bluuebunny/crossref_metadata_2025 huggingface dataset:
Non‐linear creep of polymers under superimposed static and dynamic stress
Authors: Akay G.
Published: December 2004

The non‐linear creep of polymeric materials under super‐imposed static and dynamic stress is considered theoretically. The equation of state due to Green, Rivlin, and Spencer to‐gether with the power law of time dependence for the kernel functions as suggested by Nakada is assumed to characterize the non‐linear materials. Expressions for creep strain under constant static, oscillatory dynamic and superimposed static and dynamic stress are obtained in terms of the material constants and time dependent functions, called dynamic creep functions. It is shown that the creep strain due to dynamic stressing is damped as the number of stress cycles is increased. Damping is faster if the power law of time dependence is high. Expressions for the cumulative creep strain after multiple stress cycles are also obtained in terms of cumulative strain functions. All these functions are evaluated numerically at one thousand stress cycles. Finally, a special case of stress history is considered where the stress periodically reaches zero. It is shown that the ratio of the strains due to dynamic and static stressing can be characterized by the power law parameter when the mean stress is either very high or very low. Due to the slow damping when the power law parameter is small, the decrease of the strain ratio with number of cycles is slow compared to higher power law parameters.
Morphology of random and non‐random olefin copolymers of ethylene
Authors: Bailey F. E. , Walter E. R.
Published: December 2004

Studies have been published relating the mechanical properties of ethylene copolymers with short chain branching structures. Other work has emphasized that crystallites in these copolymer systems participate in superstructures and that stress distribution during mechanical testing depends on the arrangement of these structures, the morphology. While developing structure‐property correlations, morphologies of some random and non‐random olefin copolymers have been examined. Surprising morphological differences were found depending on the length, concentration and degree of randomness of the n‐alkyl branch and on vicinal substitution with norbornene derivatives as comonomers. X‐ray diffraction yielded unexpected results which can be explained by a change in the direction of chain folding. Linear side chains randomly introduced into an otherwise high density polyethylene result in a two to three fold increase in the 110:200 x‐ray intensity ratios, a slight reduction in the 110:020 intensity and a 5 to 10 fold decrease in the 200:02 intensities. These results cannot be explained by orientations introduced in sample preparation. There are associated morphological changes. Vincinal substitution with norbornene comonomers has little effect on crystal habit but does change the morphology, tending to shift superstructures from lamellar to fibrillar in nature.
The injection molding behavior of thermoplastics in thin rectangular cavities
Authors: Kamal Musa R. , Kuo Youti , Doan P. H.
Published: December 2004

Two models are presented for simulating the injection molding of thermoplastics in thin, rectangular cavities. One is a much simplified bounded‐radial‐flow model that utilizes an existing numerical model for semi‐circular cavities to adjust for the non‐radial flow region bounded by the lateral walls. The other is a two‐dimensional analysis which assumes that both viscosity and temperature change strongly across the narrow gap but vary weakly in the directions of flow. The latter analysis allows application of the potential theory and the determination of streamlines and progressing front shapes. Both models deal with a non‐Newtonian viscosity with temperature variation. Comparisons between experimental and computational results are shown.
Characterization and piezoelectric activity, of stretched and poled poly(vinylidene fluoride). Part I: Effect of draw ratio and poling conditions
Authors: Shuford Richard J. , Wilde Anthony F. , Ricca John J. , Thomas George R.
Published: December 2004

A number of investigators have reported on the high degree of piezoelectricity manifested by oriented films of poly(vinylidene fluoride) (PVF2). To develop applications for this piezoelectric effect, our laboratory is involved in a systematic investigation of the factors responsible for this remarkable behavior of PVF2. In a unique high‐speed process, commercial PVF2 film was uniaxially stretched to a series of draw ratios ranging up to 7/1. The resulting films were characterized by techniques involving infrared spectroscopy, density, birefringence, sonic modulus, X‐ray diffraction, and dynamic mechanical response. The films were then poled at various electric field strengths, temperatures, and times. Correlations have been made between draw ratio, physical properties, poling conditions, and piezoelectric activity of the films. It was found that the piezoelectric activity increased to limiting values with draw ratio, poling voltage, poling temperature, and poling time. It was evident that for PVF2film a significant amount of oriented phase I crystalline material is required for high degrees of piezoelectric activity. The Appendix gives the apparent rate dependence observed for the piezoelectric effect when signal is measured with a voltage sensor of relatively low input impedance.
Strain‐induced crystallization I. Limiting extents of strain‐induced nuclei
Authors: Yeh G. S. Y.
Published: December 2004

This paper points out that interpretations from stress changes alone during oriented crystallization have led to widely different proposed chain conformations and consequently very different crystallization mechanisms for strain‐induced crystallization (SIC). Many of the proposals, including the one by Keller and Machin which takes into account some electron microscopy and X‐ray observations, show varying degrees of incompatibility with existing stress relaxation, kinetic or morphological data on SIC. Another problem lies in the difficulty with proper interpretation of observed morphology on samples which have been Subjected to additional thermally‐induced crystallization (TIC) after SIC, especially, in the absence of prior characterization of SIC crystallites, the finding of a fibrillar‐to‐lamellar transformation in stretched polymers upon additional TIC (Part H) also indicates that the generally‐observed oriented lamellar morphology has a much more subtle origin than‐that depicted by most crystallization models. Part I discusses our previously published morphological data on the characteristics of SIC crystallites from the melt, which includes: (a) their melting point elevation, (Tm » T°m), (b) their nearly perfect crystalline orientation function (fc ∼ 1), (c) their fast rates of crystallization (t1/2 < 1 sec), and (d) their fibrillar morphology and limited dimensions along the fibrillar stretch axis (∼100Å). Examples of morphology of SIC from the glass and from stirred solution are also included to show the overall similarity of fibrillar morphology brought about by stretching.
Optical studies of the stress‐induced crystallization of polymers
Authors: Stein Richard S.
Published: December 2004

Stress‐induced crystallization may be studied by the birefringence technique and by low‐angle light scattering. From measurements of the change in birefringence and stress during the crystallization of a polymer above its Tg and from a calculation of the intrinsic birefringence of a polymer crystal, the change in volume fraction crystallinity may be calculated. The technique is illustrated for several polymers and found to give values in reasonable agreement with other methods for the study of crystallinity. Crystallization is also accompanied by the development of a low‐angle light scattering pattern, the size and shape of which is indicative of the amount, size and morphology of the crystalline superstructure.
The interaction of fluid mechanics and crystal growth in the formation of shish kebabs from solution
Authors: McHugh A. J. , Silebi C.
Published: December 2004

Several observations are made regarding the role of the fluid mechanics involved in the process of flow‐induced, fibrillar crystallization from solution. Computational evaluation and discussion of various flow effects in the nucleation step, including combined shear and extensional flows, are presented in terms of the elastic dumbbell model. These results indicate the important role extensional kinematics in the growth process. A qualitative evaluation of concentration effects is also given. The evaluation of facts concerning the growth morphology is presented in terms of a model for the growth process along with data concerning rinsing and quench rate effects on the morphology and inciting behavior of shish kebab crystals.
An experimental study of the kinetics of polymer crystallization during shear flow
Authors: Lagasse R. R. , Maxwell B.
Published: December 2004

Some principles of rheology are applied to the study of the shear‐induced crystallization of molten polymers. A new technique is described for measuring crystallization kinetics during isothermal flow at constant shear rate in a parallel plate rheometer. The crystallization rate is characterized by the time elapsed from the start of shearing until the rise in melt viscosity due to crystallization. The measured‐viscosity and induction time for crystallization are shown to be independent of sample geometry. Kinetic data are presented for crystallization of three linear polyethylenes at shear rates of 0.03 – 30 sec−1. It is shown that shear flow has a strong accelerating effect on crystallization when the deformation rate exceeds a critical value. Comparison of results for the different polyethylenes reveals that higher molecular weight materials crystallize faster at a given shear rate and temperature. Finally, shear‐induced crystallization of propylene polymers is shown to be unaffected by the presence of either a carbon black additive or a heterogeneous nucleating agent. It is concluded that the hydrodynamic origin of the shear‐induced crystallization is elastic chain extension due to entanglement couplings between molecules. Furthermore, it is suggested that transient orientation effects during the startup of shear flow may have a dominant influence on the observed phenomena.
A dynamic melting model for a single‐screw extruder
Authors: Lindt J. T.
Published: December 2004

A new theoretical concept is given which provides an efficient and consistent method for predicting flow and heat transfer characteristics of the melting zone for a large single‐screw extruder. In contradistinction to previous, theories, significant melt accumulation in the molten films as observed experimentally is considered, instead of postulating the melt accumulation in the “melt pool”. The mathematical model has been obtained by the simultaneous solution of the momentum and energy equation of the melt flow and. solid bed, allowing for the existence of the pressure gradient: The theory is supplemented by a numerical example which shows good agreement with experimental data obtained on a 90 mm extruder with polypropylene, where the down‐channel pressure profile and me profile of the solid bed Were taken as yardsticks for the melting process. For exact determination of the area for applicability of this theory, more experimental information is required.
High performance fibers from aromatic polymers
Authors: Preston J.
Published: December 2004

Examples are given to show how molecular structures may be synthesized to prepare polymers useful for the formation of fibers intended for specific end uses, particularly for high performance applications. Current trends and developments taken from the literature are reviewed and some new work from the laboratories of Monsanto Company is presented concerning the use of aromatic polymers for heat and flame resistant fibers. Some new polymers and new routes of synthesis are described. Correlations are made between flame resistance as measured by oxygen index (O.I.) values and the structure of a large number of aromatic polymers.
Polyphenylquinoxalines—high performance thermoplastics
Authors: Hergenrother P. M.
Published: December 2004

The synthesis, processing characteristics, thermal propers, and adhesive and composite properties of polyphenvlqmnoxahnes are briefly reviewed. In addition, new high molecular weight soluble carbonate/phenylquinoxaline copolymers from the cyclopolycondensation of aromatic bis(o‐diamines) with a bis(benzilyl) carbonate are reported.
Effects of multifunctional crosslinking agents on the thermomechanical properties of polyimide Films
Authors: Delvigs Peter
Published: December 2004

Four new poly functional amines were synthesized. Polyimide films containing varying amounts of the amine crosslinking agents were prepared. Their thermomechanical properties were compared with those of control film samples prepared from difunctional amines. The effect of crosslink density on isothermal weight loss, polymer softening, and glass transition temperature is discussed.
Poly‐para‐xylelene: Its chemistry and application in coating technology
Authors: Szwarc Michael
Published: December 2004

The chemistry of p‐xylelene and of its analogues has been reviewed. Its modes of formation and polymerization to poly‐p‐xylelene have been described, and the initiation and hypothetical termination steps have been discussed. The reactions taking place in the gas phase and in the deposited film are contrasted with those taking place in solutions. The problems of crystallization of the polymer simultaneously proceeding with its growth have been stressed. Finally, the properties of poly‐p‐xylelenes and their application in the coating industry have been described in some detail, emphasizing the special advantages of gas‐deposition leading directly to a solid and coherent film. Some future fields of application of this technique have been outlined.
Lactone polymers VII. Activation energies for glass transitions of poly‐ϵ‐caprolactone and poly‐ϵ‐thiocaprolactone
Authors: Seefried C. G. , Kodeske J. V.
Published: December 2004

The activation energies for segmental motion have been determined for partially crystalline poly‐ϵ‐caprolactone and poly‐ϵ‐thiocaprolactone over a temperature range that extends from below the glass transition to near the melt region. Based on the shift factors obtained from the time‐temperature superposition of stress relaxation data, the activation energy for the glass transition of poly‐ϵ‐caprolactone is 160 kcal/mole and of poly‐ϵ‐thiocaprolactone is 200 kcal/mole. The value obtained for poly‐ϵ‐caprolactone is in excellent agreement with the reported value obtained from dielectric relaxation studies. A relatively broad secondary maximum in the activation energy spectrum was observed for each polymer in the respective temperature regions between the glass transition and the melt zone.
New techniques for mixing viscous reacting liquids. Part II. Electrical‐mechanical hybrid mixer
Authors: Rotz Christopher A. , Suh Nam P.
Published: December 2004

Based on an understanding of the basic mechanisms and governing laws for liquid mixing, a new concept for increasing the efficiency of laminar mixing is advanced. The new concept is motivated in part by the observation that in laminar flows, the interface between two fluid components tends to line up parallel to the streamlines, leading to decreased mixing efficiency. This decrease in effectiveness can be overcome by simultaneously applying mechanically induced shear forces and electrically induced destabilizing forces (which act on the interfaces) to mix the liquids. Results show a dramatic increase in mixing efficiency with the electrical‐mechanical hybrid system. Descriptions of the analytical and experimental work done to characterize the concept and prove its efficacy are presented in detail. Scale‐up factors are discussed in terms of dimensionless groups.
Powder processing of ultra‐high molecular weight polyethylene I. Powder characterization and compaction
Authors: Halldin G. W. , Kamel I. L.
Published: December 2004

Powders of ultra‐high molecular weight polyethylene from three suppliers were characterized for density, crystallinity, particle size, particle size distribution and particle morphology. The powders had molecular weights in the range 2‐5 million. Bulk powder behavior, compressibility, green strength and springback were evaluated and explained in terms of particle characteristics. The green densities of the powders were found to reach a plateau at pressures of about 100 MPa. These plateau density levels were found to depend upon powder, characteristics and to lie between 80 and 90 percent relative density. Green strength is shown to be a unique function of a densification parameter.
The role of polymers in optimizing therapeutic effectiveness of drugs
Authors: Leeper Harold , Benson Harriet
Published: December 2004

A new class of pharmaceutical dosage forms—therapeutic systems—has been developed. Unlike conventional dosage forms (i.e., pills, capsules, or sustained release preparations), which are specified only by the amount of drug they contain, therapeutic systems are described both by the rate and duration of drug delivery. The first products to embody this technology are: (1) an ocular therapeutic system that delivers the anti‐glaucoma drug, pilocarpine, to the eye for one week; (2) an intrauterine contraceptive system that delivers the fertility controlling hormone, progesterone, to the uterus for one year; and (3) an oral osmotic delivery system for administration of drug to the gastrointestinal tract. Specific furictiQns of polymers as constructional elements of therapeutic systems are discussed and consideration is given to properties critical to the intended performance, test procedures, and parameters for prediction of mechanical and physico‐chemical functionality, stability, and safety.
Distribution of some physical properties in injection‐molded thermoplastic parts
Authors: Bakerdjian Z. , Kamal Musa R.
Published: December 2004

During the injection molding process, moldings undergo shear and elongational stresses. They are also exposed to thermal gradients of varying intensity. Invariably, molded parts, or at least some sections thereof, freeze before the polymer chains can relax to a random, non‐oriented configuration. As a result, injection molded parts contain frozen strains and exhibit anisotropic physical properties. In the present work, a variety of experimental techniques have been employed to determine the three‐dimensional variation of the following properties of injection‐molded, thermoplastic, rectangular parts: density, heat shrinkage, birefringence, and tensile strength.
Parameters, affecting the sensitivity of poly(methyl methacrylate) as a positive lithographic resist
Authors: Gipstein Edward , Ouano Augustus C. , Johnson Duane E. , Need Omar U.
Published: December 2004

The contribution of parameters such as molecular weight, molecular weight distribution and stereochemistry to electron beam sensitivity of poly(methyl methacrylate), PMMA, has been investigated. The sensitivity is interpreted here as the minimum radiation dose required to obtain a predetermined solubility rate ratio, S/So = SR, of the exposed, S, and unexposed, So, material. The G‐value was foam be independent of molecular weight, molecular weight distribution, and stereochemistry. Data indicate that the weight average molecular weight ratio correlates better with SR than the number average molecular weight ratio. The tacticity of the resist and the developer solvent molecular weight or size have a large effect on solubility rate. Although the solubility rate pf isotactic PMMA is much greater than the syndiotactic and heterotactic stereoforms, the sensitivity appears to be independent of tacticity. In a homologous series of n‐alkyl acetate developer solvents, the molecular size of the solvent has a greater effect on the solubility rate than the molecular weight of the resist. A developer solvent has been selected from the n‐alkyl acetates which enhanced the sensitivity of PMMA.
The synthesis, characterization, evaluation, and processing of selected cyclo‐olefin sulfone copolymers as electron beam resists
Authors: Himics R. J. , Kaplan M. , Desai N. V. , Poliniak E. S.
Published: December 2004

The potential usefulness of electron beam sensitive polymers depends to a large extent on a variety of polymer characteristics such as solubility, sensitivity, and processability which are, in turn, required by the application. This paper details results obtained for methylcyclopentene sulfone copolymers as electron beam recording media. The determination of physical properties and the optimization of processing conditions are reported.
Thermally‐reacted poly(methacrylamide) as a positive electron beam resist
Authors: Matsuda S. , Tsuchiya S. , Honma M. , Hasegawa K. , Nagamatsu G. , Asano T.
Published: December 2004

Thermally‐reacted poly(methacrylamide) degrades under high‐energy irradiation. It has a sensitivity of 2‐3 × 10−7 C/cm2 when exposed to 10 KV electrons. The resist is thermally stable to 330°C and has a high glass transition temperature, 180 200°C. It may be used for the lift‐off process and ion‐milling.
Limiting surface tension by gas chromatography
Authors: Braun J.‐M. , Guillet J. E.
Published: December 2004

The change in retention mechanism at the glass transition temperature (Tg) of a polymer stationary phase from surface adsorption below Tg to bulk sorption above Tg allows for a separate determination of the magnitude of the solute interaction with both the bulk and the surface of the polymer. As a result, the limiting surface tension of the polymer‐probe solution can be obtained from the corresponding partition coefficients. Examples of such determinations are given for several polymer‐solute systems.
Diffusion and solution of gases and vapors in styrene‐butadiene block copolymers
Authors: Odani Hisashi , Taira Kazuo , Nemoto Norio , Kurata Michio
Published: December 2004

The diffusion, solution, and permeation behavior of a series of inert gases (helium, argon, nitrogen, krypton, and xenon) in S‐B block copolymer films was studied by transient permeability measurements and by the equilibrium desorption method. The morphologies of most of the samples used in the measurements were (a) polystyrene rods dispersed in a polybutadiene matrix and (b) alternating lamellae of styrene and butadiene components. It was indicated, as far as the kinetic nature at lower temperatures is concerned, that the diffusion and permeation processes of gases, except for helium, are governed primarily by behavior in the polybutadiene matrix. At lower temperatures, it was shown that the transient method counts only the mobile penetrant in the polybutadiene matrix, while the equilibrium method counts less diffusive species in the polystyrene domains as well. The diffusion behavior in the copolymer films was compared with that in homopolybutadiene and discussed in terms of two impedance factors: the tortuosity and the chain immobilization factors. From the homopolymer‐block copolymer comparison along with results obtained from diffusion experiments using n‐hexane as the penetrant, it was indicated that segmental motions in the polybutadiene phase in the copolymers are restricted relative to motions in homopolybutadiene. Also, from data on gas sorption in samples of various styrene contents, involving both S‐B block copolymers and binary mixtures with homopolystyrene, it was suggested that the partial mixing of component block chains occurs at the interface between the domains, resulting in rather diffuse domain boundaries.
Glass bead‐filled polypropylene part I: Rheological and mechanical properties
Authors: Faulkner D. L. , Schmidt L. R.
Published: December 2004

Five glass bead‐filled polypropylene composites were rheologically characterized at 240°C using two rotational rheometers to obtain low shear‐rate data and a capillary rheometer to obtain high shear‐rate data. Both steady and dynamic properties were measured at low shear rates. Each composite was also injection molded into tensile and flexural test bars for a mechanical properties profile at 25°C. The tensile modulus was determined from a simple extensional deformation whereas the flexural modulus was determined from a three‐point‐bend test.

The relative shear viscosity and relative loss modulus are different nonlinear functions of the volume fraction of beads at a constant shear rate, while the relative storage modulus appears to be a linear function of bead fraction. The relative viscosity decreases with increasing shear rate and the zero shear‐rate data are in very good agreement with the Guth‐Gold equation.

The relative tensile modulus and relative flexural modulus are each linear functions of bead fraction over the entire range of filler concentration, 0‐29 vol percent. From these data it is concluded that a simple correspondence between slow viscous flow and small strain elasticity does not exist for these composites.
A study of the effect of pressure, time, and temperature on high‐pressure powder molding
Authors: Bigg D. M.
Published: December 2004

High pressure room temperature molding of polymeric powders has been found to be a useful technique for producing parts from certain hard‐to‐process polymers and certain common polymers. Polymeric powders of less than 175μ were compacted at pressures up to 0.689 GPa. Subsequent heat treating of the compacted samples improves the mechanical properties of the samples to levels comparable to those attained by other techniques. Thermosetting or reactive polymers which do not evolve vapors during curing are readily processed by this technique. Semicrystalline polymers which are molded above their Tg are also easily processed by this method. Glassy polymers, in general, have not been found to be processable. The important process variables are molding pressure, molding time and heat treating temperature and time. The process relies on particle‐to‐particle fusion by either chemical reaction or localized melt fusion. For semicrystalline polymers, the annealing temperature is within the melting endotherm. Reactive polymers cure at their optimum curing temperature.
Olefin‐SO2 copolymers as biomaterials
Authors: Gray D. N.
Published: December 2004

The series of olefin‐SO2 copolymers prepared from the C8‐C18 even‐numbered carbon series of alpha‐olefins and sulfur dioxide have been studied. The C16 polymer especially has high gas permeability, good biocompatibility, low, toxicity, low thrombogenicity, ease of fabrication and potential low cost. Maximum permeabilities, maximum percent elongation at the yield point and minimum density are obtained with the C16 based polymer. An explanation is proposed involving a competitive plasticizing/side chain crystallization effect.
On the effect of mechanical energy on the degradation of polyethylene in the solid state and in the melt
Authors: Holmström Arne , Andersson Arne , Sörvik Erling M.
Published: December 2004

The influence of mechanical forces on structural changes in commercial grades of high and low density polyethylenes was investigated by using extremely high shear rates (>2 × 104S−1) during processing in a special injection molding machine and by long‐term loading at ambient temperature. Changes in molecular weight distribution (MWD) and the degree of long‐chain branching (LCB) were followed by gel permeation chromatography and viscometry. No effect of the mechanical forces was observed either in the long‐term loading or in the processing. At temperatures exceeding 280°C the small changes noted in MWD and LCB wrere found to be entirely due to thermal de radation.
Effects of post‐curing of unsaturated polyester resins in various media
Authors: Chen Show‐An , Tsai Pai‐Kuen
Published: December 2004

Experimental studies were performed on the effects of postcuring of' styrenated, unsaturated polyesters in nitrogen gas, water or its vapor, hexane vapor, dimethyl sulfoxide (DMSO), and dirnethyl formamide (DMF) all at 100°C It was found that, nitrogen gas and water are inactive, hexane acts as a chain transfer agent, and DMF and DMSO behave as solvents which extract residual styrene monomer and anhydrides from the resins. Post‐treatment in inert media can result in further reaction leading to more complete cure as free radicals and residual monomer continue to react. The post‐curing reaction is a radical copolymerization. Further condepgation polymerisation does not occur. Post‐curing in, hexane vapor1eads to a transfer of the active sites from the radicals to the hexane and to no further radical polymerization.
Light stabilization of flame retardant impact polystyrene
Authors: Bradley Richard , Farber Joseph , Testa Lawrence
Published: December 2004

This paper presents the results of a study onn the effectiveness of ultraviolet absorbers and light stabilizers in preventing the discoloration of flame retardant impact polystyrene. The deleterious effect of certain flame retardants, i.e., aromatic or aliphatic halogenated compounds, on the light stability of this polymer is shown. Light stabilizer systems resulting in substantially improved color stability during light exposure are presented. The beneficial effect df co‐additives, including antioxidants, acid acceptors, epoxies, phosphites, and thiosynergists, on light stability is also discussed.
Cobalticenium‐Bridged polybenzazoles. Part II: High‐temperature. Solution polymerization in ethylated polyphosphoric acid
Authors: Neuse Eberhard W. , Horlbeck Gernot
Published: December 2004

The polycondensation of 1,1′‐diearboxycobalticenium hexafluorophosphate with 3,3′‐diaminobenzidine in ethylated poly phosphoric acid at 100‐240°C furnishes linear, soluble polymeric intermediates for the most part composed of cobalticenium‐1,1′‐diyl and bibenzimidazoline‐2,2′‐diyl units alternating along the chain. Heating of the primary products in the solid state at 300°C results in aromatization, giving polyheterocyclics essentially composed of alternating cobalticenium‐1,1′‐diyl and bibenzimidazole‐2,2′‐diyl units. The analogous reaction of the cobalticenium salt with 3,3′dimercaptobenzidine produces primary polymers containing benzothiazoline in lieu of benzimidazoline ring systems, which, by a heat treatment as before, convert to the corresponding polymeric benzothiazoles. The primary condensation products of both reaction types are of low molecular mass (ηinh, 5‐13 ml/g) vlien prepared at temperatures not exceeding 150°C. The degree of polymerization increases (ηinh, 20‐60 ml/g) at temperatures in the 200‐250°C range; however, Lewis acid‐induced cobalt‐ring bond cleavage proceeds as a significant side reaction under these conditions, leading to random chain scission and alteration of the polymer structure by alicvclic hydrocarbon groups. In addition, the benzimidazole polymers are to a variable extent contaminated with phosphate groups stemming from the medium. These structural defects, paired with distinct reactivity of the PF6‐ anion at higher temperatures, result in a thermogravimetric analysis performance not quite equal to that of common phenylene‐bridged Polybinzazoles.
Influence of melt deformation history on orientation in vitrified polymers
Authors: Oda Kunio , White James L. , Clark Edward S.
Published: December 2004

An experimental study of the development of orientation in polystyrene melts during flow and its retention in vitrified parts is described. It is shown on the basis of elongational and shear flow experiments that orientation in vitrified polystyrenes may be predicted from a knowledge of the stress field at the time of vitrification and application of stress‐optical laws. More generally a relationship between birefringence and principal stress difference is found which correlates (1) on‐line isothermal shear flow, (2) on‐line non‐isothermal elongational flow (melt spinning), and (3) vitrified samples formed in both shear and elongational flow. It is further proposed that orientation in polymer chains in deforming melts is uniquely dependent on stress—specifically, that the ratio of the stress‐optical constant C to the intrinsic birefringence Δ° is approximately a constant.
Flammability characteristics of fiber reinforced organic matrix composites
Authors: Silvergleit M. , Morris J. G. , Larosa C. N.
Published: December 2004

The effect of resin, fiber, and fire retardant additives on flammability characteristics of organic matrix composites was evaluated. Information is presented on the flame spread index, determined by the radiant panel test, the amount of smoke generated, and products of combustion, using the National Bureau of Standards Smoke Density Apparatus, and the amount of oxygen required to support combustion using the Oxygen Index method. These methods were effective in screening the flammability characteristics of organic matrix composites. Of the materials evaluated the polyimide composites were the most resistant to flame spread, exhibited the lowest evolution of smoke and toxic products and had the highest oxygen index. No differences in flame spread and oxygen index were observed for the polyester epoxy glass‐cloth laminates. Addition of antimony trioxide and hydrated alumina to the polyester and epoxy resin systems significantly decreased the flame spread index and increased the oxygen index, but showed a marked increase in smoke evolution. Smoke properties depended on resin content whereas the type of reinforcement did not appear to affect flame spread index or smoke properties. The use of protective barriers in selected shipboard areas can reduce flame spread and lengthen the lime for generation of smoke.
Static behavior and the effects of thermal cycling in hybrid laminates
Authors: Liber T. , Daniel I. M. , Chamis C. C.
Published: December 2004

The influence of hybridization with stacking sequence variation on static stiffness, strength, ultimate strain and residual properties after thermal cycling of graphite/Kevlar 49/ epoxy and graphite/S‐glass/epoxy angle‐ply laminates was investigated. Tensile stress‐strain curves to failure and uniaxial tensile properties were determined for all laminates. Theoretical predictions of modulus, Poisson's ratio and ultimate strain were made, based on linear lamination theory, constituent ply properties and measured strength. Reasonably good agreement was found. Stacking sequence variation showed no significant systematic influence on the measured results. Specimens containing only two 0‐degree Kevlar or S‐glass plies behaved linearly to failure. Specimens containing four 0‐degree Kevlar or S‐glass plies displayed characteristic non‐linearity. One group of laminates was subjected to a tensile load and to 100 thermal cycles between room temperature and 280°F, and another group to a tensile load and 100 thermal cycles between room temperature and −100°F. All surviving specimens were tested statically to failure to determine residual properties which were compared with the properties of uncycled specimens. Specimens containing two Kevlar or two S‐glass plies behaved linearly to failure. Residual moduli in these specimens were lower than for uncycled specimens but residual strengths and ultimate strains were higher. Specimens with four Kevlar or four S‐glass plies showed some nonlinear behavior. No significant differences were found between residual and uncycled values for modulus and strength.
Crystal nucleation in sheared polymer melts
Authors: Andersen P. G. , Carr S. H.
Published: December 2004

A semiquantitative procedure has been developed for analyzing crystal nucleation in undercooled polymer melts that are undergoing flow. This analysis was applied to the specific ease of molten high density polyethylene experiencing low levels of shearing flow. A custom‐made concentric cylinder viscometer, which could be operated by the Rheometrics mechanical spectrometer instrument, was used to make simultaneous measurements of transmitted torque and optical anisotropy in isothermal melts, The result of the analytical procedure developed here was molecular Size‐dependence of chain distensions caused by prevailing shear. This distribution function was verified by testing against experimentally obtained values of birefringence. Total entropy reduction resulting from this distorted state was then calculated, and the corresponding increase in free energy was found to be at least enough to account for comparable crystal nucleation rates in flowing melts at higher temperatures and in quiescent melts at lower temperatures.
The effect of annealing on the mechanical anisotropy of cold rolled polyoxymethylene
Authors: Bahadur Shyam
Published: December 2004

The effect of cold rolling and subsequent annealing on the anisotropy of tensile properties has been studied by annealing 33 percent cold rolled polyoxymethylene at 70, 120 and 170°C and measuring the properties at 0, 45 and 90° from the rolling direction. The mechanical and thermal treatments exhibit a significant effect on the directional nature of the properties. The changes in mechanical properties have been accounted for in terms of the changes in morphology, fracture behavior and orientation as studied by a differential thermal analyzer, electron microscopy and wide‐angle X‐ray diffraction.
The effect of extrusion on the mechanical properties of polytetrafluoroethylene paste
Authors: Coleman C. J.
Published: December 2004

We consider the connection between viscous effects experienced by polytetrafluoroethylene paste during extrusion, and the final strength of the extrudate. It is shown that by reducing the viscous contribution to the pressure drop across the extruder die, we not only save on extrusion pressure, but also increase the strength of the extrudate. The shape of die that gives the minimum pressure drop is derived.
Influence of glass fibers on the extrusion and injection molding characteristics of polyethylene and polystyrene melts
Authors: Chan Yu , White James L. , Oyanagi Yasushi
Published: December 2004

A basic study is reported of extrusion and injection molding for melts of polyethylene and polystyrene filled with glass fibers. It is shown that glass fibers cause large increases in end pressure losses and decreases in extrudate swell. Flow visualization of injection molding studies indicates that polymer melts containing high loadings of glass fibers exhibit jetting in end gated molds. This is attributed to the reduction in extrudate swell. To avoid jetting, special care is required in design of gating.
Factors influencing the impact strength of high impact polystyrene
Authors: Pi Chang Eng , Takahashi Akio
Published: December 2004

The relationship between synthesis factors and the impact resistance of high impact polystyrene (HIPS) is investigated in the light of its morphology and dynamic mechanical properties. A decrease in polymerization temperature results in an increase in Tg, melt viscosity and molecular weight of the continuous polystyrene phase as characterized by gel permeation chromatography. The separated, occluded polystyrene phase however shows an invariant Tg suggesting that the grafting and/or crosslinking effect overweighs the molecular weight effect. The observed high impact strength has been correlated with the homogeneous 1‐2 μ rubber particle size distribution, a comparatively sharp rubber Tg transition at lower temperature, and a much lower occluded polystyrene content in the dispersed phase.
Vinyl chloride formation from the thermal degradation of poly(vinyl chloride)
Authors: Wakeman I. B. , Johnson H. R.
Published: December 2004

The volatile products from the thermal degradation of poly(vinyl chloride) (PVC) resins and compounds are shown to contain trace amounts of vinyl chloride. Data presented show the effect of temperature and resin type on the amount of vinyl chloride formed. At the maximum temperatures involved in PVC processing which may reach 210°C., vinyl chloride monomer (VCM) evolution amounts to less than 1 ppm (resin basis). A technique employing a thermogravimetric balance and charcoal adsorption of volatiles is described for studying thermal degradation of PVC. The volatiles are analyzed for vinyl chloride by gas chromatography. Peak identity was confirmed by mass spectrometry.
Tensile deformation of nylon 6 fibers
Authors: Oilman T. H. , Resetarits M. R. , Crist B.
Published: December 2004

Nylon 6 fibers which had been relaxed to different extents by annealing were examined at fixed strains by small angle and wide angle X‐ray techniques. It was found that the strain of the long period of the semicrystalline microfibrils is identical to the macroscopic fiber strain. Approximately 1/3 of the tensile deformation results from molecular shear of imperfectly oriented crystalline chains. Virtually no evidence for intercrystalline slip is found; the orientation of the intercrystalline amorphous regions results in a low compliance for the shear of crystals past one another. The majority of the microfibril deformation occurs by stretching these intercrystalline amorphous regions, accompanied by the flow of extrafibrillar amorphous material to maintain constant volume. In highly annealed fibers this “filling” mechanism is less efficient, as the amount of extrafibrillar material has been reduced during shrinkage. This effect leads to a decrease in Poisson's ratio after increasingly severe annealing. A related result of annealing is the dehomogenization of the microstructure, leading to the presence of more stress‐induced “microcracks” during the stretching of annealed fibers.
The role of crystallinity and morphology in the preparation of ultra‐highly oriented polyethylene
Authors: Capaccio G. , Crompton T. A. , Ward I. M.
Published: December 2004

The influence of the degree of crystallinity and morphology on the drawing behavior of linear polyethylene (LPE) has been investigated on three polymers with molecular weights ranging from 67,000 to over 300,000. Samples of similar crystallinity, but different morphology, were obtained by subjecting the polymers to two distinct preparations: slow cooling from the melt in one case and rapid quenching followed by annealing at 120°C in the other. The resulting isotropic products, characterized by optical microscopy and density measurements were drawn at constant speed and the deformation process monitored by recording the draw ratio/draw time relationship and the stress/strain curves. The results indicate that crystallinity per se does not have a primary effect in determining the rate of local deformation except in the case of polymers of very low weight average molecular weight and narrow molecular weight distribution. A dominant role seems to be played by the broad features of the sample morphology as detected by optical microscopy. A comprehensive explanation of these results is based on the concept of a network structure whose nature is affected by the annealing treatment to an extent which depends on the degree of coupling between adjacent crystalline regions in the isotropic undrawn polymer.
Time and strain dependent thermal expansion of carbon‐black filled SBR elastomers
Authors: Caruthers J. M. , Hooley C. J. , Cohen R. E.
Published: December 2004

Length‐temperature measurements were made on unfilled and carbon‐black filled vulcanizates of styrene‐butadiene rubber. As in earlier experiments on polybutadiene, transients in thermal expansion behavior were observed following abrupt changes in the temperature. The nature of this time dependence was not dramatically affected by either the level of equilibrium strain or the structure and loading of the carbon‐black filler. Both the magnitude of the linear coefficient of thermal expansion and the point of thermoelastic inversion were sensitive to the presence of carbon‐black. A model based on considerations of occluded rubber and strain amplification in the free rubbery matrix was used successfully to rationalize the observed behavior of the filled compounds.
Polymer blends containing poly(vinylidene fluoride). Part I: Poly(alkyl acrylates)
Authors: Wahrmund D. C. , Bernstein R. E. , Barlow J. W. , Paul D. R.
Published: December 2004

The transitional behavior of poly(vinylidene fluoride) (PVF2) blends with poly(methyl acrylate) and with poly(ethyl acrylate) was examined by differential thermal analysis and dynamic mechanical testing. Both blend systems were judged to be miscible on the bases of the presence of single, composition dependent glass transitions and of the strong melting point depression of the PVF2 component, Blends of poly(isopropyl acrylate) with poly(vinylidene fluoride) were found to be immiscible. These results suggest that miscibility of the acrylate series depends on a specific attractive interaction between the PVF2 and oxygen within the acrylate and the effect of this interaction is diminished as the hydrocarbon content of the ester is increased.
Studies on structural foam processing I. The rheology of foam extrusion
Authors: Han Chang Dae , Villamizar C. A.
Published: December 2004

An experimental study was carried out to investigate the flow behavior of gas‐charged molten polymers in foam extrusion. For the study, a rectangular slit die with glass windows was constructed to permit visual observations, from the direction perpendicular to flow, of the dynamic behavior of gas bubbles when a gas‐charged molten polymer flows between two parallel planes. Pictures were taken of gas bubbles in the flow channel with the aid of a camera attached to a microscope, and these were later used to determine the position at which gas bubbles start to grow. Using three melt pressure transducers mounted on the short side of the rectangular slot, pressure distributions were measured along the longitudinal centerline of the die. The polymeric materials used were high‐density polyethylene and polystyrene, and the chemical blowing agents used were a proprietary hydrazide which generates nitrogen, and sodium bicarbonate which generates carbon dioxide. It was observed that the gas‐charged molten polymer shows a curved pressure profile as the melt approaches the die exit, whereas the polymer without a blowing agent shows a linear pressure profile. The visual observations of the bubble growth in the flow channel, together with the pressure measurements, permitted us to determine the bubble inflation pressure, often referred to as the critical pressure for bubble inflation. It was found that the critical pressure decreases with increasing melt extrusion temperature, and increases with increasing blowing agent concentration. It was also found that the bulk viscosity of gas‐charged molten polymers decreases with increasing blowing agent concentration and with increasing melt temperature. A general remark is made concerning the precaution one should take when an Instron rheometer is used for determining the bulk viscosity of gas‐charged molten polymers.
Viscoelasticity of ABS samples differing in thermal history
Authors: Booij H. C. , Palmen J. H. M.
Published: December 2004

An important parameter in the various molding techniques used in polymer processing is the rate of cooling the object. The effect of the thermal history on the linear viscoelastic properties was studied, mainly by means of stress relaxation experiments on ABS samples in which the influence of other process variables was eliminated. In agreement with recent literature data, it appeared that the time dependence of the stress can be represented by a universal formula, provided that no perceptible change in temperature or relaxation of volume takes place during the experiment. The position of the momentary relaxation curve along the time scale can therefore be described by one characteristic mechanical relaxation time tr. The value of tr is affected by the temperature T, but even more by the thermal history, i.e., by the rate of cooling, q, and the annealing time, ta, at the measuring temperature previous to recording of the relaxation curve. In the case of very rapidly quenched samples, exact proportionality was found to exist between tr and ta at temperatures below Tg −15°C. In slowly cooled samples tr can be longer by a factor of ten than in quenched samples, at the same T and at annealing times of about 1 h. The difference decreases with increasing ta, but remains substantial up to ta values much longer than the total cooling time.