bluuebunny/crossref_metadata_2025_split huggingface dataset:
Direct observation of taut tie molecules in high‐strength polyethylene fibers by Raman spectroscopy
Authors: Prasad Keshav , Grubb David T. 
Published: April 2003

AbstractThe Raman spectra of four gel‐spun high‐strength polyethylene fibers were recorded as a function of stress, with detailed study of the 1063 cm−1 band. The change in the peak position of this band was linear at low stresses, but there was little change at high stresses. The center of mass of the band moved linearly with stress until fracture, and the shift per unit stress was almost the same for all the fibers studied at 4.0 ± 0.5 cm−1/GPa. The line shape is symmetric at low stresses, and the full width at half maximum increases with stress. When plastic deformation begins the band develops a broad low intensity tail extending to 1000 cm−1. This corresponds to stresses of up to 15 GPa, and the tail contains up to 18% of the band intensity when the maximum stress is applied. Equivalent wide‐angle x‐ray studies showed no such tail, so the highly stressed material is not crystalline, although it must be in the all‐trans form to contribute to the Raman peak. Such load‐bearing extended‐chain‐disordered material is described as taut tie molecules. The amount is much larger than that usually derived from mechanical modelling, it is essentially all the amorphous material if the DSC crystallinity of 85% is correct. It is thought to be interfibrillar, stressed when fibrils slide past each other in creep. On unloading the fiber, the band became more symmetrical again but did not always return to its initial position at zero stress. The peak moved to a wave number greater than its initial value on unloading and then over time relaxed to its original value. Thus tensile stresses in the disordered material, balanced by compression in the crystals, are slow to relax. Lower molecular weight fibers (Mn = 8 × 105) had small anelastic effects, probably because they relaxed within the time of the experiment.
Strain‐induced crystallization of swollen, crosslinked polyethylene
Authors: McHugh A. J. , Yung W. S. 
Published: April 2003

AbstractThe kinetics of strain‐induced crystallization of swollen polyethylene networks have been measured using dynamometry coupled with optical birefringence. Fibers were prepared by gel‐spinning ultrahigh‐molecular‐weight polyethylene followed by crosslinking in dicumyl peroxide and swelling in xylene. Retractive force on crystallization was monitored at various temperatures and draw ratios. Fiber visualization was achieved using optical illumination coupled with video recording and digital analysis of the in situ transformation. Avrami plots of the transformation data showed good linear fits for low draw ratios in the range α < 1.2 and moderate undercoolings (60 < Tc < 70°C). Time exponents of unity were found, indicating a one‐dimensional, heterogeneous growth mechanism whose temperature dependence could be described by a formalism similar to that recently used for flow‐induced growth.
Electron spin resonance and optical spectroscopic studies of the conducting polymer precursor: Poly(3,4‐diisopropylidene cyclobutene)
Authors: Kim Soon Sam , Cebe Peggy , Swager Timothy M. 
Published: April 2003

AbstractThe doping process in a recently developed conducting polymer precursor, poly(3,4‐diisopropylidene cyclobutene) (PDPCB)1 was studied by electron spin resonance and optical spectroscopy. Thin films of PDPCB were exposed to several oxidants, I2, AsF5, or a combination of AsF5 and AsCl3 or AsF3, and monitored in situ by ESR. At the initial stages of doping, the films developed broad ESR spectra with Gaussian line shape. Except in the case of doping with I2, resolved hyperfine structures from cation radicals were observed. As the doping progressed, the ESR spectra gradually transformed to narrower line widths with a Lorentzian component. The Lorentzian component can be attributed to charge carrier species developed in the film through doping. The results of optical spectroscopy (UV‐visible) are incorporated to elucidate the effect of doping on electronic transitions of the doped PDPCB.
Paracrystalline structure of poly(paraphenylene vinylene)
Authors: Granier Thierry , Thomas Edwin L. , Karasz Frank E. 
Published: April 2003

AbstractThe previous description1 of highly oriented films of poly(p‐phenylene vinylene) is augmented by close packing energy calculations and quantitative analysis of layer line intensity profiles observed via electron diffraction. The chain axial translational disorder is of the paracrystalline type: partial registration of chains occurs within the (100) layers while the layers themselves are randomly shifted from one another along the chain direction.
Network fraction and molecular motions in polymer composites: An NMR relaxation and self‐diffusion study
Authors: von Meerwall E. , Stone T. 
Published: April 2003

AbstractWe have used NMR T2 relaxation and pulsed‐gradient spin‐echo diffusion techniques to study properties of Arco R45M hydroxyl‐terminated polybutadiene, either unfilled or containing 65 wt.% filler particles (SiO2, NaCl, Al) and cured with isophorone diisocyanate (IPDI), as functions of IPDI content. A short T2 relaxation component arises from the network (gel) whose amount is greatest (up to 92%) near NCO/OH stoichiometry. Gel fraction and stoichiometry concentration both are affected slightly by filler surface reactivity but principally by filler particle size. The diffusion rate of the nonnetwork (sol) molecules has a range of 1–2 orders of magnitude. This range is narrowest near stoichiometry for the smallest filler (SiO2), i.e., the situation in which the sol molecules are least mobile. Branching theory and the hypothesis of a layer of reduced mobility in a wide vicinity of the filler particles provides semiquantitative explanations of these observations.
Polycaprolactone‐based block copolymers. II. Morphology and crystallization of copolymers of styrene or butadiene and ε‐caprolactone
Authors: Heuschen J. , Jérôme R. , Teyssié Ph. 
Published: April 2003

AbstractThree series of amorphous semicrystalline poly(styrene‐b‐ε‐caprolactone)s have been synthesized with polystyrene blocks of 6000 (series A), 40000 (series B), and 70000 (series C) molecular weight, respectively. In these materials, the polymer miscibility evolves from a situation where a diffuse interphase involves the major part of the volume of the copolymer (series A) to a sharp phase separation as observed for copolymers with the longest PS block (series C). The crystallization of PCL blocks is mainly governed by the phase morphology. In copolymers of series A, the crystallization rate of PCL blocks is slowed down the more as the miscibility with PS increases, and ultimately the degree of crystallinity Xc decreases significantly. When phase separation is sharp, Xc changes dramatically at the phase inversion and decreases when PS forms the continuous phase. At the inversion Xc depends on the mean size of the PCL microdomains as compared with the thickness of the crystalline lamellae. The periodicity of the phase morphology as observed by TEM is influenced by the solvent used in casting films, whereas monolamellar monocrystals can be obtained by a self‐seeding technique.
The morphology of the spherulitic surface in polyethylene
Authors: Lustiger A. , Lotz B. , Duff T. S. 
Published: April 2003

AbstractStudies of lamellar shapes and profiles in linear polyethylene have often been implemented by microscopic examination of appropriately etched random slices through spherulitic specimens. However, since a spherulite crystallized from a bulk specimen is a three‐dimensional assembly of radially oriented lamellae with a twisting orientation (except for very high Tcs), it may be difficult to draw appropriate conclusions from a random slice. A technique has been developed that allows one to prepare spherulitic surfaces such that their distance relative to the geometric center can be well characterized. A computer‐based graphical representation is also presented which suggests that the projection of lamellae on such surfaces is adequately represented by a radially oriented assembly of helicoidally twisted lamellae. Based on the agreement between the experimental results and the computer model, it is suggested that the C and S shapes previously observed by other researchers are not necessarily intrinsic to the lamellar profile but may arise due to geometrical effects as the lameliae project onto a surface at various angles. From these results it is also evident that in ringed spherulites lamellae undergo continuous twisting rather than successive misalignment of essentially untwisted segments.
A generalized, topologically consistent, dual‐mode transport model for glassy polymer‐gas systems
Authors: Petropoulos J. H. 
Published: April 2003

AbstractA dual‐mode transport treatment for glassy polymer‐gas systems is presented in which due account is taken of random‐walk topology. This permits elimination of the restrictive homogeneous random‐walk condition, which must hold if the existing simple dual‐mode transport model is to be considered rigorous. The results obtained from the new treatment indicate that the aforesaid simple model requires only minimal modification, as far as transport in the solution (Henry's law) mode is concerned. The simple model description of adsorption (Langmuir) mode transport, on the other hand, is shown to be subject to minor or major limitations and qualifications, according to whether the mobility of adsorbed molecules is, respectively, comparable to/lower than, or substantially higher than, that required by the homogeneous random‐walk condition. The present treatment permits reasonable semiquantitative interpretation, within the context of current dual‐mode sorption theory, of the transport behavior typically observed experimentally. The results indicate that experimental adsorption mode diffusion coefficients, as currently calculated, cannot be considered to be quantitatively meaningful.
Structure‐property relationships of copolyamides. I. Thermal properties and crystallization
Authors: Jo Won Ho , Baik Doo Hyun 
Published: April 2003

AbstractSix nylon salts [hexamethylenediammonium adipate (6.6), hexamethylenediammonium terephthalate (6.T), hexamethylenediammonium isophthalate (6.I), p‐xylylenediammonium adipate (PXD.6), m‐xylylenediammonium adipate (MXD.6), and m‐xylylenediammonium isophthalate (MXD.I)] were copolymerized with ε‐caprolactam. The resulting random copolyamides showed different modes of crystallization as confirmed by the melting temperature depression and the decrease in the isothermal crystallization rate. By selective hydrolysis it was found that the differences in Tm depression and isothermal crystallization rate were due to partial inclusion of comonomers in the crystal lattice. The effect of comonomer structure on the crystallization rate is also discussed.
A study of solvent diffusion in thin polyimide films using laser interferometry
Authors: Tong H. M. , Saenger K. L. , Durning C. J. 
Published: April 2003

AbstractUsing laser interferometry, we have determined in situ the thickness increase with time of thin supported polyimide films (4–8 μm in initial thickness) immersed in n‐methyl‐2‐pyrrolidone (NMP) as a function of NMP temperature (22–120°C). Similar experiments were also performed in dimethyl sulfoxide (DMSO) at 22°C with polyimide films of 4.1 μm in thickness. For NMP, the equilibrium fractional thickness increase (about 20%) is independent of initial polyimide thickness and temperature. The time scale for reaching equilibrium sharply decreases with temperature from 2–3 days at 22°C to 30–60 min at 120°C. Compared with NMP, the rate of DMSO sorption is considerably faster, reaching equilibrium swelling of about 28% in about 5 h at 22°C. To describe the transport process, we applied a phenomenological model proposed by Astarita and Sarti1 but reformulated in polymer fixed frame to enable straightforward comparison with the thickness data. Our analysis indicated that the transport of NMP is best described as anomalous, that is, intermediate between diffusion controlled and case II transport. The effective diffusion coefficient Deff and the front velocity U0 at 22°C were found to be 3–6 × 10−12 cm2/s and 8 × 10−9 cm/s, respectively. Our front velocity is in good accord with the value of 6 × 10−9 cm/s obtained for a similar polyimide based on gravimetric measurements.2 Both Deff and U0 show an activation energy of ∼56 kJ/mol. For DMSO, however, the transport is clearly case II. The front velocity at 22°C was found to be about 6 × 10−8 cm/s, which is about four times that obtained by Rutherford back‐scattering spectrometry.3
An indirect GPC calibration method for the low‐molecular‐weight region
Authors: Tsitsilianis C. , Mitsiani G. , Dondos A. 
Published: April 2003

AbstractA new gel permeation chromatography indirect calibration method is proposed for a polymer for which samples of known molecular weight and narrow molecular weight distribution are not available. This method is based on conversion of an experimentally obtained calibration curve for another polymer (polymer standard) through the universal calibration concept but using a new intrinsic viscosity‐molecular weight relation. This indirect calibration curve is proposed especially for the low‐molecular‐weight region, where the method based on the Mark‐Houwink‐Sakurada equation gives erroneous results.
Molecular design of multicomponent polymer systems. XIV. Control of the mechanical properties of polyethylene–polystyrene blends by block copolymers
Authors: Fayt R. , Jérôme R. , Teyssié Ph. 
Published: April 2003

AbstractThe present investigation deals with the tensile mechanical properties of the melt‐blended polyethylene/polystyrene pair as controlled by poly(hydrogenated butadiene‐b‐styrene) copolymers. It is clearly demonstrated that moderate amounts of these copolymers (2–10%) significantly increase both the ultimate strength and elongation at break of blends of polystyrene with various types of polyethylene (low‐density, high‐density, linear low‐density, and hydrogenated polybutadiene) and synergistic effects may result. The mechanical performance is strikingly dependent on the molecular characteristics of the copolymers. Over a broad range of molecular weights (60,000–270,000), diblocks are more effective than graft, triblock, or star‐shaped copolymers. It is also demonstrated that using polymeric emulsifiers under usual processing conditions in the melt state is powerful technique for preparing valuable polymer alloys.
Network structure and mechanical properties of allophanate‐cured polyurethane rubber
Authors: Oikawa Hidetoshi , Nagai Tsuyoshi , Murakami Kenkichi 
Published: April 2003

AbstractAn allophanate‐cured polyurethane (PU) has been prepared. The relationship between trifunctional network structure and mechanical properties is discussed. Mooney‐Rivlin constants C1 and C2 are determined from stress‐strain isotherms. C2 is approximately constant. The average value of structure factor A3 in the limit at large deformation is 0.432, in fair agreement with the theoretical prediction of 1/3. The structure factor A′3 of all PU samples becomes roughly unity in the limit at small deformation. The experimental results are interpreted by the recent Flory theory (a fluctuation‐constraint model of junctions). The κ parameter decreases with increasing crosslink density; the ξ parameter is small and approaches zero. Increasing crosslink density tends to cause phantom deformation for extended PU networks. However, the absolute value of the reduced stress in the limit at large deformation (or for the equivalent phantom network) [f] is believed to be overestimated for PU networks with high crosslink density. The origin of this behavior is discussed in detail.
Chemorheology of allophanate‐cured polyurethane rubber
Authors: Nagai Tsuyoshi , Oikawa Hidetoshi , Murakami Kenkichi 
Published: April 2003

AbstractChemical stress relaxation (CSR) of allophanate‐cured polyurethane (PU) was measured under two kinds of degradation conditions: (1) degradation in butylamine (BuNH2)/dimethyl sulfoxide solution; (2) degradation at high temperature in nitrogen. The allophanate groups of the crosslink sites in PU networks are selectively cleaved by BuNH2, and CSR is not influenced by the crosslink density within experimental error. This suggests the validity of Tobolosky's method for distinguishing between random scission along network chains and the cleavage of crosslinks from the dependence of CSR on crosslink densities. In addition, CSR of PU samples degraded at high temperature in nitrogen was also found to be independent of the crosslink density. It follows that thermal degradation of PU networks occurs by the cleavage of crosslink sites in the same manner. CSR is compared with the relative degree of thermal degradation of PU sample, determined by‐the amine‐decomposition method and the swelling method. Consequently, the total (or residual) crosslink density of degraded PU networks corresponds essentially to the intermittent CSR.
Rheological studies of thermotropic liquid crystal copolyesters: P‐hydroxybenzoic acid / 2‐hydroxy‐6‐naphthoic acid
Authors: Yang D.‐K. , Krigbaum W. R. 
Published: April 2003

AbstractThe rheological behavior of the thermotropic liquid crystal copolyesters composed of p‐hydroxybenzoic acid and 2‐hydroxy‐6‐naphthoic acid (indicated as HBA/HNA) is investigated for three different compositions: 75:25, 58:42, 30:70 (in mol%). The inherent viscosities are 9.2, 5.1, and 7.8 dl/g, respectively. Yield stress is observed for all three samples. This indicates the existence of crystallites in the melt which may be related to the shear thinning viscosity at low shear stress. Melt fracture, and a die swell ratio which increases with shear rate, are also observed at higher temperatures and at low shear stress for the two copolyesters having compositions 75:25 and 30:70. We have also estimated for 30HBA/70HNA at 335°C the entrance pressure loss, Δpent, and ΔPent/δw, where δw is the shear stress at the capillary wall. The large value of ΔPent/δw suggest that HBA/HNA is a highly elastic polymeric material. It is found that both ΔPent and ΔPent/δw increase with shear rate. However, the high elasticity does not account for the disappearance of melt fracture and contraction of die swell at high shear stress. This abnormal phenomenon is ascribed to the formation of a network of crystallites caused by blocky regions in the copolymer.
Pressure‐jump volume‐relaxation studies of polystyrene in the glass transition region
Authors: Tribone J. J. , O'reilly J. M. , Greener J. 
Published: April 2003

AbstractAn apparatus has been constructed that permits the measurement of time‐dependent changes in pressure near the point of vitrification. The same instrument is used for measuring steady‐state PVT properties, which are necessary for a proper analysis of the dynamic measurements. The former experiments are referred to as pressure‐jump volume‐relaxation (PJVR) measurements and serve as a direct probe of the structural relaxation process that occurs in all glasses. Experiments have been performed on polystyrene from 110 to 150°C and up to 2 kbar using pressure steps of 500 bars. The qualitative observations are analogous to those obtained at atmospheric pressure by rapid changes in temperature, namely (1) nonlinearity, (2) asymmetry, and (3) memory effects associated with complicated temperature or pressure histories. Each of these effects is accounted for semiquantitatively by a phenomenological order‐parameter model that has been extended to include the effect of pressure. Deviations between theory and experiment increase as temperature and pressure increase, this being manifest mostly in a predicted recovery curve (expansion isobar) that recovers the equilibrium volume more quickly than the experimental data; the contraction isobars are in most cases predicted within experimental error. The adjustable parameters of the model are found to vary somewhat with pressure and temperature, apparently due to variations in δ and Δκ. The activation volume suggests that 10–20 monomer segments are involved in the recovery process, assuming that the activation volume itself represents only a fraction of the dynamic unit (as observed in molecular glasses).
Sorption and dilation in poly(ethyl methacrylate)–carbon dioxide system
Authors: Kamiya Yoshinori , Mizoguchi Keishin , Hirose Takuji , Naito Yasutoshi 
Published: April 2003

AbstractSorption and dilation in the system poly(ethyl methacrylate) (PEMA) and carbon dioxide are reported for pressures up to 50 atm over the temperature range 15–85°C. The sorption isotherms were obtained gravimetrically. The dilation accompanying sorption was measured directly with a cathetometer. At low temperatures the sorption and dilation isotherms were concave toward the pressure axis in the low‐pressure region and turned to convex with increasing pressure. As the experimental temperature approached and exceeded the glass transition temperature of 61°C, both isotherms became convex or linear over the whole range of pressure. Partial molar volumes of CO2 in PEMA were obtained from sorption and dilation data, which were described well by the extended dual‐mode sorption and dilation models developed recently. The temperature dependence of the dual‐mode parameters and the isothermal glass transition are discussed.
Nylon 6 structure in solid state as studied by high‐resolution 13C‐NMR spectroscopy
Authors: Kubo Kimihiro , Yamanobe Takeshi , Komoto Tadashi , Ando Isao , Shiibashi Toru 
Published: April 2003

AbstractHigh resolution 13C‐NMR spectra of nylon 6 samples crystallized under various conditions and of a drawn sample were measured at room temperature by the cross polarization/magic angle spinning (CP/MAS) and pulse saturation transfer/magic angle spinning techniques. Additionally, 13C‐NMR spectra of the drawn sample were measured at temperatures from 20 to 100°C by the CP/MAS technique and at 20 and 100°C by the low‐power decoupling/magic angle spinning technique. The nylon 6 structure in the solid‐state is discussed on the basis of these results. The solid‐state 13C chemical shift data are used for reference in a study of conformation in solution.
Incremental vapor sorption in a phase‐segregated polyurethane elastomer
Authors: Schneider Nathan S. , Mee Caryn F. , Goydan Rosemary , Angelopoulos Anastasios P. 
Published: April 2003

AbstractIncremental vapor sorption and desorption runs have been carried out with o‐dichlorobenzene (ODCB) a strongly swelling solvent, in 2, 6, and 10 mil polyether polyurethane films. Two‐stage sorption behavior occurs at intermediate and higher concentrations but is generally absent in the desorption runs. Analysis of the two‐stage curves, using the Berens‐Hopfenberg model of independent Fickian diffusion and first‐order relaxation processes, leads to apparent diffusion coefficients which increase with thickness and show a pronounced maximum with concentration, whereas the relaxation rate constant decreases with concentration. Correction for the pressure drift during the runs, due to the low vapor pressure of ODCB, reduces the thickness dependence. The negative concentration dependence of the relaxation rate constant is related to the distribution of microdomain stabilities. Calculated values of the self‐diffusion coefficient show that the maximum in the apparent diffusion constant with concentration can be accounted for largely, but not entirely, by the thermodynamic contributions. It is proposed that the additional factor is relaxation‐controlled swelling which arises from the strong coupling between the matrix and hard‐segment responses.