bluuebunny/crossref_metadata_2025_split huggingface dataset:
Polymer fracture—A simple model for chain scission
Authors: Crist B. , Oddershede Jens , Sabin J. R. , Perram J. W. , Ratner Mark A. 
Published: April 2003

AbstractA simple model for calculating the fracture process for a single extended‐chain molecule such as polyethylene is considered. The model consists of a chain of N coupled Morse oscillators. There exists a critical overall extension ΔLc below which the fracture is energetically unfavorable but above which fracture is favored both energetically and kinetically. This elongation ΔLc scales as N1/2. For the critically stretched chain, the activation energy for rupture increases with N. Long chains must be stretched beyond this critical value to fail within experimentally meaningful times. Chains of all lengths subjected to the same force will fail with the same activation energy, provided this force is large enough to stretch each chain to ΔL > ΔLc. Observed activation energies are less than 1/3De, where De is the bond energy.
Random‐coil configurations of aromatic polyesters: Stress‐optical behavior of poly(diethylene glycol terephthalate)
Authors: Riande Evaristo , Guzmán Julio , Tarazona Maria P. , Saiz Enrique 
Published: April 2003

AbstractThe stress‐optical behaviour of an elastomeric network of poly(diethylene glycol terephthalate) (PDET) was measured over a wide range of elongation ratios α (up to 5) and temperatures (293‐353K). No evidence of strain ‐induced crystallization was found;on the contrary, the plot of birefringence versus stress exhibits negative deviations from linearity at values of α over 2.6. Values of the optical configuration parameter Δα of the order of 20 × 10−24cm3 with relative temperature coefficients of ‐1.1 × 10−3K−1 were found for the unswollen sample. The introduction of tricresylphosphate as diluent roughly doubles the birefringence of the network, presumably because of an increase in intermolecular interactions. Theoretical calculations carried out with the RIS model give values of Δα about one order of magnitude smaller than the experimental ones and temperature coefficients of about 4.1 × 10−3K−1. No reasonable modification of conformational energies or contributions to the anisotropic part of the polarizability tensor would achieve agreement between theory and experiments. The discrepancy between theoretical and experimental results may be qualitatively explained by intermolecular interactions.
On the lateral size of shish crystals
Authors: Feit K. , Karbach A. , Petermann J. 
Published: April 2003

AbstractThe lateral sizes of shish crystals crystallized from high‐molecular‐weight isotactic polystyrene and different blends with atactic polystyrene were examined with the help of transmission electron microscopy. It is found that the lateral sizes of shish crystals decrease with increasing content of the noncrystallizable aPS. A model accounting for the observation is proposed.
Oriented polyolefin laminates: A refractometry and infrared study
Authors: Gupta M. K. , Carlsson D. J. , Wiles D. M. 
Published: April 2003

AbstractA series of polypropylene‐copolymer laminates produced by biaxial orientation in the 120–155°C range has been characterized by refractometry and infrared (IR) spectroscopy. The copolymer was a random ethylene‐propylene resin and the IR techniques included tilted‐film transmission as well as internal reflection, both using polarized radiation. The IR techniques yielded spectra corresponding to the machine, transverse, and through directions. Similarly, the refractometry gave refractive indices of both components of each laminate along all three axes. Results from the three techniques were compared and contrasted and the validity of the calculated internal reflectance spectra discussed. Practical methods to estimate the source and extent of errors in the latter technique from false radiation and contact problems were explored.
Crystalline order and melting behavior of isotactic polypropylene (α form)
Authors: Guerra G. , Petraccone V. , Corradini P. , De Rosa C. , Napolitano R. , Pirozzi B. , Giunchi G. 
Published: April 2003

AbstractSamples of isotactic polypropylene in the α form may have different degrees of order in the up and down positioning of the chains. By suitable programming of the crystallization history of unoriented samples it is possible to show, from DSC and WAXS data, that a strict correlation exists between the melting behavior and the degree of ordering of the chains. The process of ordering at annealing temperatures above 150°C may lead to a “continuum” of modifications of the α form from less ordered to highly ordered ones; correspondingly we observe an increase in the melting point by as much as 15°C.
Selective permeation of CO2 and CH4 through kapton polyimide: Effects of penetrant competition and gas‐phase nonideality
Authors: Chern R. T. , Koros W. J. , Yui B. , Hopfenberg H. B. , Stannett V. T. 
Published: April 2003

AbstractSorption isotherms for pure CO2 and pure CH4 in Kapton H® polymide films at 60°C are reported for pressures up to 20 atm and are analyzed in terms of the dual‐mode sorption model. An experimental scheme for the measurement of steady‐state permeabilities of both pure and mixed gas feeds is described. Permeabilities of Kapton to the individual components at 60°C are presented for a mixture comprised of 32.2% CO2 in CH4 as functions of feed pressure up to 590 psi (absolute). The permeabilities for the individual penetrants in the mixed feed are lower than the respective purecomponent values at the corresponding partial pressures. Furthermore, the permeabilities of both penetrants drop as the feed pressure is increased at constant composition. The dual‐mobility transport model used to analyze the data postulates that the observed pressure and composition dependence of the permeabilities is due to competition between penetrants for a limited microvoid sorption capacity in the glassy polymer. It is demonstrated that in addition to flux depressions due to dual‐mode effects, nonideality of the gas phase must be accounted for to explain the substantial flux depressions observed for the CO2/CH4 mixtured used in this study.
A small‐angle X‐ray scattering study of an aromatic polyimide
Authors: Russell T. P. 
Published: April 2003

AbstractThe morphology of isotropic films of poly[N,N'‐bis(phenoxyphenyl)‐pyromellitimide] was investigated by means of small‐angle x‐ray scattering. A significantly different morphology was found to develop in these films as compared to that in films possessing as in‐plane molecular orientation. It was found that morphology could be described by a damped periodic modulation in the electron density characteristic of a chain segment aggregation. An increase in the initial imidization temperature resulted in an overall increase in the lateral packing of the chains and a reduction in the amplitude of the density modulation. Evidence was also found for segmental aggregation in the precursor polyamic acid which contained up to 35% solvent.
Crystallization kinetics for poly(ethylene terephthalate) oriented by solid‐state coextrusion
Authors: Sun Tong , Pereira Jose , Porter Roger S. 
Published: April 2003

AbstractA series of uniaxially drawn films has been prepared from isotropic, amorphous poly(ethylene terephthalate) (PET) by using solid‐state coextrusion below its Tg. To evaluating the crystallization behavior of the coextrudates, the cold‐crystallization temperature Tc, Crystallization rate constant k, and activation energy Ea have been studied as a function of extrusion draw temperature and draw ratio. The rapid crystallization of highly oriented amorphous PET in extrudates was followed by a temperature‐programmed DSC technique. This avoids the marked precrystallization often found in samples brought to appropriate isothermal temperatures. With increasing extrusion draw ratio, Tc shifts to lower temperatures, the first‐order crystallization rate constant k increases markedly (by two or three orders of magnitude) and Ea decreases markedly. For undrawn samples annealing at temperatures below or near Tg (50–70°C) does not lead to crystallization, but with increased annealing temperature both Tc and Ea decrease. In contrast, for extrusion‐drawn PET, both Tc and Ea increase with draw temperature. All these features are consistent with our previous correlations with molecular draw ratio and birefringence.
Interpretation of the moduli of trifunctional model elastomers of polydimethylsiloxane
Authors: Queslel J. P. , Mark J. E. 
Published: April 2003

AbstractA survey is made of published experimental values of the elongation moduli of trifunctional model networks of polydimethylsiloxane [Si(CH3)2O]. The results thus obtained are interpreted in light of recent molecular theories of rubberlike elasticity, particularly with regard to predicted values of the modulus, the degree of nonaffineness of the deformation, the relevance of the network sol fraction, and the efficiency and possible importance of the trapping of interchain entanglements. The most important result of this study is the conclusion that it is not necessary to invoke equilibrium contributions from interchain entanglements in order to interpret the experimental results presently available in the literature.
Optical studies of pyrolyzed polyacrylonitrile
Authors: Chung T.‐C. , Schlesinger Y. , Etemad S. , Macdiarmid A. G. , Heeger A. J. 
Published: April 2003

AbstractThe results of measurements of the optical absorption spectra, IR absorption spectra, thermogravimetric analysis, and elementary analysis of pyrolyzed polyacrilonitrile are presented as a function of the pyrolysis temperature Tp and of the duration of pyrolysis. At Tp ∼ 200°C, an intermediate phase was discovered, containing conjugated CN sequences and a completely unreacted carbon backbone. The optical absorption data imply that the resulting polymer is a semiconductor with a delocalized π‐electron system and an energy gap Eg ⋍ 2.5 eV. For Tp > 260°C, the weight loss rapidly increases, and the absorption edge gradually broadens and shifts to lower energies. The resulting polymer (after higher‐temperature pyrolysis) contains CN and CCC conjugation sequences, but appears to be a complex structure consisting of a mixture of different chemical species.
Flow‐dependent rejection of polystyrene from microporous membranes
Authors: Long T. D. , Anderson J. L. 
Published: April 2003

AbstractDilute solutions of polystyrene (molecular weight 1 × 105−2 × 107) in a mixed solvent of 90% carbon tetrachloride‐10% methanol were filtered through track‐etched porous mica membranes. The reflection coefficient σ, defined as the fraction of polymer held back by the membrane, was measured as a function of polymer size rs, pore radius ro, and solvent flow rate q through each pore. Polymer size was characterized by the Stokes‐Einstein radius, as determined from diffusion coefficients measured by quasielastic light scattering, and chain relaxation times τ were estimated from measured intrinsic viscosities. In the case of chains whose unperturbed radius was smaller than the pore, σ depended on the ratio rs/ro in the manner predicted by a hard‐sphere theory, as long as \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma \tau < < 1 $\end{document}, where \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} is the mean rate of strain of solvent at the pore entrance. However, when the polymer chains exceeded the pore in size, σ depended on flow rate and decreased from almost unity, at small q, toward zero at high q. The relationship between σ and q was nearly independent of polymer and pore size, consistent with a theory based on scaling concepts of how polymer chains deform at the entrance of a pore, but the reduction in σ as q increased was very gradual and did not exhibit the sharp transition predicted by the theory. We were able to empirically correlate all the data for σ when rs > ro by a single similarity variable \documentclass{article}\pagestyle{empty}\begin{document}$ \theta = {{({{r_s} \mathord{\left/ {\vphantom {{r_s} {r_0}}} \right. \kern-\nulldelimiterspace} {r_0}})} \mathord{\left/ {\vphantom {{({{r_s} \mathord{\left/ {\vphantom {{r_s} {r_0}}} \right. \kern-\nulldelimiterspace} {r_0}})} {(\dot \gamma \tau)^n}}} \right. \kern-\nulldelimiterspace} {(\dot \gamma \tau)^n}} \sim ({{r_s} \mathord{\left/ {\vphantom {{r_s} {r_0}}} \right. \kern-\nulldelimiterspace} {r_0}})^{1 - 3n} q^{- n} $\end{document}; a least‐squares fit gave n = 0.33, showing that σ is insensitive to polymer size for large chains.
Surface orientation of polypropylene. I. Theoretical considerations for the application of internal reflection spectroscopy
Authors: Mirabella Francis M. 
Published: April 2003

AbstractA theoretical consideration of the orientation of the transverse magnetic and the transverse electric polarization vectors of the evanescent field in attenuated total reflection spectroscopy was made. It was demonstrated that near the critical angle of total reflection the transverse magnetic polarization vector approaches a purely normal orientation to the interface between the sample and internal reflection element surfaces, while the transverse electric vector is parallel to this interface. With an appropriate experimental design a set of three orthogonal electric vectors can be obtained for the analysis of surface orientations. A method to obtain high‐quality infrared spectra of polypropylene surfaces at a fraction of a degree above the critical angle was developed using a KRS‐5 internal reflection element specially cut with a 40° entrance aperture. The surface orientation of uniaxially extended polypropylene films was studied by using the proposed experimental design. It was shown that the orientation of the helix axis of the macromolecular chains of polypropylene could be qualitatively detected. A method employing the 841‐ and 809‐cm−1 bands of polypropylene was used to show that the ratio of absorbances of these bands reflects the surface orientation of the uniaxially extended polypropylene films for a set of films extended 0–500%.
Effects of copolymer composition on the formation of ionic species, hydrogen evolution, and free‐radical reaction in γ‐irradiated styrene‐butadiene random and block copolymers
Authors: Basheer Rafil , Dole Malcolm 
Published: April 2003

AbstractBlock and random copolymers of butadiene and styrene as well as polybutadiene and polystyrene homopolymers have been investigated with respect to formation of trapped electrons, contribution of ionic species to crosslinking, and hydrogen gas evolution due to γ radiation. The decay kinetics of the disubstituted benzyl radical has also been studied. The yields of electron trapping G(e−) are measured. The G(e−) increase linearly with increased polystyrene content in block polymers, while in random copolymer a deviation from a linear relation is observed. The contribution of ionic reactions to crosslinking is about 25–35% of the total crosslinking yield. Hydrogen production in block copolymers is approximately a linear function of the weight‐fraction additivity of the yield of hydrogen formation in polystyrene and polybutadiene homopolymers. Energy transfer from butadiene units to styrene units in random copolymers resulted in a deviation from such an additivity relation. The decay of the disubstituted benzyl free radical in block copolymers is a second‐order reaction. In random copolymer, the decay is best interpreted in terms of equation based on a second‐order decay mechanism of a fraction of the free radicals decaying in the presence of other nondecaying free radicals.
Orientation of the boundary faces in nylon‐11 lamellar crystals
Authors: Dosiere M. , Point J. J. 
Published: April 2003

AbstractA semicrystalline polymer having a large repeat unit, as does nylon‐11, is particularly suitable for seeking correlation between the orientation of the lattice and the basal planes of the lamellar crystals. In filter mats of nylon‐11 single crystals, the basal planes of the lamellae are parallel to (00l) crystallographic planes; the chain axis is tilted with respect to the mat plane. By planar extrusion, bulk double oriented specimens with a nearly single texture can be prepared: the basal planes of the lamellae are parallel to (00l) planes and the chain axis is along the extrusion direction. Doubly oriented samples of nylon‐11 having a double texture have been obtained by unidirectional rolling. In these samples, the chain axes are along the rolling direction; the basal planes of the lamellar crystals are not parallel to (00l) planes. It has been proposed that lamellae consist of blocks of six hydrogen‐bonded planes shifted by one monomer unit. The parallelism between basal planes of lamellae and (00l) planes is obtained again in rolled samples annealled in contact with formic acid. Those annealed samples are similar to filter mats with respect to the orientation of the lamellar basal planes but they remain doubly oriented at the level of the unit cell; they have a long spacing larger than filter mats of single crystals.
An ESR study of the decay reaction of isotactic polypropylene peroxy radicals in air
Authors: Hori Yasuro , Shimada Shigetaka , Kashiwabara Hisatsugu 
Published: April 2003

AbstractThe decay of peroxy radicals trapped in irradiated isotactic polypropylene has been studied by ESR in air at various temperatures between 284 and 309 K. All the ESR spectra obtained at the various reaction stages are shown to be composed of two components arising from a mobile fraction and an immobile fraction. Only the mobile peroxy radicals decay; those belonging to the immobile fraction are stable. Various reaction mechanisms are examined in order to explain the experimental results; it is concluded that the decay reaction is controlled by diffusion of peroxy radicals and that the immobile peroxy radicals play no role in the decay reaction. Intermolecular hydrogen abstraction of the peroxy radicals, rather than intramolecular abstraction, is suggested as the rate‐determining reaction.
Molecular mechanics of photopolymerization of 2,5‐distyrylpyrazine in solid state
Authors: Swiatkiewicz Jacek , Prasad Paras N. 
Published: April 2003

AbstractThe molecular mechanics of photopolymerization of 2,5‐distyrylpyrazine in the solid‐state α form is investigated by a combination of Raman and electronic spectroscopic techniques. A stepwise photoreaction is used to stabilize an intermediate oligomer structure. For this purpose, the 476.5‐nm argon‐ion laser line is used to convert the monomer to the oligomer. The Raman phonon spectra reveal a homogeneous mechanism, with considerable lattice rearrangement, for the oligomerization process, which is also found to produce lattice strain. Further photoreaction of the oligomer is carried out with light of wavelength ≤400 nm from a 200‐W mercuryxenon lamp as well as with the 355‐ and 417‐nm laser lines from a Nd: Yag laser system. The Raman phonon spectra, monitored during the conversion of the oligomer to the polymer, show that initially the process starts homogeneously, with considerable lattice rearrangment. Then it turns heterogeneous, with a phase separation accompanied by a gradual ordering of the polymer product lattice. The electronic absorption spectra of the monomer are characterized and related to the stepwise photo process discussed above. Although the emission spectra show a monotonic change during the photoreaction, energy transfer from the oligomer to the monomer and from the polymer to the oligomer obscures and quantitative characterization by emission spectroscopy.
The heat of fusion of copolymers of tetrafluoroethylene and hexafluoropropylene
Authors: Starkweather Howard W. , Zoller Paul , Jones Glover A. 
Published: April 2003

AbstractThe Clapeyron equation has been applied to determine the heat of fusion of two copolymers of tetrafluoroethylene and hexafluoropropylene. The values obtained are within experimental uncertainty of that for the homopolymer, PTFE. This study supports the conclusion of earlier workers that the CF3 side groups are included in the crystal lattice. In both PTFE and polyethylene, C1 side groups are included in the crystal but have little if any effect on its density. We believe that the same relationship between the crystallinity and the heat of fusion can be used for both PTFE and copolymers of TFE and HFP.
Giant polarization in stable polymeric dielectrics
Authors: Vijayakumar Pormamilla S. , Pohl Herbert A. 
Published: April 2003

AbstractMacromolecular solids of highly aromatized structure can exhibit giant polarization. Their structures must be capable of providing long‐range electron orbital delocalization. Intermolecular conformality of the orbitals of these huge molecules can result in ready carrier formation. The ready transport of these electronic holes and electronic carriers as polarons able to range over long molecular domains will then produce the giant polarization characteristic of the nomadic polarization mechanism. Recent developments in the synthesis and characterization of such materials new make available a new and interesting class of very stable organic dielectrics having dielectric constants in excess of 100. A number of such materials are described.
Mechanical relaxation mechanism of epoxide resins cured with acid anhydrides
Authors: Shimbo Masaki , Ochi Mitsukazu , Iesako Hiroshi 
Published: April 2003

AbstractThe mechanism of low‐temperature mechanical relaxation in epoxide resins cured with various acid anhydrides has been investigated by comparing dynamic mechanical properties and chemical structures of these networks. One mechanical relaxation, denoted as the β relaxation, is observed at about −70°C. The β relaxation is affected by the chemical structure of the curing agents, but not by that of the epoxide resins. In addition, the strength of the relaxation increases linearly with increasing concentration of diester linkage introduced in the network by reaction with acid anhydrides. From these results, it is concluded that the β relaxation mechanism of the anhydride‐cured systems involves the motion of diester segments included in the network structures.
Photon correlation spectra of poly(phenylmethyl siloxane) under high pressures
Authors: Fytas George , Dorfmuller Thomas , Chu Benjamin 
Published: April 2003

AbstractWe report polarized photon correlation measurements of bulk poly(phenylmethyl siloxane) (PPMS) with a molecular weight of about 2500 at different pressures from 1 to 1750 bar and temperatures between 269 and 308 K. In the Frequency range accessible by present‐day digital correlators, the time correlation function can measure a slow q2‐dependent diffusional process due to local concentration fluctuations and a fast α‐mode structural relaxation process. The mean characteristic times of the two processes approach each other at low temperatures and high pressures, in contrast to the behavior of the α‐ and β‐relaxation times. We should be cautious in relating the mean characteristic times for the structural relaxation process obtained by different experimental techniques, such as Brillouin scattering and photon correlation spectroscopy, because of the differnces in the probe emphasis and the extremely broad frequency distribution of such relaxation processes.