bluuebunny/crossref_metadata_2025_split huggingface dataset:
Toward a quantitative theory of the rheology of concentrated solutions of stiff polymers
Authors: Larson R. G. , Mead D. W. 
Published: April 2003

AbstractPredictions of the rheological properties of concentrated isotropic solutions of stiff polymers are made that improve on those obtained with the approximate Doi theory. Predictions of the shear‐rate and concentration dependence of the viscosity η and the first and second normal stress differences N1 and N2 that avoid the closure approximations of Doi are obtained by using a Galerkin numerical solution for the orientation distribution function. The predictions for η and N1, though they neglect the effects of polydispersity in molecular weight, are shown to be in quantative agreement with recent measurements on solutions of poly‐γ‐benzyl‐L‐glutamate (PBLG). The polydispersities of a series of commercial PBLG and PBDG samples are measured by gel permeation chromatography, and found to be broad. An attempt is made to account for the effects of polydispersity on the linear viscoelastic storage and loss moduli for PBLG solutions by using the Marrucci–Grizzuti extension of the Doi theory. The attempt fails. The most probable explanation for the failure is that the rotary diffusive motions of the shorter molecules in the mixture are drastically slowed by the matrix of surrounding longer molecules.
The influence of chain structure on the equilibrium melting temperature of poly(vinylidene fluoride)
Authors: Nandi A. K. , Mandelkern L. 
Published: April 2003

AbstractThree poly(vinylidene fluoride) whole polymers were fractionated according to their head‐to‐head concentrations. Their melting temperatures and fusion properties were studied. Although small but significant differences were found among the fractions obtained from a given parent polymer, a wide range in chain compositions was not obtained. The equilibrium melting temperatures were determined by extrapolating the dependence of the observed melting temperature on the crystallization temperature. A critical analysis is given of this extrapolation method as applied to poly(vinylidene fluoride) and the results are compared with literature reports. The problems involved in explaining the dependence of the equilibrium melting temperatures on the structural irregularities of the chain are given. Possible reasons for the relatively high level of crystallinity that is observed, for what is essentially a copolymer, are also discussed.
Diffusion of polymer molecules into polymer networks: Effect of stresses and contraints
Authors: Gent A. N. , Liu Ginger L. 
Published: April 2003

AbstractFine threads of cis‐1, 4‐polyisoprene, diameter ca. 50 μm, were prepared by drawing from solution and drying. They were crosslinked by reaction with H2S and SO2 and then swollen with linear cis‐polyisoprene liquids of varied molecular weight Ms, from 1,000 to 24,000 g/mol. Diffusion coefficients were determined from the initial rate of uptake, for both unrestrained and stretched threads. They were found to be in good agreement for stretches of up to about 300%. On the other hand, the equilibrium uptake increased markedly (> 100%) over this range of strain, similar to the increase in swelling observed with low‐molecular‐weight liquids. Values of diffusion coefficient were also obtained from the rate of deswelling upon release of swollen threads from tension, and from the rate of uptake of polymer liquids by a thin coating of crosslinked polymer, bonded onto glass fibers. Satisfactory agreement was obtained in all cases. A number of simple experiments thus give similar values for the diffusion coefficient of polymer liquids in lightly crosslinked polymer networks, in the range 10−13−10−16 m2/s, depending upon the molecular weight Ms of the polymer liquid approximately weight as M−2s. The amount of liquid absorbed was strongly dependent on its molecular weight, Ms, the degree of crosslinking of the host material, and applied stresses or constraints.
A model for dynamic relaxations in amorphous polymers. II. Extensions and generalizations of formalism and application to poly(methacrylates)
Authors: Bicerano Jozef 
Published: April 2003

AbstractExtensions and generalizations of a new model for the dynamic relaxations in amorphous polymers, and its application to the poly(methacrylates), will be presented. The sizes of moving subunits will be extrapolated for the β and γ relaxations in several poly(methacrylates), by working backward from the relaxation temperatures (Tr) observed at frequencies of 1 to 100 Hz to subunits giving Tcs comparable to those Trs. A general form will be proposed for activation energy distributions; and used to derive relaxation time distributions satisfying the experimental trends. The good agreement between the calculated Tc and the Tr observed dynamically at frequencies of 1 to 100 Hz will be shown to result from the nature of these distributions. The loss peak observed at very low temperatures by isochronal sweeps at very low frequencies is therefore caused by the dissipation of applied energy in localized domains. At sweep frequencies of 1 to 100 Hz, Tr ≈ Tc, and energy dissipation begins to take place over regions spanning the entire polymer. This delocalization of the energy dissipation is relevant to the effects of molecular level factors on many mechanical and thermodynamic properties of amorphous polymers. The effects of activation entropy and of dynamic excess entropy will be shown to be small in magnitude but important in terms of fully understanding relaxation behavior. Physical aging will be shown to result in a slight increase in the calculated characteristic temperatures. Finally, it will be shown that the relaxation behavior of the moduli and the compliances share some important common features with many other physical phenomena of seemingly very different nature.
Supramolecular structure investigation of poly(γ‐benzyl, α‐l‐glutamate)‐benzyl alcohol system by static light scattering
Authors: Shukla Premal , Muthukumar M. 
Published: April 2003

AbstractThe cholesteric mesophase of the PBLG‐BA system in concentrated solutions and in the gel phase has been investigated by static laser light scattering. The observed ‘distortion’ of the patterns in terms of their dependence on the azimuthal angle was studied. Effect of form‐optical rotation on the pattern is discussed. The manner of distortion in the Hv pattern determines the sense of cholesteric twisting. The observed scattering patterns are analogous to those obtained by previous investigators in other solvents. Quantitative measurements of the intensity as a function of scattering vector provide information on the sense and pitch of the cholesteric twisting, as a function of temperature and concentration. Optical microscopy studies reveal onset and build‐up of the cholesteric structure. Optical rotation and differential scanning calorimetry (DSC) studies support the conclusions obtained from light‐scattering experiments.
Short‐range order in miscible polymer blends: A monte carlo study
Authors: Cifra P. , Karasz F. E. , Macknight W. J. 
Published: April 2003

AbstractLocal ordering in miscible binary polymer blends with strong attractive interactions between the two types of chains was investigated by computer simulation of allowable conformations on an incompressible cubic lattice. A tendency toward maximum ordering (in which chains pack in alternate parallel rows) is shown by the calculation of pair correlations and in “snapshots” of the mixtures. A specific directional interaction is not necessarily required for ordering. A comparison of these results with those recently obtained using mean‐field theory is presented. The heat capacity of mixing was also calculated and found to be positive, with a value close to that reported experimentally.
Effect of polymer binding on the spectroscopic properties of stilbene‐based fluorescent dyes
Authors: Smit K. J. , Ghiggino K. P. 
Published: April 2003

AbstractStudies utilizing absorption and emission spectrophotometry have been carried out to observe the effect of various polymer substrates on the spectral characteristics of several stilbene‐based fluorescent dyes. Poly(vinyl alcohol), cellulose acetate, cellulose fiber, and bovine serum albumin were used as substrates for two common fluorescent whitening agents and for two model compounds. Binding of these molecules to polymers leads to a marked red shift in the absorption spectra and an increase in vibrational structure. These effects are attributed to an increase in molecular planarity induced by the polymer environment. For the case of bovine serum albumin, it is concluded that the dye molecules bind to the protein largely through electrostatic interaction involving the dye sulfonate groups. The spectral characteristics of the dye molecules on wool fibers are interpreted in light of these results.
Comparison of x‐ray diffraction and thermal analysis methods for assessing order in poly(vinyl chloride)
Authors: Dawson P. C. , Gilbert M. , Maddams W. F. 
Published: April 2003

AbstractThermal analysis and wide‐angle x‐ray diffraction were used to assess the crystallinity for a series of poly(vinyl chloride) (PVC) samples, and the use of two complementary techniques was found beneficial. Two polymers were studied; a commercial suspension grade PVC, and an experimental grade of higher syndiotacticity. Both were examined as powders, and after processing. All samples were also examined after heat treatment at various temperatures between 100 and 175°C. Annealing the suspension grade of PVC appeared to cause a small change in total crystallinity, but little change in crystallite size. When the low temperature powder was annealed, some increase in crystallinity was detected. However, when this polymer was milled, there was a significant increase in crystallite size, and crystallinity seemed to increase further. Particularly noticeable was the increase in the amount of high melting crystallites.
Phase separation in mixtures of poly(ethylene glycol monomethylether) and poly(propylene glycol) oligomers
Authors: Takahashi H. , Kyu T. , Tran‐Cong Q. , Yano O. , Soen T. 
Published: April 2003

AbstractTime‐resolved light scattering was employed to investigate kinetics of phase separation in mixtures of poly (ethylene glycol monomethylether) (PEGE)/poly (propylene glycol) (PPG) oligomers. Phase diagrams for PEGE/PPG of varying molecular weights were established by means of cold point measurements. The oligomer mixtures reveal an upper critical solution temperature (UCST). Several temperature quench experiments were carried out with a 60/40 PEGE/PPG blend by rapidly quenching from a single phase (69°C) to two‐phase temperatures (66–61°C) at 1°C intervals. As is typical for oligomer mixtures, the early stage of spinodal decomposition (SD) was not detected. The kinetics of phase decomposition was found to be dominated by the late stage of SD. Time‐evolution of scattering intensity was analyzed in accordance with nonlinear and dynamical scaling theories. The time dependence of the peak intensity Im and the corresponding peak wavenumber qm was found to follow the power‐law {Im(t)˜ tα, qm(t)˜ t‐β} with the values of α = 3 ± 0.3 and β = 1 ± 0.2, which are very close to the values predicted by Siggia. This process has been attributed to a coarsening mechanism driven by surface tension. In the temporal scaling analysis, the structure function reveals university with time, suggesting self‐similarity. Phase separation dynamics in 60/40 PEGE/PPG resembles the behavior predicted for off‐critical mixtures.
Low‐frequency Raman spectra of odd α,ω‐disubstituted n‐alkanes
Authors: Viras Fotini , Viras Kyriakos , Campbell Carl , King Terence A. , Booth Colin 
Published: April 2003

AbstractLow‐Frequency Raman spectra of odd α,ω‐dibromo‐ and α,ω‐dihydroxy‐n‐alkanes were recorded. The longitudinal acoustic mode (LAM‐1) frequencies were assigned by references to the published results for n‐alkanes and even α,ω‐disubstituted n‐alkanes and also by taking account of the effects of end intermolecular forces and end‐group masses by use of the chain model of Minoni and Zerbi.
Study of phase separation in polyurethane using paramagnetic labels. Effect of soft segment molecular weight, structure, and thermal history
Authors: Chen Wen‐Pin , Kenney Donald J. , Frisch Kurt C. , Wong Shaio‐Wen , Moore Ralph 
Published: April 2003

AbstractThe morphology of segmented polyurethane (PU) elastomers was studied by means of ESR at various temperatures ranging from 100K‐450K. The PU's were based on 4,4'‐diphenylmethane diisocyanate (MDI), poly (oxytetramethylene) glycols (PTMO) and hydroxyl‐terminated random compolymers of tetrahydrofuran and 3‐methyl‐tetrahydrofuran (THF/Me‐THF). Purified 1,4‐butanediol (BD) was used as a chain extender. The nitroxide probe, 4‐hydroxy‐2‐2',6‐6'‐tetramethylpiperidine‐1‐oxyl (TEMPOL), was used to label the polyurethanes by reaction with an NCO group of MDI. Analysis of the electron spin resonance (ESR) spectra suggested that the label situates at heterogeneous sites, which means that a two‐phase domain is present. The morphologies at 300K and 400K were compared. It was found from ESR spectra that polyurethanes made from soft segments having higher MW exhibited greater phase separation at 300K than their corresponding elastomers made from lower MW soft segments. However, at 400K the more isotropic ESR spectra for the PUs containing higher MW soft segments suggested that the PUs were more mobile than their analogs having lower MW soft segments. The introduction of 3‐methyl side groups decreased the phase mixing at 400K due to steric hindrance.
Rheology of polystyrene/poly(vinyl methyl ether)blends near the phase transition
Authors: Ajji A. , Choplin L. , Prud'Homme R. E. 
Published: April 2003

AbstractMixtures are expected to show anomalous behavior in their viscoelastic properties close to a critical point. In this study, the reheological behavior of blends of polystyrene and poly (vinyl methyl ether) below, close to, and above the phase separation temperature Ts was investigated. Rheological measurements were carried out at three different compositions in the melt. Below and far from Ts, a satisfactory superposition of the storage and loss moduli G' and G″ was observed at all temperatures and frequencies. Close to Ts deviations were observed for G' at low frequencies (the so‐called terminal zone). Above Ts G″ values was still observed over the whole range of frequencies and temperatures. The deviations observed for G' near Ts can be interpreted as due to the presence of significant concentration fluctuations. Plots of log (G'/G″2) as a function of temperature were shown to be sensitive to this anomalous behavior.
Miscibility of poly(ϵ‐caprolactone) and of poly(styrene‐co‐acrylonitrile) with poly(styrene‐co‐acrylic acid)
Authors: Ho Jo Won , Gyun Kim Hong 
Published: April 2003

AbstractThe miscibility of poly (ϵ‐caprolactone) (PCL) with poly (styrene‐co‐acrylic acid) (SAA) and of poly (styrene‐co‐acrylonitrile) (SAN) with SAA was examined as a function of the comonomer composition in the copolymers. For PCL/SAA blends it was found that PCL is miscible with SAA within a specific range of copolymer compositions. Segmental interaction energy densities were evaluated by analysis of the equilibrium melting point depression and application of a binary interaction model. The results suggest that the intramolecular repulsion in SAA copolymer plays an important role in inducing the miscibility. Additionally, the critical AA content in SAA for the blend to be homogeneous was predicted by correlating the segmental interaction energy densities with the binary interaction model. For SAN/SAA blends, it was also found that SAA is miscible with SAN within a specific range of copolymer compositions. From the binary interaction model, segmental interaction energy denisties between different monomer units were estimated from the miscibility map and were found to be positive for all pairs, indicating that the miscibility of the blends is due to the strong repulsion in the SAA copolymers.
Determination of molecular weights and Mark–Houwink constants for soluble electronically conducting polymers
Authors: Holdcroft Steven 
Published: April 2003

AbstractIntrinsic viscosities and gel permeation chromatography data were used to evaluate the Mark–Houwink constants of the soluble electronically conducting polymer, poly(3‐hexylthiophene) (P3HT):K and a are 2.28 × 10‐3 cm3/g and 0.96, respectively, in tetrahydrofuran (THF) at 25°C. Mark–Houwink constants were used to calibrate gel permeation chromatography (GPC) columns for P3HT. Number‐average molecular weights of P3HT determined with modified calibration curves agreed well with those determined by an absolute method, embulliometry. Molecular weights estimated using unmodified polystyrene calibration procedures were significantly larger than true values.
Low‐frequency raman‐active modes in methoxy‐ended and hydroxy‐ended oligo(oxyethylene)s
Authors: Campbell Carl , Viras Kyriakos , Booth Colin 
Published: April 2003

AbstractLow‐frequency Raman spectra were recorded for α‐methyl, ω‐methoxy‐oligo(oxyethylene)s, C1EmC1 with m in the range 4–25 (i.e., 15–78 atoms). Longitudinal acoustical mode (LAM‐1) frequencies were identified and compared with those determined previously for α‐hydro, ω‐hydroxy‐oligo (oxyethylene)s. Nonlinear relationships between LAM‐1 frequency and reciprocal chain length were explained as an effect of intermolecular end forces.
Study of solid‐state polymerization by positron lifetime measurements
Authors: Millán Sonia , Sánchez Victor , Fucugauchi Luz Alicia , Ito Yasuo , Tabata Yoneho 
Published: April 2003

AbstractThe positron lifetime measurement technique was applied to analyze the structural changes occurring in solid‐state polymerization of acrylamide and trioxane. Positronium (Ps) parameters, τ2 and I2, which show a marked change as a function of the γ‐ray irradiation dose, signaled the presence of three clearly defined stages in the polymerization process: initiation, propagation, and saturation. The initial increase in τ2, may very well be the result of efficient trapping of Ps in vacancies created around the polymer nuclei. The complicated behavior of I2 may be explained as a chemical reaction between Ps precursors and the radiolysis products of this polymerization process.
2H NMR characterization of phenylene ring flip motion in poly(p‐phenylene vinylene) films
Authors: Simpson Jeffrey H. , Rice David M. , Karasz Frank E. 
Published: April 2003

AbstractSolid‐state 2H quadrupole echo nuclear magnetic resonance (NMR) spectra and measurements of 2H spin lattice relaxation times have been obtained for films of poly(p‐phenylene vinylene) deuterated in phenylene ring positions (PPV‐d4). NMR line shapes show that all the phenylene rings of PPV undergo 180° rotational jumps about the 1,4 ring axis (“ring flips”) at 225°C. The temperature dependence of the 2H line shapes show that the jump motion is thermally activated, with a median activation energy, Ea = 15 kcal/mol, and a distribution of activation energies of less than ±2 kcal/mol. The jump rate was also determined from the magnitude of the anisotropic T2 relaxation associated with 2H line shapes and from the curvature of inversion recovery intensity data. The experimental activation energy for jumps is comparable to the intramolecular potential barrier for rotation about phenylene vinylene bonds. 2H NMR provides a method for determining the phenylene‐vinylene rotational barrier in pristine PPV, and may potentially be used to study conjugation in conducting films.
The application of ultrasonic waves to the electrochemical polymerization of thiophene
Authors: Osawa Satoshi , Ito Masayoshi , Tanaka K. , Kuwano J. 
Published: April 2003

AbstractThe electrochemical polymerization of thiophene was carried out both in the presence and absence of ultrasonic waves. Without irradiation, the anode potential increased with increasing current density. Polymerization at high potential gave a low polymer yield and produced macroscopically heterogeneous films. Ultrasonic irradiation resulted in an improvement of polymer yield and in a lowering of the anode potential during polymerization. The effects were especially marked when the polymerizations were carried out at low temperature, low monomer concentration, and high current density. These results are explained by the formation of diffusion layer in the vicinity of the electrode during electrochemical reactions.
Crystallite morphology in thermotropic random copolymers: Application of transmission electron microscopy
Authors: Spontak Richard J. , Windle Alan H. 
Published: April 2003

AbstractThe morphology of nonperiodic layer (NPL) crystallites appearing in three thermotropic random copolyesters composed of 30/70, 58/42, and 75/25 monomer ratios of p‐oxybenzoate (B) and 2,6‐oxynaphthoate (N) is investigated with transmission electron microscopy. Copolymer films, produced by shearing the materials above their melting points and subsequently annealing them above their glass transition temperature, exhibit ordered entities averaging no more than 20 nm thick and 40–60 nm long, as observed with dark‐field imaging. Selected‐area electron diffractograms exhibit sharp equatorial reflections, indicating ordered packing of random intramolecular sequences of the molecules. Microdensitometry of these diffraction patterns reveal that both the equatorial and first meridional reflections are radiation sensitive, decaying exponentially with dose. Saturation doses (v) for the crystallites depend on the copolymer composition and degree of polymerization (DP) and are estimated to be approximately 130–150C/m2 for the high‐molecular‐weight copolymers (DP ≈ 150), with the N‐rich copolymer possessing greater radiation resistance, and about 430 C/m2 for a low‐molecular‐weight copolymer (DP ≈ 25).
Measurement of the density distributions in quenched polycarbonate specimens by a quantitative schlieren optical technique
Authors: Wimberger‐Friedl R. , Prast G. , Kurstjens A. V. , De Bruin J. G. 
Published: April 2003

AbstractA new technique for the determination of spatial density distributions is described. The density is derived from the refractive index gradient as determined with a quantitative Schlieren optical technique. The advantages of this technique are a high spatial resolution and sensitivity. Gapwise profiles can be measured without slicing the sample so that more reliable data are obtained quickly. This is demonstrated for the gapwise density distributions in quenched polycarbonate specimens. It is shown that the density distribution is determined by a competition between the local cooling rate variation and the cooling stress distribution. The residual density is predominantly determined by the coolant temperature. The density in the core is higher than at the surface for high coolant temperatures, whereas for low coolant temperatures the density in the core is lower than at the surface. The observed effects are explained semiquantitatively by a simple model using calculated cooling stresses and cooling rates.