bluuebunny/crossref_metadata_2025 huggingface dataset:
A thermodynamic approach to the stress‐induced crystallization in cross‐linked rubbers
Authors: Wu W. L.
Published: April 2003

The thermodynamic treatment of crystallization phenomena in a prestretched rubber was undertaken. Emphasis was put on defining conditions for the thermodynamic stability of the extendedor folded‐chain crystal structure. The extended‐chain structure is found to be stable thermodynamically at temperatures higher than the isotropic melting point of un‐cross‐linked polymer T in the stretched state, while the folded chain one is not. Below T, the stretch ratio of the network structure determines which crystal structure is more stable. The relation among the critical stretch ratio for the extended/folded crystalline structure transition, temperature, and molecular weight is also discussed. The crystallinity predicted by this work becomes zero at a temperature of T, the isotropic melting point of a cross‐linked system. The value of T decreases with increasing cross‐link density, and this is consistent with the experimental data reported in the literature.
First‐order perturbation calculations of intrinsic viscosities of flexible linear and ring polymers with excluded volume: Comments on the papers of Fujita et al.
Authors: Shimada Jiro , Yamakawa Hiromi
Published: April 2003

It is shown that the first‐order perturbation coefficient C1 of the cubed viscosity‐radius expansion factor α of nondraining flexible chains with excluded volume may be evaluated without great labor and with sufficient accuracy by solving numerically the Kirkwood–Riseman‐type integral equations with a digital computer. The value 1.18 for C1,r obtained for ring chains is in good agreement with the corresponding value of Norisuye and Fujita, while the value 1.14 for C1,l obtained for linear chains is appreciably smaller than the corresponding value 1.29 of Fujita et al. despite the fact that the starting equations are the same. However, it is shown that the calculated value of C1,l of Fujita et al. is incorrect. Thus, the present result that C1,l < C1,r is consistent with the earlier conclusion of Yamakawa and Tanaka derived in the Zimm–Hearst scheme.
On the structure of glassy polymers. VI. Electron microscopy of polycarbonate, poly(ethylene terephthalate), poly(vinyl chloride), and polystyrene
Authors: Meyer M. , Vander Sande J. , Uhlmann D. R.
Published: April 2003

High‐resolution electron microscopy studies have been carried out on four glassy polymers examined in previous small‐angle x‐ray scattering (SAXS) investigations. The polymers include polycarbonate, poly(ethylene terephthalate), poly(vinyl chloride), and polystyrene. For all four polymers, both bright field and dark‐field observations indicate the general absence of microstructural features of a size down to the resolution limit of the electron microscope. Only “pepper and salt” features on a scale ca. 5 Å are seen as characteristic of the structures. These features reflect simple interferences as the resolution limit is approached, and are seen for single crystal and oxide glasses as well as for the polymers. The present results, taken together with structural information from light scattering, SAXS, and small‐angle neutron scattering, indicate that glassy polymers should be regarded as having rendom structures. The combined results are inconsistent with heterogeneous microstructures having regions of locally high order present in large volume fractions.
Vibrational spectra and normal coordinate analysis of isotactic poly(alkyl ethylene)s. I. Modification I of poly(ethyl ethylene) and its deuterated derivatives
Authors: Holland‐Moritz K. , Sausen E.
Published: April 2003

A new calculation of the vibrational frequencies of poly(ethy1 ethylene) based on the actual molecular geometry was performed. The calculation takes into account new experimental data such far‐infrared spectra and Raman spectra of the stretched polymer, infrared spectra of poly(ethyl ethylene)‐(2,2;d2) (P‐4‐1‐d2) and poly(ethyl ethylene)‐(3,3,4,4;d4) (P‐4‐1‐d4), and Raman spectra of poly(ethyl ethylene)‐(1,3,3;d3) and poly(ethyl ethylene)‐(3,3,4,4;d4). Refinement produced a set of force constants; it is also applicable to poly(propyl ethylene).
Inverse gas chromatography of poly(cyclohexyl methacrylate)
Authors: Llorente M. A. , Menduiña C. , Horta A.
Published: April 2003

Inverse gas chromatography is applied to determine the glass transition temperature Tg of poly‐(cyclohexyl methacrylate). Both good and bad solvents for the polymer are used as molecular probes. Although the transition is clearly detected by both types of probes, only the nonsolvents yield a Tg in quantitative agreement with the value determined by differential scanning calorimetry. The relative depth of penetration of the probe in the polymer phase is calculated from retention volume data. Also, the height equivalent to a theoretical plate is calculated from peak halfwidths. Both calculated magnitudes show a temperature dependence which significantly differs from good to bad solvents. Some kind of diffusion‐limited penetration of the probe in the bulk of the glassy polymer may be responsible for the difference.
Theory of free volume in polymers
Authors: Rudnick J. , Taylor P. L. , Litt M. , Hopfinger A. J.
Published: April 2003

A number of theoretical approaches to the problem of free volume in polymers are discussed. Because some questions in the Bueche approach remain unresolved, the lattice‐gas model was studied. Inconsistencies in the independent‐vacancy approximation led to consideration of a mean‐field model and then a precise Padé‐approximant solution, neither of which gave satisfactory agreement with experiment. Introduction of an internal pressure parameter to model the effects of the chainlike structure of polymers restores agreement with experiment.
Estimations of fold surface densities using space‐filling models
Authors: Harrison I. R. , Juska T.
Published: April 2003

The model of a tight‐fold adjacent reentry fold surface for polyethylene single crystals has been questioned, because it has been speculated that a tight‐fold surface should have a density approximating that of the unit cell (∼1.00 g/cm3). This would lead to an overall lamella density of close to unity. In contrast, the majority of measured values are in the range 0.96 to 0.97+ g/cm3. These lower values have been taken as evidence to disprove a tight‐fold–surface model. The present calculations made on space filling models of tight folds indicates a fold surface density on the order of 0.75 g/cm3. This produces no inconsistency between a tight‐fold model and accepted lamella density values. Further, calculated values of the weight fraction crystallinity of lamella, as a function of the number of carbons in the fold, limits this number to approximately 9 to 11 carbons per fold.
Chain folding in crystalline syndiotactic poly(vinyl chloride)
Authors: Conte G. , D'Ilario L. , Pavel N. V. , Giglio E.
Published: April 2003

Potential‐energy calculations seem to be capable of providing information about the structural characteristics of the folds in crystalline polymers. This paper deals with the conformational analysis of syndiotactic poly(vinyl chloride) (PVC) chains in order to reproduce satisfactory models of folding in the (010), (400), and (M̄10) crystallographic planes. The results imply that tight (010) and (400) folds and, hence, adjacent reentry may occur. The folds are probably stabilized by a weak hydrogen‐bond
magnified image
type of interaction. Moreover CHClCH2CH2CHCl segments in the folds have been taken into account, thus obtaining more energetically favored models. The crystal lattice energy has been computed as a function of the PVC unit‐cell parameters a and b, releasing the fold's constraint of the stems. The energy minimum corresponds to a noticeable decrease of a in comparison with the experimental value, although the potentials used in the calculations are hard and have van der Waals radii greater than those usually accepted. As a consequence it seems reasonable to suppose that the folds strongly influence the PVC crystal packing and do not allow better interatomic contacts between stems to form.
Dielectric γ relaxation in poly(2‐hydroxyethyl methacrylate)
Authors: Diaz Calleja R.
Published: April 2003

The dielectric constant and loss were measured for poly(2‐hydroxyethyl methacrylate) from −90 to 34°C over the frequency range 30 to 3 × 105 Hz in the γ relaxation region. The relaxation was also characterized using activation energies.
Characterization of poly(styrene‐co‐divinylbenzene) microgels
Authors: Forget J. L. , Booth C. , Canham P. H. , Duggleby M. , King T. A. , Price C.
Published: April 2003

Tightly cross‐linked poly(styrene‐co‐divinylbenzene) microgels with molecular weights in the range 107 to 108 g mole−1 were studied in solution in dimethylformamide containing 7 g dm−3 LiBr. Laser light scattering photon correlation spectroscopy (PCS) was used to measure z‐average translational diffusion coefficients D̄z and dilute solution viscometry to measure intrinsic viscosities [η]; values of the equivalent hydrodynamic radii 〈R〉 and 〈RD−1〉 were derived. Weight‐average molecular weights M̄w and z‐average mean‐square radii of gyration 〈S2〉z were determined by conventional light scattering. The microgels were investigated by gel permeation chromatography, and molecular weight data obtained using the [η]M calibration procedure were in satisfactory agreement with results obtained by electron microsocopy and conventional light scattering.
The influence of pendent chains on the thermodynamic and viscoelastic properties of swollen networks
Authors: Bastide J. , Picot C. , Candau S.
Published: April 2003

Polystyrene networks containing controlled amounts of pendent chains and swollen to equilibrium in benzene have been investigated by inelastic light scattering and stress‐strain experiments. Both the cooperative diffusion constant and compression modulus have been shown to decrease drastically when the proportion of pendent chains increased, while the equilibrium segment concentration exhibits only a small variation. The experimental results are discussed in terms of a change of structure and dimensions of the macromolecular strands of the network.
Statistical mechanical model of diffusion of complex penetrants in polymers. I. Theory
Authors: Pace R. J. , Datyner A.
Published: April 2003

A previously developed model of simple penetrant diffusion is extended to encompass complex penetrants of idealized molecular shape, characterized by dimensions of length, width, and thickness. Expressions are obtained for D(0,T), the diffusion coefficient at zero penetrant concentration (c), and the fractional increase in D(0,T) as a function of c and temperature (T). The model predicts that D(0,T) will exhibit Arrhenius behavior at temperatures well above Tg and gives the limiting activation energy as a function of penetrant thickness and the polymer energy/distance constants used previously. For Tg < T ≲ Tg + 150 K the model requires two new disposable parameters, in addition to the jump‐length parameter of the simple penetrant theory. These parameters, however, have precise physical meanings (all are lengths) and together with the penetrant dimensions and polymer constants determine the absolute magnitude of the diffusion coefficient as well as its relative dependence on c and T. For T ≳ Tg + 40 the relative concentration dependence may be calculated in terms of the penetrant dimensions and polymer constants only.
Induction and measurement of glassy‐state relaxations by vapor sorption techniques
Authors: Berens A. R. , Hopfenberg H. B.
Published: April 2003

Recent gravimetric studies of the sorption of organic vapors by poly(vinyl chloride) and polystyrene powders have demonstrated several features which promise to be generally useful in studying the structure and properties of the glassy state. The uptake of vapor can be significantly altered by prior thermal or vapor treatment of the polymer, apparently reflecting changes in the microvoid content or free volume of the polymer. Fickian sorption in sufficiently fine powders proceeds to equilibrium in a few minutes. Upon exposure of a polymer powder to an appreciable pressure of vapor, both a rapid Fickian sorption and a slower, relaxation‐controlled sorption are observed. Superposition of these processes leads to widely varied sorption kinetics; a model comprising Fickian diffusion and first‐order relaxation terms accurately describes the data and allows estimation of equilibrium and rate constants for both processes. After prolonged exposure, removal of a swelling vapor induces a slow reconsolidation of the polymer structure; this deswelling relaxation can be monitored by the decreasing amounts of vapor sorbed in repeated brief exposures to low vapor pressures, and can also be described by a first‐order relaxation model. In this regard, the penetrant vapor serves as a molecular probe, monitoring glassy‐state relaxation occurring in the absence of penetrant. The same, presumably true equilibrium is ultimately reached both by swelling from a low free‐volume state and by consolidation from a preswollen state of high free volume. The rates of both swelling and consolidation relaxations appear to be retarded by the presence of low concentrations of vapor in the polymer, suggesting that vapor molecules may preempt some of the free volume required for relaxation.
Evaluation of the mosaic model for polymer single crystals
Authors: Dorset Douglas L.
Published: April 2003

By analogy with observed alteration of electron diffraction intensities from other crystalline long‐chain materials (e.g., paraffins) the mosaic model for polymer single crystals is evaluated. When many strong diffracted beams are excited simultaneously, the two‐beam dynamical diffraction theory (which then must be a consequence of the mosaic model) is shown not to be an adequate explanation of the electron diffraction intensities from such crystals; the n‐beam dynamical theory is much more adequate. Further, observed bend contours in these crystals indicate significant bend distortion. This is confirmed by observed modulation of diffraction intensities which are well explained by Cowley's model for diffraction from elastically bent crystals. Other experiments on these crystals reveal the bends to be irregular. Rocking curves reveal that bends also explain diffraction line broadening, as also proposed by White. The most accurate description of molecular organic microcrystals is therefore that of a deformed foil and not a mosaic.
Morphology and properties of low‐density (branched) polyethylene
Authors: Mandelkern L. , Maxfield J.
Published: April 2003

The different types of morphology that can be developed in a large number of low‐density (branched) polyethylene whole polymers, as well as in a series of fractions, have been studied for two different extreme crystallization modes. Concomitantly, thermodynamic properties of the same samples have also been determined. After isothermal crystallization at elevated temperatures, spherulitic structures are found in all the whole polymer samples. On the other hand, after rapid crystallization a variety of different types of supermolecular structures are observed which are shown to depend systematically on the concentration of side‐chain branches and the relative proportion of high molecular weight species in the sample. This temperature dependence of the morphological forms is opposite to that previously reported for linear polyethylene. The studies with the fractions show that the individual species are not the cause of this behavior; rather, the total composition is the important factor. The thermodynamic properties are also quite different from those of linear polyethylene in showing virtually no molecular weight dependence and being governed primarily by the concentration of short‐chain branches. The degrees of crystallinity as determined from density and enthalpy of fusion measurements do not vary much with the two extreme crystallization conditions employed, are not sensitive to the morphology, and differ from one another, even when well‐developed spherulites are formed. A major influence of the branching concentration on these properties is clearly indicated.
Random‐Coil configurations of the polymormals [CH2O(CH2)yO]. II. Dipole moments of poly(1,3–dioxepane) [CH2O(CH2)4O;]
Authors: Riande E. , Mark J. E.
Published: April 2003

Dielectric constant measurements were carried out on a sample of poly(1,3–dioxepane) [CH2O(CH2)4O], in benzene over the range 25—60°C. This chain molecule was found to be very similar to poly(1,3–dioxolane) [CH2O(CH2)2O] in having a relatively small dipole moment, which increases markedly with increasing temperature. Rotational isomeric‐state calculations show that these two characteristics are due to very strong preferences for gauche states in the two central bonds in the sequence CH2OCH2OCH2. Such pairs of gauche states, necessarily of the same sign, place neighboring‐group dipoles in essentially antiparallel orientations. The resulting attenuation of dipole vectors is the origin of the small dipole moment, and the disruption of these preferred conformations with increasing temperature gives rise to the large positive temperature coefficient of the dipole moment.
Characterization of semicrystalline random copolymers by small‐angle neutron scattering
Authors: Kalepky U. , Fischer E. W. , Herchenröder P. , Schelten J. , Lieser G. , Wegner G.
Published: April 2003

A new method is introduced to determine the degree of partitioning of noncrystallizable comonomer units (B units) between the two phases of a semicrystalline random copolymer. The method is based on the comparison of the intensities of small‐angle neutron and x‐ray scattering (SANS and SAXS, respectively). By this technique two quantities can be evaluated: the difference Δρ of the mass densities between the crystalline and the disordered regions, and the concentration fluctuations of the B units in the two phases. It is found that SANS is very sensitive to the presence of small amounts of B units if their scattering length is sufficiently different from that of the A units. This will be the case for copolymers with B units, in which a hydrogen is substituted by another atom. But in addition it can also be achieved generally by deuteration of the comonomer units. So a wide range of copolymer systems can be studied by this method. The capability of the method was proved by measurements on chlorinated polyethylene and on 1,3,5‐trioxane–1,3‐dioxolane copolymers. Both copolymers are distinguished by a random distribution of the co‐units. The results show that even at relatively low concentrations xB of the comonomer units a remarkable fraction of the B units is incorporated into the crystalline A phase and that this fraction rises if xB is increased.
Solid‐state coextrusion of high‐density polyethylene. I. Effects of geometric factors
Authors: Kanamoto Tetsuo , Zachariades Anagnostis E. , Porter Roger S.
Published: April 2003

The solid (crystalline) state coextrusion of two high‐density polyethylenes (HDPE) having weight‐average molecular weights (Mw) of 59,000 and 200,000 have been studied as a function of the geometrical arrangement and the volume fraction of the components. The extrusion rate increased nonlinearly with the volume fraction of the low‐Mw, component. The rate was faster when the low‐Mw, component was the core rather than the sheath in the initial cylindrical concentric billet. Thus the slow extrusion rate of high‐Mw HDPE alone was increased up to ten times by coextrusion with a small fraction of the low‐Mw, HDPE component in its center. Generally, the deformation flow profile changed gradually from a parabolic to a W‐shaped pattern as the volume fraction of the high‐Mw, component increased. However, the geometric arrangement of the two different Mw components also had a pronounced effect on the deformation. The deformation patterns showed that upon coextrusion the low‐ and high‐Mw HDPE's were extruded at the same rate and extrusion draw ratio. The geometrical arrangement had no substantial effects on the tensile modulus and strength of the extrudates; i.e., they increased linearly with volume fraction of the high‐Mw HDPE.
Orientable polymer fluids—suggestions from Ericksen's theory of anisotropic fluids
Authors: Abhiraman A. S. , George Waller
Published: April 2003

Ericksen's theory incorporates in the constitutive equations a vector ni that characterizes the anisotropy of a fluid particle arising from flow induced orientation. The present study is an appraisal of the potential of such an approach in studying orientable polymeric fluids. The behavior of a normalized version of ni (ni → 0 at rest and ni → 1 when fully stretched) is studied in simple shear, elongational, and converging flow fields. Following a discussion of the merits and the problems with this approach, a modification is proposed where directional anisotropy and stretching of a fluid particle are separated. Empirical extensions include the incorporation of a simple structure parameter in viscoelastic fluid theories.
Polarized Raman study of molecular orientation in uniaxially stretched atactic polystyrene
Authors: Jasse B. , Koenig J. L.
Published: April 2003

Polarized Raman intensities have been obtained from thin films of uniaxially oriented atactic polystyrene. The study was made using the 623 cm−1 radial skeletal η6b vibration of the benzenering. Values of the fourth‐order as well as second‐order terms in the expansion of the orientation distribution were obtained for draw ratios between 1.5 and 7.7. Values for P2(cosθ) agree well with the values obtained from infrared dichroism.
Partial miscibility of liquid mixtures of protonated and deuterated high polymers
Authors: Buckingham A. D. , Hentschel H. G. E.
Published: April 2003

Certain low‐angle neutron scattering experiments on bulk protonated polymers containing a fraction of the deuterated species (or vice versa) suggest that clustering of like isotopes is occurring. In order to see whether this is likely, a form for the Helmholtz free energy of mixing is proposed, using thermodynamics and the ideal solution (Flory‐Huggins) entropy of mixing. From this free energy the upper consolute point Tc is found. Tc is calculated for several isotopic molecular solutions. The results suggest that while isotope segregation phenomena are impossible for small hydrocarbons, they may become important for large polymer systems.
Rigid backbone polymers. X. Transitions in bulk polyisocyanates
Authors: Aharoni Shaul M.
Published: April 2003

Data obtained by hot‐stage cross‐polarized light microscopy and thermogravimetric analysis were used to plot a transition map for the poly(n‐alkylisocyanate) homologous family. Most members had a low‐temperature transition (or relaxation). The middle members of the family, with side‐chain length of 4 ≤ n < 13 carbon atoms, exhibit a mesomorphic range of temperatures. Members with sidechains of n ≥ 13 in length, go directly from the low‐temperature transition to the melting point without passage through a liquid crystalline state. A transition map for two homopolymers and a series of their copolymers, poly[x mol% n‐butyl + (100‐x)mol% p‐anisole‐3‐propyl]isocyanate, was also prepared. The range of mesomorphicity as a function of composition and temperature is clearly indicated.
Biaxial drawing and orientation of polymers. I. Theoretical and mathematical considerations
Authors: Jungnickel Bernd‐J.
Published: April 2003

Analytical expressions are derived which relate the degree of orientation caused by a biaxial nonorthogonal pseudoaffine deformation to the conditions and parameters of drawing. They contain relations previously given by Kuhn et al. and Sasaguri et al. as special cases and can be evaluated in a way which allows a proper analysis of experimental data. The plastic and elastic components of the deformation can be estimated on the basis of these equations. The equations are useful for analysis of biaxial drawing, including technical drawing of polymer films and of the phenomenon of “bowing.”
Morphology of melt‐crystallized linear polyethylene fractions and its dependence on molecular weight and crystallization temperature
Authors: Voigt‐Martin I. G. , Fischer E. W. , Mandelkern L.
Published: April 2003

Replicas and thin‐section electron microscopic studies were made of fractions of linear polyethylene covering the molecular weight range 2.78 × 104 to 6.0 × 106 for a variety of crystallizing conditions. Lamellar crystallites were found under all circumstances; and the supermolecular structure, or crystalline morphology, is in agreement with that previously reported from an analysis of the small‐angle light‐scattering patterns of the same samples under similar crystallization conditions. Details of the crystalline microstructure are also described, which range from truncated hollow pyramids which degenerate as the molecular weight or the undercooling are increased. From these results, it is possible to describe the mechanism of formation of polyethylene spherulites.
Infrared study of high‐pressure crystallization of polyethylene
Authors: Hsu T. S.
Published: April 2003

The high‐pressure crystallization of polyethylene in a diamond cell has been studied by infrared spectroscopy. The splitting of the CH2 rocking band at 720–730 cm−1 as a function of pressure was analyzed. It was found that pressure alone up to 3 kbar will not change the distance between methylene groups in the unit cell. However, this distance can be shortened by crystallization at this pressure. Intensities of selected crystalline (1176 and 1050 cm−1) and amorphous (1303, 1352, and 1368 cm−1) bands were measured on samples before and after high‐pressure crystallization, and also on samples of various densities crystallized under atmospheric pressure. The increase in the intensities of crystalline bands and concomitant decrease in amorphous bands, together with density changes, indicate that the crystallinity can be enhanced by crystallization under high pressure. Nevertheless, the crystallinity of polyethylene crystallized at high pressure is comparable with that of polyethylene crystallized at atmospheric pressure at low undercooling for long periods of time.
Adsorption of acrylic acid copolymers with hydrophobic chain substituents on polystyrene latex
Authors: Awad Nagi M. , Morawetz Herbert
Published: April 2003

Adsorption of three copolymers of acrylic acid with N‐1‐naphthylacrylamide on monodisperse polystyrene latex was studied as a function of pH, ionic strength, and latex concentration. A comparison of the observed adsorptive capacities with predictions based on a model in which the adsorbed polymers retain their shape in solution showed that this model leads to an increasing underestimate as the density of the hydrophobic groups on the polymer is increased and as the pH is reduced.
Time‐dependent step–step correlations in a self‐avoiding random walk
Authors: Khwaja Y. , Sadiq A.
Published: April 2003

The space–time step–step correlations of a polymer chain, described by a self‐avoiding random walk on a cubic lattice, are studied by simulating its behavior on a computer. Chains of N = 16 and N = 32 beads are studied. The short‐time behavior of the correlations is characterized by peaks, which become depressed and broadened with increasing interstep distance. The long‐time behavior is characterized by extremely slow decay, which goes to zero in about the same time for all interstep distances. The short‐time behavior is a consequence of the rotation of parts of the chain smaller than or equal to the static coherence length ζ. These short‐time processes also seem to be responsible for the relaxation of the square of the end‐to‐end distance, since in both cases the relaxation is of the order of 6N2. The long‐time behavior of the correlations is a consequence of the rotations of parts of the chain greater than the coherence length. The correlations vanish when the entire chain has had time to rotate in space. Our results suggest that this corresponds to the relaxation of the end‐to‐end vector.
Structural studies of ultrahigh‐modulus linear polyethylene using nitric acid etching and gel permeation chromatography. I. Determination of the crystal size distribution
Authors: Capaccio G. , Ward I. M.
Published: April 2003

The technique of nitric acid etching followed by gel permeation chromatography (GPC) has been used to study the structure of ultrahigh‐modulus linear polyethylene (LPE) tapes drawn to draw ratio λ of 20. For comparison, lower draw ratio (λ = 11) samples were also examined. The etching was carried out in fuming nitric acid at 60°C and the progress of the reaction was monitored by measuring weight loss and molecular weight distributions as a function of time over a period up to 25 days. Consistent with previous work by us and other workers, notably Porter and Peterlin and co‐workers, the ultrahigh‐modulus products exhibit an exceptional resistance to the acid attack, i.e., after 3 days their weight loss is still negligible while at lower draw ratios it could be as high as 30%. At longer times, however, the rate of weight loss becomes comparable for the two sets of samples, even if the absolute values are much smaller for the products of λ = 20. During the early stages of the etching treatment a rapid decrease in molecular weight and narrowing of the molecular weight distribution is observed in all cases. Eventually the molecular weight distribution becomes time independent, while the weight loss continues to increase. This stage coincides with the attack of the lateral surfaces of the crystals becoming the dominant process and it is considered that the observed molecular length distribution then reflects the distribution of crystal thicknesses. The values of the weight average crystal thickness derived from the GPC experiments (L̄w) are in very good agreement with those obtained from wide‐angle x‐ray determinations. Furthermore the ratio of weight‐average to number‐average crystal thickness (L̄w/L̄n) is about 2 for the high draw (λ = 20) samples, i.e., the value predicted by the simple statistical model proposed by Gibson, Davies, and Ward for the structure of ultrahigh‐modulus LPE. It is therefore concluded that the nitric acid etching/GPC technique can be used for reliable measurements of crystal size and crystal size distribution in ultraoriented LPE.
Intrinsic viscosities of four homopolymers and one copolymer in nonpolar solvents. I. Effect of correlations of molecular orientations
Authors: Filiatrault Danielle , Phuong‐Nguyen H. , Delmas Geneviève
Published: April 2003

Intrinsic viscosities [η] of four homopolymers, polyisobutylene (PIB), polypentene‐1 (PP‐1), polypentenamer (PPmer), and polydimethylsiloxane (PDMS), and of an ethylene‐propylene copolymer containing 81% ethylene (81% E) have been measured at 25°C in seven linear alkanes ranging from n‐C6 to n‐C16 and three highly branched alkanes, 2,2,4‐trimethylpentane, 2,2,4,6,6‐pentamethylheptane, and 2,2,4,4,6,8,8‐heptamethylnonane. Correlation of molecular orientations (CMO) in the polymers was investigated. The difference Δ[η] = [η](lin) − [η](br) is used as a test of CMO with the supporting assumption that CMO lowers the free energy and the destruction of CMO raises it. The positive value of Δ[η], which varies from 20% to 40% for PPmer and 81% E, is indicative of orientational order in these two polymers. The negative value of Δ[η] for PDMS results from the disordering of linear alkanes by the nonordered PDMS. δ[η] is near zero for PP‐1 and small for PIB implying that these two polymers are indifferent to solvent molecular shape. The variation of [η] with alkane chain length of the linear alkanes gives additional information about size and solvent quality. The dependence is small for ordered polymers due to the short range of CMO. [η] diminishes rapidly with n for PDMS probably because of the increased difference of cohesive energy between polymer and solvent. The dependence is small for PIB but very large for PP‐1. The much better quality of small‐molecule solvents for PP‐1 may be an indication of a helicoidal conformation of this polymer in solution.
Effect of junction constraints on rubber elasticity in multiaxial states of stress
Authors: Erman Burak
Published: April 2003

The strains obtained in an amorphous polymer network subject to multiaxial states of stress are calculated according to a recent molecular theory which takes account of constraints on junctions in real networks. Experimental measurements for biaxial states of stress, for pure shear superposed on tension, and for combined torsion and simple extension, are seen to compare favorably with the predictions of the theory.
An ESR study of polyisobutylene radicals produced by γ‐irradiation at 77 K
Authors: Hori Yasurō , Kashiwabara Hisatsugu
Published: April 2003

An electron spin resonance (ESR) study has been made of radicals trapped in polyisobutylene irradiated at 77 K. It is concluded that only the C(Me)2CH radical remains after heat treatment above 213 K. This radical shows a very sharp doublet ESR spectrum with hyperfine splitting of about 2.2 mT. A broad doublet ESR spectrum observed without warming, which was previously assigned as C(Me)2CHis explained as a mixture of a doublet and a triplet due to the C(Me)2CH and
magnified image
radicals. γ‐irradiation at 77 K produces these major radical components and other carbon radicals as minor species. It has also been found that a conversion of other radicals into C(Me)2CHoccurs without decay on annealing below 183 K. Heating in the range 183–213 K causes both conversion and decay of the radicals. On annealing above 213 K, only decay occurs. The ESR spectra of polyisobutylene radicals are easily saturated by microwave power even at 1.6 μW. The mechanism of radiolysis for polyisobutylene is briefly discussed.
Glass transition temperature and heat capacity of heterotacticlike PMMA
Authors: Fernandez‐Martin F. , Fernandez‐Pierola I. , Horta Arturo
Published: April 2003

The glass transition temperature and heat capacity have been determined by differential scanning calorimetry on samples of heterotacticlike monodisperse poly(methyl methacrylate) (PMMA). The nuclear magnetic resonace spectra of these samples reveal that they contain a larger percentage of mr triads (ca. 51%) than atactic polymers and that their microstructure may be predominantly stereoregular because of an abundance of mrrr sequences. We combine our Tg values with data from the literature for PMMAs of other tacticities, in order to analyze the correlation of Tg with tacticity. In this analysis, we consider PMMA as a steric copolymer formed by mm, mr, and rr triads.
The laser‐raman spectrum and a normal‐coordinate analysis of isotactic poly(tertiary butyl acrylate)
Authors: Aylward N. N. , Ti S. S.
Published: April 2003

The coincidence of many bands in the Raman and infrared spectra of isotactic poly(tertiary butyl acrylate) and the crystallographic identity period suggests that it may be treated as a 31 helix whose line group is isomorphous with the factor group C3. A normal‐coordinate analysis of a possible helical 31 conformation allows many of the bands to be provisionally assigned and the potential‐energy distribution to be calculated.
Use of quantum chemistry relations in cationic polymerization: A critical examination
Authors: Bunel C. , Marechal E.
Published: April 2003

The temperature dependence of the reactivity ratio r2 (subscript 2 for styrene) is reported for 1‐and 2‐vinyl naphthalenes, 4‐halogeno‐1‐vinyl naphthalenes, 2‐ and 3‐vinyl phenanthrenes, and o‐methylstyrene. The monomers are classified with respect to In (1/r2) and ΔH # and ΔS# (the differences in activation parameters for homopropagation and crosspropagation). It is shown that the reaction is entropically controlled. It appears that there is no correlation between stabilization energy (ΔE)rs and ΔH# and that the correlation with In(1/r2) is only apparent. The origin of this lack of correlation is analyzed and a new index of reactivity is proposed; its correlation with ΔH# is quite satisfying.
A study of the PE‐PTFE system. I. IR and NMR measurements
Authors: Nagarajan S. , Stachurski Z. H.
Published: April 2003

Polyethylene‐polytetrafluoroethylene blends have been prepared by sintering at a temperature of 200°C and various times. Detailed description of the preparation of these samples is given. IR optical and NMR line‐broadening properties of these samples have been measured. The results indicate interaction between the two highly crystalline polymers on the molecular level. The properties of the sintered samples are compared with a mechanical mixture (not sintered) of the two polymers, and the deviations from the law of simple mixtures are interpreted in terms of molecular interactions between the constituent phases.
Dynamic birefringence studies of high‐density polyethylene
Authors: Cembrola Robert J. , Kyu Thein , Suehiro Shoji , Kawai Hiromichi , Stein Richard S.
Published: April 2003

A rheo‐optical investigation has been carried out on a sample of high‐density polyethylene (HDPE) in an attempt to examine the nature of the α‐relaxation mechanism. Dynamic mechanical and bi‐refringence behavior was measured over the frequency range of 0.008‐4.3 Hz and temperature range −40 to 100°C. The dynamic mechanical and birefringence data were reduced to a reference temperature of 50°C by a combination of horizontal and vertical superposition. The significance of the vertical shift factor has been discussed extensively in previous papers and is not dealt with here. An Arrhenius plot was made of the log of the horizontal shift factor versus reciprocal temperature for the mechanical and optical data. The mechanical data exhibited three distinct regions, the slopes of which led to activation energies of 70, 90, and 150 kJ mol−1. The temperature at which these dispersions occurred suggested the observation of the β, α1, and α2 relaxation processes. The optical data contained two distinct regions from which activation energies of 55 and 95 kJ mol−1 were obtained. The high‐temperature α2 process was not observed in the Arrhenius plot; however, a maximum in K′ and a change in sign of K″ probably reflects a contribution from the α2 relaxation mechanism.
Effects of high‐pressure CO2 on the glass transition temperature and mechanical properties of polystyrene
Authors: Wang Wen‐Chou V. , Kramer Edward J. , Sachse Wolfgang H.
Published: April 2003

Hydrostatic pressure usually increases the glass transition temperature Tg of a polymer glass by decreasing its free volume; if the pressurizing environment is soluble in the polymer, however, one might expect an initial decrease in Tg with pressure as the polymer is plasticized by the environment. Just such a minimum in the Tg of polystyrene (PS) is observed as the pressure of CO2 gas is increased over the range 0.1–105 MPa from both ultrasonic (1 MHz) measurements of Young's modulus E and static measurements of the creep compliance J. A time‐temperature‐pressure superposition law is obeyed by PS which allows a master curve for the compliance to be constructed and shift factors to be determined. A master curve for E is then obtained by using the Boltzmann superposition principle. The compliance J reaches a maximum, and E and Tg reach minima, at a CO2 pressure of ca. 20 MPa at both 34 and 45°C, which are above the critical temperature (31°C) of CO2. At the minimum, Tg is 41 at 45°C and 36 at 34°C, the larger depression at 34°C evidently corresponding to the higher solubility of CO2 at the lower temperature. The plasticization effect due to CO2 can be isolated by subtracting the effect of hydrostatic pressure alone from the experimental data. The results leave no doubt that at high pressures CO2 gas is a severe plasticizer for polystyrene.
Some evidence against the existence of the liquid‐liquid transition. I. Transitions or dispersions: A quantitative explanation of one torsional braid log decrement peak
Authors: Plazek Donald J.
Published: April 2003

Some of the history of the proposed liquid‐liquid transitions Tll and T′ll that have been reported to exist above the glass transition temperature Tg and the role played by the torsional braid analyzer are outlined. Confusion has been created by the failure to clearly distinguish such kinetic phenomena as viscoelastic and dielectric loss processes from thermodynamic transitions. It is shown quantitatively how the log decrement of an oscillating system can exhibit one more peak than is found in the loss tangent–temperature curve of a viscoelastic liquid.
Some evidence against the existence of the liquid‐liquid transition. II. Properties of and measurements with a torsional braid analyzer
Authors: Plazek Donald J. , Gu Guo‐Fang
Published: April 2003

A torsional braid analyzer (TBA) has been constructed for an investigation into the behavior of this instrument. An explanation was desired for the presence of the proposed second liquid‐liquid transition T′ll peak in the log decrement–temperature curve of a low‐molecular‐weight polystyrene whose loss tangent increases monotonically as a function of increasing temperature at constant frequency. The response of the TBA was studied as a function of amplitude of oscillation, the nature of the braid, the braid tension, frequency, and the nature of the impregnant. All of the braids examined were shown to be extremely nonlinear in their response. Two peaks in the damping of the composite system as a function of temperature are attributed to the interaction between the impregnant properties and the nonlinear response of the glass fiber braid.
Some evidence against the existence of the liquid‐liquid transition. IV. The temperature dependence of shear viscosity
Authors: Orbon Stephen J. , Plazek Donald J.
Published: April 2003

The viscosity data available for four anionically polymerized polystyrenes ranging in molecular weight from 1100 to 47,000 for the temperature range Tg to Tg + 100°C have been fitted by computer programs to both the Vogel, Fulcher, Tamman, and Hesse (VFTH) equation and to two optimum intersecting Arrhenius equations. The intersection point has been interpreted as a manifestation of a liquid‐liquid transition. The fits to the VFTH equation were in every case found to be far superior. Systematic deviations of the residuals were observed for the best Arrhenius fits which indicate the lack of any validity for such a representation of the data.
Assignment of an anomalous peak in the brillouin spectrum of oriented polymer films
Authors: Cavanaugh D. B. , Wang C. H.
Published: April 2003

The origin of a previously unassigned peak in the Brillouin spectrum of an oriented polymer film has been clarified. This peak is shown to be due to backscattering from a quasitransverse acoustic phonon from the reflected laser beam. Studies of this peak are expected to provide the information concerning the birefringence effect of the oriented film.
Refinement of linear polymer crystal structures determined from electron diffraction data
Authors: Moss Barbara , Dorset Douglas L.
Published: April 2003

The reliability of linear polymer structures determined by using electron diffraction data is investigated. The results of n‐beam dynamical calculations and kinematic calculations which take account of crystal bending are compared with experimental structure factors for five published structures. In specific cases, both dynamical scattering and crystal bending effects are found to be important. Finally, guidelines are given for obtaining electron diffraction data which are optimal for structure solution.
Epitaxial morphologies of polyoxymethylene. II. Deposition energetics on alkali halide (001) surfaces
Authors: Balik C. M. , Tripathy S. K. , Hopfinger A. J.
Published: April 2003

Molecular mechanics energy calculations have been used to assess the relative affinities that an iodobenzene solvent molecule and a segment of a polyoxymethylene chain have for five different alkali halide (001) surfaces. Dispersive‐repulsive and coulombic contributions to the interaction energy have been included. The calculated interaction deposition energy correlates well with the nucleation density of rodlike epitaxial crystals formed on these substrates in epitaxial crystallization experiments from solution. Substrates which have a higher preference for solvent (LiF and NaF) than for polymer exhibit relatively low polymer nucleation densities. As the degree of solvent preference decreases, and polymer preference increases, higher nucleation densities are observed (KCI and KI). These results suggest that preferential solvent adsorption may account for the low nucleation densities observed on some substrates in polymer epitaxial crystallization experiments from solution.
Stress relaxation in styrene‐butadiene‐styrene block copolymers containing unattached linear polybutadiene and styrene‐butadiene diblocks
Authors: Kamykowski Gregory W. , Ferry John D. , Fetters L. J.
Published: April 2003

Stress relaxation has been studied in networks of styrene‐butadiene‐styrene triblock copolymers with spherical styrene domain structure containing 0.10 weight fraction of unattached linear polybutadiene (Mw = 389,000) or styrene‐butadiene diblocks with very long butadiene segments (M = 225,000 or 510,000). The stretch ratio (uniaxial extension) was usually 1.15 and the temperature ranged from −20 to +20°C. The contribution of the linear polybutadiene species to relaxation was essentially the same in two triblock networks with very different butadiene block lengths, as expected if the configurational rearrangements are dominated by reptation. In the diblock‐triblock mixtures, in which the diblock butadiene segments are free at one end but anchored at the other and therefore incapable of reptation, there was no contribution to relaxation from the dangling butadiene segments of the diblock component; this would be expected if there are no relaxation mechanisms alternative to reptation for these very long semiattached species within the experimental time scale.
Crystallization kinetics of syndiotactic polypropylene
Authors: Miller Robert L. , Seeley Evelyn G.
Published: April 2003

Melting‐point and spherulite growth rate measurements for a sample of syndiotactic polypropylene (S = 0.716 and η = 0.356) were analyzed for the parameters characterizing crystal formation and growth: Tm = 159 ± 2°C, σe = 47 erg cm −2, σ = 4.4 erg cm−2, and q = 5.6 kcal per mole of folds. The q and σe values place syndiotactic polypropylene in the group of “unhindered” polymers. Failure of the isotactic‐polypropylene spherulite growth rate data to follow current theories of crystal growth precluded a comparison of crystal parameters of the two stereoisomers. At comparable degrees of supercooling, the absolute growth rates for the two forms are of the same order of magnitude and exhibit one or more crossover(s) in relative position.
Crystallization under stress: A theory of fibrillar crystallization
Authors: Smith K. J.
Published: April 2003

A theory of crystallization in stretched polymer networks is developed. In it, four principal features are incorporated: (i) crosslinks are displaced by growing crystallites, (ii) network chains are constrained to positions compatible with fixed sample shape and volume, (iii) some network chains remain amorphous, and (iv) the relative direction of a chain through a crystallite may not be the same for all chains. The derived network force exhibits a V or U shape with changing temperature in the crystallization zone that is a close replica of the behavior of gutta percha networks. Postulates of fibrillar‐lamellar transitions are not introduced into the calculations.