bluuebunny/crossref_metadata_2025_split huggingface dataset:
Carbon‐13 NMR relaxation and transitions in polymers
Authors: Dekmezian A. , Axelson D. E. , Dechter J. J. , Borah B. , Mandelkern L. 
Published: April 2003

AbstractHigh‐resolution proton‐decoupled carbon‐13 nuclear magnetic resonance relaxation parameters have been obtained as a function of temperature for a set of completely amorphous polymers, semicrystalline polymers, and a series of ethylene–vinyl acetate copolymers. With these samples the nature of the glass temperature, other postulated amorphous transitions, and the β transition were investigated. For the completely amorphous polymers, the average correlation times depend on temperature according to the Williams–Landel–Ferry relation. Spectral collapse occurs at temperatures whose ratio to Tg is in the range 1.2–1.4 and corresponds to a correlation time of about 10−7s. The loss of resolvable spectra is demonstrated to be a consequence of experimental methods and is not due to the occurrence of another amorphous transition. Both the methylene and methine carbons can be resolved for the ethylenevinyl acetate copolymers. Although the correlation time for the methylene carbon is continuous and resolvable through the β transition region, the methine branch‐point resonance is lost. The implication of these results to the molecular nature of the β transition is discussed.
NMR self‐diffusion study of polyethylene and paraffin melts
Authors: Zupancic I. , Lahajnar G. , Blinc R. , Reneker D. H. , Vanderhart D. L. 
Published: April 2003

AbstractThe self‐diffusion coefficient D of paraffin and polyethylene melts—covering the range between N = 19 and 103 where N is the number of monomeric units—was measured by the pulsed‐magnetic‐field‐gradient NMR method for diffusion times between 3 ms and 1 s. For the paraffins, D is proportional to N−2 though the molecular weights are smaller than the critical molecular weight for entanglement. In polyethylene, melts a strong dependence of the diffusion coefficient on the diffusion time is observed, whereas no such dependence is found in paraffin melts. A mathematical formalism for describing spin‐echo attenuation in terms of a velocity autocorrelation function is shown to yield qualitative agreement with the experimental results.
Rheo‐optical Fourier‐transform infrared spectroscopy of polymers. VII. Uniaxial deformation of heat‐set poly‐(ethylene terephthalate) film
Authors: Gupta V. B. , Ramesh C. , Siesler H. W. 
Published: April 2003

AbstractPoly(ethylene terephthalate) (PET) film was uniaxially stretched to a draw ratio of 2.5. Two samples were prepared from this oriented film by heat‐setting it at 493 K while free to relax and when held at constant length. The structural changes occurring during uniaxial elongation up to fracture of these two oriented crystalline samples and their stress–strain characteristics were simultaneously monitored by rapid‐scanning Fourier‐transform infrared (FTIR) spectroscopy. In the free‐annealed sample, the orientation of the molecules in the amorphous phase shows a gradual improvement throughout the test, while chain unfolding occurs above 20% strain. This indicates that the predominant mechanism of deformation in this sample could be chain uncoiling in the amorphous phase followed by longitudinal slip processes within the sample. In the taut‐annealed sample, chain unfolding occurs at low strains accompanied by slight improvements in amorphous and crystalline orientation. Thus, in this sample longitudinal slip would appear to be the main deformation mechanism. The results of the FTIR measurements are discussed with reference to the dependence of the deformation mechanisms on the initial structure of the samples.
High‐resolution solid‐state 13C‐NMR spectroscopy of cellulose nitrates
Authors: Patterson P. M. , Patterson D. J. , Blackwell J. , Koenig J. L. , Jamieson A. M. , Carignan Y. P. , Turngren Edward V. 
Published: April 2003

AbstractSolid‐state 13C‐NMR spectroscopy has been used to investigate the structure of cellulose nitrates prepared from cotton linters. The solid‐state technique has the advantage that the entire range of substitution can be studied, which is not possible at present by solution methods. The spectra change progressively with increasing degree of substitution (DOS = 3 for cellulose trinitrate), and can be used to quantify the extent of substitution at C6, C2, and C3. Progressive nitration leads to elimination of the original C6 resonances of native cellulose by DOS = 1.39. The first nitration of C6 occurs in the amorphous regions, and this is complete by DOS = 0.50. Substitution at C6 is accompanied by decrystallization, as indicated by the decrease in the resonance assigned to crystalline C4, which also disappears at DOS = 1.39. A new resonance assigned to C1 appears at DOS = 0.28 at 101 ppm; the original C1 resonance for cellulose declines steadily and disappears by DOS = 2.65. This change is assigned to nitration of C2, based on the published solution spectra. Finally, development of intensity at 82 ppm at DOS = 1.83 is assigned to the effect of nitration at C3. There is no indication of any rearrangement of the unsubstituted regions analogous to those that occur on Mercerization of native cellulose.
Macromolecular pair correlation functions from fluorescence depolarization experiments
Authors: Fredrickson Glenn H. , Andersen Hans C. , Frank Curtis W. 
Published: April 2003

AbstractDepolarization of fluorescence resulting from the transport of electronic excitations in chromophore‐containing polymers is investigated as a technique for detecting deviations from ideal‐chain statistics. An approximate expression for the fluorescence anisotropy that depends only on the pair correlation function of chromophore labels is presented. This approximation is shown to be accurate for particular cases of long‐range correlations, short‐range correlations, and no correlations among chromophore positions. The formalism allows fluorescence depolarization experiments to be used as a probe of macromolecular pair correlation functions.
Low‐frequency excitations in amorphous poly(vinyl acetate)
Authors: Viras F. , Viras K. G. , King T. A. 
Published: April 2003

AbstractRaman‐active modes have been detected in amorphous poly(vinyl acetate) in the frequency region 20–160 cm−1. The low‐frequency spectrum appears as a broad band for all three molecular weights examined. Spectra were recorded at two temperatures, 293 and 77 K, and found to show similar characteristics at both temperatures. The presence in solid amorphous polymers of broad structureless bands in general and their significance are discussed.
Structural studies on the polymorphism of nylon 3
Authors: Mun̄oz‐Guerra Sebastian , Fernandez‐Santin Jose Maria , Rodriguez‐Galan Alfonso , Subirana Juan A. 
Published: April 2003

AbstractThe structure and morphology of crystalline nylon 3 [poly(β‐alanine)] have been studied by electron microscopy and x‐ray diffraction. Two clearly defined forms are detected. Form I appears as spherulites made up of ribbonlike lamellae upon crystallization at high temperature from a solution in phenol–butanediol‐1,4. They have monoclinic unit cell with a = 9.60 Å, c = 8.96 Å, and β = 122.5°. The hydrogen‐bonded planes run parallel to the long dimension of the crystals. Form II is observed when the samples are prepared from formic acid solution at room temperature. A second type of spherulite with a microfibrillar structure is formed in this case. The isolated crystalline structures obtained from Form II appear to grow along the intersheet direction rather than along the hydrogen bond direction, which constitutes anomalous behaviour. Our results for this second form are consistent with an orthorhombic lattice with a = 9.56 Å and c = 7.56 Å. No clear information is obtained on the b dimension of the unit cell (chain axis) in either case. We assume a value of 4.78 Å, which corresponds to fully extended chains. The two forms coexist in films prepared from a formic acid–water solution as well as in samples recovered immediately after polymerization.
Comparative study of dielectric, mechanical, and nuclear magnetic relaxations of linear polyethylene. I. Broad‐line NMR investigation of the fine structure of the α and γ loss bands observed by dielectric and mechanical measurements
Authors: Kakudate Takuo , Kakizaki Maeko , Hideshima Teruo 
Published: April 2003

AbstractDetailed investigation of the linewidth, second moment, and mobile fraction by straightline decomposition of NMR spectra of linear polyethylene from ca. −160 to ca. 100°C reveals five molecular processes denoted γ1, γ2, β, α′, and α. Relaxation maps show that the γ1, γ2, and β processes correspond to the dielectric and mechanical relaxations given the same names, while the α and α′ processes correspond respectively to the mechanical α2 process due to molecular motion in the interior of crystals and to the dielectric α process. Close relations are found between the mass fractions of protons for the γ1 and β processes and also between those for the γ2 and α′ processes. From the effects of diluent on these processes and the annealing‐time dependence of mass fractions of protons, the γ1, and β processes are attributed to two modes of molecular motion in an interlamellar amorphous region, while the γ2 and α′ processes are attributed to motions in the lamellar surface layers. The surface‐layer thickness obtained by applying the two‐phase model for the data on mobile fraction agrees quite well with that reported in the literature. The Bergmann–Nawotki three‐component analysis of NMR spectra and symmetric‐line decomposition for determining mobile fraction are shown to be unsuitable for studying the fine structure of the α and γ loss bands.
Effect of polydispersity on the cloud‐point curves of polymer mixtures
Authors: Roe Ryong‐Joon , Lu Learong 
Published: April 2003

AbstractA method is presented for the calculation of cloud‐point curves of polymer–polymer mixtures when the polymers involved are polydisperse. The method is based on the Flory–Huggins free energy of mixing with a concentration‐independent χ parameter. Numerical results are given for cases in which the molecular weight distributions are represented by the Schulz–Flory type. When the two polymers have similar average molecular weights and polydispersities, the cloud‐point curves become flatter as the polydispersity increases. When the two polymers have similar average molecular weights but differ in polydispersity, the cloud‐point curves become more skewed as the difference in the polydispersity increases. The results point out that, if the polydispersity effect is not properly accounted for, the value of χ deduced from experimental cloud points is liable to be in error, especially with regard to its temperature coefficient and its concentration dependence.
Influence of the interphase on block copolymer thermodynamics: Extension of the leary model
Authors: Henderson Clayton P. , Williams Michael C. 
Published: April 2003

AbstractThe Leary–Williams model for the microphase thermodynamics of triblock ABA copolymers has been modified to accommodate deviations from homogeneous random‐coil configurations in the B‐chain dimensions as well as in those of the A chains, and has also been extended to cover the case of diblock AB copolymers. Only planar morphology is considered, but qualitative conclusions reported herein are expected to hold for other morphologies as well. The focus is on interphase thickness ΔT, with predictions made also for separation temperatureTsand planar repeat distanceD. Results are presented as systematic functions of copolymer composition (0 ≤ ϕA≤ 1), total molar volume (25,000 ≤Ṽ≤ 4 × 106cm3/g mol), block architecture (AB vs. ABA), temperature (298, 373 K), and for five different interphase composition profiles. In most cases, A represents a polystyrene block and B a butadiene block in these calculations. Predictions for ΔTincrease with temperature and depend on architecture, profile, andṼ; comparisons with data are close, in the range 15–30Å. It is shown thatTsdepends strongly on profile choice and ϕA, reaching a maximum in the ϕAmidrange but always with ϕA> 0.5. The major parameter influencingD(at constantṼ) is architecture, withD(SB) ≈ 2D(SBS), andD(Ṽ) varies fromD∝Ṽ0.75at lowṼtoD∝Ṽ0.5at highṼ.
Stress‐optical behavior of poly(cis/trans‐1, 4‐cyclohexane dimethanol‐alt‐formaldehyde) (PCDO) networks
Authors: Riande E. , Guzman J. , Saiz E. , Tarazona M. P. 
Published: April 2003

AbstractTwo samples of poly(cis/trans‐1,4‐cyclohexane dimethanol‐alt‐formaldehyde), having fractions of trans configuration of cyclohexylene rings Ft = 0.1 and 0.7, were synthesized by polycondensation of the appropriate mixtures of glycols with paraformaldehyde. These two samples were crosslinked by a chemical reaction using 2,4‐bis(p‐isocyanatebenzyl) phenylisocyanate as the crosslinking agent. The stress‐optical behavior of the resulting networks was studied at several temperatures in the range 10–80°C. The values of the optical configuration parameter Δα were 9.9 and 7.2 in units of 10−24 cm3 for Ft = 0.7 and 0.1, respectively. The temperature coefficient of this quantity was roughly zero. Theoretical analysis, performed using the rotational isomeric state model, proved that the only parameters that have an appreciable effect on the calculated values of Δα are those concerning the cyclohexane ring (i.e., Ft and the optical parameter ΓCC = ΔαCC − 2ΔαCH). Conformational energies, geometrical parameters, and contributions to the optical anisotropies from the oxymethylene oxide have no noticeable effect on the value of Δα calculated for the polymer. In fact, values of Δα calculated for the polymer at Ft = 0 and 1 are very close to those obtained, respectively, for cis and trans cyclohexane dimethylene. The theoretical values of Δα are roughly one order of magnitude lower than the experimental results; however, the theory reproduces satisfactorily both the variation of Δα with Ft and its temperature coefficient.
Morphological studies of liquid‐crystalline cellulose derivatives. I. Liquid‐crystalline characteristics of hydroxypropyl cellulose in 2‐hydroxyethyl methacrylate solutions and in polymer composites prepared by bulk polymerization
Authors: Nishio Y. , Yamane T. , Takahashi T. 
Published: April 2003

AbstractThe formation of liquid‐crystalline structure in hydroxypropyl cellulose (HPC) in the solvent 2‐hydroxyethyl methacrylate (HEMA) is described. In addition, an attempt is made to preserve the ordered structure of HPC in a composite by polymerizing the solvent. Optical evidence, including refractive index, absorption spectra, polarized‐light microscopy, and x‐ray diffraction, indicates that HPC‐HEMA solutions exhibit the cholesteric nature of the mesophase over limited concentration and temperature ranges. The polymer composite (HPC‐PHEMA) prepared from the liquid‐crystalline solution by polymerization of HEMA is endowed with anisotropic organization reflecting liquid‐crystalline character. Detailed morphological observations of the composite by electron microscopy show many round particles composed of parallel‐stacked, disklike lamellae, and aggregate bodies developed by coalescence of neighboring particles.
Simultaneous differential scanning calorimetry and small‐angle x‐ray scattering
Authors: Russell T. P. , Koberstein J. T. 
Published: April 2003

AbstractA method is described for performing simultaneous measurements of small‐angle x‐ray scattering and differential scanning calorimetry. The experiment is made possible through a combination of the high flux afforded by the storage ring at the Stanford Synchrotron Radiation Laboratory, a linear position‐sensitive detector with rapid response time, and a differential scanning calorimeter developed for optical microscopy. The feasibility of the technique is illustrated by examining the melting and crystallization of a polyethylene specimen. This example demonstrates the power of the technique and the accuracy and reliability of the scattering and thermal data.
Dilute‐solution properties of ring polystyrenes
Authors: Roovers J. 
Published: April 2003

AbstractThe temperature ΘA2 at which the second virial coefficient A2 is zero for ring polystyrenes is 28.5°C in cyclohexane, independent of molecular weight in the range 2 × 104 to 4.5 × 105. This cannot be explained solely by the Candau–Rempp–Benoit theory, which takes into account the effect of segment density on ΘA2 The radius of gyration of a ring is found to be approximately one‐half that of a linear polymer with the same molecular weight. The intrinsic viscosities [η] and intrinsic translational friction coefficients [f] of ring polystyrenes with molecular weights ranging from 7 × 103 to 4.5 × 105 have been measured in cyclohexane at 34.5°C (Θ, the Flory theta temperature for linear polystyrenes) and in toluene (a good solvent). The results are compared with those for linear polystyrene. It is found that the Mark–Houwink exponent is less than one‐half in cyclohexane at Θ. In toluene it is 0.67 compared to 0.73 for linear polystyrene. The hydrodynamic measurements suggest that large rings are less expanded than the linear polymers with the same molecular weight, contrary to many predictions.
The influence of tetraalkylammonium counterions on the conformational transition of an alternating copolymer of maleic acid and n‐butyl vinyl ether in aqueous solutions
Authors: Shimizu Toshio , Kwak Jan C. T. , Minakata Akira 
Published: April 2003

AbstractpH titration curves for the neutralization of an alternating co‐polymer of maleic acid and n‐butylvinylether (MAnBVE) with tetrabutylammoniumhydroxide (TBAOH) are reported, and compared to the case of neutralization with NaOH or tetraethylammoniumhydroxide (TEAOH). With TBA+ counterions the compact form of the polymer is stabilized, remaining the preferred form up to higher net charge densities. This tendency is enhanced at higher temperature. Free energy changes of the conformational transition are higher for TBA+ than for TEA+ or Na+ as counterions.
The effect of heating rate on the melting of polytetrafluoroethylene
Authors: Starkweather Howard W. 
Published: April 2003

AbstractWhen virgin polytetrafluoroethylene is heated at intermediate rates, two melting peaks are observed. As the heating rate is increased, the higher‐temperature peak grows at the expense of the lower‐temperature one without any significant change in the total heat of fusion. It is suggested that the higher‐temperature peak represents a transition to a strained melt which subsequently changes to the more stable equilibrium melt. After recrystallization from the melt, there is only a single melting peak which occurs at a lower temperature than peaks for the virgin polymer. All of these transitions are subject to superheating.
The determination of longitudinal crystal moduli in polymers by spectroscopic methods
Authors: Fanconi Bruno , Rabolt John F. 
Published: April 2003

AbstractExperimental methods for determining longitudinal crystal moduli of polymers were evaluated in light of recent proceesing methods that produced macroscopic Young's moduli which exceeded ultimate values as found by the x‐ray diffraction method. The spectroscopic techniques of Raman and coherent inelastic neutron scattering yielded higher longitudinal crystal moduli than x‐ray diffraction, and from calculations described herein it is concluded that these spectroscopic values are better estimates of the maximum Young's moduli in fully aligned and crystalline polymeric materials.
Corrections to calculations of the radius of gyration of polydisperse gaussian coils when scattering measurements at low angle are missing
Authors: Ullman Robert 
Published: April 2003

AbstractThe initial slopes and intercepts of Zimm plots of scattering experiments yield the centroidal radius of gyration of the scattering particle. In many cases, it is not possible to measure at sufficiently low angle to obtain accurate initial slopes and intercepts. Errors for missing data can be corrected by using calculated scattering functions as models. Such corrections are given here for polydisperse Gaussian coils.
Concentration distribution of elements in heterogeneous polymer systems determined by electron microprobe analysis
Authors: Popova E. D. , Avdeev N. N. , Chalych A. E. 
Published: April 2003

AbstractA technique for analysis of composition variations in heterogeneous polymer systems permits investigation of materials with both uniform and nonuniform distribution of heterostructural components. To illustrate the method, determinations have been made of concentration profiles of dibromo‐diallyl‐phthalate formed in porous PVC in two different ways.
Model calculation of the thermal conductivity of polymer crystals
Authors: Choy C. L. , Wong S. P. , Young K. 
Published: April 2003

AbstractA linear lattice model without adjustable parameters provides an accurate description of the magnitude and temperature dependence of the thermal conductivity of Kc of polyethylene crystals parallel (∥) and perpendicular (⟂) to the chain direction. The model shows that heat is transported principally by phonons polarized transverse (T) to the chain direction. Phonons polarized longitudinal (L) to the chain direction contribute about 20% to the heat transport along the chain direction, and negligibly to heat transport perpendicular to the chain direction. Thermal resistance is caused by LTT three‐phonon umklapp scattering in the parallel direction, and by TTT scattering in the perpendicular direction. The calculated values for large crystals are K = 465 W m−1 K−1, K = 0.16 W m−1 K−1 at 300 K, in agreement with experimental estimates and implying an anisotropy ratio of K/K ≈ 3000. The axial thermal conductivity of polyethylene crystals is extremely high and comparable to that of copper. Comparison with experimental data on semicrystalline samples at lower temperature yields a crude value of mean free path for boundary scattering of about 50 nm, agreeing in order of magnitude with the size of crystalline blocks.
Analysis of the effect of electrical conductance of ferroelectric polymers on D‐E hysteresis curves measured by the sawyer–tower method
Authors: Ikeda Susumu , Kobayashi Shin'ichi , Wada Yasaku 
Published: April 2003

AbstractThe D‐E (electric displacement vs. field) hysteresis curve of a ferroelectric polymer is conventionally measured by the Sawyer–Tower method. The effect of the electrical conductance of the polymer on the measured hysteresis curve is theoretically analyzed and a new method is proposed for elimination of the error arising from the conductance. Examples of original and corrected hysteresis curves of vinylidene fluoride–trifluoroethylene copolymer are presented.