bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of interfacial bonding on the strength of adhesion of elastomers. III. Interlinking by molecular entanglements
Authors: Gent A. N. , Tobias R. H. 
Published: April 2003

AbstractPolydimethylsiloxane (PDMS) networks have been formed by end‐linking linear PDMS molecules. When a second layer is cast on top of a fully gelled lower layer, the new molecules diffuse into the surface of the lower layer and form molecular loops (“entanglements”) in the course of end‐linking with themselves. The two layers are then joined only by these macromolecular loops. Measurements have been made of the work required to separate such layers under threshold conditions, i.e., at low rates, high temperatures, and, in some cases, in the swollen state. Values of the work of detachement have been found to be 15–25 J/m2, generally about one‐half of the work of fracture of the layers themselves, and consistent with the inferred density of interlinking molecular loops at the interface. Values of the work of detachment were higher for higher densities of interlinking, roughly in proportion, and for interlinking molecular strands of higher molecular weight, in accordance with the theory of Lake and Thomas. In the absence of interlinking the work of detachment was extremely small, only about 70 mJ/m2.
Determination of elastic shear constants of polyethylene at room temperature by inelastic neutron scattering
Authors: Heyer D. , Buchenau U. , Stamm M. 
Published: April 2003

AbstractThe elastic shear constants of both the amorphous and crystalline regions of polyethylene have been measured at room temperature. A newly developed method is used which allows the determination of elastic constants from the coherent inelastic neutron scattering of polycrystals. A deuterated and partially oriented sample is investigated on a triple‐axis spectrometer and a time‐of‐flight instrument. The elastic constants of the crystalline regions of polyethylene are c44 = 2.1 ± 0.3, c55 = 2.2 ± 0.3, c66 = 1.8 ± 0.2, and c′ = 1/4(c11 + c22 − 2c12) = 0.92 GPa. The shear modulus of the amorphous regions is obtained as G = 0.55 ± 0.03 GPa. In connection with other experimental results the elastic constant matrix is given and compared with theoretical estimates. With simple models, macroscopic moduli are calculated which are in good agreement with published experimental data.
Transient and steady rheology of polydisperse entangled melts. Predictions of a kinetic network model and data comparisons
Authors: Liu Tony Y. , Soong David S. , Williams Michael C. 
Published: April 2003

AbstractThe kinetic network (KN) model discussed previously in the context of monodisperse and bimodal polymer systems is extended to polymers of arbitrary molecular weight distribution. A generalization is proposed for the flow‐dependent entanglement loss term in the structure equation, replacing the shear rate \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} by a new variable \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \Gamma $\end{document} which reduces to\documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} for simple shear and is more appropriate for elongational and other flows. New data are obtained on shear stress transients of many kinds, using a recently developed parallel‐plate rheometer. These data and others on steady and transient flows of well‐characterized polydisperse polymers in shear and in elongation are used to demonstrate that the KN model predictions are valid. Comparisons with predictions for monodisperse polymers having the same as Mw polydisperse systems show that transient behavior—especially stress overshoot—is particularly sensitive to details of the molecular weight distribution. Further possible improvements in the theory are suggested, and the relationship of the KN model to other recent network models is discussed. The KN model has greater data fitting capabilities, with fewer parameters, than any other model available at present.
High‐modulus polydiacetylene single‐crystal fibers
Authors: Galiotis C. , Read R. T. , Yeung P. H. J. , Young R. J. , Chalmers I. F. , Bloor D. 
Published: April 2003

AbstractThe relationship between structure and mechanical properties of polydiactetylene single‐crystal fibers has been studied in detail. It is shown by transmission electron microscopy that the fibers have a high degree of internal perfection, with the polymer molecules aligned parallel to the fiber axes. The fibers of the dicarbazolyl derivative investigated were found to have a Young's modulus of 45 GPa and fracture strengths of up to 1.5 GPa, the strengths being controlled by defects such as surface steps. It is shown that the stiffness of the polymer backbone is similar to that of polyethylene, and the theoretical strength of the polydiacetylene singlecrystal fibers is determined to be about 3 GPa, corresponding to a fracture strain of between 6 and 8% and a force required to break molecules to the order of 3 nN. The derivative studied is also found to have good thermal stability, not degrading below 300°C, and excellent creep resistance up to at least 100°C. The possibility of using the single‐crystal fibers in composites is also discussed.
The heat of fusion of 66 nylon
Authors: Starkweather Howard W. , Zoller Paul , Jones Glover A. 
Published: April 2003

AbstractThe heat of fusion ΔHf of 66 nylon has been determined by use of the Clapeyron equation. Measurements of ΔHf and the unit‐cell parameters on molding pellets show that this material contains the α2 crystal phase, which is less dense than the α1 phase obtained by crystallization from solution. The value of ΔHf‐45–46 cal/g, is in good agreement with earlier reports.
Tearing of unvulcanized natural rubber
Authors: Kadir A. , Thomas A. G. 
Published: April 2003

AbstractThe tear behavior of unvulcanized natural rubber has been studied by using established techniques normally adopted for the study of vulcanized rubbers. Unvulcanized rubber has been found to tear in a relatively steady manner, in contrast to the stick‐slip tear behavior of the vulcanized rubber, the tearing energy being dependent on the rate of tearing. Crystallization seems to be an important factor in determining the tear behavior since it has not been found possible to tear unvulcanized SBR under the same conditions. The effect of the pronounced imperfect elastic nature of the material was studied under conditions where the driving force for tearing was solely governed by the rate of release of elastic energy. Under such conditions, it has been found that the tearing energy is determined not by the strain energy required to stretch the material but by the energy which can be recovered on retraction. The set developed in the test piece, due to imperfect elasticity, has also to be taken into account.
On the CO2 permeation of uniaxially drawn polymers
Authors: Wang L. H. , Proter Roger S. 
Published: April 2003

AbstractThe CO2 permeation coefficient and the difficient were measured using the permeation time‐lag method for films of atactic polystyrene and high‐density polyethylene, each as a function of uniaxial draw ratio. The reduction of permeability with draw ratio is observed for polystyrene and for polyethylene. In the latter it is associated with an increase in crystallinity. In both cases the premeability decreases and the solubility constant remains unchanged. The reduction of permeability is thus caused only by the reduction in diffusion of CO2 in the drawn polymers. The mechainism is different for the two polymers, as is confirmed by measurements of birefringence, glass transition temperature, and crystallinity.
Hydrodynamic radius and intrinsic viscosity of polystyrene in the crossover region from θ to good‐solvent conditions
Authors: Varma Brijesh Kumar , Fujita Yutaka , Takahashi Masato , Nose Takuhei 
Published: April 2003

AbstractThe radius of gyration Rg, the hydrodynamic radius Rh, and the intrinsic viscosity [η] have been measured for polystyrene in cyclohexane (at the θ temperature and 44.5°C), 2‐butanone (at 30°C), and toluene (at 30°C) in the molecular weight range 2.38 × 105 ≤ Mw ≤ 5.47 × 106 to elucidate the excluded‐volume effects on dynamical behavior in dilute polymer solutions in the crossover region from θ to good‐solvent conditions. The results are compared with theoretical predictions of current thermal blob theories and the Pade approximant theory of Tanaka. It is found that the ratio of Rh/Rg decreases with an increase in the excluded‐volume effect, following the prediction of the simple blob theory, but that its magnitude is about 15% higher than the theoretical value. Experimental variation of [η] with Rh and/or Rg lies in between predictions of the scaling law and the Pade approximant. The concentration dependence of the diffusion coefficient is also compared with predictions of current theories.
Miscible blends of amorphous polymers
Authors: Tremblay C. , Prud'homme R. E. 
Published: April 2003

AbstractThirty‐five polymethacrylate/chlorinated polymer blends were investigated by differential scanning calorimetry. Poly(ethyl), poly(n‐propyl), poly(n‐butyl), and poly(n‐amyl methacrylate)s were found to be miscible with poly(vinyl chloride) (PVC), chlorinated PVC, and Saran, but immiscible with a chlorinated polyethylene containing 48% chlorine. Poly(methyl) (PMMA), poly(n‐hexyl) (PHMA), and poly(n‐lauryl methacrylate)s were found to be immiscible with the same chlorinated polymers, except the PMMA/PVC, PMMA/Saran, and PHMA/Saran blends, which were miscible. A high chlorine content of the chlorinated polymer and an optimum CH2/COO ratio of the polymethacrylate are required to obtain miscibility. However, poly(methyl), poly(ethyl), poly(n‐butyl), and poly(n‐octadecyl acrylate)s were found to be immiscible with the same chlorinated polymers, except with Saran, indicating a much greater miscibility of the polymethacrylates with the chlorinated polymers as compared with the polyacrylates.
The role of molecular diffusion in the adhesion of elastomers
Authors: Ellul M. D. , Gent A. N. 
Published: April 2003

AbstractButyl rubber (polyisobutylene‐co‐isoprene) mixed with polyisobutylene was crosslinked to yield elastomeric macromolecular networks containing dissolved linear macromolecules. Adhesion of these materials to themselves (self‐adhesion) and to an inert substrate was investigated over a wide range of peel rates and test temperatures. Greatly enhanced self‐adhesion was found when linear polyisobutylene molecules of high molecular weight were present, but the strength of adhesion to a rigid inert substrate was hardly affected. The enhancement of self‐adhesion is attributed to interdiffusion of polyisobutylene molecules. It was greatest at intermediate peel rates and temperatures, becoming insignificant at extremely low rates, probably because the diffusing species can then migrate readily, and at high effective rates of peel when the polymer approaches the glassy state and the strength of adhesion is high in all cases. A transition to somewhat lower levels of adhesion at relatively high rates of peel is tentatively ascribed to the onset of molecular fracture in place of pullout. The presence of large amounts of low‐molecular‐weight polyisobutylene (M̄v = 50,000 g/mol) increased the level of self‐adhesion and of adhesion to an inert substrate to a similar degree, over a broad range of peel rates. This effect is attributed primarily to enhanced viscous losses in the elastomeric layer during separation. Application of these results to crack and weld‐line healing in glassy plastics is discussed.
Magic‐angle 13C‐NMR of cured and degraded acrylic copolymer/melamine formaldehyde coatings
Authors: Bauer D. R. , Dickie R. A. , Koenig J. L. 
Published: April 2003

AbstractMagic‐angle carbon‐13 NMR has been used to probe the structure and dynamics of acrylic copolymer melamine formaldehyde crosslinked coatings. Changes in chemical composition that occur in conventional accelerated weathering tests were found to be dominated by hydrolysis of acrylic‐melamine crosslinks and subsequent formation of melamine‐melamine crosslinks. Evidence for photo‐oxidation was also observed. These results are in substantial agreement with infrared measurements also made on these coatings. Gated high‐power decoupling experiments were used to determine relative mobilities of the different carbon resonances as a function of acrylic copolymer composition and as a function of degradation. It was found that motion of the side‐chain carbons on the acrylic copolymer was not sensitive either to the chemical composition of the coating or to the extent of degradation. Mobilities of the main‐chain carbons of the acrylic copolymer decrease with increasing glass transition temperature of the acrylic copolymer. For acrylic copolymers containing styrene, it is found that the main‐chain carbon mobilities decrease with weathering. The melamine triazine ring becomes very rigid after degradation consistent with the formation of short melamine‐melamine crosslinks.
Regularities in x‐ray scattering patterns from amorphous polymers
Authors: Miller Robert L. , Boyer Raymond F. 
Published: April 2003

AbstractIt is known that the x‐ray scattering pattern of an amorphous polymer may contain one or more halos corresponding to van der Waals spacings or larger. In this work we found that (i) the halo positions (and equivalent Bragg spacings) vary systematically with the cross‐sectional area of the polymer chain in the crystal and (ii) there are at least two families of such halos with spacings larger than that of van der Waals contacts of atoms. From the wide variety and nature of the polymers considered, this behavior is believed to be representative of the amorphous state existing in polymeric samples. This work indicates that there is a degree of packing regularity in amorphous polymers that should be considered in models and theories of polymer chain conformation.
Chemical healing in poly(ethylene terephthalate)
Authors: Fakirov S. 
Published: April 2003

AbstractA new molecular mechanism for the healing phenomenon in semicrystalline linear polycondensates (healing resulting from chemical reactions between macromolecules located in the interfacial surface) is demonstrated. Strips of commercial poly(ethylene terephthalate) are annealed at 258°C in order to avoid melt sticking. Two such strips are partially overlapped, pressed, and heated in a vacuum at 240°C for 10, 20, 30, and 100 h. By measuring the stress at break outside the contact area and the debonding shear stress the critical overlapping length is computed. It is concluded that transreaction contributes more than solid‐state post‐condensation to chemical healing.
Preparation, fractionation, and dilute‐solution behavior of ABA poly(methyl methacrylate‐b‐α‐methylstyrene) block copolymers
Authors: Corner T. , Peaker F. W. 
Published: April 2003

AbstractThe preparation by anionic polymerization of six ABA poly(methyl methacrylate‐b‐α‐methylstyrene) block copolymers and of sixteen poly(α‐methylstyrene)s is described. The block copolymers, of similar molecular weight but with different chemical compositions, were fractionated by preparative gel permeation chromatography and their behavior in dilute solution was investigated using viscometry. The results obtained indicate that the intramolecular phase separation does not occur under the conditions utilized, the block copolymers assuming randomcoil configurations in all of the copolymer/solvent systems studied. Consequently the block copolymer molecules are more expanded than homopolymers of the same molecular weight. The series of poly(α‐methylstyrene)s covered the molecular weight range 2.7 × 103–1.3 × 106and enabled the determination of Mark–Houwink–Sakurada constants for poly(α‐methylstyrene) in the solvents chosen for the block copolymer studies.
Small‐angle polarized light scattering from amorphous spheres
Authors: Meeten G. H. , Navard P. 
Published: April 2003

AbstractSmall‐angle light‐scattering measurements have been made using micron‐diameter isotropic and spherical polymer latex particles placed between crossed polarizers. Four‐leaf clover patterns are obtained, reminiscent of those commonly found for spherical birefringent scatterers. The experimental results compare closely with predictions of Mie scattering theory for isotropic spheres.
The effect of crosslinking on the reactivity of free radicals in isotactic polypropylene
Authors: Klimová Marta , Tin̆O Jozef , Borsig Eberhard , Ambrovic̆ Peter 
Published: April 2003

AbstractIsotactic polypropylene of various degrees of crosslinking was prepared using high concentrations of dicumyl peroxide as the crosslinking agent. Free radicals were generated by γ irradiation of crosslinked iso‐PP with a dose of 2.4 Mrad. It was found that the rate constant for free‐radical decay is independent of crosslinking in the temperature range from −30 to +20°C. The result is discussed from the point of view of degradation processes taking place simultaneously with crosslinking of iso‐PP.
Relaxation by reptation and tube enlargement: A model for polydisperse polymers
Authors: Marrucci G. 
Published: April 2003

AbstractModern theories of the dynamics of concentrated polymeric liquids have not yet accounted for the effects of polydispersity to a sufficient extent. In order to approach quantitatively the problem of polydispersity, a model is proposed here which is based on the concept that the “tube” of constraints around a chain enlarges as the relaxation proceeds. Predictions of linear viscoelasticity obtained with this model compare favorably with experimental results on homopolymeric blends reported in the literature. However, the theory is limited to the case of very long chains and it embodies an arbitrary, if plausible, closure assumption in the self‐consistency scheme. Thus, quantitative agreement remains incomplete.
Effects of solvent goodness and polymer concentration on rejection of polystyrene from small pores
Authors: Long T. D. , Anderson J. L. 
Published: April 2003

AbstractSolutions of polystyrene of molecular weight 4.5 × 106 and 8.4 × 106 in mixed solvents of carbon tetrachloride/methanol were filtered through track‐etched mica membranes at low membrane velocities. The unperturbed hydrodynamic radius of the polymer was always larger than the pore radius. The reflection coefficient σ, defined as the fraction of polymer held back by the membrane, was determined from material balances as a function of solvent flow rate per pore q, volume percent CCl4 of the solvent, and polymer concentration C0. In the dilute region (C0 < C*) σ was found to depend primarily on q and was essentially independent of chain size (or solvent goodness), molecular weight, and pore radius. In the semidilute region (C0 > C*) σ decreased significantly as C0 was increased.
Crystal structure of poly(γ‐methyl L‐glutamate) at high temperature
Authors: Sasaki Shintaro , Uematsu Ichitaro 
Published: April 2003

AbstractA unique crystal structure of the α‐helical form of poly(γ‐methyl L‐glutamate) appearing above 170°C is analyzed by x‐ray diffraction. At room temperature, the unit cell determined for a well‐annealed sample is hexagonal with dimension a = 11.82 Å and contains one α helix with pitch P = 5.42 Å and unit height p = 1.493 Å. This form is reversibly transformed at about 170°C into a modification characterized by a trigonal unit cell (a = 21.27 Å) having three times the cross section of the single‐chain unit cell and containing three α helices with P = 5.50 Å and p = 1.504 Å. In a narrow temperature range below 180°C, the three chains in the unit cell are not interrelated by any crystallographic symmetry element, but they are reasonably associated with one another so that all helices are surrounded by other helices in the same way. The axial and azimuthal displacements between neighboring chains are expressed by z = ±4p/3 and ϕ = ± (2π/3)(4p/P − 1), respectively. The segmental main‐chain motion, which becomes appreciable above 150°C, is composed of screw‐type motion along the α‐helix fold and correlated P/3 axial jumps. The relation between the structural transition and thermal motion is discussed.
Free‐volume theories for self‐diffusion in polymer–solvent systems. I. Conceptual differences in theories
Authors: Vrentas J. S. , Duda J. L. , Ling H.‐C. 
Published: April 2003

AbstractConceptual differences in two recent versions of the free‐volume theory for self‐diffusion in polymer–solvent systems are identified and discussed. The validity of the assumptions of these two theories is tested using experimental data. In all cases, the experimental evidence favors the Vrentas–Duda free‐volume theory over the Paul version of this theory.
Free‐volume theories for self‐diffusion in polymer–solvent systems. II. Predictive capabilities
Authors: Vrentas J. S. , Duda J. L. , Ling H.‐C. , Hou A.‐C. 
Published: April 2003

AbstractThe predictive and correlative capabilities of two recent versions of the free‐volume theory for self‐diffusion in polymer–solvent systems are examined by comparisons with experimental data. Neither the Vrentas–Duda free‐volume theory nor the Paul version generally provides satisfactory predictions for the temperature and concentration variations of solvent self‐diffusion coefficients. However, the Vrentas–Duda theory does provide good correlations of solvent self‐diffusion data, and, furthermore, this theory can provide good predictions if a small amount of solvent self‐diffusion data is used to help estimate the parameters of the theory. New diffusivity and equilibrium data were collected for the toluene‐PVAc system to provide a broader database for evaluation of the self‐diffusion theories.
Morphology of poly(phthalocyaninatogermoxane) crystals
Authors: Zhou X. , Marks T. J. , Carr S. H. 
Published: April 2003

AbstractThe morphology of fully and partially polymerized poly(phthalocyaninatogermoxane), [Ge(Pc)O]n, crystals was studied by both scanning and transmission electron microscopy. It was found that the morphological units are lath‐like crystals which aggregate into particles. Generally speaking, the thickness, width, and length of the laths are in the range of 1000–2000 Å, 2000–10,000 Å, and 1–5 μm, respectively. Each lath may possess a mosaic substructure. Selected‐area electron diffraction patterns indicate that the rigid, extended [Ge(Pc)O]n chains are parallel to the large surface of the lath, and in most crystals the chains lie parallel to the lengthwise direction of the lath. However, in several cases, the chain orientation is at an angle of about 60° with respect to the long edge of the lath. The electron diffraction results are in accord with a tetragonal crystal structure (P4/m).
Crystallization of bisphenol‐A polycarbonate induced by organic salts: Chemical modification of the polymer. I. Model reactions
Authors: Bailly Ch. , Daumerie M. , Legras R. , Mercier J. P. 
Published: April 2003

AbstractThe chemical modifications induced in diphenyl carbonate (DPC) by sodium arylcarboxylates between 200 and 250°C were studied to model the behavior of bisphenol‐A polycarbonate – salt systems. Reaction between the salt and DPC produces sodium phenoxide, the phenyl arylcarboxylate corresponding to the salt, and carbon dioxide. The two latter compounds probably result from the decarboxylation of an unstable intermediate compound, viz., a mixed carboxylic carbonic anhydride. CO2 and sodium phenoxide act as catalysts transforming DPC into phenyl salicylate via the formation of a small amount of sodium salicylate. Electrophilic acylation of sodium phenoxide by DPC is another possible but minor source of phenyl salicylate. Above 250°C, phenyl salicylate becomes unstable and pyrolyzes into o‐phenoxybenzoic acid, which is immedicately esterified in the presence of DPC into phenyl o‐phenoxybenzoate. In DPC + sodium o‐chlorobenzoate systems, reaction between phenyl o‐chlorobenzoate and sodium phenoxide is another source of phenyl o‐phenoxybenzoate.