bluuebunny/crossref_metadata_2025_split huggingface dataset:
Solute and penetrant diffusion in swellable polymers. II. Verification of theoretical models
Authors: Korsmeyer Richard W. , Von Meerwall Ernst , Peppas Nikolaos A. 
Published: April 2003

AbstractA mathematical model developed to describe diffusion of a penetrant and a solute in a swellable polymer slab was applied to the case of a hydrophilic polymer loaded with a soluble drug in which the penetrant (water) is sorbed and solute (drug) is desorbed. An exponential dependence of the penetrant and solute diffusion coefficients on penetrant concentration was chosen and shown to be adequate for description of the systems studied. Experimental verification of the model was conducted by using copolymers of 2‐hydroxyethyl methacrylate (HEMA) and N‐vinyl‐2‐pyrrolidone (NVP). The monomers were bulk polymerized with benzoyl peroxide initiator and cut into thin disks. Monomer mole fractions of HEMA in the copolymers were 0.707, 0.446, and 0.211. Swelling behavior of the samples was observed in water at 37 and 0°C. Solute‐containing samples were prepared and solute release from these samples into water was followed by monitoring the UV absorption of the release medium. The concentration dependence of the diffusivity of water and two model solutes, sodium trifluoroacetate and sodium heptafluorobutyrate, in the gels was studied by using the pulsed‐gradient spin‐echo NMR technique. The diffusivities measured by this technique followed the concentration dependence predicted by the free‐volume theory. The simple exponential dependence used in the model was an adequate approximation of this behavior in the case of a transient diffusion experiment.
Formation of band textures in hydroxypropylcellulose liquid crystals
Authors: Navard P. 
Published: April 2003

AbstractUpon shearing, some polymeric liquid crystals develop a particular texture, called “band texture,” consisting of fine equidistant black lines as viewed between crossed polarizers. In mesomorphic hydroxypropylcellulose–water solutions, these bands are due to relaxation of the shear. The band texture appears after shearing at low shear rates and seems to exist during shearing at high shear rates (> 200 S−1). In the latter case, the steady‐state first normal‐stress difference is negative. The bands relax in a few minutes and parts of the bands form elongated domains. This texture gives a Hv light‐scattering pattern which can be interpreted by the theory of scattering by anisotropic rods. These domains lose their elongated shape with time.
Solvent self‐diffusion, polymer NMR relaxation, and free volume in polymer solutions
Authors: Blum Frank D. , Durairaj Bojayan , Padmanabhan Alapat S. 
Published: April 2003

AbstractThe self‐diffusion coefficient of chloroform in poly(isopropyl acrylate)—chloroform solutions has been studied as a function of concentration and temperature by using the pulsed‐field‐gradient spin‐echo NMR method. It is found that the self‐diffusion coefficient of the solvent can be adequately fitted by using a simple free‐volume approach with either a concentration or temperature superposition. It was noted that the free‐volume parameters derived from the self‐diffusion data are the same as those derived from deuterium NMR transverse relaxation‐time measurements of the polymer in the same system. The equality of these two sets of experiments suggests a fundamental relationship between the two different processes. The simplest explanation is that the free volume necessary for the local segmental motion of the polymer and the translation of the solvent are similar.
Diffusion of Cu2+ catalysts from Cu‐metal polymer or Cu‐oxide/polymer interfaces into isotactic polypropylene
Authors: Jellinek H. H. G. , Wei You Ching 
Published: April 2003

AbstractDiffusion coefficients of Cu2+ in the form of its carboxylate have been measured in isotactic polypropylene as a function of temperature (90–128°C) and extent of preoxidation. Diffusion take place from the metal catalyst/polymer interface into the bulk polymer. The diffusion is dependent on the extent of preoxidation and temperature but not on the type of catalyst (Cu, CuO, CuO0.67). Analysis of polymer sections for Cu2+ ions was carried out with a selective Cu2+ electrode. Diffusion in isotactic polypropylene is about 1000 times faster than in lowdensity polyethylene. The carboxylate anion appears to have about 7 C‐atoms for diffusion in isotactic polypropylene compared with 29 C‐atoms for low‐density polyethylene.
Analysis of meridional x‐ray diffraction pattern of the γ form of nylon 6 and comparison of paracrystalline and microstrain models of lattice disorder
Authors: Murthy N. S. 
Published: April 2003

AbstractAnalysis of the increase in the width of the seven meridional (OkO) reflections of the γ form of nylon 6 suggests microstrain rather than paracrystallinity as the source of lattice disorder along the chain axis. It is shown that the intensity of the (020) reflection depends not only on the amount of γ phase present in the fiber, but is also influenced by orientation, Lorentz factor, and crystallite size. Calculations confirm the presence of a weak, diffuse meridional band at 2θ=6° (Cu Kα) when the coherence length along the chain axis corresponds to less than two unit cells.
An application of an homogenization method to a model of diffusion in glassy polymers
Authors: Sangani Ashok S. 
Published: April 2003

AbstractThe sorption of gases in polymers below their glass‐transition temperature Tg is known in many cases to be described by the “dual sorption” theory, according to which the gas is held in accordance with both the Langmuir and Henry's laws. Based on this theory, expressions for the “effective diffusion coefficient” in the glassy polymers have been obtained by investigators in the past, notably by Paul and Koros.2 The present analysis regards the glassy polymers as inhomogeneous with regions on which the gas sorption follows the Langmuir law. Assuming that the linear dimensions of these regions, which are often referred to as “microvoids” (although they are not space filled by vacuum), are small compared to the macroscopic length of interest but large compared to the mean free path of the penetrant gas molecules, we derive a rigorous relation between the average flux and the concentration gradient in the polymer and show that this relation can be expressed in terms of an “effective diffusion coefficient” Deff which depends on the details of the microstructure, i.e., the size, shape and spatial distribution of the “microvoids.” This expression for Deff is shown to reduce to that of Paul and Koros2 in two situations: (1) when the “voids” consist of slabs running parallel to the concentration gradient, and (2) when the “voids” are spherical and the temperature of the polymer is not too different from Tg. The results of the present study lead to an alternative procedure for interpreting the experimental data on sorption and permeation which may have some advantages over the procedure currently employed. Finally, the analysis presented here is also applicable to polymers containing adsorptive fillers.
Small‐angle x‐ray scattering from polyethylene
Authors: Kifle Z. , Harrison I. R. , Herman R. M. 
Published: April 2003

AbstractA new theoretical model for the description of small‐angle x‐ray scattering from oriented polymer systems is developed and intensity functions are derived. The scattering system considered can be represented by a set of stacks of approximately parallel lamellae where the stack dimensions are assumed large in comparison with correlation distances within the stacks. Each point within the sample is envisioned as being surrounded by such a domain, so the theory is based on a concept of continuously varying local conditions. Fluctuations in the local lamellar distribution parameters are taken into account through a gross disorder parameter which specifies a spread in the local mean long periods. In addition, a long‐range disorder parameter has been included as a measure of the randomness of the actual amorphous layer spacings about the local mean lamellar spacing within each lamellar stack. Important characteristics of the diffraction curves such as the peak positions, heights, and widths; the number of observable orders; and the higher‐angle behavior allow one to obtain the mean lamellar spacing, the fluctuation parameters, and the effective widths of the refractive index perturbations associated with the amorphous regions. The number of lamellae that scatter x‐rays coherently can be predicted with ease. We show that geometric corrections are negligible for oriented samples. In addition, the Lorentz correction is shown to be unnecessary in all cases. We have measured diffraction patterns for polyethylene (PE) precipitated from solution using a Kratky system. Good agreement between theoretical and experimental results on PE 87 and PE 91 is achieved for three‐parameter fits. Finally, we have considered non‐Gaussian distributions of the local mean long periods, with excellent results being generated for a slightly positively skewed three‐Gaussian distribution.
Ordered structures of syndiotactic poly(methyl methacrylates) studied by a combination of infrared, Raman, and NMR spectroscopy.
Authors: Dybal Jiří , Spěváček Jiří , Schneider Bohdan 
Published: April 2003

AbstractThe C  O stretching vibrations in Raman and infrared spectra of poly(methyl methacrylate) (PMMA) samples of various stereoregularity in solutions and in the solid state were measured. It was found that for ordered structures of syndiotactic (s) PMMA, transition dipole coupling of ester groups in ordered s‐PMMA sequences leads to a splitting of the C  O stretching vibration into three components with wavenumbers equal in the solid state and in solution, and of different activity in Raman and infrared spectra. A comparison of the time dependences of self‐aggregation of s‐PMMA as measured by NMR and infrared spectroscopy indicates that the first phase of the aggregation process is detected by both methods. NMR analysis of the effect of the degree of stereoregularity on the self‐aggregation of s‐PMMA shows that the prerequisite for the generation of a stable ordered structure is the mutual interaction of two s‐sequences of a minimum length of 9 monomer units. The experimental data obtained lead to the conclusion that, in self‐aggregated s‐PMMA, some of the ester groups are in close contact, with the interacting s‐sequences probably forming a double helix.
Kinetics of transitions involving condis crystals
Authors: Grebowicz Janusz , Cheng Stephen Z. D. , Wunderlich Bernhard 
Published: April 2003

AbstractThe kinetics of condis (conformationally disordered) crystal formation from the melt and of rigid crystals from condis crystals has been studied by differential scanning calorimetry for trans‐1,4‐polybutadiene and polytetrafluoroethylene. Generally very low two‐stage Avrami exponents between 0.02 and 1.74 were needed to describe the fast and often incomplete crystallizations. None of the commonly accepted crystal nucleation and growth models seems to fit the data.
Kinetics of phase separation in polymer blends for deep quenches
Authors: Voigt‐Martin I. G. , Leister K.‐H. , Rosenau R. , Koningsveld R. 
Published: April 2003

AbstractElectro microscopy was used to study the phase separation kinetics of a polystyrene/polyvinylmethylether system subjected to a critical deep quench. The size of the phase‐separated domains was found to increase linearly with time, implying that hydrodynamic effects control the rate of growth of the domains in the time scale and temperature range under consideration. From these measurements the growth velocity and approximate diffusion constants can be determined for three different temperatures. Comparison of these results with those obtained by light scattering on other systems and with theoretical predictions is possible by replotting in dimensionless units.
Cumulative erasing effect in the melt memory of nylon 6
Authors: Avramova N. , Fakirov S. 
Published: April 2003

AbstractThe effect of melt annealing over a wide range of temperatures (240–300°C) and durations (0.5–120 min) on the induction period and the heat of crystallization of nylon 6 was studied by means of differential scanning calorimetry. It was found that the crystallization characteristics are determined by the total annealing time experienced by the melt at a given temperature; i.e., a cumulative effect on the erasure of the melt memory is observed. Complete destruction of the crystalline nuclei is achieved after annealing for 90 min at 280°C, the equilibrium melting point of nylon 6; the nuclei cannot be regenerated by nonisothermal crystallization. All the effects observed are explained by the existence of a wide distribution of size and perfection of nuclei.
Volumetric properties of water–poly(acrylic acid) salt systems from the pure solid to highly concentrated solutions
Authors: Hiraoka Kyoko , Yokoyama Tetsuo 
Published: April 2003

AbstractVolumetric properties of poly(acrylic acid) alkali‐metal salts (Li, Na, K, Rb, and Cs) with different degrees of neutralization and water contents were studied in the range from pure solid to highly concentrated solutions. The apparent partial molar volume ϕ2 of the polymer and the partial molar volume V1 of water were calculated from density data. The value of ϕ2 decreased with decreasing polymer concentration and eventually leveled off. Values of V1, which at low water contents were much smaller than that of free water, increased with increasing water content and eventually reached a constant value equivalent to that of free water, thus indicating the appearance of free water. Water contents corresponding to the appearance of free water increased in the order of Li < Na < K < Rb < Cs, differing from the usual trend of hydration numbers observed in dilute solutions. The change of the slope of the plots of V1 versus composition suggested a change in the hydration mechanism. For Li, Na, and K salts, the limiting values of V1 at very low water content is considerably smaller than the 18 cm3/mol of free water. In contrast, for Rb and Cs salts, these values were relatively large, indicating the relatively weak electrostriction effects of these ions.
Solution and solid‐state NMR characterization of poly(p‐phenylene terephthalamide)
Authors: English A. D. 
Published: April 2003

AbstractNMR spectroscopy has been used to characterize poly(p‐phenylene terephthalamide) in the solid state and in solution in sulfuric acid. Solid‐state13C NMR spectra illustrate that the chain structure is highly ordered in the solid state and is of lower symmetry than in solution. Solid‐state13C and1H NMR results show that only very limited motion takes place over the temperature range of −170 to +200°C. High‐resolution NMR spectra can be observed only in very dilute isotropic solutions because it is the overall rotational motion of the polymer, not segmental motion, that averages the nuclear spin interactions to their isotropic values. These results demonstrate that previous solution NMR studies that were interpreted as reflecting the presence of isotropic and anisotropic high‐molecular‐weight polymer phases over a wide range of concentrations actually are representative of polymer degradation.
Translational diffusion of polystyrene in 1,4‐dioxane at infinite dilution determined with the extended Taylor dispersion method
Authors: Barooah A. , Sun C. K. J. , Chen S. H. 
Published: April 2003

AbstractThe extended Taylor dispersion method was used to determine the translational diffusion coefficients of a series of polystyrene standards in 1,4‐dioxane at infinite dilution at temperatures from 323.2 to 378.2 K. The experimental data suggest that in the context of Kirkwood's theory the Monte Carlo chain represents real polystyrene molecules better than does the Guassian chain with physically reasonable values for the hydrodynamic interaction parameter h*. The feasibility and accuracy of the simultaneous determination of the diffusivities of several polystyrene standards from a single dispersion measurement were also analyzed using a binary mixture of polystyrenes with molecular weights of 2000 and 100,000 as an example.
Fourier‐transform infrared study of the nematic order in thermotropic main‐chain polyesters
Authors: Wu P. P. , Hsu S. L. , Thomas O. , Blumstein A. 
Published: April 2003

AbstractInfrared spectroscopy has been used to analyze the microstructures of liquid‐crystalline polymers in various mesophases. It was quite evident that there is little indication of conformational order associated with the flexible polymethylene sequences in the nematic or isotropic phases. However, additional spectroscopic evidence does suggest that polyesters with an even number of methylene units have a higher degree of lateral order when compared to those with an odd number in the liquid‐crystalline state.
Electric relaxation currents in a cured polyester resin with nitroaniline additives
Authors: Sodolski H. , Słupkowski T. 
Published: April 2003

AbstractThe influence of a small amount of nitroaniline additive on the thermally stimulated discharge current in a cured polyester resin was investigated. The height of dipole relaxation and space charge relaxation peaks increased and the maxima of the dipole peaks shifted to lower temperatures. The observed effects are explained by the nitroaniline—polyester resin interaction, which causes a structural plasticization of the polymer.
Effect of introducing acrylic and methacrylic acid groups on molecular relaxation of poly(ethyl methacrylate)
Authors: Kihira Yasuhasru , Yamamura Hitoshi , Murakami Ichiro 
Published: April 2003

AbstractDielectric and infrared data have been obtained over a wide temperature range on copolymers of ethyl methacrylate with methacrylic and acrylic acid synthesized by radical copolymerization. The dissociation energy ΔH0 for the acid dimer in the copolymer is estimated from the temperature dependence of the relaxation strength Δεα of the α relaxation, which is associated with the glass transition. The value of δH0 obtained by this method is in fair agreement with that determined by infrared (IR) spectroscopy. The strength of the α relaxation and its activation energy are both increased by the incorporation of methacrylic acid units but are decreased by acrylic acid units. This behavior is attributed to the restriction of main‐chain motions by hydrogen‐bonded acid dimers in the copolymers with methacrylic acid and to the incorporation of more flexible links in the copolymers with acrylic acid. The β relaxation observed below the glass transition temperatures is almost unaffected by the incorporation of methacrylic acid.
Electrical resistivity correlations in rigid and plasticized polymers
Authors: Hartmann Bruce , Warfield R. W. 
Published: April 2003

AbstractEmpirical correlations in the literature between room‐temperature electrical resistivity and activation energy in various polymers are shown to be equivalent to the compensation law. Assuming that electrical conduction is an ionic process, following the Eyring theory, the compensation law can be expressed as a direct proportionality between activation energy and activation entropy. It is found that plasticizers do not change the ion number density or the ionic jump distance but only the temperature at which energy and entropy effects balance. An approximation of the ion number density is consistent with the idea that the mobile ions result from the dissociation of absorbed water.
Dilation of polycarbonate by carbon dioxide
Authors: Sefcik Michael D. 
Published: April 2003

AbstractA new technique is described for dilatometry under high pressure. The technique is based on optical interferometry and is analogous to measuring the thickness of thin, nonabsorbing films and coatings. The procedure is demonstrated for the well‐characterized system of n‐pentane sorption by polyisobutylene, and then results for the dilation of polycarbonate by the sorption of carbon dioxide are presented. The dilation of polycarbonate by CO2 is nearly linear with concentration; the partial molar volume of CO2 decreases slightly with increasing pressure. This result indicates that all sorbed CO2 molecules contribute equally to the dilation of the polymer matrix and that none reside in microvoids or in preexisting free‐volume elements which do not contribute to volume expansion of the polymer.
Dilation and plasticization of polystyrene by carbon dioxide
Authors: Sefcik Michael D. 
Published: April 2003

AbstractWe have used an optical interference technique to measure the dilation of polystyrene films in the presence of carbon dioxide or helium at pressures up to 20 atm. Dilation isotherms (plots of dilation versus gas pressure at constant temperature) were obtained for three samples of polystyrene which had widely differing molecular weights. The dilation isotherms have the same general shape as sorption isotherms, which means that all of the sorbed gas molecules contribute to volume dilation and non can be thought of as occupying molecular‐sized voids in the polymer. Using sorption results from the literature we show that the partial molar volume of CO2 at 35°C is about 39 cm3 mol−1 and appears to be independent of polystyrene molecular weight. For a polystyrene sample with Mn = 3600, the partial molar volume of sorbed CO2 increases to 44 and 50 cm3 mol−1 at 45 and 55°C, respectively. The sorption of CO2 in polystyrene is shown to depress the glass transition temperature of the mixture, consistent with theoretical predictions. The shape of the dilation and sorption isotherms are consistent with the depression of the glass transition temperature.
Molecular design of multicomponent polymer systems. X. Emulsifying effect of poly(styrene‐b‐methyl methacrylate) in poly(vinylidene fluoride)/noryl blends
Authors: Ouhadi T. , Fayt R. , Jérôcme R. , Teyssié Ph. 
Published: April 2003

AbstractThe emulsifying activity of block copolymers is investigated in blends of incompatible engineering polymers such as polyvinyldene fluoride (PVF2) and Noryl (high impact polystyrene‐poly‐2,6‐dimethyl‐1,4‐phenylene oxide mixture). When the blends are modified by a poly(styrene‐b‐methyl methacrylate) copolymer, (PS‐PMMA) each block of which being selectively miscible with Noryl and PVF2 respectively, Phase dispersion, interfacial adhesion and ultimate mechanical properties are significantly improved; major effect is observed upon addition of 12 percent copolymer, resulting in intermingled and firmly adherent phases and a rough additivity of both tensile strength and elongation at break. All these features clearly demonstrate the important emulsifying effect of PS‐PMMA in these very attractive PVF2/Noryl blends.