bluuebunny/crossref_metadata_2025_split huggingface dataset:
Direct signal analysis applied to the determination of the relaxation parameters from TSDC spectra of polymers
Authors: Aldana M. , Laredo E. , Bello A. , Suarez N. 
Published: April 2003

AbstractA new method to evaluate the thermally stimulated depolarization current curves based on the direct signal analysis is first applied here to the fitting of computer‐generated curves to test its validity and accuracy. The method consists in finding the N elementary curves that best fit the experimental spectrum. The adjustment of the τ0 values associated with each energy bin is also included in the fitting. The results are presented for a monoenergetic peak and a peak generated with a Gaussian broadened energy distribution both with a single τ0. Good agreement is also found when a random noise of width σ0 ≤ 0.01 Jmax is added to the data to simulate experimental errors. Experimental complex curves corresponding to the γ and β overlapping relaxations in DGEBA‐EDA epoxy resin system are separated into their fine structure components. The complex band corresponding to water dipoles physisorbed in different sites of the microporosity of bituminous coal is analyzed and different processes are identified. The results are compared to those performed on cleaned peaks. Finally, the TSDC peak corresponding to the glass transition temperature in bisphenol‐A polycarbonate is analyzed and could only be fitted by using a Vogel‐Fulcher expression for the temperature dependence of the relaxation times. © 1994 John Wiley & Sons, Inc.
Application of proton multiple‐quantum NMR to the study of dynamics in bulk chloral polycarbonate
Authors: Roy Ajoy K. , Gleason Karen K. 
Published: April 2003

AbstractProton multiple quantum (MQ) nuclear magnetic resonance (NMR) is applied for the first time to probe chain dynamics in the polycarbonate of 1,1‐dichloro‐2,2‐bis(4‐hydroxyphenyl)ethylene (chloral) over a temperature range of 195–280 K. Since chloral polycarbonate contains only one essentially indistinguishable proton, any dynamical information is associated with the phenylene group motion, the dominant sub‐Tg motion. First, the fractional multiple quantum (fMQ) signal which can be refocused at a fixed MQ preparation time decreases with increase in temperature from 195 to 200 K, associated with the onset of the phenylene π flips. Since π flips do not modulate dipolar couplings between phenylene protons at positions 2 and 3 on the same ring, this decrease in fMQ can be attributed to modulation of the remaining couplings on the time scale of the MQ experiment. Next, over the range 200–240 K, the fMQ maintains a reduced, nonfluctuating value. This suggests a broad distribution of correlation times exists for the π flips, since at any one temperature in this intermediate range, only a small portion of the phenylene groups undergo motion at the precise kHz frequency required to refocus the MQ signal. The increase in the signal strength as the temperature rises above 240 K is due to the majority of π flips occurring in the fast motional limit. These observations are in agreement with previous single quantum proton NMR studies on phenylene group rotation in choral polycarbonate. Furthermore these fMQ measurements show several similarities to those for PTFE (poly(tetrafluoroethylene)), measured by 19F MQ‐NMR. One notable difference is that in as‐polymerized PTFE at intermediate temperatures (200–220 K) rapid oscillations of fMQ were noted as the temperature was changed by a few degrees. Such behavior is not observed in the chloral polycarbonate. © 1994 John Wiley & Sons, Inc.
Carbon‐13 NMR measurments of polybutadiene local dynamics in dilute solution: Further evidence for non‐Kramers' behavior
Authors: Zhu Wei , Gisser Daniel J. , Ediger M. D. 
Published: April 2003

AbstractThe local segmental dynamics of cis‐polybutadiene in dilute solutions have been investigated using natural abundance 13C NMR spectroscopy. The time integral of the CH orientation autocorrelation function 〈σ〉 is extracted from T1 measurements in five solvents covering a viscosity range of two decades. The hydrodynamic Kramers' equation, which is generally assumed to be appropriate for local polymer dynamics in dilute solution, cannot describe these results. 〈σ〉 is found to depend upon the solvent viscosity η raised to the 0.33 power whether η is varied by changing the temperature or the solvent. This apparent power law behavior has been observed for other polymers in dilute solution and can be rationalized if the solvent is viewed as viscoelastic. This view is supported by experimental results which show that local polymer dynamics can occur as much as an order of magnitude faster than solvent reorientation. Trends for a mixed microstructure polybutadiene are similar to those of cis‐polybutadiene. © 1994 John Wiley & Sons, Inc.
Molecular level study of the crystallization of a thermoplastic polyimide by infrared spectroscopy
Authors: Ishida H. , Huang M. T. 
Published: April 2003

AbstractThe crystallization process of the thermoplastic polyimide derived from 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride (BTDA) and 2,2‐dimethyl‐1,3,‐(4‐aminophenoxy) propane (DMDA) is studied at the molecular level by utilizing infrared spectroscopy's sensitivity to local molecular environment. Assignments of the unique spectral features appearing upon crystallization are made with additional verification from crystalline model imide compounds. A direct comparison of an annealed polyimide film studied by both infrared spectroscopy and x‐ray diffraction shows fairly close agreement in the amount of crystallinity obtained. The unique crystalline infrared spectral features provide insight into the molecular mechanism involved with the crystallization process. Spectral shifts occurring upon crystallization reflect intramolecular rotations toward increased coplanarity as well as intermolecular chain packing of certain molecular groups at specific sites of the chain backbone. The site specific information is then used to study the relative kinetic behavior of the intramolecular and intermolecular effects occurring during crystallization. © 1994 John Wiley & Sons, Inc.
Blends of metal acetates and polyurethanes containing pyridine groups II. SAXS and EXAFS studies
Authors: Grady B. P. , O'Connell E. M. , Yang C. Z. , Cooper S. L. 
Published: April 2003

AbstractPolyurethanes containing pendant pyridine units were blended with various metal acetates and studied by small‐angle x‐ray scattering (SAXS) and extended x‐ray absorption fine structure spectroscopy (EXAFS) to better understand the microscopic effect of blending on these materials. An earlier investigation found a dramatic enhancement in mechanical properties after blending, which suggests at least two pyridine units were coordinating to a single cation. This coordination would enable the cation to act as a cross‐linking site, which could then cause the observed changes in mechanical properties. To determine the effect of complexation on the microphase‐separated domain structure, small‐angle x‐ray scattering patterns were collected. Neutralization with a metal acetate increased the scattered intensity, which can be explained by an increase in electron density contrast but may also have been due to an improvement in phase separation. The distance between lamellar domains was basically unaffected by the addition of metal acetate, with the exception of nickel acetate. In this instance the distance decreased, which was caused by an improvement of packing inside the hard segments. EXAFS at the nickel and zinc edges indicated that the same qualitative changes occurred in the local environments around both cations after blending versus the unblended acetates. The magnitude of the first shell peak in the radial structure function (RSF) increased significantly upon blending, a result that is difficult to rationalize. The higher shell peaks exhibited significant changes in position and magnitude upon blending, which indicates substantial local rearrangement around the metal cation These fundamental changes in the EXAFS spectra may have been due to complexation between the cation and the pyridine group, but the results were not conclusive. © 1994 John Wiley & Sons, Inc.
Sorption and transport of water in nylon‐6 films
Authors: Hernandez Ruben J. , Gavara Rafael 
Published: April 2003

AbstractSorption and transport properties of water through films of Nylon‐6 were obtained at 5, 23, and 40°C. Commercially available films were used and a Cahn electrobalance was employed for measuring the water uptake by the polymer samples. Values of the water sorption isotherms are accurately described by the Langmuir/Flory‐Huggins dual‐mode sorption model. At water activity values below 0.15, the volume fraction of water described by the Langmuir portion of the model was greater than the Flory‐Huggins population. Solubility and diffusion coefficients of water, as well as the diffusion activation energy and enthalpy of dissolution of water for Nylon‐6, were determined from the sorption experiments. Values obtained support the hypothesis of a bimodal water sorption mode, and the formation of water clusters. © 1994 John Wiley & Sons, Inc.
Hopping conduction in “pure” polypropylene
Authors: Okoniewski A. , Perlman M. M. 
Published: April 2003

AbstractA recently developed theory of steady‐state conduction in high‐density polyethylene is applied to “pure” polypropylene (PP) in the temperature range 50–93°C. Morphological changes occur in PP, including a disordered‐amorphous to monoclinic‐amorphous transition between 50 and 80°C, where, with increasing temperature T, free volume increases, and decreases with decreasing amorphous fraction. The latter competing processes lead to large increases in hopping site separation, λ, in the transition region, followed either by saturation or a maximum in λ vs. T. We speculate that segmental and/or main chain molecular motions lower apparent activation energies, are “pinned“ by applied field, and impeded by dangling bonds in regions surrounding the surfaces of crystallites. Our analysis is semi‐quantitative only, because the latter mechanisms have not been adequately quantified, and the relative contributions of each are unknown. Measurements were carried out on heated and cooled disordered‐amorphous, and 106°C, 17‐h annealed, 43% crystalline samples. Hopping distances, obtained from measured current vs. applied field characteristics, ranged from 1.2 to 5.2 nm. Apparent activation energies up to 1.80 eV were obtained from In (I/T) vs. (1/T) plots. Remarkable plateaus in the temperature range of the transition were observed in these plots, implying some carrier conduction with near zero activation energy. Possible explanations for the latter, and the electronic nature of the carriers are given. X‐ray and density flotation measurements enabled crystallinity determinations. © 1994 John Wiley & Sons, Inc.
Thermophysical properties of azomethine ether main‐chain liquid crystalline polymers at pressures to 200 MPa
Authors: Fakhreddine Youssef A. , Zoller Paul 
Published: April 2003

AbstractThis article reports on an experimental investigation of the equation of state and the transition behavior of main‐chain thermotropic liquid crystalline polymers over a wide temperature range, and at pressures to 200 MPa. The materials studied were a series of azomethine ether polymers. A varying number n (= 4, 7, 8, 9, 10 and 11) of methylene spacer units in the backbone provided systematic variation of the structure.Experimental techniques used included high‐pressure dilatometry (PVT measurements) to 200 MPa, high‐pressure differential thermal analysis, also to 200 MPa, and conventional (atmospheric‐pressure) differential scanning calorimetry (DSC).The equation of state of the materials can be well represented by the Tait equation in distinct regions, separated by a glass transition, Tg(P), a first‐order transition to a nematic state, Tk‐n(P), and a first‐order transition to an isotropic melt state Tc(P). The atmospheric pressure values of Tk‐n and Tc decreased with increasing number of spacer units and showed a clear odd‐even effect. Tg and Tk‐n both increased with pressure. The pressure dependence of Tc could not be observed due to the onset of degradation in the same temperature region. On isobaric cooling at 3°C/min, the crystallization from the nematic state occurred a few tens of degrees below Tk‐n. This supercooling was independent of pressure for some materials, while for others it increased with increasing pressure. The values of the enthalpy and entropy associated with the first‐order transition into the nematic state were lower than those of typical isotropic polymers at their melting transitions. The transition enthalpy did not have any systematic variation with increasing number of spacer units. Values of the transition enthalpy calculated from the Ciapeyron equation did not always agree with the values measured by DSC. This may be due to the two‐phase nature of the low‐temperature state. At the transition to the isotropic state, the transition enthalpy at P = 0 decreased with n and showed an odd‐even effect. © 1994 John Wiley & Sons, Inc.
Structure and dynamics of concentration fluctuations in a polymer blend solution under shear flow
Authors: Lai Janet , Fuller Gerald G. 
Published: April 2003

AbstractThe technique of small‐angle light scattering (SALS) has been employed to investigate the time‐dependent behavior of a single‐phase, semidilute solution of polystyrene and polybutadiene in dioctyl phthalate under shear flow. Concentration fluctuations in the polymer blend solution are found to grow with time in the direction of flow, and their orientation angles evolve from 45° from the flow direction toward 0°, with the steady‐state value being dependent on shear rate. SALS patterns are simulated using a modified Cahn‐Hilliard‐Cook model, with an additional collective restoring force to account for polymer elasticity. Predictions from this modified model for the orientation angles of the concentration fluctuations are in excellent agreement with the experimental results. Our model also predicts that the quiescent structure factor has a Gaussian form and that the steady‐state orientation of the scattering patterns is dependent on shear rate. These predictions are also in good agreement with our experimental observations. © 1994 John Wiley & Sons, Inc.
Effect of polymerization catalysts on the microstructure of P(LLA‐co‐εCL)
Authors: Choi Eui‐Jun , Park Jung‐Ki , Chang Ho‐Nam 
Published: April 2003

AbstractThe copolymerization of L,L‐lactide and ε‐caprolactone was carried out using antimony trioxide and stannous octoate as catalysts. The effect of polymerization catalysts on the physical and the chemical microstructures of this copolymer was investigated by 13C NMR and DSC analysis. Antimony trioxide causes more random sequence distribution within the copolymer chain due to its higher transesterification characteristic than stannous octoate. The copolymer samples made with the antimony trioxide catalyst seem to have more amorphous phase structure, than those prepared using stannous octoate which are semicrystalline for the entire compositional range due to blocky copolymer sequences. © 1994 John Wiley & Sons, Inc.
Impedance spectroscopy of reactive polymers. 1
Authors: Bellucci Francesco , Valentino Maio , Monetta Tullio , Nicodemo Luigi , Kenny Jose , Nicolais Luigi , Mijovic Jovan 
Published: April 2003

AbstractDielectric measurements were utilized to follow the advancement of cure in an epoxy/amine formulation. In contrast to earlier studies, complex impedance was measured during cure and used to calculate ionic resistivity. By using complex impedance were able to separate according to their frequency dependence the contributions to overall polarization from electrode blocking layers, migrating charges, and dipole relaxations. At any stage of cure, there is a unique frequency at which ionic resistivity can be singularly measured. Our approach does not involve trial‐and‐error frequency search and is conducive to the development of phenomenological models based on equivalent circuits. Excellent agreement was reported between the calculated values of normalized degree of cure obtained by dielectric and calorimetric measurements. © 1994 John Wiley & Sons, Inc.
Orientation and relaxation in uniaxially stretched styrene‐co‐methyl methacrylate random copolymers
Authors: Oultache A. K. , Jasse B. , Monnerie L. 
Published: April 2003

AbstractOrientation and relaxation behavior in uniaxially stretched styrene‐co‐methyl methacrylate random copolymers was investigated. When compared at a reference temperature T = Tg + constant, orientation of methyl methacrylate units (MMA) decreases while styrene units orientation increases with a decrease in the styrene percentage. This behavior can be related to intermolecular interactions between MMA units and to the stiffness of styrene‐MMA units, which do not undergo conformational changes upon stretching. Both monomer units relax the same in a given copolymer and chain relaxation increases when the styrene percentage increases. Orientation relaxation of styrene and MMA units can be reduced to two general relaxation master curves whatever the blend composition, when the results are compared at same monomeric friction coefficient. © 1994 John Wiley & Sons, Inc.
On the plasticization of poly(2,6‐dimethyl phenylene oxide) by CO2
Authors: Handa Y. Paul , Lampron Steeve , O'neill Mark L. 
Published: April 2003

AbstractChange in the glass transition temperature, Tg, of poly(2,6‐dimethyl phenylene oxide), PPO, due to the dissolved CO2 has been measured as a function of the gas pressure, p, using a high‐pressure DSC cell. At 61.2 atm, the highest pressure studied, Tg is depressed by 31.6°C. The depression in Tg is found to be linear with pressure, with dTg/dp of −0.5°C atm−1. The experimental results are in fair agreement with those calculated from a quasilattice solid‐solution model for polymer‐diluent systems. The present results, however, differ markedly from a recent investigation on PPO‐CO2 system which reported a depression in Tg of 226°C at 60 atm and a dTg/dp of −3.8°C atm−. © 1994 John Wiley & Sons, Inc.
Crystallization of poly(aryl ether ketone ketone) copolymers containing terephthalate/isophthalate moieties
Authors: Hsiao Benjamin S. , Gardner Kenncorwin H. , Cheng Stephen Z. D. 
Published: April 2003

AbstractIn a previous study, we have investigated the structure, crystallization, and morphology of poly(aryl ether ketone ketone), PEKK, copolymers prepared from diphenyl ether (DPE), terephthalic acid (T), and isophthalic acid (I) with T/I ratios from 100/0 to 50/50. These materials were considered as having ‐DPE‐T‐DPE‐T‐ (TT) and/or ‐DPE‐T‐DPE‐I‐ (TI) “phthalate diads.” In this work, we continue the study of this copolymer series with six different T/I ratios (40/60, 30/70, 20/80, 15/85, 10/90, and 0/100), which are viewed as having TI and/or ‐DPE‐I‐DPE‐I‐ (II) “diads.” The I moieties (1,3‐linked isomers) were always found to be incorporated in the crystals and acted as “entropy or symmetry” defects that effectively decreased the equilibrium melting temperature Tmo and the rate of crystallization. However, the retardation of crystallization in PEKK 0/100 (the homopolymer with pure II diads) was significantly less than expected, which was attributed to the segregation of I moieties between the chains leading to a reduction of total entropy in the unit cells. The evidence of segmental segregation in PEKK 0/100 was seen in x‐ray diffraction patterns, where several extra reflections were seen that could only be indexed by the published unit cell modified with a larger c‐axis dimension (3.048 nm, corresponding to the length of six phenyl residues or 1.524 nm, the length of three phenyl residues). The composition of 15/85 was found to have the lowest value of Tmo and the slowest crystallization rate. Upon heating, the “II” crystals (T/I from 30/70 to 0/100) exhibited the conventional double‐melting behavior rather than the triple‐melting behavior as in the “TI” crystals (50/50 to 40/60). No indication of the second polymorph form 2 was found in “II” crystals. © 1994 John Wiley & Sons, Inc.
In situ monitoring of free‐radical polymerization by fluorescence quenching of fluorene and reactive derivatives
Authors: Miller Keith E. , Burch Eric L. , Lewis Frederick D. , Torkelson John M. 
Published: April 2003

AbstractA new approach for monitoring in situ the progress of an addition polymerization has been developed based on the fluorescence quenching of fluorene. Fluorene is quenched by the enone functionality on acrylates and methacrylates, but is not quenched after the carbon‐carbon double bond in this group is broken by incorporation into the polymer backbone. Ethyl (2‐fluorenyl)methacrylate was used as a self‐quenching comonomer during the 2,2'‐pazo bis(2‐methylpropionitrile)‐initiated free‐radical copolymerization of methyl methacrylate at 60°C. The fluorescence intensity increases by 60% up to the onset of the gel effect (defined as the sudden increase in the temperature profile). The system shows sensitivity well into the glassy state, with fluorescence increasing more than two orders of magnitude from the beginning of the reaction. This sensitivity is compared with that of two free‐volume‐dependent probes, 1,3‐bis(1‐pyrene)propane and dimethylaminobenzylidenemalononitrile, and to fluorene. The temperature profile of the test‐tube‐scale reaction was used as an internal reference for characterizing the sensitivity of the probes with respect to the gel effect region. © 1994 John Wiley & Sons, Inc.
Molecular weight‐dependence of free volume in polystyrene studied by positron annihilation measurements
Authors: Yu Z. , Yahsi U. , McGervey J. D. , Jamieson A. M. , Simha R. 
Published: April 2003

AbstractPositron annihilation lifetime measurements are reported for four monodisperse polystyrenes with molar mass M = 4,000, 9,200, 25,000, and 400,000. The temperature dependences of orthopositronium (o‐Ps) lifetime (τ3) and intensity (I3) were measured from 5°C to Tg + 30°C for each sample. From these data, the free volume hole size, 〈vf(τ3)〉, and fractional free volume hps=CI3〈vf(τ3)〉 were calculated. The temperature dependences of τ3, 〈vf(τ3)〉 and hps show a discrete change in slope at an effective glass transition temperature, Tg,ps, which is measurably below the conventional bulk Tg. This suggests that τ3 is sensitive to large holes which retain their liquid‐like mobility in the glassy state. Good agreement was found for T > hg,ps between hps and the theoretical free volume fraction hth deduced from experimental P‐V‐T data for polystyrene using the statistical mechanical theory of Simha and Somcynsky. Below Tg,ps, deviations between hps and hth are observed, hps falling increasingly below hth as temperature decreases. Whereas hps and hth depend strongly on M in the melt, each essentially independent of M in the glass. A free volume quantity, computed from the bulk volume, which is in good numerical agreement with the Simha‐Somcynsky h‐function in the melt, gives improved agreement with hps in the glassy state. © 1994 John Wiley & Sons, Inc.