bluuebunny/crossref_metadata_2025 huggingface dataset:
Modelling climate change in West African Sahel rainfall (1931–90) as an artifact of changing station locations
Authors: Chappell Adrian , Agnew Clive T.
Published: April 2004

Since the major droughts in the West African Sahel during the 1970s, it has been widely asserted that mean annual summer rainfall has declined since the late 1960s. Explanation of this persistent regional drying trend was important for famine early‐warning and global climate models. However, the network of rainfall stations changed considerably during that recent period of desiccation. Furthermore, it was difficult to reconcile the calculation of a simple mean value for a region known to have a complex spatial and temporal rainfall pattern.

A simple model separated the Sahel into ‘wet’ and ‘dry’ regions. This model was inverted against mean annual summer rainfall for the Sahel between 1931 and 1990. Model predictions were found to be insensitive to initial starting conditions. The optimized parameters explained 87% of the variation in observed mean annual summer rainfall. The model predicted the mean annual rainfall in the wet ‘coastal’ and dry ‘continental’ regions of the Sahel to be 973 mm and 142 mm respectively. Consequently, the predicted long‐term mean annual summer rainfall was 558 mm, 15% greater than that of the observed long‐term mean (417 mm).

The mean annual summer rainfall for the region was corrected by removing the influence of changing station locations over the study period. No persistent decline was found in mean annual summer rainfall, which suggested that the perceived drying trend was an artifact of the crude statistical aggregation of the data and historical changes in the climate station networks. The absence of a decline in rainfall questioned the validity of the hypotheses and speculations for the causes of the drying trend in the region and its effects on global climate change. It also increased the likelihood that changes over time in other regional and global climate station networks have influenced the performance and interpretation of global climate models. Copyright © 2004 Royal Meteorological Society
Impact of regular LDL apheresis on the development of restless legs syndrome
Authors: Tings Tobias , Schettler Volker , Canelo Monica , Paulus Walter , Trenkwalder Claudia
Published: April 2004

We examined 25 hyperlipidaemic patients with coronary heart disease undergoing regular low‐density lipoprotein apheresis (LA) treatment in weekly intervals. In this patient population, half were found to have concomitant restless legs syndrome (RLS). Laboratory investigations suggest that iron metabolism is modified by regular LA treatment and this change may be involved in the pathogenesis of this previously unrecognised form of secondary RLS. Substitution of iron therefore may be a promising line of treatment for LA‐induced RLS. © 2004 Movement Disorder Society
Deficits in ocular and manual tracking due to episodic ataxia type 2
Authors: Engel Kevin C. , Anderson John H. , Gomez Christopher M. , Soechting John F.
Published: April 2004

Four patients with a novel mutation leading to episodic ataxia type 2 were studied in a task that required them to track target motion either with the eyes or with the index finger of the right hand. The target initially moved in a straight line and then changed direction at an unpredictable time by an unpredictable amount. On the day of testing, 3 of the patients were evaluated as normal on a neurological exam, whereas the fourth was severely ataxic. Nevertheless, all 4 showed deficits in tracking behavior with common features. Ocular tracking tended to result in hypermetric saccades at longer than normal latencies. Smooth pursuit tracking was absent in 1 patient and had lower than normal gain in the others. Deficits in manual tracking showed similarities to the deficits in ocular tracking, with hypermetric compensations for changes in target direction. The similarities in the deficits in manual and ocular tracking suggest that they are subject to similar control by the cerebellar structures. © 2004 Movement Disorder Society
Involvement of glial cells in rhythmic size changes in neurons of the housefly's visual system
Authors: Pyza Elzbieta , Górska‐Andrzejak Jolanta
Published: April 2004

In the housefly's first optic neuropile, or lamina, the axons of two classes of monopolar cell interneurons, L1 and L2, exhibit a daily rhythm of size changes: swelling during the day, and shrinking by night. At least for the L2 cells this rhythm is circadian. Moreover, epithelial glial cells that enwrap each lamina cartridge, its monopolar cell axons, and their surrounding crown of input photoreceptor terminals also change size, but in the opposite direction to the changes in L1 and L2—swelling by night and shrinking by day. The rhythmic changes in glia indicate the possible involvement of these cells in the lamina's circadian system. To examine their role in regulating the rhythmic changes of L1 and L2's axon sizes we have injected three chemicals into the haemolymph of the fly's head: fluorocitrate (FL) and iodoacetate (IAA), which affect the metabolism of glial cells, and octanol (OC), which closes gap junction channels. All chemicals exerted an effect on L1 and L2, which depended on the time of injection, the drug concentration, and the postinjection times at which we examined the fly's brains. Moreover, day/night changes in the axon sizes of L1 and L2 were increased in FL‐ and IAA‐treated flies, indicating that glial cells may normally inhibit these changes by regulating the sizes of L1 and L2's axons during the day and night. In turn, lack of a day/night rhythm in L1 and L2 after OC injections shows that the rhythm's persistence depends on communication between the lamina cells through gap junction channels. © 2004 Wiley Periodicals, Inc. J Neurobiol 59: 205–215, 2004
Novel changes in gene expression following axotomy of a sympathetic ganglion: A microarray analysis
Authors: Boeshore Kristen L. , Schreiber Rebecca C. , Vaccariello Stacey A. , Sachs Hilary Hyatt , Salazar Rafael , Lee Junghee , Ratan Rajiv R. , Leahy Patrick , Zigmond Richard E.
Published: April 2004

Neurons of the peripheral nervous system are capable of extensive regeneration following axonal injury. This regenerative response is accompanied by changes in gene expression in axotomized neurons and associated nonneuronal cells. In the sympathetic nervous system, a few of the genes affected by axonal injury have been identified; however, a broad sampling of genes that could reveal additional and unexpected changes in expression has been lacking. We have used DNA microarray technology to study changes in gene expression within 48 h of transecting the postganglionic trunks of the adult rat superior cervical ganglion (SCG). The expression of more than 200 known genes changed in the ganglion, most of these being genes not previously associated with the response to injury. In contrast, only 10 genes changed following transection of the preganglionic cervical sympathetic trunk. Real‐time RT‐PCR analysis verified the upregulation of a number of the axotomy‐induced genes, including activating transcription factor‐3 (ATF‐3), arginase I (arg I), cardiac ankyrin repeat protein, galanin, osteopontin, pituitary adenylate cyclase‐activating polypeptide (PACAP), parathyroid hormone‐related peptide, and UDP‐glucoronosyltransferase. Arg I mRNA and protein were shown to increase within neurons of the axotomized SCG. Furthermore, increases in the levels of putrescine and spermidine, a diamine and polyamine produced downstream of arg I activity, were also detected in the axotomized SCG. Our results identified many candidate genes to be studied in the context of peripheral nerve regeneration. In addition, the data suggest a potential role for putrescine and spermidine, acting downstream of arg I, in the regenerative process. © 2004 Wiley Periodicals, Inc. J Neurobiol 59: 216–235, 2004
Estrogen increases calcitonin gene‐related peptide‐immunoreactive sensory innervation of rat mammary gland
Authors: Blacklock A.D. , Smith P.G.
Published: April 2004

Estrogen plays important roles in preparing mammary tissue for lactation. However, estrogen also influences innervation in some tissues. We examined the effect of estrogen on peripheral innervation of mammary tissues of ovariectomized adult virgin female rats. Seven days after ovariectomy, 17β‐estradiol or placebo pellets were implanted subcutaneously, and tissues were harvested 1 week later. Estrogen treatment decreased mammary gland mass and adipocyte content, while ductal content increased and vascular composition was unaffected. Estrogen increased total areas occupied by nerves in mammary gland sections immunostained for the pan‐neuronal marker protein gene product 9.5, and this increase persisted after normalizing for treatment‐induced differences in gland mass. Although a significant increase in tyrosine hydroxylase‐immunoreactive sympathetic nerve area was observed, no difference was detected following correction for differences in gland size, implying a conserved number of sympathetic nerves in the face of reduced gland volume. Calcitonin gene‐related peptide‐immunoreactive sensory nerve sectional area was also increased, and corrected nerve area remained 88% greater, indicating nerve proliferation during estrogen treatment. Total, sensory, and sympathetic innervation of the nipple and adjacent dermal tissue were unaffected by estrogen. We conclude that chronic estrogen elevation induces selective proliferation of rat mammary gland calcitonin gene‐related peptide‐containing nerves, which are associated primarily with blood vessels and are probably nociceptors. Because they are likely to subserve a vasodilatory function, increased innervation may promote increased blood flow necessary for milk formation during suckling. Moreover, these findings may help explain abundant anecdotal reports of increased breast sensitivity in humans under high estrogen conditions. © 2004 Wiley Periodicals, Inc. J Neurobiol 59: 192–204, 2004
Effects of early song experience on song preferences and song control and auditory brain regions in female house finches (Carpodacus mexicanus)
Authors: Hernandez Alexandra M. , MacDougall‐Shackleton Scott A.
Published: April 2004

We examined the effects of song tutoring on adult song preferences, volume of song‐control brain regions, and activity of auditory brain regions in female house finches (Carpodacus mexicanus). Hand‐reared females were tutored with local songs, foreign songs, or no song. We then examined adult song preferences, determined the Nissl‐defined volume of the song‐control nuclei HVc, Area X, and RA, and compared the number of cells immunoreactive for Zenk protein in the auditory regions NCM and cmHV, following playback of songs heard early in life (Tutor/Playback Match) versus not heard (Tutor/Playback Nonmatch). All hand‐reared birds exhibited preferences for locally recorded song over foreign or heterospecific song. We found no difference in the volume of song‐control nuclei among the three groups. As well, we found no difference in the number of Zenk immunoreactive cells in NCM and cmHV between females in the Tutor/Playback Match group and females in the Tutor/Playback Nonmatch group. Isolate‐reared birds showed greater Zenk immunoreactivity following song playback than either tutored group. Thus, early auditory experience may not play a role in adult geographic song preferences, suggesting that genetic factors can lead to preferences for songs of local dialects. Song tutoring did not influence the size of song‐control regions nor Zenk induction levels following song playback, suggesting that early experience with particular songs does not influence Zenk expression. However, overall greater activation in isolate females in auditory areas suggests that exposure to song early in life may increase the selectivity of Zenk activation to song playback in auditory areas. © 2004 Wiley Periodicals, Inc. J Neurobiol 59: 247–258, 2004
Some remarks about the hierarchical a posteriori error estimate
Authors: Achchab B. , Achchab S. , Agouzal A.
Published: April 2004

The saturation assumption is widely used in a posteriori error analysis of finite element methods. It asserts, in its simplest form, that the solution can be approximated asymptotically better with quadratic than with linear finite elements. In this article, we show that a simple modification of this “hypothesis” is valid, and the proof of many authors can be made rigorous with this simple modification. We prove also the robustness of the estimator for a singularly perturbed problem. © 2004 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2004
High‐dose methotrexate therapy of childhood acute lymphoblastic leukemia: Lack of relation between serum methotrexate concentration and creatinine clearance
Authors: Joannon Pilar , Oviedo Iris , Campbell Myriam , Tordecilla Juan
Published: April 2004

Background

The objectives of this study were: (1) to analyze the relation of serum methotrexate (MTX) concentration with creatinine clearance, (2) to compare the leucovorin rescue dose administered to the patients based on creatinine clearance, with the one calculated according to serum MTX levels, and (3) to determine MTX‐related toxicity.

Procedure

Thirty children with high‐risk non‐B acute lymphoblastic leukemia (ALL) treated according to the national protocol (PINDA 92) based on ALL BFM 90, were randomized to receive consolidation with four doses of either 1 or 2 g/m2 MTX as a 24‐hr infusion, at 2‐week intervals (group M1 and M2, respectively). Serum MTX concentrations were measured at 24, 42, and 48 hr after beginning the infusion and were analyzed retrospectively. The creatinine clearance was calculated after 12‐hr intravenous hydration prior to each MTX dose. Leucovorin dosage was adjusted according to creatinine clearance.

Results

Serum MTX concentrations at 24, 42, and 48 hr after starting the infusion were not related to creatinine clearance in both treatment groups. Leucovorin rescue administered according to creatinine clearance was excessive in 43% in group M1 and in 51% in group M2, as compared to the dose calculated according to serum MTX levels. No serious clinical complications were observed.

Conclusions

These results suggest that creatinine clearance is not a good parameter to calculate leucovorin rescue. MTX‐related toxicity in this group of patients receiving a dose of 1 or 2 g/m2 and rescued with leucovorin without monitoring serum MTX levels was acceptable. © 2004 Wiley‐Liss, Inc.
An integrated analysis for a coextrusion process
Authors: Chiou Jong‐Yee , Wu Ping‐Yao , Tsai Chin‐Chin , Liu Ta‐Jo
Published: April 2004

An integrated numerical approach is proposed to simulate the polymeric liquid flow in a bicomponent coextrusion die with a feedblock inlet system. The three‐dimensional finite element method is applied to determine the fluid motion and the interfacial position of the two polymeric liquids before entering the die body, whereas the two‐dimensional Hele‐Shaw model is assumed to be applicable for fluid flow in the manifold and the slot section of the die body. An optimal design procedure is incorporated into the simulation and the dimensions of the manifold can be properly selected such that the die can deliver the composite liquid film with acceptable uniformity. A specially designed choker bar can also be inserted to reduce the flow non‐uniformities caused by production variations. The contraction of the composite liquid film emanating from the die exit is also predicted under different draw ratios.
Experimental investigation and modeling of the dissolution of polymers and filled polymers
Authors: Cao Zhihua , Kovenklioglu Suphan , Kalyon Dilhan M. , Yazici Rahmi
Published: April 2004

Dissolution of Polymers is important in various areas, including microlithography, controlled drug and herbicide/fertilizer delivery, and recycling. The dissolution rates of an oxetane polymer in ethyl acetate were obtained and well correlated with a quasi‐stationary dissolution model. Equilibrium solubility values obtained from the mathematical model on the basis of the best fit to the dissolution data were found to be in good agreement with equilibrium solubilities obtained in independent experiments. Mass transfer coefficients were also obtained from the mathematical model on the basis of the best fit, and the calculated activation energies were typical for diffusion controlled dissolution. The dissolution of highly filled polymers in various solvents was also investigated using the oxetane polymer filled with ammonium sulfate and aluminum fillers. The dissolution rates for the highly filled polymer were well correlated with a pseudo‐homogenous diffusion model.
The “sharkskin effect” in polymer extrusion
Authors: Koopmans Rudolf J. , Molenaar Jaap
Published: April 2004

Melt flow instabilities associated with surface defects for polymer extrudates and commonly referred to as the “Sharkskin Effect,” are modeled. The relaxation oscillation model of Molenaar and Koopmans, developed to describe “Spurt” defects is expanded to provide the theoretical framework for the description of surface defects. It is shown that a nonlinear viscoelastic constitutive equation may give rise to a boundary layer, which is unstable. The presence of an oscillating boundary layer is one of the many possible solutions of the momentum equation indicating the need for additional selection criteria to define which one will be selected by the polymer system. The energy balance equation is suggested to provide the extra criterion. The current macroscopic approach may form the basis for the reconciliation of the debate on the origin of extrudate surface melt flow instabilities as either a “slip‐at‐the‐wall” or a non‐monotone “constitutive equation” phenomenon.
Blends and interpenetrating polymer networks of polypropylene and polystyrene‐block‐poly(ethylene‐stat‐butylene)‐block‐polystyrene. 2: Melt flow and injection molding properties
Authors: Ohlsson Bertil , Törnell Bertil
Published: April 2004

Injection molding and melt flow properties of a set of blends prepared by mixing the triblock thermoplastic elastomer polystyrene‐block‐poly(ethylene‐stat‐buty‐lene)‐block‐polystyrene with polypropylene and a processing oil are reported. Despite the high viscosity of the thermoplastic elastomer, the melt viscosity of the blends was similar to or lower than that of the pure polypropylene. The rheological behavior of the molten blends seems to be dominated by a low‐viscosity melt phase containing mainly polypropylene and oil. The surface of solidified extrudates from the capillary rheometer was rich in polypropylene and free from large domains of the thermoplastic elastomer. Injection molded plates had a similar surface morphology, although in this case, larger domains of the elastomeric phase were found in narrow, band‐shaped surface regions. These local surface heterogeneities were probably caused by flow‐induced phase segregation during mold filling. Weld lines in injection molded test pieces prepared from blends within the composition range giving interpenetrating network structures had no noticeable effect on either stress‐strain behavior or falling dart impact strength.
Cushioning performance of flexible polyurethane foams
Authors: Sims G. L. A. , Bennett J. A.
Published: April 2004

Impact cushioning and deformation of flexible open‐cell polyester polyurethane (PU) foams were studied as a function of specimen geometry, including the incorporation of controlled voids. It was shown that cushioning behavior is dependent on sample geometry, which was in trun due to a complex balance of air compression and air flow, which changes with surface area‐to‐volume ratio of the impact specimen. Deformation studies show that impact compression proceeded initially by crushing the surface layers with little or no deformation of the center layers. As bulk compression was increased, deformation progressively propagated for the collapsed layers tending to a more uniform strain distribution at high bulk compression strains. Local asymmetric strain patterns were exaggerated using square cushions, because of cornr effects which complicated air flow paths. It was concluded that cushion curve determination of open‐cell foams would be more accurately performed using circular samples and deflecting air pressure form the top surface of the cushion to more closely simulate practical conditions. When designing at or near the margin, the number of cushions should be kept to a minimum and open surface area to volume ratios minimized by adopting square rather than strip cushions.
Optimized data collection for determination of the MWD from the viscosity data of polymer melts
Authors: Liu Yongming , Shaw Montgomery T. , Tuminello William H.
Published: April 2004

Numerical differentiation is one source of loss of resolution in the differential transformation of viscosity to MWD. Resolution can be improved by increasing the amount of viscosity data, keeping in mind that optimizing this process is necessary for practical reasons. This problem was approached by finding the minimum point for practical reasons. This problem was approached by finding the minimum point density and coverage required to resolve bimodality. For given point density and range, the higher the superposition between the viscosity data and the “knee” in the viscosity function produced by each mode, the better the resolution. Testing time can be reduced and resolution can still be preserved by placing more points in the “knee” region in the viscosity function and by skipping the unnecessary points at low shear rates.
Properties and kinetics of thermally crystallized oriented poly(ethylene terephthalate) (PET). II: Physical and optical properties
Authors: Varma Pankaj , Lofgren E. A. , Jabarin S. A.
Published: April 2004

Recent processing advances, developed to improve barrier properties and thermal stability of oriented poly(ethylene terephthalate) (PET), present a need to further study and understand the resultant material changes. The mechanical, optical, and infrared characteristics of PET sheets, previously oriented and subsequently thermally crystallized (under unconstrained conditions) have been investigated. The effects on properties, with respect to the initial mode of extension and extension ratio, have been studied. Results indicate that mechanical properties of this form of PET are dependent on the initial extension ratio, in addition to the initial mode of extension. Among thermally crystallized samples, optical property variations were found to be strong functions of initial extension ratios as well as the levels of thermal crystallinity achieved. Unoriented samples became completely opaque at low levels of thermally induced crystallinity, while highly oriented samples remained optically clear even with high levels of additional crystallinity induced by thermal exposure. Moderate levels of initial orientation were also found to inhibit the development of haze, resulting from additional thermal crystallization, although not to the extent observed for highly oriented samples. Crystal growth resulting from unconstrained thermal exposure was monitored using internal reflectance infrared spectroscopy. This growth was found to selectively occur in the directions of polymer chain orientation (alignment) for samples prepared under conditions of both uniaxial and equibiaxial stretching.
As‐molded shrinkage measurements on polystyrene injection molded products
Authors: Jansen K. M. B. , Pantani R. , Titomanlio G.
Published: April 2004

An experimental study of shrinkage in injection molded products is presented in this paper and documented with all information needed for any further analysis. In particular, the effect of holding pressure, holding time and mold geometry on product shrinkage was investigated for amorphous polystyrene. It turned out with increasing holding pressure the in‐plane shrinkages varied from 0.6% to 0.1%, while the product thickness increased from about 1% to 10%. The holding time only affected product shrinkage for settings less than gate freeze‐off time. Shrinkage usually increases along the flow path unless back flow sets in at the end of the holding stage. It was also found that if a constraint prevents in‐mold shrinkage to take place, final shrinkage may decrease if holding pressure and time are small. The effects of all the variables analyzed were described by a thermomechanical model recently proposed.
Dog‐legging in the melt spinning process
Authors: Rwei Syang‐Peng , Tang Hsiang‐In , Hu Yeh‐Ron , Bai Chi‐Chung
Published: April 2004

Experiments were performed in a continuous capillary rheometry to investigate the dog‐legging mechanism in a melt spinning process. The bending angle of the spinline was measured under various spinning conditions while spinning PP, PET, and PEN fibers. Experimental results indicate that obstacles inside the capillary of the spinnerette are the primary cause of bending of the spinline. The bending angle is greatly influenced by the obstacle's dimensions and position inside the capillary. An increase in the obstacle's radius or a decrease in the obstacle's length caused the bending angle to increase. The closer the obstacle was to the exit of the capillary implied the bending angle would be the wider except for the block immediately above the exit. Also at high shear rates, PEN was found to be relatively insensitive to spinline‐bending than PET polymers. In accounting for the dog‐legging behavior, our results suggest that the die swell model is more realistic than the conventional vortex model.
Durability of in‐plant recycled glass fiber reinforced polyamide 66
Authors: Eriksson P. A. , Albertsson A.‐C. , Boydell P. , Månson J.‐A. E.
Published: April 2004

A study of the effect of in‐plant recycling on the durability of glass fiber reinforced polyamide 66 is reported. Injection‐molded test bars were exposed to thermal agin, coolant aging, and creep testing. It was shown that thermal aging and coolant aging induced similar effects. The onset of embrittlement was revealed by both a reduction in elongation at break and loss of impact resistance. Samples based on 100% in‐plant regrind exhibited a more intense embrittlement compared to virgin samples during both thermal and coolant aging. For samples based on 25% recycled material, the deterioration rate was similar to that of virgin samples during thermal aging but slightly faster during coolant aging. Creep experiments indicated that inplant recycled materials can display reduced dimensional stability, which may be explained by a reduced degree of reinforcement due to fiber breakage during the recycling operation. Knock‐down (KD) factors were defined and determined for the in‐plant recycled samples. For creep resistance, tensile strength and modulus, the KD factors were strongly affected by fiber length. The highest KD factor, as determined for samples based on 25% and 100% recycled material, was 8% and 16% respectively.
Infrared imaging of stress‐crazing in rubber modified polystyrene
Authors: Telenkov S. A. , Wang Yingxia , Lu Yuesheng , Favro L. D. , Kuo P. K. , Thomas R. L.
Published: April 2004

The plastic yield and fracture of rubber‐modified polystyrene specimens under tensile load have been investigated at room temperature. The experimental procedure consisted of rapidly alternating measurements of transmitted and emitted infrared (IR) radiation from samples under tensile stress. The load‐displacement data were simultaneously recorded. Both single‐edge‐notched and dog bone‐shaped test coupons were studied. In each tensile test run, we observed a significant decrease of material transparency in the IR which occurred near the low yield point. A further increase in material deformation was accompanied by a substantial temperature increase. After material failure, the surface morphology of the test specimens in the vicinity of the crack was examined using atomic force microscopy (AFM). Our results are explained in the context of existing models of material crazing.
Degradation behavior of poly (caprolactone)–poly(ethylene glycol) block copolymer/low–density polyethylene blends
Authors: Tjong S. C. , Bei J. Z.
Published: April 2004

Blends of poly(carprolactone)‐poly(ethylene glycol) block polymer (PCE) with low‐density polyethylene (LDPE) were prepared by extrusion followed by compression molding into thin film specimens. The morphology, thermal properties, degradation, and mechanical behavior of the blends were investigated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), water immersion, static tensile testing, and dynamic mechanical analysis (DMA). The LDPE/PCE blends were immiscible for all chemical compositions. A LDPE/PCE (75/25 wt%) blend exhibited small reductions in weight and tensile strength after immersion in a buffer solution (pH = 5.0) at 50°C for extended periods of time. However, grafting maleic anhydride onto the LDPE/PCE blends improved the compatibility between the LDPE and PCE phases. Consequently, a 75/25 wt% blend of maleated LDPE/PCE exhibited significant losses in weight and tensile strength after immersion in the buffer solution. For comparison, blends of poly(caprolactone) (PCL) with LDPE were fabricated by similar techniques. The effect of compatibilizer on the degradation of LDPE/PCE and LDPE/PCL is discussed.
Plane strain fracture toughness of polyphenylene sulfide
Authors: Nishihata N. , Koizumi T. , Ichikawa Y. , Katto T.
Published: April 2004

The plane strain fracture toughness of polyphenylene sulfide (PPS), tested with different molecular structures such as linear PPS, heat‐treated PPS, and branched PPS, was investigated over a wide range of melt viscosity and crystallinity and compared with other mechanical properties. The fracture toughness of linear PPS increased with increasing melt viscosity, and it reached the highest fracture toughness calculated from linear elastic fracture mechanics (LEFM) observed in this study. In this high melt viscosity region, the linear PPS specimen showed slow crack growth instead of brittle fracture. Although the tensile properties of heat‐treated PPS and branched PPS are the same as those of linear PPS, the fracture toughness of linear PPS is superior. The fracture toughness is affected by the crystallinity of the specimen, and the effect of crystallinity on fracture toughness is smaller than that of melt viscosity and molecular structure.
A study on the ternary blends of nylon 6, a thermotropic liquid crystalline polymer, and a thermoplastic elastomer
Authors: Seo Yongsok , Kim Kwang Ung
Published: April 2004

Reactive extrusions were performed with blends of nylon 6 and a liquid crystalline copolyesteramide (LCP) including a functionalized elastomer (maleic anhydride grafted ethylene‐propylene‐diene terpolymer). The functionalized elastomer acts as a compatibilizer at the interface. The thermal behavior of the blends suggests partial compatibility of the components endowed by the produced graftcopolymer. A finer dispersion was observed for the compatibilized blends. Fine fibrils were also observed even when no extensional force was applied, indicating the importance of interfacial adhesion for the dispersed phase deformation. The shear viscosity of the ternary blend was lower than those of the neat polymers. The mechanical properties of the compatibilized blends, however, were not significantly improved except when an optimum amount of compatibilizer was added. Explanations for the compatibilizing action of the elastomer and mechanical performance of the ternary blends are presented.
Shear viscosity of carbon black filled polypropylene at very low shear stresses
Authors: Araki Takumi , White James L.
Published: April 2004

The shear viscosity of carbon black filled polypropylene with a range of different carbon blacks was investigated. This was accomplished using (a) a constant shear stress creep instrument, (b) a cone‐plate rotational rheometer, (c) a capillary extrusion rheometer. It was found that stresses exist for these compounds below which there are only finite deformations and no steady flow, Much attention was given to measurements of creep at low stresses, especially in the neighborhood of yield values. The magnitudes of yield stresses obtained from such creep measurements are significantly lower than those obtained using standard extrapolation to zero shear rate of higher stress data from rotational instruments. The shear viscosity behavior near the yield value differs significantly depending upon the carbon black used. A high viscosity was observed (∼109 to 1010 Pa.s) plateau in some compounds.
Polymer blends of PA6 and PPE compatibilized by poly[methylene (phenylene isocyanate)] (PMPI) coupler
Authors: Chiang Chih‐Rong , Ju Ming‐Yih , Chang Feng‐Chih
Published: April 2004

An oligomeric isocyanate, poly[methylene(phenylene isocyanate)] (PMPI), is demonstrated to be an efficient reactive compatibilizer for blends of polyamide‐6 (PA6) and poly(2,6dimethyl‐1, 4‐phenylene ether)(PPE). Use of this compatibilizer results in substantial improvements in mechanical properties. The multiple functional isocyanate acts as a coupling agent and reacts with PA6 and PPE endgroups during melt processing to produce a PA6‐co‐PMPI‐co‐PPE mixed copolymer at the interface. This in situ‐formed copolymer tends to reside at the interface to reduce the melt interfacial tension and enhance interface adhesion.
Burst pressure prediction of polyamide pipes
Authors: Germain Y.
Published: April 2004

The maximum pressure borne by a pressurized polymer pipe is a fundamental factor in the applications. This paper is aimed at showing that this factor can be calculated using a minimum knowledge of the mechanical behavior of the polymer. According to previous works developed for metal pipes, ductile rupture proceeds by a plastic instability. This theory is reviewed in the paper. A close form equation provides the burst pressure or critical hoop stress as a function of initial dimensions of the pipe and tensile test data. The model is checked against burst experiments performed on plasticized polyamide 12 pipes. The agreement is good at room and elevated temperature. Finally, generalization to other polymers is discussed. This approach can be used as a rapid method of assessing the performance of new polymer compounds.
A model for the thermoviscoelastic behavior of physically aged polymers
Authors: Drozdov Aleksey D.
Published: April 2004

A constitutive model is derived for the viscoelastic behavior of physically aged amorphous polymers under non‐isothermal loading. The model is based on the concept of transient reversible networks, where a polymeric material is treated as a network of active chains (adaptive links) that break and reform because of micro‐Brownian motion. The breakage and reformation rates for active chains in an aged medium are assumed to depend on the current temperature and the time after cooling below the glass transition temperature. To validate the model, the long‐term viscoelastic response is predicted on the basis of data obtained in the standard short‐term creep tests and compared with data from long‐term experiments. Fair agreement is demonstrated between numerical results and observations. The model is used to calculate residual stresses in a polymeric spherical pressure vessel cooled down (after curing) to the room temperature. It is shown that physical aging of the polymer significantly affects stresses and displacements in the vessel.
Effects of processing conditions and copolymer molecular weight on the mechanical properties and morphology of compatibilized polymer blends
Authors: Hermes H. E. , Higgins J. S.
Published: April 2004

The mechanical properties and morphology of melt mixed polystyrene (PS)/polyethylene (PE) blends that were modified by the addition of up to 16% of a semicrystalline PS‐b‐hPB (hydrogenated polybutadiene) diblock copolymer with varying molecular weight are reported. As a result of the blocks of the copolymer penetrating the corresponding homopolymers, these diblock copolymers are capable of reinforcing the PS/PE interface significantly. This increase in interfacial strength between the immiscible blend components does not necessarily result in an improvement in the mechanical properties of the blends as measured by Izod or tensile tests. This may be because the effect of the copolymers on the rheological properties of the blends during processing outweighs their emulsifying/reinforcing effects. If found to be universally true for polymer blends, these results suggest that the relationship between the effects of copolymers on interfacial strength, their emulsifying effects, and the mechanical properties of copolymer modified blends are not as simple as suggested by many statements found in the literature.
The onset of nonlinear viscoelasticity in multiaxial creep of glassy polymers: A constitutive model and its application to PMMA
Authors: Dooling P. J. , Buckley C. P. , Hinduja S.
Published: April 2004

A physically based, isostructural, constitutive model is described for simulating the onset of nonlinear viscoelasticity in multiaxial creep of glassy polymers, as needed in stress analyses of load‐bearing components. In the linear viscoelastic limit, shear response reduces to that of a generalized Maxwell model, while hydrostatic response is Hookean. Nonlinearity enters through Eyring‐type rate process kinetics. The equations of the model are solved numerically using a pseudo‐linear approximation through each time step, leading to an incremental equation for stress that would be convenient for use in finite element analyses. The model and its assumptions were tested using tension, shear and combined tension/shear creep experiments on well‐aged poly(methyl methacrylate) at 70°C. Reproducibility tests confirmed the assumption of constant glass structure for strains up to ∼ 1.5 × 10−2. Shear and pressure activation volumes were obtained by fitting the dependence of the shear compliance on stress invariants. The data showed unequivocally that shear activation volumes vary with log(relaxation time), and excellent agreement was obtained for a linear variation, consistent with the “compensation rule” of polymer thermo‐viscoelasticity. The activation volumes are large (many monome units), indicating the cooperative nature of the elementary flow process. Interestingly, they are of the same order as those applying to yield and plastic flow. Although the model finds success in simulating creep, it fails to describe so accurately the strain recovery on unloading. Possible explanations are suggested.
Distributive mixing in conveying elements of a ZSK‐53 co‐rotating twin screw extruder
Authors: Cheng Hongfei , Manas‐Zloczower Ica
Published: April 2004

A fluid dynamics analysis package (FIDAP), using the finite element method, was implemented to simulate the 3‐D isothermal flow patterns in the conveying element of a ZSK‐53 co‐rotating twin screw extruder. The fluid was described by a power‐law model. The dynamics of distributive mixing was studied numerically by tracking the motion of particles in the mixer. The extent of distributive mixing was characterized in terms of length and area stretch as well as strain distributions. The length stretch reflects the overall capability of the mixing device to spread minor component particles away from their neighbors originally present in the same cluster. The area stretch reflects the evolution of intermaterial area when mixing two fluids with a passive interface. We observed an oscillatory behavior for the average intermaterial area stretch, which was explained in terms of a stretching and folding mechanism. Folding occurs during material takeover from one screw to the other. Operating at higher rotational speeds enhances distributive mixing efficiency.
Polymer weld strength predictions using a thermal and polymer chain diffusion analysis
Authors: Ezekoye O. A. , Lowman C. D. , Fahey M. T. , Hulme‐Lowe A. G.
Published: April 2004

A new simplified model for strength prediction of welded polymer‐polymer interfaces is proposed. Two schools of thought exist for strength prediction of polymer‐polymer interfaces: microscopic analysis of polymer chain behavior, and macroscopic analysis of bulk polymer properties during welding. The microscopic analysis is based on De Gennes' reptation theory for macromolecules (1), and has been described extensively by both analytical and empirical techniques. Reptation models are based on constant temperature interfaces, which are not found in any actual welding process. Conventional macroscopic analyses of welding empirically relate strength to important thermal process parameters, such as power density and heating time, but do not address the behavior of the polymer chains. Little interaction exists between these two schools of thought. This study seeks to combine these two areas, using reptation theory to explain the polymer chain interactions on a macromolecular level, and relate the interface thermal signature to strength prediction. The result of the new model is a method for strength prediction that takes into account fundamental materials properties as well as the engineering conditions imposed in a realistic welding process to predict weld strength. The model is adapted to run for similar conditions as two separate empirical endeavors. The results show that the model is effective in predicting overall trends, which emphasizes the importance of examining heat transfer effects in any polymer welding process.
Effects of pressure sensitivity and notch geometry on notch‐tip fields
Authors: Al‐Abduljabbar A. , Pan J.
Published: April 2004

Plane‐strain stress and slip‐line fields near the sharp and round tips of wedge‐shaped notches in perfectly plastic pressure‐sensitive materials are investigated to illustrate the effects of pressure sensitivity and notch geometry on notch‐tip fields. The Drucker‐Prager yield criterion is adopted to describe the material yielding behavior. Notch‐tip stress and slip‐line fields are first established for a sharp notch. Then the notch‐tip stress and slip‐line fields for a round notch tip are obtained. The results indicate that for sharp wedge‐shaped notches, as the pressure sensitivity increases, the opening stress and hydrostatic tension ahead of the tip decrease. As the wedge angle increases, the opening stress and hydrostatic tension ahead of the tip decrease. For round wedge‐shaped notches, as the pressure sensitivity increases, the extent of the exponential spiral zone ahead of the tip increases. As the wedge angle increases, the extent of the exponential spiral zone ahead of the tip decreases and the maximum opening stress and hydrostatic tension ahead of the tip decrease. The closed‐form notch‐tip stress solutions are useful for design of plastic structural components.
Investigation of gas‐assisted injection molding. Part III: Effect of gas channel design on part bending strength
Authors: Chen Shia‐Chung , Hu Sheng‐Yan , Huang Jeng‐Sheng , Chien Rean Der
Published: April 2004

Numerical simulation and experimental measurements were carried out to investigate the effect of gas channel design on the bending performance of gas‐assisted injection molded parts. Plate parts designed with various channel geometries were gas‐assisted injection molded. Part flexible strength were measured via bending tests. It was found that part stiffness basically increases linearly with the inertia moment of the plate. The gas channel introduces an additional moment of inertia, the amount of which is determined by the shape and the dimension of the channel section as well as the hollowed core geometry. An analysis algorithm based on DKT/VRT elements superimposed with beam elements representing gas channels of various section geometries was developed to evaluate part bending behavior. An equivalent diameter was assigned to the beam element so that both the original gas channel and the circular beam have the same moment of inertia. The analyzed results from this model of 2 1/2‐D characteristics were also verified with both 3‐D and 2 1/2‐D analyses using ANSYS. The present simulations show reasonable accuracy as compared with experimental measurements and predictions from ANSYS. This investigation also indicates that it may be feasible to use the same CAE finite element model implemented for process simulation of gas‐assisted injection when performing part structural analysis as well as warpage calculation, resulting in great computational efficiency for industrial application.
Hot plate welding of polypropylene. Part II: Process simulation
Authors: Nieh Jenn‐Yeu , Ni Jun , James L.
Published: April 2004

A volume of fluid finite difference method based computer code was developed to simulate the heat transfer and squeezing flow in the hot‐plate welding process. The simulation results include melt displacement, temperature distribution, and stress build‐up and relaxation. By implementing the crystallization kinetic model developed in Part I, the formation of stress‐induced crystal structure in the heat affected zone can be revealed.
Effects of surface modification by Ar+ irradiation on wettability of surfaces of poly(ethylene terephthalate) films
Authors: Park Sung‐Chul , Koh Seok‐Keun , Pae Kook D.
Published: April 2004

Surfaces of poly(ethylene terephthalate); PET, films were irradiated with Ar+ at 1 keV using various ion doses (ID) from 1014 to 1017 ions/cm2 (isc) with and without an O2 environment. The wettability of the modified surfaces of PET was determined by measuring the contact angle between water droplets and the modified surfaces. The modified surfaces were also characterized by AFM (atomic force microscopy) and XPS (X‐ray photoelectron spectroscopy) for changes in the surface morphology, and the chemical composition and molecular structure, respectively. The contact angle decreased from 70° for unmodified surfaces to 45° for modified surface with ID = 1014 isc without O2 and remained relatively constant with higher ID. The contact angle, however, reached a minimum value of 8° for modified surfaces with ID = 1016 isc with O2. The improved wettability may be due to a combination of the formation of hydrophilic groups, chemical and molecular structural changes, physical structural or morphological changes, and increased roughness of the surface. The wettability of the modified surfaces also depended on the time of exposure to air. The wettability worsened with exposure time to air, but was revived by immersing the films into water. Possible mechanisms for the change of the wettability of the modified surfaces are given.
Determination of the degree of fill in a counter‐rotating twin screw extruder
Authors: Van Der Goot A. J. , Poorter O. , Janssen L. P. B. M.
Published: April 2004

Current residence time distribution models for counter‐rotating extruders are mainly developed for fully filled machines. However, in practical situations, these extruders operate under starved conditions. To overcome this missing link in the current models, this paper describes the average degree of fill in the situation of starvation. This degree of fill is determined by the length over which the extruder is fully filled and the degree of fill in the partially filled zone. The comparison of the experiments with the theory shows that the existing theory underestimates the length of the fully filled zone by about 30%. The degree of fill in the partially filled zone is well predicted by taking into account all non‐pressure driven leakage flows.