bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of molecular weight on conformation of poly(β‐benzyl L‐aspartate) in films
Authors: Kyotani Hiroko , Kanetsuna Hisaaki 
Published: April 2003

AbstractIn order to study the effect of the molecular weight on the crystallinity and conformational changes of poly(β‐benzyl L aspartate) in films, a previous study on high molecular weight samples has been extended to included polymers of low molecular weight, about 3.3 × 103. Films were prepared from chloroform solution by quick or slow evaporation at room temperature. The conformation and the thermal behavior were studied by means of infrared spectroscopy and differential scanning calorimetry. All films dried quickly are composed of polymer in the left‐handed α‐helical form. All samples studied which have molecular weights above 2.3 × 104 are similar in crystallinity and the left‐handed α‐helices in them crystallize to ω‐helices during slow evaporation. In the low molecular weight region, however, the left‐handed α‐helices reverse to right‐handed α‐helices during slow evaporation, and the right‐handed α‐helices, in turn, reverse and crystallize to highly ordered ω‐helices upon heat treatment, although there is some simultaneous conversion to the β‐form. The transition temperatures of the quick‐dried films for conversion from the left‐handed α‐helix to the ω‐helix and from the ω‐helix to the β‐form increase linearly with increasing molecular weight up to about 2 × 104, but no large molecular weight dependence is observed beyond that region.
Thermoluminescence and thermally stimulated conductivity in polymers
Authors: Ranicar J. H. , Fleming R. J. 
Published: April 2003

AbstractThermoluminescence (TL) and thermally stimulated conductivity (TSC) glow curves in poly(vinyl chloride), polyethylene, polystyrene, polytetrafluoroethylene, and polyimide have been compared, and many similarities have been observed. Comparison with available NMR, dynamic mechanical loss, and dielectric loss, molecular mobility data shows that most TL and TSC peaks occur at temperatures similar to those assigned to the onset of specific molecular motions, suggesting that the peaks are due to the liberation of electrons from traps formed by the polymer chains themselves, e.g., potential wells or cavities due to chain entanglement in amorphous regions, or main‐chain branching points. Peaks for which correlation with molecular motion is not apparent are tentatively assigned to liberation of electrons from traps centered on impurities. The TSC peak temperatures in PVC were not affected in any consistent fashion by the application of high‐strength electric fields during the warming process, indicating that the electron traps are electrically neutral when empty and charged when filled; the direction of the TSC currents appears to be determined by temperature gradients existing within the samples. The TL glow‐curves are generally in good agreement with the results of other workers. The dark dc conductivity of PVC not exposed to ionizing radiation rises sharply in the temperature region assigned to the β‐relaxation process, suggesting that the electron mobility in that polymer is dominated by molecular chain motion, i.e., the interchain charge transport process is probably best described in terms of a hopping process.
Surface tension of polymers with long unbranched side chains
Authors: Schwarcz Andor , Farinato R. S. 
Published: April 2003

AbstractThe surface tension of polymers having long unbranched side chains was measured by a contact‐angle technique. It ranges between 20–29 dynes/cm and is almost entirely due to dispersion forces. At the lower end of this range the side chains are attached to every other carbon atom of the main chain and the structure is characterized by extended alkyl side chains at right angles to the main chain with methyl groups forming the interface. As the spacing frequency of the side chains increases and the main chain becomes the site of other bulky groups, this structure becomes less ordered and the surface tension increases to the high twenties. The same effect, to a lesser extent, is observed when the length of the side chains is gradually diminished to 12 carbon atoms from 21. A sudden increase of the polar force contribution of the surface tension at 10–12 carbon atoms indicates collapse of the ordered side‐chain structure.
Broad‐line NMR of aromatic polyamides
Authors: Woodward A. E. , Landis J. , Frosini V. 
Published: April 2003

AbstractBroad‐line NMR spectra have been obtained at 90 MHz in the temperature region 140–530°K for four aromatic polyamides: poly(1,3‐phenylene isophthalamide), poly(1,4‐phenylene terephthalamide), poly(4,4,‐diphenylene terephthalamide), and poly(4,4,‐diphenylene isophthalamide). For the latter three materials, a broad‐line narrowing process occurs in the 210–370°K region. At temperatures above 340°K a complex line shape is observed for all samples. Possible causes of these processes are discussed, and comparison made with dynamic mechanical results.
Effect of domain morphology of butadiene—styrene block copolymers on viscoelastic properties and in tension shear
Authors: Kraus Gerard , Rollmann K. W. , Gardner J. O. 
Published: April 2003

AbstractDynamic viscoelastic properties of S—B—S block copolymers were measured in the tensile and shear deformation modes. Between the glass transitions of the polybutadiene and polystyrene domains the ratio of storage moduli E'/G' in tension and in shear for the same polymer varied from 3 to more than 30, depending on sample preparation. For films cast from good solvents this ratio was near 3; large ratios resulted from deposition from poor polybutadiene solvents or from compression molding. Above the polystyrene glass transition, E'/G' approached 3 for all samples. The effect is ascribed to various degrees of polystyrene domain connectivity. Electron micrographs confirm this interpretation. For morphologies of high polystyrene domain connectivity, the loss tangent in tension is heavily weighted by mechanical losses in the polystyrene phase; the loss tangent in shear is affected only moderately by differences in domain morphology.
Aging of quenched polypropylene
Authors: Kapur Sudhir , Rogers C. E. 
Published: April 2003

AbstractStructural changes occurring during the aging of polypropylene quenched from the melt have been investigated by measurements of gas transport parameters and mechanical properties. Major changes in diffusion and sorption behavior during the first 30 hr of aging at room temperature were found to be strongly dependent on the atomic size of the inert gas penetrants. Tensile mechanical properties increased with aging time, while resilience and mechanical damping showed more complex behavior. The interpretation of the combined results suggests that the aging process may involve molecular rearrangements generally comparable to those for secondary crystallization processes, but within domains comparable in size to that of the smaller penetrant species. The relaxation of a residual stress distribution related to nonuniform plastic flow during quenching may be superimposed on the rearrangement phenomena.
Molecular reorientation in polypropylene
Authors: Duckett R. A. , Goswami B. C. , Ward I. M. 
Published: April 2003

AbstractThe molecular reorientation occurring within deformation bands in one‐way drawn polypropylene films has been studied by optical microscopy. The deformation bands formed at the yield stress in tension tests on specimens cut at various angles to the initial drawing direction. It is shown that the principal axes of the refractive index ellipsoid in the deformation band coincide with the principal axes of the plastic strain ellipsoid, provided that the strains are referred to the initial isotropic state, i.e., prior to the initial drawing process. This result is similar to the results obtained previously for poly(ethylene terephthalate), but differes somewhat from the results obtained by other workers for poly(vinyl chloride). A possible reason for these differences is discussed.
Polymerization in catalyst particles: Calculation of molecular weight distribution
Authors: Schmeal W. R. , Street J. R. 
Published: April 2003

AbstractFactors which affect and broaden molecular weight distributions are investigated for polymerizations in catalyst particles which are accumulating polymer, such as Ziegler polymerizations of olefins. Expressions for the Q value, or ratio of weight‐average to number‐average molecular weight, are derived for simple, but important linear models. These expressions are of the form Q = FSX for a kinetic scheme in which polymer chains have an infinite growing lifetime, and Q = 2FGSX for a kinetic scheme in which termination by hydrogen is dominant and each site produces many chains. The factors F, G, S, and X are equal to or greater than unity: F depends on particle geometry and the nature of diffusion and reaction; G depends on environmental history of the particle; S depends on the distribution of site activities; and X depends on the distribution of particle sizes. Effects of reactor type are also studied quantitatively.
Specific heats of poly(vinyl chloride) compositions
Authors: Dunlap L. H. , Foltz C. R. , Mitchell A. G. 
Published: April 2003

AbstractSpecific heats of a series of plasticized and unplasticized poly(vinyl chloride) compositions have been determined by a differential scanning calorimeter over the temperature range 200–400°K. The glass temperatures were higher and the specific heat values at 300°K lower with phthalate esters then with esters of aliphatic acids; those values were also higher and lower, respectively, with benzyl‐containing esters than with the methyl or octyl esters. The data were treated by a computer technique and plotted. A mathematical expression was developed which would describe the curves obtained when specific heat was plotted against temperature, and provided values in each case for the glass temperature and the specific heat at the glass temperature and at 300°K, as well as other descriptive parameters.
Estimation of parameters of the scattering particles in horn keratin by low‐angle X‐ray scattering
Authors: Ratho T. , Torasia S. 
Published: April 2003

AbstractLow‐angle x‐ray scattering methods have been applied to determine the parameters of the scattering materials in horn keratin. A low‐angle camera of Kratky's latest design has been used for the experimental measurements. By treating the substance as a densely packed system, the evaluation of the parameters has been made after theories of Porod and Kratky. Pore analysis of the substance yields a specific inner surface of 4.7 × 10−1m2/cm3, a heterogeneity distance lc = 290.2Å and a reduced inhomogeneity length lr = 99.4Å. The volume of the pores has also been determined approximately.
Critical molecular weight for onset of non‐newtonian flow and upper newtonian viscosity of polydimethylsiloxane
Authors: Ito Yoshio , Shishido Shunsuke 
Published: April 2003

AbstractThe flow curves of fractionated polydimethylsiloxanes of different molecular weights were obtained over a wide range of shear rates, from 3 × 10−1 to 4.3 × 106 sec−1, by use of a gas‐driven capillary viscometer designed to decrease the experimental error in high shear rate region. Non‐Newtonian flow can occur at molecular weights below the critical molecular weight Mc for the entanglement of polymer chain. The critical molecular weight M′c for the onset of the non‐Newtonian flow is identical with that of the segment of viscous flow. For the polymer of molecular weights from M′c to Mc, the upper Newtonian viscosity increases with an increase in molecular weight. Above Mc, the upper Newtonian viscosity is almost independent of the molecular weight.
Electron paramagnetic resonance study of mechanically degraded poly(ethylene terephthalate) and nylon 6 fibers
Authors: Chiang T. C. , Sibilia J. P. 
Published: April 2003

AbstractPeroxide radical concentrations were measured from both PET and nylon 6 fibers mechanically stretched in air at room temperature and quickly quenched into liquid nitrogen. The radical concentrations depend on degree of stretching as well as conditions under which the fibers were made, i.e., morphology. Drawn fibers of PET and nylon 6 produced peroxide radical concentrations of the same order of magnitude at the breaking points. These results indicate that chain scissions occur both in PET and nylon 6 under mechanical stretching.
Rigidity of polycarbonate in the presence of sulfur dioxide
Authors: Davis E. G. , Rooney M. L. 
Published: April 2003

AbstractA torsional pendulum method was used to study the transient effects of sulfur dioxide at pressures up to 700 torr on the rigidity of Bisphenol‐A polycarbonate at 25°C. Incremental increases in pressure led to a decrease and then to an increase in rigidity. Removal of sulfur dioxide at the rigidity minimum led to a rapid recovery in rigidity, but after apparent equilibrium had been reached the rigidity was not fully reversible. These observations provide evidence that sorption of sulfur dioxide results in structural reorganization of the polymer. A qualitative discussion of the data is presented.
Study of the influence of concentration on the time for first appearance of crystallization of polyethylene from xylene
Authors: Cryer N. B. , Zajícová V. 
Published: April 2003

AbstractAn experiment is reported in which a simple laser light‐scattering technique is used to measure the time for a critical turbidity to appear during the crystallization of polyethylene in xylene. The effect is examined over the entire range of solution concentration, and it is found that the relation between this concentration and temperature is linear for all solutions, provided that the time for turbidity to develop is arranged to be the same in all cases. In the appendix it is reasoned that similar results could have been obtained if, instead, times had been measured to a given degree of crystallinity rather than of turbidity. Departures from this linearity at low concentrations are taken as indicating multimolecular nucleation for all concentrations greater than about 1%.
Band structure and limiting behavior of linear polyanilines
Authors: Zach P. 
Published: April 2003

AbstractThe band structure of an infinite linear polyaniline is investigated by using the HCO approximation. The degree of modelling of the limiting structure for the series by the linear oligomers so far synthesized is investigated. The upper bound of the wave‐length of the long‐wave ultraviolet absorption band for the limit is found, and the possibility of estimation of the conduction properties of its complex with iodine is indicated. The problem of replacement of the NH group in the chains by a more general group X is dealt with, and the dependence of the energy gap between the highest occupied and the lowest unoccupied band on the parameters of this group is determined.
Investigation of mechanism of initiation of polymerization of cyclic acetals by protic and aprotic acids
Authors: Rozenberg B. A. , Komarov B. A. , Ponomareva T. I. , Enikolopyan N. S. 
Published: April 2003

AbstractThe kinetics of polymerization of 1,3‐dioxolan and degradation of polydioxolan catalyzed by boron trifluoride etherate have been studied in methylene chloride solution. The S‐shape kinetic curves is found to be due to the autocatalytic action of the macromolecules formed. Effective participation of macromolecules in the initiation process has been confirmed on the basis of the relationship of the rate constants of boron trifluoride etherate reactions with cyclic monomer and with the polymer chain as well as by the decrease of the induction period on addition of the polymer or its low molecular weight analog, methylal, to the polymerization system.
Effect of graft copolymer on demixing of immiscible polymers in solution
Authors: Horii Fumitaka , Ikada Yoshito , Sakurada Ichiro 
Published: April 2003

AbstractThe effect of graft copolymer on the demixing of solutions of two immiscible homopolymers and critical conditions for emulsion formation were studied. The graft copolymer used in the present work consists of one backbone poly(vinyl acetate) (PVAc) and one branch polystyrene (PS). PVAc and PS of various degrees of polymerization were used as immiscible homopolymers. The common solvent was benzene. When the concentration of homopolymer blend was not sufficiently higher than the critical concentration for demixing of the blend solution, no stable emulsion was formed, even when a considerable amount of graft copolymer was present, and the added graft copolymer merely reduced the demixing rate. However, as the blend concentration was increased, a stable emulsion could readily be obtained by addition of rather small amounts of graft copolymer. The radius of emulsion droplets was inversely proportional to the weight ratio of the graft copolymer to the dispersed component polymer, in accordance with the theoretical prediction. It was concluded that the emulsions were stabilized against coagulation by graft copolymer molecules fixed strongly as a monolayer on the interface of the emulsion.
Structure of the tetramer of α‐methylstyrene
Authors: Richards D. H. , Williams R. L. 
Published: April 2003

AbstractThe structure of the tetrameric dianion formed by α‐methylstyrene in tetrahydrofuran by reaction with sodium has been examined. Mass spectral, NMR, infrared, and kinetic data all indicate that the structure is
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rather than the structure
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which had previously been assumed for this species.
Photodecomposition of polystyrene on long‐wave ultraviolet irradiation: A possible mechanism of initiation of photooxidation
Authors: Lawrence J. B. , Weir N. A. 
Published: April 2003

AbstractThe long‐wave (λ < 3000 Å) photo‐oxidation of polystyrene in solution at 25°C has been studied osmometrically. Two types of chain scission have been observed: a purely photo process which occurs completely independently of oxygen and which is attributed to fission of photolabile groups in the polymer, and another process associated with random photolyses of the products of oxidation Scavenger experiments with 131I2 have shown that approximately two iodine atoms are incorporated per chain scission when photolysis is carried out in solution (benzene, hexafluorobenzene, methylene chloride) under high vacuum conditions in the presence of 131I2. No iodine incorporation or chain scission occurs when ionically prepared polystyrenes are treated similarly. The nature of the photolabile bond has been discussed, and there is some evidence for a peroxidic linkage arising from oxygen copolymerization in the chains. It is suggested that fission of the photolabile groups contributes to the initiation of the long‐wave photooxidation of the polymer.
Polymerization of olefin oxides with zinc n‐butyl xanthate and zinc dimethyldithiocarbamate catalysts: Mechanism of initiation
Authors: Lal Joginder , Devlin Edward F. 
Published: April 2003

AbstractXanthates and dithiocarbamates of metals have been reported to be effective as catalysts for the polymerization of olefin oxides. To investigate the mechanism of initiation, the bulk polymerization at 50°C of 1,2‐butene oxide with zinc n‐butyl xanthate (ZBX) at a monomer: ZBX molar ratio of 584: 1 was studied by ultraviolet and infrared spectroscopy. There is a rapid conversion of the sulfur–zinc bond in the metal xanthate to a structure
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containing sulfur–carbon and oxygen–zinc bonds, the latter acting as the site of the propagation step during polymerization. The xanthate ester moiety is subsequently converted to the oxygenated analogs, namely, O,O‐dialkylthiocarbonate and dialkyl carbonate. These changes are independent of the propagation step. The changes observed in the polymerization of butene oxide with zinc dimethyldithiocarbamate are similar, but much slower than in the case of ZBX. The presence of the carbonate ester moiety was also shown in the benzene‐insoluble, catalytically active fraction isolated from seeded catalyst, i.e., from the reaction product of propylene oxide and ZBX at a molar ratio of 8:1 or lower. This fraction also contained ionic sulfur.
Synthesis of an essentially alternating, aliphatic copolyamide
Authors: Pisanchyn John 
Published: April 2003

AbstractAn essentially alternating copolyamide of 6‐aminohexanoic acid and 11‐aminoundecanoic acid was obtained by the condensation polymerization of methyl 11‐(6‐aminohexanamido)undecanoate. The monomer was synthesized by the following reaction steps: (1) nitration of cyclohexanone to α‐nitrocyclohexanone; (2) cleavage of α‐nitrocyclohexanone by methyl 11‐aminoundecanoate to obtain methyl 11‐(6‐nitrohexanamido)undecanoate; (3) hydrogenation of the nitro group of the cleavage product. The polymer was characterized by its melting and crystallization behavior as determined by differential scanning calorimetry, by x‐ray analysis, and by comparison with an equimolar, random copolyamide of the two amino acids.
Equilibrium encapsulation of polystyrene latex particles
Authors: Keusch Preston , Williams David J. 
Published: April 2003

AbstractWe have performed a series of seeded styrene emulsion polymerizations in which the second stage of growth was initiated only after the second‐stage monomer charge had achieved equilibrium saturation with the seed particles. The final particles were observed in the electron microscope by using two means of distinguishing between the first‐ and second‐generation polymer: (a) butadiene tagging and osmium tetroxide staining coupled with ultramicrotomy and (b) tritiated‐styrene tagging coupled with autoradiographic detection. We find that the first‐ and second‐generation polymer chains are not uniformly mixed throughout the final latex product; rather, the second‐generation polymer overcoats the seed polymer in a core–shell fashion. In order to explain these results, we present the viewpoint that monomer actually concentrates at the periphery of the swollen particle to form a monomer‐encapsulated structure, rather than swelling the particle uniformly as has always been envisioned. We believe the encapsulation phenomena to be governed by the microscopic thermodynamic environment of the latex particles which has, in turn, a profound effect upon the conformational behavior of the long‐chain polymer molecules as they interact with the particle–water interface.
Evidence for and against concentration gradients in polymerizing latex particles
Authors: Gardon J. L. 
Published: April 2003

AbstractA single‐charge emulsion polymerization involving a monomer which is a good solvent for its polymer is considered. It is shown to be unlikely that within the polymerizing latex particles there are concentration gradients large enough measurably to affect the kinetics of the reaction. The average displacement of monomers due to Brownian motion within the latex particles and in the absence of concentration gradients is calculated. This diffusive mean free path, corresponding to an interval involving less than 1% change in conversion, is shown to be much longer than the radius of the latex particle. Consequently, loci where monomer concentration is perturbed by conversion to polymer are immediately swamped by unreacted monomer. Also, direct experimental evidence exists showing that the monomer concentration in latex particles is about the same when nonpolymerizing latex particles are saturated or during polymerization in the presence of monomer excess. The thermodynamics of saturation swelling preclude the possibility of the existence of large concentration gradients. The arguments that have been advocated in the literature for core–skin separation within polymerizing latex particles were based on conversion data which were thought to be linear with time, while a reexamination indicates that they were not. The observed core–skin separation obtained when butadiene or tritiated styrene were copolymerized with styrene in the presence of a polystyrene homopolymer seed latex has questionable relevance to single‐charge homopolymerization. There are reasons to doubt that the distribution of co‐monomers within latex particles can be frozen by their conversion to polymers in a two‐stage emulsion polymerization.