bluuebunny/crossref_metadata_2025_split huggingface dataset:
Chain folding in crystalline syndiotactic poly(vinyl chloride)
Authors: Conte G. , D'Ilario L. , Pavel N. V. , Giglio E. 
Published: April 2003

AbstractPotential‐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
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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

AbstractThe 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

AbstractTightly 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

AbstractPolystyrene 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

AbstractA 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

AbstractRecent 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

AbstractBy 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

AbstractThe 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

AbstractDielectric 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

AbstractA 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

AbstractThe 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

AbstractEricksen'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

AbstractPolarized 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

AbstractCertain 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

AbstractData 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

AbstractAnalytical 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

AbstractReplicas 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

AbstractThe 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

AbstractAdsorption 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

AbstractThe 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

AbstractThe 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.