bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship between the cohesive strength and the tack of elastomers
Authors: Hamed G. R. , Shieh C.‐H.
Published: April 2003

The autohesion (tack) and cohesion of a random styrene‐butadiene elastomer have been examined as a function of test temperature and speed using a T‐peel geometry. Both properties have been reduced to a single master curve by horizontally shifting the data with the same set of shift factors. The cohesive strength increases with increasing reduced test rate RaT and appears to approach a plateau at the highest rates. Tack also increases with RaT but decreases abruptly at a critical rate and peeling the occurs in a stick‐slip fashion. Tack again increases at sufficiently high test rates. In the range of rates where tack is maximized, its value is essentially the same as its cohesive strength. Above or below this range, tack is substantially less than the elastomer's cohesive strength. Mechanisms are proposed to explain why relative tack (i.e., tack divided by cohesive strength) is not a simple measure of the extent of completion of a tack bond and may indeed be equal to one in spite of incomplete tack bond formation.
Diffusion of iodine into PE and PET
Authors: Pender L. F. , Mak H.‐B. , Wintle H. J.
Published: April 2003

We show that the Rutherford backscattering technique is very useful for studying the kinetics of diffusion processes in polymers. We have measured the diffusion of iodine into low‐density polyethylene (LDPE) by optical absorbance and by Rutherford backscattering. The optical results appear to be normal, and show no electric field dependence, but the backscattering results reveal that this is misleading, because there is fast diffusion in the bulk accompanied by large surface concentrations of iodine. We have studied PET by the backscattering technique, and in this case the behavior corresponds to a weakly concentration‐dependent diffusion coefficient with no observable surface effects. Neither set of results gives support to the domain theories of low‐frequency electrical oscillation in these materials.
The relationship between the physical structure and the microscopic deformation and failure processes of poly(p‐phenylene terephthalamide) fibers
Authors: Morgan Roger J. , Pruneda Cesar O. , Steele Wayne J.
Published: April 2003

Relations between the physical structure and the deformation and failure processes of poly(p‐phenylene terephthalamide) (PPTA) fibers are reported. The effects of the physical processes involved in fiber fabrication, including the crystallization of PPTA‐H2SO4 dopes under stress, are considered in relation to their effect on the structure of the fiber. The deformation and failure processes together with the structure of the fiber are discussed in the light of fracture‐topography studies of fiber‐epoxy composite strands, single filaments, HCl‐etched and unetched yarns, and transmission optical microscopy studies of stressed and unstressed yarns. In view of these observations, the physical structure of PPTA fibers is discussed in terms of pleated H‐bond sheets, the macromolecular chain‐end concentration and distribution, and the presence of impurities. The structural parameters that affect the failure processes of these fibers and how such parameters can be modified by service environment conditions are also addressed.
Characterization of lattice disorder in the low‐temperature phase of irradiated PTFE by vibrational spectroscopy
Authors: Rabolt John F.
Published: April 2003

In an effort to elucidate the structure of the low‐temperature phase of PTFE, a spectroscopic study of the effect of progressive irradiation on the low‐frequency infrared spectrum was undertaken. Previous infrared and Raman measurements indicated that irradiation of PTFE decreases its crystalline content, resulting in a lowering of the 19°C phase transition temperature due to the introduction of disorder and defect structures into the lattice by chain scission. Far‐infrared studies of virgin and irradiated PTFE at liquid nitrogen temperatures show that the medium to strong bands at 45, 54, 58, and 71 cm−1, attributed to lattice vibrations, decrease in intensity as the crystalline content decreases. These findings support the assignment of these bands to intermolecular vibrations of the crystalline lattice and are an indication of the presence of more than one molecule in the crystallographic unit cell.
Morphology of poly(p‐phenylene terephthalamide) fibers
Authors: Panar M. , Avakian P. , Blume R. C. , Gardner K. H. , Gierke T. D. , Yang H. H.
Published: April 2003

Fibers of poly(p‐phenylene terephthalamide) (PPTA) have a fibrillar morphology, the individual fibrils having a high proportion of extended chains passing through periodic defect layers. A pleat structure is superimposed. The fibers are fully crystalline (within the limits of determination) with a small fraction of randomly oriented crystalline material. The major distinction between PPTA and conventional fibers lies in the high level of extended chains passing through the defect layers of the former structure. These extended chains result in crystallographic register being maintained between adjacent ordered zones. Quantitatively, a measure of this order is obtained from a comparison of the correlation length, obtained from meridional x‐ray peak widths, and the defect spacing. In conventional fibers the defect spacing, i.e., long period, is longer than the correlation length (i.e., crystal size). In PPTA, the analog of the long period, the defect spacing (about 35 nm) is smaller than the correlation length, which is over 80 nm.
Thermotropic homopolyesters. IV. Study of fiber formation
Authors: Acierno D. , La Mantia F. P. , Polizzotti G. , Ciferri A. , Krigbaum W. R. , Kotek R.
Published: April 2003

We report a melt spinning and viscosity study of two semiflexible homopolyesters containing both rigid and flexible segments in the repeating unit. Single filaments of the polyesters formed from 4,4′‐diacetoxybiphenyl and azelaic acid (PB7) and sebacic acid (PB8), and from 4′‐hydroxyphenyl‐4‐hydroxycinnamate and azelaic acid (C7), were spun at temperatures between 205 and 255°C. The temperature dependence of the Newtonian melt viscosity of PB7 and C7 was investigated, and a range of molecular weights was studied for PB7. The spinning parameters, fiber characteristics, and viscosity‐temperature behavior are related to the type of mesophase formed. The mechanical properties of fibers spun from both the nematic and smectic phases of these semiflexible chain polymers were poor. Increasing the polymer molecular weight or extrusion rate only afforded a modest improvement in fiber properties. Most polymers could not be spun at temperatures corresponding to the existence of the single mesophase. Hence the low viscosity typical of the nematic mesophase is not necessarily an advantage in fiber formation from the melt. It appears from these results that this type of polyester does not possess adequate chain extension to develop ultrahigh‐modulus properties. The director, which describes the local orientation of molecules within the mesophase, may undergo more frequent variations than is the case for rigid chain polyesters. Mechanisms relevant to flexible polymers may contribute to the development of orientation for this class of nematogenic melts.
The effect of restricted rotational diffusion on the dynamics of rodlike molecules in concentrated solution
Authors: Williams Graham
Published: April 2003

The model of Doi for the rotational dynamics of rodlike molecules in solution is compared with that of Warchol and Vaughan and Wang and Pecora. The similarities in the results for dielectric relaxation are discussed and the application of the models to the behaviour of isotropic solutions, anisotropic (lyotropic‐nematic) solutions, and mixtures of small molecules with polymers is briefly discussed.
The effect of polymer side chains on vapor sorption in polyacrylate and polymethacrylate solutions
Authors: Saeki S. , Holste J. C. , Bonner D. C.
Published: April 2003

The segment fraction Ψ1 activity coefficients, a1/Ψ1, of solvents have been determined by the piezoelectric sorption method for 0.1 ≤ Ψ1 ≤ 0.5 in binary solutions of chlorinated methanes [carbon tetrachloride (CCl4), chloroform (CHCl3), and dichloromethane (CH2Cl2)] with aromatic hydrocarbons (benzene and toluene) in poly(methyl methacrylate), poly(methyl acrylate), poly(ethyl methacrylate), and poly(n‐butyl acrylate) at 23.5°C. The present results for toluene in PMMA agree with previously published values obtained by gas‐liquid chromatography. For CCl4 and the aromatic hydrocarbons, the polymer–solvent interaction parameter χ is positive and constant, while for the polar solvents (CHCl3 and CH2Cl2), χ is negative and increases with increasing Ψ1. The effect of the polymer side chains on vapor sorption in nonpolar and polar solvent systems is discussed in terms of the χ parameter.
Orientation of isotactic polypropylene in crystalline and amorphous phases: IR methods
Authors: Kissin Y. V.
Published: April 2003

Two methods based on polarization infrared spectroscopy are described for measurement of polypropylene orientation. One method, for the crystalline phase, allows precise measurement of polypropylene orientation in three orthogonal directions. It is based on the polarization characteristics of two crystallinity bands, those at 841 and 809 cm−1, which exhibit opposite polarization characteristics. The other method, for the amorphous phase, provides a semiquantitative estimate and is suitable for uniaxially oriented films. It is based on polarization characteristics of the 1155‐cm−1 band. Both methods are applied to the measurement of orientation in polypropylene films uniaxially stretched at various speeds and temperatures.
Small‐angle x‐ray scattering measurements of diffuse phase‐boundary thicknesses in segmented polyurethane elastomers
Authors: Koberstein Jeffrey T. , Stein Richard S.
Published: April 2003

The diffuse phase boundary thicknesses of two segmented polyurethane block copolymers have been determined by small‐angle x‐ray scattering analyses of deviations from Porod's law. A self‐consistency test of these measurements is performed by comparing results of experiments carried out under a variety of collimation geometries, and using a number of different analytical procedures. Self‐consistent values of the diffuse phase boundary thickness are obtained for both of the materials examined. The discrepancies between these values and previous literature values are discussed and possible origins of these differences are presented.
Pulsed NMR observation of ultradrawn polypropylene
Authors: Ito M. , Serizawa H. , Tanaka K. , Leung W. P. , Choy C. L.
Published: April 2003

Proton spin‐spin relaxation times have been measured as a function of temperature for ultradrawn polypropylene with draw ratios λ up to 24. The three relaxation times T2a (the longest), T2i (intermediate), and T2c (the shortest), observed for all the samples, have been ascribed to the relaxations of the amorphous, constrained amorphous, and crystalline components, respectively. T2i and T2a, which reflect the changes in structure and mobility in the noncrystalline regions, decrease with increasing λ; T2i becomes saturated at λ > 9, whereas T2a shows a substantial decrease up to λ = 24. The continued decrease in T2a indicates that the constraint on the amorphous segments keeps increasing up to the highest λ. The associated mass fractions Fa, Fi, and Fc also change with λ. At λ < 9, the increasc in Fi with increasing λ is accompanied by a decrease in Fa, with Fc remaining unchanged. At higher λ, however, Fa is almost constant, and stepwise rises in Fc at about λ = 12 and 24 are accompanied by corresponding drops in Fi. It seems that, in this high draw ratio range, some of the taut molecules are fully extended and are in sufficiently good lateral register to transform into crystalline bridges. This conjecture is supported by the similarity in the λ dependence of Fc and the mass‐fraction crystallinity obtained from the heat of fusion.
Comment on the concentration and temperature dependences of the translational diffusion coefficient of flexible‐coil macromolecules in solution: Poly(2‐vinyl pyridine) in tetrahydrofuran
Authors: Van Den Berg J. W. A. , Jamieson A. M.
Published: April 2003

Concentration‐dependence coefficients kDϕ of the mutual diffusion coefficient of poly(2‐vinyl pyridine) in tetrahydrofuran in the temperature range 15–55°C are compared with predictions of recent theories of macromolecular diffusion. In this temperature range a change in local conformation of the polymer occurs. The accompanying changes in chain solvation and coil hydrodynamics result in a sizable decrease in the hydrodynamic interaction parameter X defined by Akcasu. The formulation of theory appropriate for comparison with frame‐indifferent experimental diffusivities is discussed. We find that current theories predict qualitatively, but not quantitatively, the change in temperature dependence of kDϕ that occurs at the conformational transition. The discrepancies closely parallel those reported in recent independent comparisons of theoretical kDϕ versus experimental data for flexible‐coil molecules. No theory can adequately predict kDϕ over a wide range of X.
Strength of interaction and penetration of infrared radiation for polymer films in internal reflection spectroscopy
Authors: Mirabella Francis M.
Published: April 2003

The strength of interaction of infrared radiation with relatively thick polymer films was systematically studied by internal reflection spectroscopy. The so‐called “effective thickness” was used as a measure of the strength of interaction. The experimental and theoretical effective thickness were compared, and it was found that the theoretically predicted effective thickness can be approached or achieved experimentally with film samples routinely encountered in the polymer industry when moderate attention is paid to surface preparation and clamping force on the film in the internal reflection sample holder. A series of curves for various types of film samples was developed and found to be useful in predicting the spectral contrast for KRS‐5 and germanium internal reflection elements. These data were collected for infrared radiation polarized parallel and perpendicular to the plane of incidence and for unpolarized radiation. The data for unpolarized radiation was checked for the effects of the inherent polarization of the Fourier‐Transform Infrared Spectrometer used by comparing it to the arithmetic mean of the data for parallel and perpendicular polarization. These data were in good agreement and this indicated that the inherent polarization of the instrument introduced only minor errors into the data collected with unpolarized radiation. The concept of the “sampling depth” was established where the sampling depth ds was defined as the depth normal to the surface sampled. The sampling depth was found to be on the order of three times the value of dp, the depth of penetration, at 45° for polypropylene and polystyrene on KRS‐5 and was observed to exceed the effective thickness values at the same conditions. The value of the electric field amplitude was observed to fall to 5% of its value at the surface at ds.
Birefringence and glass transition temperatures of poly(diethylene glycol terephthalate) networks
Authors: Riande E. , Guzmán J. , Llorente M. A.
Published: April 2003

A series of polymer networks were prepared by trifunctionally endlinking poly(diethylene glycol terephthalate). The elastomeric properties of these materials were studied at constant temperature using experiments that involve both the elastic force and birefringence. Whereas the stress‐strain isotherms show an anomalous increase in the modulus at very high elongation ratios, a downturn appears in the birefringence‐strain isotherms at the same extensibilities. These results suggest that the upturn that appears in the force should be attributed to maximum chain extensibility rather than to strain‐induced crystallization. A variety of additional thermoelastic experiments were carried out on these networks, to elucidate the dependence of the glass transition temperature on strain. It was found that for the elongation ratios at which the networks exhibit Gaussian behavior, the free‐volume effects on the glass transition temperature Tg (decreasing Tg with increasing free volume) offset the conformational effects (increasing Tg with decreasing entropy). However, the contrary occurs in the region where the stress increases anomalously with increasing strain.
A theoretical investigation of chain packing and electronic band structure of the rigid‐rod polymer trans‐poly(p‐phenylene benzobisthiazole) in the crystalline state
Authors: Bhaumik D. , Mark J. E.
Published: April 2003

The extended Hückel method was employed to calculate electronic band structures in trans‐poly(p‐phenylene benzobisthiazole) (trans‐PBT) in an attempt to elucidate the packing and electronic properties of these chains in the crystalline state. The unit‐cell energies thus calculated indicate that the most stable arrangement for trans‐PBT corresponds to the chains in planar configurations, at an interplanar spacing of 3.5 Å, and shifted axially by 3.0 Å relative to one another. These calculated results are in good agreement with experimental results obtained on the polymer and on relevant model compounds. No discernible dispersion of the energy bands perpendicular to the planes is observed, indicating that the neighboring chains are electronically noninteractive, as was found earlier for trans‐polyacetylene and polyethylene. Similarly, the band gap of 1.69 eV in the axial direction for one of a pair of chains was nearly the same as that, 1.73 eV, calculated previously for an isolated trans‐PBT chain. These values are in the range 1.4–1.9 eV reported for trans‐polyacetylene, which has been extensively studied because of its promise as a semiconductor.
Shear‐induced coalescence in two‐phase polymeric systems. I. Determination from small‐angle neutron scattering measurements
Authors: Roland C. M. , Böuhm G. G. A.
Published: April 2003

It has been found that flow‐induced coalescence occurs at a very rapid rate during the mixing of polymeric fluids. Furthermore, the rheological properties of the dispersed and continuous phases, as well as the nature of the flow field used in their blending, can greatly influence this coalescence. The significance of these findings is that in the development of a mixing scheme to obtain a desired morphology with minimum expenditure of time or energy, attention is usually focused only on the particle breakup aspects of the blending process. The competing coalescence can, however, be accelerated by the same conditions often employed to facilitate particle breakup (e.g., higher shear rates, reduced dispersed‐phase viscosity).

A better understanding of the mechanism for coalescence of viscoelastic fluids is clearly required. In this manner optimal blending of immiscible polymers can be achieved with respect to both the nature of the final material and the ease with which it is obtained.
The hexagonal phase and melt of low‐molecular‐weight polyethylene
Authors: Asahi Takanao
Published: April 2003

Phase diagrams of low‐molecular‐weight polyethylene (M = 1000, 2000, 6500, 16,000) and polyethylene (M ≈ 1,000) after fuming nitric acid treatment have been determined from 20 to 300°C up to pressures of about 10 kbar. The fractions with molecular weights 1000 and 2000 do not exhibit the hexagonal phase, but the others do. Effects of molecular weight and fuming nitric acid treatment on the phase diagrams are discussed in terms of the entropy of the melt.
Linear thermoviscoelasticity and characterization of noncrystalline EPDM rubber networks
Authors: Scholtens Boudewijn J. R.
Published: April 2003

An experimental study has been made to specify how the time‐temperature superposition and the linear viscoelastic characteristics vary with the degree of crosslinking for a broad class of noncrystalline peroxide‐cured EPDM networks. A new, very sensitive method is applied to determine the horizontal and vertical shift functions in an independent way. All uncrosslinked samples are thermoelasticoviscously simple with horizontal shift functions aT of the WLF type and vertical shift functions almost independent of temperature, in agreement with recent theoretical understanding. Upon crosslinking, these materials become thermoviscoelastically complex networks, but superposition can still be accomplished by assuming different temperature dependences for the relaxational strength and the equilibrium modulus. The aT functions can be taken independent of the degree of crosslinking. The vertical shift functions bT for the relaxational strength vary with the degree of crosslinking between theoretical predictions for uncrosslinked and perfectly crosslinked EPDM networks. The equilibrium moduli of the lightly cured networks decreases with increasing temperature, which is ascribed to the presence of interchain associations between ethylene sequences in the trans state. Upon further crosslinking, these effects gradually vanish and eventually the networks can be described as viscoelastically simple with an energy elastic contribution due to the ethylene trans‐gauche transitions. The linear viscoelastic characteristics, namely the storage and loss moduli and compliances and phase‐angle master curves and the relaxation and retardation spectra are discussed as a function of the degree of crosslinking. A sol/gel analysis and equilibrium swelling measurements complete the experimental characterization of three familes of five EPDM networks each.
Self‐diffusion in polymer‐solvent‐solvent systems
Authors: Vrentas J. S. , Duda J. L. , Ling H.‐C.
Published: April 2003

The ternary self‐diffusion data of Ferguson and von Meerwall are analyzed using a generalized form of the free‐volume theory of diffusion proposed by Vrentas and Duda. It is shown that the predicative capabilities of this form of the theory are satisfactory and comparable to those of the Fujita version. The lack of significant differences in the predictive capabilities of the two theories for the two ternary systems studied is explained in terms of the properties of the materials utilized. The data of Ferguson and von Meerwall are consistent with the hypothesis that the size of a polymer jumping unit is a fixed property of the polymer and that the jumping units for sufficiently large n‐paraffins are smaller than the entire solvent molecule.
Complex formation between enantiomeric polyesters
Authors: Grenier Daniel , Prud'homme Robert E.
Published: April 2003

In this study, three different polymers were blended: a racemic PMEPL (PMEPL‐RAC) which consists of a random distribution of repeat units of R or S configurations, an isotactic PMEPL made of repeat units of S configuration [PMEPL‐S(−)], and a second isotactic PMEPL made of repeat units of R configuration [PMEPL‐R(+)]. The acronym PMEPL stands for poly[α‐methyl‐α‐ethyl‐βpropiolactone]. The racemic PMEPL is optically inactive but the isotactic PMEPLs are optically active. In the first part of this study, an equimolar mixture of PMEPL‐S(−) and PMEPL‐R(+) was made. This racemate mixture leads to the formation of stereocomplex between the two polymers which has a melting point roughly 40°C higher than those of the isotactic polymers. The racemate morphology is composed of large spherulites. In the second part of this study, binary blends of PMEPL‐S(−) and PMEPL‐R(+) were prepared with an excess of one of the isotactic PMEPLs. The stereocomplex is preferentially formed between the isotactic polymers and this complex controls almost entirely the morphology of the sample. The enthalpy of fusion of the stereocomplex in the binary blends can be predicted. however, the excess isotactic polymer is trapped between the lamellae of the spherulites of the stereocomplex and its crystallization is hindered by the high viscosity of the partially crystallized medium. In the third part of this study, ternary blends were prepared. It is shown that PMEPL‐RAC does not from a complex with the isotactic polymers. The PMEPL‐S(−)/PMEPL‐R(+) stereocomplex is again preferentially formed and it controls the morphology of the sample. It decreases the degree of crystallinity of both the excess isotactic polymer and the PMEPL‐RAC. This control of the morphology is effective even in samples in which the sterocomplex accounts for only 3% of the total weight of the blend.
Study of ordered structures of syndiotactic poly(methyl methacrylate) in solution and in the solid state
Authors: Spěváček J. , Schneider B. , Dybal J. , Štokr J. , Baldrian J. , Pelzbauer Z.
Published: April 2003

From analysis of infrared spectra it was found that in syndiotactic (s) poly(methyl methacrylate) (PMMA) in solution, long s sequences contain an increased population of diads with a skeletal conformation tt (in the staggered approximation). Self‐aggregation of s‐PMMA in solution leads to a further increase of the fraction of long s sequences in the extended chain conformation, and to an ordering of easter groups. When solid s‐PMMA is isolated from a solution in which it exists in the aggregated state, these characteristics are preserved in the solid. The polymer appears partially crystalline by x‐ray scattering, and it exhibits fibrillar morphology under the electron microscope. Ordered structures of s‐PMMA melt at temperatures about 150°C, while the presence of residual solvent decreases the temperature of melting. Solid s‐PMMA obtained from solutions in which aggregation of the polymer does not take place, like s‐PMMA which did not come into contact with solvent, contains a higher proportion of syndiotactic diads with a skeletal conformation tg; these samples are amorphous and morphologically structureless. Analogies between the structure of ordered s‐PMMA and the structure of the PMMA stereocomplex are also discussed.
Viscometric behavior of mesomorphic ethyl cellulose solution in chloroform
Authors: Suto Shinichi
Published: April 2003

A simple modification of the viscometer used to determine the viscosity of concentrated solutions is described which permits the ready measurement of viscosity versus shear rate behavior above room temperature. Basic experimental viscometry was performed on the ethyl cellulose–chloroform system using a B‐type (Brookfield‐like) rotating‐cylinder viscometer. Data are compared with theoretical predictions. The flow curves exhibit yield stress at low shear rate. The yield stress is dependent on concentration and temperature. The viscosity versus concentration curves show typical mesomorphic behavior. The concentration dependences of viscosity and yield stress are similar. The critical concentration increases with temperature. In the single‐phase region (isotropic or anisotropic state) the viscosity decreases with temperature and the apparent activation energy (Ea) for flow is positive, but in the biphasic range the viscosity increases with temperature andEais negative. The experimental results for the critical concentration appear to agree with Flory's theory of rodlike molecules when the ratio of persistence length to diameter of the chain is taken as the aspect ratio.
Fourier‐transform infrared studies of polypropylene during mechanical deformation
Authors: Lee Y‐L. , Bretzlaff R. S. , Wool R. P.
Published: April 2003

Quantitative Fourier‐transform infrared (FTIR) measurements of frequency shifts Δν and absorbance profile asymmetry are reported for various polypropylene samples as a function of uniaxial stress σ. Generally, it was found that the frequency shift coefficient αχ, defined by Δν = αχσ, depended on stress rate \documentclass{article}\pagestyle{empty}\begin{document}$\dot \sigma$\end{document}, draw ratio, λ, molecular orientation f, tensile modulus E, and annealing conditions. With annealing, αχ decreased with increasing shrinkage in the case of highly oriented isotactic PP. The αχ values for the “helix bands” were less affected than those for the “liquid bands.” With increasing \documentclass{article}\pagestyle{empty}\begin{document}$\dot \sigma$\end{document}, generally αχ increased to an apparent asymptotic limit. With increasing λ, f, or E, αχ also increased from αχ ≃ 0 for λ = 1 (spherulitic) to maximum values for highly oriented isotactic PP. The observed variations in αχ can be interpreted in terms of the changes in the peak position and shape of the nonuniform molecular stress distribution. Analogous behavior with x‐ray diffraction peaks obtained for polymers under stress is discussed.
Anisotropic swelling of doubly oriented nylon‐11 with a lamellar structure
Authors: Dosiere M. , Point J. J.
Published: April 2003

Rolled smaples of nylon‐11 annealed in formic acid (90%) at 80°C remain doubly oriented and have the same allotropic form as filter mats of single crystals. the basal planes of the crystals are parallel to (00l) planes. the long spacing is larger than in filter mats. Experimental swelling results obtained on such samples are discussed on the basis of two extreme models, the lamellar and the switchboard models. The long spacing dy and the length of the sample (along 0Y) change reversibly by the same proportion during swelling. Insertion of solvent between lamellae is invoked to explain such changes of dimension. As linear swelling ratios computed from macroscopic dimensions and SAXS measurements can reach 100%, lamellar crystals are only bonded by a few chains. The large increase of the SAXS intensity observed when annealed doubly oriented samples of nylon‐11 are wetted with allylic alcohol cannot be explained on the basis of the switchboard model but only with a three‐phase lamellar model. These three phases are the crystalline phase, the fold region, and a dilute solution of polymer in the swelling agent.
Hard‐segment polymorphism in MDI/diol‐based polyurethane elastomers
Authors: Blackwell John , Lee Chun Dong
Published: April 2003

X‐ray analyses of methylene‐bis(4)phenyl isocynate(MDI)/diol/poly(tetramethylene adipate) polyurethane elastomers prepared using hexanediol (HDO), butanediol (BDO), and propanediol (PDO) point to the development of a second crystalline structure in the hard segments as a result of thermal elongation and annealing. The HDO polymer crystallizes initially in the fully extended conformation, but a second crystal structure, a contracted form, develops with stretching and annealing at 130°C. In contrast, the BDO and PDO polymers crystallize initially in contracted conformation, but fully extended forms develop as a result of elongation and annealing. Only a contracted conformation has been seen so far in hard segments prepared using ethylene glycol (EDO) as the chain extender. These results correlate very well with DSC data for the same polymers. The development of the new crystal structures is accompanied by the appearance of new hard‐segment melting peaks in the DSC traces. For the HDO polymer, the new peak for the contracted form appears at a higher temperature than that for the original extended form; for the BDO and PDO polymers the new peaks for the extended forms appear at lower temperatures than those for the contracted forms. Only a single peak is seen for the EDO polymer, for which only one crystal structure has been detected for the hard segments. These results indicate that the development of polymorphic crystal structures must be taken into account in interpreting the multiple melting phenomena seen for the hard domains in polyurethane elastomers.
Microstructural rearrangement during heat treatment of drawn nylon 66 fiber
Authors: Elad J. , Schultz J. M.
Published: April 2003

Melt‐spun nylon 66 fibers were drawn and subsequently heat treated isothermally and quenched. The heat‐treated fibers were then examined by wide‐and small‐angle x‐ray scattering (WAXS and SAXS), by differential scanning calorimetry (DSC), and by static mechanical testing. These measurements allow one to follow microstructural changes taking place during the course of the heat treatment. WAXS results show that as the treatment progresses, the crystallites become both more perfect and more disoriented with respect to the fiber axis. SAXS results show crystallite thickening. DSC results show that the melting point increases, goes through a maximum, and then decreases as the heat treatment progresses. The tensile modulus decreases with time to an asymptotic level. The changes in crystallite perfection and thickness occur more rapidly than do the changes in crystallite orientation, modulus, and melting point. A model is proposed whereby the two time frames are related to intracrystalline and intercrystalline processes, respectively.
Thermal expansion and Grüneisen parameters of isotropic and oriented polyethylene
Authors: White G. K. , Choy C. L.
Published: April 2003

The linear thermal expansion coefficients α of three isotropic samples (crystallinity v = 0.42‐0.8) and one oriented sample (v = 0.8, draw ratio 11) of polyethylene have been measured between 2 and 100 K. Together with our previous measurements at high temperatures, these data provide general patterns for the crystallinity and orientation dependence over a wide temperature range. α varies only slightly with crystallinity from 50 to 120 K, but at higher and lower temperatures, it increases significantly with decreasing crystallinity. The increase at high temperature arises from segmental motion in the amorphous regions, while the large increase near 5 K is similar to that observed in many glasses. The tranverse expansion coefficient of the oriented sample is 40‐100% higher than the values for isotropic polyethylene, but the axial expansion coefficient is much smaller and is negative over the entire temperature range. The values of these coefficients are quite close to those for the polyethylene crystal, a feature readily understood in terms of existing models. However, as a result of the much larger elastic anisotropy of the crystal, the Grüneisen parameters (especially the axial Grüneisen parameters) of these two types of polyethylene have rather different values.
Strain birefringence studies on poly(vinyl acetate) networks
Authors: Riande E. , Saiz E. , Mark J. E.
Published: April 2003

Atactic poly(vinyl acetate) prepared in a free‐radical polymerization was crosslinked by means of benzoyl peroxide. The resulting elastomeric networks were studied in elongation, both unswollen and swollen with triethylbenzene, over the range 0–90°C. The most important experimental results obtained were values of the network birefringence, which is negative, as was recently also found to be the case for networks of atactic poly(methyl acrylate).Calculations carried out to interpret the birefringence were based on Monte Carlo simulations of the atactic structure, and on the rotational isomeric‐state theory of chain configurations. The agreement between theory and experiment was very good, in fact much better than has usually been observed for other polymers studied to date.
An equation for diffusion in shrinking or swelling bodies
Authors: Viollaz Pascual E. , Suarez Constantino
Published: April 2003

The differential equation that describes the mass balance of a solvent which diffuses in a shrinking or swelling body is transformed in order to fix the domain of integration and a numerical solution is given. The numerical results obtained differ significantly from the solution of the equation without volume change commonly used in literature. Hence, the new equation could affect substantially the fields of drying, dyeing ion exchange, diffusion in polymers, and solid‐liquid extraction.
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

A 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

The 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

The 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

A 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

Samples 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

Sorption 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

The 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

A 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

A 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

The 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

Dilute 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.