bluuebunny/crossref_metadata_2025 huggingface dataset:
Miscibility and orientation behavior of poly(vinyl alcohol) / poly(vinyl pyrrolidone) blends
Authors: Nishio Yoshiyuki , Haratani Tatsuo , Takahashi Toshisada
Published: April 2003

The miscibility of poly(viny1 alcohol)/poly(vinyl pyrrolidone) (PVA/PVP) blends is investigated by differential scanning calorimetry (DSC) and wide‐angle x‐ray diffraction (WAXD). The molecular orientation induced by uniaxial stretching of the blends is also examined by WAXD and birefringence measurements. It is shown by the DSC thermal analysis that the polymer pair is miscible, since a single glass transition temperature (Tg) is situated between the Tgs of the two homopolymers at every composition. The Tg versus composition curve does not follow a monotonic function but exhibits a cusp point at a PVP volume fraction of a little under 0.7, as in a case predicted by Kovacs' theory. The presence of a specific intermolecular interaction between the two polymers is suggested by an observed systematic depression in the melting point of the PVA component. A negative value of the polymer‐polymer interaction parameter, χ12 = 0.35 (at 513 K), is estimated from a thermodynamic approach via a control experiment using samples crystallized isothermally at various temperatures. The extent of optical birefringence (Δn) of the drawn blends decreases drastically with increasing PVP content up to 80 wt %, when compared at a given draw ratio, and ultimately Δn is found to change from positive to negative at a critical PVP concentration of a little over 80 wt %. Discussion of the molecular orientation behavior takes into consideration a birefringence compensation effect in the miscible amorphous phase due to positive and negative contributions of oriented PVA and PVP, respectively.
Simultaneous scission and crosslinking of polystyrene chains on irradiation by ultraviolet light in CCl4 and CHCl3 solutions
Authors: Turzyński Zbigniew , Witkowski Konrad , Woliński Leszek , Szafko Jerzy
Published: April 2003

This work is the continuation of the investigations of changes in the structure of polystyrene initiated by radiation of wavelengths λ ≥ 270 nm in oxygen‐free solutions in CCl4 and CHCl3. Coupled integral and quasielastic light scattering techniques have been applied, enabling observations of changes in the molecular weight distribution function w(M). On the basis of the experimental data and application of Saito and Inokuti's theoretical results, it is shown that in the irradiated solutions polystyrene undergoes simultaneous chain scission and crosslinking. The average number of scissions and crosslinks per macromolecule as a function of irradiation time is calculated. The effect of macromolecular mobility on both photochemical processes is also discussed.
On the universality of sedimentation of flexible polymers in dilute and semidilute solutions
Authors: Nyström B. , Roots J.
Published: April 2003

In the present investigation, the sedimentation behavior, over an extended concentration range, in solutions of polystyrenes under good, marginal, and theta solvent conditions, is analyzed in the framework of a recent theoretical model, which takes into account the gradual screening of both hydrodynamic and excluded‐volume interactions in the semidilute regime. The model inspires the construction of universal plots of the form S/S0 versus ksc, where S is the sedimentation coefficient at polymer concentration c, S0 is that at infinite dilution, and ks is the concentration dependence coefficient. The resulting analytical expressions, without adjustable parameters, are consistent with experimental sedimentation data over the whole concentration range studied.
Polymerization of para‐xylylene derivatives. VI. morphology of parylene N and parylene C films investigated by gas transport characteristics
Authors: Yeh Y.‐S. , James W. J. , Yasuda H.
Published: April 2003

Polychloro‐p‐xylylene (Parylene C) and poly‐p‐xylylene (Parylene N) films were synthesized in vacuum with and without the presence of 42 mtorr of argon at various deposition temperatures and three different dimer sublimation rates. Depending on the synthesis conditions, the morphology of the films can vary from a homogeneous (nonporous) structure to a heterogeneous (porous) structure. The transport coefficients of the gases He, O2, N2, and CO2 through these films were measured at 25°C. The transport coefficients for both types of films vary with the deposition temperature and the dimer sublimation rate. The variation, however, cannot be solely explained by the change of crystallinity. Anomalous transport behavior is observed in the homogeneous, as‐synthesized polymers of relatively high crystalline content (above 20–30%). In many cases the permeabilities and diffusivities increase despite an increase in crystallinity. The effects of crystallization induced by isothermal and solvent annealing on the transport coefficients of polymers of Parylene C are different from those of Parylene N synthesized with or without argon. The mean pore size and effective porosity of the porous films were calculated from gas permeation data. For Parylene C and Parylene N porous films synthesized without argon, increasing the dimer sublimation rate or decreasing the deposition temperature increases the mean pore size but decreases the effective porosity. For Parylene N porous films synthesized in the presence of argon, increasing the dimer sublimation rate or decreasing the deposition temperature results in a decrease in the mean pore size but an increase in the effective porosity. Overall, no appreciable change in transport coefficients is observed upon addition of an inert gas.
Chemorheology relations for epoxy‐amine crosslinking
Authors: Bidstrup Sue Ann , Macosko Christopher W.
Published: April 2003

The goal of this study is to elucidate the relationship between the mechanism of bond formation, resulting structure, and the viscosity during the step polymerization of a thermoset. The system chosen for this work consists of a bifunctional epoxy, diglycidyl ether of bisphenol A, cured with a tetrafunctional amine, diaminodiphenyl sulfone. Steady shear and dynamic viscosity are measured as a function of extent of reaction throughout the epoxy‐amine cure. Branching theory is used to relate the extent of reaction (i.e., conversion of epoxy groups) to certain structural parameters, including the weight‐average molecular weight. The combination of the viscosity‐conversion data with the branching theory enables the correlation between changes in rheological properties and network structure during polymerization. The steady shear viscosity rise is modeled as the product of a structure‐dependent friction factor and weight‐average molecular weight. The structural significance of the point in cure when the dynamic storage and loss modulus are equal (i.e., tan δ = 1) is also explored.
Bubble nucleation in polymeric liquids. II. theoretical considerations
Authors: Han James H. , Dae Han Chang
Published: April 2003

A theory based on classical nucleation theory is developed for bubble nucleation in polymer solutions. The theory requires information on solubility, diffusivity, concentration, surface tension, temperature, and degree of supersaturation. The effects of supersaturation and of the presence of large molecules in a liquid mixture on the free energy of bubble formation are included in the theoretical development. A semiempirical equation for the determination of bubble nucleation rate is developed, with the aid of experimental results reported in part I of this series. Using the experimental data, computer simulations of bubble nucleation in polymer solutions are performed. The consumptions of the volatile component in a liquid mixture, due to bubble nucleation and subsequent growth, and the variation of bubble nucleation rate during the expansion process are included in the simulation of the bubble nucleation process.
Crystallization mechanisms for LLDPE and its fractions
Authors: Wilfong Debra L. , Knight G. W.
Published: April 2003

The crystallization behavior of two ethylene/octene copolymers, which differ in hexyl branch concentration, and their fractions were assessed. Fractionation of the crystalline linear low density polyethylenes (LLDPEs) was achieved by temperature rising elution fractionation. As the column temperature was raised, the eluted fractions exhibited a reduction in branch concentration and an increase in molecular weight. This was attributed to the difference in reactivity between ethylene and octene and the subsequent depletion of the ethylene monomer in the solution process. Spherulites formed during the crystallization of the whole polymers were well developed, banded, and displayed a wide distribution of sizes. However, spherulites of the LLDPE fractions were less well developed, more uniform in size, and tended to progressively deteriorate and become smaller as the concentration of branches increased. The ethylene and octene blocks of the copolymer crystallized independently, and it was proposed that the octene portion formed short, curved lamellae in the interfacial region of the lamellae formed from the linear ethylene portion of the molecule. Decreases in d spacing for fractions with increased short chain branching corresponded with similar drops in molecular weight.
Solution and thermal properties of poly(1,3‐dioxocane)
Authors: Alamo Rufina G. , Bello Antonio , Fatou José G. , Obrador Carmen
Published: April 2003

Poly( 1,3‐dioxocane) was synthesized by cationic ring‐opening polymerization with triphenyl‐methane hexafluoroantimoniate as the initiator and was studied with regard to its solubility, unperturbed chain dimensions, and thermal transitions. The intrinsic viscosity and Flory‐Huggins interaction parameter were used to determine the solubility parameter, δp = 9.6 cal1/2cm−3/2, a value that agrees with that calculated empirically. Fractions were obtained from the solvent/non‐solvent system benzene/methanol at 25°C. The number‐average molecular weight Mn and intrinsic viscosity [η] were measured in toluene at 25°C. The relation [η] = 1.459. 10−4 Mn0.79 was found. A value of 5.3 was obtained for the characteristic ratio 〈r2〉0/nl2. Results are correlated with the main thermal transitions of this polyformal.
Intrinsic crazing and anisotropy of maximum recoverable extension in oriented glassy polycarbonate
Authors: Heymans Nicole
Published: April 2003

The critical draw ratio for initiation of intrinsic crazing in polycarbonate below the glass transition temperature has been determined after preorientation in the melt. Even when referred to the isotropic state, this ratio is found to depend not only on the extent of preorientation but also to be distinctly anisotropic. Under the assumption that intrinsic crazing occurs at the maximum extension of the entanglement network, these results are incompatible with a strain dependent but homogeneous breakdown of the entanglement network, but can be interpreted in terms of departure from affine transformation of the radius of gyration of an entangled length.
On the theory of the effective nuclear magnetic relaxation time T2e and its application to polymer melts
Authors: Schneider H. , Hiller W.
Published: April 2003

A general expression for the magnetization decay of a multipulse group is derived. This formula is applied to a three‐component model of molecular motions in polymer melts. The influence of the several components on the magnetization decay is discussed. The relation of the effective nuclear magnetic relaxation time T2e to the Anderson‐Weiss formula is also shown, and an analytical expression for the transverse relaxation in melts is derived. Finally T2e is compared with the relaxation time in the rotating frame T1ρ in the melt. The theoretical results for T2e are tested with measurements of frequency dependence in polyethylene melts.
Influence of migration of plasticizer during melt mixing on the transitions and properties of PC/CAB blends
Authors: Jurado M. J. , Gaztelumendi M. , Nazabal J. , Mondragon I.
Published: April 2003

Static and dynamic mechanical properties, morphology, and thermal behavior of polycarbonate (PC)/plasticized cellulose acetobutyrate (CAB) blends were investigated to determine whether the plasticizer of the CAB modifies the miscibility of the blend and the mechanical properties of this essentially incompatible blend. In spite of the lack of transparency of the blends, both dynamic mechanical and thermal analysis results show the presence, at all blend compositions studied, of a single glass transition temperature which varies with the composition of the blend. Considering the ternary nature of the blends, we propose that plasticizer migration and the difficulty of discerning the presence of one or two peaks in a narrow temperature range may account for the observed behavior. Scanning electron microscopy confirms the immiscibility of the blends. The blends show large positive deviations of the tensile moduli from linearity and very low ductility. The reported tensile strength data are discussed in terms of several different equations for composites. This mechanical behavior is explained as a consequence of the migration of the plasticizer and of its subsequent antiplasticizing effect on the properties of the blend.
A temperature window of extrudability and reduced flow resistance in high‐molecular weight polyethylene; interpretation in terms of flow‐induced mobile hexagonal phase
Authors: Waddon A. J. , Keller A.
Published: April 2003

An unexpected feature of melt flow behavior has been identified in high‐molecular weight (≥ 4.105) polyethylenes such as are being considered unprocessable at conventional temperatures (> 160°C) and at practicable extrusion rates (> 1 cm/min). In addition to a lower temperature window of smooth extrudability, lying in the range of 138–155°C already observed in previous works, we now discovered within this window a narrow temperature interval (150–152°C) of minimum flow resistance (viscosity). The new effect has the attributes of being associated with a new phase of increased fluidity. This, in turn, we attribute to a transient mesophase arising through the chain‐orienting effect of the elongational flow within the extrusion orifice; from the experiments presented here, this mesophase depends critically on both molecular weight and strain rate. The hexagonal form of polyethylene, known to exist under other circumstances, is suggested as this mesophase. The relevance of the new findings for applications (extrusion, melt rheology) and for fundamentals (orientation‐induced phase transformations, liquid crystals from flexible chains in particular) should be obvious and are accordingly highlighted.
Local order in a globally deformed polymer network with nematiclike interactions
Authors: Walasek Janusz
Published: April 2003

The influence of nematiclike interactions, that is, preference for the alignment of neighboring chain segments, is considered for a globally deformed polymer network. The local order is characterized by an orientation factor which is the average of second Legendre polynomial, being a function of segment orientations, calculated with the actual distribution of chain segment orientations. For the description of interactions between segments, a molecular mean‐field approach is used. The orientation‐dependent potential is assumed proportional to cos2θ, where θ is the angle between the direction of the orienting field and the chain segment vector. The local orientation factor of deformed polymer network, is calculated for various values of interaction strength and network structural parameters (contraction coefficent, length of network chains). For some values of these parameters, the local order can be considerable.
Dilation of substituted polycarbonates caused by high‐pressure carbon dioxide sorption
Authors: Fleming G. K. , Koros W. J.
Published: April 2003

Mass sorption/desorption and volume dilation/consolidation measurements are reported for tetramethyl and hexafluoro polycarbonate samples in the presence of carbon dioxide pressures up to 900 psia at 35°C. These results are combined with previous results for bisphenol‐A‐polycarbonate. The combined sorption and volume dilation measurements are used to analyze the thermodynamics of the three gas‐polymer systems, and the partial specific volumes of carbon dioxide and the polymer are reported as functions of sorption level. Desorption and subsequent resorption measurements with the substituted polycarbonates show large and long‐lived hysteretic effects similar to those seen with standard bisphenol‐A‐polycarbonate. Finally, assumptions inherent in the dual‐mode model are shown to provide a satisfactory description of the volume dilation behavior for the substituted polymers.
Phase transformation and structure of n‐C50H102/n‐C60H122 solid solutions formed from the vapor phase
Authors: Zhang W. P. , Dorset D. L.
Published: April 2003

Binary solid solutions of n‐paraffins (n‐C50H102/n‐C60H122) epitaxially prepared on potassium hydrogen phthalate substrate from the vapor phase have been studied by electron diffraction to characterize their phase transitions and structure. The continuity of solid solution in the n‐C50H102/n‐C60H122 system is demonstrated once lamellar ordering of the crystal packing is achieved. However, such ordering is achieved only by annealing and proceeds through a series of intermediate chain packings. At first, the electron diffraction patterns from all samples prepared at room temperature resemble those from polyethylene, in which no spots corresponding to the interlayer spacings appear. Longitudinal chain translations are induced by annealing to cause the lamellar reflections to appear, while the “polyethylene” subcell reflections remain unsplit until the crystal structure with well‐defined methyl end planes is reached.
High‐temperature, high‐resolution nuclear magnetic resonance of poly (p‐phenylene sulfide)
Authors: Wade Bruce , Abhiraman A. S. , Wharry Stephen , Sutherlin Dirk
Published: April 2003

A high‐temperature, high‐resolution 13C nuclear magnetic resonance spectroscopy technique was developed for the analysis of poly (p‐phenylene sulfide) (HT/HR NMR of PPS). This technique can be applied to the identification and quantitative analysis of end groups and polymer structure in high‐temperature polymers where solution temperatures above 200°C are required for analysis. Verification of calculated 13C NMR shift values of chloro‐terminated and hydrogen‐terminated end groups was made by HT/HR NMR of two oligo (p‐phenylene sulfide) model compounds. Identification of the chlorine end group was made in high‐molecular weight PPS. On the high‐molecular weight PPS, identification and quantitative analysis of amino and N‐alkylamino end groups were possible only after derivatization of the polymer with 13C‐enriched benzoyl chloride.
Solid‐state phase behavior and molecular‐level mixing phenomena in a strongly interacting polymer blend
Authors: Belfiore Laurence A. , Lutz Thomas J. , Cheng Chihmin , Bronnimann Charles E.
Published: April 2003

This research contribution addresses mixing phenomena in a polymer blend that exhibits strong intermolecular association and bieutectic phase behavior. Molecular‐level observations of specific interactions between dissimilar blend components have been obtained from high‐resolution solid‐state proton and carbon‐13 nuclear magnetic resonance (NMR) experiments at ambient temperature. Results illustrate mixing effects on the isotropic chemical shifts of the critical component in a completely or partially phase‐mixed blend. Perturbations in the NMR spectra result from conformational changes, hydrogen bonding, molecular complexation, or altered packing geometries that occur concomitantly with the mixing process. More convincing evidence that two components of a strongly interacting blend reside in a near‐neighbor environment is obtained from the measurement of proton spin diffusion between dissimilar species. Proton spin diffusion is measured directly via the high‐resolution CRAMPS experiment (Combined Rotation and Multiple Pulse Spectroscopy) in a molecular complex of poly (ethylene oxide) and resorcinol. A primary objective of this research endeavor is to bridge the gap between macroscopic and molecular‐level probes of phase behavior and intermolecular association in mixtures that form molecular complexes. In this respect, the temperature ‐composition projection of the thermodynamic phase diagram is generated for binary mixtures of poly (ethylene oxide) and resorcinol, whose interaction sites are characterized via solid‐state NMR. Under fortuitous conditions that are related to the overall mixture composition, two morphologically and crystallographically inequivalent phenolic 13C NMR signals are identified for resorcinol when the blends exist in a two‐phase region below the eutectic solidification temperature. The success of this proposed structure–property relationship scheme, which bridges molecular‐level mixing phenomena (via NMR) with solid‐state phase behavior (via differential scanning calorimetry) depends on our ability to understand material properties at a level where continuum hypotheses are no longer valid.
Study of segmental orientation in poly (vinyl chloride)/poly (α‐methyl‐α‐n‐propyl‐β‐propiolactone) blends by fourier‐transform infrared spectroscopy
Authors: Chabot Paul , Prud'Homme Robert E. , Pézolet Michel
Published: April 2003

Fourier‐transform infrared (FTIR) spectroscopy has been used to investigate the segmental orientation of poly(vinyl chloride) (PVC)/poly (α‐methyl‐α‐n‐propyl‐β‐propiolactone) (PMPPL) blends in uniaxially stretched samples over a wide range of compositions and draw ratios. The results indicate that for pure PVC, syndiotactic segments reach a higher degree of orientation than isotactic segments and gauche conformations. Similarly, for pure PMPPL, crystalline segments orient more than amorphous segments at any given elongation. Thus, for both polymers, the higher orientation is obtained for the more rigid segments or those located in a more rigid (crystalline) phase. The addition of the macromolecular plasticizer PMPPL has no effect on the orientation of PVC syndiotactic segments, but it lowers the orientation of PVC gauche conformations, suggesting that the polyester is located in the amorphous regions of PVC. Finally, the PMPPL orientation function initially decreases with the addition of PVC and thereafter remains constant. The results are discussed in terms of interpenetrating networks and relaxation processes.
Mass transfer modeling of asymmetric membrane formation by phase inversion
Authors: Tsay C. S. , Mchugh A. J.
Published: April 2003

A derivation is presented of a ternary diffusion model to describe the mass transfer processes associated with the quench bath period of the phase inversion process for membrane formation. The complete governing equations, initial conditions, and boundary conditions in the casting film and coagulation bath are presented. Equations for ternary chemical potentials and diffusion coefficients are consistently based on constant specific volume formulations. The model is applied to the analysis of mass transfer paths and their effects on membrane structure formation. Precipitation times are determined for given sets of conditions by superposing calculated mass transfer paths on the ternary phase diagram and observing when the miscibility gap is crossed. Comparisons are made with an earlier reported study on the membrane‐forming system: water‐acetone‐cellulose acetate (CA). Agreement between predicted and measured precipitation times is found to be excellent. The polymer film composition profile at the moment of precipitation is shown to be a useful indicator of both skin and sublayer structures, allowing distinctions to be made between conditions leading to spongelike and fingerlike morphologies. The influence of model parameters on the mass transfer paths and associated polymer profiles is also discussed.
Star polymer diffusion in concentrated solutions: Dependence on functionality via the coupling model
Authors: Ngai K. L. , Rajagopal A. K. , Lodge T. P.
Published: April 2003

The coupling model is formulated in terms of the dynamic constraint entropy, Sc(t), and subsequently compared with measured tracer diffusion coefficients D for linear, 3‐arm star, and 12arm star polystyrenes in solutions of poly (vinyl methyl ether) (PVME) /o‐fluorotoluene. Inequalities between the dynamic constraint entropies, Scf and Scf′(t), respectively, for f‐arm and f′‐arm stars, enable us to explain the experimental observation that at constant arm molecular weight, the ratio Dstar/Dlinear decreases substantially with increasing concentration above the entanglement concentration for the PVME. This work indicates that not only the reptation model, but also the coupling model, can account for the experimental observation that in entangled solutions the mechanism for diffusion depends on diffusant architecture.
Compatibility studies of blends of polycarbonate and poly(ethylene terephthalate)
Authors: Kim Woo Nyon , Burns Charles M.
Published: April 2003

Blends of bisphenol‐A polyarbonate (PC) and poly(ethylene terephthalate) (PET) has been investigated by differential scanning calorimetry and scanning electron microscopy. Blends were prepared by screw extrusion and solution casting with weight fractions of PC in the blends varying from 0.90 to 0.10. From the measured glass transition temperature (Tg) and apparent weight fractions of PC and PET dissolved in each phase, it appears that PET dissolves more in the PC‐rich phase than does the PC in the PET‐rich phase. The composition‐dependent values of the Flory–Huggins polymer–polymer–interaction parameter were determined and found to be from 0.054 to 0.037 for extruded blends at 275°C and from 0.058 to 0.040 for solution casting at 25°C. The interaction parameter decreases with increasing PET concentration. This result is consistent with the values of the Tgs, the microscopy study, and the measured extrudate swell ratios which show that compatibility increases more in the PET‐rich compositions than in the PC‐rich compositions. The PC–PET blends are not microscopically miscible for all the blend compositions.
Influence of polymer composition on the thermal characteristics of sulfonated EPDM ionomers
Authors: Maurer J. J. , Harvey G. D. , Agarwal P. K.
Published: April 2003

Detailed thermal characterization of sulfo‐EPDM ionomers was carried out. The endotherms observed via differential scanning calorimetry (DSC) and multiple inflections observed via thermal mechanical analysis (TMA) are speculated to be due to associated ionic species in the polymers. A large time dependence of the DSC endotherm was observed, suggesting substantial variations in the nature of the ionic species over this period. The influence of ionomer composition on thermal characteristics was probed with whole polymers and polymer fractions. In general, trends in the DSC and TMA data paralleled those expected on the basis of the anticipated effect of polymer sulfonation level, molecular weight, and counterion type on the strength of the physical crosslinking of these polymers. Both the DSC endotherms and the TMA inflection regions are rationalized in terms of a previously developed ionic‐bond exchange model.
Influence of initial polymer concentration in solution and weight‐average molecular weight on the drawing behavior of polyethylenes
Authors: Bastiaansen C. W. M.
Published: April 2003

The influence of initial polymer concentration in solution (c), weight‐average molecular weight (Mω), and drawing temperature on the solid‐state drawing behavior of linear polyethylenes was investigated. Optimum conditions, with respect to maximum attainable draw ratio, are observed in isothermal drawing experiments. Moreover, it is shown that high maximum attainable draw ratios can also be obtained upon multistage drawing of UHMW‐PE (ultrahigh‐molecular‐weight polyethylene, Mω > 106 g/mol) gel films cast from concentrated solutions. The high maximum attainable draw ratio in combination with the high molecular weight (Mω > 106 g/mol) and polymer concentration (c = 10% w/v) is of particular interest because it results in tapes or fibers with a high Young's modulus (100 GPa) and tensile strength (2.5–3.5 GPa). It is also shown that the maximum attainable draw ratio of polyethylenes scales with the Bueche parameter (c · Mω) to the −0.5 power. This experimental observation indicates that intermolecular interactions not only dominate the rheological properties of polyethylene melts and concentrated solutions, but also strongly influence the solid‐state drawing behavior of linear polyethylenes.
A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers
Authors: Stutz H. , Illers K.‐H. , Mertes J.
Published: April 2003

A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers has been developed, which takes into account the influences of end groups, branching, and crosslinking, and their functionality distribution. DiBenedetto's theory was found to correctly characterize the influence of crosslinks on the glass temperature. Normalized to constant crosslink functionality, the crosslink constant is a universal parameter suggesting that the entropic theory of glasses is applicable to crosslinked systems. Data on linear polymers and networks from the crosslinking of polymer chains, vinyl/divinyl‐copolymers and step‐growth polymers, such as polyurethanes, amine‐cured epoxies, or inorganic glasses, are presented.
Rheological and thermal properties of a commercial liquid‐crystalline polyesteramide
Authors: Gonzalez J. M. , Muñoz M. E. , Cortazar M. , Santamaría A. , Peña J. J.
Published: April 2003

A commercial main‐chain liquid‐crystalline, naphthalene‐based polyesteramide, was studied by three experimental techniques: extrusion capillary rheometry, dynamic viscoelasticity, and differential scanning calorimetry (DSC). From capillary rheometry a maximum at 360°C was observed in viscosity temperature curve. This result is compared with literature data for other thermotropics, and the possibility of a transition from a nematic to an isotropic phase is considered. The results obtained from dynamic viscoelasticity and DSC agree, and reveal the existence of a glass transition at 128°C (by DSC) and 137–147°C (by viscoelastic measurements, depending on frequency) as well as melting at 282°C. Annealing below 230°C produces two types of crystals, whereas annealing above this temperature gives rise to only one type of crystal, the melting temperature of which can be, depending on annealing time, as high as 340°C. The results are compiled in a phase diagram with six regions, four of them corresponding to the solid state, one to a liquid mesophase, and one to an isotropic phase.
Optical anisotropy of a thermotropic liquid‐crystalline polymer in transient shear
Authors: Hsiao Benjamin S. , Stein Richard S. , Deutscher Klaus , Winter H. Henning
Published: April 2003

A rheo‐optical apparatus, based on a linear shear rheometer, has been constructed to study the deformation of liquid–crystalline polymers. This apparatus uses optical techniques such as flow birefringence, small‐angle light scattering, and optical microscopic image analysis. The rheological responses were simultaneously measured under varying temperatures and deformation conditions. The modified Debye‐Bueche equation for scattering, in the nonspherically symmetrical form, was adapted to analyze small‐angle light‐scattering data. The orientation correlation lengths, determined by this method, reveal the deformation mechanism in nematic melts. Flow birefringence results are in agreement with the proposed mechanism.
Changes in crystal structure parameters and thermal mechanical properties of poly(p‐phenylene terephthalamide) fibers under different annealing conditions
Authors: Wu Zongquan , Zhang Anqiu , Cheng Stephen Z. D. , Huang Bing , Qian Baojun
Published: April 2003

As‐spun poly(p‐phenylene terephthalamide) (PPTA) fibers have been annealed by three different methods: (1) the relaxation mode; (2) the constant temperature mode; and (3) the constant tension mode. Wide‐angle x‐ray diffraction (WAXD) was used with all three modes to characterize the changes in three crystal structure parameters: the degree of crystal orientation, the apparent crystal size, and the lattice strain along the c‐axis of the crystal unit cell. It was found that internal stresses in the fibers, which are frozen‐in during either the fiber spinning process or in later annealing, is the key factor causing these structure parameter changes. Our thermal shrinkage stress measurements may reasonably represent the fiber's internal stresses. The thermal shrinkage modulus and the tensile modulus of the PPTA fibers under these annealing modes have been determined. Based on a serial model, quantitative structure–property relationships can be established. The effects of annealing temperature and applied tension on both the microscopic crystal structures and the macroscopic properties are also discussed.
Probe diffusion in poly (vinyl acetate) / toluene solutions
Authors: Lodge T. P. , Lee J. A. , Frick T. S.
Published: April 2003

The technique of forced Rayleigh scattering has been used to examine the tracer diffusion of probe photochromic dyes, azobenzene and methyl red, in poly (vinyl acetate)/ toluene solutions, as a function of polymer concentration (0–96% by weight) and temperature (5–45°C). It is demonstrated that the dye diffusivity can be quantitatively scaled to the solvent diffusivity, thus permitting direct application of predictions for two‐component systems. The results have been interpreted using the free volume theories of Fujita and Vrentas and Duda. While both approaches are able to describe the concentration dependence extremely well, the Vrentas‐Duda expression has the virtue of being employed without any freely adjustable parameters. However, neither theory is particularly successful in predicting the observed temperature dependence.
Probe diffusion in polystyrene/toluene solutions
Authors: Frick T. S. , Huang W. J. , Tirrell M. , Lodge T. P.
Published: April 2003

The technique of forced Rayleigh scattering has been used to measure the tracer diffusion coefficient, Dt, for two photochromic dyes, azobenzene and Aberchrome 540, in polystyrene / toluene solutions, as a function of polymer concentration (79–90% by weight) and temperature (−15–+60°C). The dye diffusivity can be scaled quantitatively to the solvent diffusivity, permitting direct application of the Vrentas‐Duda free volume predictions for binary systems. However, the scaling factors are not in agreement with the model assumptions. The temperature and concentration dependence of Dt above the solution glass transition temperature, Tgs, is in good agreement with the model, without any freely adjustable parameters. The value of Dt at Tgs is a very strong function of concentration, increasing by a factor of 103 as the polymer concentration is reduced from 90 to 80%; this is also in quantitative agreement with the model. It is demonstrated that the temperature dependence of Dt exhibits a distinct break in the vicinity of Tgs, with both the existence and the character of the break in qualitative agreement with the extension of the Vrentas‐Duda approach to glassy solutions. However, a fundamental difficulty in the calculation of the polymer contribution to the solution free volume at temperatures well below the bulk polymer Tg is exposed.
Synthesis and characterization of polydimethylsiloxane polyurea‐urethanes and related zwitterionomers
Authors: Yang C. Z. , Li C. , Cooper S. L.
Published: April 2003

A series of segmented polyurea urethane and polyurea block copolymers based on a hexane diisocyanate (HDI) modified aminopropyl terminated polydimethylsiloxane soft segment was synthesized. The hard segments consisted of 4,4′‐methylene diphenylene diisocyanate (MDI) which was chain extended with 1,4‐butanediol (BD), N‐methyldiethanolamine (MDEA), or ethylene diamine. Zwitterionomers were prepared by quaternizing the tertiary amine of the MDEA extended material with γ‐propane sultone. The effect of chemical structure on the extent of phase separation and physical properties was studied using a variety of techniques including thermal analysis, dynamic mechanical spectroscopy, tensile testing, and small‐angle x‐ray scattering. It was observed that the compatibility between the nonpolar polydimethylsiloxane soft segments and the polar urethane hard segments was improved by inserting HDI linkages into the polydimethylsiloxane soft segments. The aggregation of hard segments was enhanced by increasing hard‐segment content or by the introduction of ionic functionality. The tensile strength and modulus of these materials was higher than those of polyurethanes containing soft segments based on polydimethylsiloxane and its derivatives.
Fiber formation via solution spinning of the cellulose/ammonia/ammonium thiocyanate system
Authors: Liu Cheng‐kung , Cuculo John A. , Allen Thomas C. , Degroot A. Willem
Published: April 2003

Fiber formation via the cellulose/ammonia/ammonium thiocyanate system by wet spinning has been investigated. This report presents a characterization of the structure and tensile properties of fibers spun under various coagulation conditions. Microscopic observations showed that the molecular size of coagulant was the dominant factor governing the crosssectional shape of the fibers. Density, birefringence, and crystallinity data indicated that a higher cellulose concentration and lower coagulation temperature favored development of a fiber with a denser and more oriented structure. Under optimum conditions, a welldefined fibrillar structure was obtained. Fiber tensile property measurements suggested the existence of a linear relationship between the fiber breaking tenacity and the product of the square of the Hermans' orientation factor and the infrared crystallinity index.
Complexation thermodynamics of free and graft oligomers with complementary polymers
Authors: Scranton Alec B. , Klier John , Peppas Nikolaos A.
Published: April 2003

The dilute solution complexation equilibrium between linear macromolecules and smaller complementary oligomers is considered when: (1) the oligomers are free in solution; and (2) the oligomers are covalently attached at one end to the polymer. A general statistical mechanical framework is developed and is illustrated using a simple random walk model for polymer conformation. The statistical mechanical partition functions are formulated using a generating function technique, allowing thermodynamic averages in the complexed state to be calculated. Loops, trains, and tails of all possible length are allowed in the conformation of a complexed oligomer. Simulation results for the free oligomer case are compared with those obtained for oligomers covalently attached to the polymeric molecular. The model provides a theoretical explanation for the experimentally observed enhancement of complexation of oligomers grafted to the complementary polymers.
Argon sorption and partial molar volume in poly(ethyl methacrylate) above and below the glass transition temperature
Authors: Kamiya Yoshinori , Mizoguchi Keishin , Naito Yasutoshi , Bourbon Dominique
Published: April 2003

Sorption and dilation isotherms for argon in poly(ethyl methacrylate) (PEMA) are reported for pressures up to 50 atm over the temperature range 5–85°C. At temperatures below the glass transition (Tg=61°C), sorption isotherms are well described by the dual‐mode sorption model; and isotherms above Tg follow Henry's law. However, isotherms for dilation due to sorption are linear in pressure at all temperatures over the range investigated. Partial molar volumes of Ar in PEMA are obtained from these isotherms. The volumes are approximately constant above Tg (about 40 cm3/mol), whereas the volumes below Tg are smaller and dependent on both temperature and concentration (19–26 cm3/mol). By analyzing the experimental data according to the dual‐mode sorption and dilation model, the volume occupied by a dissolved Ar molecule and the mean size of microvoid in the glass are estimated to be 67 129 Å3, respectively. The cohesive energy density of the polymer is also estimated as 61 cal/cm3 from the temperature dependence of the dual‐mode parameters.
Structure‐property relationships in poly(n‐vinyl pyrrolidone)‐phenoxy‐water gels
Authors: Tsang Cynthia J. , Clark John C. , Wellinghoff Stephen T. , Miller Wilmer G.
Published: April 2003

Miscible 80/20 and 90/10 by weight blends of poly (n‐vinyl pyrrolidone) [PNVP] and phenoxy polymer were swollen with 73–85% water to produce rubbery hydrogels with shear moduli as high as 103dyne/cm2. Small‐angle x‐ray scattering, rheological, and calorimetric measurements showed that the gel consisted of tie chains, most probably of PNVP embedded in glassy phenoxy particles with radius of gyration 50–200 Å. A ternary phase diagram was calculated assuming athermal mixing between PNVP–H2O and PNVP and the known endothermic interaction between H2O–phenoxy. Phase separation into a phenoxy phase containing minimal H2O and PNVP and a water‐swollen PNVP phase was predicted in accordance with experimental results. Phase separation was suspected as forming first an interconnected phenoxy phase which was later dispersed by swelling forces transmitted through PNVP tie chains. The presence of localized internal stress in the swollen network was thought to be responsible for the irreversible decrease in the shear moduli observed at shear strain above 10%.
Effects of blending on the polymorphic behavior of melt‐crystallized syndiotactic polystyrene
Authors: Guerra Gaetano , De Rosa Claudio , Vitagliano Vincenzo M. , Petraccone Vittorio , Corradini Paolo
Published: April 2003

The complex polymorphic behavior of syndiotactic polystyrene (s‐PS) in melt‐crystallized samples is altered by blending with poly (2,6‐dimethyl‐1,4‐diphenylene oxide) (PPO). In particular, to render the beta form in these blends, starting with samples containing the α or γ froms, requires much lower temperatures and shorter melting times than for pure s‐PS. On the basis of the results, it is suggested that this phenomenon is due to more rapid loss of the memory of the α form, for the same temperature and time, in the melt in the presence of PPO molecules.
The solubility of Carbon Dioxide and Methane in polyimides at elevated pressures
Authors: Hirose T. , Mi Y. , Stern S. A. , St. Clair Anne K.
Published: April 2003

The solubility of CO2 and CH4 in five polyimides was measured at 35.0°C and at pressures up to 10 atm (147 psia). The concentration of the penetrant gases dissolved in the polymers can be represented satisfactorily as a function of penetrant pressure by the “dual‐mode sorption” model. The solubility coefficients for CO2 and CH4, S(CO2) and S(CH4), increase in the polyimide order:

The magnitude of the solubility coefficients appears to depend primarily on the intermolecular forces between the penetrant gases and the polymers. The values of these coefficients are greater for the polyimides with larger mean interchain spacings, but no one‐to‐one correspondence appears to exist in this respect. The lower solubility of CO2 in PMDA‐4,4'‐m‐APPS compared with that in the 6FDA polyimides may be due to the lower “excess” free volume of the former polymer. The ratio S (CO2)/S (CH4) varies relatively little for a variety of PMDA and 6FDA polyimides.
Biaxial extension measurements on bimodal elastomeric networks
Authors: Xu P. , Mark J. E.
Published: April 2003

Bimodal elastomeric networks were prepared by tetrafunctionally endlinking mixtures of short and long hydroxyl‐terminated poly (dimethylsiloxane) chains having number‐average molecular weights of 500 and 18,000 g/mol−1, respectively, over a composition range of 0–98 mol % of the short chains. Biaxial extension (compression) measurements were carried out by inflation of circular sheets of these materials at 23°C. The unimodal network (0 mol % short chains) showed the same behavior reported by other workers for noncrystal‐lizable networks: as the compression increased, the reduced stress or modulus [f*] went through a rather slight maximum followed by a gradual leveling off to the rupture point. The bimodal networks, however, showed much more pronounced maxima with another, significant increase in [f*] at very high compressions. This final increase is presumably due to non‐Gaussian effects from the very limited extensibility of the short chains, and thus parallels the upturns in [f*] frequently reported for bimodal networks at very high elongations.
ESR and electrical conduction in pyrolyzed polyimides
Authors: Noro Sumiko , Yamadaya Tokio
Published: April 2003

Electrical conductivity and electron spin resonance (ESR) of pyrolyzed polyimides change drastically in air after passage of a characteristic time, which depends on the temperature and time of pyrolysis. Samples must be kept from air throughout the measurements. ESR and resistivity measurements in vacuum show that there are two types of magnetic species: localized and delocalized. A. variable‐range hopping model is proposed as a possible mechanism for conduction.
Polyurethane network structure from tensile tests
Authors: Pegoraro M. , Di Landro L. , Severini F. , Cao N. , Donzelli P.
Published: April 2003

The mechanical behavior of polyurethane networks based on polyether (PE‐PU) and polyester (PES‐PU) diols is studies in light of recent molecular theories of rubber elasticity. The relationship between reduced stress (or modulus), extension ratio α, and network structure is discussed. In the range of low extensions, the deformation behavior of PESPU appears to be more affinelike than PE‐PU networks. When crosslinking agents with higher functionality are used, behavior closer to the affine limit is observed. Data in the low deformation range (α → 1) allow the estimation of network molecular weights predicted by the limiting phantom and affine networks. However, an exact determination of the true chain length cannot be obtained. The uncertainty in the molecular weight is due to the uncertainty in the theoretical structural parameter A'ϕ, which can assume values 1–2/ϕ ≤ A'ϕ ≤ 1. The range of possible network chain molecular weights can be narrowed when the real network, which always presents defects, is formed starting from building blocks of known molecular weight Ms, as in our case. In these real cases the molecular weight is between the value Ma Predicted by the affine model and the stoichiometric value Ms.
Isothermal heat treatment of a thermotropic LCP fiber
Authors: Sarlin Juha , Törmälä Pertti
Published: April 2003

The effect of the heat treatment on as‐spun fibers made of a commercial liquid crystalline polymer, Vectra®, was examined through tensile properties and differential scanning calorimetry (DSC) measurements. The heat treatment increased the fiber strength considerably, and it also increased the elongation at break. On the other hand, the treatment had a weaker effect on the modulus. In general, the effects of treatment depended on the temperature and on the treatment time used. DSC measurements showed the effect of the treatment on the behavior of the melting endotherm. In light of the development of this endotherm, it seems possible that the structure formed during treatment depends essentially on the treatment temperature. On the whole, an unambiguous correlation between tensile properties and calorimetric quantities could not be concluded.
Miscibility of solution‐chlorinated polyethylene with poly(α‐methylstyrene‐co‐acrylonitrile)
Authors: Cowie John M. G. , Elexpuru Elisa M. , McEwen Iain J.
Published: April 2003

Statistical copolymers of α‐methylstyrene and acrylonitrile (MSAN) form single‐phase blends with solution‐chlorinated polyethylene within a restricted composition range, even though no combination of the corresponding homopolymers is miscible. The phase behavior can be successfully described using the general mean‐field expression which predicts miscibility on the basis of the intramolecular interactions of the comonomers. By deriving the absolute value of one of the six system interactions from a subsidiary determination of the miscibility limits of poly (methylmethacrylate) in MSAN, values of all the remaining interaction parameters have been determined using a novel phase boundary‐fitting technique.