in such an experiment is independent of the draw ratio; however, the scattering intensity depends on it quite strongly even after prolonged annealing.
Thermoluminescence in poly(methyl methacrylate) γ‐irradiated at 77°K
Authors: Fleming R. J. 
Published: April 2003

AbstractIsotactic and atactic poly(methyl methacrylate) containing varying amounts of polymerization initiator and chain‐transfer additives, and carefully purified radiation‐polymerized material, were γ‐irradiated in the dark at liquid nitrogen temperature. On warming to room temperature at a near‐linear rate of 20°K./min., a glow peak having a maximum luminescence intensity at 162 ± 8°K. was observed in all samples; an additional peak at 239 ± 7°K. was resolved relatively infrequently. Low luminescence yields precluded an investigation of the spectral distribution of the glow peaks. It is shown that the electron traps associated with the lower temperature peak are most probably the main‐chain methyl groups, while the higher temperature glow peak is tentatively associated with escape of electrons from structural defects in a few small crystalline regions of the samples. It is postulated that, for the peak at 162°K., the trapped electrons combine radiatively with luminescence centers when thermally induced rotational motion of the main‐chain methyl groups permits sufficient wave‐function overlap. Assuming that methods of glow curve analysis proposed for inorganic materials are applicable to organics, first‐order recombination kinetics and an activation energy of 0.084 ± 0.006 e. v. are deduced for the intensity maximum at 162°K. This activation energy is in fair agreement with that obtained by Bordoni et al. for an unspecified side‐chain motion occurring at similar temperatures, but is only about half the values quoted with some reservations by Powles and by Kawai on the basis of NMR experiments. It was not possible to determine the kinetics of the 239°K. peak with certainty, but an activation energy of 0.432 ± 0.085 e. v. follows from a first‐order assumption. The additives present in the samples did not appear to play any significant part as electrontrapping agents; this observation contrasts with some recent work on the visible and ultraviolet absorption spectra of the same materials which showed a close correlation of additional bands developed following irradiation with the presence of specific additives.
Dielectric relaxation and electrical conduction of polymers as a function of pressure and temperature
Authors: Saito Shogo , Sasabe Hiroyuki , Nakajima Tatsuji , Yada Koji 
Published: April 2003

AbstractThe dielectric properties and the d. c. conductivity of poly(vinyl chloride), poly(vinyl acetate), polychlorotrifluoroethylene, and poly(ethylene terephthalate) were measured at temperatures above and below the glass transition temperature and at various pressures up to 3000 atm. The α relaxation associated with the micro‐Brownian motion of amorphous chain segments depends strongly upon temperatures and pressure, while the β relaxation due to local‐mode motion of the frozen main chain shows weak dependence on temperature and pressure. It is found that the free volume concept is valid for description of the temperature and the pressure dependence of the relaxation time for the α process. Activation energy and volume for both relaxation processes are determined from the experimental data. A simple relation between activation energy and volume for the α relaxation and pressure dependence of the glass transition temperature are derived. Temperature and pressure dependence of the d. c. conductivity in the rubbery state are notably different from those in the glassy state. Ionic conduction appears to be dominant in these polymers.
Effects of temperature and water on the γ → α crystalline transition of nylon 6 caused by stretching in the chain direction
Authors: Miyasaka Keizo , Ishikawa Kinzo 
Published: April 2003

AbstractThe effects of temperature and water on the crystal transition of nylon 6 from the γ phase to the α phase, which is caused by stretching along the chain direction, were investigated. The γ‐phase fibers with high crystallite orientation were stretched at constant load under various conditions. An inversion of the effect of water on the transition occurs at about −60°C. Stretching in the wet state is more effective for the transition at higher temperature. In contrast, at low temperatures water in the crystalline regions actsasa cohesive agent for the chains and increases the activation energy for the transition. Thus, dry stretching is more effective than wet stretching at very low temperature. The fraction of transformed α‐phase crystallites increases abruptly over a narrow range of stress. Thus the critical stress can be determined for the transition. The critical stress changes appreciably with temperature; the higher the temperature, the lower the critical stress. The relation between stretching temperature and critical stress was analyzed by Flory's equation for the shift of transition temperature by stress. About 220°C. was estimated as the zero‐stress transition temperature. The heat content of the γ‐phase crystal was estimated to be smaller by 500 cal./mole than that of the α‐phase crystal. This result suggests that the free energy of the γ‐phase crystal is lower than that of the α‐phase crystal at temperatures lower than the transition point. The irreversible strain of a sample in which the crystal transition has taken place is very small at low temperature. This small extension of the sample is evidence that the γ → α transition produced by stretching along the chain axis is a crystal‐crystal transition.
Relaxation characteristics and intrinsic birefringence and viscosity of polystyrene solutions for a wide range of molecular weights
Authors: Thurston G. B. , Schrag J. L. 
Published: April 2003

AbstractA comparison is made between measurements on polystyrene solutions and the relaxation characteristics and intrinsic birefringence and viscosity given by the theory for the flexible Gaussian chain of variable number of segments and with internal viscosity and internal hydrodynamic interaction. This is done in order to determine the applicability of the theory to polymers over a wide range of molecular weights, including the low molecular weight range in which there may be conflict with the theoretical assumption of chains having a large number of segments. The longest, terminal relaxation time and the number of chain segments are determined from measurements of the frequency dependence of oscillatory flow birefringence while the intrinsic birefringence and viscosity are determined from steady flow measurements. The range of molecular weights studied is from approximately 900 at 106. It is found that the segment weight is approximately 1000 and the number of segments is in direct proportion to the molecular weight for the range from 1 to 1000 segments. The terminal relaxation time has a molecular weight dependence of the type given by the theory but with better agreement for higher molecular weights. While the measured dependences of the intrinsic viscosity and birefringence are in agreement with theory for molecular weights greater than 5 × 104, they deviate significantly for molecular weights below 1 × 104. The ratio of the intrinsic birefringence to intrinsic viscosity, which in theory is a constant independent of molecular weight, is found to change at the lower molecular weights.
Flow‐rate dependence of gel permeation chromatography
Authors: Yau W. W. , Suchan H. L. , Malone C. P. 
Published: April 2003

AbstractAnalysis of polystyrene standards by gel permeation chromatography over a wide range of flow rates revealed two sources of error in volume measurement. These errors arise from solvent evaporation in the siphon chamber and from solvent continuing to flow into the siphon during discharge. Appropriate corrections are discussed, and a vapor feedback loop to eliminate the solvent evaporation error is described. The flow rate dependence of the GPC calibration curve, expressed in the corrected elution volumes, appears different from that reported in the literature. The corrected flow rate dependence of peak elution volumes is in agreement with the expectation of diffusion and exclusion theories.
Calibration procedure for gel permeation chromatography
Authors: Frank F. C. , Ward I. M. , Williams T. 
Published: April 2003

AbstractA novel procedure is presented for calibrating a gel permeation chromatograph by using broad polymer fractions. The limit in accuracy is set by the accuracy of number‐average and weight‐average molecular weight measurements and the reproducibility of the chromatograph. In the particular calibration for linear polyethylene described here, low molecular weight fractions were prepared by nitric acid oxidation of annealed single crystals. The calibration is then of particular value in morphological studies of polyethylene. The influence in the calibration of polar endgroups on the polymer, as produced in these oxidized samples is discussed.
Low‐frequency losses in rubbers cross‐linked in the presence of diluent
Authors: Langley Neal R. , Dickie Ray A. , Wong Chu‐Ping , Ferry John D. , Chasset R. , Thirion P. 
Published: April 2003

AbstractMeasurements of the complex shear compliance have been made from 0.1 to 7 cycles/sec. and from −5° to 45°C. on several samples of natural rubber cross‐linked by dicumyl peroxide in the presence of a diluent oil (volume fraction of rubber, v2, = 0.63 and 0.76) which was subsequently extracted. The properties of the extracted vulcanizates were compared with those having the oil still present and with those of conventional undiluted vulcanizates. Measurements of the diffusion coefficient of radioactively tagged n‐hexadecane in trace amounts through the polymer structure were also made both before and after extraction of the oil. The diffusion coefficient was higher in the presence of the oil by an amount consistent with the Fujita theory for concentration dependence of diffusion rate based on free volume considerations. The low‐frequency mechanical losses (reduced to 25°C. by the method of reduced variables), as measured by the loss tangent, were shifted to higher frequencies by the presence of oil to a much larger degree than would be expected from the difference in local mobility gauged by the diffusion coefficient. The equilibrium modulus, derived by extrapolation to zero frequency, was diminished by the presence of oil to a greater extent than the factor of v2⅓ expected from the simple theory of rubberlike elasticity. The low‐frequency losses in the extracted vulcanizates were smaller than those in conventional vulcanizates with comparable degrees of cross‐linking; the differences are attributed to differences in network topology.
Growth and drawing of polyacrylonitrile crystals grown from solution
Authors: Klement J. J. , Geil P. H. 
Published: April 2003

AbstractPolyacrylonitrile crystals were grown from dilute solutions of propylene carbonate by a technique similar to that described by Holland et al. Crystals were also grown on various substrates and sampled at various stages of growth after washing away the uncrystallized solution. The initial single lamellae rapidly develop into complex twinned structures with many overgrowths. The morphology of the full‐grown crystal varied considerably with crystallization temperature. Small‐angle x‐ray and electron microscopy results indicate only a small change in lamellar thickness with crystallization temperature for our polymer. Electron and x‐ray diffraction results indicate lateral order can be quite good in these crystals. Our x‐ray results indicate that the previously reported a and b parameters should be doubled to give a = 21.18 Å and b = 11.60 Å. However, there was no evidence for chain‐axis order. The crystals were stretched at various stages of growth on Mylar film. Multilamellar crystals having their long axis parallel to the draw direction formed large cracks perpendicular to the long axis. Fibers of large diameter (100–250 Å) were observed across these cracks. When the long axis of these crystals was at an angle to the draw direction no large cracks or fibers were observed. Single lamellae generally cracked perpendicular to the draw direction, irrespective of their orientation to the draw direction. Definite fibers were not observable across cracks in single lamellae.
Design of beams and beam columns
Authors: Lindner J 
Published: April 2003

AbstractBeams and beam columns are very important elements in steel structures. This paper gives an introduction to mathematical solution methods, to the typical behaviour of these elements and to the design methods used in new codes in Europe. Special attention is given to the checking of cross‐sectional capacity and to the use of simplified design formulae. Additional information, such as diagrams for the lateral torsional buckling parameter k will help to solve practical design problems. For the case of lateral torsional buckling, different types of constructional details are discussed with regard to the load‐carrying capacity of beams and beam columns. The new European rules for interaction equations which are of great importance for practical design, are explained in detail, distinguishing between members which are susceptible to torsional deformations or not. Copyright © 2003 John Wiley & Sons, Ltd.